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

Re: ADSL and SDSL?

Robert Bonomi (
Fri, 18 Feb 2005 09:50:43 -0000

In article <>, Dean
<> wrote:

> Robert Bonomi wrote:

>> In article <>,
>> T. Sean Weintz <> wrote:

>>> Robert Anderson wrote:

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

>>>> For example, is the CIR higher with SDSL?

>>>> 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.

>>>> Robert Anderson

>>> ADSL is asymmetrical

>> *not*necessarily*

>>> SDSL is symetrical

>> *not*necessarily*

>> As "commonly deployed", SDSL has same speed up and down, and ADSL has
>> mis-matched (almost invariably higher in the 'down' direction) speeds.

>> However, I have had "symmetric rate" ADSL, and "Asymmetric rate" SDSL,
>> service, at various times, from various providers. Not terribly
>> common, but such services do/did exist. :)

> Can you elaborate? I thought the 'A' in ADSL and the 'S' in SDSL stood
> for asymmetrical and symmetrical respectively. What you're describing
> seems to negate that doesn't it? What am I missing?

Your understanding of the _names_ underlying the acronyms is correct.

The names reference two different, incompatible, implementations of
technology that fall under the broad categorization of 'DSL'.

The names do -not- dictate the actual link speeds in either direction.
Nor the 'relationship' between them.

I don't know enough about the 'deep innards' of the specific
technologies to discuss differences in detail. I'm _guessing_ that
the names refer to the allocation of the available spectrum-space to
the signals in each direction, Now, obviously, the 'allocation' puts
an _upper_limit_ on the data-transmission rate, *BUT* nothing says
that you have to be using the maximum rate within that allocation.

I know SDSL _does_ use the 'voice spectrum' bandwidth, and thus
_cannot_ be shared on the same wire-pair with POTS voice service.
ADSL runs totally at 'supersonic' frequencies, and thus can use
'shared' wiring. DSL ("low-pass") 'filters' are needed for shared
lines to prevent 'non-linearities' (amplifiers, other bi-metallic
junctions, etc.) from generating 'noise' in the voice spectrum, due to
the supersonics.

I *do* know what service I have actually had.

1) 512kbit up / 512kbit down. on a non-shared line.
CPE was a "Cisco 678" ADSL modem.
(I'm using that _same_ modem on a 1.5mbit down/ 768k up 'shared'
circuit presently.)

2) 768k down / 384k up, on a non-shared line.
CPE was an Efficient Networks 'SpeedStream 5150" *SDSL* modem.
(*not* a "5160", which is an identical-looking ADSL unit. :)

You can "google" up the specs of those specific modems to confirm the
which type of DSL are used for.

I had numerous occasions where I saturated those links for extended
periods, both upstream and downstream (at different times), to
_nearby_ end-points with "big" pipes -- the speed figures above stand
'confirmed by actual usage measurements'.

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