Let’s review the process of message exchange when a call comes from a Public Switched Telephone Network (PSTN)


PSTN sends an Initial address message (IAM)to a GMSC. The message among other things contains its unique MSISDN. GMSC forwards, via Send Routing Info (SRI), MSISDN to HLR and requests the information that is required for the following call routing. HLR defines IMSI via the MSISDN, and passes it to VLR using the message ‘Provide Roaming Number’ that MS is registered in. HLR requests MSRN (Mobile Subscriber Roaming Number), that is similar to MSISD and is essential for GMSC to identify which MSC is operating the MS. HLR defines IMSI via the MSISDN, and passes it to VLR using the message ‘Provide Roaming Number’ that MS is registered in. HLR requests MSRN (Mobile Subscriber Roaming Number), that is similar to MSISD and is essential for GMSC to identify which MSC is operating the MS. VLR identifies MSRN and sends it to HLR with the ‘Provide Roaming Number Ack’ message. After that HLR forwards ‘Routing Informaton Ack’ message to GMSC. Once GMSC knows the MSC, it sends MSRN of the mobile station. The Gateway MSC routes the number to the correct MSC/VLR. When MSC/VLR accepts the GMSC message, MSRN can be used again.


Once MSC receives the message with MSRN, it requests VLR, in the message SFIC, or ‘Send Info for Incoming Call Setup’ , the identifiers that are required to process the call, they are LAI and TMSI (or IMSI). . VLR посылает эту информацию обратно на MSC в сообщении «page». VLR sends information back to MSC with the message ‘Page’. MSC broadcasts the message «‘Paging Request’ within the Location Area, as only LAI is known.


MSC scans the Paging Channel (PCH), once it detects TMSI responds with a Channel Request. BSS sends a message ‘DCCH Assign’, assigning a SDCCH to a MSC. SDCCH is assigned within a BSS and MSC requests access to a Network. Once MSC switches to the assigned SDCCH, it responds ‘Paging Response’ in the BSS that afterwards send the message to VLR.


Once all these steps are completed, MS goes through the authentication and ciphering.


Once authentication is over and ciphering is set up, MSC send a message ‘Complete Call’to BSS, that notifies that the call has to be process further. BSS forwards ‘SETUP’ message to MS via SDCCH. Then MS sends the message ‘Call Confirmed’to BSS, that means the MS is ready to receive an Inbound Call.


Call Set Up BSS sends to MS via SDCCH a message ‘Assignment Complete’, assigning TCH (Traffic Channels) for a MS. MS sends an ‘ALERT’ to BSS, and then forwards it to MSC. A Switch forwards a message ‘Address Complete’ via Public Switched Telephone Network (PSTN)to a calling subscriber. At this point, the subscriber who is making the call, hears dial tones.


Once the second subscriber answered the call, MS sends ‘Connect’ to MSC. MSC approves the message is received and responds back ‘Connect Ack’, and sends ‘Answer’ to GMSC and PSTN. At this point GMSC and MSC connect TCH GSM and Voice Traffic PSTN together (bring together). The connection between the mobile subscriber and PSTN subscriber is established.