Contention Ratios Conundrum

DMT

Consider this simplified hypothetical situation:
Suppose there are only 3 telephone lines out of a possible 480 connected to an exchange, each with a 48:1 contention ratio 512kbps DSL product. Do each of the 3 lines get a full 512kbps connection for themselves or do they have to share the same connection?

To put it another way, suppose whoever is providing the DSL service at the exchange treats each 512kbps like a bin - do they add new customers to the same bin until it's full and then start adding customers to the next bin or do they spread out the customers over all the bins until all the bins (and thus the exchange) are full?

Ryaner

Its my understanding that each is added to the same slot until it's full and then start on the new one.

bk

Yes the three would share the 512k, however contention is actually far more complicated then that, the conention dosen't actually start at the ports, but instead it usually occurs further back in the system, I believe it starts at the ADSL Geographical POP's of which there are 11 in Ireland. The contention occurs in the Virtual Paths on the backhaul from the ADSL Geographical POP's back to Dublin HQ.
Virtual Path's are actually supplied in 2mb chunks, so when they say 48:1 they actually mean that a maximum of 192 users share the 2MB VP (48 x 4 = 192).

However there should never really be 192 users on one VP, as the VP reaches about 90% capacity, the ISP would order a new 2mb VP and this would be added to the existing VP, giving us a combined 4MB VP this would then be shared by the 90%, plus any new users, so you would then see the contention drop down to about 20:1 and then start increasing again as new users are added.

This is my understanding fromk quickly reading the Eircom wholesale documentation, it may or may not be correct.

Also remember that conention can and does happen at a lot of other places. Such as in the ISP's network, out onto national and international backhaul and on the server you are communicating with.

longword

The first point of contention will be on the link from the DSLAM in the local exchange to the regional POP. You can expect that to be contended at the specified rate, but I believe it's implemented in blocks of 2Mbit.

iano

God help us if Eircom are still provisioning exchanges with discrete 2megs of bandwidth to be shared among certain customers.

It virtually guarantees that one or two heavy users will screw up everyone else on their 2meg, regardless of the "contention ratio" that they have bought or which ISP they use.

Statistically, there is a far better chance of ensuring that everyone gets decent service if the pipes are provisioned in 34, 45 or 155 mbps chunks.

Also remember that the local exchange is only part of the issue, cheapskate ISPs have the option of provisioning 2mbps circuits between themselves and Eircom rather than decent pipes, again virtually guaranteeing an "unfair" service.

That only leaves performance/contention on the ISPs connections to the Tier 1 and 2 providers (Internet backbone)!

When things settle down, it will be interesting to see whether 24:1 customers actually get a better service than 48:1 customers or whether local bottlenecks make everyone's service a random function of their "neighbour"s behaviour.

DMT

/me goes off to revise how datagrams, virtual circuits, packet switching, flow control and queuing theory all fit into all of this....