Contention ratios how do ISP come up with this number

[nslalu]

Hi

I have been trying to find a good explanation on how ISP define contention ratio. I came across one of the best explanations and if it is true then most ISP sales staff are incorrectly advising clients.

The examples : If Two users each have a 2M link and sharing a 2M back haul each user will get half the 2M giving the contention ratio of 2:1 ( the demand for bandwidth is 2 times greater than the link )

another example : if 1000 people each with a 2 meg link share a 40M pipe ( 1000 people x 2M = 2000M / by total pipe size 40m = 50)

contention ratio is 50 :1 so the demand for bandwidth is 50 times greater on the single 40 meg line for 1000 users each with a 2meg line.

So why do so many of use believe that this 50:1 contention ratio means 50 users to one link...... this to me is totally incorrect. It should mean the demand on bandwidth over the back haul link or any other link is 50 times greater than the link.

:wtf

 

[MickeyD]

And what if those users are indeed on one link, but it is a 10Gbps link?

 

[quovadis]

The contention ratio generally represents the number of subscribers who share a common bandwidth allocation. This allocation is not necessarily the available backhaul but allocation of bandwidth on the backhaul or at the breakout point. eg. There may be a 10gbit backhaul but 50 users share 2mbit of allocated bandwidth on that 10gbit/s link. Common sense dictates that the backhaul should be at least at this allocated capacity but link sizes are generally dictated by actual utilisation. Contention ratios also generally exist at various points and at various levels.

 

[Pavan]

Agreed. Contention happens on the IP layer these days and not on the transmission layer itself.

 

[MickeyD]

Agreed. Contention happens on the IP layer these days and not on the transmission layer itself.

That's the point I'm getting at...