T1 - Digital Signal 1
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 transmission protocol used over a physical T1 line; however, the terms "DS1" and "T1" are often used interchangeably. Also, the 24 channels of traffic in a T1 line are sometimes called a T-span.
A DS1 circuit is made up of twenty-four 8-bit channels (also known as timeslots or DS0's), each channel being a 64 kbit/s DS0 multiplexed pseudo-circuit. A DS1 is also a full-duplex circuit, meaning, in theory, the circuit can send 1.544 Mbit/s and receive 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 DS0's 8000 times per second.
Inband T1 versus T1 PRI
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, required channel type, 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 PRI can actually dial more calls faster because of the dedicated data channel.
PRI vs. BRI
In the Integrated Services Digital Network (ISDN), there are two levels of service: the Basic Rate Interface (BRI), intended for the home and small enterprise, and the Primary Rate Interface (PRI), for larger users. Both rates include a number of B-channels and a D-channel. Each B-channel carries data, voice, and other services. The D-channel carries control and signaling information. The Basic Rate Interface consists of two 64 kbit/s B-channels and one 16 kbit/s D-channel. Thus, a Basic Rate Interface user can have up to 128 kbit/s service. The Primary Rate Interface consists of 23 B-channels and one 64 kbit/s D-channel using a T1 line ( standard of North America ) or 30 B-channels and one D-channel using an E1 line( standard of Europe ). Thus, a Primary Rate Interface user on a T1 line can have up to 1.544 Mbit/s service or up to 2.048 Mbit/s service on an E1 line. PRI uses the Q.931 protocol over the D-channel.
Fractional T1
Fewer active B channels (also called user channels) can be used for a fractional T1. More channels can be used with more T1s, within certain design limits.
Application
The Primary Rate Interface channels are typically used by medium to large enterprises with digital PBXs to provide them digital access to the Public Switched Telephone Network (PSTN) and to the Digital Switched Network. The 23 (or 30) B-channels can be used flexibly and reassigned when necessary to meet special needs such as videoconferences. The Primary Rate user is hooked up directly to the telephone company central office.
