APconnections Field Guide to Contention Ratios


In a recent article titled “The White Lies ISPs Tell about Broadband Speeds,” we discussed some of the methods ISPs use when overselling their bandwidth in order to put on their best face for their customers. To recap a bit, oversold bandwidth is a condition that occurs when an ISP promises more bandwidth to its users than it can actually deliver. Since the act of “overselling” is a relative term, with some ISPs pushing the limit to greater extremes than others, we thought it a good idea to do a quick follow-up and define some parameters for measuring the oversold condition. 

For this purpose we use the term contention ratio. A contention ratio is simply the size of an Internet trunk divided by the number of users. We normally think of Internet trunks in units of megabits. For example, 10 users sharing a one megabit trunk would have a 10-to- 1 contention ratio. If sharing the bandwidth on the trunk equally and simultaneously, each user could sustain a constant feed of 100kbs, which is exactly 1/10 of the overall bandwidth.

So what is an acceptable contention ratio?

From a business standpoint, it is whatever a customer will put up with and pay for without canceling their service. This definition may seem ethically suspect, but whether in the bygone days of telecommunications phone service or contemporary Internet bandwidth business, there are long-standing precedents for overselling. What do you think a circuit busy signal is caused by? Or a dropped cell phone call? It’s best to leave the moral debate to a university assignment or a Sunday sermon.

So, without pulling any punches, what exactly will a customer tolerate before pulling the plug?
Here are some basic observations:
  • Rural customers in the US and Canada: Contention ratios of 50 to 1 are common
  • International customers in remote areas of the world: Contention ratios of 80 to 1 are common
  • Internet providers in urban areas: Contention ratios of 20 to 1 are to be expected
  • The numbers above are a good, rough starting point, but things are not as simple as they look. There is a statistical twist as bandwidth amounts get higher.

    Contention ratios can actually increase as the overall Internet trunk size gets larger. For example, if 50 people can share one megabit without mutiny, it should follow that 100 people can share two megabits without mutiny as the ratio has not changed. It is still 50 to 1.

    However, from observations of hundreds of ISPs, we can easily conclude that perhaps 110 people can share two megabits with the same tolerance as 50 people sharing one megabit. What this means is that the larger the ISP, the more bandwidth at a fixed cost per megabit, and thus the larger the contention ratios you can get away with.

    Is this really true? And if so, what are its implications for your business?

    This is simply an empirical observation, backed up by talking to literally thousands of ISPs over the course of four years and noticing how their oversubscription ratios increase with the size of their trunk.

    A conservative estimate is that, starting with the baseline ratio listed above, you can safely add 10 percent more subscribers above and beyond the original contention ratio for each megabit of trunk they share.

    Thus, to provide an illustration, 50 people sharing one megabit can safely be increased to 110 people sharing two megabits, and at four megabits you can easily handle 280 customers. With this understanding, getting more from your bandwidth becomes that much easier.

    12 Responses to “APconnections Field Guide to Contention Ratios”

    1. Taming YouTube how ISPs can comprimise with consumers « NetEqualizer News Blog Says:

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      […] The True Price of Bandwidth Monitoring What Is Deep Packet Inspection and Why the ControversyAPconnections Field Guide to Contention RatiosA Detailed Case Study of Packet Shaper and NetEqualizerNetEqualizer White Paper Comparison with […]

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      […] Factoid: Typically for a business in an urban area, we would expect about 10 megabits of bandwidth for every 100 employees. If you fall below this ratio, 10/100, you can still take advantage of cloud computing but you may need a some form of QOS device to prevent the recreational or non essential Internet access from interfering with your cloud applications. , see our article on contention ratios. […]

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      […] Factoid: Typically for a business in an urban area, we would expect about 10 megabits of bandwidth for every 100 employees. If you fall below this ratio, 10/100, you can still take advantage of cloud computing but you may need a some form of QOS device to prevent the recreational or non essential Internet access from interfering with your cloud applications. , see our article on contention ratios. […]

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      […] I would skip my studies for some social actively (drinking) .   Only now it applies to bandwidth contention ratios.   Every body wants to know how they compare with the Industry average in their sector. Are they […]

    12. NetEqualizer News: March 2013 | NetEqualizer News Blog Says:

      […] when I would skip my studies for some social activity (drinking). Only now it applies to bandwidth contention ratios. Everybody wants to know how they compare with the industry average in their sector. Are they […]


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