Issues about Circuit Pool
1. Acronym
TRAU Transcoder and Rate Adaptation Unit
BSC Base Station Controller
BTS Base Transreceiver Station
BSS Base Station Subsystem
MS Mobile Station
MSC Mobile Switch Center
PCM Pulse Code Modulation
IE Information Element
2. Background
Codecs transferred in PCM bearer and air interface are different. So entity that performs transcoding function for speech channels and RA( Rate Adaption ) for data channels is needed. The entity is so called TRAU. Logically the entity exists between MSC and BSC. Physically, the entity may locate at the MSC side or BSC side.
Circuit pool is a concept introduced in A-interface (MSC - BSC). In other places, such as between BSC and BTS, BTS and MS, it is not necessary at all.
The Circuit Pool provides the capability of managing different types of TRAU between an MSC and the BSC it controls. The PCM links between the MSC and the BSC are organized in pools according to the capabilities of the TRAU they are connected to.
There are predefined circuit pools. Their capabilities are also predefined. They are identified by circuit pool ID.
A "pool" is a group of circuits supporting the same channel types. Channel types stand for kinds of capability. The ability is determined by TRAU’s ability. So, when configuring a BSS Trunk Group’s circuit pool ID, TRAU’s ability should be considered.
3. Predefined Circuit Pools
There are over 30 predefined circuit pools currently. A few of them are listed below. From the circuit pool ID, a list of speech capabilities can be concluded. Speech capabilities overlap between circuit pools. So from speech capabilities, a list of circuit pool ID supporting them can be concluded. The action to be concluded from each other will be used during a call.
1
FR speech version 1
FR data (12, 6, 3.6kbit/s)3
FR speech
version 1
FR data (12, 6, 3.6kbit/s)
HR speech version 1
HR data (6,
3.6kbit/s)4
FR speech version 2
FR data (12, 6, 3.6kbit/s)5
FR speech version 1
FR speech version 2
FR data (12, 6,
3.6kbit/s)6 FR speech version 2
FR data (12, 6, 3.6kbit/s)
HR
speech version 1
HR data (6, 3.6kbit/s)7
FR speech version 1
FR speech version 2
FR data (12, 6, 3.6kbit/s)
HR speech version 1
HR data (6, 3.6kbit/s)
4. Procedure
4.1. Originating Call
During call setup, MS will send SETUP with bearer-capacity IE. The MSC will return CALL-PROCEEDING with negotiated bearer-capacity IE. In bearer-capacity IE, there is a list of speech capabilities. From the negotiated bearer-capacity IE, a list of circuit pools can be concluded. If any speech capability in bearer-capacity IE is included in a circuit pool, then the circuit pool will be added to the result set.
MSC find a circuit which supports at least one in the list of circuit pools concluded from bearer-capacity IE. That is, MSC only need to select the circuit which supports at least one speech capability in bearer-capacity IE. When sending Assignment-Request to BSC, the speech capabilities are included in Channel-Type IE.
If, on reception by the BSS of an ASSIGNMENT REQUEST message allocating a circuit, the circuit pool implied by the CIC information element is incompatible with the channel type indicated (that is, the pool does not support any of the radio resources indicated by the channel type) an ASSIGNMENT FAILURE shall be returned to the MSC with the failure cause set to "circuit pool mismatch".
If, on reception by the BSS of an ASSIGNMENT REQUEST message allocating a circuit, the circuit pool implied by the CIC is compatible with the channel type indicated (that is, the pool supports at least one of the radio resource types indicated by the channel type), but the BSS still wishes to change the circuit pool, it sends an ASSIGNMENT FAILURE with the cause "switch circuit pool" and the "circuit pool list" information element.
4.2. Terminating Call
MSC sends SETUP with Bearer-Capacity IE, MS reply Call-Confirmed with negotiated Bearer-Capacity IE. The negotiated Bearer-Capacity IE will be used for MSC to select a circuit. The latter flows are nearly the same with Originating Call.
4.3. Handover
For anchor side handover, the circuit pool can be derived from stored Bearer-Capacity IE. For serving side handover, it can be derived from Channel-Type IE of previous received messages.
5. Reference
3GPP 48.008
AAC030654400DS_02_En_RL