Link Status Signal Unit

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The Link Status Signal Unit (LSSU) is used to convey status information between adjacent signaling points (SP) on a link. The Status field in the LSSU can be either 1 or 2 octets, but the status information itself consists of only the low order three bits of the field. The following list shows the available Link Status codes and their descriptions.

  • 000: Out of alignment
  • 001: Normal alignment
  • 010: Emergency alignment
  • 011: Out of service
  • 100: Processor outage
  • 101: Busy

Note that several conditions deal with alignment. A link is considered out of alignment if 64 consecutive signal units (SU) are received in error or if the error rate exceeds 1 error per 256 SUs. Loss of alignment also occurs when more than 7 consecutive 1 bits are received or if an SU with a Signaling Information field longer than 272 octets is received. Regardless of the cause, a link with one of these conditions is declared to be out of alignment in an LSSU and action is taken to correct the situation. To establish alignment, the SPs exchange FISUs and count errors. For normal alignment, no more than 4 errors can be sustained in a 2.3 second period. For emergency alignment, no more than 1 error can occur within 0.6 seconds.

An LSSU carrying a status of Busy is signaling congestion to its neighbors. An SP receiving this indication must halt sending MSUs and commence sending FISUs. An SP can send a Busy LSSU every 80-120 ms until the congestion is relieved. If the congestion continues for 3 to 6 seconds then Level 3 is notified of a link failure. The congested SP can signal the end of congestion by resuming the sending of MSUs.

See Also