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This deployment incorporates the following solutions to multisite deployment issues:
• Availability issues are solved by Cisco Unified Survivable Remote Site Telephony
(SRST), which may include Media Gateway Control Protocol (MGCP) fallback or
H.323, both using dial peers.
• Quality and bandwidth issues are solved by quality of service (QoS), call admission
control (CAC), Real-Time Transport Protocol (RTP) header compression, and local
media resources.
Dial plan solutions include access and site codes, as well as digit manipulation.
Network Address Translation (NAT) and security issues are solved by the deployment
of a Cisco Unified Border Element.
Quality of Service
QoS refers to the capability of a network to provide better service to selected network traffic
at the direct expense of other traffic. The primary goal of QoS is to provide better serviceincluding
dedicated bandwidth, controlled jitter and latency (required by some real-time
Quality of Service 25
and interactive traffic), and improved loss characteristics—by giving priority to certain
communication flows. QoS can be thought of as "managed unfairness," because whenever
one type of traffic is given a higher priority, another is implicitly given a lower priority. The
QoS designer must assess the level of each type of traffic and prioritize them best to suit the
business needs of each organization.
Fundamentally, QoS enables you to provide better service to certain flows. This is done by
either raising the priority of a flow or limiting the priority of another flow. When using
congestion-management tools, you try to raise the priority of a flow by queuing and servicing
queues in different ways. The queue-management tool used for congestion avoidance
raises priority by dropping lower-priority flows before higher-priority flows. Policing and
shaping provide priority to a flow by limiting the throughput of other flows. Link efficiency
tools prevent large flows (such as file transfers) from severely degrading small flows such
as voice.
When implementing QoS, you must do the following:
• Identify traffic, such as voice, signaling, and data.
• Divide traffic into classes such as real-time traffic, mission-critical traffic, and
any less-important traffic where QoS policy is implemented.
• Apply QoS policy per class, usually on a router interface, specifying how to serve
each class.
QoS Advantages
QoS can improve the quality of voice calls when bandwidth utilization is high by giving
priority to Real-Time Transport Protocol (RTP) packets. Figure 2-2 demonstrates how
voice (audio) traffic is given absolute priority over all other traffic with Low-Latency
Queuing (LLQ). This reduces jitter, which is caused by variable queuing delays, and lost
voice packets, which are caused by tail drops that occur when buffers are full. To avoid the
complete blocking of other traffic, voice bandwidth should be limited by defining the
bandwidth used by the maximum number of calls with the priority command within the
LLQ configuration. The number of voice calls should also be limited by a CAC mechanism.
Therefore, additional calls will not try to further saturate the WAN link, and ideally they
will be configured with Automated Alternate Routing (AAR) to route the additional calls
through the public switched telephone network (PSTN).
Chapter 2: Identifying Multisite Deployment Solutions
Figure 2-2 QoS Advantages
As an analogy, QoS builds a car-pool lane for prioritized drivers with LLQ. CAC is the
mechanism that limits the maximum number of cars that can be in the car-pool lane at once.
NOTE Video frames containing visual images are also sent over RTP and should also
be configured with their own priority queue in LLQ. The Cisco best practice is that the
priority queue should not exceed 33% of the interface bandwidth.
Finally, to ensure proper service for voice calls, you should configure QoS to guarantee a
certain bandwidth for signaling traffic such as Session Initiation Protocol (SIP) or Skinny
Client Control Protocol (SCCP) with class-based weighted fair queuing (CBWFQ). Otherwise,
despite the fact that the quality of active calls may be okay, calls cannot be torn down,
and new calls cannot be established.
NOTE QoS is not discussed further in this book. For more information, refer to the
Cisco Quality of Service or Optimizing Converged Cisco Networks courses.
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