How Diff-Serv prioritizes frame traffic within a single PVC
 
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In an "IP-aware" frame relay service, such as those currently in service provider trials with Nortel Networks equipment, the egress queue is managed by the Differentiated Services - marked priorities of the various traffic flows, not just by PVC. The major advantage to this approach is that a particular port can be oversubscribed while ensuring that critical flows are not delayed too much.
For simplicity, assume there are two flows within an IP stream - a high-priority customer service application and low-priority Web browsing. If both are in the same PVC, a traditional frame relay can't distinguish the higher-priority traffic from the lower-priority traffic.
When the switches are able to manage the output queues, based on the Diff-Serv bits, the various flows may be assigned different priorities. Thus, traffic generated by the business- critical application on any PVC would always leave the queue ahead of Web-browsing traffic.
This technology has particularly strong implications for voice over IP and voice over frame relay. Making Diff-Serv based queuing work for voice over IP also brings up the question of whether the traffic is being fragmented - and, if so, where. For now, we'll simply state that frame-based (FRF.12) fragmentation needs careful attention when deployed in conjunction with Diff-Serv based queuing, and we'll discuss this in more detail in later newsletters.
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Steven Taylor, consultant and broadband packet evangelist, and Joanie Wexler, an independent networking technology editor and writer, team up to bring you this analysis and commentary. Taylor specializes in education and market analysis, and Wexler adds incisive reporting and research. For more detailed information on most of the topics discussed in this newsletter, connect to www.webtorials.com, the first Web site dedicated exclusively to market studies and technology tutorials in the Broadband Packet areas of Frame Relay, ATM, and IP.
Feedback and additional topic ideas are welcome. Please contact taylor@webtorials.com or joanie@jwexler.com.
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Frame Relay newsletter,
10/09/00
