Wikipedia has the below information about T1's -

Digital signal 1 (DS1, also known as T1, sometimes "DS-1") is a T-Carrier signaling scheme devised by Bell Labs.  DS1 is a widely used standard in telecommunications in North America and Japan to transmit voice and data between devices. E1 is used in place of T1 outside of North America, Japan, and South Korea. Technically, DS1 is the logical bit pattern used over a physical T1 line; however, the terms "DS1" and "T1" are often used interchangeably.

A DS1 circuit is made up of twenty-four 8-bit (also known as timeslots or DS0s, each channel being a 64 kbit/s DSO multiplexed carrier circuit. A DS1 is also a full duplex circuit, which means the circuit transmits and receives 1.544 Mbit/s concurrently. A total of 1.536 Mbit/s of bandwidth is achieved by sampling each of the twenty-four 8-bit DS0s 8000 times per second. This sampling is referred to as 8-kHz sampling. An additional 8 kbit/s of overhead is obtained from the placement of one framing bit, for a total of 1.544 Mbit/s.

Before the jump in Internet traffic in the mid 1990s, DS1s were found mostly in larger businesses and telephone company central offices as a means to transport voice traffic between locations. DS1s have been and still are the primary way cellular phone carriers connect their central office switches (MSCs) to the cell sites deployed throughout a city.

Today, many smaller companies often use an entire DS1 for Internet traffic, providing 1.544 Mbit/s of shareable synchronous connectivity (allowing for 1.536 Mbit/s of usable traffic, and 8 kbit/s of framing overhead). However, DS1 can be ordered as a channelized circuit, and any number of channels can be reserved for non-data (for example, voice) traffic.

Many radio stations also use this technology in their broadcasting. A T1 telephone line can be used as a link to convey the broadcast audio from the studio to the transmitter/tower site, a distance that can be quite a few miles in length. T1-based solutions, as opposed to IP-based, remain very attractive to broadcasters because the data is transported in effective real-time.

Additionally, for voice T1s there are two types: so-called "plain" or Inband T1s and PRI (Primary Rate Interface). While both carry voice telephone calls in similar fashion, PRIs are commonly used in call centers and provide not only the 23 actual usable telephone lines (Known as "B" channels) but also a 24th line that carries signaling information (Known as the "D" channel for Data.) This special "D" channel carries: Caller ID (CID) and Automatic Number Identification (ANI) data (commonly referred to in industry parlance as "signaling data"), required channel type (usually a B channel), call handle, DNIS info, requested channel number and a request for response.

Inband T1s are also capable of carrying CID and ANI information if they are configured by the carrier to do so but PRI's handle this as a standard and thus the PRI's CID and ANI information has a much better chance of getting through to the destination. While an Inband T1 seemingly has a slight advantage due to 24 lines being available to make calls (as opposed to a PRI that has 23), each channel in an Inband T1 must perform its own set up and tear-down of each call. A PRI uses the 24th channel as a data channel to perform all the overhead operations of the other 23 channels (including CID and ANI). So even though an Inband T1 has 24 channels, the 23 channel PRI can actually dial more calls faster because of the dedicated 24th data (also called "D") signaling channel.

The name T1 came from the carrier letter assigned by AT&T to the technology. Essentially, the "T" is a part number that was assigned by AT&T. Just as there is the generally knownL-carrier and N-carrier systems, T-carrier was next letter available and T1 is the first level in the hierarchy. DS-1 meant "Digital Service - Level 1", and had to do with the service to be sent (originally 24 digitized voice channels over the T1). The terms T1 and DS1 have become synonymous and include a plethora of different services from voice to data to clear-channel pipes. The line speed is always consistent at 1.544 Mbit/s, but the payload can vary greatly.

Related Topics:

T-carrier, E-carrier, Time-division multiple access, Pulse-code modulation, Federal Standard 1037C, DS1 Encoding schemes: B8ZS, HDB3, AMI, Line code, Time-division multiplexing,  Multiplexing, Physical layer, Data frame, Quantization (signal processing),  Digital Signal 3, Digital Signal 0

(For more information see http://en.wikipedia.org/ and search for Digital_Signal_1 or T1)