On receipt of congestion indication [contained in MTP status primitives (see also 11.2.3/Q.704 [13]] the ISDN User Part should reduce traffic load (e.g. call attempts) into the affected destination point code in several steps.
When the first congestion indication is received by the ISDN User Part, the traffic load into the affected destination point code is reduced by one step. At the same time two timers T29 and T30 are started. During T29 all received congestion indications for the same destination point code are ignored in order not to reduce traffic too rapidly. Reception of a congestion indication after the expiry of T29, but still during T30, will decrease the traffic load by one more step and restart T29 and T30. This stepwise reduction of the ISDN User Part signalling traffic is continued until maximum reduction is obtained by arriving at the last step. If T30 expires (i.e. no congestion indications having been received during the T30 period), traffic will be increased by one step and T30 will be restarted unless full traffic load has been resumed.
Timers T29 and T30 have the following values:
• T29 = 300-600 ms;
• T30 = 5-10 s.
The number of steps of traffic reduction and the type and/or amount of increase/decrease of traffic load at the various steps are considered to be an implementation matter.