[asterisk-biz] VoIP Provider

[Alex Balashov]

Steve Totaro wrote:
> On Tue, Jun 24, 2008 at 12:50 PM, Kevin P. Fleming <kpfleming at 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
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