Ok, now let’s study the last section, handover sequence.
In this section, the content consists of two parts, inter-BSS handover sequence and inter-MSC handover sequence.
At first, I will explain the inter-BSS handover sequence.
The MS is in the conversation state and is continuously compiling measurements both current transmission and broadcast control channels of up to sixteen surrounding cells.
The measurements from the six best cells are reported back to the BSS, every 480ms.
When a handover is required, due to low Receive Signal Strength Indication (RSS) or poor signal quality the existing “originating” BSS(oBSS) notifies the MSC using message “Handover Required”.
The target or the new BSS (nBSS) is alerted with the message “Handover Request” tagged with the TMSI or IMSI.
Then the new BSS allocates a Handover Reference Number which is used to determine whether the correct mobile gains access to the air-interface channel which it allocates, and acknowledges the MSC’s request with “Handover Request Ack.” This is tagged with the HO Reference number. The nBSS assigns a traffic channel.
The MSC, via the oBSS orders the MS to change to the new channel with the message “Handover Command” on FACCH.
There is an information interchange between nBSS and MS. This uses the FACCH channel but an access burst is used. The messages and information carried depend upon the type of handover being performed.
Once all necessary information has been transferred the message “Handover Complete” is sent to the MSC.
The MSC now sends a “Clear Command” to the oBSS, this frees the radio resources for another MS. The channel is not cleared until this point incase nBSS can not accommodate the MS being handed over.
The MS, still in the conversation mode, then continues to prepare periodic measurement reports and sends them to the nBSS.
Ok, after the inter-BSS handover sequence, we’ll study inter-MSC handover sequence.
There are two types of inter-MSC handover sequence, basic inter-MSC sequence and subsequent inter-MSC sequence.
In the inter-MSC handover , here devoted to introduce the sequence between different MSC/VLR.
When the MSCA receives the “Handover required” from the oBSS, the MSCA finds that the new cell belongs to the MSCB, then sends the “Prepare Handover” message to the MSCB. This request may optionally contain an indication that a handover number allocation is not required, target Cell Id, for compatibility reasons, and all information required by MSCB to allocate the necessary radio resources.
The VLRB responds with the message “Send Handover Report” which handover number is included in.
Then the MSCB transfers the handover number to the MSCA with the message “Prepare Handover Ack”.
The MSCB sends the acknowledgement to the VLRB. The handover number will be reserved until a “Send Handover Report” confirmation is received from MSC-B.
The MSCA then sends the message “Initial Address Message” tagged with handover number allocated by VLRB. The MSCB sends the “Address Complete Message” and “Answer” to the MSCA. The connection between the MSCA and MSCB has been established.
Optionally MSC-A can receive, after a “Prepare Handover” confirmation, a “Process Access Signaling” indication containing BSSAP information.
When the connection has been established between the MS and MSCB, MSCA will be informed by a “Send End Signal” indication.
If required, the MSCA requests the “Forward Access Signaling” request containing the information to be transferred to the A-interface of MSCB (e.g. call control information).
The “Forward Access Signaling” is a non-confirmed service.
The “Forward Access Signaling” is composed in such a way that the information can be passed transparently to the A-interface for call control and mobility management information.
Any response received in MSC-B from the A-interface that should be brought to MSC-A will require a new independent request from the MSCB to the MSCA by invoking a Process Access Signaling request.
When the conversation is ended, the MSCA sends the “Release” message to the MSCB and MSCB responds with the message “Release Complete”.
When MSCA wants to clear the connection with BSSB, and then sends the “Send End Signal” response to MSCB to close the sequence.
After the release of the resources for the call and handover, the inter-VLR location update sequence is followed.
Ok, let’s see the subsequent inter-MSC handover sequence.
The procedure is used when the MSCB has decided that a call is to be handed over to another MSC (either back to the controlling MSC (MSCA) or to a third MSC (MSCC)).
When MSCA receives a “Prepare Subsequent Handover” request, it will start the procedure of handing the call over to a third MSC (MSCC), or back to the controlling MSC (MSCA). In this example, handover to the MSCC.
The controlling MSC(MSCA) sends the “Prepare Handover” to the MSC/VLRC, and MSC/VLRC responds with the handover number in the acknowledgement
The MSCA then sends the message “Initial Address Message” tagged with handover number allocated by the VLRC. The MSCC sends the “Address Complete Message” and “Answer” to the MSCA. The connection between the MSCA and MSCC has been established.
And then the MSCA will release the connection with the MSCB. The MSCA sends “Release” message to the MSCB, and MSCB responds with “Release Complete” message.
If required , MSCC invokes the “Process Access Signaling” request containing the information received on the A-interface that should be transferred to MSCA (e.g. call control information).
“Process Access Signaling” is a non-confirmed service and any response from MSC-A will require a “Forward Access Signaling” request.
When the conversation is ended, the MSCA sends the “Release” message to MSCC and MSCC responds with the message “Release Complete”.
Actually, after the establishment of the connection between the MSCC and MSCA, the MSCC sends the message “Send End Signal” to the MSCA.
If the new handover procedure towards MSCC (or MSCA) is successful, the MSCA will request the release of the MSC-B by sending the “Send End Signal” confirmation.
Ok, hereto, we have finished the study of the communication flow in the GSM system.
Finally, let’s make a summary.
In this course, the GSM security management was introduced at first, it consists of the authentication, ciphering, TMSI reallocation and equipment identification. It is usually used in the other sequence.
Then the explanation for the GSM basic call sequence, it is the most important sequence in the GSM system.
In the third section, the location update sequence was introduced. And then an introduction to the basic SM sequence.
At last a brief introduction to the handover sequence.
Actually, for each kind of sequence, there are many abnormal communication flows. If you want to know more information, please refer to the relative specifications.