Digital Telephony 101

Can the great SD folks give me some basic info on the different ways T-1 lines can be programmed for telephony, such as PR1 lines? And, please say how many channels does it allow for calling? And, which formats, if any, use robbed bit?

Also, I’d like to know some basics on the following related items:
ISDN and/or ISDN-PR1? And, in relation to ISDN, perhaps someone can explain in simple terms what is meant by a “pipe”?

And, what is meant by “wink start” and “loop start”. (What are the pros and cons?)

Any links to some primer info for the novice (that would also serve to make the explaining easier on the SD as well) would be great, too. FYI: Yes, while I have googled for such material, it is hard to find something written for the novice.

As you can see, I am just a tad bit confused by all these terms,

  • Jinx

A T1 line has a bandwidth of 1.544 Mbps.

An ISDN line has 2x 64Kbps channels and a 16Kbps channel.

When you say a T1 line has a certain bandwidth, is that dependent on how it is configured, whether as a PR1 or not? (And, pardon my total ignorance here, but does bandwidth apply to telephony…or just IT applications?)

Jinx, you’ve asked about 2 chapters-worth of questions. I’s gonna be real hard to untangle this into something understandable.

I’ll take the real high-level appraoch.

A T1 is a digital telephony standard from the 1970s. In other words, from when dinosaurs we commonplace.

It runs as a digital service at a blazing 1.554 megaBITS/sec. It runs over 4 wires from the CO to your demarc. In the old days, a hardware splitter (“interface unit”) was installed which then exposed that as 24 POTS phone lines (24 pairs = 48 wires) which you could hook ordinary phones to, or a traditional key system PBX. As far as the PBX or phone knew, it was looking at a regular single-pair-back-to-the-CO line.

later (the 80s), there came things like fractional T1s, where you could split the mighty bandwidth between data & a reduced number of hone lines.

Eventually, when the pbxs became dumb hard-coded computers in the 90s, you could connect the T1 directly to the PBX & it could do smarter things with the bandwidth, including trying to get extra bandwidth via a hack called “robbed bit”.
wink start / loop start: In days of yore (ie 1880s to 1950s) all phone lines were loop start. The CO provided DC power to one leg of the two wires to your phone (the “loop”). When your phone went off-hook, it completed the circuit and the DC flowed back to the CO. When the CO detected the closed loop, it changed into dialing mode, sent you a dial tone, etc. In other words, you closed the loop to start a call. Hence the name. Dirt simple.

But that mean the COs were pushing DC down all those dead lines to every unused phone in America. So they invented wink start.

With wink start, the CO is listening passively on the wire. The phone end is required to send a brief pulse of DC (a “wink”) to the CO. The CO interprets that as a signal to chang into dialing mode, sent you a dial tone, etc. In other words, you winked at the CO to start a call. Hence the name. Still kinda simple.

But … An ordinary phone can’t produce a wink. only a key system, PBX, or similar can produce a wink.

When you order line service from the telco, or set up a programmable T1 splitter you control, you ned to specify loop or wink start to match the needs of the downstream device it connects to. That’s all ther eis to it froma “what should I do” POV.

I’ll let som,ebody else tackel teh ISDN & the rest of the T1, T3, PRI, etc. soup.

As you get into this stuff, remember to set your technology wayback machine to about 1965. That is about how old the thinking is on all the interface stuff of the phone system. So things which seem complex or stupid are that way because they had to be built with at most 4 relays at each end, not a multi giga-transistor CPU.

Damn. Ran out of time improving my abysmal typing. I think it all still makes sense.

Some links to some basic reference material would have been easier, if they exist. Thanks for trying to keep it short!