Packet Switching Types: ATM, Frame Relay, TCP/IP, X.25

Transmission: SONET T-Carrier

Services: [3G] [4G] [Bluetooth] [I-Mode] [WAP] [Wireless and packet switching]

T-Carrier

Ah, T-Carrier, the first time division multiplexing scheme in the public switched telephone network. Huh? To explain briefly what I've written about in much greater depth internal link), transmission means sending information, intelligence, from one point to another. If twisted pair and cable are the tracks that information rides on, transmission is the train that carries the information. It's how we send information, how we package it, the kind of train we build, that makes up the different transmission schemes. This can be simple or complicated.

Excellent writing on T-Carrier by Regis Bates. More detailed than the discussion by Russell listed below (14 pages, 273K in .pdf)

The simplest telephone system is an intercom. Communication from one room to the next. Nothing fancy required. Pick up a phone and its transmitter, a microphone, modulates or varies the current in response to your speech. This electrical signal is then passed on a wire to the next phone. The fluctuating current so passed causes the receiver, a miniature speaker, in the other telephone to vibrate, moving its diaphragm, reproducing sound. This is analog transmission, a long, continuous train with a single box car of great length.

In time division multiplexing we go digital, chopping up several conversations into bits, 0s and 1s. We then pass parts of each call along, one part at a time. It's like filling a boxcar, one after another, with bits from each call. Part one of call one goes into boxcar one, part one of call two goes into boxcar two, and so on. Thus the calls are divided by time. This lets us use a single track, a single telephone line, to carry several calls at once, making for great efficiency. Using just four wires, two twisted pairs, 24 telephone calls can be carried at once.

Once used just between telephone switching offices, T-carrier has been long been used between businesses and telephone company switching equipment. A company PBX or an internet service provider may both use T-1 to send their voice or data traffic to the public switched telephone network. Designed for voice signals over noise inducing copper cables, T-carrier is now being replaced by the data oriented, fiber optic based SONET.

T-1 uses a confusing terminology. T-1's bandwidth is 1.54Mbps. 24 channels fit into that bandwidth, each being 64Kbps wide. T-carrier calls those individual channels a DS0. Twenty four channels or DS0s make up what's called the DS1, or digital signal one, sometimes called level one. Bundles of DS0s, 48, 96, 672, and so on, make up higher levels of increasing bandwidth. It's what's called a digital signal hierachy and you really need a table to keep it all straight. I've put one below from Jade Clayton's McGraw-Hill Illustrated Telecom Dictionary (external link to Amazon.com) OR more information on the book from Tom Farley.

Note how SONET, designed for fiber optic cable, can carry far more information than T-Carrier. Click here to go to the SONET page --->

Russell explains why the present telephone network is configured the way it is, reviews T-Carrier, and introduces another transmission scheme, SONET. (11 pages, 275K, .in .pdf)

Ordering information for the book this file is from: Signaling System #7 by Travis Russell, McGraw Hill is here; I am puzzled by the reviews at Amazon on this book. I think this selection is quite clear, moves logically, and is well written. What am I missing?

Packet Switching Types: ATM, Frame Relay, TCP/IP, X.25

Transmission: SONET T-Carrier

Services: [3G] [4G] [Bluetooth] [I-Mode] [WAP] [Wireless and packet switching]