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.