Queue management, Use of frame relay and atm services – Nortel Networks NN43001-563 User Manual
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ITG engineering guidelines
Queue management
Queueing delay
From
, it can be seen that queueing delay is a
major contributor to delay, especially on highly-utilized and low-bandwidth
WAN links. Routers that are TOS-aware and support class-based queuing
can help reduce queueing delay of voice packets when these packets are
treated with preference over other packets.
Class-based Queueing
To this end, Class-Based Queueing (CBQ) can be considered for
implementation on these routers, with the IP Trunk 3.01 traffic prioritized
against other traffic. CBQ, however, can be CPU-intensive and might not
scale well when applied on high-bandwidth link. Therefore, if implementing
CBQ on the intranet for the first time, do so selectively. Usually CBQ is
implemented at edge routers or at entry routers into the core.
Buffer management and WRED
The global synchronization situation described in
can be countered using a buffer management scheme which
discards packets randomly as the queue starts to exceed some threshold.
Weighted Random Early Detection (WRED), an implementation of
this strategy, additionally inspects the TOS bits in the IP header when
considering which packets to drop during buffer build up. In an intranet
environment where TCP traffic dominates real-time traffic, WRED can be
used to maximize the dropping of packets from long-lived TCP sessions and
minimize the dropping of voice packets.
As in CBQ, check the configuration guidelines with the router vendor for
performance ramifications when enabling WRED. If global synchronization
is to be countered effectively, implement WRED at core and edge routers.
Use of Frame Relay and ATM services
IP can be transported over Frame Relay and ATM services, both of which
provide QoS-based delivery mechanisms. If the router can discern IP
Trunk 3.01 (and later) traffic by inspecting the TOS field or observing the
UDP port numbers, it can forward the traffic to the appropriate Permanent
Virtual Circuit (PVC) or Switched Virtual Circuit (SVC). At the data link layer,
the differentiated virtual circuits must be provisioned. In Frame Relay, the
differentiation is created by having both "zero-Committed Information Rate
(CIR)" and CIR-based PVCs; in ATM, differentiation is created by having
VCs with different QoS classes.
Nortel Communication Server 1000
IP Trunk Fundamentals
NN43001-563
01.01
Standard
Release 5.0
30 May 2007
Copyright © 2007, Nortel Networks
.