Physical Layer. Communication Channel

Communication Channel. The physical medium of a communication channel is used to transmit electromagnetic signals from the transmitter to the receiver. The physical media can be either guided media for wired transmission, or unguided media for wireless transmission. A signal that propagates along a guided medium is directed and is contained within a physical conductor. An unguided medium transports signal without using a physical conductor.

Guided media include twisted-pair cable, coaxial cable, and optical fiber. Figure 2 shows the electromagnetic spectrum used by wired transmission in these guided media. Consisting of two insulated copper conductors twisted together; twisted-pair cables commonly are used in subscriber loops to telephone networks.

Figure 2. Electromagnetic spectrum used for wired and wireless transmission

Coaxial cable consists of an inner solid wire conductor surrounded by an outer conductor of metal braid, with an insulting ring between the two conductors. Coaxial cables are used widely in cable TV networks, local area networks, and long-distance telephone networks. Coaxial cable offers a larger bandwidth and less interference or crosstalk than that of twisted-pair cable.

Consisting of a thin glass or plastics central core surrounded by a cladding layer, optical fiber transports light signal through the core by means of reflection. Optical fiber offers a wider bandwidth, lower attenuation, and lighter weight than that of coaxial or twisted-pair cables. The wider bandwith is achieved at a relatively higher cost. Optical fiber cables commonly are used in backbone networks and in long-haul undersea links because its wide bandwidth is cost effective.

Unguided media include the air atmosphere, outer space, and water. Wireless transmission and the reception of electromagnetic signals in unguided media are accomplished through an antenna. For unidirectional wireless transmission, the transmitted signal is a focused electromagnetic beam and the receiving antenna must be aligned properly for signal reception.

For omni-directional wireless transmission, the transmitted signal spreads out in all directions to enable reception by many antennas. Antenna size depends on the operating frequency of electromagnetic wave.

Wireless transmission in the air medium commonly is classified according to the operational frequency ranges or bands of the electromagnetic signals. These signals include radio waves that range from 3 KHz to 1 GHz, microwaves that range from 1 GHz to 300 GHz, and infrared waves that range from 300 GHz to 400 THz. Radio waves mostly are omni-directional; these waves are appropriate for multicasting or broadcasting communication.

Their applications include AM and FM radio, television, and cordless phones. Microwaves are unidirectional and are appropriate for unicast communication. Their applications include cellular, satellite, and wireless local area networks. Infrared waves are unidirectional and cannot penetrate physical barriers, thus they can be used for short-range unicast communication.

Their applications include communications between computers and peripheral devices such as keyboards, mice, and printers. Figure 2 shows the electromagnetic spectrum used by wireless transmission in the air medium.

Channel capacity is the maximum rate at which data can be transmitted over a medium, using whatever means, with negligible errors. For binary digital data, the transmission rate is measured in bits-per-seconds (bps). Channel capacity increases with transmission bandwidth (W in Hz) and increases with the ratio of received signal power (S in Watts) to noise power (R in Watts); also it is limited by the Shannon theorem: Channel Capacity = W log₂ (1 + S/R).