WAN Oversubscription

Because data connections tend to be used on an intermittent basis (for example, "bursty data"), oversubscription of a WAN (most often Frame Relay) port can be effective for achieving more economical network connectivity in an enterprise organization. Oversubscription lets network designers "play the averages" by designing and provisioning a total committed information rate (CIR) to a given port that is greater than the port speed. For example, a Frame Relay port might be provisioned for 56 Kbps, with the total CIR measuring 112 Kbps.

Oversubscription is often measured as a percentage, such as, 200 or 400 percent oversubscription. For example, four 32 Kbps permanent virtual circuits (PVCs) a total CIR of 128 Kbps attached to a 64 Kbps port represents a 200 percent subscription.

This 200 percent, or 2:1, oversubscription rate is illustrated in Figure 14-11.

Figure 14-11. 200 Percent (2:1) Oversubscription

If two 32-Kbps PVCs were attached to a 64-Kbps port, the total CIR equals the size of the attached port: 2 x 32 Kbps = 64 Kbps, 64 Kbps/64 Kbps equals 1, or a 100 percent (1:1) subscription rate, representing no oversubscription.