Oversubscription
In a WAN network (for example, frame relay or ATM), 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.
From Oversubscription basics, Network World on Frame Relay, 08/30/00.
Additional resources
More benefits of oversubscription
Network World Frame Relay Newsletter, 06/21/99.
Oversubscription: Balancing risk and benefit
Network World Frame Relay Newsletter, 12/01/99.
Oversubscription and contention
Network World Frame Relay Newsletter, 09/20/00.
Topic: Frame Relay
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