In a DS1 carrier (the kind you're using for chan. T1 or PRI), you get 1.536 Mbps
of bandwidth. That's "Mega" in the sense of 1,000,000, not that marvelous
base-2 math we all would prefer. Remember: The phone company invented this
shit. :)
When the digital signal is inverse multiplexed at your end, the switching
equipment (your PM3, MAX, T1 channel bank, dog, whatever) needs to have
some bandwidth for sending control information.
The PRI solution takes the viewpoint that having full-bandwidth channels is
a nice thing to do. Therefore, a PRI gives you 23 64k channels, and then
sacrifices one of those as a control channel (the D channel). So the pipe
looks like:
-------------------
/ \ 1 64k D channel \
| | 23 64k B channels|
\/___________________/
The "B" channels are "Bearer" channels, btw.
With a channelized T1, the phone company said, "Hey. Voice calls don't need
that much bandwidth", so they rob bits from the other channels - one of every
eight bits is used for control messages. Therefore, on a channelized T1
circuit, you get 24 lines, each of which is 56k wide.
Of course, the more observant out there might notice that 8*24 is 192, not
64k. It's also possible to share a single D channel between two PRIs to
wean one more B channel out of the group of them. Livingston has taken
the position that it's not worth the risk. *shrug* 1/46th of your
bandwidth is *not* that big deal - with most pricing that I've seen, it
comes to somewhere around $4/month that you'd be "saving" by taking the
risk of losing both channels.
The two line types use completely different control methods. The DS1 carrier
itself probably uses the same signaling methods (B8ZS is nearly universal with
major US phone carriers, etc), but they've still got to get the other
half of it done.
> You want to sell the product and we want to buy it, so you really need to
> educate us, it will help you out in the long run.
Depends. You could make a good case that anyone who's handling the physical
interface side of your ISP should have an understanding of telecommunications
concepts, as well. I think it's quite resonable for any manufacturer in
Livingston's position to assume that their audience knows what the terms
"Channelized T1" and "PRI" mean - why else would you be LOOKING at their
equipment, after all?
I suppose that there are some people who merely want a plug-and-play solution.
So be it - I believe there's a service ordering guide stuffed somewhere.
If you're not sure if you can support what you're buying, talk to an
engineer at your teleco (if you can). They should be able to tell
you "Yes, we support PRI," or "No, it's not really true that US West
likes to have people shot for talking about them on mailing lists.
What did you say your address was again?" :)
-Dave