13.1.3 Full-Duplex Systems
As illustrated in Figure 13.6, a full-duplex communication system carries information simultaneously in both directions. Each terminal can transmit and receive continuously without waiting for the other terminal to finish transmitting. Unlike half-duplex systems, no channel turn-taking is required because separate communication channels are provided for each transmission direction.
The two directions of transmission do not necessarily use identical physical media. A full-duplex connection may employ separate wire pairs for transmit and receive, different radio frequencies, different optical wavelengths, separate time slots, or independent spatial paths. The essential characteristic is not the physical implementation, but that simultaneous communication is possible in both directions.
Historically, many wireline communication systems achieved full-duplex operation by using two separate pairs of conductors (a four-wire circuit), while some radio systems employed one radio frequency for each transmission direction. Modern digital communication systems may instead achieve full-duplex operation using techniques such as Frequency Division Duplex (FDD), Time Division Duplex (TDD), wavelength-division multiplexing in optical fibre, or sophisticated echo-cancellation techniques that allow simultaneous transmission and reception over a shared physical medium.
Full-duplex communication is required whenever both ends of the link must exchange information continuously or with minimal delay. Typical examples include telephone systems, Ethernet networks, optical fibre links, microwave radio systems, and many satellite communication links. Because each direction of transmission is always available, interactive applications such as voice, video conferencing, remote control, and high-speed computer networking operate much more naturally than over half-duplex links.
Unlike half-duplex communication, full-duplex systems require no protocol to determine which terminal may transmit at any given instant because both terminals may transmit whenever information is available. Higher-layer communication protocols remain necessary, however, to perform functions such as framing, addressing, sequencing, flow control, congestion management, and error recovery.
The principal advantage of full-duplex operation is improved communication efficiency. Because neither terminal waits for the channel to become available, transmission delay is reduced and the available capacity can be utilised continuously in both directions. This makes full-duplex particularly well suited to applications requiring high throughput or frequent exchanges of short messages.
The principal disadvantage is that full-duplex operation generally requires additional communication resources. Separate transmission paths must be provided for each direction of communication, whether by using additional conductors, different radio frequencies, separate time slots, optical wavelengths, or more sophisticated signal-processing techniques. Consequently, full-duplex systems often involve greater infrastructure complexity and higher implementation cost than equivalent half-duplex systems.
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