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Quality of Service guidelines
Traffic shaping and policing
Traffic shaping is a mechanism to reduce the rate at which data is transmitted over an interface.
When people discuss traffic shaping, they are usually referring to the related technology of
traffic policing. Policing works by either adjusting the priority of excess traffic to a lower queue,
or discarding it. As with RED, discarding TCP traffic has the effect of throttling the stream by
forcing window size to shrink, and decreasing its transmission rate. Because RTP is a
fixed-bandwidth application, discarding RTP packets reduces voice quality without altering the
transmission rate. Adjusting the priority of voice traffic removes the strict priority protection that
reduces latency and jitter, and offers the highest voice quality. Thus, in most cases, it is
beneficial to use the QoS mechanisms listed above, rather than traffic shaping and policing, to
offer the highest quality for voice.
Frame Relay traffic shaping
Traffic shaping is important in technologies that implement virtual circuits (VCs), such as Frame
Relay or ATM, where the Committed Information Rate (CIR) might be less than the physical
speed of the interface, the port speed. In such scenarios, it is possible for traffic to burst above
the CIR. Depending on the Service Level Agreement (SLA), a carrier might mark excess traffic
as Discard Eligible (DE), and either delay or discard it if congestion is detected within the
network of the carrier. This behavior is unacceptable for voice traffic, which must minimize delay
and jitter to achieve optimal voice quality. To solve this issue, Frame Relay traffic shaping gives
an administrator tools to limit the transmission rate on a Frame Relay virtual circuit to the CIR.
A popular misconception is that voice traffic can be confined to the CIR, while data traffic can be
allowed to burst. Unfortunately, that is not how Frame Relay works. There is not a QoS
mechanism for Frame Relay that is negotiated between service providers and customers.
Service providers view all traffic equally, and mark any packet that exceeds the CIR as DE,
even if the packet is high-priority voice. Thus, the only way to guarantee optimal performance
for voice traffic is to restrict the traffic rate to the CIR.
On a Cisco router, do the following to ensure proper handling for voice:
1. Disable Frame Relay adaptive shaping. This technique reduces the CIR in response to
backwards explicit congestion notification (BECN) messages from the service provider.
Because traffic is being transmitted at the CIR in the first place, it does not need to be
throttled.
2. Set cir and mincir to the negotiated CIR. If FRF.12 fragmentation is implemented, reduce
the cir and mincir values slightly to account for the fragment headers.
3. Set be, the excess burst rate, to 0
4. Set bc, the committed burst rate, to cir/100. This accounts for at most a 10-ms serialization
delay.
5. Apply this map class to an interface, subinterface, or VC.
322 Avaya Application Solutions IP Telephony Deployment Guide
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