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GSM Issue-Radio link failure (radio link timeout)

In this page we will analyze the cases of loss of communication link usually referred as radio link failure. This can happen when UE(mobile) is in the idle state or in connected state or trying to connect from idle state to obtain the network service.

Following are the various scenarios where in radio link failure can happen in wireless network such as GSM,CDMA or LTE.
•  Failure at lower layer either RF or physical layer(e.g. layer 1) due to various factors such as higher frequency offset, high timing offset or poor C/I conditions leading to CRC failures. Due to this upper layer decoding fails and both mobile and base station could not able to negotiate minimum messages required to establish the radio link connectivity.
•  As mentioned data link layer failure as specified in 3GPP GSM specification document version 04.06.
•  During channel assignment procedure, cell reselection procedure or handover procedure from one RAT to the other RAT or within the same RAT from one base station to the other.
•  Timers are started in the cases where response from the other entity is not available immediately. This prompts for initiation of retransmissions and later expiry of timers after waiting for set timer value.

RF and layer-1 failure is also referred as lower layer failure.

Mobile side

When a radio link failure is detected by mobile (UE) following actions are performed by it.
- Release of signalling links unless otherwise specified by base station.
- Deactivation of all the channels.
- Mobile indicates failure of RR connection to MM sub layer unless informed by base station.

Network side

In RR connected mode, due to lower layer failure, network releases connection using channel release procedure. The network starts the gsm timer T3109 and deactivate the SACCH channel so that SACCH transmission will get stop.

Once radio link failure occurs, the indication is passed to upper MM sublayer (Mobility Management sublayer).

When gsm timer T3109 will expire, the base station (i.e. network) will release the channels and their allocations so that they can be utilized by other requesting mobile subscribers. Consecutively if a mobile subsriber(MS) will not receive SACCH for some time, it will release the allocated channels completely.

Summary

Following are the two conditions under which the connection is considered to be terminated and radio link is said to be failed.

Due to Layer-1 failure

The most frequent reason for the declaration of a radio link failure on Layer 1 is simply the incapacity to decode the SACCH [RADIO_LINK_TIMEOUT] times. This definition applies to both uplink and downlink. For SACCHs that cannot be read, the channel decoder sets the BFI flag to 1. The parameter RADIO_LINK_TIMEOUT can be set per BTS and is broadcast in the BCCH / SYS_INFO 3. When a radio link failure on Layer 1 is detected on a dedicated channel (see TCH and SDCCH), the BTS sends a CONN_FAIL message with cause 1 = radio link failure to the BSC. The BSC consequently sends a CLR_REQ message cause 1 = radio interface failure to the MSC to indicate the connection loss and to release the resources. Figure G.49 shows this relation.

Due to Layer-2 failure

The criteria for radio link failure on Layer 2 are similar to those for Layer 1. When an MS or a BTS sends a Layer 2 frame (I-frame, SABME-frame, etc.) as often as indicated in N200 without receiving a response, Layer 3 receives an error indication that requests release of that channel. An ERR_IND message cause 1 = timer T200 expired (N200 + 1) times is sent to the BSC to release the channel. The BSC then performs the channel release procedure. This results frequently in a radio link failure on Layer 1, due to the fact that the radio link is so bad that the SACCH cannot be decoded anymore.

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