Transmission/Communication Medium or Channel:

  • The transmission medium is the physical path/connection between transmitting and receiving devices in a data transmission system.
  • Transmission media is one of the major components of a communication system.
  • The characteristics and quality of a data transmission are determined both by the characteristics of the medium and the characteristics of the signal transmits.

Classification of Transmission Medium

  • Transmission media can be broadly classified as guided/wired/dedicated or unguided/wireless/Non-dedicated medium.
[A] Guided Transmission Medium:
  • Guided media provide a physical connection between two devices.
  • With guided media, the waves/signals are guided as solid medium/cable goes.
  • These are uni-directional transmission.
  • In the case of guided media, the medium itself is more important in determining the limitations of transmission.
  • Examples are – twisted pair cable, coaxial cable, and optical fiber.

(a) Twisted Pair Cable(TPC):


    • The pair of wires in tpc forms a circuit that can transmit data.
    • A twisted pair has a bandwidth to distance ratio of about 1 MHz per kilometer.
    • The performance of the twisted pair can be substantially improved by adding a metallic shield around the wires.
    • Structure:
      • It consists of two separate wires/conductors which are normally made of copper and twisted with each other and makes single one.
      • Here, each conductor has its own plastic insulation typically 1 mm thick.
      • These cables are twisted together. The wires are twisted in a helical form, similar to a DNA molecule.
      • Twisting of wire is done to reduce crosstalk and noise generated by adjacent pairs during data transmission. 

    • Working Mechanism:
      • When electrical current flows through a tpc wire, it creates a small, circular magnetic field around the wire which are nullified by these two wires itself due to they are placed close together and their magnetic fields are the exact opposite of each other. Thus, the two magnetic fields cancel each other out and they also cancel out any outside magnetic fields.
      • Twisting the wires can enhance this cancellation effect.


    • Twisted pair cabling comes in several varieties, two of which are very important: Category 3 and Category 5. Category 5 has more twists per centimeter resulting in less crosstalk and a better quality signal.
    • There are two types of TPC –

(i) Unshielded Twisted Pair[UTP] Cable and (ii) Shielded Twisted Pair[STP] Cable.

(i) Unshielded Twisted Pair[UTP] Cable:

      • It is ordinary type of tpc and is used in a variety of  networks.
      • It is the most common kind of copper telephone wiring.
      • Mainly use in the home network.
      • Each of the eight individual copper wires (4 pairs) in UTP cable is covered by an insulating material. In addition, the wires in each pair are twisted around each other.
      • UTP cable is create by considering the cancellation effect produced by the twisted wire pairs to limit signal degradation caused by electromagnetic interference (EMI) and radio frequency interference (RFI). To further reduce crosstalk between the pairs in UTP cable, the number of twists in the wire pairs varies per meter/3.28feet. 

(ii) Shielded Twisted Pair[STP] Cable:

      • It is specialized type of tpc i.e. shielded twisted pair is a special kind of copper telephone wiring used in some business installations.
      • Shielded wires are much more resistant to thermal noise and crosstalk effects.
      • Mainly use in the business network.
      • STP is similar to UTP in that the wire pairs are twisted around each other i.e. STP also has shielding around the cable to further protect it from external interference.
      • The extra shielding further reduces the chance of crosstalk but the shielding increases the  overall diameter and weight of the cable.
      • The maximum segment length of STP cable is 100 meters.
      • An outer covering or shield is added to the ordinary twisted
        pair telephone wires; the shield functions as a ground.
      • Shielded twisted-pair (STP) cable combines the techniques of shielding, cancellation and wire twisting. Each pair of wires is wrapped in a metallic foil. The four pairs of
        wires then are wrapped in an overall metallic braid or foil.
      • It is usually a 150-ohm cable, as specified for use in Ethernet network installations.
      • STP reduces electrical noise both within the cable (pair-to-pair coupling, or crosstalk) and from outside the
        cable (EMI and RFI).


    • Twisted pair cable is the most widely used transmission media for data transmission in local area network mainly.
    • Twisted Pairs are very effective for relatively short distances (a few hundred feet), but can be used for up to a few kilometers.
    • It is the least expensive type of local-area network (LAN) cable.
    • Most of networks today contain some part of twisted-pair cabling at some point along the network.
    • It is easier to handle, install and run.


    •  TPC is limited in distance, bandwidth, and data rate.
    • Higher attenuation.
    • Low durability cable and hence must be maintained at regular interval.
    • UTP requires proper grounding at both the ends.
    • STP cable is heavier and costlier.


    • Twisted-pair cable is mainly used for long distance connections in (old) telephone communication line and in most modern Ethernet LAN networks.

(b) Coaxial Cable (Coax):


    • Structures:
        • Coaxial cable consists of two conductors i.e. it consists of a hollow outer cylindrical conductor that surrounds a single inner wire conductor. The inner conductor is held in place by either regularly spaced insulating rings or a solid dielectric material. The outer cond-uctor is covered with a jacket or shield.
        • A single coaxial cable has a diameter of from 0.4 to about 1 inch. Because of its shielding, concentric construction, coaxial cable is much less susceptible to interference and cross-talk than is twisted pair.
    • Coaxial cable is used to transmit both analog and digital signals.


    • It operates over a wider range of frequencies i.e. using frequency-division multiplexing, a coaxial cable can carry over
      10,000 voice channels simultaneously.
    • Coaxial cable can be used over longer distances and supports more stations on a shared line than twisted pair.
    • Coaxial cable is perhaps the most versatile transmission medium and has widespread use in a wide variety of applications.
    • Using digital signaling, coaxial cable can be used to provide high-speed I/O channels on computer systems.


    • They are bulky.
    • It is expensive to install for longer distances because of its thickness and stiffness.
    • It needs to be grounded to limit interference.
    • They are not really supported by fast networking standards.
    • It is easy to tap the coaxial cable by breaking it and inserting T-joint in between.


The most important use of this cable is-

i) In Television/Cable TV distribution.
ii) Long-distance telephone transmission with quality.
iii) Short-run computer system links.
iv) In Local Area Networks
v) short-range connections between devices.

(c) Optical Fiber Cable (OFC):


    • One of the most significant technological breakthroughs in data transmission system has come into existence when the development of fiber optic cable occurred.
    • Various types of glasses and plastics can be used to make different types of optical fibers.
    • An optical fiber is a thin (2 to 125 nm(nano meter), flexible medium capable of conducting an optical ray.
    • Optical fibers can provide bandwidth to distance ratios in order of 100s of MHz per kilometer.
    • Like other cables, hundreds of optical fibers are usually housed/set within one cable.
    • It is believed that they will replace twisted pair residential loops in the near future.
    • Structure:
      • An optical fiber cable has a cylindrical shape and consists of three concentric sections: the core, the cladding, and the jacket.
      • In another words, an optical fiber consists of two concentric cylinders: an inner core surrounded by a cladding. Both the core and the cladding are made of transparent plastic or glass material which transmit signals in the form of light.
      • The core is the innermost section and consists of one or more very thin strands, or fibers, made of glass or plastic.
      • Each fiber is surrounded by its own cladding, a glass or plastic coating that has optical properties different from those of the core.
      • The outermost layer, surrounding one or a bundle of cladded fibers, is the jacket. The jacket is composed of plastic and other material layered to protect against moisture, abrasion, crushing and other environmental dangers.
      • An optical transmission system has three components- the light source, the transmission medium and the detector.
      • The transmitter generates the light pulses based on the input electrical signal. The transmission medium (usually light) carries the pulse with the speed of light. The detector regenerates the electrical signal based on the light signal it detects on the transmission medium.
    • Working Mechanism:
      • Optical fiber use the principle of reflections of light to guide light through a channel where the density of the core and cladding must differ sufficiently to reflect the beam of light instead of refracting.
      • Here, the core is used for guiding a light beam, whereas the cladding (which has a different refractive index) acts as a reflector to prevent the light signal instead of electrons.
      • The signal is usually generated by a laser or Light Emitting Diode (LED).
      • In this transmission, a pulse of light indicates a bit 1 and absence of light indicates bit 0.
      • Transmission medium is an ultra-thin fiber of glass like a thin needle.
      • By attaching a light source to one end of an optical fiber and a detector to the other, we have an unidirectional data transmission system that accepts an electrical signal, converts and transmits it by light pulse, and then reconverts the output to an electrical signal at the receiving end.
    • A fiber optics with ultrapure fused silica components show lowest rate of data losses, which is difficult to manufacture. But higher-loss multi-component glass fibers are more economical and still provide good performance which is normally used at large scale. Plastic made fiber is even less costly and can be used for short-haul links, for which moderately high losses are acceptable.
    • Fiber optic cables are much more secure for data transfer, since it is very difficult to tap fiber optic cables.
    • Fiber optic cables are much thinner and lighter than copper cables, so less weight.


    • Optical fiber is of two types-

a) Single Mode Optical Fiber.
b) Multimode Optical Fiber.

a) Single Mode Optical Fiber

      • This fiber itself is manufactured with a much smaller diameter than that of multimode fibers, and with substantially lowers density (index of refraction).
      • This uses step-index fiber and a highly focused source of light that limits beams to a small range of angles, all close to the horizontal.
      • The decrease in density results in a critical angle that is close enough to 90 degrees to make the propagation of beams delays are negligible.
      • All of the beams arrive at the destination together and can be recombined without distortion to the signal.

b) Multimode Optical Fiber


    • Optical fiber cable is widely used as a back bone for network due to –
      • It has highest data transmission rate.
      • Bandwidth is higher.
      • It has lighter in weight.
      • It shows low interferences.
      • It required less number of repeaters in its connection.
      • It covers long distance connection/telecommunications due to  low power loss.
      • it does not suffer from the various noise problems associated with electromagnetic signals.
      • Since there is no electricity used, there is no electrical interference and related noise.
      • The optical cable is resistance for electromagnetic  interference.
      • Higher Band width – it can support higher band width and hence can transfer data at a higher rate.
        2) Less signal attenuation – its transmission distance is greater than the twisted pair and it can run for 50Kms without regeneration.
        3) Immunity to electromagnetic interface
        4) These cables are much lighter than the copper cables
        5) These cables are more immune to tapping than the copper cables.


    • They are fragile and more vulnerable to damage. If we bend fiber optic cables beyond a limit, it will break.
    • High manufacturing cost.
    • Setup/installation cost is high i.e. It needs costly splicing/joining tools or machines. The cables and interfaces used are relatively expensive.
    • Fiber optic cables are difficult to splice/join.
    • Maintenance cost is also high.
    • Working mechanism is comparatively complex.
    • Installation or maintenance – it needs trained specialists to install fiber optic cables which is not available everywhere.
    • Unidirectional – Propagation of light is unidirectional and we need two fibers for bidirectional communication.


    • Widely used in making fast & sensitive data transmission network.
    • They are being increasingly used as telecommunication carriers for long distance digital trunk lines. 
[B] Unguided Transmission Medium:
  • It is often called wireless communication.
  • Unguided media transport data/signal mainly through electromagnetic waves without using any physical media or a physical conductor.
  • Its Signals are broadcast though air(mostly) or water(rarely), and thus are available to anyone who has a device capable of receiving them.
  • These Electromagnetic spectrum covers frequencies from 3 Hz (ELF) to gamma rays (30 ZHz[Zetta Hertz] – 1021 Hz) and beyond (cosmic rays). But only frequencies ranging from 3 KHz to 900 THz are used for wireless communication.
  • With un-guided media, the waves/signals are transmitted in all the direction as air passes.
  • These are omni-directional transmission.
  • The atmosphere and outer space supports unguided transmission that
    provide a means of transmitting electromagnetic signals but do not guide them.
  • For unguided media, the bandwidth of the signal produced by the transmitting antenna is more important than the medium in determining transmission characteristics.
  • Examples are – Infra red wave, Microwave, Radio wave etc.
  • Working Mechanism of Radio Transmission Technology:
    • Radio technology considers the earth as surrounded by two layers of atmosphere: the troposphere and the ionosphere.
    • Troposphere:
      • The troposphere is the portion of the atmosphere extending outward approximately 30 miles from the earth’s surface.
      • The troposphere contains what we generally think of as air. Clouds, wind, temperature variations, and weather in general occur in the troposphere.
    • Ionosphere:
      • The ionosphere is the layer of the atmosphere above the troposphere but below space.
  • Unguided radio signals can travel from the source to destination in several ways. There is ground propagation, sky propagation, and line-of-sight propagation.
    • Ground Propagation :
      • In ground propagation, radio waves travel through the lowest portion of the atmosphere, hugging the earth.
      • These low-frequency signals emanate in all directions from the transmitting antenna and follow the curvature of the earth.
      • The distance depends on the power of the signal.
    • Sky Propagation
      • In Sky propagation, higher-frequency radio waves radiate upward into the ionosphere where they are reflected back to earth.
      • This type of transmission allows for greater distances with lower power output.
    • Line-of-Sight Propagation
      • In Line-of-Sight Propagation, very high frequency signals are transmitted in straight lines directly from source antenna to destination antenna.
      • Antennas must be directional, facing each other and either tall enough or close enough together not to be affected by the curvature of the earth.

(a) Infra-red Wave:


    • A wave having frequencies from 300 GHz to 400 THz[Tera-Hertz] (wavelengths from 1 mm to 700 nm) called Infra-red.
    • Data transmission rates are near to twisted pair cables.
    • Infrared signals are generated and received using optical transceivers.
    • This transmission cannot be affected by another system in the next room.
    • The infrared band, almost 400 THz, has an excellent potential for
      data transmission. Such a wide bandwidth can be used to transmit digital data with a very high data rate.
    • They are useless for long range communication.
    • Infrared waves cannot be used outside a building because the sun’s rays contained infrared waves can interfere with the communication. 


    • They have high frequencies therefore prevents interference
      between one system and another.
    • These can be used for short range communication.
    • Infrared systems represent a cheap alternative to most other methods, because there is no cabling involved and the necessary equipment is relatively cheap.
    • Infrared having wide bandwidth can be used to transmit digital data with a very high data rate.


    • Infrared signals can be used for short-range communication.
    • They cannot penetrate walls because they are weak signals.
    • It cannot be used outside the building as sun rays contain infrared which leads to interference in communication.


    • Infrared signals can be used for communication between keyboards, mouse and printers.
    • One recent use of infra-red has been for interfacing hand-held and portable computing devices to Local Area Networks.
    • The infrared Data Association (IrDA), an association for sponsoring the use of infrared waves, has established a standard for using these signals for communication between devices such as the keyboard, mice, PCs, and printers. Infrared signals defined by the IrDA transmit through line of sight; the IrDA port on the keyboard needs to point to the PC for transmission occurs.

(b) Microwave:


    • A wave having frequencies between 1 GHz to 300 GHz are called microwaves.
    • Microwaves are unidirectional i.e. the sending and receiving antennas need to be aligned.
    • Microwave is by far the most widely used form of radio transmission.
    • It operates in the GHz range with data rates in order of hundreds of Mbps per channel.
    • Structure:
      • An important form of microwave system is a satellite system, which is essentially a microwave system plus a large repeater as satellite in the sky. The signals transmitted by earth stations are received, amplified, and retransmitted to other earth stations by the satellite which acts as a repeater.

    • The bandwidth of microwave systems is subdivided into channels of 10s of MHz each, providing data rates in order of 100s of mbps. Because of their high bandwidths, satellites are capable of supporting an enormous number and variety of channels, including TV, telephone, and data.
    • Microwave transmission occurs through two ways – (i) Through unidirectional antenna and (ii) Through artificial satellite relay.
    • (i) Microwave antenna are two types: the parabolic dish antenna and the horn antenna .
      • A parabolic dish antenna is based on the geometry of the parabola. Every line parallel to the line of symmetry reflects off the curve at angles such that all the lines intersect in
        a common point called focus. The parabolic dish works as a funnel, catching a wide range of waves and directing them to a common point.
      • A horn antenna on the other hand looks like a gigantic scoop. Here, outgoing transmissions are broadcast up a stem and deflected outward in a series of narrow parallel beams by the curved head. Received transmissions are collected by the scooped shape of the horn, in a manner similar to the parabolic dish, and are deflected down into the stem.
    • (ii) Satellite relay
      • It requires geo-stationary orbit with the height of 35,784km to match the earth’s rotation.
      • It has uplink that receives transmission on one frequency and a downlink that transmits on a second frequency.
      • It Operates on a number of frequency bands known
        as transponders.
      • It can operate in two ways:-
        a) Point to point- Ground station to satellite to ground station.
        b) Multipoint (Broadcast link)- Ground station to satellite to multiple receiving stations. 
    • Working Mechanism:
      • When an antenna transmits microwave waves, they can be
        narrowly focused. This means that the sending and receiving antennas need to be aligned.
      • Here a pair of antennas can be aligned without interfering with another pair of aligned antennas.


    • Microwaves are widely used for point-to-point communications because their small wavelength allows conveniently-sized antennas to direct them in narrow beams, which can be pointed directly at the receiving antenna.
    • Higher data rates are transmitted/achieved as the bandwidth is more.
    • Low power consumption as the signals are of higher frequencies.
    • Unidirectional property of microwave helps in avoiding interference by a pair of aligned antenna to another.


    • The propagation of microwave is line-of-sight. The problem with this propagation is that towers that are far apart from each other need to be very tall.
    • Line-of-sight will be disrupted if any obstacle, such as new buildings, are in the way.
    • Microwaves suffer from attenuation due to atmospheric conditions. 
    • Signals are absorbed by the atmosphere.
    • For long distance communication, repeaters are often needed.
    • Even high frequency microwaves cannot penetrate walls.
    • Towers are expensive to build.
    • High frequency micro waves cannot be received inside the building.


    • Microwave transmission is commonly used in point-to-point communication systems on the surface of the Earth, in satellite communications, and in deep space radio communications.
    • They are also used for radars, radio navigation systems, sensor systems, and radio astronomy.
    • Microwaves are also used in unicast communication such as cellular telephones, satellite networks, and wireless LANs.
    • Telecommunication carriers and TV stations are the primary users of microwave transmission.

(c) Radio Wave:


    • Electromagnetic waves ranging in frequencies between 3 KHz(Kilo-Hertz) and 1 GHz(Giga-Hertz) are normally called radio waves.
    • They use reflected and refracted principle of light to change their direction and hence ideal for communications.
    • It uses omnidirectional antenna for data transmission that travel in all directions from the source, so the transmitter and receiver do not have to be carefully aligned physically.
    • Radio waves transmitted by one antenna are susceptible to interference by another antenna due to its omni-directional property.
    • Here, omnidirectional antenna propagates signal in all direction i.e. the sending and receiving antennas do not have to be aligned. 


    • Radio waves are easy to generate and can travel long distances and can penetrate buildings easily, therefore widely used for communication.
    • Radio signals have been used for a long time to transmit analog information.
    • They are particularly preferred for long distance communication over difficult terrain or across the oceans.
    • Because of the ability to pass through the ionosphere (top) layer of atmosphere, radio wave is suitable for satellite to earth transmission. 
    • Radio waves are suitable for long-distance wireless data transmission.
    • Radio waves/signal can be received both inside and outside of the building.


    • It is susceptible to interference wherein a radio wave transmitted by one antenna may be interfered by another antenna that may send signals using the same frequency or band.


    • The short wave form (2400 to 2480 MHz) of Radio wave is mainly used in Bluetooth technology which is a proprietary wireless technology standard used for exchanging data over short distances in mobile phones and other related devices. It allows wireless devices to be connected to wireless host which may be a computer over short distances. It helps in transferring data between a mobile phone and a computer provided both have Bluetooth technology.
    • Radio waves are used for multicast communications such as radio (AM and FM radio), maritime radio, television, cordless phones and paging systems.
    • Radio waves are very useful in multicasting and hence used in AM and FM radios, cordless phones and paging.
    • They are used in standard broadcast radio and television, shortwave radio, navigation and air-traffic control, cellular telephony, and even remote-controlled toys.
    • Recently, an increasingly-popular usable form of radio wave is in cellular radio, which is currently being used by carriers for providing mobile telephone networks. These operate in the VHF (Very High Frequency) band and subdivide their coverage area into conceptual cells, where each cell represents a limited area which is served by a low-power transmitter and receiver station. As the mobile user moves from one cell area to another, its communication is handed over from one station to another. 


  • The connectors are tools or interface that is used to connect different types of data communication cables(such as twisted pair cable, coaxial cable, fiber optic cable etc.) at one end with the computer or other network devices (such as hub, switch, router etc.)at other end in a network environment for data communication.
  • Connectors may be used in LAN,MAN and WAN.
  • There are different types of connectors used uniquely to connect different cables.
  • In LAN, common used connector are –

(i) RJ-45/TPC Connector:

    • RJ stands for registered jack.
    • RJ45 connectors are the most commonly used standard type of connectors with Ethernet/TPC cables and networks.
    • RJ45 connectors consists of eight pins to which the wire strands of a cable interface electrically.
    • Standard RJ-45 pin-outs define the arrangement of the individual wires needed when attaching connectors to a cable.
    • RJ-45 connectors are of two types: Male RJ-45 and Female RJ-45. Here, Male RJ-45 is connected with the cables where as Female RJ-45 is attached with the devices.

(ii) BNC/Coax connector:

    • The BNC connector (Bayonet Neill–Concelman) is used to connect/disconnect coaxial cable.
    • It features two bayonet lugs on the female connector; mating is achieved with only a quarter turn of the coupling nut.
    • BNCs are ideally suited for cable termination for miniature-to-subminiature coaxial cable (e.g., RG-58, 59, to RG-179, RG-316).
    • It is used with radio, television, and other radio-frequency electronic equipment, test instruments, video signals, and was once a popular computer network connector.
    • BNC connectors are made to match the characteristic impedance of cable at either 50 ohms or 75 ohms.
    • It is usually applied for frequencies below 3 GHz and voltages below 500 Volts.

(iii) Fiber Optic Cable Connector

    • In order for information to be transmitted efficiently, the fiber cores must be properly aligned.
    • They are usually devices that can be connected and disconnected repeatedly.
    • There are many types of fiber optic cable connectors –

1.) ST (Straight Tip) Connectors:

      • It is slotted bayonet type connector with long ferrule, a common connector for multi-mode fibers.
      • The ST connector has been the main stay of optical fiber connectors for many years.
      • It can be found in almost every communications room worldwide, but used mainly in data communications systems.
      • The simple to use bayonet locking mechanism reduces the risks of accidental disconnection of fiber connections.

2.) SC (Standard Connector) Connectors:

      • It is a push/pull type of connector that can also be used with duplex fiber connection.
      • The SC connector comprises a polymer body with ceramic ferrule barrel assembly plus a crimp over sleeve and rubber
      • These connectors are suitable for, 900μm and 2-3mm cables.
      • The connector is precision made to demanding specifications.
      • The combination of a ceramic ferrule with precision polymer housing provides consistent long-term mechanical and optical performance.

3.) MT (Multi-fiber) Connector:

      • The MT-RJ connector is a development of the now legendary
        MT ferrule.
      • The MT ferrule in its various designs has the ability to connect anything from 2 fibers in the MTRJ to 72 fibers in the latest versions of the MPO connector.


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