What is T1 and
how does it work?

Since the World Wide Web excitement started, there has been a lot of talk about T1 (or T-1) data circuits, that can let an office-full of people have full-time high-speed (1.544 megabits per second) connections to the net.

T1 has been around since before the web boom, and it can also carry voices and faxes.

It allows businesses and institutions to save money by sending long distance calls directly to a long distance carrier (bypassing the local phone company), and can tie two branches of the same enterprise together for easy communication. The technology is also used between phone company facilities.

T1 uses two pairs of copper wires (four individual wires) to carry up to 24 simultaneous conversations ("channels"), that would normally need one pair of wires each. Each 64Kbit/second channel can be configured to carry voice or data traffic. Most telephone companies allow customers to buy just some of these individual channels, a service called fractional T1. Typically, fractional T1 lines are sold in increments of 56 Kbps (the extra 8 Kbps per channel is used for administration). The monthly charge for T1 service is usually about $1000, give or take a few hundred. The longer the T1 circuit, the more you pay. Installation will probably also cost about $1000, plus the price of equipment.

Each end of the T1 circuit typically uses a piece of equipment called a channel bank, that changes normal analog voice signals into digital data and multiplexes the conversations to flow together to the other end, where the data bits are then decoded and conversations are separated though a similar piece of equipment. All this complex processing happens lightning-fast, and phone calls sound just fine.

A channel bank is called a "bank"  because it can convert a bank (a bunch of similar stuff, as in "elevator bank," "snow bank," and "cloud bank") of up to 24 individual channels to a digital format, and then back to analog again.

A transmitting portion of a channel bank digitally encodes the 24 analog channels, adds signaling information into each channel, and multiplexes the digital stream onto the wires. The receiving portion reverses the process.

If the equipment on your end of the T1 circuit can handle digital signals (such as a network router or a Panasonic KX-TD1232 with a KX-TD187 module), you'll probably use a CSU/DSU ("Channel Service Unit / Data Service Unit"  or "Customer Switching Unit / Digital Switching Unit"), which is normally much less expensive than a channel bank.