Message structure and function 4-23
Digital Switching Systems DMS-250 ISDN Reference Manual MWC14 (CSP14)
Congestion and Network Flow Control
There are two kinds of congestion control, user congestion and network
congestion. The DMS-250 switch can flow control the transfer of User
Information messages by way of a Congestion Control message. The
Congestion Control message contains a Congestion level parameter that can
be set to “Receiver Not Ready,” indicating Congestion or “Receiver Ready,”
indicating Congestion Clearing.
User Congestion
Status messages are not generated during user congestion. All user
information messages are discarded.
Network Flow Control
Flow control of User Information messages at the DMS-250 switch is done
using the following algorithm:
• burst parameter X, initialized to office parameter UUS_BURST
• User Information message counter N, initialized to X
• the replenishment parameter Y, initialized to X/2
• the DMS-250 timer TUUS is 10 seconds
A burst capability of sending N User Information messages will be available
to each access/egress agent. The value of N is decremented by one for each
User Information message sent by the user and incremented by Y at regular
interval of TUUS subject to the limitation that N should not exceed X. If the
DMS-250 receives more than N User Information messages, within the
TUUS timer setting, “network congestion” occurs. The network sends a
Congestion Control message to the user with Congestion Level parameter
set to “Receiver Not Ready”. The Cause Parameter in the Congestion
Control message is set to a value of 42, “Switching Equipment Congestion,”
Location “Transit,” and Coding Standard “CCITT”.
When network congestion clears, a Congestion Control message is sent to
the user with Congestion Level parameter in the Congestion Control
message set to “Receiver Ready”. The Cause Parameter is set to a value of
42, “Switching Equipment Congestion”, Location “Transit” and Coding
Standard “CCITT”. Only the user who was previously notified of
congestion is sent a Congestion Control message with “Receiver Ready”.
User Congestion and Network Flow Control Interaction
The DMS-250 switch considers the congestion status of both end users and
itself before sending or forwarding Congestion Control messages to the end
users.
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