Data circuit-terminating equipment
Data circuit-terminating equipment (DCE) provides functions necessary for the DTE to make use of a transmission facility. These include converting the DTE’s signal to the form required by the line, carrying out procedures for establishing and terminating the connection, and providing clocking to the DTE. The term DCE applies to many different types of devices for use in various types of transmission facilities.
The DCE falls into two categories based on function. The device that provides conversion from a digital source to an analog transport is known as a modulator/demodulator (modem). The device that provides conversion from one digital line code to another is commonly known as a data service unit (DSU). They are most often associated with carrier services like DS-0, DS-1, and DS-3. Another digital-to-digital DCE is the NT1 used to terminate an ISDN circuit, either BRI or PRI.
Two Wire vs. Four Wire Operation
As the word circuit implies, an electrical circuit requires a complete round-trip path. If two devices (such as two DCEs) want to communicate using signals on a circuit, then they require at least two wires between them.
A two-wire connection between DCEs means that a single electrical circuit exists between the DCEs. A four wire connection means two circuits exist. In the case of four-wire operation, the two DCEs can easily establish simultaneous two-way communication (full duplex) by agreeing on which DCE will transmit on which circuit. Thus, in the four wire example shown to the right, the DCE on the left uses the upper circuit to transmit its signals, while the DCE on the right uses the lower.
In two-wire operation, if both DCEs transmit at the same time, the resulting signal is the sum of the two inputs. If the two signals are modulated analog signals using non-overlapping frequencies (e.g., frequency division multiplexing, or FDM), both can happily coexist without interfering with each other. This is the approach used by some modems for full duplex operation over a two-wire connection (e.g., DSL. Sophisticated echo cancellation procedures can be used if the two DCEs wish to use the same set of frequencies. In this approach, the two DCEs transmit at the same time, but the receiving circuitry in each DCE subtract its own transmitted signal from the summed signal, leaving only the other DCE’s signal.
If the two signals are digital, the situation is more complicated, since digital signals cannot be confined to a particular passband. One approach is to take turns using the circuit, like a game of ping-pong; if the turns are fast enough, the two DCEs can be given the illusion that they are communicating over a full-duplex link. This approach is called time compression multiplexing (TCM) and is found on the ISDN BRI interface.
- Data circuit-terminating equipment (DCE)
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