"Playing the averages" with oversubscription
 
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Because data connections tend to be used intermittently, oversubscription of a frame relay port can be an effective strategy for achieving more economical network connectivity. Oversubscription lets network designers "play the averages" by assigning a total committed information rate (CIR) to a given port that is greater than that port's speed. This approach is particularly attractive for saving money on the port connection, which tends to be the most costly component in the frame relay pricing equation.
Oversubscription is usually measured in percentages. Thus, four 32K bit/sec permanent virtual circuits (PVC) - a total CIR of 128K bit/sec - attached to a 64K bit/sec port represents a 200% subscription. Two 32K bit/sec PVCs attached to a 64K bit/sec port equals 100% subscription, which represents no oversubscription.
How do you know how much you can oversubscribe your port without causing too high a risk to your application performance? Unfortunately, there is no hard and fast recommended ratio. It all depends on the traffic characteristics. Although, you will likely find that in many applications, you can easily use a ratio concentration of 2-to-1 to 4-to-1.
If the total CIR exceeds the speed of the port connection and all virtual circuits concurrently transmit at the CIRs, the port obviously will not have enough capacity to accommodate all the traffic. However, for typical bursty data applications, the chances of this happening are low. Leased-line data connections, for example, tend to have a 30% utilization or less on average. However, your challenge lies in choosing the appropriate level of oversubscription for your own unique network. To do that, you need to measure your usage - either using performance data generated by your own enhanced DSU/CSU, WAN probe, or router, or by using the network information in the monthly performance reports from your frame relay service provider.
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_wexler@mindspring.com>
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