Nortel Networks NN43001-563 Queue management Queueing delay, Class-based Queueing, Buffer management and WRED, Use of Frame Relay and ATM services

Implement QoS in IP networks 129

Queue managementQueueing delay

From "Queuing delay"(page 133), 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 voicepackets 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 Trunk3.01 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 "TCP trafﬁc behavior"

(page 127) 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 bufferbuild up. In an intranet

environment where TCP trafﬁc dominates real-time trafﬁc, WRED can be

used to maximize the dropping of packetsfrom long-lived TCP sessions and

minimize the dropping of voice packets.

As in CBQ, check the conﬁguration guidelines with therouter 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 ATMservices, both of which

provide QoS-based delivery mechanisms. If the router can discer n IP

Trunk3.01 (and later) trafﬁc by inspecting the TOS ﬁeld or observing the

UDP port numbers, it can forward the trafﬁc tothe appropr iate 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-CommittedInformation Rate

(CIR)" and CIR-based PVCs; in ATM,differentiation is created by having

VCs with different QoS classes.

Nortel Communication Server 1000

IP TrunkFundamentals

NN43001-563 02.01 Standard

Release 5.5 21December 2007