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Contention ratio and quality of service

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Jan 31st, 2019 at 1:52pm  
Here are some examples of texts used to describe the notion of contention ratio ..

Contention ratio is the term used for the number of users that are currently sharing data capacity.  For example, a contention ratio of 75:1 means that 75 broadband users are currently sharing the same bandwidth.

In computer networking, the contention ratio is the ratio of the potential maximum demand to the actual bandwidth.

If your contention ratio is 20:1, for instance, that means twenty households are using one line.
Standard contention ratios used to be around 50:1 for home broadband, and 20:1 for business broadband.

Contention Ratio (or over subscription ratio) is the number of users sharing the same data capacity. The lower the contention ratio the higher the quality of service. A 50:1 contention ratio means that up to 50 broadband customers are sharing the same bandwidth at any one time.

My comments ...

1. If you have 40 Mbit/s downlink carrier, shared with 1333 customer PCs, each customer "up to 40 Mbit/s"
Contention ratio = (1333 x 40 ) / 40 = 1333

2. If you have 2 Mbit/s downlink carrier, shared with 66 customer PCs, each customer "up to 2 Mbit/s"
Contention ratio = (1333 x 20 ) / 40 = 66

Looking at these three examples, the third example with a contention ratio = 66 appears to be the best with the highest quality of service. But I doubt this. Reason: Both examples are designed the same, with 30 kbit/s per PC (total bit rate = number of PCs x 30 kbit/s ), so both are the same.   Reason: Example 1 offers up to 40 Mbit/s, example 2 offers only up to 2 Mbit/s, so 1 is better.  Reason: In example 1 there are very many customers so the average load will be steady.  In example 2 there are few customers so there will be big variatins in traffic load and congestion at times. so 1 is better.

These considerations suggest that "contention ratio" alone is not a good measure of quality of service.

Your comments welcome below.  If you need to register and login to post a reply go to the bottom of the forum home page.

Best regards, Eric.
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