TELECOM Digest OnLine - Sorted: Re: ADSL and SDSL?

Re: ADSL and SDSL?

Robert Bonomi (
Tue, 15 Feb 2005 23:50:42 -0000

In article <>, Robert Anderson
<> wrote:

> Do you more reliably get the higher bandwidth with SDSL than with

There is no inherent superiority in either of the technologies.

> For example, is the CIR higher with SDSL?

*WHAT* CIR? <grin>

> We are a very small business using VoIP and our connection is
> ADSL. People have problems hearing us but not the other way around.

> We were thinking of switching from ADSL over to SDSL, to see if that
> helps.

There are a whole sh*tload of issues, some 'technical', but *most* are
'administrative, that affect the situation.

ADSL vs. SDSL affects only the connection between the customer
premises and the DSLAM (at the telco C.O.) -- and obviously, the
specific equipment used at each of those two places. If the CPE and
the DSLAM 'sync up' at a specified data-rate, whatever that rate is,
you _will_ have that speed of transmission *between*those*two*points*.
What you will get, going any further than that, is *anybody's* GUESS.
Emphasis on the word "guess". It's simply not possible to predict in
advance, nor is an end-to-end throughput at one point in time any sort
of a guarantee that you will be able to get anything approaching that
same throughput (between those *same* endpoints) at any other time.
This is the "nature" of the Internet -- performance is 'statistical',
at best.

Separately, there is the matter of the speed of the 'upstream' link
from the DSLAM, and the number of users (and at what speeds) sharing
that uplink. The 'uplink' is *almost*always* "over-subscribed" --
i.e. the aggregate bandwidth for the 'sold' DSL lines is _much_ higher
than the bandwidth of the upstream link. This "works" because it is
*very*rare* for all the end- users to be *trying* to use all their
bandwidth at the same time.

Similarly, there is 'yet another layer' of 'over-subscribing', with
regard to the DSLAM uplinks vs. the size of the pipe that the provider
has "to the outside world".

Note: The over-all situation is *exactly* similar with a 'cable modem'
connection. The 'details' are slightly different -- in that the
'over-subscribing' (and 'contention' for access) occurs one stage
earlier in the process. The 'uplink' from the cable-modem is a
'shared' resource, with all the users on the same run of cable
competing for the limited bandwidth available on the cable. The
cable can support _maybe_ a dozen or two users at the full advertised
'up to' download speed. With several _hundred_ to possibly a few
*thousand* customers sharing a single cable, the reason one =rarely=
gets anything approaching the advertised maximum rate is *not* hard
to understand. <wry grin>

DSL is commonly sold in a couple of different "grades". Commonly
referred to as "consumer class", and "business class". The underlying
technology (ADSL, SDSL) is relatively inconsequential to the class of
service, or to the pricing.

The differences include:

The degree of 'over-subscribing' the uplink -- "business class"
service generally gets a much _lower_ degree of
over-subscription; thus you have a *much* better chance of
actually getting the claimed bandwidth (or close to it). A
lower 'over subscription' ratio also translates into lower
latency (from queuing), and fewer dropped packets.

The level of 'monitoring', and/or proactive preventative
maintenance involved. i.e., "catching things *before* they
fail". Also, watching the traffic levels, and (potentially)
moving customers between racks so as to reduce the amount of

The _timeliness_ of repairs, if the circuit fails. "consumer
grade" service usually has *NO* 'guaranteed' repair time,
whatsoever. An outage can go for *weeks*, literally.
"Business" DSL may have a repair time "promise" of 'a couple of
days'. Which just means that one *cannot* get the problem
'escalated' before that interval has elapsed; and only if you,
the customer *complain* about it still being down. OTOH, true
"T-1" service has a _4-hour_ window, after which *automatic*
escalation occurs. (The proverbial "you get what you pay for",
applies to repairs on data circuits -- with a *vengeance* :)

Note: "People have problems hearing us" is simply NOT a good enough
statement of the problems you are experiencing, for _anyone_ to
*guess* at what's going on.

Do you mean:
1) insufficient audio level?
2) sufficient level, but momentary 'gaps' in the audio?
3) sufficient level, but 'unintelligible'?
4) "something else"?

And, of course, there is the "incidental" question of how much
_upstream_ bandwidth you have on your ADSL connection, and how many
simultaneous calls you're trying to make, Overloading the uplink is a
*great* way to end up suffering from dropout gaps and other
intelligibility issues.

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