The network or user will flow control,when needed, the transfer of USER INFORMATION messages from a user or network by means of a CONGESTION CONTROL message containing a congestion level information element. Two indications of congestion level are specified:"receive not ready" and "receive ready". On receipt of the former, the user or network should suspend sending USER INFORMATION messages; on receipt of the latter, sending may recommence. After having sent a "receive not ready"indication, the network or user shall discard USER INFORMATION messages which are subsequently received. The network or user will send a CONGESTION CONTROL message with a "receive not ready" indication whenever a USER INFORMATION message is locally discarded, if it is possible. The CONGESTION CONTROL message shall also include a cause No. 43 "access information discarded".
The network shall notify the user that flow control restriction has been removed by sending a CONGESTION CONTROL message with the congestion level specified as "receive ready" to indicate that further messages may be sent. This message may be sent, as an implementation option:
a) immediately upon removal of the flow control restriction;
b) in response to the first USER INFORMATION message received following the removal of the flow control restriction; or
c) in both cases.
The receipt of the "receive ready"indication shall be interpreted as an indication that no more than n USER INFORMATION messages may be sent before another receive ready indication is received.
In each direction a Burst Capability of sending n messages is immediately available where n initially equals the value of the Burst parameter x. The value of n shall be decremented by one for every message sent by the user and incremented by y at regular intervals of T (T = 10 seconds) subject to the limitation that n may not exceed x, i.e. n + y ≤ x.
The Burst parameter x is a variable which shall be set to a value of x = 16.
The replenishment parameter y shall be capable of taking a value y = 8.
Note - While some networks may support higher values of x and y, the value of x and y across international interfaces shall be set as above. It is up to the network using higher values to take the appropriate actions, unless bilateral agreements exist.
If USER INFORMATION messages are received at a rate which exceeds the flow control limit set by the network, the network shall discard the messages that cannot be handled and respond to the first discarded message with a control indication. The network shall also respond to the first USER INFORMATION message received following the removal of flow control restriction by returning an indication that further messages may be sent.
The Congestion control procedure should be regarded as local. Congestion control procedure for remote applications is for further study.