Transcript
Transmission Media
Dr. Indranil Sen Gupta
Transmission Media
Slide 1
Introduction • Physical path between transmitter and receiver in a data transmission system. • May be classified into two types: – Guided:- waves are guided along a solid medium, such as copper twisted pair, copper coaxial cable, or optical fiber. – Unguided:- provides a means for transmitting electromagnetic signals but do not guide them. • Wireless transmission Dr. Indranil Sen Gupta
Transmission Media
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Contd. • Characteristics and quality of data transmission are determined by medium and signal characteristics. – For guided media, the medium is more important in determining the limitations of transmission. – For unguided media, the bandwidth of the signal produced by the transmitting antenna is more important than the medium. • Signals at lower frequencies are omnidirectional – propagate in all directions • For higher frequencies, focussing the signals into a directional beam is possible.
Dr. Indranil Sen Gupta
Transmission Media
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Guided Transmission Media • Transmission capacity depends critically on the distance, and whether the medium is point-to-point or multipoint (e.g. LAN). • Point-to-point transmission performances: Transmission Medium Twisted pair Coaxial cable Optical fiber Dr. Indranil Sen Gupta
Total Data Rate
Bandwidth
Repeater Spacing
4 Mbps
3 MHz
2-10 Km
500 Mbps
350 MHz
1-10 Km
2 Gbps
2 GHz
10-100 Km
Transmission Media
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Figure 3.2 from the book by Stallings
Dr. Indranil Sen Gupta
Transmission Media
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Guided Media: Twisted Pair • A twisted pair consists of two insulated copper wires arranged in a regular spiral pattern. – Twisting decreases the crosstalk interference between adjacent pairs in a cable. – Typically, a number of pairs are bundled together into a cable by wrapping them in a tough protective sheath.
• Most common transmission media for both analog and digital signals. – Telephone network – Connecting computers in a LAN (upto 100 Mbps)
Dr. Indranil Sen Gupta
Transmission Media
Slide 6
Twisted Pair (contd.) • Two common types: – Unshielded Twisted Pair (UTP) • ordinary telephone wire • subject to external electromagnetic interference
• Two commonly used categories (vide Electronic Industries Association standard EIA-568) – Category 3: upto 16 MHz – Category 5: upto 100 MHz
• A key difference is the number of twists in the cable per unit distance. – Cat-3 ==> 3-4 twists per foot; Cat-5 ==> 3-4 twists per inch – Tighter twisting of Cat-5 provides much better performance, but also increases the cost. Dr. Indranil Sen Gupta
Transmission Media
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Guided Media: Coaxial Cable • 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 conductor is covered with a jacket or shield.
• Due to its shielding, coaxial cables are much less susceptible to interference or crosstalk than twisted pair. • Used in a variety of applications: – Television distribution (cable TV) – Long-distance telephone transmission (10,000 voice channels per cable) – Local Area Networks Dr. Indranil Sen Gupta
Transmission Media
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Guided Media: Optical Fiber • An optical fiber is a thin (2-125 micrometer), flexible medium capable of conducting an optical ray. – Made of ultrapure fused silica, glass fiber or even plastic.
• It has a cylindrical shape and consists of three concentric sections: the core, the cladding, and the jacket. – The core consists of one or more very thin strands, or fibers, made of glass or plastic. – The cladding is a glass or plastic coating that has optical properties different from that of the core. – The jacket surrounds one or a bundle of cladded fibers.
Dr. Indranil Sen Gupta
Transmission Media
Slide 9
Optical Fiber (contd.) • Main advantages of optical fiber: – – – – –
Greater capacity (2Gbps over tens of kilometers) Smaller size and lighter weight Lower attenuation Electromagnetic isolation Greater repeater spacing
• Long-haul fiber transmission is becoming increasingly common in the telephone network. – About 900 miles with 20,000 to 60,000 voice channels
Dr. Indranil Sen Gupta
Transmission Media
Slide 10
Optical Fiber (contd.) • Optical fiber systems operate in the range of 10^14 to 10^15 Hz. – Light from a source enters the cylindrical glass or plastic core. – Rays at shallow angles are reflected and propagated along the fiber; other rays are absorbed by the surrounding material.
• Three types of communication: – Multimode: Refers to the variety of angles that will reflect. Multiple propagation path exists, signal elements spread out in time, and hence limits the data rate. – Single-mode:- When the fiber code radius is reduced, fewer angles will reflect. By reducing the radius of the core to the order of a wavelength, only a single angle or mode can pass (the axial ray). – Multimode graded index: By varying the refractive index of the core, rays may be focussed more efficiently than multimode. Dr. Indranil Sen Gupta
Transmission Media
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Attenuation of Typical Guided Media
Figure 3.3 from the book by Stallings
Dr. Indranil Sen Gupta
Transmission Media
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Wireless Transmission • Transmission and reception are achieved by means of antenna. – For transmission, the antenna radiated electromagnetic radiation in the air. – For reception, the antenna picks up electromagnetic waves from the surrounding medium.
• Basically two types of configuration: – Transmitting antenna puts out a focussed electromagnetic beam. • Transmitter & receiver must be carefully aligned.
– Transmitted signal spreads in all directions. • Can be received by many antennas. Dr. Indranil Sen Gupta
Transmission Media
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Contd. • Three general ranges of frequencies: – 2-40 GHz • microwave frequencies • highly directional beams are possible • suitable for point-to-point and satellite transmission
– 30 MHz - 1 GHz • broadcast radio range • suitable for omnidirectional applications
– 0.3 - 200 THz • infrared range • useful for point-to-point and multipoint applications within confined areas, such as a single room. Dr. Indranil Sen Gupta
Transmission Media
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Wireless: Terrestial Microwave • Most common type of microwave antenna is the parabolic dish. – Antenna is fixed rigidly – Focuses a narrow beam to achieve line-of-sight transmission
• Primarily used in long-haul communication, as an alternative to coaxial cable or optical fiber. • Maximum distance between antennas d = 7.14 sqrt(kh)
where d is the distance in Km, h is the antenna height in meters, and k is an adjustment factor (= 1.33 approx.) Dr. Indranil Sen Gupta
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Wireless: Satellite Microwave • A communication satellite is basically a microwave relay station. – Used to link two or more earth stations. – Satellite receives transmission on one frequency band (uplink), amplifies or repeats the signal, and transmits it on another frequency (downlink). – A single orbiting satellites operate on a number of frequency bands, called transponders.
• Geostationary orbit – often necessary for it to function effectively – height of 35,784 Km Dr. Indranil Sen Gupta
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Satellite Communication Configurations Point-to-point link
Broadcast link
Multiple Receivers
Multiple Receivers
Transmitter Dr. Indranil Sen Gupta
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Satellite (contd.) • Two satellites using the same frequency band, if close enough, will interfere with each other. – Minimum 4 degrees spacing in the 4/6 GHz band. – Minimum 3 degrees spacing in the 12/14 GHz band.
• Most important applications of satellite: – Television distribution – Long-distance telephone transmission – Private business network (using VSATs).
Dr. Indranil Sen Gupta
Transmission Media
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Very Small Aperture Terminal (VSAT) • A relatively low-cost solution. • A number of subscriber stations are equipped with low-cost VSAT antennas. – Using some protocol, these stations share a satellite channel for transmission to a hub station. – The hub station can exchange messages with each of the subscribers as well as relay messages between subscribers.
Dr. Indranil Sen Gupta
Transmission Media
Slide 19
VSAT Configuration 256 Kbps 256 Kbps 56 Kbps
56 Kbps
HUB
Dr. Indranil Sen Gupta
Transmission Media
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Wireless: Broadcast Radio • Omnidirectional • Covers VHF and part of UHF band – 30 MHz -- 1 GHz
• Apart from FM radio as well as UHF and VHF TV, also used in data networking applications.
Dr. Indranil Sen Gupta
Transmission Media
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Wireless: Infrared • Uses transceivers that modulate non-coherent infrared light. • Must be in light of sight, either directly or via reflection from a light-colored surface such as the ceiling of a room. • Does not penetrate walls. • No frequency allocation issue. – No licensing required.
Dr. Indranil Sen Gupta
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