Networking Glossary

1000Base-CX	A two-wire implementation of Gigabit Ethernet using shielded twisted pair cables.
1000Base-LX	A two-wire implementation of Gigabit Ethernet using optical fibers transmitting long-wave laser signals.
1000Base-SX	A two-wire implementation of Gigabit Ethernet using optical fibers transmitting short wave laser signals.
1000Base-T	A four-wire implementation of Gigabit Ethernet using twisted-pair cables.
100Base-FX	A two-wire implementation of Fast Ethernet using fiber-optic cable.
100Base-T4	A four-wire implementation of Fast Ethernet using twisted-pair cable.
10Base2	The IEEE 802.3 standard for Thin Ethernet.
10Base5	The IEEE 802.3 standard for Thick Ethernet.
10Base-FL	The IEEE 802.3 standard for fiber-optic Ethernet.
10Base-T	The IEEE 802.3 standard for twisted-pair Ethernet.
1-persistent strategy	A CSMA persistence strategy in which a station sends a frame immediately if the line is idle.
2B1Q encoding	A line encoding technique in which each pulse represents 2 bits.
4B/5B encoding	A block coding technique in which 4 bits are encoded into a 5-bit code.
4-dimensional, 5-level pulse amplitude modulation (4D-PAM5)	An encoding scheme used by 1000Base-T.
56K modem	A modem technology using two different data rates: one for uploading and one for downloading from the Internet.
800 service	A telephone service free to the caller.
8B/10B encoding	A block coding technique in which 8 bits are encoded into a 10-bit code.
8B/6T encoding	A block coding technique in which 8 bits are encoded into a 6-bit code.
900 service	A telephone service paid by the caller.
access control	The determination of link control through a data link protocol.
access rate	In Frame Relay, the data rate that can never be exceeded.
access point	A central base station in a BSS.
acknowledgment (ACK)	A response sent by the receiver to indicate the successful receipt and acceptance of data.
active close	In the client-server model, the closing of a communication by the client.
active document	In the World Wide Web, a document executed at the local site using Java.
active open	In the client-server model, the opening of a communication by the client.
add/drop multiplexer	A SONET device that multiplexes signals from different sources or demultiplexes a signal to multiple destinations.
additive increase	With slow start, a congestion avoidance strategy in which the window size is increased by just one segment instead of exponentially.
Address Resolution Protocol (ARP)	In TCP/IP, a protocol for obtaining the physical address of a node when the Internet address is known.
address space	The total number of addresses used by a protocol.
address-mask request and reply ICMP	Messages that find the network mask.
Advanced Mobile Phone System (AMPS)	A North American analog cellular phone system using FDMA.
Advanced Research Projects Agency (ARPA)	The government agency that funded ARPANET.
Advanced Research Projects Agency Network (ARPANET)	The packet-switching network that was funded by ARPA.
ALOHA	The original random multiple access method in which a station can send a frame any time it has one to send.
alternate mark inversion (AMI)	A digital-to-digital bipolar encoding method in which the amplitude representing 1 alternates between positive and negative voltages.
American National Standards Institute (ANSI)	A national standards organization that defines standards in the United States.
American Standard Code for Information Interchange (ASCII)	A character code developed by ANSI and used extensively for data communication.
amplitude	The strength of a signal, usually measured in volts, amperes, or watts.
amplitude modulation (AM)	An analog-to-analog conversion method in which the carrier signal's amplitude varies with the amplitude of the modulating signal.
amplitude shift keying (ASK)	A modulation method in which the amplitude of the carrier signal is varied to represent binary 0 or 1.
analog	A continuously varying entity.
analog data	Data that are continuous and smooth and not limited to a specific number of values.
analog hierarchy	A telephone company system in which multiplexed signals are combined into successively larger groups for more efficient transmission.
analog leased service	A service featuring a dedicated line between two users.
analog signal	A continuous waveform that changes smoothly over time.
analog switched service	A temporary analog connection between two users.
analog-to-analog modulation	The representation of analog information by an analog signal.
analog-to-digital conversion	The representation of analog information by a digital signal.
angle of incidence	In optics, the angle formed by a light ray approaching the interface between two media and the line perpendicular to the interface.
anonymous FTP	A protocol in which a remote user can access another machine without an account or password.
anycast address	An address that defines a group of computers with addresses that have the same beginning.
aperiodic signal	A signal that does not exhibit a pattern or repeating cycle.
applet	A computer program for creating an active Web document. It is usually written in Java.
application adaptation layer (AAL)	A layer in ATM protocol that breaks user data into 48-byte payloads.
application layer	The fifth layer in the Internet model; provides access to network resources.
application programming interface (API)	A set of declarations, definitions, and procedures followed by programmers to write client-server programs.
area	A collection of networks, hosts, and routers all contained within an autonomous system.
area border router	A router inside an area that summarizes the information about the area and sends it to other areas.
area identification	A 32-bit field that defines the area within which the routing takes place.
asymmetric digital subscriber line (ADSL)	A communication technology in which the downstream data rate is higher than the upstream rate.
asynchronous balanced mode (ABM)	In HDLC, a communication mode in which all stations are equal.
asynchronous connectionless link (ACL)	A link between a Bluetooth master and slave in which a corrupted payload is retransmitted.
Asynchronous Transfer Mode (ATM)	A wide area protocol featuring high data rates and equal-sized packets (cells); ATM is suitable for transferring text, audio, and video data.
asynchronous transmission	Transfer of data with start and stop bit(s) and a variable time interval between data units.
ATM LAN	A LAN using ATM technology.
ATM layer	A layer in ATM that provides routing, traffic management, switching, and multiplexing services.
ATM switch	An ATM device providing both switching and multiplexing functions.
attachment unit interface (AUI)	A 10Base5 cable that performs the physical interface functions between the station and the transceiver.
attenuation	The loss of a signal's energy due to the resistance of the medium.
authenticating state	In PPP, an optional state that verifies the identity of the receiver.
Authentication Header (AH) Protocol	A protocol defined by IPSec at the network layer that provides integrity to a message through the creation of a digital signature by a hashing function.
authentication server (AS)	The KDC in the Kerberos protocol.
authentication	Verification of the sender of a message.
automatic repeat request (ARQ)	An error-control method in which correction is made by retransmission of data.
automatic tunneling	Tunneling in which the receiving host has an IPv6 compatible address; no reconfiguration is necessary.
autonegotiation	A Fast Ethernet feature that allows two devices to negotiate the mode or data rate.
autonomous system (AS)	A group of networks and routers under the authority of a single administration.
autonomous system boundary router	Routers responsible for dissipating information about other autonomous systems into the current system.
available bit rate (ABR)	The minimum data rate in ATM at which cells can be delivered.
back off	In multiple access, waiting before re-sending after a collision.
backbone router	A router inside the backbone.
backbone	A network that connects smaller networks in an organization.
backward explicit congestion notification (BECN)	A bit in the Frame Relay packet that notifies the sender of congestion.
bandwidth on demand	A digital service that allows subscribers higher speeds through the use of multiple lines.
bandwidth	The difference between the highest and the lowest frequencies of a composite signal. It also measures the information-carrying capacity of a line or a network.
bandwidth-delay product	A measure of the number of bits that can be sent while waiting for news from the receiver.
band-pass channel	A channel that can pass a range of frequencies.
base header	In IPv6, the main header of the datagram.
basic service set (BSS)	The building block of a wireless LAN as defined by the IEEE 802.11 standard.
baud rate	The number of signal elements transmitted per second. A signal element consists of one or more bits.
best-effort delivery	The unreliable transmission mechanism by IP that does not guarantee message delivery.
bidirectional authentication	An authentication method involving a challenge and a response from sender to receiver and vice versa.
bidirectional frame (B-frame)	An MPEG frame that is related to the preceding and following I-frame or P-frame.
bipolar encoding	A digital-to-digital encoding method in which 0 amplitude represents binary 0 and positive and negative amplitudes represent alternate 1s.
bipolar n-zero substitution (BnZS)	An encoding method to provide synchronization for long strings of 0s.
bit	binary digit; the smallest unit of information; 1 or 0.
bit interval	The time required to send one bit.
bit padding	In TDM, the addition of extra bits to a device's source stream to force speed relationships.
bit rate	The number of bits transmitted per second.
bit stuffing	In HDLC, the addition of an extra 0 to prevent the receiver from mistaking the data for a flag. In TDM, a technique that adds bits for synchronization purposes.
bits per second (bps)	A measurement of data speed; bits transmitted per second.
block cipher	An encryption/decryption algorithm that has a block of bits as its basic unit.
block coding	A coding method to ensure synchronization and detection of errors.
blocking	An event that occurs when a switching network is working at its full capacity and cannot accept more input.
blocking port	A port on a bridge that does not forward a frame.
Bluetooth	A wireless LAN technology designed to connect devices of different functions such as telephones and notebooks in a small area such as a room.
BNC connector	A common coaxial cable connector.
Bootstrap Protocol (BOOTP)	The protocol that provides configuration information from a table (file).
Border Gateway Protocol (BGP)	An interautonomous system routing protocol based on path vector routing.
bridge	A network device operating at the first two layers of the Internet model with filtering and forwarding capabilities.
broadcast address	An address that allows transmission of a message to all nodes of a network.
broadcasting	Transmission of a message to all nodes in a network.
browser	An application program that displays a WWW document. A browser usually uses other Internet services to access the document.
BSS-transition mobility	In a wireless LAN, a station that can move from one BSS to another but is confined inside one ESS.
burst error	Error in a data unit in which two or more bits have been altered.
bursty data	Data with varying instantaneous transmission rates.
bus topology	A network topology in which all computers are attached to a shared medium (often a single cable).
byte	A group of eight bits.
cable modem	A technology in which the TV cable provides Internet access.
cable modem transmission system (CMTS)	A device installed inside the distribution hub that receives data from the Internet and passes them to the combiner.
cable TV	A system using coaxial cable that brings multiple channels of video programs into homes.
caching	The storing of information in a small, fast memory used to hold data items that are being processed.
carrier sense multiple access (CSMA)	A contention access method in which each station listens to the line before transmitting data.
carrier sense multiple access with collision avoidance (CSMA/CA)	An access method in which collision is avoided.
carrier sense multiple access with collision detection (CSMA/CD)	An access method in which stations transmit whenever the transmission medium is available and retransmit when collision occurs.
carrier signal	A high frequency signal used for digital-to-analog or analog-to-analog modulation. One of the characteristics of the carrier signal (amplitude, frequency, or phase) is changed according to the modulating data.
cell	A small, fixed-size data unit; also, in cellular telephony, a geographical area served by a cell office.
cell network	A network using the cell as its basic data unit.
cellular telephony	A wireless communication technique in which an area is divided into cells. A cell is served by a transmitter.
Certification Authority (CA)	An agency such as a federal or state organization that binds a public key to an entity and issues a certificate.
Challenge Handshake Authentication Protocol (CHAP)	In PPP, a three-way handshaking protocol used for authentication.
channel	A communications pathway.
channelization	A multiple access method in which the available bandwidth of a link is shared in time.
checksum	A field used for error detection. It is formed by adding bit streams using one's complement arithmetic and then complementing the result.
chip	In CDMA, a number in a code that is assigned to a station.
choke point	A packet sent by a router to the source to inform it of congestion.
cipher	An encryption/decryption algorithm.
cipher block chaining (CBC) mode	A DES and triple DES operation mode in which the encryption (or decryption) of a block depends on all previous blocks.
cipher feedback mode (CFM)	A DES and triple DES operation mode in which data is sent and received 1 bit at a time, with each bit independent of the previous bits.
cipher stream mode (CSM)	A DES and triple DES operation mode in which data is sent and received 1 byte at a time.
ciphertext	The encrypted data.
circuit switching	A switching technology that establishes an electrical connection between stations using a dedicated path.
cladding	Glass or plastic surrounding the core of an optical fiber; the optical density of the cladding must be less than that of the core.
class A address	An IPv4 address with the first octet between 0 and 127.
class B address	An IPv4 address with the first octet between 128 and 191.
class C address	An IPv4 address with the first octet between 192 and 223.
class D address	An IPv4 multicast address.
class E address	An IPv4 address reserved for special purposes.
classful addressing	An IPv4 addressing mechanism in which the IP address space is divided into 5 classes: A, B, C, D, and E. Each class occupies some part of the whole address space.
classless addressing	An addressing mechanism in which the IP address space is not divided into classes.
Classless InterDomain Routing (CIDR)	A technique to reduce the number of routing table entries when supernetting is used.
client process	A running application program on a local site that requests service from a running application program on a remote site.
client-server model	The model of interaction between two application programs in which a program at one end (client) requests a service from a program at the other end (server).
closed-loop congestion control	A method to alleviate congestion after it happens.
coaxial cable	A transmission medium consisting of a conducting core, insulating material, and a second conducting sheath.
code division multiple access (CDMA)	A multiple access method in which one channel carries all transmissions simultaneously.
collision	The event that occurs when two transmitters send at the same time on a channel designed for only one transmission at a time; data will be destroyed.
collision domain	The length of the medium subject to collision.
committed burst size	The maximum number of bits in a specific time period that a Frame Relay network must transfer without discarding any frames.
committed information rate (CIR)	The committed burst size divided by time.
common carrier	A transmission facility available to the public and subject to public utility regulation.
Common Gateway Interface (CGI)	A standard for communication between HTTP servers and executable programs. CGI is used in creating dynamic documents.
community antenna TV (CATV)	A cable network service that broadcasts video signals to locations with poor or no reception.
competitive local exchange carrier (CLEC)	A telephone company that cannot provide main telephone services; instead, other services such as mobile telephone service and toll calls inside a LATA are provided.
complementary code keying (CCK)	An HR-DSSS encoding method that encodes four or eight bits into one symbol.
composite signal	A signal composed of more than one sine wave.
concurrent client	A client running the same time as another client of the same process.
concurrent server	A server that can process many requests at the same time and share its time between many requests.
congestion avoidance	In Frame Relay, a method using two bits that explicitly notify the source and destination of congestion.
congestion control	A method to manage network and internetwork traffic to improve throughput.
congestion	Excessive network or internetwork traffic causing a general degradation of service.
connecting device	A tool that connects computers or networks.
connection control	The technique used by the transport layer to deliver segments.
connection establishment	The preliminary setup necessary for a logical connection prior to actual data transfer.
connection termination	A message sent to end a connection.
connectionless iterative server	A connectionless server that processes one request at a time.
connectionless service	A service for data transfer without connection establishment or termination.
connection-oriented concurrent server	A connection-oriented server that can serve many clients at the same time.
connection-oriented service	A service for data transfer involving establishment and termination of a connection.
constant bit rate (CBR)	The data rate of an ATM service class that is designed for customers requiring real-time audio or video services.
constellation	A graphical representation of the phase and amplitude of different bit combinations in digital-to-analog modulation.
Consultative Committee for International Telegraphy and Telephony (CCITT)	An international standards group now known as the ITU-T.
contention	An access method in which two or more devices try to transmit at the same time on the same channel.
control connection	The FTP connection used for control information (commands and responses).
controlled access	A multiple access method in which the stations consult one another to determine who has the right to send.
convergence sublayer (CS)	In ATM protocol, the upper AAL sublayer that adds a header or a trailer to the user data.
core	The glass or plastic center of an optical fiber.
Core-Based Tree (CBT)	In multicasting, a group-shared protocol that uses a center router as the root of the tree.
country domain	A subdomain in the Domain Name System that uses two characters as the last suffix.
CRC checker	The process that validates the CRC remainder.
CRC generator	The process that creates the CRC remainder.
critical angle	In refraction, the value of the angle of incidence that produces a 90-degree angle of refraction.
crossbar switch	A switch consisting of a lattice of horizontal and vertical paths. At the intersection of each horizontal and vertical path, there is a crosspoint that can connect the input to the output.
crosspoint	The junction of an input and an output on a crossbar switch.
crosstalk	The noise on a line caused by signals traveling along another line.
cryptography	The science and art of transforming messages to make them secure and immune to attacks.
CSNET	A network sponsored by the National Science Foundation originally intended for universities.
cycle	The repetitive unit of a periodic signal.
cyclic redundancy check (CRC)	A highly accurate error-detection method based on interpreting a pattern of bits as a polynomial.
data connection	The FTP connection used for data transfer.
data encryption standard (DES)	The U.S. government standard encryption method for nonmilitary and nonclassified use.
data exchange protocol	A protocol that uses the secret key to encrypt the data for secrecy and to encrypt the message digest for integrity.
data level	The number of different symbols used to represent a digital signal.
data link connection identifier (DLCI)	A number that identifies the virtual circuit in Frame Relay.
data link control	The responsibilities of the data link layer: flow control and error control.
data link layer	The second layer in the Internet model. It is responsible for node-to-node delivery.
Data Over Cable System Interface Specifications (DOCSIS)	A standard for data transmission over an HFC network.
data transparency	See transparency.
datagram approach (to packet switching)	A data transmission method in which each data unit is independent of others.
datagram	In packet switching, an independent data unit.
datagram socket	A structure designed to be used with a connectionless protocol such as UDP.
DC component	See direct current.
de facto standard	A protocol that has not been approved by an organized body but adopted as a standard through widespread use.
de jure standard	A protocol that has been legislated by an officially recognized body.
decibel (dB)	A measure of the relative strength of two signal points.
decryption	Recovery of the original message from the encrypted data.
default mask	The mask for a network that is not subnetted.
default routing	A routing method in which a router is assigned to receive all packets with no match in the routing table.
Defense Advanced Research Projects Agency (DARPA)	A government organization, which, under the name of ARPA funded ARPANET and the Internet.
delayed response strategy	A technique used by IGMP to prevent unnecessary traffic on a LAN.
demodulation	The process of separating the carrier signal from the information-bearing signal.
demodulator	A device that performs demodulation.
demultiplexer (DEMUX)	A device that separates a multiplexed signal into its original components.
dense wave-division multiplexing (DWDM)	A WDM method that can multiplex a very large number of channels by spacing channels closer together.
destination-unreachable message	An ICMP error-reporting message sent to a source when a router cannot route a datagram or a host cannot deliver a datagram.
dibit	A unit of data consisting of two bits.
differential Manchester encoding	A digital-to-digital polar encoding method that features a transition at the middle of the bit interval as well as an inversion at the beginning of each 1 bit.
Differentiated Services (DS or Diffserv)	A class-based QoS model designed for IP.
Diffie-Hellman protocol	A key management protocol that provides a one-time session key for 2 parties.
digest	A condensed version of a document.
digital AMPS (D-AMPS)	A second-generation cellular phone system that is a digital version of AMPS.
digital data	Data represented by discrete values or conditions.
digital data service (DDS)	A digital version of an analog leased line with a rate of 64 Kbps.
digital service unit (DSU)	A device that allows the connection of a user's device to a digital line.
digital signal (DS) service	A telephone company service featuring a hierarchy of digital signals.
digital signal	A discrete signal with a limited number of values.
digital signature	A method to authenticate the sender of a message.
digital subscriber line (DSL)	A technology using existing telecommunication networks to accomplish high-speed delivery of data, voice, video, and multimedia.
digital subscriber line access multiplexer (DSLAM)	A telephone company site device that functions like an ADSL modem.
digital-to-analog modulation	The representation of digital information by an analog signal.
digital-to-digital encoding	The representation of digital information by a digital signal.
Dijkstra's algorithm	In link state routing, an algorithm that finds the shortest path to other routers.
direct current (DC)	A zero-frequency signal with a constant amplitude.
direct sequence spread spectrum (DSSS)	A wireless transmission method in which each bit to be sent by the sender is replaced by a sequence of bits called a chip code.
discard eligibility (DE)	A bit that defines that a packet can be discarded if there is congestion in the network.
discrete cosine transform (DCT)	A JPEG phase in which a transformation changes the 64 values so that the relative relationships between pixels are kept but the redundancies are revealed.
discrete multitone technique (DMT)	A modulation method combining elements of QAM and FDM.
Distance Vector Multicast Routing Protocol (DVMRP)	A protocol based on distance vector routing that handles multicast routing in conjunction with IGMP.
distance vector routing	A routing method in which each router sends its neighbors a list of networks it can reach and the distance to that network.
distortion	Any change in a signal due to noise, attenuation, or other influences.
distributed interframe space (DIFS)	In wireless LANs, a period of time that a station waits before sending a control frame.
distributed processing	A strategy in which services provided for the network reside at multiple sites.
distribution hub	In an HFC network, a site that modulates and distributes signals.
DNS server	A computer that holds information about the name space.
domain	A subtree of the domain name space.
domain name	In the DNS, a sequence of labels separated by dots.
domain name space	A structure for organizing the name space in which the names are defined in an inverted-tree structure with the root at the top.
Domain Name System (DNS)	A TCP/IP application service that converts user-friendly names to IP addresses.
dotted-decimal notation	A notation devised to make the IP address easier to read; each byte is converted to its decimal equivalent and then set off from its neighbor by a decimal.
downlink	Transmission from a satellite to an earth station.
downloading	Retrieving a file or data from a remote site.
downstream data band	In an HFC network, the 550 to 750 MHz band for data from the Internet to the subscriber premises.
dual stack	Two protocols (IPv4 and IPv6) on a station.
duplex mode	See full-duplex mode.
dynamic document	A Web document created by running a CGI program at the server site.
Dynamic Domain Name System (DDNS)	A method to update the DNS master file dynamically.
Dynamic Host Configuration Protocol (DHCP)	An extension to BOOTP that dynamically assigns configuration information.
dynamic mapping	A technique in which a protocol is used for address resolution.
dynamic routing	Routing in which the routing table entries are updated automatically by the routing protocol.
E lines	The European equivalent of T lines.
echo-request and reply message	An ICMP query message that determines whether two systems (hosts or routers) can communicate with each other.
effective bandwidth	The bandwidth that the network needs to allocate for the flow of traffic; a function of three values: average data rate, peak data rate, and maximum burst size.
electromagnetic spectrum	The frequency range occupied by electromagnetic energy.
electronic code block (ECB) mode	A DES and triple DES operation method in which a long message is divided into 64-bit blocks before being encrypted separately.
electronic mail (email)	A method of sending messages electronically based on mailbox addresses rather than a direct host-to-host exchange.
Electronics Industries Association (EIA)	An organization that promotes electronics manufacturing concerns. It has developed interface standards such as EIA-232, EIA-449, and EIA-530.
email	See electronic mail.
Encapsulating Security Payload (ESP)	A protocol defined by IPSec that provides privacy as well as a combination of integrity and message authentication.
encapsulation	The technique in which a data unit from one protocol is placed within the data field portion of the data unit of another protocol.
encryption	Converting a message into an unintelligible form that is unreadable unless decrypted.
end office	A switching office that is the terminus for the local loops.
ephemeral port number	A port number used by the client.
error control	The detection and handling of errors in data transmission.
error correction by retransmission	The process of correcting bits by resending the data.
error-reporting message	An ICMP message sent to the source to report an error.
ESS-transition mobility	A station in a wireless LAN that can move from one ESS to another.
establishing state	In PPP, a state in which communication begins and options are negotiated.
Ethernet	A local area network using CSMA/CD access method. See IEEE Project 802.3.
even parity	An error-detection method in which an extra bit is added to the data unit so that the total number of 1s becomes even.
excess burst size	In Frame Relay, the maximum number of bits in excess of Bc that the user can send during a predefined period of time.
Extended Service Set (ESS)	A wireless LAN service composed of two or more BSSs with APs as defined by the IEEE 802.11 standard.
extension header	Extra headers in the IPv6 datagram that provide additional functionality.
exterior routing	Routing between autonomous systems.
external link LSA	A message that announces all the networks outside the AS.
extranet	A private network that uses the TCP/IP protocol suite that allows authorized access from outside users.
Fast Ethernet	See 100Base-T.
Federal Communications Commission (FCC)	A government agency that regulates radio, television, and telecommunications.
fiber distributed data interface (FDDI)	A high-speed (100-Mbps) LAN, defined by ANSI, using fiber optics, dual ring topology, and the token-passing access method. Today an FDDI network is also used as a MAN.
fiber link Ethernet	Ethernet using fiber optic media.
fiber node	In an HFC network, the location of the optical fiber and coaxial fiber juncture.
fiber-optic cable	A high-bandwidth transmission medium that carries data signals in the form of pulses of light. It consists of a thin cylinder of glass or plastic, called the core, surrounded by a concentric layer of glass or plastic called the cladding.
File Transfer Protocol (FTP)	In TCP/IP, an application layer protocol that transfers files between two sites.
filtering	A process in which a bridge makes forwarding decisions.
finite state machine	A machine that goes through a limited number of states.
firewall	A device (usually a router) installed between the internal network of an organization and the rest of the Internet to provide security.
first-in, first-out (FIFO) queue	A queue in which the first item in is the first item out.
flag field	In an HDLC frame, an 8-bit synchronization sequence that identifies the beginning or end of a frame.
flat name space	A method to map a name to an address in which there is no hierarchical structure.
flooding	Saturation of a network with a message.
flow control	A technique to control the rate of flow of frames (packets or messages).
flow label	An IPv6 mechanism to enable the source to request special handling of a packet.
footprint	An area on Earth that is covered by a satellite at a specific time.
forward error correction	Correction of errors at the receiver.
forward explicit congestion notification (FECN)	A bit in the Frame Relay packet that notifies the destination of congestion.
forwarding port	A port on a bridge that forwards a received frame.
Fourier analysis	The mathematical technique used to obtain the frequency spectrum of an aperiodic signal if the time-domain representation is given.
fragmentation offset	A field in the IP header used in fragmentation to show the relative position of the fragment with respect to the whole datagram.
fragmentation	The division of a packet into smaller units to accommodate a protocol's MTU.
frame	A group of bits representing a block of data.
frame check sequence (FCS)	The HDLC error-detection field containing either a 2- or 4-byte CRC.
Frame Relay	A packet-switching specification defined for the first two layers of the Internet model. There is no network layer. Error checking is done on end-to-end basis instead of on each link.
Frame Relay assembler/disassembler (FRAD)	A device used in Frame Relay to handle frames coming from other protocols.
framing bit	A bit used for synchronization purposes in TDM.
frequency division multiple access (FDMA)	A multiple access method in which the bandwidth is divided into channels.
frequency hopping spread spectrum (FHSS)	A wireless transmission method in which the sender transmits at one carrier frequency for a short period of time, then hops to another carrier frequency for the same amount of time, hops again for the same amount of time, and so on. After N hops, the cycle is repeated.
frequency modulation (FM)	An analog-to-analog modulation method in which the carrier signal's frequency varies with the amplitude of the modulating signal.
frequency shift keying (FSK)	A digital-to-analog encoding method in which the frequency of the carrier signal is varied to represent binary 0 or 1.
frequency	The number of cycles per second of a periodic signal.
frequency-division multiplexing (FDM)	The combining of analog signals into a single signal.
frequency-domain plot	A graphical representation of a signal's frequency components.
full-duplex mode	A transmission mode in which communication can be two way simultaneously.
full-duplex switched Ethernet	Ethernet in which each station, in its own separate collision domain, can both send and receive.
fully qualified domain name (FQDN)	A domain name consisting of labels beginning with the host and going back through each level to the root node.
fundamental frequency	The frequency of the dominant sine wave of a composite signal.
gatekeeper	In the H.323 standard, a server on the LAN that plays the role of the registrar server.
gateway	A device used to connect two separate networks that use different communication protocols.
general header	A part of an HTTP request or response message that gives general information about the message.
generic domain	A subdomain in the domain name system that uses generic suffixes.
geographical routing	A routing technique in which the entire address space is divided into blocks based on physical landmasses.
geosynchronous Earth orbit	An orbit that allows a satellite to remain fixed above a certain spot on earth.
Gigabit Ethernet	Ethernet with a 1000 Mbps data rate.
gigabit medium independent interface (GMII)	In Gigabit Ethernet, a specification that defines how the reconciliation sublayer is to be connected to the transceiver.
global Internet	The Internet.
Global Positioning System (GPS)	An MEO public satellite system consisting of 24 satellites and used for land and sea navigation. GPS is not used for communications.
Global System for Mobile Communication (GSM)	A second-generation cellular phone system used in Europe.
Globalstar	An LEO satellite system with 48 satellites in six polar orbits with each orbit hosting eight satellites.
Go-Back-N ARQ	An error-control method in which the frame in error and all following frames must be retransmitted.
grafting	Resumption of multicast messages.
ground propagation	Propagation of radio waves through the lowest portion of the atmosphere (hugging the earth).
group	An analog signal created by 12 voice channels multiplexed together.
group membership	Belonging to a group.
group-shared tree	A multicast routing feature in which each group in the system shares the same tree.
guard band	A bandwidth separating two signals.
guided media	Transmission media with a physical boundary.
H.323	A standard designed by ITU to allow telephones on the public telephone network to talk to computers (called terminals in H.323) connected to the Internet.
half-duplex mode	A transmission mode in which communication can be two-way but not at the same time.
Hamming code	A method that adds redundant bits to a data unit to detect and correct bit errors.
handoff	Changing to a new channel as a mobile device moves from one cell to another.
handshake protocol	A protocol to establish or terminate a connection.
harmonics	Components of a digital signal, each having a different amplitude, frequency, and phase.
hash function	An algorithm that creates a fixed-size digest from a variable-length message.
head end	A cable TV office.
header	Control information added to the beginning of a data packet. Also, in an email, the part of the message that defines the sender, the receiver, the subject of the message, and other information.
header translation	Conversion of the IPv6 header to IPv4.
hertz (Hz)	Unit of measurement for frequency.
hexadecimal colon notation	In IPv6, an address notation consisting of 32 hexadecimal digits, with every four digits separated by a colon.
hierarchical name space	A name space made of several parts, with each succeeding part becoming more and more specific.
hierarchical routing	A routing technique in which the entire address space is divided into levels based on specific criteria.
high bit rate digital subscriber line (HDSL)	A service similar to the T1-line that can operate at lengths up to 3.6 km.
High Rate Direct Sequence Spread Spectrum (HR-DSSS)	A signal generation method similar to DSSS except for the encoding method (CCK).
High-level Data Link Control (HDLC)	A bit-oriented data link protocol defined by the ISO. It is used in X.25 protocol. A subset, called link access procedure (LAP), is used in other protocols. It is also a base for many data link protocols used in LANs.
homepage	A unit of hypertext or hypermedia available on the Web that is the main page for an organization or an individual.
hop count	The number of nodes along a route. It is a measurement of distance in routing algorithms.
hop limit	An IPv6 field that limits the number of routers that a packet can visit.
hop-to-hop delivery	Transmission of frames from one node to the next.
horn antenna	A scoop-shaped antenna used in terrestrial microwave communication.
host	A station or node on a network.
host file	A file, used when the Internet was small, that mapped host names to host addresses.
hostid	The part of an IP address that identifies a host.
host-specific routing	A routing method in which the full IP address of a host is given in the routing table.
hub	A central device in a star topology that provides a common connection among the nodes.
Huffman encoding	A statistical compression method using variable-length codes to encode a set of symbols.
hybrid network	A network with a private internet and access to the global Internet.
hybrid-fiber-coaxial (HFC) network	The second generation of cable networks; uses fiber optic and coaxial cable.
hypermedia	Information containing text, pictures, graphics, and sound that are linked to other documents through pointers.
hypertext	Information containing text that is linked to other documents through pointers.
HyperText Markup Language (HTML)	The computer language for specifying the contents and format of a web document. It allows additional text to include codes that define fonts, layouts, embedded graphics, and hypertext links.
HyperText Transfer Protocol (HTTP)	An application service for retrieving a web document.
idle state	In PPP, a state in which the link is inactive.
image file	In FTP, the default format for transferring binary files. The file is sent as continuous streams of bits without any interpretation or encoding.
incumbent local exchange carrier (ILEC)	A telephone company that provided services before 1996 and is the owner of the cabling system.
infrared wave	A wave with a frequency between 300 GHz and 400 THz; usually used for short-range communications.
inner product	A number produced by multiplying two sequences, element by element, and summing the products.
Institute of Electrical and Electronics Engineers (IEEE)	A group consisting of professional engineers which has specialized societies whose committees prepare standards in members' areas of specialty.
Integrated Services (IntServ)	A flow-based QoS model designed for IP.
integrity	A data quality of being noncorrupted.
interactive audio/video	Real-time communication with sound and images.
interautonomous system routing protocol	A protocol to handle transmissions between autonomous systems.
interexchange carrier (IXC)	A long-distance company that, prior to the Act of 1996, provided communication services between two customers in different LATAs.
interface	The boundary between two pieces of equipment. It also refers to mechanical, electrical, and functional characteristics of the connection.
Interim Standard 95 (IS-95)	One of the dominant second-generation cellular telephony standards in North America.
interior routing	Routing inside an autonomous system.
interleaving	Taking a specific amount of data from each device in a regular order.
International Organization of Standardization (ISO)	A worldwide organization that defines and develops standards on a variety of topics.
International Telecommunications Union-Telecommunication Standardization Sector (ITU-T)	A standards organization formerly known as the CCITT.
internet	A collection of networks connected by internetworking devices such as routers or gateways.
Internet	A global internet that uses the TCP/IP protocol suite.
Internet address	A 32-bit or 128-bit network-layer address used to uniquely define a host on an internet using the TCP/IP protocol.
Internet Architecture Board (IAB)	The technical adviser to the ISOC; oversees the continuing development of the TCP/IP protocol suite.
Internet Assigned Numbers Authority (IANA)	A group supported by the U.S. government that was responsible for the management of Internet domain names and addresses until October 1998.
Internet Control Message Protocol (ICMP)	A protocol in the TCP/IP protocol suite that handles error and control messages.
Internet Control Message Protocol, version 6 (ICMPv6)	A protocol in IPv6 that handles error and control messages.
Internet Corporation for Assigned Names and Numbers (ICANN)	A private, nonprofit corporation managed by an international board that assumed IANA operations.
Internet draft	A working Internet document (a work in progress) with no official status and a six-month lifetime.
Internet Engineering Steering Group (IESG)	An organization that oversees the activity of IETF.
Internet Engineering Task Force (IETF)	A group working on the design and development of the TCP/IP protocol suite and the Internet.
Internet Group Management Protocol (IGMP)	A protocol in the TCP/IP protocol suite that handles multicasting.
Internet Mail Access Protocol, version 4 (IMAP4)	A complex and powerful protocol to handle the transmission of electronic mail.
Internet Mobile Communication for year 2000 (ITM-2000)	An ITU issued blueprint that defines criteria for third generation cellular telephony.
Internet model	A 5-layer protocol stack that dominates data communications and networking today.
Internet Network Information Center (INTERNIC)	An agency responsible for collecting and distributing information about TCP/IP protocols.
Internet Protocol (IP)	The network-layer protocol in the TCP/IP protocol suite governing connectionless transmission across packet switching networks.
Internet Protocol, version 6 (IPv6)	The sixth version of the Internetworking Protocol; it features major IP addressing changes.
Internet Research Task Force (IRTF)	A forum of working groups focusing on long-term research topics related to the Internet.
Internet service provider (ISP)	Usually, a company that provides Internet services.
Internet Society (ISOC)	The nonprofit organization established to publicize the Internet.
Internet standard	A thoroughly tested specification that is useful to and adhered to by those who work with the Internet. It is a formalized regulation that must be followed.
Internetwork Protocol Control Protocol (IPCP)	In PPP, the set of protocols that establish and terminate a network layer connection for IP packets.
internetwork (internet)	A network of networks.
internetworking	Connecting several networks together using internetworking devices such as routers and gateways.
intracoded frame (I-frame)	An independent frame that is not related to any other frame and appearing at regular intervals.
intranet	A private network that uses the TCP/IP protocol suite.
inverse domain	A subdomain in the DNS that finds the domain name given the IP address.
inverse multiplexing	Taking data from one source and breaking it into portions that can be sent across lower-speed lines.
IP datagram	The Internetworking Protocol data unit.
IP Security (IPSec)	A collection of protocols designed by the IETF (Internet Engineering Task Force) to provide security for a packet carried on the Internet.
IrDA port	A port that allows a wireless keyboard to communicate with a PC.
Iridium	A 66-satellite network that provides communication from any Earth site to another.
iterative resolution	Resolution of the IP address in which the client may send its request to multiple servers before getting an answer.
iterative server	In the client-server model, a server that can serve only one client at a time.
Java	A programming language used to create active Web documents.
jitter	A phenomenon in real-time traffic caused by gaps between consecutive packets at the receiver.
Joint Photographic Experts Group (JPEG)	A standard for compressing continuous-tone picture.
jumbo group	An analog signal created by six multiplexed master groups.
Karn's Algorithm	An algorithm that does not include the retransmitted segments in calculation of round-trip time.
keepalive message	A message that establishes a relationship between the two routers.
keepalive timer	A timer that prevents a long idle connection between two TCPs.
Kerberos	An authentication protocol used by Windows 2000.
key	A number that a cipher operates on.
key distribution center (KDC)	In secret key encryption, a trusted third party that shares a key with each user.
layered architecture	A model based on ordered tiers.
leaky bucket algorithm	An algorithm to shape bursty traffic.
least-cost tree	An MOSPF feature in which the tree is based on a chosen metric instead of shortest path.
leave report	An IGMP message sent by a host when no process is interested in a specific group.
legacy ATM LAN	LAN in which ATM technology is used as a backbone to connect traditional LANs.
line-of-sight propagation	The transmission of very high frequency signals in straight lines directly from antenna to antenna.
Link Control Protocol (LCP)	A PPP protocol responsible for establishing, maintaining, configuring, and terminating links.
link state advertisement (LSA)	In OSPF, a method that disperses information.
link state database	In link state routing, a database common to all routers and made from LSP information.
link state packet (LSP)	In link state routing, a small packet containing routing information sent by a router to all other routers.
link state routing	A routing method in which each router shares its knowledge of changes in its neighborhood with all other routers.
link state update packet	A packet that provides information about a specific route or routes.
link	The physical communication pathway that transfers data from one device to another.
load	The number of packets sent to a network.
local access and transport area (LATA)	An area covered by one or more telephone companies.
local access	Using a terminal directly connected to the computer.
local address	The part of an email address that defines the name of a special file, called the user mailbox, where all of the mail received for a user is stored for retrieval by the user agent.
local area network (LAN)	A network connecting devices inside a single building or inside buildings close to each other.
local area network emulation (LANE)	Software that enables an ATM switch to behave like a LAN switch.
local call service	A telephone service handling local calls, usually charging a flat monthly fee.
local exchange carrier (LEC)	A telephone company that handles services inside a LATA.
local Internet service provider	The same as an Internet service provider.
local ISP	The same as an Internet service provider.
local loop	The link that connects a subscriber to the telephone central office.
local management information (LMI)	A protocol used in Frame Relay to provide. management features.
logical address	An address defined in the network layer.
logical link control (LLC)	The upper sublayer of the data link layer as defined by IEEE Project 802.2.
Logical Link Control and Adaptation Protocol (L2CAP)	A Bluetooth layer used for data exchange on an ACL link.
logical tunnel	The encapsulation of a multicast packet inside a unicast packet to enable multicast routing by non-multicast routers.
low Earth orbit (LEO)	A polar satellite orbit with an altitude between 500 and 2000 km. A satellite with this orbit has a rotation period of 90 to 120 minutes.
low-pass channel	A channel that passes frequencies between 0 and f.
mail transfer agent (MTA)	An SMTP component that transfers the mail across the Internet.
Manchester encoding	A digital-to-digital polar encoding method in which a transition occurs at the middle of each bit interval for the purpose of synchronization.
man-in-the-middle attack	A key management problem in which an intruder intercepts and sends messages between the intended sender and receiver.
mask	For IPv4, a 32-bit binary number that gives the first address in the block (the network address) when ANDed with an address in the block.
master	The one Bluetooth station in a piconet that controls all the others.
master group	An analog signal created by 10 multiplexed supergroups.
maximum burst size	The maximum length of time traffic is generated at the peak rate.
maximum transfer unit (MTU)	The largest size data unit a specific network can handle.
media player	A help application that plays an audio/video file; used by a browser.
media server	A server accessed by a media player to download an audio/video file.
medium access control (MAC) sublayer	The lower sublayer in the data link layer defined by the IEEE 802 project. It defines the access method and access control in different local area network protocols.
medium attachment unit (MAU)	See transceiver.
medium bandwidth	The difference between the highest and lowest frequencies a medium can support.
medium dependent interface (MDI)	In Fast Ethernet, implementation-specific hardware that connects the transceiver to the medium.
medium Earth orbit (MEO)	A satellite orbit positioned between the two Van Allen belts. A satellite at this orbit takes six hours to circle the earth.
medium independent interface (MII)	In Fast Ethernet hardware that connects an external transceiver to the reconciliation layer.
membership report	An IGMP message sent by a host or router interested in joining a specific group.
mesh topology	A network configuration in which each device has a dedicated point-to-point link to every other device.
message authentication	A security measure in which the sender of the message is verified for every message sent.
metric	A cost assigned for passing through a network.
metropolitan area network (MAN)	A network that can span a geographical area the size of a city.
microwave	Electromagnetic waves ranging from 2 GHz to 40 GHz.
minislot	In an HFC network, a time slot for timesharing of the upstream channels.
mixer	A device that combines real-time signals from different sources into one signal.
mobile host	A host that can move from one network to another.
mobile switching center (MSC)	In cellular telephony, a switching office that coordinates communication between all base stations and the telephone central office.
mobile telephone switching office (MTSO)	An office that controls and coordinates communication between all of the cell offices and the telephone control office.
modem	A device consisting of a modulator and a demodulator. It converts a digital signal into an analog signal (modulation) and vice versa (demodulation).
modulation	Modification of one or more characteristics of a carrier wave by an information-bearing signal.
modulator	A device that converts a digital signal to an analog signal suitable for transmission across a telephone line.
monoalphabetic substitution	An encryption method in which each occurrence of a character is replaced by another character in the set.
motion picture experts group (MPEG)	A method to compress videos.
MT-RJ	A fiber-optic cable connector.
multicast address	An address used for multicasting.
multicast backbone (MBONE)	A set of internet routers supporting multicasting through the use of tunneling.
Multicast Open Shortest Path First (MOSPF)	A multicast protocol that uses multicast link state routing to create a source-based least cost tree.
multicast router	A router with a list of loyal members related to each router interface that distributes the multicast packets.
multicasting	A transmission method that allows copies of a single packet to be sent to a selected group of receivers.
multiline transmission, 3-level (MLT-3) encoding	A line coding scheme featuring 3 levels of signals and transitions at the beginning of the 1 bit.
multimode graded-index fiber	An optical fiber with a core having a graded index of refraction.
multimode step-index fiber	An optical fiber with a core having a uniform index of refraction. The index of refraction changes suddenly at the core/cladding boundary.
multiple access (MA)	A line access method in which every station can access the line freely.
multiplexer (MUX)	A device used for multiplexing.
multiplexing	The process of combining signals from multiple sources for transmission across a single data link.
multiplicative decrease	A congestion avoidance technique in which the threshold is set to half of the last congestion window size, and the congestion window size starts from one again.
Multipurpose Internet Mail Extension (MIME)	A supplement to SMTP that allows non-ASCII data to be sent through SMTP.
multistage switch	An array of switches designed to reduce the number of crosspoints.
n2 problem	A problem due to the large number of keys needed in symmetric key distribution.
Nagle's algorithm	An algorithm that attempts to prevent silly window syndrome at the sender's site; both the rate of data production and the network speed are taken into account.
name space	All the names assigned to machines on an internet.
name-address resolution	Mapping a name to an address or an address to a name.
national service provider (NSP)	A backbone network created and maintained by a specialized company.
Needham-Schroeder protocol	A key management protocol using multiple challenge-response interactions between 2 entities.
negative acknowledgment (NAK)	A message sent to indicate the rejection of received data.
netid	The part of an IP address that identifies the network.
network	A system consisting of connected nodes made to share data, hardware, and software.
network address	An address that identifies a network to the rest of the Internet; it is the first address in a block.
network address translation (NAT)	A technology that allows a private network to use a set of private addresses for internal communication and a set of global Internet addresses for external communication.
network allocation vector (NAV)	In CSMA/CA, the amount of time that must pass before a station can check the line for idleness.
Network Control Protocol (NCP)	In PPP, a set of control protocols that allows the encapsulation of data coming from network layer protocols.
network interface card (NIC)	An electronic device, internal or external to a station, that contains circuitry to enable the station to be connected to the network.
network layer	The third layer in the Internet model, responsible for the delivery of a packet to the final destination.
network link LSA	An LSA packet that announces the existence of all of the routers connected to the network.
Network Virtual Terminal (NVT)	A TCP/IP application protocol that allows remote login.
networking state	A PPP state in which packets of user data and packets for control are transmitted.
network-specific routing	Routing in which all hosts on a network share one entry in the routing table.
network-to-network interface (NNI)	In ATM, the interface between two networks.
next header	In IPv6, an 8-bit field defining the header that follows the base header in the datagram.
next-hop routing	A routing method in which only the address of the next hop is listed in the routing table instead of a complete list of the stops the packet must make.
node	An addressable communication device (e.g., a computer or router) on a network.
node-to-node delivery	Transfer of a data unit from one node to the next.
noise	Random electrical signals that can be picked by the transmission medium and result in degradation or distortion of the data.
nonce	A large random number that is used once to distinguish a fresh authentication request from a used one.
nonpersistent connection	A connection in which one TCP connection is made for each request/response.
nonpersistent strategy	A random multiple access method in which a station waits a random period of time after a collision is sensed.
nonrepudiation	A security aspect in which a receiver must be able to prove that a received message came from a specific sender.
nonreturn to zero (NRZ)	A digital-to-digital polar encoding method in which the signal level is always either positive or negative.
nonreturn to zero, invert (NRZ-I)	An NRZ encoding method in which the signal level is inverted each time a 1 is encountered.
nonreturn to zero, level (NRZ-L)	An NRZ encoding method in which the signal level is directly related to the bit value.
normal response mode (NRM)	In HDLC, a communication mode in which the secondary station must have permission from the primary station before transmission can proceed.
notification message	A BGP message sent by a router whenever an error condition is detected or a router wants to close the connection.
no-transition mobility	In wireless LANs, mobility confined inside a BSS or non-mobility.
Nyquist theorem	A theorem that states that the number of samples needed to adequately represent an analog signal is equal to twice the highest frequency of the original signal.
odd parity	An error-detection method in which an extra bit is added to the data unit such that the sum of all 1-bits becomes odd.
omnidirectional antenna	An antenna that sends out or receives signals in all directions.
on-demand audio/video	Another name for streaming stored audio/video.
one's complement	A representation of binary numbers in which the complement of a number is found by complementing all bits.
open message	A BGP message sent by a router to create a neighborhood relationship.
open shortest path first (OSPF)	An interior routing protocol based on link state routing.
open system	A model that allows two different systems to communicate regardless of their underlying architecture.
Open Systems Interconnection (OSI) model	A seven-layer model for data communication defined by ISO.
open-loop congestion control	Policies applied to prevent congestion.
optical carrier (OC)	The hierarchy of fiber-optic carriers defined in SONET. The hierarchy defines up to 10 different carriers (OC-1, OC-3, OC-12, . . . , OC-192), each with a different data rate.
optional attribute	A BGP path attribute that need not be recognized by every router.
orbit	The path a satellite travels around the earth.
Orthogonal Frequency Division Multiplexing (OFDM)	A multiplexing method similar to FDM, with all the subbands used by one source at a given time.
Otway-Rees protocol	A key management protocol with less steps than the Needham-Schroeder method.
overhead	Extra bits added to the data unit for control purposes.
packet switching	Data transmission using a packet-switched network.
packet	Synonym for data unit, mostly used in the network layer.
packet-filter firewall	A firewall that forwards or blocks packets based on the information in the network-layer and transport-layer headers.
packet-switched network	A network in which data are transmitted in independent units called packets.
page	A unit of hypertext or hypermedia available on the Web.
parabolic dish antenna	An antenna shaped like a parabola used for terrestrial microwave communication.
parallel transmission	Transmission in which bits in a group are sent simultaneously, each using a separate link.
parameter-problem message	An ICMP message that notifies a host that there is an ambiguous or missing value in any field of the datagram.
parity bit	A redundant bit added to a data unit (usually a character) for error checking.
parity check	An error-detection method using a parity bit.
partially qualified domain name (PQDN)	A domain name that does not include all the levels between the host and the root node.
passive open	The state of a server as it waits for incoming requests from a client.
Password Authentication Protocol (PAP)	A simple two-step authentication protocol used in PPP.
path layer	A SONET layer responsible for the movement of a signal from its optical source to its optical destination.
path overhead	Control information used by the SONET path layer.
path	The channel through which a signal travels.
path vector routing	A routing method on which BGP is based; in this method, the ASs through which a packet must pass are explicitly listed.
P-box	A hardware circuit used in encryption that connects input to output.
peak amplitude	The maximum signal value of a sine wave.
peak data rate	The maximum data rate of the traffic.
peer-to-peer process	A process on a sending and a receiving machine that communicate at a given layer.
per hop behavior (PHB)	In the Diffserv model, a 6-bit field that defines the packet-handling mechanism for the packet.
period	The amount of time required to complete one full cycle.
periodic signal	A signal that exhibits a repeating pattern.
permanent virtual circuit (PVC)	A virtual circuit transmission method in which the same virtual circuit is used between source and destination on a continual basis.
persistence timer	A technique to handle the zero window-size advertisement.
persistent connection	A connection in which the server leaves the connection open for more requests after sending a response.
persistent strategy	In CSMA, a strategy in which the station sends a frame after sensing the line.
Personal Communication System (PCS)	A generic term for a commercial cellular system that offers several kinds of communication services.
phase modulation (PM)	An analog-to-analog modulation method in which the carrier signal's phase varies with the amplitude of the modulating signal.
phase shift keying (PSK)	A digital-to-analog modulation method in which the phase of the carrier signal is varied to represent a specific bit pattern.
phase	The relative position of a signal in time.
PHY sublayer	The transceiver in Fast Ethernet.
physical address	The address of a device used at the data link layer (MAC address).
physical layer signaling (PLS) sublayer	An Ethernet sublayer that encodes and decodes data.
physical layer	The first layer of the Internet model, responsible for the mechanical and electrical specifications of the medium.
physical topology	The manner in which devices are connected in a network.
piconet	A Bluetooth network.
piggybacking	The inclusion of acknowledgment on a data frame.
pipelining	In Go-Back-n ARQ, sending several frames before news is received concerning previous frames.
pixel	A picture element of an image.
plaintext	In encryption/decryption, the original message.
playback buffer	A buffer that stores the data until they are ready to be played.
point coordination function (PCF)	In wireless LANs, an optional and complex access method implemented in an infrastructure network.
point of presence (POP)	A switching office where carriers can interact with each other.
point-to-point access	See point-to-point connection.
point-to-point connection	A dedicated transmission link between two devices.
point-to-point link	A dedicated transmission link between two devices.
Point-to-Point Protocol (PPP)	A protocol for data transfer across a serial line.
polar encoding	A digital-to-analog encoding method that uses two levels (positive and negative) of amplitude.
policy routing	A path vector routing feature in which the routing tables are based on rules set by the network administrator rather than a metric.
poll	In the primary/secondary access method, a procedure in which the primary station asks a secondary station if it has any data to transmit.
poll/final (P/F) bit	A bit in the control field of HDLC; if the primary is sending, it can be a poll bit; if the secondary is sending, it can be a final bit.
poll/select	An access method protocol using poll and select procedures. See poll. See select.
polyalphabetic substitution	An encryption method in which each occurrence of a character can have a different substitute.
polynomial	An algebraic term that can represent a CRC divisor.
port	In a URL, the port number of the server.
port address	In TCP/IP protocol an integer identifying a process.
port number	An integer that defines a process running on a host.
Post Office Protocol, version 3 (POP3)	A popular but simple SMTP mail access protocol.
p-persistent strategy	A CSMA persistence strategy in which a station sends with probability p if it finds the line idle.
preamble	The 7-byte field of an IEEE 802.3 frame consisting of alternating 1s and 0s that alert and synchronize the receiver.
predicted frame (P-frame)	An MPEG frame which contains only the changes from the preceding frame.
predictive encoding	In audio compression, encoding only the differences between the samples.
presentation layer	The sixth layer of the OSI model responsible for translation, encryption, authentication, and data compression.
Pretty Good Privacy (PGP)	A protocol that provides all four aspects of security in the sending of email.
primary server	A server that stores a file about the zone for which it is an authority.
primary station	In primary/secondary access method, a station that issues commands to the secondary stations.
priority queueing	A queuing technique in which packets are assigned to a priority class, each with its own queue.
privacy	A security aspect in which the message makes sense only to the intended receiver.
private key	In conventional encryption, a key shared by only one pair of devices, a sender and a receiver. In public-key encryption, the private key is known only to the receiver.
private network	A network that is isolated from the Internet.
process	A running application program.
process-to-process delivery	Delivery of a packet from the sending process to the destination process.
product block	A combination of P-boxes and S-boxes to get a more complex cipher block.
Project 802	The project undertaken by the IEEE in an attempt to solve LAN incompatibility. See also IEEE Project 802.
propagation speed	The rate at which a signal or bit travels; measured by distance/second.
propagation time	The time required for a signal to travel from one point to another.
Protocol Independent Multicast (PIM)	A multicasting protocol family with two members, PIM-DM and PIM-SM; both protocols are unicast-protocol dependent.
Protocol Independent Multicast, Dense Mode (PIM-DM)	A source-based routing protocol that uses RPF and pruning/grafting strategies to handle multicasting.
Protocol Independent Multicast, Sparse Mode (PIM-SM)	A group-shared routing protocol that is similar to CBT and uses a rendezvous point as the source of the tree.
protocol	Rules for communication.
protocol suite	A stack or family of protocols defined for a complex communication system.
proxy firewall	A firewall that filters a message based on the information available in the message itself (at the application layer).
proxy server	A computer that keeps copies of responses to recent requests.
pruning	Stopping the sending of multicast messages from an interface.
public key	In public-key encryption, a key known to everyone.
public key infrastructure (PKI)	A hierarchical structure of CA servers.
public-key cryptography	A method of encryption based on a nonreversible encryption algorithm. The method uses two types of keys: The public key is known to the public; the private key (secret key) is known only to the receiver.
pulse amplitude modulation (PAM)	A technique in which an analog signal is sampled; the result is a series of pulses based on the sampled data.
pulse code modulation (PCM)	A technique that modifies PAM pulses to create a digital signal.
pulse rate	The number of symbols per second.
quadbit	A unit of data consisting of four bits.
quadrature amplitude modulation (QAM)	A digital-to-analog modulation method in which the phase and amplitude of the carrier signal vary with the modulating signal.
quality of service (QoS)	A set of attributes related to the performance of the connection.
quantization	The assignment of a specific range of values to signal amplitudes.
query message	An ICMP message that helps a host or a network manager get specific information from a router or another host. Or, an IGMP message that requests group information from a router or a host. Or, a DNS message that requests information.
queue	A waiting list.
radio wave	Electromagnetic energy in the 3-KHz to 300-GHz range.
random access	A medium access category in which each station can access the medium without being controlled by any other station.
ranging	In an HFC network, a process that determines the distance between the CM and the CMTS.
rate adaptive asymmetrical digital subscriber line (RADSL)	A DSL-based technology that features different data rates depending on the type of communication.
raw socket	A structure designed for protocols that directly use the services of IP and use neither stream sockets nor datagram sockets.
read-only memory (ROM)	Permanent memory with contents that cannot be changed.
Real-Time Streaming Protocol (RTSP)	An out-of-band control protocol designed to add more functionality to the streaming audio/video process.
Real-time Transport Control Protocol (RTCP)	A companion protocol to RTP with messages that control the flow and quality of data and allow the recipient to send feedback to the source or sources.
Real-time Transport Protocol (RTP)	A protocol for real-time traffic; used in conjunction with UDP.
receiver window	In the TCP sliding window protocol, the window at the receiver site.
reconciliation sublayer	A Fast Ethernet sublayer which passes data in 4-bit format to the MII.
recursive resolution	Resolution of the IP address in which the client sends its request to a server that eventually returns a response.
redirection	An ICMP message type that informs the sender of a preferred route.
redundancy	The addition of bits to a message for error control.
reflection	The phenomenon related to the bouncing back of light at the boundary of two media.
refraction	The phenomenon related to the bending of light when it passes from one medium to another.
regenerator	A device that regenerates the original signal from a corrupted signal. See also repeater.
regional cable head (RCH)	In an HFC network, the main distribution site.
regional ISP	A small ISP that is connected to one or more NSPs.
registrar server	In SIP, a server that knows the IP address of the callee.
reliability	A QoS flow characteristic; dependability of the transmission.
remote access	Using a terminal that is not directly connected to a computer.
remote bridge	A device that connects LANs and point-to-point networks; often used in a backbone network.
remote host	The computer that a user wishes to access while seated physically at another computer.
remote server	A program run at a site physically removed from the user.
rendezvous router	A router that is the core or center for each multicast group; it becomes the root of the tree.
rendezvous-point tree	A group-shared tree method in which there is one tree for each group.
repeater	A device that extends the distance a signal can travel by regenerating the signal.
replay attack	The resending of a message that has been intercepted by an intruder.
Request for Comment (RFC)	A formal Internet document concerning an Internet issue.
request header	A part of the HTTP request message that specifies the client's configuration and the client's preferred document format.
resolver	The DNS client that is used by a host that needs to map an address to a name or a name to an address.
Resource Reservation Protocol (RSVP)	A signaling protocol to help IP create a flow and make a resource reservation to improve QoS.
response header	A part of the HTTP response message that specifies the server's configuration and special information about the request.
response message	A DNS message type that returns information.
retransmission timer	A timer that controls the waiting time for an acknowledgment of a segment.
return to zero (RZ)	A digital-to-digital encoding technique in which the voltage of the signal is zero for the second half of the bit interval.
reuse factor	In cellular telephony, the number of cells with a different set of frequencies.
Reverse Address Resolution Protocol (RARP)	A TCP/IP protocol that allows a host to find its Internet address given its physical address.
reverse path broadcasting (RPB)	A technique in which the router forwards only the packets that have traveled the shortest path from the source to the router.
reverse path forwarding (RPF)	A technique in which the router forwards only the packets that have traveled the shortest path from the source to the router.
reverse path multicasting (RPM)	A technique that adds pruning and grafting to RPB to create a multicast shortest path tree that supports dynamic membership changes.
ring topology	A topology in which the devices are connected in a ring. Each device on the ring receives the data unit from the previous device, regenerates it, and forwards it to the next device.
Rivest, Shamir, Adleman (RSA) encryption	See RSA encryption.
RJ45	A coaxial cable connector.
roaming	In cellular telephony, the ability of a user to communicate outside of his own service provider's area.
root server	In DNS, a server whose zone consists of the whole tree. A root server usually does not store any information about domains but delegates its authority to other servers, keeping references to those servers.
rotary dialing	Accessing the switching station through a phone that sends a digital signal to the end office.
round-trip time (RTT)	The time required for a datagram to go from a source to a destination and then back again.
route	A path traveled by a packet.
router	An internetworking device operating at the first three OSI layers. A router is attached to two or more networks and forwards packets from one network to another.
router link LSA	An LSA packet that advertises all of the links of a router.
router-solicitation and advertisement message	An ICMP message sent to obtain and disperse router information.
Routing Information Protocol (RIP)	A routing protocol based on the distance vector routing algorithm.
routing table	A table containing information a router needs to route packets. The information may include the network address, the cost, the address of the next hop, and so on.
routing	The process performed by a router; finding the next hop for a datagram.
RSA encryption	A popular public-key encryption method developed by Rivest, Shamir, and Adleman.
sampling rate	The number of samples obtained per second in the sampling process.
sampling	The process of obtaining amplitudes of a signal at regular intervals.
satellite network	A combination of nodes that provides communication form one point on the earth to another.
S-box	An encryption device made of decoders, P-boxes, and encoders.
scatternet	A combination of piconets.
search algorithm	A rule for finding the next hop.
secondary server	In DNS, a server that transfers the complete information about a zone from another server (primary or secondary) and stores the file on its local disk.
secondary station	In poll/select access method, a station that sends a response in answer to a command from a primary station.
secret-key encryption	A security method in which the key for encryption is the same as the key for decryption; both sender and receiver have the same key.
Security Association (SA)	An IPSec signaling protocol that creates a logical connection between 2 hosts.
security	The protection of a network from unauthorized access, viruses, and catastrophe.
segment	The packet at the TCP layer. Also, the length of transmission medium shared by devices.
segmentation and reassembly (SAR)	The lower AAL sublayer in the ATM protocol in which a header and/or trailer may be added to produce a 48-byte element.
segmentation	The splitting of a message into multiple packets; usually performed at the transport layer.
select	In poll/select access method, a procedure in which the primary station asks a secondary station if it is ready to receive data.
selective-repeat ARQ	An error-control method in which only the frame in error is resent.
self-synchronization	Synchronization of long strings of 1s or 0s through the coding method.
semantics	The meaning of each section of bits.
sender window	In the TCP sliding window protocol, the window at the sender site.
sequence number	The number that denotes the location of a frame or packet in a message.
serial transmission	Transmission of data one bit at a time using only one single link.
server	A program that can provide services to other programs, called clients.
service-point address	See port address.
Session Initiation Protocol (SIP)	In voice over IP, an application protocol that establishes, manages, and terminates a multimedia session.
session layer	The fifth layer of the OSI model, responsible for the establishment, management, and termination of logical connections between two end users.
setup phase	In virtual circuit switching, a phase in which the source and destination use their global addresses to help switches make table entries for the connection.
S-frame	An HDLC frame used for supervisory functions such as acknowledgment, flow control, and error control; it contains no user data.
Shannon capacity	The theoretical highest data rate for a channel.
shielded twisted-pair (STP)	Twisted-pair cable enclosed in a foil or mesh shield that protects against electromagnetic interference.
short interframe space (SIFS)	In CSMA/CA, a period of time that the destination waits after receiving the RTS.
shortest path tree	A routing table formed by using the Dijkstra algorithm.
signal	Electromagnetic waves propagated along a transmission medium.
signal level	The number of values allowed in a particular signal.
signal-to-noise ratio (SNR)	The signal strength divided by the noise, both in decibels.
silly window syndrome	A situation in which a small window size is advertised by the receiver and a small segment sent by the sender.
simple bridge	A networking device that links two segments; requires manual maintenance and updating.
Simple Mail Transfer Protocol (SMTP)	The TCP/IP protocol defining electronic mail service on the Internet.
Simple Network Management Protocol (SNMP)	The TCP/IP protocol that specifies the process of management in the Internet.
simplex mode	A transmission mode in which communication is one way.
sine wave	An amplitude-versus-time representation of a rotating vector.
single-bit error	Error in a data unit in which only one single bit has been altered.
single-mode fiber	An optical fiber with an extremely small diameter that limits beams to a few angles, resulting in an almost horizontal beam.
sky propagation	Propagation of radio waves into the ionosphere and then back to earth.
slash notation	A shorthand method to indicate the number of 1s in the mask.
slave	In a piconet, a station under control of a master.
sliding window	A protocol that allows several data units to be in transition before receiving an acknowledgment.
sliding window ARQ	An error-control protocol using sliding window concept.
slow convergence	A RIP shortcoming apparent when a change somewhere in the internet propagates very slowly through the rest of the internet.
slow start	A congestion-control method in which the congestion window size increases exponentially at first.
socket address	A structure holding an IP address and a port number.
socket	An end point for a process; two sockets are needed for communication.
socket interface	An API based on UNIX that defines a set of system calls (procedures) that are an extension of system calls used in UNIX to access files.
solicited response	A RIP response sent only in answer to a request.
source address (SA)	The address of the sender of the message.
source quench	A method, used in ICMP for flow control, in which the source is advised to slow down or stop the sending of datagrams because of congestion.
source routing bridge	A source or destination station that performs some of the duties of a transparent bridge as a method to prevent loops.
source routing	Explicitly defining the route of a packet by the sender of the packet.
source-based tree	A tree used for multicasting by multicasting protocols in which a single tree is made for each combination of source and group.
source quench message	An ICMP message sent to slow down or stop the sending of datagrams.
source-to-destination delivery	The transmission of a message from the original sender to the intended recipient.
space propagation	A type of propagation that can penetrate the ionosphere.
space-division switching	Switching in which the paths are separated from each other spatially.
spanning tree	A tree with the source as the root and group members as leaves; a tree that connects all of the nodes.
spanning tree algorithm	An algorithm that prevents looping when two LANs are connected by more than one bridge.
spatial compression	Compressing an image by removing redundancies.
special-query message	An IGMP query message sent by a router to ensure that no host or router is interested in continuing membership in a group.
specific host on this network	A special address in which the netid is all 0s and the hostid is explicit.
spectrum	The range of frequencies of a signal.
split horizon	A method to improve RIP stability in which the router selectively chooses the interface from which updating information is sent.
spread spectrum	A wireless transmission technique that requires a bandwidth several times the original bandwidth.
star backbone	A backbone in which the logical topology is a star.
star topology	A topology in which all stations are attached to a central device (hub).
start bit	In asynchronous transmission, a bit to indicate the beginning of transmission.
start frame delimiter (SFD)	A 1-byte field in the IEEE 802.3 frame that signals the beginning of the readable (nonpreamble) bit stream.
state transition diagram	A diagram to illustrate the states of a finite state machine.
static document	On the World Wide Web, a fixed-content document that is created and stored in a server.
static mapping	A technique in which a list of logical and physical address correspondences is used for address resolution.
static routing	A type of routing in which the routing table remains unchanged.
stationary host	A host that remains attached to one network.
status line	In the HTTP response message a line that consists of the HTTP version, a space, a status code, a space, a status phrase.
stop bit	In asynchronous transmission, one or more bits to indicate the end of transmission.
stop-and-wait ARQ	An error-control protocol using stop-and-wait flow control.
store-and-forward switch	A switch that stores the frame in an input buffer until the whole packet has arrived.
straight tip connector	A type of fiber-optic cable connector using a bayonet locking system.
stream socket	A structure designed to be used with a connection-oriented protocol such as TCP.
streaming live audio/video	Broadcast data from the Internet that a user can listen to or watch.
streaming stored audio/video	Data downloaded as files from the Internet that a user can listen to or watch.
STS multiplexer/demultiplexer	A SONET device that multiplexes and demultiplexes signals.
stub link	A network that is connected to only one router.
subnet address	The network address of a subnet.
subnet mask	The mask for a subnet.
subnetwork	A part of a network.
subscriber channel connector	A fiber-optic cable connector using a push/pull locking mechanism.
substitution	A bit-level encryption method in which n bits substitute for another n bits as defined by P-boxes, encoders, and decoders.
suffix	For a network, the varying part (similar to the hostid) of the address. In DNS, a string used by an organization to define its host or resources.
summary link to AS boundary router LSA	An LSA packet that lets a router inside an area know the route to an autonomous boundary router.
summary link to network LSA	An LSA packet that finds the cost of reaching networks outside of the area.
supergroup	A signal composed of five multiplexed groups.
supernet	A network formed from two or more smaller networks.
supernet mask	The mask for a supernet.
supervisory frame	See S-frame.
switch	A device connecting multiple communication lines together.
switched Ethernet	An Ethernet in which a switch, replacing the hub, can direct a transmission to its destination.
switched virtual circuit (SVC)	A virtual circuit transmission method in which a virtual circuit is created and in existence only for the duration of the exchange.
switched/56	A temporary 56-Kbps digital connection between two users.
switching office	The place where telephone switches are located.
symmetric digital subscriber line (SDSL)	A DSL-based technology similar to HDSL, but using only one single twisted-pair cable.
symmetric key	The key used for both encryption and decryption.
symmetric-key cryptography	A cipher in which the same key is used for encryption and decryption.
synchronization points	Reference points introduced into the data by the session layer for the purpose of flow and error control.
synchronous connection oriented (SCO) link	In a Bluetooth network, a physical link created between a master and a slave that reserves specific slots at regular intervals.
Synchronous Digital Hierarchy (SDH)	The ITU-T equivalent of SONET.
Synchronous Optical Network (SONET)	A standard developed by ANSI for fiber optic technology that can transmit high-speed data. It can be used to deliver text, audio, and video.
synchronous payload envelope (SPE)	The part of the SONET frame containing user data and transmission overhead.
synchronous transmission	A transmission method that requires a constant timing relationship between the sender and the receiver.
synchronous transport signal (STS)	A signal in the SONET hierarchy.
syntax	The structure or format of data, meaning the order in which they are presented.
tag	A formatting instruction embedded in an HTML document.
TCP timer	The timers used by TCP to handle retransmission, zero window-size advertisements, long idle connections, and connection termination.
TCP/IP protocol suite	A group of hierarchical protocols used in an internet.
TDM bus	A time-division switch in which the input and output lines are connected to a high-speed bus through microswitches.
teardown phase	In virtual circuit switching, the phase in which the source and destination inform the switch to erase their entry.
telecommunications	Exchange of information over distance using electronic equipment.
teleconferencing	Audio and visual communication between remote users.
Teledesic	A system of satellites that provides fiber-optic communication (broadband channels, low error rate, and low delay)
TELNET	See Terminal Network.
temporal compression	An MPEG compression method in which redundant frames are removed.
Terminal Network (TELNET)	A general purpose client-server program that allows remote login.
terminating state	A PPP state in which several packets are exchanged between the two ends for house cleaning and closing the link.
terminator	An electronic device that prevents signal reflections at the end of a cable.
thick Ethernet	See 10Base5.
thin Ethernet	See 10Base2.
three-layer switch	A switch at the network layer; a router.
three-way handshake	A sequence of events for connection establishment or termination consisting of the request, then the acknowledgment of the request, and then confirmation of the acknowledgment.
throughput	The number of bits that can pass through a point in one second.
ticket	An encrypted message containing a session key.
ticket-granting server (TGS)	A Kerberos server that issues tickets.
time division duplexing TDMA (TDD-TDMA)	In a Bluetooth network, a kind of half-duplex communication in which the slave and receiver send and receive data, but not at the same time (half-duplex).
time division multiple access (TDMA)	A multiple access method in which the bandwidth is just one time-shared channel.
time to live (TTL)	The lifetime of a packet.
time-division multiplexing (TDM)	The technique of combining signals coming from low-speed channels to share time on a high-speed path.
time-division switching	A circuit-switching technique in which time-division multiplexing is used to achieve switching.
time-domain plot	A graphical representation of a signal's amplitude versus time.
time-exceeded message	An ICMP message sent to inform a source that (1) its datagram has a time-to-live value of zero, or (2) the fragments of a message have not been received within a set time limit.
time-slot interchange (TSI)	A time-division switch consisting of RAM and a control unit.
timestamp	An IP header option used to record the time of datagram processing by a router. Also, a method to handle jitter in interactive real-time audio/video.
timestamp-request and reply message	An ICMP message sent to determine the round-trip time or to synchronize clocks.
time-waited timer	A TCP timer used in connection termination that allows late segments to arrive.
T-lines	A hierarchy of digital lines designed to carry speech and other signals in digital forms. The hierarchy defines T-1, T-2, T-3, and T-4 lines.
token	A small packet used in token-passing access method.
token bucket	An algorithm that allows idle hosts to accumulate credit for the future in the form of tokens.
token passing	An access method in which a token is circulated in the network. The station that captures the token can send data.
Token Ring	A LAN using a ring topology and token-passing access method.
toll call service	An inter-LATA or intra-LATA telephone service charged to the caller.
topology	The structure of a network including physical arrangement of devices.
touch-tone dialing	A telephone dialing method in which each key is represented by two small bursts of analog signals.
traffic	Messages on a network.
traffic control	A method for shaping and controlling traffic in a wide area network.
traffic shaping	A mechanism to control the amount and the rate of the traffic sent to the network to improve QoS.
trailer	Control information appended to a data unit.
transceiver	A device that both transmits and receives.
transceiver cable	In Ethernet, the cable that connects the station to the transceiver. Also called the attachment unit interface.
transient link	A network with several routers attached to it.
transition state	The different phases through which a PPP connection goes.
translation	Changing from one code or protocol to another.
Transmission Control Protocol (TCP)	A transport protocol in the TCP/IP protocol suite.
Transmission Control Protocol/Internetworking Protocol (TCP/IP)	A five-layer protocol suite that defines the exchange of transmissions across the Internet.
transmission medium	The physical path linking two communication devices.
transmission path (TP)	In ATM, the physical connection between two switches.
transmission rate	The number of bits sent per second.
transparency	The ability to send any bit pattern as data without it being mistaken for control bits.
transparent bridge	Another name for a learning bridge.
transparent data	Data that can contain control bit patterns without being interpreted as control.
Transport Layer Security (TLS)	A security protocol at the transport level designed to provide security on the WWW.
transport layer	The fourth layer in the Internet and OSI model; responsible for reliable end-to-end delivery and error recovery.
transpositional cipher	A character-level encryption method in which the position of the character changes.
trellis-coded modulation	A modulation technique that includes error correction.
triangulation	A two-dimensional method of finding a location given the distances from 3 different points.
tribit	A unit of data consisting of three bits.
triple DES	An algorithm compatible with DES that uses three DES blocks and two 56-bit keys.
Trivial File Transfer Protocol (TFTP)	An unreliable TCP/IP protocol for file transfer that does not require complex interaction between client and server.
trunk	Transmission media that handle communications between offices.
tunneling	In multicasting, a process in which the multicast packet is encapsulated in a unicast packet and then sent through the network. In VPN, the encapsulation of an encrypted IP datagram in a second outer datagram. For IPv6, a strategy used when two computers using IPv6 want to communicate with each other when the packet must pass through a region that uses IPv4.
twisted-pair cable	A transmission medium consisting of two insulated conductors in a twisted configuration.
twisted-pair Ethernet	An Ethernet using twisted-pair cable; 10Base-T.
two-dimensional parity check	An error detection method in two dimensions.
two-layer switch	A bridge with many ports and a design that allows better (faster) performance.
type of service (TOS)	A criteria or value that specifies the handling of the datagram.
U-frame	An HDLC unnumbered frame carrying link management information.
unbalanced configuration	An HDLC configuration in which one device is primary and the others secondary.
unguided medium	A transmission medium with no physical boundaries.
unicast address	An address belonging to one destination.
unicast message	A message sent to just one destination.
unicast routing	The sending of a packet to just one destination.
unidirectional antenna	An antenna that sends or receives signals in one direction.
Uniform Resource Locator (URL)	A string of characters (address) that identifies a page on the World Wide Web.
unipolar encoding	A digital-to-digital encoding method in which one nonzero value represents either 1 or 0; the other bit is represented by a zero value.
unnumbered frame	See U-frame.
unshielded twisted-pair (UTP)	A cable with wires that are twisted together to reduce noise and crosstalk. See also twisted-pair cable and shielded twisted-pair.
unspecified bit rate (UBR)	The data rate of an ATM service class specifying only best-effort delivery.
update message	A BGP message used by a router to withdraw destinations that have been advertised previously or to announce a route to a new destination.
uplink	Transmission from an earth station to a satellite.
uploading	Sending a local file or data to a remote site.
upstream data band	In an HFC network, the 5 to 42 MHz band for data from the subscriber premises to the Internet.
user agent (UA)	An SMTP component that prepares the message, creates the envelope, and puts the message in the envelope.
user authentication	A security measure in which the sender identity is verified before the start of a communication.
User Datagram Protocol (UDP)	A connectionless TCP/IP transport layer protocol.
user datagram	The name of the packet in the UDP protocol.
user network interface (UNI)	The interface between a user and the ATM network.
user support layers	The session, presentation, and application layers.
user-to-network interface (UNI)	In ATM, the interface between an end point (user) and an ATM switch.
V series	ITU-T standards that define data transmission over telephone lines.
variable bit rate (VBR)	The data rate of an ATM service class for users needing a varying bit rate.
very high bit rate digital subscriber line (VDSL)	A DSL-based technology for short distances.
video band	In an HFC network, the band from 54 to 550 MHz for downstream video.
Vigenere cipher	A polyalphabetic substitution scheme that uses the position of a character in the plaintext and the character's position in the alphabet.
virtual channel identifier (VCI)	A field in an ATM cell header that defines a channel.
virtual circuit (VC)	A logical circuit made between the sending and receiving computer. The connection is made after both computers do handshaking. After the connection, all packets follow the same route and arrive in sequence.
virtual circuit approach to packet switching	A packet switching method in which all packets of a message or session follow the exact same route.
virtual circuit identifier (VCI)	A field in an ATM cell header that defines a channel.
virtual circuit switching	A switching technique used in switched WANs.
virtual connection identifier	A VCI or VPI.
virtual link	An OSPF connection between two routers that is created when the physical link is broken. The link between them uses a longer path that probably goes through several routers.
virtual local area network (VLAN)	A technology that divides a physical LAN into virtual workgroups through software methods.
virtual path (VP)	In ATM, a connection or set of connections between two switches.
virtual path identifier (VPI)	A field in an ATM cell header that identifies a path.
virtual path identifier/virtual channel identifier (VPI/VCI)	Two fields used together to route an ATM cell.
virtual private network (VPN)	A technology that creates a network that is physically public, but virtually private.
virtual tributary (VT)	A partial payload that can be inserted into a SONET frame and combined with other partial payloads to fill out the frame.
Voice Over Frame Relay (VOFR)	A Frame Relay option that can handle voice data.
voice over IP	A technology in which the Internet is used as a telephone network.
Walsh table	In CDMA, a two-dimensional table used to generate orthogonal sequences.
wavelength	The distance a simple signal can travel in one period.
wave-division multiplexing (WDM)	The combining of modulated light signals into one signal.
web page	A unit of hypertext or hypermedia available on the Web.
Web	Synonym for World Wide Web (WWW).
weighted fair queueing	A packet scheduling technique to improve QoS in which the packets are assigned to queues based on a given priority number.
well-known attribute	Path information that every BGP router must recognize.
well-known port	A port number that identifies a process on the server.
wide area network (WAN)	A network that uses a technology that can span a large geographical distance.
wide area telephone service (WATS)	A telephone service in which the charges are based on the number of calls made.
window size field	The size of the sliding window used in flow control.
wireless communication	Data transmission using unguided media.
wireless LAN	A LAN which uses unguided media.
World Wide Web (WWW)	A multimedia Internet service that allows users to traverse the Internet by moving from one document to another via links that connect them together.
X.25	An ITU-T standard that defines the interface between a data terminal device and a packet-switching network.
X.509	An ITU-T standard for public key infrastructure (PKI).
zone	In DNS, what a server is responsible for or has authority over.