Re: How many on a T1?
From: Steve Wolfe (unt_at_codon.com)
Date: 02/07/05
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Date: Sun, 6 Feb 2005 23:42:33 -0700
> Sorry to be off-topic, but I know that so many network aware folk read
> this list :)
>
> I'm involved in a community broadband inititive. Read all about it:
>
> http://mypage.uniserve.com/~bvdp/broadband
>
> One big unknown we're trying to figure out is how many users we can
> comfortably server off a single, 1.5mps, T1 line? We're hearing some
> folks say that a T1 is good for one user, 20 to 50, and the government
> line (they are supplying the line for the low cost of $900.00 per
> month... what a deal :) ) and they say that with caching we can easily
> do 100+.
>
> I think that the govt. guy might have been right in the year 2000, but
> today he's full of whatever.
>
> So, what's the scoop for 2005?
It greatly depends on what your clients will be doing with the t1. If
you're talking about general web surfing, you can get a *lot* from a t1.
One of the offices for which I manage the connectivity uses a t1 for ~40
desktops, where the general workflow of most of them involves HTTP
transactions all day, and the 5-minute average on the router rarely rises
above 300 kilobits - or at least it doesn't when P2P/bit torrents aren't
being used.
P2P/bit torrents are the largest killers of bandwidth. The biggest
problem isn't that they're using lots of bandwidth (one or two people can
saturate the t1 with single FTP downloads, and nobody in the office even
notices) - it's that they open up such a large number of concurrent
connections. If Joe Schmoe with a P2P client has 300 concurrent connections
vying for bandwidth, and you've got two, you're going to get the short end
of it. If your clients will be using P2P *without* traffic shaping, a t1 is
good for about one, maybe two clients.
Luckily, you have a lot of powerful options for traffic-shaping. The most
rudimentary, simple bandwidth-limitting, can be the most simple, and most
effective - at the cost of lower peak for each user. My quick-and-dirty
solution in said office was to limit each user to 256k sustained, with
something like a 1- or 2-megabyte burst to full speed. With that in place,
it would take at least six users at full usage to flood the t1, but in that
population, that doesn't happen - 5 minute averages stay around 500-600
kilobits/second.
There are plenty of more advanced ways of doing the shaping, but the
options available will depend on the router you choose. A caching proxy is
a very good thing on such a network - even if for nothing else, when
Micro$haft releases a 100+ megabyte service pack, your t1 will only have to
choke along for ten minutes downloading the file for each and every user.
In implementing such a network, you really do want to route between
individual user connections, don't just connect them all to the same switch.
That will greatly help contain viral spread/outbreaks, as well as a very
good deal of other potential problems. Low-cost, low-power machines with
quad-port Ethernet cards from ebay will fill the bill on a budget, and let
you use very advanced firewalling/traffic shaping features of Linux (or the
BSDs) without having to sell organs to pay for Ciscos.
(It's sort of a shame that you're only getting a t1 - at $900, that's $600
per megabit. I've had commercial carriers offer me full t3s at less than
$140 per megabit, including line and termination charges. Your costs go
down GREATLY as the size of the pipe increases.)
steve
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