ADSL uncontended versus SDSL contended

We're moving to a new office and we're taking the opportunity to look at our
internet connection. We currently have:

Demon 2MBps/256kbps ADSL uncontended
BT 512kbps/256kpbs ADSL contended 20:1

The Demon link is used primarily for email and downloads and actually is
probably overkill being uncontended. The BT link though is where we want to
improve the upload speed as we use that for Citrix/Terminal Services where
response is critical.

Demon appeared to have a 1MB uncontended service but when they did the line
checks, they couldn't guarantee 1MB, only 512K. Their 1MB price was fine so
we said, fine, we don't care - we'll pay for 1MB but accept 512K. But then
their legal got in the way and said they couldn't do that, i.e. charge for
1MB but only give 512K.

Nobody else (unless someone can advise differently) appears to do an
uncontended SDSL link so we're probably back at BT with 512K SDLS 10:1
contention.

So the tricky question is - is 512K SDSL 10:1 contended "better" than
2M/256k ADSL uncontended... I'm assuming 512K SDSL 10:1 is certainly no
worse than our existing 512K/256K 20:1 BT ADSL :-)

Cheers, Rob.

Rob Nicholson <rob.nicholson@nospam_informed-direct.com> wrote:
> So the tricky question is - is 512K SDSL 10:1 contended "better" than
> 2M/256k ADSL uncontended... I'm assuming 512K SDSL 10:1 is certainly no
> worse than our existing 512K/256K 20:1 BT ADSL :-)

Stating the obvious: SDSL gives you 512k both ways, but 2m ADSL gives
you only 256k upstream. Ignoring contention.

Personally, I don't think that contention really matters unless you're
hammering your connection or you need to guarantee the availability of
a certain bandwidth. This, of course, depends on your requirements for
the link.

You were talking about email and downloads. If you have a local email
server I'd suggest you go for the 2m ADSL, since it doesn't affect your
users how long it takes to push emails out to your ISP, and it gives
you a better downloads speed. If your users talk SMTP directly to your
ISP you might be better off with the SDSL line.

If you've enough budget, consider 512k SDSL for email *and* 2m ADSL
for downloads.

Chris

> Personally, I don't think that contention really matters unless you're
> hammering your connection or you need to guarantee the availability of
> a certain bandwidth. This, of course, depends on your requirements for
> the link.

Hi - on a Citrix link you are very concerned about availability which is why
we separate out Citrix traffic from the rest. Whilst Citrix is very
bandwidth lean, you do need to worry about latency. Otherwise, users
complain about slow response etc.

Out of interest, on an ADSL line if you're downloading a full tilt, do you
still get the upload speed on top?

We've signed with Zen for the ADSL 2Mbps/256kbps service and they've just
announce availability of 8Mbps/834kbps (theoretical maximum) for just
another £5 a month - very nice! We'll use this for downloads and email.
Email speed is important but invisible unless you're waiting for something
important.

I think we'll go with the 512kbps SDSL 10:1 as it does double our upstream
speed which, with Citrix, is where you want the bandwidth.

Thanks, Rob.

Rob Nicholson wrote:
> [...]
> Out of interest, on an ADSL line if you're downloading a full tilt, do you
> still get the upload speed on top?
> [...]

"Sort of". The upload bandwidth runs on a different frequency, so it's
theoretically possibly to use both at the same time, but due to the way
TCP/IP works (for every packet - (or is it connection? I've just rubbed
some curry in my eye, so I'm a bit distracted at the moment - owwwww!).
Ahem... that you send out, you get a short reply from the recipient
saying "I got that one, thanks"), if you max out one side of the link,
the other side tends to suffer from latency quite badly, because the
responses to or from the other PC can't get through. If you're using
UDP/IP, then I'd imagine that the problem is less severe.

Another possibility, if you want to increase your upload, is bonding two
ADSL (or SDSL, if you're rich!) lines together... only a few ISPs do it
(Nildram is one, there are a couple of others but I forget who). That
turns your two ADSL lines into one virtual line, at twice the speed. You
can then set up the Quality of Service on your router to prioritise the
Citrix traffic, so if the line is busy the emails take a bit longer to
be sent, but your remote users aren't phoning you up and complaining .
Mind you, the hardware can be pretty expensive...

xF,

....Nick

Rob Nicholson <rob.nicholson@nospam_informed-direct.com> wrote:
> I think we'll go with the 512kbps SDSL 10:1 as it does double our upstream
> speed which, with Citrix, is where you want the bandwidth.

Hmm. I've just been upgraded to (residential) MaxDSL and my uplink
sync speed is now 484k, which is not really that shy of your 512:512
SDSL. (Mind you I've got almost 8m downlink sync, so I've got a pretty
good line. And I'm on 50:1 contention, not that I've noticed this over
the past three years.)

Chris

> Hmm. I've just been upgraded to (residential) MaxDSL and my uplink
> sync speed is now 484k, which is not really that shy of your 512:512
> SDSL. (Mind you I've got almost 8m downlink sync, so I've got a pretty
> good line. And I'm on 50:1 contention, not that I've noticed this over
> the past three years.)

This Zen upgrade has just arrived at possibly a good time. We're going to
get the 2Mbps service installed this week and then upgrade to the 8Mbps with
~834kbps up. We'll transfer the existing BT line across for now so we're no
worse off but then we can try some trials on the 834kbps upstream speed at
our leisure. If it works then 2 x Zen ADSL are a lot cheaper than 1 x Zen +
1 x SDSL with someone.

Cheers, Rob.