TRANSMISSION MEDIA
Computers and other telecommunication devices s use signals to represent data. These signals s are transmitted rom one device to another in the form of electromagnetic energy. electromagnetic signals can travel through vacuum, air or other transmission media.
Transmission media can be classified as guided or unguided. Guided media provide a physical path along which the signals are propagated; these include twisted pair, coaxial cable, and optical fibers. Unguided media employ an antenna for transmitting through air, vacuum or water.
The characteristics and quality of data transmission are determined both by the characteristics of the medium and characteristics of the signal. In the case of guided ed media, the limitations of transmission are decided by the medium. For unguided media, -the anlog signal produced by the transmitting antenna is more important than the medium in determine ing transmision characteristics ics. In considering the design of data transmission systems, the key concerns are data rate and distance. The greater the data rate and distance, the better.
a) Twisted Pair
The most common application of the twisted pair is the tele hone system. Nearly all te ep ones are connected to the telephone company office by a twisted pair. Twisted pairs can run several kilometres without _amplification but for longer distances, repeaters needed.
A twisted pair consists of two insulated copper wires typically about 1 mm thick. The wires are twisted together in a helical form. The purpose of twisting the wires is to reduce electrical interference from similar pairs close by. The twisted pair (two parallel wires constitute a simple antenna; a twisted pair does not).
Twisted pairs can be used for either analog or digital transmission. The bandwidth depends on the thickness of the wire and the distance travelled, but in many cases several megabits/sec, can be achieved for a few kilometres. Due to such satisfactory performance, and low cost, twisted pairs are widely used.
The Electronic Industries Association (EIA) has developed the standards for twisted pair cable (UTP-Unshielded Twisted Pair). Twisted pair cabling comes in several varieties, two of which are important for computer networks. These are:
Category 3 twisted pairs. These consist of two insulated wires gently twisted together. Four such pairs are typically grouped together in a plastic cover for protection and to keep the eight wires together. Category 3 can be used at speed up to 16 Mbps.
Category 5 twisted pairs. These are similar to Category 3 pairs, but have more twists per centimetre and Teflon insulation, which results in less cross talk and better quality of signals over long distances, making them more suitable for high speed computer communication. Category 5 can be used at speed up to 100 Mbps.
b)Baseband Coaxial Cable
Another common transmission medium is the coaxial cable or coax. Coaxial cables carry signals of higher frequency ranges than twisted pair cable.
coaxial cable copper wire as t e core, surroun e an insulation material we are making outer er con ductor of metal foil, braid, or a combination of the two (also usually copper). The outer metallic wrapping serves both as a shield against noise and as the second conductor, which completes the circuit. This outer conductor is also enclosed in an insulating cover, and the whole cable is protected by a plastic cover. Figure 2.17 illustrates the coaxial cable.
The construction and shielding of the coaxial cable give it a good combination of high bandwidth and excellent noise immunity. Two kinds of coaxial cable are widely used: 50 ohm cable, commonly used for digital transmission; and 75 ohm cable, commonly used for analog transmission.
For 1 kilometre of cable, a data rate of 1 to 2 Gbps is feasible. Longer cables can also be used but only at lower data rates. Coaxial cables, once widely used in the telephone system, have now largely been replaced by fibre optics.
2.4.3 Broadband Coaxial Cable
The term "broadband" comes from the telephone world, where it refers to anything wider than 4 kHz used in transmission. Coaxial cable systems use analog transmission on standard cable television cabling. It is called broadband.
One key difference between baseband and broadband is that broadband systems need analog amplifiers to strengthen the signal periodically. Two types of broadband systems have been developed considering that amplifiers can transmit signals only in one direction: Dual cable systems; and Single cable systems.
Dual cable systems have two identical cables running next to each other. A computer transmits the data onto cable 1, which runs to a device called the Headend at the root of the cable tree. The Headend then transfers the signals to cable 2 for transmission back down the tree. All computers transmit data on cable 1 and receive on cable 2.
Single cable systems use different frequency bands for inbound and outbound communication on a single cable. The low frequency band is used for communication from the computer to the Headend, which then shifts the signal to the high frequency band and rebroadcasts it. A single cable system is shown in Figure
2.4.4 Optical Fibre
An optical fibre is a thin, flexible medium capable of guiding an optical ray. Optical fibre uses light, not electricity, to transmit information.
An optical fibre cable has a cylindrical shape and consists of three concentric sections; the core, the cladding, and the jacket, as shown in Figure.
The core is the innermost section and consists of fibre made of glass or plastic. The core has diameter in the range 8-100 km Each fibre is surrounded by its own c ad ding, which is a glass or plastic coating that has optical properties different from
!hose of the core. The interface between the core and the cladding acts as a reflector to confine light that would otherwise escape the core. The jacket is composed of plastic and other material layered to protect against moisture, abrasion, crushing and other nvironmental amage.
There are two categories of fibres:
• Single mode fibre
• Multi mode fibre
The single mode fibre is manufactured in much smaller diameter than that of multi-mode fibres, and with substantially lower density (index of refraction). In single mode fibres, the core is 8 to 10 microns. In multi mode fibres, the core is 50 microns in diameter, about the thickness of human hair.
The multi mode fibre is so named because in them, light from source moves through the core in different paths. How these beams move within the cable depends on the structure of the core.
An optical transmission system has three components
• Light source
• Transmission media
• Detector
A pulse of light indicates a 1 bit and absence of light indicates a 0 bit. The transmission medium is an ultra-thin fibre of glass. The detector generates an electrical pulse when light falls on it. By attaching a light source to one end of an optical fibre and a detector to the other, we have an unidirectoin data transmission system that accepts an electrical signal, converts and transmits it by light pulses, and then reconverts the output to an electrical signal at the receiving end_
Light ray incident at or above the critical angle is trapped inside the fibre and can propagate for many kilometres with virtually no loss.
Figure shows only one trapped ray but since any light ray incident on the boundary above the critical angle will be reflected internally many different rays will be bouncing around at different angles. Each ray is said to have a different mode' and so, a fibre having this property is called a multi mode fibre.
However, if the fibre diameter is reduced to a few wave lengths of light, the fibre acts like a wave guide and the light can only propagate in a straight line, without bouncing, in which case the fibre is called a single mode fibre.
2.5 WIRELESS COMMUNICATIONS
Wireless communication or unguided media transmission, transports electromagnetic waves without using a physical conductor. There are several Wireless media available for transmitting network packets: radio waves, infrared signals and microwave. All of these technologies transmit a signal through the air or the atmosphere. The characteristics of these media make them a good alternative in Situations where it is difficult or impossible to use cables.
2.5.1 Radio Waves
In a radio network transmission, a signal is transmitted in one or multiple directions, depending on the type of antenna. Network signals are transmitted over radio waves in a fashion similar to the way your local radio station broadcasts; only that network applications use much higher frequencies. Electromagnetic waves having frequencies between 3 kHz and 1 GHz are called radio waves.
The advantages of radio waves are:
- An inexpensive alternative where communications cabling cannot be installed easily.
- An option for portable communications.
- The major disadvantages are:
- May not be feasible when higher speed communications are needed.
Subject to interference from the military, amateur radio, cell phones and other sources.
2.5.2 Microwaves
Electromagnetic waves having frequencies between 1 and 300 GHz are called microwaves. A communication satellite is, in effect, a microwave relay station. It is used to link two or more ground based microwave transmitter/receivers, known as earth stations, or ground stations. The satellite receives transmissions on one frequency band (uplink), amplifies or repeats the signal, and transmits it on another frequency (downlink). A single orbiting satellite will operate on a number of frequency bands called transponder channels, or simply transponders.
In general, there are two configurations for satellite communication. In the first, the satellite is used to provide a point-to-point link between two distant ground-based antennas as shown in Figure 2.22. In the second, the satellite provides communication.sbetween one ground-based transmitter and a number of ground-based receivers as shown in Figure.
Communication satellite is an alternative way where communications cabling cannot be installed easily, such as over long distances.
For a communication satellite to function effectively, it is generally required that it remains stationary with respect to its position over the earth. Otherwise, it would not be within the line of sight of its earth stations at all times. To remain stationary, the satellite must have a period of rotation equal to the earth's period of rotation. Two satellites using the same frequency band, if close enough together, will interfere with each other. Very high-frequencies microwave cannot penetrate walls. This characteristic can be a disadvantage if receivers are inside buildings. Microwaves are used for unicast communication such as cellular telephones, satellite networks and wireless LANs.
2.5.3 Infrared
Infrared light can also be used as a medium for network communications. This technology is probably most familiar to you in the remote control devices for your television and stereo. Infrared communications is achieved using transmitter/receivers that modulate non-coherent infrared light. Transmitter/receivers must be within the line of sight of each other directly or via reflection from the light-coloured surface such as the ceiling of a room.
One important difference between infrared and microwave transmission is that the former does not penetrate walls. Thus the security and interference problems encountered in microwave systems are not present with infrared. However, infrared transmission may not be feasible when high-speed communication is needed.
BACK |
