Contention/Oversubscription maths

Adam Armstrong lists at
Fri May 27 13:01:46 UTC 2011

On 27/05/2011 13:45, Jared Mauch wrote:
> On May 27, 2011, at 8:14 AM, Adam Armstrong wrote:
>> No SLA, residential customers.
> I would watch out for the 'abusers' in this case, and have the capability to rate-limit the ports if necessary.  Some hardware doesn't deal well with 'small' buckets of rate-limiting, eg: taking a 1G port to 1M.
> I'm interested in your operational results, as i've had a few general ASSumptions i've offered in this space:
> - Most people are going to be limited by their wireless gear (few people care to run wired very far)
This results in people complaining *about* us, as the crappy performance 
of 802.11g made a lot of people complain about ADSL2+ ISPs (because G 
can't really do 24Mbit!)
> - Most servers on the far-end aren't going to be fast enough to cope
This holds true until you look at services delivered by farms, such as 
Akamai, Usenet or P2P. I'd be interested to see what happens with 1000 
people with 1000Mbit services start torrenting the next major HD movie 
> - Most people aren't going to spend time debugging network problems
Indeed, they just moan about us instead :(
> - Some percentage of users are going to run a torrent or something else and flatline their port.  You need to be able to police them at a reasonable bandwidth cap, eg: 10M if they have 1G, but that's 1% and many dumber switches won't go under 10%.
Policing isn't something we want to do (other than to police 100Mbit 
ports to 20Mbit for those who buy the 20Mbit product. Policing is 
against the ethos of the company.)
> The other solution is to just throw more bandwidth at the problem.  Make sure your switches can easily take a 10G or n*10G uplink.
> I'd expect that the single bursty user is going to drown out the rest of them into the noise, as they will likely plug in directly and have no wireless hops that limit their speeds.
This what i see elsewhere in buildings with similar mixes of customers 
as we're expecting. For example in a building with ~1000 2-100Mbit 
customers a single 100mbit customer can pretty much double the peak 
traffic usage of a building if they decide to download a lot.
> One of my friends operates a WISP in this area and has periodic issues with the heavy usage users and wonders what they're doing with that 150GB of data they "download" in a month.

I've been known to download ~1.5TB in  month on my 24Mbit ADSL service.

It's people like *me* that I'm worried about. Our deployment model means 
that individual customers can't chose us, we chose which buildings we 
connect and then are likely to get most people within that building, so 
I'm hoping our hoggers should be fairly spread out. Though, I'm sure 
this won't be the case as certain areas of the city are likely to have 
more of each specific kinds of network user.

Our upgrade plans include 10GE to buildings when the building has a 
large number of occupants (>1000), but before then it's cost prohibitive.


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