Mobile originates a call
Figure no.1 below shows the trace of the messages that have been sent while the mobile originating a call to another mobile registered in the same MSC. These messages will be described in details.
1- CM- Service Request message (MS -> MSC):
This message is sent by the mobile equipment to the MSC through the BSC (Using the A-interface). CM means Call Management; this message is sent when the Mobile equipment requires making a call. This message contains the following data:
o Identification for the mobile which can be the IMSI (International Mobile Subscriber Identity) or TMSI (Temporary Mobile Subscriber Identity), also it contains the type of the number of digits of this identification number (it can be even or odd number of digits that consist the identification number).
o Another Important information that is contained in this message is the type of service requested by the mobile station.
o The SPC (Signaling Point Code) and the SSN (Sub System Number) of the MSC (Mobile Switching Centre) is contained in this message, also there may be the GT (Goble Title) of the MSC contained in this message.
o The available encryption algorithm that the mobile station can use, which can be A5.1, A5.2 and A5.3.
o The global cell identification that the mobile is in its coverage area while doing this call.
2- Class mark request message(MSC -> MS):
This message is sent by the MSC to the MS requesting MS to send some information such that: the RF (Radio Frequency) power class used, the supported encryption algorithms….etc.
3- CM-service accept (MSC -> MS):
As the mobile station has been already registered in this MSC and a location update process has been taken place before the subscriber request a service, so the MSC check the subscriber data and if the subscriber is not barred from doing the required service; the MSC will send this message to the MS informs it that the service it requested is allowed.
4- Class mark update (MS -> MSC):
This message is sent by the MS as a response of the MSC’s class mark request message. Some of the information contained in this message is as following:
o The RF power Capability.
o The encryption algorithms that are allowed or not allowed.
o If the mobile can support or not the SMS (Short Message Service) service.
5- Setup(BSS -> MSC):
This message is sent from the BSS (Base Station Sub-System) side to the MSC informing it with the requested traffic channel, the dialed number and some other information. The data contained in this message is as following:
o CM Message type: here it is setup
o GSM bearer capability: speech, Transfer mode and coding standard.
o Radio channel required: in this field MS specified the preferred channel and the supported one. For example: full rate speech version 1 preferred, half rate speech version 1 is also supported.
o Speech version octet: The speech versions supported and the coding techniques used in sending this speech. For example: GSM half rate speech version 1.
o The called subscriber information: the type of the dialed number and the dialed number itself. For example: the number attribute is the ISDN (Integrated Service Digital Network) telephony number plan, number: 2012100006.
6- MAP open request(MSC -> HLR): MAP (Mobile Application Part)
The MSC sends the HLR this message to start a new session. The data contained in this message is as following:
o The sender MSC number.
o The type of the address attached in this message, for example: MSISDN.
o The number of the called subscriber.
7- MAP send routing information request (MSC -> HLR):
The MSC sends this message to the HLR requesting the routing information (MSRN: Mobile Station Roaming Number) for the called subscriber. The data contained in this message is as following:
o The MAP service request type: the type of service requested from HLR, for example: send routing information.
o Send routing information argument: if the service requested from the HLR is to send routing information, the MSC should specify the arguments of this service. Simply, the arguments may be represented in the MSISDN of the called subscriber. Also, the nature of the address should be specified.
8- Call proceeding(MSC -> BSS):
This message is sent by the MSC to the BSS. This message is the response of the message setup, it is sent to confirm the setting of the connection sent by the MS. This message is sent after the MSC sends the “SRI: Send Routing Information” message to the HLR
9- MAP provide roaming information request(HLR-> MSC):
This message is sent by the HLR to the MSC requesting MSRN for the called mobile entity. As the called mobile is registered in the same MSC, the HLR requests the MSRN from the MSC of the calling MS. The data contained in this message is as following:
o The called MS data, which includes the IMSI and MSISDN of the called party.
o The called mobile MSC data, which is the MSC number.
10- MAP provide Roaming Number Response (MSC -> HLR):
This message sent by the MSC to the HLR as a response on the message named as provide roaming information request. The content of this message is the MSRN requested by the HLR for the called subscriber.
11- MAP close request (MSC -> HLR):
This message sent by the MSC to HLR informs it to close the map session. Included in this message is the reason of the closing of the session. If the close of session is not a result of any error occurring in the session, the reason would be “Normal Release”.
12- Map send Route information(HLR -> MSC):
This message is sent by the HLR to the MSC of the calling subscriber. This message contains the MSRN allocated for the called subscriber.
13- Send route information response(HLR -> MSC):
This message is sent by HLR to the MSC, it contains the IMSI and MSRN of the called subscriber.
14- Paging(MSC -> MS):
As the MSC received the MSRN for the called MS, MSC start searching for the called MS. The data contained in this message is as following:
o The MSC data: The signaling point of the MSC of the called mobile. Also, the subsystem number of the MSC. Also, it may contain the global title in this message (GT).
o The BSC data: The signaling point of the BSC where the MS is registered, also its SSN. Also, it may contain the global title for the BSC.
o The Cell identification of the cell where the mobile is registered.
15- ADD REQ(MSOFT -> UMG):
This message is sent by the MSOFT to UMG, asking it to allocate circuit resources for the call.
16- Assignment REQ(MSC -> BSS):
This message is sent by the MSC to the BSS requesting it to assign traffic channel for the call. The data included in this message is as following:
o The channel type: in this field the MSC specifies the type of data that would be transferred along this link, for example this could be “speech”. Also, we specified the channel rate (the preferred and the allowed), so that if the preferred is not available it can use the allowed. For example: half or full rate TCH, full rate preferred.
o The actual Circuit identity code that are going to be used.
17- ADD Reply(UMG -> MSOFT):
This message is sent by the UMG as a response for the ADD REQ message. This is message to inform the MSOFT that the resource reservation is done. Also, the UMG specifies the TID allocated.
18- Assignment complete (BSS -> MSOFT):
This message is sent from BSS side to the MSOFT to inform the MSOFT that the requested channel has been allocated. The TCH type is specified in this message, also the circuit pool number and the message type which would be BSSMAP assignment complete message.
19- Paging response(BSS -> MSC):
This message is sent form BSS side to the MSC as a response for the paging message sent by the MSC. This message contains the location area identification of the called mobile, also the cell identification of the called mobile.
20- Setup(MSC -> BSS):
This message is sent for this time by MSC not from BSS, this is because the message is generated from the side that originates the transactions and in this case it is the MSC side. This message is used to inform the called party with the characteristics of the connection. For example: the radio channel required field that is supported.
21- Call confirmed(BSS -> MSC):
This message is sent by the BSS side, this message is similar to the message “Call proceeding” sent by the MSC in response to the message setup sent by the BSS.
22- ADD REQ (MSOFT -> UMG):
This message is used to allocate resources for the called party in the UMG, as UMG resources have already been allocated to the calling party.
23- ADD Reply(UMG -> MSOFT):
This message is sent by the UMG to confirm the allocation of the resources and give the allocated TID.
24- Assignment REQ(MSC -> BSS):
This message is used to allocate radio resources for the called party, as radio resources have already been allocated to the calling party.
25- Assignment complete (BSS -> MSC):
This message is sent by the BSS side to confirm the allocation of the RF resources. In this message we will find the type of the channel allocated to this connection, as the MSC when requesting RF resources specifies the allowed and the preferred channel rather than specifying a certain channel.
26- Alert (MS -> MSC):
This message is sent from the MS to the MSC side. The message is sent to inform the MSC that the called mobile is now ringing; I know that the called mobile is the one mentioned here form two things:
o First, the user connected id in the message is for the called mobile.
o Second, the transaction flag of the message indicates that the message is sent to the side that originates the transaction which is the MSC in case of the called mobile.
27- Alert (MSC -> MS):
This time the “Alert” message is sent from the MSC to the MS, this message is used to make the caller hearing alerting tone. I know that this message is sent to the calling mobile from the transaction flag as it indicates that the message is sent to the side that start the transaction which is BSS in case of the calling mobile.
28- MOD REQ (MSOFT -> UMG):
This message is sent to the UMG to send the ringing tone to the calling mobile. I know that this message is for the calling mobile not the called from the TID specified in this message is for the calling not the called, and from the signal type I know that this message is to send ring tone signaling to the called mobile, because these tones are stored in the UMG.
29- Connect(BSS -> MSC):
This message is sent from the BSS side to the MSC informing the MSC that the called mobile answers the call.
30- Connect (MSC -> BSS):
This message is sent by the MSC to the BSS informing the calling mobile that the called party is connected.
31- Connect ACK (MSC -> BSS):
This message is sent by the MSC to the BSS to acknowledge the connect message.
32- MOD REQ(MSOFT -> UMG):
This message is sent from the MSC to UMG to stop the ringing tone that is sent to the calling mobile, as the signal field in this message is empty.
33- MOD Reply (UMG -> MSOFT):
This message is just an acknowledgment to the “MOD REQ” message.
34- Connect ACK (BSS -> MSOFT):
This message is sent by the BSS to the MSC to acknowledge the connect message.
35- Disconnect (BSS -> MSC):
This message is sent by the BSS side to inform the MSC that mobile disconnects the call. The ID of the mobile subscriber can be found in the user ID filed in this message. An important field in this message is the cause of the disconnection which in normal disconnecting cases should be “Normal Clear”. Note that this message is from the BSS side because the called subscriber is the one who disconnects first.
36- Release (MSC -> BSS):
This message comes as a response for the message “Disconnect”, to release the traffic channel used.
37- Disconnect (MSC -> BSS):
This message is sent by the MSC to the BSS to inform the calling mobile station the disconnecting of the call.
38- Release (BSS -> MSC):
This message is sent by the BSS as an acknowledgment for the Disconnect message sent by the MSC. This is to inform the MSC that the calling mobile signaling resources are now released.
39- Release complete (MSC -> BSS):
This message is sent by the MSC to BSS to inform it that the calling mobile signaling resources are now released from its side.
40- Release complete (BSS -> MSC):
This message is sent by the BSS to MSC to inform it that the calling mobile resources are now released from its side.
41- Clear command(MSC -> BSS):
This message is sent by the MSC to the BSS side to ask it to release the RF resources allocated to a mobile station. The mobile is defined by its SCCP ID.
42- Clear complete(BSS -> MSC):
This message is sent by the BSS to confirm the release of the mobile station RF resources. The mobile is identified by its “User connected ID” filed in the message.
43- SUB REQ(MSOFT -> UMG):
This message is sent by the MSOFT to the UMG to release the resources of UMG allocated to a certain mobile. The resource is identified in the TID field.
44- SUB Reply(UMG -> MSOFT):
This is a reply for the request from MSOFT to clear the UMG resources.
Note 1: the Alert for the calling mobile is originating form the MSC side, this alert is sent by the UMG to the mobile through the BSS part.
Note 2: the above situation is for a MS calling a MS in the same BSC and registered in the same MSC.
Note 1: the Alert for the calling mobile is originating form the MSC side, this alert is sent by the UMG to the mobile through the BSS part.
Note 2: the above situation is for a MS calling a MS in the same BSC and registered in the same MSC.
