Chapter 7

1. Chapter 7 The Design of Long-Distance Links Bahman R. Alyaei

2. 1 Introduction • The network may be defined as a group of switching nodes interconnected by links. • We may refer to a link as a transmission highway between switches carrying one or more traffic relations. • Links: provide connection for a multiple of switch inputs from one stage to a multiple of switch outputs in another stage. Bahman R. Alyaei

3. Continue… Examples of links. Bahman R. Alyaei

4. 2 The Bearer • Bearer: is what carries the information signals (digital domain). • It could be a pair of wires or two pairs on a four-wire basis, a radio carrier in each direction, a coaxial cable, or a fiber-optic cable. • In the text that follows, it is assumed that the bearer will be transporting some sort of multiplex configuration, probably in a digital format (TDM). Bahman R. Alyaei

5. Continue… • Modern long-distance links use either radio or fiber-optic cable as the medium of choice. • The decision on which one to use is driven by economics and capacity. • Optical fiber is so far superior as transport for telecommunication digital configurations that coaxial cable must be removed from discussion. Bahman R. Alyaei

6. Continue… • Coaxial cable requires many more active repeaters per unit length than fiber optics. • Jitter: is the deviation from the ideal timing of an event. • Jitter is a major transmission impairment on digital systems, builds up as a function of the number of repeaters in tandem. • Optical fiber needs no equalization, whereas coaxial cable needs equalizers. Bahman R. Alyaei

7. 3 Introduction to Radio Transmission • Cable transmission medium (metallic and fiber): • Display little variability in performance • Need not be shared. • Do not require licensing (but often require right-of-way). • Right-of-way: is a free path assigned by the switch for a subscriber. Bahman R. Alyaei

8. Continue… • Radio transmission medium: • It displays notable variability in performance. • Shared with other users (not secure). • Requires licensing (different frequency bands are allocated for different applications and to different service providers by telecommunication authorities). Bahman R. Alyaei

9. Continue… • A major factor in the medium selection process is information bandwidth. • Fiber optics seems to have nearly an infinite bandwidth. • Radio systems have very limited information bandwidth (bandpass). Bahman R. Alyaei

10. Continue… • Then, why use radio in the first place if it has so many drawbacks? • Often, radio turns out to be less expensive than fiber-optic cable. • No requirement for right-of-way. • Less vulnerable to vandalism. Bahman R. Alyaei

11. Continue… • Not susceptible to “accidental” cutting of the link. • Often more suited to crossing rough terrain (hilly areas). • Often more practical in heavily urbanized areas (inside the city). • As a backup to fiber-optic cable links. Bahman R. Alyaei

12. Radio links Bahman R. Alyaei

13. Continue… Microwave link tower Bahman R. Alyaei

14. Continue… • Fiber optic cable systems provide strong competition with line-of-sight (LOS) microwave, but LOS microwave does have a place and a good market. • Satellite communications is an extension of LOS microwave. • It is also feeling the “pinch” of competition from fiber optic systems. Bahman R. Alyaei

15. Continue… • Satellite link has two drawbacks: • Limited information bandwidth. • Excessive delay when the popular geostationary satellite systems are utilized. Bahman R. Alyaei

16. 4 Design Essential For LOS Microwave Systems • LOS microwave provides broadband bearer connectivity over a link or series of links in tandem. • LOS phenomenon at frequencies from 150 MHz and upwards into the millimeter spectrum. • Each link can be up to 30 miles (46 km) long or more depending on terrain topology. • Some links extend over 100 miles (160 km). Bahman R. Alyaei

17. Continue… Frequency bands and their applications Bahman R. Alyaei

18. Continue… Refraction phenomena A sketch of an LOS microwave radio relay system Bahman R. Alyaei

19. Continue… • LOS implies that the antenna of the radio link on one end has to be able to “see” the antenna on the other end. • This may not necessarily be true, but it does give some idea of the problem. • Let us suppose smooth earth (i.e. no mountains or ridges, buildings, or sloping ground of any sort). • Here our LOS distance is limited by the horizon. Bahman R. Alyaei

20. Continue… • The horizon (or skyline): is the apparent line that separates earth from sky. • Is the line that divides all visible directions into two categories: those that intersect the Earth's surface, and those that do not. • At many locations, the true horizon is obscured by trees, buildings, mountains, etc., and the resulting intersection of earth and sky is called the optical or visible horizon. Bahman R. Alyaei

21. Continue… Horizon Bahman R. Alyaei

22. Continue… • Given a LOS microwave antenna of a heighthin feet above ground surface, then, the distance to optical horizond in miles to the horizon just where the ray beam will graze the rounded earth surface horizon can be calculated using Bahman R. Alyaei

23. Continue… • But, the real Earth is surrounded by an atmosphereof air, the density and refractive index of which vary considerably depending on the temperature and pressure. • Usually, the density of the air just above the surface of the Earth is greater than its density at greater altitudes. Bahman R. Alyaei

24. Continue… • This makes its refractive index greater near the surface than higher, which causes light that is traveling roughly horizontally to be refracted downward, so it goes, to some small degree, around the curvature of the Earth's surface. • This makes the air refract light to varying extents, affecting the appearance of the horizon. Bahman R. Alyaei

25. Continue… Radio and optical horizon (smooth earth) Bahman R. Alyaei

26. Continue… • This phenomena led to define a new horizon which is called radio horizon. • In this case, at a height hin feet the distance to radio horizon d in miles is usually calculated by Bahman R. Alyaei

27. Continue… • The design of a microwave LOS link involves five basic steps: • Setting performance requirements. • Site selection and preparation of a path profile to determine antenna tower heights. • Carrying out a path analysis, also called a link budget . • Running a path/site survey. • Test of the system prior to cutover to traffic. Bahman R. Alyaei

28. 4.2 Setting Performance Requirements • Often a microwave link is part of an extensive system of multiple links in tandem. • Thus we must first set system requirements based on the output of the far-end receiver of the several or many links. • The specification would be a bit error rate (BER) on a digital bit stream. Bahman R. Alyaei

29. Continue… • The specification should be based on an existing standard. • BER on a single link may have a 1×10−12 requirement during unfaded conditions. • For many digital links, a threshold floor of no worse than 1×10−3 is set. • This value is related to supervisory signaling where, if further degraded, supervisory signaling is lost and the link drops out (i.e., dial-tone is returned to the subscriber). Bahman R. Alyaei

30. 4.3 Site Selection and Preparation of a Path Profile • A path profile: is a graphic representation of the physical features of a propagation path in the vertical plane containing both endpoints of the path, showing the surface of the Earth and including trees, buildings, and other features that may obstruct the radio signal. • After site selection, we will prepare a path profile of each link to determine the heights of radio towers to achieve LOS. Bahman R. Alyaei

31. Continue… • For long distance (hundreds of miles or kilometers), there will be two terminal sites, where the system begins and ends. • Along the way, repeater sites will be required. • At some repeater sites, we may have need to drop and insert traffic. • Other sites will just be repeaters. • These drop and insert points may just as well be buildings or other facilities in a private/corporate network. Bahman R. Alyaei

32. Simplified functional block diagram of the LOS microwave system Bahman R. Alyaei

33. Continue… 24 ft x 24 ft Tower Base Communication Shelter Bahman R. Alyaei

34. Continue… Tower Base Communication Shelter Bahman R. Alyaei

35. Continue… Inside of the Tower Base Communication Shelter Bahman R. Alyaei

36. Continue… • In gross system design • Exchange location, particularly with tandem/transit exchanges, must be considered in light of probable radio and cable routes. • Another consideration is electromagnetic interference (EMI). Bahman R. Alyaei

37. Continue… • Midcity Repeater-Relay or terminal sites have the following advantages: • Colocation with a local or toll exchange. • Use of tall buildings as natural towers. • And have the following disadvantages: • Wave reflections (multipath) off buildings. • Electromagnetic interference (EMI) problems, particularly from other nearby emitters and industrial emission. Bahman R. Alyaei

38. Continue… • Terminal sites will be in or near heavily populated areas and preferably collocated with a toll exchange. • The tops of modern large office buildings, if properly selected, are natural towers. • Repeater-Relay sites are heavily influenced by intermediate terrain. • Accessible hilltops or mountain tops are good prospective locations. Bahman R. Alyaei

39. Continue… Building top and hilltop repeaters Bahman R. Alyaei

40. Continue… • High towers are the rule over flat country. • The higher the tower, the longer the LOS distance. • Thus, on a given link, fewer repeaters would be required if towers could be higher. • Hence there is a trade-off between tower height and number of repeaters. Bahman R. Alyaei

41. Continue • A rule-of-thumb for maximum tower height is 300 ft. • Certainly, towers can be built higher. • For example, there are broadcast towers in excess of 1000 ft. • As a tower goes above 300 ft, the cost of maintaining twistand sway requirements begins to escalate. Bahman R. Alyaei

42. 5 The Satellite • Most of the presently employed communication satellites are RF repeaters. • It may decode and recode a digital bit stream. • It also may have some bulk switching capability, switching to cross-links connecting to other satellites. Bahman R. Alyaei

43. Continue… Satellite as an RF repeater Bahman R. Alyaei

44. Continue… • Satellites in general are used for: • Mobile applications such as communications to ships, vehicles, planes and hand-held terminals, • TV broadcasting. • Radiobroadcasting. Bahman R. Alyaei

45. Continue… • In commercial telecommunications there are three methods of handling digital communication by satellite: • TDMA (Time Division Multiple Access). • FDMA (Frequency Division Multiple Access). • VSAT Network (Very Small Aperture Antenna). Bahman R. Alyaei

46. Continue… • There are two types of radio station on the earth: • Earth Station: is a radio frequency facility located on the earth’s surface that communicates with satellites. • Terrestrial Station: is a radio facility on the earth’s surface that communicates with other similar facilities on the earth’s surface. Bahman R. Alyaei

47. Continue… Earth station Bahman R. Alyaei

48. Continue… Earth station Bahman R. Alyaei

49. Continue… Earth station Bahman R. Alyaei

50. Continue… Earth and terrestrial stations Bahman R. Alyaei