Flow Control

Flow control allows incoming traffic to be throttled when the MTP2 receive buffer becomes congested. When an SP detects that the number of received MSUs in its input buffer exceeds a particular value—for example, because MTP3 has fallen behind in processing these MSUs—it begins sending out LSSUs with the status indicator set to busy (SIB). These LSSUs are transmitted at an interval set by timer T5, sending SIB (80 to 120 ms), until the congestion abates. The congested SP continues sending outgoing MSUs and FISUs but discards incoming MSUs. It also "freezes" the value of BSN and the BIB in the SUs it sends out to the values that were last transmitted in an SU before the congestion was recognized. This acknowledgment delay would normally cause timer T7, excessive delay of acknowledgment, at the distant SP to time out; however, timer T7 restarts each time an SIB is received. Therefore, timer T7 does not time out as long as the distant SP receives SIBs.

Timer T6, remote congestion, is started when the initial SIB is received. If timer T6 expires, it is considered a fault, and the link is removed from service. Timer T6 ensures that the link does not remain in the congested state for an excessive period of time.

When congestion abates, acknowledgments of all incoming MSUs are resumed, and periodic transmission of the SIB indication is discontinued. When the distant SP receives an SU that contains a negative or positive acknowledgment whose backward sequence number acknowledges an MSU in the RTB, timer T6 is stopped, and normal operation at both ends ensues. Figure 6-14 depicts flow control using LSSUs with status indication busy.