Home Telecoms & NBN Contention ratios alone 'not enough to judge ISP'

Contention ratios alone 'not enough to judge ISP'

Contention ratios alone would not be enough for consumers to make a judgement as to which ISP would provide sufficient bandwidth for their needs, an academic claims.

Rod Tucker, Laureate Emeritus Professor at the University of Melbourne and a member of Labor's Expert Panel that advised on the NBN, told iTWire that the claim made by Robin Eckermann, an adjunct professor at Canberra University and one of the people behind the TransACT network, was misleading.

Eckermann had suggested that if the NBN Co, the company rolling out Australia's national broadband network, made contention ratios available to consumers, then that would be of help for them to understand whether a particular ISP could meet their needs.

He gave the following example: "For example, if an RSP was supporting (say) 100 customers at 100 Mbps connection speed, 100 at 50 Mbps, 500 at 25 Mbps and 300 at 12 Mbps over a CVC, the total *theoretical* demand that these customers could generate is 31,100 Mbps. 

"If the RSP was buying only (say) 300Mbps CVC capacity to support all these 1000 customers, they would be operating the CVC at a contention ratio of around 104:1 – and at this level, it is predictable that there would be congestion at busy times."

But Tucker said it was not as simple as that. 

"Under the scenario that Robin mentioned (100 users at 100 Mbps connection speed, 100 at 50 Mbps, 500 at 25 Mbps and 300 at 12Mbps), and with a contention ratio of 104:1, the 100 users with 100 Mbps plans would be disadvantaged more than other users with lower-speed plans," he said.

"Assume, for example, that 5% of all users are trying to download data at the same time (i.e. 5 users with 100Mbps plans, 5 with 50Mbps plans, 25 with 25 Mbps plans, and 15 with 12 Mbps plans." These 50 users would be competing for a share of the 300Mbps CVC allowance in Robin’s example.  

"On average, these 50 users would end up with a connection speed of 6Mbps. For those customers who signed up for a 12Mbps plan, this might not be too much of an inconvenience, but for the 5 users on 100 Mbps plans, an actual speed of 6Mbps would be a great annoyance." 

Tucker said Eckermann's solution was not really going to work.

"For a user to understand what kind of service they are going to get from their RSP, they need more information than just the contention ratio. Other information would include details of the relative numbers of customers on different plans, and whether the RSP has ways to provide priority to customers on higher speed plans."


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Sam Varghese

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A professional journalist with decades of experience, Sam for nine years used DOS and then Windows, which led him to start experimenting with GNU/Linux in 1998. Since then he has written widely about the use of both free and open source software, and the people behind the code. His personal blog is titled Irregular Expression.


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