Please explain what you mean about "touching the switched network."
If you dial a phone number you are accessing what we call
the PSTN, or Public Switched Telephone Network.
We usually classify services as either switched or unswitched.
A T1 can be either depending on configuration. In a switched
voice environment it could be a digital handoff, or 24 56K voice
channels, either inbound, outbound or two-way. That is not
as common now as it once was but it's out there. Signalling is
what we call "in band", that is the 8K of bandwidth above the
56K is used for signalling, e. g. call setup and teardown, timing,
etc. Very much like a set of POTS lines or trunks.
ISDN PRI is also a flavor of T1, properly referred to as DS1.
This is a channelized T1 consisting of 23 64K DS0 channels
and a dedicated D channel for signalling. Because the signalling
is routed over a separate channel it's referred to as "out of band,"
you get the whole 64K of each channel for either voice or data.
There's more to it than that but that's the basics as far as switched
telephony over T1/DS1 level carrier.
Do you mean that T1 can be used for both data and POTS?
Well, you're not going to be able to make a phone call (other
than a VOIP call) over a data only T1. It depends on how your
T1 is designed. I consider "T1" to be too broad a term for
specific use. It's really a descriptor of an interface level, not an
application level.
Would using T1 for POTS save any money?
If the T1 costs $500 per month (a wild guess without actually
looking it up) and you get 24 channels, that's $21.00 per month
per channel. Which is about the cost of a business POTS line.
If T1 costs 10 times more than DSL or FIOS do I really want to
make any effort to get T1..?
I'd say not, unless you have a mission critical data or voice
application that's going to benefit from the Service Level
Agreement.
If you need a significantly higher level of bandwidth you are not
shopping for T1s.
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