Biblio5

200 LOCAL AND LONG-DISTANCE NETWORKS

Figure 8.20 Example of echo round-trip delay (5.5 + 5.5 c 11 ms round-trip delay).

conductors). The question arises as to what measures can be taken to reduce losses and still keep echo within tolerable limits. One obvious target is to improve return losses at the hybrids. If all hybrid return losses are improved, the echo tolerance curve shifts; this is because improved return losses reduce the intensity of the echo returned to the talker. Thus the talker is less annoyed by the echo effect.

One way of improving return loss is to make all two-wire lines out of the hybrid look alike—that is, have the same impedance. The switch at the other end of the hybrid (i.e., on the two-wire side) connects two-wire loops of varying length, thus causing the resulting impedances to vary greatly. One approach is to extend four-wire transmission to the local office such that each hybrid can be better balanced. This is being carried out with success in Japan. The U.S. Department of Defense has its Autovon (automatic voice network), in which every subscriber line is operated on a four-wire basis. Two-wire subscribers connect through the system via PABXs (private automatic branch exchanges).

As networks evolve to all-digital, four-wire transmission is carried directly through the local serving switch such that subscriber loops terminate through a hybrid directly to a PCM channel bank. Hybrid return losses could now be notably improved by adjusting the balancing network for its dedicated subscriber loop.

8.5.7 Loss Plan for Digital Networks (United States)

For digital connections terminated in analog access lines, the required loss values are dependent on the connection architecture:

•For interLATA or interconnecting network connections, the requirement is 6 dB.

•For intraLATA connections involving different LECs (local exchange carriers), 6 dB is the preferred value, although 3 dB may apply to connections not involving a tandem switch.

•For intraLATA connections involving the same LEC, the guidelines are 0 –6 dB (typically 0 dB, 3 dB, or 6 dB).

The choice of network loss value depends on performance considerations, administrative simplicity, and current network design (Ref. 4).

Loss can be inserted in a digital bit stream by using a digital signal processor involving a look-up table. By doing this, the bit sequence integrity is broken for each digital 8-bit time slot. Some of these time slots may be carrying data bit sequences. For this

REVIEW EXERCISES

201

reason we cannot break up this bit integrity. To avoid this intermediate digital processing (which destroys bit integrity), loss is inserted on all-digital connections on the receiving end only, where the digital-to-analog conversion occurs (i.e., after the signal has been returned to its analog equivalent). Devices such as echo cancelers, which utilize digital signal processing, need to have the capability of being disabled when necessary, to preserve bit integrity.

In Section 8.5.6 round-trip delay was brought about solely by propagation delay. In digital networks there is a small incremental delay due to digital switching and digital multiplexing. This is due to buffer storage delay, more than anything else.

REVIEW EXERCISES

1. Name at least three factors that affect local network design.

2. What are the three basic underlying considerations in the design of a long-distance (toll) network?

3. What is the fallacy of providing just one high-capacity trunk group across the United States to serve all major population centers by means of tributaries off the main trunk?

4. How can the utilization factor on trunks be improved?

5. For long-distance (toll) switching centers, what is the principal factor in the placement of such exchanges (differing from local exchange placement substantially)?

6. How are the highest levels of a national hierarchical network connected, and why is this approach used?

7. On a long-distance toll connection, why must the number of links in tandem be limited?

8. What type of routing is used on the majority of international connections?

9. Name two principal factors used in deciding how many and where long-distance (toll) exchanges will be located in a given geographic area.

10. Discuss the impacts of fan-outs on the number of hierarchical levels.

11. Name the three principal bases required at the outset for the design of a longdistance network.

11. Once the hierarchical levels have been established and all node locations identified, what is assembled next?

12.	Define a final route.
13.	A grade of service no greater than		% per link is recommended on a final
	route.

14. There are two generic types of routing schemes. What are they?

15. Name three different types of dynamic routing and explain each in one sentence.

16. What is crankback?

17. Give an example of state-dependent routing.

202 LOCAL AND LONG-DISTANCE NETWORKS

18. What is the principal cause of echo in the telephone network?

19. What causes singing in the telephone network?

20. Differentiate balance return loss from the point of view of stability (singing) from echo return loss.

21. How can we control echo? (Two answers required).

22. The stability of a telephone connection depends on three factors. Give two of these factors.

23. Based on the new loss plan for North America for the digital network, how much loss is inserted for interLATA connections?

REFERENCES

1. International Telephone Routing Plan, CCITT Rec. E.171, Vol. II, Fascicle II.2, IXth Plenary Assembly, Melbourne, 1988.

2. The Transmission Plan, ITU-T Rec. G.101, ITU Geneva, 1994. 3. Traffic Routing, CCITT Rec. E.170, ITU Geneva, 1992.

4. BOC Notes on the LEC Networks—1994, Special Report SR-TSV-002275, Issue 2, Bellcore, Piscataway, NJ, 1994.

5. Influence of National Systems on Stability and Talker Echo in International Connections, ITU-T Rec. G.122, ITU Geneva, 1993.

6. Control of Talker Echo, ITU-T Rec. G.131, ITU Geneva, 1996.

7. Terms and Definitions, CCITT Rec. B.13, Fascicle I.3, IXth Plenary Assembly, Melbourne, 1988.

Fundamentals of Telecommunications. Roger L. Freeman

Copyright 1999 Roger L. Freeman

Published by John Wiley & Sons, Inc.

ISBNs: 0-471-29699-6 (Hardback); 0-471-22416-2 (Electronic)

9

CONCEPTS IN TRANSMISSION

TRANSPORT

9.1 CHAPTER OBJECTIVE

A telecommunication network consists of customer premise equipment (CPE), switching nodes, and transmission links, as illustrated in Figure 9.1. Chapter 5 dealt with one important type of CPE, namely, the telephone subset. The chapter also covered wirepair connectivity from the telephone subset to the local serving switch over a subscriber loop. Basic concepts of switching were reviewed in Chapter 4, and Chapter 6 covered digital switching. In this chapter we introduce the essential aspects for the design of long-distance links.

There are four different ways by which we can convey signals from one switching node to another:

1. Radio;

2. Fiber optics;

3. Coaxial cable; and

4. Wire pair.

Emphasis will be on radio and fiber optics. The use of coaxial cable for this application is deprecated. However, it was widely employed from about 1960 to 1985 including some very-high-capacity systems. One such system (L5) crossed the United States from coast- to-coast with a capacity in excess of 100,000 simultaneous full-duplex voice channels in FDM configurations (see Section 4.5.2). Fiber-optic cable has replaced the greater portion of these coaxial cable systems. There is one exception. Coaxial cable is still widely employed in cable television configurations (see Chapter 15). Wire pair remains the workhorse in the subscriber plant.

At the outset, we can assume that these transmission links are digital and will be based on the PCM configurations covered in Chapter 6, namely, either of the DS1 (T1) or E1 families of formats. However, the more advanced, higher-capacity digital formats such as synchronous optical network (SONET) or synchronous digital hierarchy (SDH) (see Chapter 17) are now being widely deployed on many if not most new fiber-optic systems, and with lower capacity configurations on certain radio systems.

203

204 CONCEPTS IN TRANSMISSION TRANSPORT

Figure 9.1 A telecommunication network consists of customer premise equipment (CPE), switching nodes, and interconnecting transmission links.

9.2 RADIO SYSTEMS

9.2.1 Scope

The sizes, capacities, ranges, and operational frequency bands for radio systems vary greatly. Our discussion will be limited to comparatively high-capacity systems. Only two system types meet the necessary broadband requirements of the long-distance network. These are line-of-sight (LOS) microwave and satellite communications. Satellite communications is really nothing more than an extension of LOS microwave.

9.2.2 Introduction to Radio Transmission

Wire, cable, and fiber are well-behaved transmission media, and they display little variability in performance. The radio medium, on the other hand, displays notable variability in performance. The radio-frequency spectrum is shared with others and requires licensing. Metallic and fiber media need not be shared and do not require licensing (but often require right-of-way).

A major factor in the selection process is information bandwidth. Fiber optics seems to have nearly an infinite bandwidth. Radio systems have very limited information bandwidths. It is for this reason that radio-frequency bands 2 GHz and above are used for PSTN and private network applications. In fact, the U.S. Federal Communications Commission (FCC) requires that users in the 2-GHz band must have systems supporting 96 digital voice channels where bandwidths are still modest. In the 4- and 6-GHz bands, available bandwidths are 500 MHz, allocated in 20and 30-MHz segments for each radio-frequency carrier.

One might ask, why use radio in the first place if it has so many drawbacks? Often, it turns out to be less expensive compared with fiber optic cable. But there are other factors such as:

•No requirement for right-of-way;

•Less vulnerable to vandalism;

•Not susceptible to “accidental” cutting of the link;

•Often more suited to crossing rough terrain;

•Often more practical in heavily urbanized areas; and

•As a backup to fiber-optic cable links.

Fiber-optic cable systems provide strong competition with LOS microwave, but LOS microwave does have a place and a good market.

9.2 RADIO SYSTEMS	205
Satellite communications is an extension of LOS microwave. It	is also

feeling the “pinch” of competition from fiber-optic systems. It has two drawbacks. First, of course, is limited information bandwidth. The second is excessive delay when the popular geostationary satellite systems are utilized. It also shares frequency bands with LOS microwave.

One application showing explosive growth is very small aperture terminal (VSAT) systems. It is very specialized and has great promise for certain enterprise networks, and there are literally thousands of these networks now in operation.

Another application that is becoming widely deployed is large families of low earth orbit (LEO) satellites such as Motorola’s Iridium, which provide worldwide cellular/ PCS coverage. Because of its low altitude orbit (about 785 km above earth’s surface), the notorious delay problem typical of GEO (geostationary satellite) is nearly eliminated.

9.2.3 Line-of-Sight Microwave

9.2.3.1 Introduction. Line-of-sight (LOS) microwave provides a comparative broadband connectivity over a single link or a series of links in tandem. We must be careful on the use of language here. First a link, in the sense we use it, connects one radio terminal to another or to a repeater site. The term link was used in Figure 9.1 in the “network” sense. Figure 9.2 illustrates the meaning of a “link” in LOS microwave. Care must also be taken with the use of the expression line-of-sight. Because we can “see” a distant LOS microwave antenna does not mean that we’re in compliance with line-of- sight clearance requirements.

We can take advantage of this “line-of-sight” phenomenon at frequencies from about 150 MHz well into the millimeter-wave region.1 Links can be up to 30 miles long depending on terrain topology. I have engineered some links well over 100 miles long. In fact, links with geostationary satellites can be over 23,000 miles long.

On conventional LOS microwave links, the length of a link is a function of antenna height. The higher the antenna, the further the reach. Let us suppose smooth earth. This means an earth surface with no mountains, ridges, buildings, or sloping ground whatsoever. We could consider an over-water path as a smooth earth path. Some paths on the North American prairie approach smooth earth. In the case of smooth earth, the LOS distance from an antenna is limited by the horizon. Given an LOS microwave

Figure 9.2 A sketch of an LOS microwave radio relay system.

1Millimeter-wave region is where the wavelength of an equivalent frequency is less than 1 cm.

206 CONCEPTS IN TRANSMISSION TRANSPORT

Figure 9.3 Radio and optical horizon (smooth earth).

antenna of hft or h′m above ground surface, the distance dmi or dkm to the horizon just where the ray beam from the transmitting antenna will graze the rounded surface of the horizon can be calculated using one of the formulas given as follows:

To optical horizon (k c 1):

d

h

3h

(9.1a)

c

2

and to the radio horizon (k

c

4/ 3):

d

c

f2h

(9.1b)

d′

2.9(2h′)1/ 2,

(9.1c)

c

where k expresses the bending characteristic of the path.

These formulas should only be used for rough estimates of distance to the horizon under smooth earth conditions. As we will find out later, the horizon clearance must be something greater than (n feet or meters) of grazing. The difference between formulas (9.1a) and (9.1b) and (9.1c) is that formula (9.1a) is “true” line-of-sight and expresses the optical distance. Here the radio ray beam follows a straight line. Under most circumstances the microwave ray beam is bent toward the earth because of characteristics of the atmosphere. This is expressed in formulas (9.1b) and (9.1c), and assumes the most common bending characteristic. Figure 9.3 is a model that may be used for formulas (9.1). It also shows the difference between the optical distance to the horizon and the radio distance to the horizon.

The design of an LOS microwave link involves five basic steps:

1. Setting performance requirements;

2. Selecting site and preparing a path profile to determine antenna tower heights;

3. Carrying out a path analysis, often called a link budget. (Here is where we dimension equipment to meet the performance requirements set in step 1);

4. Physically running a path/ site survey; and

5. Installing equipment and testing the system prior to cutting it over to carry traffic.

In the following subsections we review the first four steps.

9.2 RADIO SYSTEMS

207

9.2.3.2 Setting Performance Requirements. As we remember from Chapter 6, the performance of a digital system is expressed in a bit error rate (bit error ratio) (BER). In our case here, it will be expressed as a BER with a given time distribution. A time distribution tells us that a certain BER value is valid for a certain percentage of time, percentage of a year, or percentage of a month.

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 in tandem. If the system were transmitting in an analog format, typically FDM using frequency modulation (FM), the requirement would be given for noise in the derived voice channel; if it were video, a signal-to-noise ratio specification would be provided. In the case we emphasize here, of course, it will be BER on the far-end receiver digital bit stream.

The requirements should be based on existing standards. If the link (or system) were to be designed as part of the North American PSTN, we would use a Bellcore standard. (Ref. 1).2 In this case the BER at the digital interface level shall be less than 2 × 10−10, excluding burst error seconds. Another source is CCIR/ ITU-R. For example, CCIR Rec. 594-3 (Ref. 2) states that the BER should not exceed 1 × 10−6 during more than 0.4% of any month and 1 × 10−3 during more than 0.054% of any month. We will recall that the bottom threshold for bit error performance in the PSTN is 1 × 10−3 to support

supervisory signaling, even though 8-bit PCM (Chapter 6) is intelligible down to a BER of 1 × 10−2.

A common time distribution is 99.99% of a month to be in conformance with ITU- R/ CCIR recommendations (e.g., 0.054% of a month). This time distribution translates directly into time availability, which is the percentage of time a link meets its performance criteria.

9.2.3.3 Site Selection and Preparation of a Path Profile

9.2.3.3.1 Site Selection. In this step we will select operational sites where we will install and operate radio equipment. After site selection, we will prepare a path profile of each link to determine the heights of radio towers to achieve “line of sight.” Sites are selected using large topographical maps. If we are dealing with a long system crossing a distance of hundreds of miles or kilometers, we should minimize the number of sites involved. 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. This concept is illustrated in Figure 9.4. The figure shows the drops and inserts (also called add-drops) of traffic at telephone exchanges. These drop and insert points may just as well be buildings or other facilities in a private/ corporate network. There must be considerable iteration between site selection and path profile preparation to optimize the route.

In essence, the sites selected for drops and inserts will be points of traffic concentration. There are several trade-offs to be considered:

1. Bringing traffic in by wire or cable rather than adding additional drop and insert (add-drop) capabilities at relay point, which provides additional traffic concentration;

2. Siting based on propagation advantages (or constraints) only, versus colocation with exchange (or corporate facility) (saving money for land and buildings); and

2Bellcore is Bell Communications Research, Piscataway, NJ.

208 CONCEPTS IN TRANSMISSION TRANSPORT

Figure 9.4 Simplified functional block diagram of the LOS microwave system shown in Figure 9.2.

3. Choosing a method of feeding (feeders): by light-route radio, fiber optic cable, or wire-pair cable.3

9.2.3.3.2 Calculation of Tower Heights. LOS microwave antennas are mounted on towers. Formula (9.1) allowed us to calculate a rough estimate of tower height. Towers and their installation are one of the largest cost factors in the installation of LOS microwave systems. Thus we recommend that actual tower heights do not exceed 300 ft (92 m). Of course, the objective is to keep the tower height as low as possible and still maintain effective communication. The towers must be just high enough to surmount obstacles in the path. High enough must be carefully defined. What sort of obstacles might we encounter in the path? To name some: terrain such as mountains, ridges, hills, and earth curvature—which is highest at midpath—and buildings, towers, grain elevators, and so on. The path designer should consider using natural terrain such as hilltops for terminal/ relay sites. The designer should also consider leasing space on the top of tall buildings or on TV broadcast towers. In the following paragraphs we review a manual method of plotting a path profile.

From a path profile we can derive tower heights. Path profiles may be prepared by a PC with a suitable program and the requisite topological data for the region stored on a disk. Our recommendation is to use ordinary rectangular graph paper such as “millimeter” paper or with gradations down to 1/ 16 inch or better. “B-size” is suggested. There are seven steps required to prepare a path profile:

3Here the word feeders refers to feeding a mainline trunk radio systems. Feeders may also be called spurs.

9.2 RADIO SYSTEMS

209

1. Obtain good topo(logical) maps of the region, at least 1 : 62, 500 and identify the two sites involved, one of which we arbitrarily call a “transmit” site and the other a “receive” site.

2. Draw a straight line with a long straightedge connecting the two sites identified.

3. Follow along down the line identifying obstacles and their height. Put this information on a table, labeling the obstacles “A,” “B,” etc.

4. Calculate earth curvature (or earth bulge) (EC). This is maximum at midpath. On the same table in the next column write the EC value for each obstacle.

5. Calculate the Fresnel zone clearance for each obstacle. The actual value here will be 0.6 of the first Fresnel zone.

6. Add a value of additional height for vegetation such as trees; add a growth factor as well (10 ft or 3 m if actual values are unavailable).

7. Draw a straight line from left to right connecting the two highest obstacle locations on the profile. Do the same from right to left. Where this line intersects the vertical extension of the transmit site and the vertical extension of the receive site defines tower heights.

In step 4, the calculation of EC, remember that the earth is a “sphere.” Our path is a tiny arc on that sphere’s surface. Also in this calculation we must account for the radio ray path bending. To do this we use a tool called K-factor. When the K-factor is greater than 1, the ray beam bends toward the earth, as illustrated in Figure 9.3. When the K-factor is less than 1, the ray beam bends away from the earth.

The EC value (h) is the amount we will add to the obstacle height in feet or meters to account for that curvature or bulge. The following two formulas apply:

hft c 0.667d1d2	/ K (d in miles)	(9.2a)
hm c 0.078d1d2	/ K (d in km)	(9.2b)

where d1 is the distance from the “transmit” site to the obstacle in question and d2 is the distance from that obstacle to the receive site.

Table 9.1 is a guide for selecting the K-factor value. For a more accurate calculation of the K-factor, consult Ref. 3. Remember that the value obtained from Eq. (9.2) is to be added to the obstacle height.

In step 5, calculation of the Fresnel zone clearance, 0.6 of the value calculated is added to the obstacle height in addition to earth curvature. It accounts for the expanding properties of a ray beam as it passes over an obstacle. Use the following formulas to calculate Fresnel zone (radius) clearance:

	Rft c 72.1 h	d1d2	,	(9.3a)
		FD

where F is the frequency in gigahertz, d1 is the distance from transmit antenna to obstacle (statute miles), d2 is the distance from path obstacle to receive antenna (statute miles), and D c d1 + d2. For metric units: