CSL fault is the alarm generated in telecommunication when BTS doesn’t sends the handshake acknowledgement signal to the BSC. BSC sends the link handshake signal to the BSC in every 15 seconds in-order to verify that the BTS is working well.
If CSL link is generated, the BTS cannot provide all the services that it need to.
CSL Alarm Details:
The data link layer protocol between the BSC and the BTS is the SCTP protocol. To enable the communication between the data link layers of the BSC and the BTS based on the SCTP protocol, the SCTP link must be established. An SCTP link that carries operation and maintenance information is called a CSL.
An SCTP link that carries carrier signaling is called an RSL. The BSC periodically sends the BTS handshake messages and the BTS is expected to send the BSC handshake response messages. The handshake mechanism helps detect CSL faults on layer 3.
The BSC sends the BTS handshake messages in every 15 seconds through the CSL, and the BTS is expected to send the BSC handshake response messages. If the BSC does not receive any response messages or other linkup messages from the BTS within 30 seconds, the BSC considers the CSL as faulty. This alarm is reported when the fault persists for more than 90s (The value of alarm report delay after a BTS resets is 180s). This alarm will not be reported when the fault is recovered within 90s (The value of alarm report delay after a BTS resets is 180s).
If the SCTP link that carries the CSL is faulty, the BSC sends the BTS handshake messages every 5 seconds over the CSL. If the BSC receives handshake response messages or other up-link messages from the BTS, the BSC considers that the CSL works properly and reports the clear alarm with a 90-second delay. If the BSC detects that the CSL becomes faulty within a 90-second delay, the BSC does not report the clear alarm.