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SDCCH congestion occurs

Publication Date:  2018-04-11 Views:  58 Downloads:  0
Issue Description
The BSC32 is used for a railway. SDCCH congestion occurs at some time segments in certain cells along the railway. Signaling analysis shows that there are a large number of SDCCH access requests within an extremely short period.
Handling Process
Cell 15 on network B is selected for analysis.

Signaling analysis shows that the BSC allocates SDCCHs upon the first eight channel requests. As shown in the following figure, the BSC allocates SDCCHs and delivers the immediate assignment command.



Then, the BSC waits for MSs to access the network. However, all network access attempts of the MSs fail. Consequently, the BSC releases the SDCCH when the specified timer expires. The following figure shows the signaling generated when the BSC releases the SDCCH. 



All SDCCHs remain in use from the time a channel request is initiated to the time it is released by the BSC. In this situation, no SDCCH can be allocated to new channel requests initiated during this period. Consequently, the BSC directly delivers an immediate assignment reject command, as shown in the following figure. 



The above analysis indicates that congestion in some cells is caused by a large number of channel requests initiated within an extremely short period, causing all SDCCHs to be occupied. If dynamic SDCCH allocation is enabled and no SDCCH is available, an ongoing channel request will definitely fail. Due to the extremely short interval between channel requests, the dynamic conversion of a new SDCCH cannot be completed in a timely manner. As a result, subsequent channel requests are rejected. In general situations, it is impossible to initiate dozens of channel requests within a short period. In addition, no MS accesses the network after the BSC allocates SDCCHs and delivers the immediate assignment command. However, subsequent channel requests are rejected.

It is suspected there are abnormal MSs on the live network based on the above analysis.
Suggestions
More than 30 channel requests are initiated within an extremely short period. When the first eight channel requests are initiated, the BSC successfully allocates the SDCCH and delivers the immediate assignment command. During this procedure, BSC processing is normal. However, the MS does not access the SDCCH allocated by the BSC. Consequently, the SDCCH is always in use and is then released by the BSC when the specified timer expires. If dynamic SDCCH allocation is enabled and no SDCCH is available, an ongoing channel request will definitely fail. Due to the extremely short interval between channel requests, the dynamic conversion of a new SDCCH cannot be completed in a timely manner. As a result, subsequent channel requests are rejected. The signaling analysis shows that the access causes in the channel requests are the same, the TA values in the requests are also basically the same, and MSs do not respond to the immediate assignment command. Therefore, this problem is caused by abnormal MSs. 

END