Quality of Service Guide QoS and QoS Policies
Edition: 01 3HE 11014 AAAC TQZZA 25
There is a need to distinguish between high-priority (that is, mission-critical traffic like
signaling) and best-effort traffic priority levels when managing traffic flow. Within
these priority levels, it is important to have a second level of prioritization, that is,
between a certain volume of traffic that is contracted/needed to be transported, and
the amount of traffic that is transported if the system resources allow. Throughout this
guide, contracted traffic is referred to as in-profile traffic. Traffic that exceeds the
user-configured traffic limits is either serviced using a lower priority or discarded in
an appropriate manner to ensure that an overall quality of service is achieved.
The 7705 SAR must be properly configured to provide QoS. To ensure end-to-end
QoS, each and every intermediate node together with the egress node must be
coherently configured. Proper QoS configuration requires careful end-to-end
planning, allocation of appropriate resources and coherent configuration among all
the nodes along the path of a given service. Once properly configured, each service
provided by the 7705 SAR will be contained within QoS boundaries associated with
that service and the general QoS parameters assigned to network links.
The 7705 SAR is designed with QoS mechanisms at both egress and ingress to
support different customers and different services per physical interface or card,
concurrently and harmoniously (refer to Egress and Ingress Traffic Direction for a
definition of egress and ingress traffic). The 7705 SAR has extensive and flexible
capabilities to classify, police, shape and mark traffic to make this happen.
The 7705 SAR supports multiple forwarding classes (FCs) and associated
class-based queuing. Ingress traffic can be classified to multiple FCs, and the FCs
can be flexibly associated with queues. This provides the ability to control the priority
and drop priority of a packet while allowing the fine-tuning of bandwidth allocation to
individual flows.
Each forwarding class is important only in relation to the other forwarding classes. A
forwarding class allows network elements to weigh the relative importance of one
packet over another. With such flexible queuing, packets belonging to a specific flow
within a service can be preferentially forwarded based on the CoS of a queue. The
forwarding decision is based on the forwarding class of the packet, as assigned by
the ingress QoS policy defined for the service access point (SAP).
Note: The characteristics and nature of traffic flows in the ingress and egress directions are
usually totally different. As an example, traffic is usually shaped at egress for pacing
purposes and jitter tolerance imposed by the network transport rules, whereas at ingress,
traffic is usually policed to ensure it fits into the traffic volumes defined in the Service Level
Agreement. Thus, segregation between ingress and egress offers not only the seamless
flexibility to address different requirements but as well allows fine-tuning of appropriate
parameters in each direction.
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