Transmission modes and data-flow direction
In communication between two devices, the direction of information flow is a basic property of the link. It determines whether data moves in one direction, in both directions alternately, or in both directions simultaneously. There are three main transmission modes.
1. Simplex transmission
is a mode with one preassigned direction of data flow. Information is transferred from the transmitting device to the receiving device. The transmitter and receiver roles remain assigned to the two ends of the link.
Examples of simplex communication
- Radio and TV broadcasting: A central station sends a signal, and home receivers receive it. The radio receiver channel does not provide a path for sending a signal back to the station.
- Computer and monitor: The computer sends an image to the monitor. In the classic simplex model, the monitor does not return user data, although modern interfaces may provide a separate return channel.
- GPS: Satellites transmit signals to GPS receivers on Earth, which receive them without a reply on the GPS channel. A car GPS receiver does not transmit to satellites through that system.
2. Half-duplex transmission
mode allows transmission in both directions with alternating access. Both devices can transmit and receive, but only one of them transmits at a given moment.
Examples of half-duplex communication
- Walkie-talkies and CB radio: One person must press a button to talk, and the other person can only listen. To respond, the second person must wait for the first to finish and release the button. Saying "Over" signifies the end of a transmission to let the other person know it's their turn to speak.
3. Full-duplex transmission
mode allows both devices to transmit and receive simultaneously. Data can therefore flow in both directions without switching the transmission direction.
Examples of full-duplex communication
- Telephone Conversation: Both parties can speak and hear at the same time, allowing for interruptions and a natural flow of conversation.
- Full-duplex Ethernet networks: A computer connected to a switch can send and receive packets simultaneously. Independent traffic in both directions increases throughput compared with half-duplex operation.
Techniques for bidirectional transmission
Simultaneous transmission in both directions requires separating the directions in space, frequency, or time. The technique depends on the medium:
- Two physical channels: Separate physical paths can carry the two directions, such as two pairs of wires in a 100BASE-TX link, one for sending and one for receiving.
- Frequency-division duplex (FDD): The available band is split into two sub-bands. One carries transmission and the other reception, allowing both directions at the same time. This is common in mobile phone systems.
- Time-division duplex (TDD): Transmission time is divided into short, alternating slots. One slot carries transmission and the next carries reception. TDD provides both directions, but the directions use the medium in turn, so this mode is half-duplex.
- Echo cancellation: A technique for full-duplex operation over a single wire pair, used for example on DSL lines. In systems with overlapping transmit and receive spectra, the device transmits and receives on the same frequency at the same time. A digital signal processor (DSP) estimates the echo of its own transmitted signal and subtracts it from the received signal, leaving the component from the other end.