Oversubscription basics

Recently, a reader requested advice on how much to oversubscribe his network, and we were reminded that previous newsletters on the topic also yielded many reader questions. So perhaps it is time to review this important network design component.

The term "oversubscription" is used to mean assigning a total committed information rate to a given port that is greater than that port's speed. The upside to this approach is you save money on the port connection, which tends to be the most expensive component in the frame relay cost.

The oversubscription concept is as old as time (well, in telecom years, anyway). User connections to telephone switch ports, for example, have always been oversubscribed, relying on the concept that all users will never be on the phone at the same time.

The downside to oversubscription is that you run the risk that users will attempt to use all your capacity concurrently, and you will run into network congestion. The risks are low, though, if you do not oversubscribe too much. And without oversubscription, you lose significant economic benefits of frame relay and should consider using leased lines.

How much should you oversubscribe? It all depends on the traffic characteristics of your applications. In many applications, you can easily use a ratio concentration of 2-to-1 to 4-to-1. Oversubscription is often expressed in percentages, such as 100% or 200% subscription. For example, subscribing two 32K bit/sec permanent virtual circuits to a 64K bit/sec port is 100% subscription and represents no oversubscription; both PVCs could run to their max and the port on the carrier's frame-relay access switch would have the capacity to support them.

Assigning four 34K bit/sec PVCs to that 64K bits/sec port, on the other hand, is 200% subscription (2-to-1 oversubscription). Some carriers put a cap on how much oversubscription you are allowed, so you need to factor that into your equation.

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.

Frame Relay archive
Past newsletters.

DSL oversubscription
Network World on Internet Services, 05/22/00

Belts, suspenders, or both?
Network World on Frame Relay, 12/08/99

White paper: Frame Relay over geosynchronous satellites

Frame relay: market motivation for its development