> On Tue, Jun 24, 2008 at 12:50 PM, Kevin P. Fleming <kpfleming@digium.com> wrote:
>> Trixter aka Bret McDanel wrote:
>>
>>> I think there is a symantic difference between T3 and DS3, although I
>>> could be wrong. I think that a T3 uses the T1 and T2 framing bits for
>>> data, why a T1 is 1.544Mbps not 1.536Mbps. Where a DS3 uses them for
>>> framing.
>>
>> DS3 is the definition of how to frame and combine 28 DS1s into a single
>> stream of bits; T3 is the definition of how to transport that over
>> copper cabling. DS3 can be transported over other mechanisms as well
>> (for example, three of them can be bundled together into a SONET OC3).
>>
>> --
>> Kevin P. Fleming
>> Director of Software Technologies
>> Digium, Inc. - "The Genuine Asterisk Experience" (TM)
>>
>
> Getting totally OT
>
> Thanks for the clarification but I am still not clear.
>
> So what is a DS1 then? Is the usage of the term DS1 synonymous with T1?
>
> It would seem if someone is speaking of T1s then they should also use
> T3, if they are speaking of DS1 then DS3 would be the proper naming
> convention.
DSn is an abstract specification of things common to the low-level data
representation characteristics (i.e. frame definition) associated with
certain signals. That's why it stands for "Digital Signal" -- it
defines the signal, as such, per se.
Tn is a specification for how to transmit that signal down a particular
physical medium - in the case of T-carrier, a certain type of copper wire.
So, a DS1, by definition, cannot actually refer to any kind of physical
facility; you can't "order a DS1." There is no such tangible, palpable
entity that would bear that name, in a technically accurate sense. DS1
does not refer to an electrical circuit, or an optical fibers, or a
system or span of interconnected circuits or fibers. T1 is the term for
an electrical copper facility to deliver a DS1 signal standalone.
Confusion exists because although physical T1s tend to be called T1s,
physical DS3 circuits tend to be called DS3s, when in fact, they are
rightfully T3s if they are built over copper and DS3s if built over
something else, such as an STS-1 carrier signal over SONET. If single
DS3s were delivered over a physical SONET fiber facility, the
appropriate correspondent term to a T3 would be an OC-1, although you
can't really get < OC-3, so that's somewhat moot. If a DS3 signal could
be delivered over shortwave radio, perhaps it'd be called a SHRAD-3.
Who knows.
Confusion also exists because often, DS1s that exist purely in "virtual"
form as pure channelised multiplexing entities are referred to as "T1s"
even though no copper T1 facility is involved. For instance, ISPs that
provide T1 services often get them delivered wholesale on channelised
DS3 loops from the telco and terminate them straight into a router with
a channelised T3 card, so no actual T1s ever see the light of day
demuxed except virtually within a set of circuitry. Yet they are fond
of saying that "we've got T1s coming in on this (DS3|T3)." They don't,
really.
In reality, "we ordered a DS3" should not have much more semantic
content and validity than "we ordered a DS1."
The reason this distinction is particularly significant is because of
the role of these signals as channelised components in a higher-order
multiplexing hierarchy. For instance, when a CLEC interconnects with
the ILEC and turns up some fiber meet of OC-n size, the constituent DS3s
and their constituent DS1s become an important factor in the arrangement
of trunk groups, TCICs, SS7 links, etc. Yet none of these DS1s or DS3s
ever leave the fiber muxes, DACSs, switches, gateways, etc. among which
they exist merely as carrier waves multiplexed into a higher-order
signal. Considerable confusion could be introduced by suggesting, "We
run 16 T1s from our DACS to our main switch," when in fact, there is a
short channelised DS3 cross-connect that has 16 bearer DS1 channels
turned up.
-- Alex
--
Alex Balashov
Evariste Systems
Web : http://www.evaristesys.com/
Tel : (+1) (678) 954-0670
Direct : (+1) (678) 954-0671
Mobile : (+1) (706) 338-8599
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