Belts, suspenders, or both? The carrier view on oversubscription

In several of our recent newsletters, we've discussed the oversubscription of frame relay port speeds by enterprise customers, exploring to what extent it is safe for the aggregated sum of the committed information rates for a given port to exceed the port speed. As a follow-up to those discussions, we thought you might be interested in the carrier attitude toward oversubscription. It's obvious from what we've found out that the carriers don't think that most enterprise users consume a very large fraction of their CIRs.

According to an extremely knowledgeable source we contacted, many carriers oversubscribe the CIRs by a ratio of about 3-to-1. That is, even though they may guarantee a CIR of "n"K bit/sec, the network is generally engineered to have about "n"/3K bit/sec available for that CIR.

If you think your frame relay carrier is being bold in their oversubscription, the same source pointed out that Internet services are often oversubscribed by a ratio of 10-to-1. In fact, when we were writing about ADSL access to frame relay and the Internet, another knowledgeable source pointed out that some ISPs would like to oversubscribe their ADSL services by a ratio of 150-to-1.

All of a sudden, the oversubscription ratio of 4-to-1 we analyzed in a previous newsletter sounds like something appropriate for even a guy who is so cautious that he wears a belt and suspenders. But the bottom line on "safe" oversubscription lies in a careful analysis of your actual traffic patterns. The enhanced DSU/CSU products (as covered extensively in our columns) that perform analysis of the actual traffic patterns can give you a precise answer.

How much do you oversubscribe your networks? We'd like to hear from you, either via e-mail, or, even better, via the frame relay newsletter discussion area at www.webtorials.com/htm.

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