CHAPTER 3 TRANSMISSION MEDIA ; SIGNAL POWER

Introduction Communications network cannot exist without a medium to connect the source and the receiver to provide a path over which messages can be sent. Medium : 1) Physical wire or cable 2) Wireless transmission medium

Guided Media Twisted Pair Wire Coaxial Cable Fiber Optic Cable

Twisted Pair Wire: 1) Unshielded Twisted Pair (UTP) The cable has four pairs of wires inside the jacket. Each pair is twisted with a different number of twists per inch to help eliminate interference from adjacent pairs and other electrical devices. The tighter the twisting, the higher the supported transmission rate and the greater the cost per foot. The EIA/TIA (Electronic Industry Association/Telecommunication Industry Association) has established standards of UTP and rated six categories of wire (additional categories are emerging).

The standard connector for unshielded twisted pair cabling is an RJ-45 connector. This is a plastic connector that looks like a large telephone-style connector A slot allows the RJ-45 to be inserted only one way. RJ stands for Registered Jack, implying that the connector follows a standard borrowed from the telephone industry. This standard designates which wire goes with each pin inside the connector

Shielded Twisted Pair (STP) Each pair of wires is placed into a metallic shield Which is a wrapping made of thin wires to absorb any interference. The wires then are put into a plastic outer shell. STP typically is used when many wires need to be packed together in a small space or in an environment with considerable electrical equipment. STP wires are reliable for transmitting data in high-interference area.

Coaxial Cable Coaxial cabling has a single copper conductor at its centre. A plastic layer provides insulation between the centre conductor and a braided metal shield Although coaxial cabling is difficult to install, it is highly resistant to signal interference. In addition, it can support greater cable lengths between network devices than twisted pair cable.

Fiber Optic Cable Fiber optic cabling consists of a center glass core surrounded by several layers of protective materials. It transmits light rather than electronic signals eliminating the problem of electrical interference. This makes it ideal for certain environments that contain a large amount of electrical interference

Fibre optic cable has the ability to transmit signals over much longer distances than coaxial and twisted pair. It also has the capability to carry information at vastly greater speeds. This capacity broadens communication possibilities to include services such as video conferencing and interactive services.

 There are three common types of fibre cables  Multimode step index – is an early form of fiber optic cable that uses a plastic coating or a mirror- like cladding around the core to reflect the light from the laser and LED.As the light is reflected off the sides of the cables, it moves down the cable to its destination.  Multimode graded index- the core of the varies in density, which bends the light.  Single mode can provide more distance, but it is more expensive.

Unguided Media Type of unguided Media :Air,Vacuum of space Forms of transmission that use unguided media : broadcast radio, terrestrial and satellite microwave transmission, cellular radio, spread spectrum radio, and infrared transmission.

Wireless (Unguided Media) Transmission transmission and reception are achieved by means of an antenna directional transmitting antenna puts out focused beam transmitter and receiver must be aligned omnidirectional signal spreads out in all directions can be received by many antennas

Wireless Examples terrestrial microwave satellite microwave broadcast radio infrared

Terrestrial Microwave A system, method, technology, or service, such as Multichannel Multipoint Distribution Service, which utilizes microwave line of sight communications between sending and receiving units located on the ground or on towers, as opposed to a sender and/or receiver antenna being located on a communications satellite. Used, for instance, for telephone, TV, and/or data services. Also called Terrestrial Microwave radio. uses radio frequency spectrum, from 2 to 40 Ghz parabolic dish transmitter, mounted high used by common carriers as well as private networks requires unobstructed line of sight between source and receiver curvature of the earth requires stations (repeaters) ~30 miles apart

Satellite Microwave Transmission Microwave transmission refers to the technology of transmitting information or energy by the use of radio waves whose wavelengths are conveniently measured in small numbers of centimeter; these are called microwaves. a microwave relay station in space can relay signals over long distances geostationary satellites remain above the equator at a height of 22,300 miles (geosynchronous orbit) travel around the earth in exactly the time the earth takes to rotate

Satellite Microwave Applications Television distribution Long-distance telephone transmission Private business networks

Microwave Transmission Disadvantages line of sight requirement expensive towers and repeaters subject to interference such as passing airplanes and rain

Satellite Transmission Links In satellite communication, signal transferring between the sender and receiver is done with the help of satellite. In this process, the signal which is basically a beam of modulated microwaves is sent towards the satellite. Then the satellite amplifies the signal and sent it back to the receiver’s antenna present on the earth’s surface. So, all the signal transferring is happening in space. Thus this type of communication is known as space communication.

dish uplink stationdownlink station satellite transponder 22,300 miles Satellite Transmission Process

Satellite Transmission Applications television distribution a network provides programming from a central location direct broadcast satellite (DBS) long-distance telephone transmission high-usage international trunks private business networks

Principal Satellite Transmission Bands C band: 4(downlink) - 6(uplink) GHz the first to be designated Ku band: 12(downlink) -14(uplink) GHz rain interference is the major problem Ka band: 19(downlink) - 29(uplink) GHz equipment needed to use the band is still very expensive

Fiber vs Satellite

Radio radio is omnidirectional and microwave is directional Radio is a general term often used to encompass frequencies in the range 3 kHz to 300 GHz. Mobile telephony occupies several frequency bands just under 1 GHz.

Infrared Infrared (IR) light is electromagnetic radiation with longer wavelengths than those of visible light, extending from the nominal red edge of the visible spectrum at 0.74 micrometers (µm) to 0.3 mm. Uses transmitters/receivers (transceivers) that modulate noncoherent infrared light. Transceivers must be within line of sight of each other (directly or via reflection ). Unlike microwaves, infrared does not penetrate walls.

Attenuation It is the gradual loss in intensity of any kind of flux through a medium. For instance, sunlight is attenuated by dark glasses, X-rays are attenuated by lead, and light and sound are attenuated by water In electrical engineering and telecommunications, attenuation affects the propagation of waves and signals in electrical circuits, in optical fibers, as well as in air (radio waves)

Attenuation (Cont.) Reduction of signal strength during transmission. Attenuation is the opposite of amplification, and is normal when a signal is sent from one point to another. If the signal attenuates too much, it becomes unintelligible, which is why most networks require repeaters at regular intervals. Attenuation is measured in decibels