Simplex:The signal may travel in only one direction, (one way or unidirectional communication).Football coach using a megaphone to shout instructions, voice travels onlyone-way.Half duplex:signals may travel in both directions over a medium but in only one direction at a time.Walkie-talkieFull duplex (aka duplex or bidirectional transmission):signals are free to travel in both directions over a medium simultaneously.
Telephone conversation, both people can talk simultaneously.Channel: A distinct communication path between nodes, much as a lane is a distinct transportation path on a freewayMultiplexingMultiplexing:A form of transmission that allows multiple signals to travel simultaneously over one mediumTo carry multiple signals, the medium’s channel is logically separated into multiple smaller channels, or sub-channels.For each type of multiplexing, a device that can combine many signals on a channel, a multiplexer (mux),is required at the transmitting end of the channel.At the receiving end, a demultiplexer (demux)separates the combined signals and regenerates them in their original form.Networks rely on multiplexing to increase the amount of data that can be transmitted in a given time span over a given bandwidth.TDM (time division multiplexing),divides a channel into multiple intervals of time, or time slots. It then assigns a separate time slot to every node on the net- work and, in that time slot, carries data from that node.Statistical multiplexingis similar to time division multiplexing, but rather than assigning a separate slot to each node in succession, the transmitter assigns slots to nodes according to priority and need. This method is more efficient than TDM, because in statistical multiplexing time slots are unlikely to remain empty.FDM (frequency division multiplexing)is a type of multiplexing that assigns a
unique frequency band to each communications sub-channel. Signals are modulated with different carrier frequencies, then multiplexed to simultaneously travel over a single channel.WDM (wavelength division multiplexing)is a technology used with fiber-optic cable, which enables one fiber-optic connection to carry multiple light signals simultaneously. Using WDM, a single fiber can transmit as many as 20 million telephone conversations at one time. WDM can work over any type of fiber-optic cable.DWDM (dense wavelength division multiplexing):a single fiber in a fiber-opticcable can carry between 80 and 160 channels. It achieves this increased capacity because it uses more wavelengths for signaling. In other words, there is less separation between the usable carrier waves in DWDM than there is in the original form of WDM. Because of its extraordinary capacity, DWDM is typicallyused on high-bandwidth or long-distance WAN links, such as the connection between a large ISP and its (even larger) network service provider.