Radio Link Timeout

 Value range: 4~64, the step size is 4

Unit: SACCH period (480ms)

Content: This parameter is used for MS to decide down-link disconnection in case of SACCH decoding failures.

Recommendation: 20~56

 

Once assigned with a dedicated channel, MS will start counter S. From then on, S will decrease by 1 when a SACCH message fails to be decoded, and will increase by 2 when decoded correctly. When S decreases to 0, there will be a radio link failure. This allows either re-establishment or release of the connection. If the value of this parameter is too small, the radio link will easily get failed which will result in call drops. If it is too large, MS will not release for a long time which will lower the availability of resources (this parameter functions for the downlink).

For area with little traffic (remote area), it is recommended to be between 52~62.

For area with light traffic and large coverage(suburb or countryside), it is recommended to be between 36~48.

For area with heavy traffic (urban), it is recommended to be between 20~32.

For the area with very heavy traffic (area covered by microcell), it is recommended to be between 4~16.

For the cell with obvious coverage hole or the area where the call drops is serious during movement, this parameter can be increased appropriately in order to increase the possibility to resume the conversation.

Note: Radio link timeout is the parameter used to judge the downlink failure. Likewise, the uplink will be monitored at BTS, either based on the uplink SACCH error or based on the receiving level and quality of the uplink.

 

 

Application of Radio Link Timeout 

 

 

 

If cell A and B are adjacent to each other, assume that one MS moves from point P to point Q during a conversation, usually an outgoing cell handover will occur. If the value of parameter “radio link timeout” is too small and the quality of signal at the edge of cells A and B is poor, it is likely that the radio link will be timeout before the handover starts, thus resulting in call drops.

If it is too large, when MS stays near point P and makes a conversation, though the voice quality is unacceptable, the network still has to wait for a long time before the related resources can be released, thus the resources utilization rate is lowed.