SS7 LINK ALIGNMENT

 

Before signaling units can be transmitted over a link, the link must be aligned. Once LSSUs and FISUs can be successfully sent between two signaling points over a link, alignment has been achieved and MSUs can be sent.                       

If a link is being brought into service for the first time, or if it is being re-activated after a failure, MTP Level 2 conducts an Initial Alignment Procedure (IAP). This procedure has 2 variations, normal and emergency, but the basic order of events and criteria for link alignment are the same.

 

During an IAP, a link's goal is to "prove" itself reliable. The IAP begins when a link between two signaling points is assumed to be ready for "proving." The signaling point starts sending LSSUs to the other signaling point. It then waits for confirmations or retransmission requests.

 

If no confirmations or retransmission requests are received, the signaling point will generate an "Out of Alignment" LSSU (This might happen if a problem exists with the physical connection of the link.). If signaling units are received from the other point, the link will be monitored to determine whether it can be put into service.                          

 

Alignment Error Rate Monitor

During the IAP, the Alignment Error Rate Monitor (AERM) is responsible for determining whether a link has proven itself. The AERM tracks the number of messages sent without error over the link being established. If too many errors occur, the link will not be placed into service.

 

 

Signaling Unit Error Rate Monitor 

Once a link is aligned and placed into service, MTP Level 2 will continue to monitor it with the Signaling Unit Error Rate Monitor (SUERM). If too many message errors occur over a link, it will be considered "out of alignment" and will be taken out of service.

 

Since errors are expected to occur, the SUERM employs an error counter that is incremented or decremented. If a message error occurs, the counter is incremented. To decrement the counter, 256 messages must be received without error. If the counter reaches its threshold (e.g., "3"), MTP Level 2 will notify MTP Level 3 and the link will be taken out of service.   

 

 

SUERM Example

To illustrate how the SUERM operates:

 

Signaling point A is sending messages to signaling point B over Link A. An error message suddenly occurs. The SUERM counter maintained by MTP L2 is then incremented to 1. Now it monitors for the 256 messages to be transmitted without another error.

 

However, another error occurs within the next 256 messages, so the counter is incremented to 2. Another error occurs and the counter reaches 3. Link A is now taken out of service until it is re-aligned.

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