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Simply Speaking
Athena Dynamic CIR

Article Number: A015
Article Type: Application
Modified: November 4, 1999

SUMMARY

Traffic Shaping. At the root of traffic shaping is the ability of a system to control (shape) a traffic flow to provide a predictable quality of service and quantity of bandwidth.

Athena implements traffic shaping using a variety of controllable and definable shaping properties. These properties operate together to model the traffic shape desired. In a recent software release, Athena has been given another property that yields exceptional performance in the presence of other voice traffic. This property is called Dynamic CIR.

MORE INFORMATION

Before we discuss Dynamic CIR, we will briefly list some of the other traffic shaping properties presently available in Athena:

• Commited Information Rate (CIR)
• Excess Information Rate (EIR)
• Commited Burst
• Priority Queuing
• Queue Depth
• Protocol Streaming

and now,

• Dynamic CIR

An Overview

Dynamic CIR was created to allow the Athena to scale sub-channel CIRs to make best use of available bandwidth. In FRF.11, CIR is still only referenced at the PVC (DLCI) level. This can be a problem when dealing with multiple traffic streams, and particularly problematic when these streams originate from seperate DLCIs in some other location on the network.

In this particular instance, each stream would have had their own CIR level defined for the inbound stream, but would effectively lose control of CIR on the outbound leg as they converged onto their respective FRF.11 sub-channels. Now, this stream of sub-channels will be managed under the CIR of the one PVC containing the sub-channels. Without an over-suscription condition on each incoming PVC, the outgoing bandwidth may not be effectively used.

Voice

The presence of voice traffic can complicate this scenario. Voice demands a level of responsiveness that, if not given, poor voice quality will result. With the previous scenario in mind, voice would have to contend on an equal basis with other normal data traffic. This would negatively affect the responsiveness of the circuit, leading to voice quality problems, and that would not be acceptable.

Priority

To deal with voice traffic and its contention for bandwidth, Athena ensures that voice traffic is given priority. By default, voice traffic will always lead other forms and types of data. This ensures that voice is delivered the responsiveness it requires.

However, this further affects how CIR is divided between all other sub-channel-based streams. From a management perspective, the CIR of all other streams could be lowered to make room for the possibility of voice traffic. But this leads to inefficiencies in bandwidth utilization, if not at the outgoing port, perhaps further downstream. Also, by inappropriately lowering CIR on each PVC, traffic in excess of the prescribed CIR will be marked Discard Eligible. That could have an effect on the quality of service delivered to each PVC further downstream.

To combat this, Athena doesn't just pre-allocate bandwidth to voice traffic, it senses and adapts the flow of normal data flows to accommodate the voice flows.

How does it do that effectively? Dynamic CIR.

Dynamic CIR

Dynamic CIR is simply the property that allows Athena to automatically adjust the level of CIR on a per stream basis. It uses this technique to dynamically adapt the CIR of each merged channel in the presence of voice traffic. When a voice flow is present, Athena lowers the prescribed CIR of the other data channels automatically giving way to the voice flow. In the absense of voice, Athena will re-raise the prescribed CIR to allow the data channels to use the remaining bandwidth.

Benefit

The benefit of this new feature should now be obvious. Data channels need not be configured for an over-subscription condition, nor do CIRs have to be lowered unnecessarily. That means, that effective use of available bandwidth is provided, and quality of service is supplied for both data and voice traffic.

Now, that's traffic shaping.

REFERENCES

For additional information on this important traffic management feature, please refer to the Athena Reference Guides. For information on FRF.11, please refer to article A014, which discusses both FRF.11 and FRF.12. §

Copyright © 1999 Develcon Electronics Ltd.  All Rights Reserved.