Signaling Link Initial Alignment

The purpose of the signaling link alignment procedure is to establish SU timing and alignment so that the SPs on either side of the link know where SUs begin and end. In doing so, you must inherently test a link's quality before putting it into use. Example L-1 in Appendix L shows a trace file of two aligned SPs.

The signaling link alignment procedure ensures that both ends have managed to correctly recognize flags in the data stream.

Initial alignment is performed for both initial activation of the link (power on) to bring it to service and to restore a link following a failure. Alignment is based on the compelled exchange of status information and a proving period to ensure that SUs are framed correctly. MTP3 requests initial alignment, which is performed by MTP2. Because MTP2 operates independently on each link, the initial alignment procedure is performed on a single link without involving other links. There are two forms of alignment procedures: the emergency procedure and the normal alignment procedure. The emergency procedure is used when the link being aligned is the only available link for any of the routes defined within the SSP. Otherwise, the normal alignment procedure is used.

Status Indications

LSSUs are exchanged as part of the alignment procedure. There are six different status indications, as shown earlier in Table 6-1. Only the first four indications are employed during the initial alignment procedure.

The alignment procedure passes through a number of states during the initial alignment:

• Idle

• Not Aligned

• Aligned

• Proving

• Aligned/Ready

• In Service

Idle

When an SP is powered up, the links are initially put in the idle state. The idle state is the first state entered in the alignment procedure; it indicates that the procedure is suspended. If the procedure fails at any time, it returns to the idle state. Timer T17 (MTP3) prevents the rapid oscillation from in service to out of service. Timer T17 is started when the link begins the alignment procedure. No further alignment attempts are accepted from a remote or local SP until T17 has expired. LSSUs of SIOS (out of service) are sent during the idle state. LSSUs of this type are sent continuously until the link is powered down or until an order to begin initial alignment is received from MTP3. The FIB and the BIB of the LSSUs are set to 1, and the FSN and BSN are set to 127.

Not Aligned

When MTP2 receives an order to begin initial alignment, the SP changes the status of the transmitted LSSUs to indication SIO (out of alignment) and starts the timer T2. If T2 expires, the status of the transmitted LSSUs reverts to SIOS.

Aligned

During T2 SIO, if SIN (normal alignment) or SIE (emergency alignment) is received from the remote SP, T2 is stopped, and the transmission of SIO ceases. The SP then transmits SIN or SIE, depending on whether normal or emergency alignment has been selected and timer T3 is started. The link is now aligned, indicating that it can detect flags and signal units without error. If T3 expires, the alignment process begins again, transmitting LSSUs with a status field of SIOS. The aligned state indicates that the link is aligned and can detect flags and signal units without error.

Proving

Timer T4 governs the proving period, and the Alignment Error Rate Monitor (AERM) is used during this period.

The proving period is used to test the signaling link's integrity. FISUs are sent and errors (CRC and signaling unit acceptance) are counted during the proving period. LSSUs are also sent, indicating whether this is a SIN or SIE alignment.

The proving period is shorter for emergency alignment and as a result is not as thorough. As previously stated, emergency alignment is selected if only one in service (or none) exists between two SPs. If the local SP detects an emergency alignment situation, emergency alignment is used regardless of whether an SIN or SIE is received from the distant SP. Similarly, emergency alignment is used if an SIE is received from the distant SP, even when the local MTP3 indicates a normal alignment situation (more than one in-service link between the two adjacent nodes).

If four errors are detected during the proving period, the link is returned to state 00 (idle), and the procedure begins again.

Aligned/Ready

When T4 expires, the transmission of SIN/SIE ceases, timer T1 is started, and FISUs are transmitted. If timer T1 expires, the transmission of FISUs ceases, and LSSUs of type SIOS are transmitted.

In Service

Timer T1 stops upon receiving either FISUs or MSUs. When it stops, the SUERM becomes active. Figure 6-11 shows the initial alignment procedure.

Figure 6-11. Procedure for Signaling Link Alignment

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