The bones of contention...

Now I know what contention is, but what I do want to know is the
following:

1. Why do BT Wholesale use 50:1 and 20:1 for ADSL? The higher is for
single-user connections whereas the lower for small networks - but
does anyone know why these particular values? Any BTW or DSL
engineers out there able to enlighten me?

2. Now I use to think that say a 50:1 512k service would be shared
between 50 users at the DSLAM resulting in a minimum bandwidth of 10k
a sec! I'm now somewhat better-off knowledge wise but I'd like to
know how the backhaul is configured: is the line speed set by the
DSLAM operator (ie: BTW in most cases) then the ISP purchases a
virtual path dependent on the number of subscribers? If so, what the
rule of thumb or how do they work out the capacity required?

For example, if an ISP has 50 512k customers from one serving
exchange, how much bandwidth do they have to have going back to their
POP?

I'm a bit confused to say the least... also if ISP's marketed this
information then maybe they'd attract the more discerning customer.

"Ben" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed) om
> Now I know what contention is, but what I do want to know is the
> following:
>
> 1. Why do BT Wholesale use 50:1 and 20:1 for ADSL? The higher is for
> single-user connections whereas the lower for small networks - but
> does anyone know why these particular values? Any BTW or DSL
> engineers out there able to enlighten me?

BT Wholesale carried out various trials and looked at contention ratio's
abroad and decided that 50:1 and 20:1 were appropiate for the UK.

When introduced, BT Wholesale wasn't 100% sure, so their was a vague promise
to review these if the ratio's were too high.

In practice, BT Wholesale seem to operate the 50:1 service at closer to 20:1
and the 20:1 service at around 10:1.

> 2. Now I use to think that say a 50:1 512k service would be shared
> between 50 users at the DSLAM resulting in a minimum bandwidth of 10k
> a sec! I'm now somewhat better-off knowledge wise but I'd like to
> know how the backhaul is configured: is the line speed set by the
> DSLAM operator (ie: BTW in most cases) then the ISP purchases a
> virtual path dependent on the number of subscribers? If so, what the
> rule of thumb or how do they work out the capacity required?

Line speed is set at the DSLAM by BT Wholesale.

Most ISP's use BT IPStream to provide the backhaul from exchanges - that
means BT Wholesale "look after" the contention through their network - until
it reaches the ISP.

If an ISP uses DataStream instead (there are not many of these now) of
IPStream, then they(or the telco they use i.e. not BT) get to choose the
size of virtual paths from exchanges themselves - using their own
"confidential" planning rules.

I have previously posted how DataStream works on this newsgroup - you'd have
to google for a post..

> For example, if an ISP has 50 512k customers from one serving
> exchange, how much bandwidth do they have to have going back to their
> POP?

Its not really possible to say as its commercially sensitive.

Regards
Sunil

On 6 Oct 2004 10:54:22 -0700, (E-Mail Removed) (Ben) wrote:

>Now I know what contention is, but what I do want to know is the
>following:
>
>1. Why do BT Wholesale use 50:1 and 20:1 for ADSL? The higher is for
>single-user connections whereas the lower for small networks - but
>does anyone know why these particular values? Any BTW or DSL
>engineers out there able to enlighten me?

Several years ago I had a meeting with a senior BT IP engineer, just
prior to ADSL being launched in the UK, and he told me that these
figures were, at the time, fairly arbitrary, but based on anticipated
usage and cost of the network.

>
>2. Now I use to think that say a 50:1 512k service would be shared
>between 50 users at the DSLAM resulting in a minimum bandwidth of 10k
>a sec! I'm now somewhat better-off knowledge wise but I'd like to
>know how the backhaul is configured: is the line speed set by the
>DSLAM operator (ie: BTW in most cases) then the ISP purchases a
>virtual path dependent on the number of subscribers? If so, what the
>rule of thumb or how do they work out the capacity required?

IIRC the contention is set at the ATM level. ATM is the networking
protocol that encapsulates the IP traffic on the WAN side of your ADSL
router, so in effect it's configured in software rather than by the
number of users on a particular exchange. If BT decided to change the
contention for any reason then they would do this by changing the
parameters of the ATM configuration, not by making any physical
alterations at local exchange level.

>
>For example, if an ISP has 50 512k customers from one serving
>exchange, how much bandwidth do they have to have going back to their
>POP?
>

ISPs (those on IP Stream, anyway) don't have to be concerned about how
many customers are connected to a particular serving exchange. They
purchase a single (or sometimes multiple) connection(s) from BT,
called BT Central, and all the aggregated end-user connections are
delivered over that. The size of the BT Central determines how many
concurrent sessions they can terminate. For example, an ISP with a
622Mbps BT Central can handle around 25,000(ish) concurrent sessions
(or could be 27,500, I can't remember), which can be any mixture of
the home and office products.

Some ISPs will accept more customers than their BT Central can handle
on the assumption that not everybody will want to connect at the same
time. The extent to which they're prepared to stretch this
oversubscription margin varies from ISP to ISP.

>I'm a bit confused to say the least... also if ISP's marketed this
>information then maybe they'd attract the more discerning customer.

Hope the above helps.

Jake