Information Technology Reference
In-Depth Information
The following example shows the configuration of WRED:
class class-default
fair-queue
random-detect dscp-based
The
random-detect
[
dscp-based
|
prec-based
] command is used in the example to configure RFC 2597-compliant DSCP-based
WRED for all traffic assigned to class-default (all traffic not matched by other classes [not shown]). When using the
dscp-based
keyword, DSCP values are used to calculate drop probability, and when using the
prec-based
keyword, IP Precedence values are
used to calculate drop probabilities.
Call Admission Control
The congestion management and avoidance mechanisms discussed in previous sections can be essential in ensuring that voice traffic
is protected from (excess) data traffic. But, although congestion management and avoidance mechanisms can protect voice from data,
they cannot protect voice traffic from other voice traffic.
The function of CAC is to ensure that too many voice traffic flows are not sent across the network. That is, voice calls are admitted
to the network only as long as the network can ensure sufficient quality of service (in terms of delay, jitter, and packet loss) for those
voice calls. If the network is unable to ensure sufficient quality for a voice call, that voice call must not be admitted to the network.
CAC is needed for voice (and video) and not data traffic because voice traffic is particularly sensitive to delay, jitter, and packet loss,
while data traffic typically is not. Similarly, data traffic can typically be retransmitted, while voice media traffic cannot (arrival of
voice media is very time sensitive, so any retransmission would take too long).
There are three overall types of CAC:
■
Reservation-based CAC
is based on the reservation or calculation of required resources before a call is admitted.
Mechanisms that can be categorized as resource-based CAC mechanisms include the Resource Reservation Protocol (RSVP),
gatekeeper zones, and CUCM locations.
■
Local CAC
is based on a local determination on a device as to whether a call can be admitted to the network based on local
information, such as the state of an outgoing interface or link.
