Mailinglist Archive: opensuse-edu (225 mails)
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Re: [suse-linux-uk-schools] ADSL speeds into school
- From: "Mark Evans" <mpe@xxxxxxxxxxxxxxxxxxxxxxxxxxx>
- Date: Tue, 6 May 2003 08:42:06 +0000 (UTC)
- Message-id: <20030506084143.GB30706@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx>
On Tue, May 06, 2003 at 04:20:42AM +0100, David Bowles wrote:
> ...err, I didn't realise this. Is this specification practice
> documented anywhere on the web?
>
> > Contention are frequently misunderstood. 5:1 actually means something
> > more along the lines of 250 2M connections being multiplex over
> > an ATM network onto one 100M ethernet port at the ISP end.
>
> So I assume one also needs to take into account the maximum real-world
Why, since the figures arn't exactly based on "real world"
throughput, in the first place? They are theoretical
"worst case senarios".
> throughput of a 100Mb ethernet port is nothing like 100Mbits / second.
> Most likely this is going to peak at 20Mbits / second, max. What this
> implies is when applying this 'hypothetical' definition of
> contention ratios, in practice this is more likely to become 25:1.
So far as the calculations are concerned the figure used
is more likely to be 100M than 20M. Or even 45M to a
router with an ethernet interface. (In which case the
ADSL side would be up to 112 2M, 225 1M, 450 0.5M or
indeed any mixture of these totalling up to 225M.)
The basic point is that you don't have anything like
5 2M ADSL lines connected to one 2M line at an ISP.
> Also, if the ISP decides to simply slot a 1Gbit ethernet link into the
> data stream, can they legitimately claim their users now benefit
> from an uprated link with a contention ratio of "0.5 to 1" ?!!!!!!!
No, since that wouldn't change the rate of their connection
to the ATM network. If they did change that connection
from 100M to 1G BT would feel free to direct 10 times as
much traffic to that port.
The way it works is that the ISP has one or more connections
to BT's ATM network. N.B. the ISP connections have considerably
greater bandwidth than an ADSL line. A connection ratio of 5:1
means that BT can direct ADSL lines to a total bandwidth of
5 times the bandwidth of the ISP connection to that connection.
This figure won't tell you anything about contention ratios
for the ISP's peering bandwidth or contention ratios for
a specific ISP service. e.g. a filtering proxy. Which are
probably far more relevent than anything to do with the ADSL
figures. Especially with a general purpose ISP which has a
broad range of different types of customers.
--
Mark Evans
St. Peter's CofE High School
Phone: +44 1392 204764 X109
Fax: +44 1392 204763
> ...err, I didn't realise this. Is this specification practice
> documented anywhere on the web?
>
> > Contention are frequently misunderstood. 5:1 actually means something
> > more along the lines of 250 2M connections being multiplex over
> > an ATM network onto one 100M ethernet port at the ISP end.
>
> So I assume one also needs to take into account the maximum real-world
Why, since the figures arn't exactly based on "real world"
throughput, in the first place? They are theoretical
"worst case senarios".
> throughput of a 100Mb ethernet port is nothing like 100Mbits / second.
> Most likely this is going to peak at 20Mbits / second, max. What this
> implies is when applying this 'hypothetical' definition of
> contention ratios, in practice this is more likely to become 25:1.
So far as the calculations are concerned the figure used
is more likely to be 100M than 20M. Or even 45M to a
router with an ethernet interface. (In which case the
ADSL side would be up to 112 2M, 225 1M, 450 0.5M or
indeed any mixture of these totalling up to 225M.)
The basic point is that you don't have anything like
5 2M ADSL lines connected to one 2M line at an ISP.
> Also, if the ISP decides to simply slot a 1Gbit ethernet link into the
> data stream, can they legitimately claim their users now benefit
> from an uprated link with a contention ratio of "0.5 to 1" ?!!!!!!!
No, since that wouldn't change the rate of their connection
to the ATM network. If they did change that connection
from 100M to 1G BT would feel free to direct 10 times as
much traffic to that port.
The way it works is that the ISP has one or more connections
to BT's ATM network. N.B. the ISP connections have considerably
greater bandwidth than an ADSL line. A connection ratio of 5:1
means that BT can direct ADSL lines to a total bandwidth of
5 times the bandwidth of the ISP connection to that connection.
This figure won't tell you anything about contention ratios
for the ISP's peering bandwidth or contention ratios for
a specific ISP service. e.g. a filtering proxy. Which are
probably far more relevent than anything to do with the ADSL
figures. Especially with a general purpose ISP which has a
broad range of different types of customers.
--
Mark Evans
St. Peter's CofE High School
Phone: +44 1392 204764 X109
Fax: +44 1392 204763
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