Nortel DMS-250 Reference Manual page 309

NCAUUI 15-19
are ready to receive User Information messages. The originating ISDN user
only receives one Q.931 Congestion Control message with an indication of
"Receiver Ready". Once a user receives a Q.931 Congestion Control
message with an indication of "Receiver Ready", the next Q.931 Congestion
Control message for that user must have an indication of "Receiver Not
Ready".
If, for example, the access DMS-250 switch remains congested and the
terminating ISDN user congestion is cleared, the terminating ISDN user
sends a Q.931 Congestion Control message with an indication of "Receiver
Ready". Once the terminating switch receives the Congestion Control
message, the terminating switch sends an SCCP DT1 message with a
Congestion Control Parameter Code of #20 and a Congestion Level of
"Receiver Ready" to the access DMS-250 switch. The access DMS-250
switch does not forward this user generated Q.931 Congestion Control
message with "Receiver Ready" to the access ISDN user because the access
DMS-250 switch is still in congestion. Once the network flow control is
cleared, the access DMS-250 switch sends the Q.931 Congestion Control
message with "Receiver Ready" to the access ISDN user. At this point both
users can resume sending and receiving User Information messages.
Note: The DMS-250 switch only sends cause value of 42 "Switch
Equipment Congestion" in the Q.931 Congestion Control message. The far
end user includes any cause value in the Q.931 Congestion Control message
with the limitation that some cause values are not supported for AT&T
variant.
NCAUUI TSC Clearing
User Clearing
A Q.931 DISConnect, RELease, or RELease COMPlete message is utilized
by either of the end users to clear a NCAUUI connection. The user that
receives the Q.931 RELease message responds with a Q.931 RELease
COMPlete message. A Q.931 DISConnect message is not expected since
there is not a B-channel to disconnect but is honored as a valid indication of
TSC clearing if received.
If the originating user clears an established NCA-TSC with a Q.931
RELease COMPlete message, the originating DMS-250 switch formulates
and sends an SCCP RLSD message towards the terminating switch. Once
the terminating switch receives the SCCP RLSD message, it sends an SCCP
RLC message to the originating switch and a Q.931 RELease message to the
terminating ISDN user. Any cause value received by the originating switch
is forwarded in the SCCP RLSD message and is delivered to the terminating
ISDN user in the corresponding Q.931 RELease message.
Digital Switching Systems DMS-250 ISDN Reference Manual MWC14 (CSP14)