Sdcch congestion rate optimization guide

1 When SDCCH are required?
The requests for the SDCCH in GSM are mainly made in the following two procedures:
A. The procedure for requesting the SDCCH for services such as point-to-point calls, location updating (only SDCCH), call re-establishment, or short messages;
B. The procedure of SDCCH handovers (including both the intra-BSC and inter-BSC handovers)

2 Principles of SDCCH Congestion Rate: 
SDCCH congestion rate is one of the important counters that indicate accessibility in circuit service. This counter provides the ratio of failed SDCCH seizures due to busy SDCCH to the total requests for the SDCCH. SDCCH congestion rate indicates the failed requests for the SDCCH for various reasons. SDCCH congestion rate also indicates the status of the SDCCH resource utilization.

3 Definition & Formula of SDCCH Congestion : 
The definition and formula of SDCCH congestion rate are as follows:
• Definition:
Ratio of failed SDCCH seizures due to busy SDCCH to the total requests for the SDCCH
• Formula:
SDCCH congestion rate = Failed SDCCH seizures due to busy SDCCH/Total requests for the SDCCH x 100%

4 Factors of SDCCH Congestion Rate in GSM : 

4.1 Congestion Caused by Faults on Equipment or Transmission : 
The faults on BTS, BSC, and Abis interface, such as broken LAPD link, cause the SDCCH congestion. The alarm "Excessive Loss of E1/T1 Signals in an Hour" also causes the SDCCH congestion.

4.2 Congestion Caused by Insufficient Signaling Resources : 
The heavy traffic and burst traffic cause the SDCCH congestion. Proper setting of the number of SDCCHs and TCHs, and the SDCCH dynamic conversion function can relieve the congestion.

4.3 Congestion Caused by Improper Data Configuration : 
The SDCCH congestion relates to the relevant parameters of the BSC such as SDCCH Availability, LAC and T3101 (the timer used in the immediate assignment procedure), and T3212 (the timer used for periodic updating). If these parameters are set correctly, the SDCCH congestion can be relieved. In addition, if the assignment procedure is set to Late Assignment, the time of the SDCCH being occupied increases, which may lead to congestion.

4.4 Congestion Caused by Interference : 
Interference on the Um interface also causes congestion. For example, if the main BCCH in the serving cell and the TCH in the neighboring cell share the same TRX frequency and BTS BSIC, the handover access on this TCH may be mistaken as random access. As a result, the SDCCH is abnormally allocated and congestion occurs. The excessive receive sensibility can also make the interference signal mistaken as access signal, which leads to congestion.

5 Analysis Procedure of SDCCH Congestion Rate : 
SDCCH congestion rate is classified into BSC-level and cell-level according to the statistic object of the counter. The SDCCH congestion rate of a BSC is composed of the SDCCH congestion rate of a series of cells. Procedure of SDCCH congestion rate analysis is as shown below:



First, determine the level of the SDCCH congestion rate. If the congestion occurs in a large area, check the traffic volume and the setting of T3212. Then, calculate the SDCCH capacity to check whether it meets the system demand and whether the faults exist on system-level equipment or transmission. If congestion occurs in only a few cells, check the hardware, data configuration, and Um interface quality of the cell.

Below given is the analysis & optimization procedure for SDCCH Congestion Rate : 

1 Checking Channel Configuration : 
Query the traffic statistics to see whether the traffic volume on the SDCCH and TCH are higher than the normal value. If the congestion is caused by excessive traffic on the SDCCH, for multi-TRX BTSs, enable SDCCH dynamic allocation function or increase the number of SDCCH channels to relieve the congestion. For cells with only one TRX or without extra channels, enable the very early immediate assignment procedure. In the very early immediate assignment procedure, the TCH channel is immediately assigned when the SDCCH has no available resource during the processing of access request. In this case, a TCH can be used as only one SDCCH, which is a waste of the TCH resources.

All the services such as location updating, MS attach/detach, call setup, short messages are performed on the Schiff a certain service causes burst traffic, the SDCCH congestion may occur.
In this case, check whether the abnormity of Channel Requests, Successful Immediate Assignments or Successful SDCCH Seizures is caused by the service of location updating, MOC, paging, or short message. Note that you should check the history traffic statistics when querying these traffic counters to see whether they fluctuate during a certain period.

If the SDCCH congestion is caused by certain burst services such as location updating and short messages, use the following methods according to the specific configuration of the BTSs.

a. For the BTS with multiple TRXs, enable the SDCCH dynamic allocation function or increase the number of SDCCH channels.

b. For the cell with only one TRX or without extra channels, enable the immediate assignment procedure. In the immediate assignment procedure, the TCH is immediately assigned when the SDCCH has no available resource during the processing of access request.

c. Increase the number of TRXs.
It is difficult to avoid the SDCCH congestion caused by network burst services; however, you can take some relief measures such as increasing the number of SDCCHs or enabling the SDCCH dynamic conversion function.