Data and Computer Communications

1. Data and Computer Communications Chapter 4 –Transmission Media Eighth & Ninth Editions by William Stallings

2. Transmission Media Communication channels in the animal world include touch, sound, sight, and scent. Electric eels even use electric pulses. Ravens also are very expressive. By a combination voice, patterns of feather erection and body posture ravens communicate so clearly that an experienced observer can identify anger, affection, hunger, curiosity, playfulness, fright, boldness, and depression. —Mind of the Raven, Bernd Heinrich

3. Overview media that are used to convey information can be classified as guided or unguided • guided - wire / optical fiber (provide Physical path) • unguided – wireless employ an antenna for transmitting through air, vacuum, or water. • characteristics and quality determined by medium and signal • in unguided media - bandwidth produced by the antenna is more important • in guided media - medium is more important • key concerns are data rate and distance

4. Directionality. • One key property of signals transmitted by antenna is directionality. • Signals at lower frequencies are omnidirectional:the signal propagates in all directions from the antenna. • At higher frequencies :it is possible to focus the signal into a directional beam. • In considering the design of data transmission systems, key concerns are data rate and distance: the greater the data rate and the less the distance the better.

5. Design Factors • bandwidth • higher bandwidth gives higher data rate • transmission impairments • eg. attenuation(limit the distance) • Interference (competing signals in overlapping frequency bands ) • number of receivers in guided media • more receivers introduces more attenuation and distortion

6. Electromagnetic Spectrum frequencies at which various guided media and unguided transmission techniques operate

7. Frequency Range Typical Attenuation Typical Delay Repeater Spacing Twisted pair (with loading) 0 to 3.5 kHz 0.2 dB/km @ 1 kHz 50 µs/km 2 km Twisted pairs (multi-pair cables) 0 to 1 MHz 0.7 dB/km @ 1 kHz 5 µs/km 2 km Coaxial cable 0 to 500 MHz 7 dB/km @ 10 MHz 4 µs/km 1 to 9 km Optical fiber 186 to 370 THz 0.2 to 0.5 dB/km 5 µs/km 40 km Transmission Characteristics of Guided Media Based on this chart a decision can be made on what media to choose!

8. Guided Transmission Media

9. Twisted Pair • Most common guided transmission medium for both analog and digital signals • Most commonly used medium in the telephone network (linking residential telephones to the local telephone exchange, or office phones to a PBX • Much less expensive than the other commonly used guided transmission media (coaxial cable, optical fiber) and is easier to work with. • The twist length typically varies from 5 to 15 cm

10. Twisted Pair - Transmission Characteristics • analog • needs amplifiers every 5km to 6km • digital • can use either analog or digital signals • needs a repeater every 2-3km • limited distance • limited bandwidth (1MHz) • limited data rate (100MHz) • susceptible to interference and noise

11. Unshielded vs Shielded TP • unshielded Twisted Pair (UTP) • ordinary telephone wire • cheapest • easiest to install • suffers from external EM interference • is ordinary telephone wire • shielded Twisted Pair (STP) • metal braid or sheathing that reduces interference • more expensive • harder to handle (thick, heavy) • in a variety of categories - see EIA-568

12. UTP Categories In response to the need to support higher speeds, Standard EIA-568-A was issued in 1995. The new standard reflects advances in cable and connector design and test methods

13. Comparison of Shielded and Unshielded Twisted Pair Table 4.2 summarizes the performance of Category 3 and 5 UTP, as well as the STP specified in EIA-568-A The strength of a signal falls off with distance over any transmission medium Near-end crosstalk see next slide

14. Near End Crosstalk • coupling of signal from one pair to another • occurs when transmit signal entering the link couples back to receiving pair • ie. near transmitted signal is picked up by near receiving pair

15. Coaxial Cable like twisted pair, consists of two conductors, but is constructed differently to permit it to operate over a wider range of frequencies used in a wide variety of applications, including: • Television distribution - aerial to TV & CATV systems • Long-distance telephone transmission - traditionally used for inter-exchange links, now being replaced by optical fiber/microwave/satellite • Short-run computer system links • Local area networks

16. Coaxial Cable - Transmission Characteristics • superior frequency characteristics to TP • performance limited by attenuation & noise • analog signals • amplifiers every few km • closer if higher frequency • up to 500MHz • digital signals • repeater every 1km • closer for higher data rates

17. Optical Fiber An optical fiber is a thin flexible medium capable of guiding an optical ray. Various glasses and plastics can be used to make optical fibers. An optical fiber cable has a cylindrical shape and consists of three concentric sections: 1. the core 2. the cladding, 3. the jacket

18. Optical Fiber • The core: is the innermost section and consists of one or more very thin strands, or fibers, made of glass or plastic; the core has a diameter in the range of 8 to 50 µm. • The cladding: a glass or plastic coating that has optical properties different from those of the core and a diameter of 125 µm. The interface between the core and 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 environmental dangers. • .

19. Optical Fiber - Benefits • greater capacity • data rates of hundreds of Gbps • smaller size & weight • lower attenuation • electromagnetic isolation • not vulnerable to interference, impulse noise, or crosstalk • high degree of security from eavesdropping • greater repeater spacing • 10s of km at least

20. Where is Fiber used? • long-distance telecommunications • In military applications is growing. • local area networking. • Long-haul trunks, Metropolitan trunks, Rural exchange trunks, Subscriber loops & Local area networks

21. Optical Fiber - Transmission Characteristics • uses total internal reflection to transmit light • effectively acts as wave guide for 1014 to 1015 Hz • can use several different light sources • Light Emitting Diode (LED) • cheaper, wider operating temp range, lasts longer • Injection Laser Diode (ILD) • more efficient, has greater data rate • relation of wavelength, type & data rate Both single mode and multimode can support several different wavelengths of light and can employ laser or LED light sources.

22. Optical Fiber Transmission Modes

23. Optical Fiber Transmission Modes (Step-index multimode) • 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. • Multiple propagation paths exist, each with a different path length and hence time to traverse the fiber. This causes signal elements (light pulses) to spread out in time, which limits the rate at which data can be accurately received. • This type of fiber is best suited for transmission over very short distances.

24. Optical Fiber Transmission Modes (Single-mode) • When the fiber core 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. • provides superior performance Because there is a single transmission path with single-mode transmission. • the distortion found in multimode cannot occur. • used for long-distance applications, including telephone and cable television.

25. Optical Fiber Transmission Modes (Single-mode) • light rays moving down the axis advance more slowly than those near the cladding. • light in the core curves helically because of the graded index, reducing its travel distance. • often used in local area networks.

26. Attenuation in Guided Media attenuation versus wavelength for the various types of wired media attenuation for twisted pair is a very strong function of frequency coaxial cable has frequency characteristics that are superior to those of twisted pair and can be used effectively at higher frequencies and data rates. The unusual shape of the curve is due to the combination of a variety of factors that contribute to attenuation. absorption and scattering, which is the change in direction of light rays after they strike small particles or impurities in the medium.

27. Wireless Transmission Frequencies • 2GHz to 40GHz • microwave • highly directional • point to point • Used in satellite • 30MHz to 1GHz • Omnidirectional (radio range) • 3 x 1011 to 2 x 1014 • Infrared used for LAN

28. Antennas For unguided media, transmission and reception are achieved by means of an antenna • electrical conductor used to radiate or collect electromagnetic energy • transmission antenna • radio frequency energy from transmitter • converted to electromagnetic energy by antenna • radiated into surrounding environment • reception antenna • electromagnetic energy impinging on antenna • converted to radio frequency electrical energy • fed to receiver • same antenna is often used for both purposes

29. Radiation Pattern • power radiated in all directions • not same performance in all directions • an isotropic antenna is a (theoretical) point in space • radiates in all directions equally • with a spherical radiation pattern

30. Parabolic Reflective Antenna Paraboloid surfaces are used in headlights, optical and radio telescopes, and microwave antennas parabolic reflective antenna, used in terrestrial microwave and satellite applications. A parabola is the locus of all points equidistant from a fixed line (the directrix) and a fixed point (the focus) not on the line, If a parabola is revolved about its axis, the surface generated is called a paraboloid.

31. Antenna Gain • measure of directionality of antenna • power output in particular direction verses that produced by an isotropic antenna (a reference radiator with which other sources are compared) • measured in decibels (dB) • Effective area relates to size and shape • Note: Antenna gain does not refer to obtaining more output power than input power but rather to directionality.

32. Terrestrial Microwave • used for long haul telecommunications • and short point-to-point links • requires fewer repeaters but line of sight • use a parabolic dish to focus a narrow beam onto a receiver antenna • 1-40GHz frequencies • higher frequencies give higher data rates • main source of loss is attenuation • distance, rainfall also interference

33. Satellite Microwave • satellite is relay station • receives on one frequency, amplifies or repeats signal and transmits on another frequency • eg. uplink 5.925-6.425 GHz & downlink 3.7-4.2 GHz • typically requires geo-stationary orbit • height of 35,784km • spaced at least 3-4° apart • typical uses • Television • long distance telephone • private business networks • global positioning

34. Satellite Point to Point Link Figure 4.6 depicts in a general way two common configurations for satellite communication. In the first, the satellite is being used to provide a point-to-point link between two distant ground-based antennas.

35. Satellite Broadcast Link the satellite provides communications between one ground-based transmitter and a number of ground-based receivers.

36. Broadcast Radio • Radio is a general term used to encompass frequencies in the range of 3 kHz to 300 GHz. We are using the informal term broadcast radio to cover the VHF and part of the UHF band • use broadcast radio, 30MHz - 1GHz, for: • FM radio • Ultra high frequency (UHF)and VHF (Very high frequency)television • is omnidirectional • still need line of sight • suffers from multipath interference • reflections from land, water, other objects

37. Infrared Communications • Achieved using transmitters/receivers (transceivers) that modulate noncoherent light(space time codes are a way of transmitting data in wireless communications)infrared • Transceivers must be within the line of sigh • are blocked by walls • no licenses required • typical uses • TV remote control • IRD port

38. Wireless PropagationGround Wave A signal radiated from an antenna travels along one of three routes: ground wave, sky wave, or line of sight Ground wave propagation more or less follows the contour of the earth and can propagate considerable distances, well over the visual horizon. This effect is found in frequencies up to about 2 MHz. The best-known example of ground wave communication is AM radio.

39. Wireless PropagationSky Wave Sky wave propagation is used for amateur radio, CB radio, and international broadcasts such as BBC and Voice of America. A sky wave signal can travel through a number of hops, bouncing back and forth between the ionosphere and the earth's surface (Refraction).

40. Wireless PropagationLine of Sight • Neither ground wave nor sky wave propagation modes operate, and communication must be by line of sight . • The transmitting and receiving antennas must be within an effective line of sight of each other.

41. Refraction • velocity of electromagnetic wave is a function of density of material ~3 x 108 m/s in vacuum, less in anything else • speed changes as move between media • Index of refraction (refractive index) is • sin(incidence)/sin(refraction) • varies with wavelength • have gradual bending if medium density varies • density of atmosphere decreases with height • results in bending towards earth of radio waves • hence optical and radio horizons differ

42. Line of Sight Transmission Some impairments specific to wireless line-of-sight transmission. • Free space loss • loss of signal with distance • Atmospheric Absorption • from water vapour and oxygen absorption • Multipath • multiple interfering signals from reflections • Refraction • bending signal away from receiver

43. Free Space Loss Free space loss can be expressed in terms of the ratio of the radiated power Pt to the power Pr received by the antenna or, in decibels, by taking 10 times the log of that ratio.

44. Multipath Interference Figure 4.11 illustrates in general terms the types of multipath interference typical in terrestrial, fixed microwave and in mobile communications. For fixed microwave, in addition to the direct line of sight, the signal may follow a curved path through the atmosphere due to refraction and the signal may also reflect from the ground. For mobile communications, structures and topographic features provide reflection surfaces.

45. Summary • looked at data transmission issues • frequency, spectrum & bandwidth • analog vs digital signals • transmission impairments