Oversubscribed (communications)

Oversubscribed (communications)

In a communications system in which multiple users share a common resource, oversubscription refers to the ratio of the allocated bandwidth per user to the guaranteed bandwidth per user. Underlying the oversubscription model is the fact that statistically few users will attempt to utilize their allocated bandwidth simultaneously. Calculation and management of oversubscription ratios is common in the CATV industry[1][2][3].

In a cable network utilizing DOCSIS 1.1, for example, the full 38 Mbit/s download bandwidth [4] is typically shared by some 500 subscribers [5], each of which may be allocated 7 Mbit/s [6]. Calculating the guaranteed bandwidth per subscriber in this case is accomplished by dividing the maximum total bandwidth of 38 Mbit/s by 500, the maximum number of simultaneous users. The advertised peak bandwidth per user of 7 Mbit/s is 92.1 times the guaranteed bandwidth per user of 0.076 Mbit/s. In this example, the download oversubscription ratio is 92.1.

A similar calculation can be performed using upstream bandwidths. Typically, advertised upload rates are 1/4 the download rates [7] however the available upload bandwidth using DOCSIS 1.1 or 2.0 is also lower. If 100 users allocated 1.75 Mbit/s of upload bandwidth share a single DOCSIS 1.1 upstream, the upload oversubscription is 17.1. One article found a typical DOCSIS download oversubscription ratio to be 28.125, while a best case oversubscription may be 12.0[1].

Oversubscription is not the same as overselling, provided that the oversubscription ratio, for a given number of subscribers traffic, which is multiplexed over time, does not significantly impact performance. A significant impact to performance might be one in which the performance of the oversubscribed portion of the network is less than that of the end to end service that the subscriber is using. In other words, oversubscription works because not everyone uses it at the same time. Oversubscription is the basis for all non-dedicated telecommunications services. Even if a subscriber had a dedicated access network, that network is still oversubscribed against many others with respect to Internet access for example. The same is true in telephony and in wireless.

It is not economically practical, environmentally reasonable, or technically feasible to provide dedicated access for every service to every customer. A well engineered oversubscribed services appears to function as a dedicated service to a subscriber. Another way of saying this is that a good engineer designs the oversubscription in a way that makes the service appear to be a dedicated service for that customer. That means no dropped calls on your cell phone, no busy signals when dialing, and no failed downloads on the Internet (for example).

In contrast, PON access networks are not typically oversubscribed, that is, the oversubscription ratio is 1.0[1].

References

  1. ^ a b c Victor Blake,Chasing Verizon FiOS,[1], Accessed 10/3/2009
  2. ^ More on DOCSIS,[2], Accessed 10/3/2009
  3. ^ Cisco,Appendix A: DOCSIS and CMTS Architectural Overview,[3], Accessed 10/3/2009
  4. ^ CableLabs, Radio Frequency Interface Specification,[4], Accessed 10/3/2009
  5. ^ Mountain Cablevision, Deploying Commercial Services, Tools and Techniques,10/21/2008,[5], Accessed 10/2/2009
  6. ^ Mountain Cable Internet Offerings,[6], Accessed 10/3/2009
  7. ^ Comcast High Speed Data Products "[7]",Accessed 10/3/2009
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