How to solve the problem that the assignment success rate is low due to TCH congestion?

According to the result of signaling tracing over the A interface, the BSC sends the MSC an assignment failure message containing the cause value of No radio resource available. In addition, the values of the following counters increase significantly: ZTA3129J:Failed Assignments per BSC (Channel Unavailable), CA312:Failed Assignments (Channel Unavailable), and K3011A:Failed TCH Seizures due to Busy TCH (Traffic Channel).

Background Information

• The following is the formula for calculating the TCH congestion rate:

[K3021:Failed TCH Seizures due to Busy TCH (Signaling Channel) + K3011A:Failed TCH Seizures due to Busy TCH (Traffic Channel) + K3011B:Failed TCH Seizures in TCH Handovers due to Busy TCH (Traffic Channel)]/[K3020:TCH Seizure Requests (Signaling Channel) + K3010A:TCH Seizure Requests (Traffic Channel) + K3010B:TCH Seizure Requests in TCH Handovers (Traffic Channel)] x 100%

• If the TCH congestion rate of a cell with a high assignment success rate is greater than 10%, assignment failures are mainly caused by TCH congestion. The following are the common causes of TCH congestion in a cell:

  • The traffic volume in a cell increases.

  • The cell does not support the half-rate speech versions 1 and 3 in an effort to ensure speech quality.

  • Traffic volume in the cell is large because the subscriber density is high or coverage overlap occurs in the cell.

  • The traffic volume in the cell sharply increases because emergencies occur or any neighboring cells are out of service.

  • The cell is configured with a large number of static packet data channels (PDCHs) or dynamic PDCHs and processes PS services preferentially.

  • Some TRXs of the cell are faulty or some channels of the cell are blocked.

  • Very early assignment is enabled.

• When TCHs are congested, new access requests are rejected because TCHs are unavailable, decreasing the assignment success rate.

Location Procedure

Figure 1 shows the procedure for locating low assignment success rates due to TCH congestion.

Figure 1 Procedure for locating low assignment success rates due to TCH congestion

Troubleshooting Procedure

1. Run the MML command LST GCELLCCACCESS to check whether a cell with a low assignment rate supports the half-rate speech versions 1 and 3. Run the MML command LST GTRXDEV to check whether TCH Rate Adjust Allow is set to YES(Yes).

• If yes, no further action is required.

• If no, go to Step 2.

• If yes, go to Step 2.

• If no, perform the following operations to enable the half-rate speech versions 1 and 3 when the speech quality meets the requirement:

1. Run the MML command SET GCELLCCACCESS with Speech Version set to HALF_RATE_VER1(Half-rate VER 1) and HALF_RATE_VER3(Half-rate VER 3).

2. Run the MML command SET GTRXDEV with TCH Rate Adjust Allow set to YES(Yes).

3. Check whether the assignment success rate returns to normal.

Check whether coverage overlap or unbalanced traffic distribution occurs in the cell where TCHs are congested according to drive test logs and traffic statistics.

• If yes, reduce the cell coverage by performing operations such as decreasing the power, reducing the antenna tilt, and improving CS RACH Min. Access Level. Then, check whether the assignment success rate returns to normal. If yes, no further action is required. If no, go to Step 3.

• If no, go to Step 3.

Check whether any neighboring cells are out of service according to alarm logs.

• If yes, resolve the problem of out-of-service cells. Then, check whether the assignment success rate returns to normal. If yes, no further action is required. If no, go to Step 4.

   NOTE:

  If TCHs become congested abruptly in the cell because of emergencies such as a gathering, no action is required.

• If no, go to Step 4.

Run the MML command LST GTRXCHAN to check whether the cell is configured with a large number of static packet data channels (PDCHs) or dynamic PDCHs and processes PS services preferentially.

• If yes, reduce the number of PDCHs. Then, check whether the assignment success rate returns to normal. If yes, no further action is required. If no, go to Step 5.

• If no, go to Step 5.

Run the MML command LST GCELLBASICPARA to check whether TCH Immediate Assignment is Yes, that is, whether very early assignment is enabled.

• If yes, run the MML command SET GCELLBASICPARA with TCH Immediate Assignment set to NO(No) to disable very early assignment. Alternatively, perform capacity expansion. Then, check whether the assignment success rate returns to normal. If yes, no further action is required. If no, go to Step 6.

• If no, go to Step 6.

Check whether RR307:TCH Availability is less than 100%.

• If yes, some TRXs are faulty or some channels are blocked. Check the TRX status or run the MML command DSP CHNSTAT to check channel status, and rectify TRX or channel faults. Then, check whether the assignment success rate returns to normal. If yes, no further action is required. If no, return to Procedure for locating access faults.

• If no, return to Procedure for locating access faults.

Typical Case

Symptom

A customer complains that TCHs are congested in some cells during off-peak hours.

Cause Analysis

TCH congestion is generally caused by problems such as improper parameter settings, hardware faults, and large traffic volumes.

Troubleshooting Procedure

1. Analyze the traffic statistics of one of the cells where TCHs are congested. The cell is configured with 12 TCHs, and the TCHs are congested twice at a certain time and R3561:Maximum Number of Busy Channels (TCHF) measured at the time is 12. This indicates that all TCHs are occupied at the time, resulting in TCH congestion. This indicates that all TCHs are occupied at the time, resulting in TCH congestion.

2. Analyze the traffic statistics of all cells where TCHs are congested. The traffic volume of these cells during the measurement period is low but all the TCHs are occupied within a short period. In this case, TCHs are congested when new calls access these cells, leading to assignment failures.

3. Check the data configuration of these cells. TCH Rate Adjust Allow are all No.

4. Perform the following operations to resolve the problem:

   1. Run the MML command SET GTRXDEV with TCH Rate Adjust Allow set to YES(Yes).

   2. Run the MML command SET GCELLCHMGAD with TCH Traffic Busy Threshold set to a required value.

   3. Run the MML command SET GCELLCHMGBASIC with Enhanced TCH Adjust Allowed set to YES(Yes).