Serial Transmission - In serial transmission, bits are sent sequentially on the same channel (wire) which reduces costs for wire but also slows the speed of transmission. Also, for serial transmission, some overhead time is needed since bits must be assembled and sent as a unit and then disassembled at the receiver.
Parallel Transmission - In parallel transmission, multiple bits (usually 8 bits or a byte/character) are sent simultaneously on different channels (wires, frequency channels) within the same cable, or radio path, and synchronized to a clock. Parallel devices have a wider data bus than serial devices and can therefore transfer data in words of one or more bytes at a time. As a result, there is a speedup in parallel transmission bit rate over serial transmission bit rate.
The disadvantage of parallel transmission is that it is very expensive, as it requires several wires for both sending, as well as receiving equipment. Secondly, it demands extraordinary accuracy that cannot be guaranteed over long distances.
Simplex Operation - In simplex operation, a network cable or communications channel can send information in only one direction; it’s a one-way street. For example, a radio station usually sends signals to the audience but never receives signals from them, thus a radio station is a simplex channel. The good part of simplex mode is that its entire bandwidth can be used during the transmission.
Half-Duplex Operation - In half-duplex operation information can be send in both directions between two nodes, but only one direction at a time. At a certain point, it is actually a simplex channel whose transmission direction can be switched. Walkie-talkie is a typical half duplex device. It has a “push-to-talk” button which can be used to turn on the transmitter but turn off the receiver. Therefore, once you push the button, you cannot hear the person you are talking to but your partner can hear you. An advantage of half-duplex is that the single track is cheaper than the double tracks.
Full-Duplex Operation - In full-duplex operation, a connection between two devices is capable of sending data in both directions simultaneously. Full-duplex channels can be constructed either as a pair of simplex links (as described earlier) or by using one channel that’s designed to permit bidirectional simultaneous transmissions. Take telephone as an example, people at both ends of a call can speak and be heard by each other at the same time because there are two communication paths between them. Thus, using the full duplex mode can greatly increase the efficiency of communication.
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