Five components of data communication

Figure 3.1.A Transmission medium and physical layer Tx media is located below physical layer and is controlled by it. So Tx media can be said to belong to layer zero. Tx media is anything that can carry info from src to dst in form of EM signal.

Figure 3.1.B Classes of transmission media

Topics discussed in this section: 3.1 GUIDED MEDIA Guided media, which are those that provide a conduit from one device to another . The signal is directed and contained by physical limits of the medium . Metallic media carries signal in form of electric current while optical media in the form of light. Topics discussed in this section: Twisted-Pair Cable Coaxial Cable Fiber-Optic Cable http://www.youtube.com/watch?v=hjVNKeVdKcs&list=PL374944B232C0B48E

Figure 3.1.1 Twisted-pair cable Two wires : one for signal and one for reference or ground. Insulator to avoid shorting of lines . Why twists? How many twists? Noise affects both lines eqally thus at Rx the effect of noise gets cancelled. Number of twists per unit length decides the quality of cable.

Figure 3.1.2 UTP and STP cables Unshielded is more in use . Shielded is by IBM . seldom used outside IBM. STP has metal braided mesh to prevent noise and crosstalk, but bulkier & expensive.

Table 3.1.1 Categories of unshielded twisted-pair cables by EIA

Figure 3.1.3 UTP performance

Figure 3.1.4 UTP connector RJ stands for registered jack. Has 8 lines. Used in LAN RJ11 has 2 lines , used in land telephone system.

Figure 3.1.5 Coaxial cable It carries higher freq than TP Outer conductor has two functions: protection from noise & complete the circuit.

Table 3.1.2 Categories of coaxial cables RG : radio govt ratings. Specifies wire gauge , type of insulator, size of casing etc. Table 3.1.2 Categories of coaxial cables

Figure 3.1.6 BNC connectors BNC: Bayone-Neill –concelman Three types: connector to extend , T connector for taps & terminator What is the need of terminator? To prevent reflection of signal else it works as antenna

Figure 3.1.7 Coaxial cable performance Applications: Was used in analog telephone networks( to carry 1000 voice channels, then for digital telephone networks ( upto 600Mbps) But now replaced by FOC 2) Cable TV networks also used co-ax , but there also replaced by FOC 3) Ethernet LAN( Thin 10 base2 10Mbps over 200 meters , Thick 10base5 10Mbps over 500mtr) but here also it replaced by UTP

Optical fiber Is made of glass or plastic ( glass is costlier than plastic but better quality). Transmits signal in form of light.

Figure 3.1.8 Bending of light ray Light travels stright in single uniform substance , but when enters into different density material, the ray changes direction. As long as angle of incidence (I) is less than or equal to critical angle , ray gets refracted, if greater then it gets reflected. Analogy : throwing a flat stone piece in water

Figure 3.1.9Optical fiber Cladding has less density than core

Figure 3.1.10 Propagation modes of transmission of light in FOC

Figure 3.1.11 Modes

Multi-mode fiber Named because multiple beams from light source move through core in different paths. Fiber with large core diameter (greater than 10 micrometers) Divided in two sub types : step and graded index. Step Index : where density changes abruptly . So rays get reflected suddenly Graded index : where density changes gradually, So rays get reflected smoothly.

Single mode fiber: The most common type of single-mode fiber has a core diameter of 8–10 micrometers and is designed for use in the near infrared Uses highly focused light , (laser), and propagation of beam almost horizontal. All beams arrive at Rx together , so least distortion as compared to multimode. But most costly

Acceptance angle Is the maximum angle ,within which all light rays having angle less than this ,will get reflected internally.

Table 3.1.3 Fiber types

Figure 3.1.11 Fiber construction The structure of a typical single-mode fiber. 1. Core: 8 µm diameter 2. Cladding: 125 µm dia. 3. Buffer: 250 µm dia. 4. Jacket: 400 µm dia.

Figure 3.1.12 Fiber-optic cable connectors Subscriber channel Straight tip

Figure 3.1.13 Optical fiber performance Attenuation per Km is very less as compared to UTP and co-ax, so requires less no. of ( 10 times less) repeaters .

Advantages of FOC: Higher BW limited only because of transducers at the ends. Less signal attenuation: repeater after 50Km Immunity to EM interference : since propagation by light Glass is not corrosive but copper Light weight Immunity to tapping : copper cable acts as antenna Disadvantages of FOC: Expensive installation and maintenance: expertise required Unidirectional light propagation: so for duplex two FOCs required. Cost: costlier cable and interfaces Requires additional devices at both ends to convert electric signal to light and light back to electric signal . To convert electric signal to light : 1) LED 2) Laser diode To convert light to electric signal : 1) PIN photodiode 2) PiPN Avalanche diode

1) LED

2) LASER diode

1)PIN photodiode

2) PiPN Avalanche