LSSUs provide link status information over the signaling links between two adjacent signaling endpoints. You use this information to maintain link alignment and to identify a processor outage at the remote endpoint. The LSSU contains the L2 interface link status and L3 status of the transmitting endpoint. LSSUs maintain reliability because these endpoints are not synchronized and, instead, run independently of each other. The LSSU identifies the status of the remote endpoint link interfaces and processors. If the endpoints receive an LSSU with errors, the signal unit is discarded. Retransmission of LSSUs is not required, as these signal units do not provide any information.

The LSSU has the same fields as the FISU, with the addition of the Status field, as illustrated in Figure 4-10. The Status field in the LSSU relays the link status information between endpoints. LSSUs are not transmitted across the network and only are carried on links between two adjacent endpoints.

Figure 4-10. LSSU Fields

Figure 4-10. LSSU Fields

MTP3 uses the L2 information the LSSU provides to track the status of the link and of the remote endpoint processor, both of which are responsible for maintaining link alignment. You use the link alignment procedures to correct a misalignment or problem on the link.

The following list describes fields that are unique to the LSSU:

• The LI field determines the type of signal unit. In the case of the LSSU, the LI is set to a value of 1 or 2. The LI value is 0 for an FISU and from 3 to 63 for an MSU.

• The Status field carries the information regarding the status of the link between endpoints. This field is either 1 octet (8 bits) or 2 octets (16 bits) and provides status on the link on which it is carried. In the Status field, only 3 bits are actually used to identify link status; the remaining bits are set to zero.

The SF identifies the following indicators:

• Status Indicator Busy (SIB)—Identifies L2 congestion at the transmitting endpoint. Receiving an SIB causes the receiving end to stop sending MSUs and start sending FISUs.

• Status Indicator Processor Outage (SIPO)—Identifies that the transmitting endpoint can't communicate with the upper-level protocols. Processor failures or other endpoint component failures can cause this to occur. Receiving an SIPO causes the receiving end to stop sending MSUs and start sending FISUs.

• Status Indicator Out-of-Alignment (SIO)—Identifies that a link failed and alignment procedures need to be initiated.

• Status Indicator Out-of-Service (SIOS)—Identifies that the transmitting endpoint cannot send or receive any MSUs. An SIOS is used when the problem is not related to a processor failure. Receiving an SIOS causes the receiving end to stop sending MSUs and start sending FISUs.

• Status Indicator Normal (SIN) and Status Indicator Emergency (SIE)—Identify that the transmitting endpoint initiated alignment procedures. The FISU packets are continually transmitted until the link alignment procedure is complete and MSUs are again transmitted on the link.

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