RFTroubleshooting GSM CM Service Re-Establishment Failures
This article addresses a common problem in GSM networks: frequent call drops due to CM (Connection Management) Service Re-Establishment failure, which occurs when the network releases the channel.
When this happens, the mobile station (MS) or User Equipment (UE) attempts to re-establish the connection.
Here’s a breakdown:
CM Re-Establishment Request
In GSM, there’s an option to re-establish a call if the connection is dropped. The process involves the following:
- The MS sends a
CHANNEL REQUESTto the BTS (Base Transceiver Station). - It then sends a
CM_RES_REQ(CM Re-Establishment Request) to try and re-establish the RR (Radio Resource) connection, while maintaining the existing MM (Mobility Management) and CC (Call Control) connections.
The table below summarizes the key message related to mobility management during re-establishment:
| ID (HEX) | Name of mobility management | Direction | Description |
|---|---|---|---|
| 28/68 | CM Re-Establishment Request | MS (UE) to BTS | An option in GSM is to allow for a call reestablishment in case of a dropped connection. Here first a CHANNEL REQUEST has to be sent to the BTS and then it is tried with the CM_RES_REQ to reestablish an RR connection for the still existing and active MM and CC connection. |
GSM Timer T3230
- Starts: Upon transmission of the CM service request OR CM Re-Establishment Request.
- Stops: When the mobile receives a CM service accept/CM service reject.
- Timeout: If the timer expires, the call is cleared by the UE(MS).
- Default value: 15 seconds.
CM RE-ESTABLISHMENT REQUEST Message Content
The CM Re-Establishment Request message contains crucial information for the network to understand the context of the re-establishment attempt. The message typically includes these elements:
- MM Protocol Discriminator
- Skip indicator
- CM RE-ESTABLISHMENT REQUEST message type
- Ciphering key sequence number
- Spare half octet
- MS classmark
- Mobile identity
- Location Area ID
Troubleshooting CM Re-Establishment Failures
If radio link issues are ruled out, the first thing to check is whether the network or cell actually supports call re-establishment. If the network cannot associate the CM Re-establishment request with an existing call from the MS, it will send a CM SERVICE REJECT message, indicating the reason for the rejection. The cause code #38 "call cannot be identified" is a common indicator of this.
Furthermore, you should also consider the other possible reject causes:
| HEX VALUE | REJECT CAUSE | DESCRIPTION |
|---|---|---|
| #4 | IMSI unknown in VLR | |
| #6 | Illegal ME | |
| #17 | Network failure | |
| #22 | Congestion | |
| #25 | Not authorized for this CSG | |
| #32 | Service option not supported | |
| #34 | Service option is temporarily Out of Order |
This information helps in pinpointing the root cause of the CM Service Re-Establishment failure and applying the necessary corrective measures.
