Thread: Assignment procedure in GSM call flow
04-15-2012, 02:26 PM#1
Assignment procedure in GSM call flow
The assignment procedure is used to assign the MS to the correct TCH. The selection of the TCH is based on the information received from the MSC.
The assignment procedure is presented in the following figure:
ASSIGNMENT REQUEST message
The MSC sends an ASSIGNMENT REQUEST (a) message to the BSC for the assignment of channels on the A interface and the radio interface. The MSC also runs a supervisory timer for the assignment procedure.
The BSS responds by sending the MSC the ASSIGNMENT COMPLETE, ASSIGNMENT FAILURE or the QUEUEING INDICATION message. If the MSC fails to receive any of the messages within the time limit defined by the timer, it clears the call with a CLEAR COMMAND message. The BSS releases the MS connection with the channel release procedure.
The BSC expects to receive the following fields in the ASSIGNMENT REQUEST message:
- channel type: radio channel required for the call type
- L3 header info
- priority (optional)
- circuit identification code (CIC) indicating the channel to be used on the A interface
- downlink DTX flag (optional)
- radio channel identity (optional)
- interference band to be used (optional).
The MSC uses the priority element to define whether the request is allowed to be queued and, if so, to define the queueing priority level. If the priority element is not present, it is assumed that the request can be queued if it cannot be served immediately. Furthermore, if the priority is not indicated, the message is queued according to the priority associated with the request type, which is defined internally within the BSS. If the request is queued, a QUEUEING INDICATION message is returned to the MSC.
The downlink DTX flag element is also optional: it is present only when the MSC controls the downlink DTX in a speech call.
If the BSC identifies any of the following situations on analysing the ASSIGNMENT REQUEST message, it rejects the request by sending the ASSIGNMENT FAILURE message to the MSC, without taking any action towards the BTS:
1. If the BSC is not able to use the A channel indicated in the CIC, due to it being used for another call, the BSC rejects the request with the cause requested terrestrial circuit already allocated.
2. If the BSC is not able to use the A channel indicated in the CIC, due to it being marked as undefined, the BSC rejects the request with the cause requested terrestrial resource unavailable.
3. If the CIC is blocked, the BSC rejects the request with the cause requested terrestrial resource unavailable and sends a BLOCK message to the MSC. For reasons when a circuit is blocked by the BSC, see 3GPP 48.008.
4. If there is no circuit pool on the A interface that is able to support the requested TCH type, the BSC rejects the request with the cause requested transcoding/rate adaption not available.
5. If there is a circuit pool on the A interface that is able to support the requested TCH type, but the pool implied by the CIC is totally contradictory with the required TCH type, the BSC rejects the request with the cause circuit pool mismatch.
6. If the circuit pool implied by the CIC is at least partially compliant with the requested TCH type, change of the circuit pool may be required. Reasons for the change include the actual TCH resource situation of the cell and the attempt to allocate the resources in the most optimal way. In this case the BSC rejects the request with the cause switch circuit pool.
7. If the requested speech coder versions are not supported, the BSC rejects the request with the cause requested speech version unavailable.
8. If the radio interface data rate requirement is totally contradictory with the requested TCH rate, the BSC rejects the request with the cause invalid message contents.
On receiving the ASSIGNMENT FAILURE message, the MSC can either issue a CLEAR COMMAND or retry.
If the MSC receives the ASSIGNMENT FAILURE message with a cause indicating a CIC problem, the MSC may send another ASSIGNMENT REQUEST for the same call using a different circuit.
TCH assignment procedure in the BSS
After the ASSIGNMENT REQUEST, the BSC proceeds to perform the TCH assignment procedure.
The BSC sends the PHYSICAL CONTEXT REQUEST message (b) to the BTS. In this message it requests for the timing advance of mobile transmission. The SDCCH sends the timing advance via the TCH and the BTS to the MS, which requires it when moving onto a new radio channel.
If the BSC fails to receive the PHYSICAL CONTEXT CONFIRM message (c) within the time limit defined by the GSM timer T9108, the BSC returns an ASSIGNMENT FAILURE message to the MSC with the cause radio interface message failure. This applies also when the PHYSICAL CONTEXT CONFIRM message does not contain all the expected information. The MSC either aborts with a CLEAR COMMAND or retries.
On receiving the PHYSICAL CONTEXT CONFIRM message, the BSC searches for the relevant TCH using the channel reservation procedure described in Channel reservation procedure.
If the request cannot be acted upon, the BSC returns an ASSIGNMENT FAILURE message with the cause no radio resource available. The reason for this is that all radio channels are reserved, the queue is full, the request has been in the queue for too long, or exceptional conditions such as restart or reset.
After a successful channel reservation, the BSC sends a CHANNEL ACTIVATION message to the BTS. In this message it tells the BTS to activate the required radio channel. The BSC also starts the GSM timer T9103.
The CHANNEL ACTIVATION message contains the following data:
- message type
- channel number: Lm/Bm + AGCH
- activation type: normal assignment
- channel mode: DTX indication and channel type. The channel type is either a speech or data channel. If it is a speech channel, the speech encoding algorithm is included; if it is a data channel, the transparent or the non-transparent mode is included. The data rate is also given.
- channel identification (optional)
- encryption information if received from the MSC (optional)
- BS power as received in the PHYSICAL CONTEXT CONFIRM (optional)
- MS power as received in the PHYSICAL CONTEXT CONFIRM (optional)
- timing advance as received in the PHYSICAL CONTEXT CONFIRM (optional)
- SACCH information (optional)
- multirate configuration (optional).
The channel identification information is included only when activating a TCH under DFCA TRX. For more information, see Dynamic Frequency and Channel Allocation.The SACCH information is used in a dual band cell or when IMSI Based Handover is enabled in the BSC.
The multirate configuration element is present in calls using multirate speech codec.
The BTS returns the CHANNEL ACTIVATION NACK message in the following situations. The BSC indicates the failure to the MSC by sending the ASSIGNMENT FAILURE message with the relevant assignment failure cause value, which can be one of the following:
- radio interface message failure, due to a message format error
- requested transcoding/rate adaption not available, due to the requested service not being supported
- radio resource not available, due to radio channel problems
- radio channel already activated; the radio channel is already in use
- equipment failure
- service or option not available
- not implemented unspecified
- mandatory IE error
- general IE error.
If no response is received from the BTS within the time limit defined by the GSM timer T9103, the BSC sends the MSC an ASSIGNMENT FAILURE message with the cause equipment failure. The MSC either aborts or retries.
When the BTS has successfully activated the radio traffic channel, it returns the CHANNEL ACTIVATION ACKNOWLEDGE message (containing the current frame number) to the BSC.
TCH radio channel assignment in the BSS
The BSC sends the ASSIGNMENT COMMAND message (f, g) to the MS and starts the GSM timer T3107. This command is transparent to the BTS and is transferred as the DATA REQUEST message over the Abis interface.
The ASSIGNMENT COMMAND gives the MS all the information necessary for it to change over to the TCH. The message consists of:
- channel description
- power command: max. MS power
- frequency list (optional)
- cell channel description (optional)
- channel mode: speech full rate, half rate or data (optional)
- mobile allocation (optional)
- cipher mode setting (optional)
- multirate configuration (optional).
The cipher mode setting element is present if ciphering is to be applied and ciphering algorithm is changed during TCH assignment.
On receiving the ASSIGNMENT COMMAND over the radio interface, the MS changes over from the SDCCH to the TCH it has been assigned to. The MS sends a layer 2 SABM (h) on the TCH. The BTS acknowledges the SABM with a UA frame (j) and sends the ESTABLISH INDICATION (i) message to the BSC. Synchronisation with the transcoder starts at this point and the BSC connects the Abis channel to the A channel and the speech path is switched through.
On receiving the UA frame (j), the MS indicates to the BSC with an ASSIGNMENT COMPLETE message (k) that it has successfully changed over to the TCH. This message is transparent to the BTS and is transferred over the Abis interface as a DATA INDICATION message (l). The BSC sends the information to the MSC in the DT1 (ASSIGNMENT COMPLETE) message (m) with the RR cause normal release.
After a successful assignment procedure, the BSC releases the SDCCH channel.
04-18-2012, 02:47 AM#2Level 1 Users
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05-17-2012, 12:30 PM#3
12-11-2012, 09:49 PM#4Junior Member
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its very useful easy to understand and details information .................like it.....
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