T1/E1 and fractional T1 are two kinds of valid connection alternatives.
A T1 connection bundles together twenty-four 64 Kbps (DS0) time-division multiplexed (TDM) channels over 4-wire copper circuit. This creates a total bandwidth of 1.544 Mbps. An E1 circuit in Europe and other parts of the world bundles together thirty-two 64-Kbps channels for a total of 2.048 Mbps. TDM allows multiple users to share a digital transmission medium by using pre-allocated time slots. Many digital private branch exchanges (PBX) take advantage of T1 service to import multiple call circuits over one T1 line instead of having 24 wire pairs routed between the PBX and telephone company. It is important to note that T1 can be shared between voice and data. A telephone service may come over a subset of the 24 channels of a T1 link, for instance, leaving remaining channels for Internet connectivity. A T1 multiplexer device is needed to manage the 24 DS0 channels when a T1 trunk is shared between multiple services. For a single data-only connection, the circuit can be run unchannelized (no TDM is performed on the signal). Consequently, a simpler CSU/DSU device can be used. Typically, you can connect to a T1/E1 CSU/DSU or multiplexer over V.35 or RS 449 serial interface with synchronous protocol at rates at a multiple of 64 Kbps to 1.544 Mbps or 2.048 Mbps. The CSU/DSU or multiplexer provides the clocking in the network.
With Fractional T1 (FT1), a customer can lease any 64-Kbps sub-multiple of a T1 line. FT1 is useful whenever the cost of dedicated T1 will be prohibitive for the actual bandwidth customer uses. With FT1 you pay only for what you need. Additionally, FT1 has the following feature that is unavailable with a full T1 circuit: Multiplexing DS0 channels at the central office of the telephone company. The remote end of an FT1 circuit is at a Digital Access Cross-Connect Switch that is maintained by the telephone company. Systems that share the same digital switch can switch among DS0 channels. This scheme is popular with ISPs that use a single T1 trunk from their location to the digital switch of a telephone company. In these cases, multiple clients can be served with FT1 service. Typically, you can connect to a T1/E1 CSU/DSU or multiplexer over V.35 or RS 449 serial interface with synchronous protocol at some multiple of 64 Kbps. With FT1, you are pre-allocated a subset of the 24 channels. The T1 multiplexer must be configured to fill only the time slots that are assigned for your service.