Multiplexing

Every computer has a port to connect a communication line or cable from a peripheral device. Computers have limited number of ports, so multiplexers are used. They combine signals from multiple terminals and put them all on a single communications line. This increases the efficiency of communication and is cost-effective. The communication line is divided into different sections and the data from each terminal is coded and sent and then received, decoded and directed to the correct section from where it came from. There are four commonly used types of multiplexing, the first is (FDM) frequency division multiplexing. FDM divides the bandwidth of the communication line so that each terminal can transmit data. The individual terminals are separated from guardbands to ensure the signals do not interfere with each other. Multiplexers often have external modems or internal modems built in to convert the data to analog before transmission takes place. The local cable television companies use FDM to send multiple stations over the same line at once.

Another method for combining data on a transmission line is time division multiplexing, instead of dividing frequencies to transmit signals, TDM divides the transmission line into time segments. Each terminal is assigned a fixed time slot on a rotation. When the terminals time slot appears it receives the full transmissions capacity for that line. TDM is like packet switching except the time slots are pre-allocated. Instead of using gaurdbands to separate signals TDM uses guard times. If a terminal has no data to transmit at its specific time, the line remains idle. In TDM terminals rarely transmit continuous data which means some sections of the line are almost always idle. In STDM (statistical time division multiplexing) the same concepts apply except if the terminal doesn't have any information to share it's time slot will not be created and will move on to the next available terminal, producing less down time. STDM is driven by data, they also perform more operations than TDM. They provide buffers, and can compress data and report statistics, support digital data, and modem capabilities.

The first wavelength division multiplexing system combined two signals and was created 1985, modern systems can handle 160 signals. Telecommunications companies like this technique because they can easily upgrade there lines without having to overhaul the network backbone. It uses same idea of