The document describes speech channel assignment and channel mode modification procedures in 3 G mobile networks. It discusses (1) how the BSC assigns TCH channels to an MS based on a service request, (2) the internal BSC signaling for channel assignment, and (3) how the BSC modifies the channel mode based on an assignment request from the MSC.

1. HUAWEI BSC6000 Base Station Subsystem
Signaling Analysis Guide 10 Speech Channel Assignment
10 Speech Channel Assignment
About This Chapter
The speech channel assignment is the procedure for assigning TCHs to the MS according to the
service requests.
10.1 Speech Channel Assignment Procedure
This topic describes the speech channel assignment procedure.
10.2 Channel Mode Modification Procedure
This topic describes the channel mode modification procedure.
10.3 Internal BSC Signaling Procedure of Speech Channel Assignment
This topic describes the internal BSC signaling procedure of speech channel assignment.
10.4 Abnormal Speech Channel Assignment Cases
This topic describes the abnormal speech channel assignment cases.
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2. HUAWEI BSC6000 Base Station Subsystem
10 Speech Channel Assignment Signaling Analysis Guide
10.1 Speech Channel Assignment Procedure
This topic describes the speech channel assignment procedure.
Figure 10-1 shows the speech channel assignment procedure.
Figure 10-1 Speech channel assignment procedure
MS BTS BTS BSC
Assignment
Request(1)
Channel Activation(2) Start T10
Channel Activation
Acknowledge(3)
Assignment Command(4)
main DCCH
Start T3107
SABM(5)
FACCH Establishment Indication(7)
UA(6)
FACCH
Assignment
Complete(8)
FACCH
Stop T3107
Assignment
Complete(9)
Stop T10
The speech channel assignment procedure is as follows:
1. The MSC requests the radio resources from the BSS through the SCCP connection and
starts timer T10. The Assignment Request message carries the Channel Type, Priority,
Circuit Identity Code, and Downlink DTX Flag.
l The Channel Type parameter has five to ten bytes. The third byte "speech / data
indicator" indicates speech, data, or signaling. The fourth byte "channel rate and type"
indicates the number of half-rate channels or full-rate channels and the number of related
physical channels. The remaining bytes indicate speech versions 1–5.
l The Priority parameter indicates different priority levels and whether queuing and pre-
emption is performed.
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Signaling Analysis Guide 10 Speech Channel Assignment
l The Circuit Identity Code parameter is used for the physical connection between the
BSC and the MSC.
l The Downlink DTX Flag parameter indicates whether discontinuous transmission is
allowed.
2. For details about the Channel Activation message, refer to 4 Immediate Assignment.
3. For details about the Channel Activation Acknowledge message, refer to 4 Immediate
Assignment.
4. Upon receiving the Channel Activation Acknowledge message, the BSC sends the MS an
Assignment Command message on the SDCCH and starts timer T3107. The Assignment
Command message carries the Channel Description, Power Command, and Channel Mode.
For details about these parameters, refer to the immediate assignment procedure.
5. Upon receiving the Assignment Command message from the BTS, the MS adjusts the
signaling to the TCH and sends the SABM frame on the FACCH. The SABM frame is the
first frame during the setup of multi-frame verification mode and does not carry layer 3
messages.
6. Upon receiving the SABM frame, the BTS responds to the MS with a UA frame on the
FACCH. The UA frame does not carry layer 3 messages.
7. Then the BTS sends the BSC an Establishment Indication message. The Establishment
Indication message does not carry layer 3 messages. The system responds with a UA
Acknowledgement frame.
8. Upon receiving the UA frame, the MS sends an Assignment Complete message on the
FACCH to notify the BSC of the channel assignment completion. Then it stops Timer
T3107.
9. The BSC sends the MSC the Assignment Complete message and stops timer T10. The
Assinment Complete message carries the Chosen Channel, Circuit Pool, and Speech
version. The meanings of the parameters are as follows:
l Chosen Channel
Inidicates the type of the assigned channel
l Circuit Pool
Indicates the circuit pool information
l Speech Version
Indicates the speech coding algorithm
10.2 Channel Mode Modification Procedure
This topic describes the channel mode modification procedure.
Figure 10-2 shows the channel mode modification procedure.
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4. HUAWEI BSC6000 Base Station Subsystem
10 Speech Channel Assignment Signaling Analysis Guide
Figure 10-2 Channel mode modification procedure
MS BTS BSC MSC
Assignment
Request(1)
Start T10
Mode Modify(2)
Mode Modify
Acknowledge(3)
Channel
Mode Modify(4)
main DCCH
Channel Mode
Modify Acknowledge(5) Assignment
main DCCH Commplete(6)
StopT10
The channel mode modification procedure is as follows:
1. For details about the Assignment Request message, refer to 10.1 Speech Channel
Assignment Procedure.
2. Upon receiving the Assignment Request message, the BSC sends the BTS a Mode Modify
message. The Mode Modify message carries the Channel Mode parameter.
3. If the channel mode is changed, the BTS sends the BSC a Mode Modify Acknowledge
message to the BSC. If the channel mode cannot be changed, the BTS sends the BSC a
Mode Modify Negative Acknowledge message.
4. Upon receiving the Mode Modify Acknowlege message, the BSC sends the MS a Channel
Mode Modify message. The Channel Mode Modify message carries the information of the
channel to be changed and the mode of channel after change.
5. If the channel mode is changed, the MS sends the BSC a Channel Mode Modify
Acknowledge message. If the MS does not support the specified mode, it stays in the
original mode. The Channel Mode Modify Acknowledge message carries the mode
information.
6. For details about the Assignment Complete message, refer to 10.1 Speech Channel
Assignment Procedure.
10.3 Internal BSC Signaling Procedure of Speech Channel
Assignment
This topic describes the internal BSC signaling procedure of speech channel assignment.
The internal BSC signaling procedure of speech channel assignment is as follows:
1. Upon receiving the Assignment Request message from the MSC, the BSC checks the
channel type. Choose Other Attributes > Advanced > Other Parameters. Check
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whether the data services are supported. If not, the BSC responds with an Assignment
Failure message.
2. Based on the CIC in the Assignment Request message, the BSC checks the E1 timeslots
on the A interface and confirms the existence of CIC. It also checks whether the circuit
pool of the configured CIC, the channel type in the Assignment Request message, and the
capacity of the GDPUC are collided. If they are collided, the BSC responds with an
Assignment Failure message.
3. Upon receiving the Assignment Complete message from the MS, the BSC complements
the message with the A Interface Phase Flag and sends the message to the BSC. You can
choose BSC Property > Basic Data to display the A Interface Phase Flag.
10.4 Abnormal Speech Channel Assignment Cases
This topic describes the abnormal speech channel assignment cases.
10.4.1 BSC Sending an Assignment Failure Message
This topic describes the fault, probable causes, and handling suggestions.
10.4.2 BSC Receiving a Connection Failure Indication Message
This topic describes the fault, probable causes, and handling suggestions.
10.4.3 BSC Receiving an Error Indication Message
This topic describes the fault, probable causes, and handling suggestions.
10.4.1 BSC Sending an Assignment Failure Message
This topic describes the fault, probable causes, and handling suggestions.
Fault
Upon receiving the Assignment Request message, the BSC responds with an Assignment Failure
message.
Probable Causes
The probable causes are as follows:
1. Radio Interface Message Failure
The MS fails to access the speech channel. It sends an Assignment Failure message on the
signaling channel. Due to the particularity of the transmission on the Um interface, this
case happens the most frequently and cannot be solved.
2. Equipment Failure
The BSC hardware is faulty.
3. No Radio Resource Available
The BSC has no appropriate speech channels for assignment. The possible reason is that
all the speech channels are busy or that the speech channels are blocked.
4. Requested Terrestrial Resource Unavailable
5. Requested Transcoding/Rate Adaption Unavailable
6. Terrestrial Resource Already Allocated
7. Invalid Message Contents
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8. Radio Interface Failure - Reversion To Old Channel
9. Requested Speech Version Unavailable
10. Timer T3107 expiry
Handling Suggestions
The respective handling suggestions are as follows:
l Check the access-related parameters of the antenna system, BTS boards, and BSC data
configuration.
l Check the following:
– Communication between modules and subracks in the BSC
– Communication on the A interface, for example, the cable connections between
GEIUAs and subracks
l Add TRXs to the BTS, modify the access threshold, and enable the directed retry.
l The circuits on the A interface on the BSC side are abnormal, for example, the CIC in the
Assignment Request message is not available. Check the consistency of the A interface
data on the MSC and the BSC. Check also the CIC state.
l Check the transmission on the A interface.
l Check whether the CIC configuration on the BSC and the MSC is consistent. If not,
manually reset the CIC on the LMT.
l Check whether the message conforms to the protocols.
l Check the environment quality on the Um interface. For example, check the quality of the
level.
l Check whether the BSC supports the requested speech version.
l Check whether the timeout value of timer T3107 is appropriate.
10.4.2 BSC Receiving a Connection Failure Indication Message
This topic describes the fault, probable causes, and handling suggestions.
Fault
During the assignment procedure, the BSC receives a Connection Failure Indication message.
Probable Causes
The probable causes are as follows:
l Radio link failure
l Hardware failure
Handling Suggestions
The handling suggestions are as follows:
l Check the resources on the Um interface.
l Check the following:
– Communication between subracks
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7. HUAWEI BSC6000 Base Station Subsystem
Signaling Analysis Guide 10 Speech Channel Assignment
– Communication on the A interface, for example, the cable connections between
GEIUBs and subracks
10.4.3 BSC Receiving an Error Indication Message
This topic describes the fault, probable causes, and handling suggestions.
Fault
During the assignment procedure, the BSC receives an Error Indication message.
Probable Causes
The probable causes are as follows:
l Protocol disconformity
l Link layer failure
l Timer T200 expiry
l MS failure in receiving a UA frame
Handling Suggestions
The respective handling suggestions are as follows:
l Check the contents of the frames at the data link layer.
l Check the status of the data link layer.
l Check the timeout value of timer T200.
l Check the contents of the SABM frame.
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