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`material from which the texts reproduced here are extracted, can be obtained from
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`As referenced, some figures and tables throughout this text are copyrighted by The Institute of
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`As referenced, some figures and tables are copyrighted by Bellcore and are reprinted with permission.
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`This book is printed on acid-free paper. @
`
`Copyright © 1999 by Roger L. Freeman
`
`Published by John Wiley & Sons, Inc.
`Published simultaneously in Canada.
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`10987654
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`Library of Congress Cataloging-in-Publicatian Data:
`Freeman, Roger L.
`Fundamentals of telecommunications / Roger L. Freeman.
`p.
`cm.
`“A Wiley-Interscience publication."
`Includes index.
`ISBN 0-471-29699-6 (cloth : alk. paper)
`1. Telecommunication.
`I. Title.
`TK5101.F6595
`1999
`621382—ch1
`
`Printed in the United States of America.
`
`98-4272
`CIP
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`INTRODUCTORY CONCEPTS
`
`1.1 WHAT IS TELECOMMUNICATION?
`
`Many people call telecommunication the world’s most lucrative industry. If we add cel-
`lular and PCS users,1 there are about 1800 million subscribers to telecommunication
`services world wide (1999). Annual expenditures on telecommunications may reach
`900,000 million dollars in the year 2000.2
`Prior to divestiture, the Bell System was the largest commercial company in the United
`States even though it could not be found on the Fortune 500 listing of the largest com-
`panies. It had the biggest fleet of vehicles, the most employees, and the greatest income.
`Every retiree with any sense held the safe and dependable Bell stock. In 1982, Western
`Electric Co., the Bell System manufacturing arm, was number seven on the Fortune 500.
`However, if one checked the Fortune 100 Utilities, the Bell System was up on the top.
`Transferring this information to the Fortune 500, again put Bell System as the leader
`on the list.
`-
`
`from.
`
`We know telecommunication is big business; but what is it? Webster’s (Ref. 1) calls it
`communications at a distance. The IEEE dictionary (Ref. 2) defines telecommunications
`as “the transmission of signals over long distance, such as by telegraph, radio or tele—
`vision.” Another term we often hear is electrical communication. This is a descriptive
`term, but of somewhat broader scope.
`'Some take the view that telecommunication deals only with voice telephony, and
`the typical provider of this service is thelocal telephone company. We hold with a
`wider interpretation. Telecommunication encompasses the electrical communication at
`a distance of voice, data, and image information (e.g., TV and facsimile). These media,
`therefore, will be major topics of this book. The word media (medium, singular) also is
`used to describe what is transporting telecommunication signals. This is termed trans-
`mission media. There are four basic types of medium: (1) wire-pair, (2) coaxial cable,
`(3) fiber optics, and (4) radio.
`
`1.2 TELECOMMUNICATION WILL TOUCH EVERYBODY
`
`In industrialized nations, the telephone is accepted as a way of life. The telephone is con- '
`nected to the public switched telecommunications network (PSTN) for local, national,
`
`lPCS, personal communication services, is a cellular-radiolike service covering a smaller operational area.
`2W6 refrain from using billion because it is ambiguous. Its value differs, depending on where you come
`
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`INTRODUCTORY CONCEPTS
`
`.
`
`a major growth segment.
`There is a growing trend for users to bypass the PSTN partially or completely. The
`use of satellite links in certain situations is one method for PSTN bypass. Another is
`to lease capacity from some other provider. Other provider could be a power company
`with excess capacity on its microwave or fiber optic system. There are other examples,
`such as a railroad with extensive rights-of—way that are used by a fiber optic network.
`Another possibility is to build a private network using any one or a combination of
`fiber optics, line~of-sight-microwave, and satellite connnunications. Some private net-
`works take on the appearance of a mini-PSTN.
`
`
`
`and international voice communications. These same telephone connections may also
`carry data and image informatiOn (e.g., television).
`-
`The personal computer (PC) is beginning to take on a similar role as the telephone,
`that of being ubiquitous. Of course, as we know, the two are becoming married. In
`most situations, the PC uses telephone connectivity to obtain intemet and e-mail ser-
`viccs. Radio adjuncts to the telephone, typically cellular and PCS, are beginning to offer
`similar services such as data communications (including internet) and facsimile (fax), as
`well as voice. The popular press calls these adjuncts wireless. Can we consider wireless
`in opposition to being wired?
`'
`'
`Count the number of devices one has at hOme that carry out some kind of controlling
`or alerting function. They also carry out a personal communication service. Among
`these devices are television remote controls, garage-door Openers, VCR and remote radio
`and CD player controllers, certain types of home security systems, pagers, and cordless
`telephones. We even take cellular radios for granted.
`‘
`In some countries, a potential subscriber has to wait months or years for a telephone.
`Cellular radio, in many cases, provides a way around the problem, where equivalent
`telephone service can be established in an hour——just enough time to buy a cellular
`radio in the local store and sign a contract for service.
`The PSTN has ever-increasing data communications traffic, where the network is
`used as a conduit for data. PSTN circuits may be leased or used in a dial-up mode for
`data connections. Of course, the Internet has given added stimulus to data circuit usage
`of the PSTN. The PSTN sees facsimile as just another data circuit, usually in the dial-up
`mode. Conference television traffic adds still another flavor to PSTN traffic and is also
`
`or combined device, facsimile, or conference TV equipment. It may also be some type
`
`1.3
`
`INTRODUCTORY TOPICS IN TELECOMMUNICATIONS
`
`An overall telecommunications network (i.e., the PSTN) consists of local networks inter-
`connected by a long—distance network. The concept is illustrated in Figure 1.1. This is
`the PSTN, which is open to public correspondence. It is usually regulated by a gov-
`ernment authority or may be a government monopoly, although there is a notable trend
`toward privatization. In the United States the PSTN has been a commercial enterprise
`since its inception.
`
`1.3.1 End-Users, Nodes, and Connectivities
`
`End-users, as the term tells us, provide the inputs to the network and are recipients of
`network outputs. The end—user employs what is called an I/O, standing for input/output
`(device). An I/O may be a PC, computer, telephone instrument, cellular/PCS telephone
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`Local Network
`
`Local Network
`
`Long Distance Network
`
`Local Network
`
`Local Network
`
`Figure 1.1 The PSTN consists of local networks interconnected by a long-distance network.
`
`of machine that provides a stimulus to a coder or receives stimulus from a decoder in,
`say, some sort of SCADA system.3
`-
`End-users usually connect to nodes. We will call a node a point or junction in a
`transmission system Where lines and trunks meet. A node usually carries out a switching
`function. In the case of the local area network (LAN), we are stretching the definition.
`In-this case a network interface unit is used, through which one or more end-users may
`be connected.
`I
`A connectivity connects an end—user to a node, and from there possibly through other
`nodes to some final end—user destination with which the initiating end-user wants to
`communicate. Figure 1.2 illustrates this concept.
`
`3SCADA stands for supervisory control and data acquisition.
`
`To/from other nodes
`or end users
`
`Ed-ser
`
`Figure 1.2
`
`Illustrating the functions of end-users, nodes. and connectivity.
`
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`INTRODUCTORY CONCEPTS
`
`The IEEE (Ref. 2) defines a connection as “an association of channels, switching sys-
`tems, and other functional units set up to provide means for a transfer of information
`between two or more points in a telecommunications network." There would seem to
`be two interpretations of this definition. First, the equipment, both switching and trans—
`mission facilities, is available to set up a path from, say, point A to point B. Assume A
`and B to be user end-points. The second interpretation would be that not Only are the
`circuits available, but they are also connected and ready to pass information or are in
`the information-passing mode.
`At this juncture, the end-users are assumed to be telephone users, and the path that
`is set up is a speech path (it could, of course, be a data or video path). There are three
`sequential stages to a telephone call:
`
`1. Call setup;
`
`2. Information exchange; and
`3. Call take down.
`
`(e.g., the PSTN) network.
`
`Call setup is the stage where a circuit is established and activated. The setup is facilitated
`by signaling, which is discussed in Chapter 7.4 It is initiated by the calling subscriber
`(user) going oficrhook. This is a term that derives from the telephony of the early 1900s.
`It means “the action of taking the telephone instrument out of its cradle.” Two little
`knobs in the cradle pop up, pushed by a Spring action, causing an electrical closure. If
`we turn a light on, we have an electrical closure allowing electrical current to pass. The
`same thing happens with our telephone set; it now passes current. The current source
`is a “battery” that resides at the local serving switch. It is connected by the subscriber
`loop. This is just a pair of copper wires connecting the battery and switch out to the sub-
`scriber premises and then to the subscriber instrument. The action cf current flow alerts
`the serving exchange that the subscriber requests service. When the current starts to
`flow, the exchange returns a dial tone, which is audible in the headset (of the subscriber
`instrument). The calling subscriber (user) now knows that she/he may start dialing dig-
`its or pushing buttons on the subscriber instrument. Each button is associated with a
`digit. There are 10 digits, 0 through 9. Figure 1.3 shows a telephone end instrument
`connected through a subscriber loop to a local serving exchange. It also shows that all-
`important battery (battery feed bridge), which provides a soilrce of current for the sub-
`scriber 100p.
`If the called subscriber and the calling subscriber are in the same local area, only
`
`Battery
`feed bridge
`
`SUbscnber Imp
`
`Subscriber
`
`Figure 1.3 ,A subscriber set is connected to a telephone exchange by a subscriber loop. Note the battery
`feed in the telephone sewing switch. Distance D is the loop length discussed in Section 5.4.
`
`“Signaling may be defined as the exchange of information specifically concerned with the establishment and
`control of connections, and the transfer of user-to-user and management information in a circuit-switched
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`5
`
`Subscriber Loops
`
`Trunks
`
`Subscriber Loops
`
`Figure 1.4 Subscriber loops connect telephone subscribers to their local sewing exchange; trunks inter-
`
`seven digits need be dialed. These seven digits represent the telephone number of the
`called subscriber (user). This type of signaling, the dialing of the digits, is called address
`signaling. The digits actuate control circuits in the local switch, allowing a connectivity
`to be set up. If the calling and called subscribers reside in the serving area of that local
`switch, no further action need be taken. A connection is made to the called subscriber
`line and the switch sends a special ringing signal down that loop to the called subscriber,
`and her/his telephone rings, telling her/him that someone wishes to talk to her/him on
`the telephone. This audible ringng is called alerting, another form of signaling. Once
`the called subscriber goes off-hook (i.e., takes the telephone out of its cradle), there is
`activated connectivity, and the call enters the information-passing phase, or phase 2 of
`the telephone call.
`When the call is completed, the telephones at each end are returned to their cradles,
`breaking the circuit of each subscriber loop. This, of course, is analbgous to turning off
`a light; the current stops flowing. Phase 3 of the telephone call begins. It terminates the
`call, and the connecting circuit in the switch is taken down and freed-up for another
`user. Both subscriber loops are now idle. If a third user tries to call either subscriber
`during stages 2 and 3, she/he is returned a busy-back by the exchange (serving switch).
`This is the familiar “busy signal,” a tone with a particular cadence. The return of the
`busy-back is a form of signaling called call-progress signaling.
`Suppose now that a subscriber wishes to call another telephone subscriber outside the
`local serving area of her/his switch. The call setup will be similar as before, except that
`at the calling subscriber serving switch the call will be connected to an outgoing trunk.
`As shown in Figure 1.4, trunks are transmission pathways that interconnect switches. To
`repeat: subscriber loops connect end-users (subscriber) to a local serving switch; trunks
`interconnect exchanges or switches.
`The IEEE (Ref. 2) defines a trunk as “a transmission path between exchanges or
`central offices.” The word transmission in the IEEE definition refers to one (or several)
`transmission media. The medium might be wire-pair cable, fiber optic cable, micro—
`wave radio and, stretching the imagination, satellite communications. In the conven-
`tional telephone plant, coaxial cable has fallen out of favor as a transmission medium
`for this application. Of course, in the long-distance plant, satellite communication is
`
`connect exchanges (switches).
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`INTRODUCTORY CONCEPTS
`
`fairly widely employed, particularly for international service. Our preceding reference
`was for local service.
`
`1.3.2 Telephone Numbering and Routing
`
`EVery subscriber in the world is identified by a number, which is geographically tied to
`a physical location.5 This is the telephone number. The telephone number, as we used
`it here, is seven digits long. For example:
`
`234 — 5678
`
`
`
`The last four digits identify the subscriber line; the first three digits (i.e., 234) identify
`the serving switch (or exchange).
`For a moment, let’s consider theoretical numbering capacity. The subscriber number,
`those last four digits, has a theoretical numbering capacity of 10,000. The first telephone
`number issued could be 0000, the second number, if it were assigned in sequence, would
`be 0001, the third, 0002, and so on. At the point where the numbers ran out, the last
`number issued would be 9999.
`
`The first three digits of the preceding example contain the exchange code (or central
`office code). These three digits identify the exchange or switch. The theoretical maxi—
`mum capacity is 1000. If again we assign numbers in sequence, the first exchange would
`have 001, the next 002, then 003, and finally 999. However, particularly in the case of
`the exchange code, there are blocked numbers. Numbers starting with 0 may not be
`desirable in North America because 0 is used to dial the operator.
`The numbering system for North America (United States, Canada, and Caribbean
`islands) is governed by the NANP or North American Numbering Plan. It states that
`central office codes (exchange codes) are in the form NXX where N can be any number
`from 2 through 9 and X can be any number from 0 through 9. Numbers starting with 0
`or 1 are bleeked numbers. This cuts the total exchange code capacity to 800 numbers.
`Inside these 800 numbers there are five blocked numbers such as 555 for directory
`
`a day when telephone numbers are issued at birth, much like social security numbers?
`
`assistance and 958/959 for local plant test.
`When long—distance service becomes involved, we must turn to using still an addi-
`tional three digits. Colloquially we call these area codes. In the official North American
`terminology used in the NANP is NPA for numbering plan area, and we call these area
`codes NPA codes. We try to assure that both exchange codes and NPA codes do not
`cross political/administrative boundaries. What is meant here are state, city, and county
`boundaries. We have seen exceptions to the county/city rule, but not to the state. For
`example, the exchange code 443 (in the 508 area code, middle Massachusetts) is exclu-
`sively for the use of the town of Sudbury, Massachusetts. Bordering towns, such as
`Framingham, will not use that number. Of course, that exchange code number is meant
`for Sudbury’s singular central office (local serving switch).
`There is similar thinking for NPAs (area codes). In this case it is that these area codes
`may not cross state boundaries. For instance, 212 is for Manhattan and may not be used
`for northern New Jersey.
`Return now to our example telephone call. Here the calling party wishes to speak
`
`5This will change. At least in North America, we expect to have telephone number portability. Thus,
`whenever one moves to a new location, she/he takes her/his telephone number with them. Will we see
`
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`7
`
`234
`Exchange
`‘
`
`447
`Exchange
`
`’
`
`Called subscriber
`
`Calling subscriber
`
`Figure 1.5 Example connectivity subscriber-to-subscriber through two adjacent exchanges.
`
`to a called party that is served by a different exchange (central office).6 We will assign
`the digits 234 for the calling party’s serving exchange; for the called party’s serving
`exchange we assign the digits 447. This connectivity is shown graphically in Figure 1.5.
`We described the functions required for the calling party to reach her/his exchange. This
`is the 234 exchange. It examines the dialed digits of the called subscriber, 447—8765.
`To route the call, the exchange will only work upon the first three digits. It accesses
`its local look-up table for the routing to the 447 exchange and takes action upon that
`information. An appropriate vacant trunk is selected for this route and the signaling for
`the call advances to the 447 exchange. Here this exchange identifies the dialed number
`as its own and connects it to the correct subscriber loop, namely, the one matching the
`8765 number. Ringing current is applied to the loop to alert the called subscriber. The
`called subscriber takes her/his telephone off-hook and conversation can begin. Phases
`' 2 and 3 of this telephone call are similar to our previous description.
`
`7The erlang is a unit of traffic intensity. One erlang represents one hour of line (circuit) occupancy.
`
`1.3.3 Use of Tandem Switches in a Local Area Connectivity
`
`Routing through a tandem switch is an important economic expedient for a telephone
`company or administration. We could call a tandem switch a mafia concentrator. Up to I
`now we have discussed direct trunk circuits. To employ a direct trunk circuit, there must
`be sufficient traffic to justify such a circuit. One reference (Ref. 3) suggests a break point
`of 20 erlangs.7 For a connectivity with traffic intensity under‘ZO erlangs for the busy
`hour (BH), the traffic should be routed through a tandem (exchange). For traffic inten-
`sities over that value, establish a direct route. Direct route and tandem connectivities
`are illustrated in Figure 1.6.
`
`1.3.4 Busy Hour and Grade of Service
`
`The PSTN is very inefficient. This inefficiency stems from the number of circuits and
`the revenue received per circuit. The PSTN would approach 100% efficiency if all the
`circuits were used all the time. The fact is that the PSTN approaches total capacity
`utilization for only several hours during the working day. After 10 PM. and before 7
`AM. capacity utilization may be 2% or 3%.
`The network is dimensioned (sized) to meet the period of maximum usage demand.
`
`6The'term ofiice or central ofi‘ice is commonly used in North America for a switch or an exchange. The
`terms switch, office, and exchange are synonymous.
`
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`8
`
`iNTFiODUCTOFiY CONCEPTS
`
`EXChanQe
`
`Exchange
`
`Direct route
`
`Exchange
`
`Figure 1.6 Direct route and tandem connectivities.
`
`This period is called the busy hour (BH). There are two periods where traffic demand on
`- the PSTN is maximum-one in the morning and one in the afternoon. This is illustrated
`in Figure 1.7.
`Note the two traffic peaks in Figure 1.7. These are caused by business subscribers. If
`the residential and business curves were combined, the peaks would be much sharper.
`Also note that the morning peak is somewhat more intense than the afternoon busy hour.
`In North America (i.e., north of the Rio Grande river), the busy hour BH is between
`9:30 AM. and 10:30 AM. Because it is more intense than the afternoon high-traffic
`period, it is called the BH. There are at least four distinct definitions of the busy hour.
`We quote only one: “That uninterrupted period of 60 minutes during the day when the
`traffic offered is maximum”. Other definitions maybe found in (Ref. 4).
`BH traffic intensities are used to dimension the number of trunks required on a con-
`nectivity as well as the size of (a) switch(es) involved. Now a PSTN company (admin-
`istration) can improve its revenue versus expenditures by cutting back on the number
`
`Figure 1.7 The busy hour.
`
`
`
`In
`
`12
`Time
`
`—-— Residential subscribers
`----- Business subscribers
`
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`9
`
`of trunks required and making switches “smaller.” Of course, network users will do a
`lot of complaining about poor service. Let’s just suppose the PSTN does just that—cuts
`back on the number of circuits. Now, during the BH period, a user may dial a number
`and receive either a voice announcement or a rapid—cadence tone telling the user that all
`trunks are busy (ATB) and to try again later. From a technical standpoint, the user has
`encountered blockage. This would be due to one of two reasons, or may be due to both
`causes. These are: insufficient switch capacity and not enough trunks to assign during
`the BH. There is a more in—depth discussion of the busy hour in Section 4.2.1.
`Networks are sized/dimensioned for a traffic load expected during the busy hour.
`The sizing is based on probability, usually expressed as a decimal or percentage. That
`probability percentage or decimal is called the grade of service. The IEEE (Ref. 2)
`defines grade of service as “the proportion of total calls, usually during the busy hour,
`that cannot be completed immediately or served within a prescribed time.”
`Grade of service and blocking probability are synonymous. Blocking probability,
`objectives are usually stated as B = 0.01 or 1%. This means that during the busy hour
`1 in 100 calls can be expected to meet blockage.
`
`SCalled both-way in the United Kingdom and in ccm documentation.
`
`Trunks can be configured for either one-way or two-way operation.8 A third option is
`a hybrid, where one-way circuits predominate and a number of two—way circuits are
`provided for overflow situations. Figure 1.8a shows two-way trunk operation. In this
`case any trunk can be selected for operation in either direction. The insightful reader
`will observe that there is some fair probability that the same trunk can be selected from
`either side of the circuit. This is called double seizure. It is highly undesirable. One way
`to reduce this probability is to use normal trunk numbering (from top down) on one side
`of the circuit (at exchange A in the figure) and to reverse trunk numbering (from the
`bottom up) at the opposite side of the circuit (exchange B).
`Figure 1.8b shows one-way trunk operation. The upper trunk group is assigned for
`the direction from A to B and the lower trunk group for the opposite direction, from
`exchange B to exchange A. Here there is no possibility of double seizure.
`Figure 1.8c illustrates a typical hybrid arrangement. The upper trunk group carries
`traffic from exchange A to exchange B exclusively. The lowest trunk group carries traf-
`fic in the opposite direction. The small, middle trunk group contains two-way circuits.
`Switches are programmed to select from the one-way circuits first, until all these circuits
`become busy, then they may assign from the two-way circuit pool.
`Let us clear up some possible confusion here. Consider the one-way circuit from A
`
`1.3.5 Simplex, Half—Duplex and Full Duplex
`
`These are operational terms, and they will be used throughout this text. Simplex is one-
`way operation; there is no reply channel provided. Radio and television broadcasting
`are simplex. Certain types of data circuits might be based on simplex operation.
`Half-duplex is a two-way service. It is defined as transmission over a circuit capable
`of transmitting in either direction, but only in one direction at a time.
`Full duplex or just duplex defines simultaneous two-way independent transmission on
`a circuit in both directions. All PSTN-type circuits discussod in this text are considered
`using full duplex operation unless otherwise specified.
`
`1.3.6 One-Way and Two-Way Circuits
`
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`INTRODUCTORY CONCEPTS
`
`
`
`Exchange
`A
`
`Exchange
`A
`
`Exchange
`A
`
`Exchange
`B
`
`Exchange
`B
`
`Exchange
`B
`
`
`
`tenth and so on. The increasing complexity becomes very obvious.
`
`to B, for example. In this case, calls originating at exchange A bound for exchange B
`in Figure 1.819 are assigned to the upper trunk group. Calls originating at exchange B
`destined for exchange A are assigned from the pool of the lower trunk group. Do not
`confuse these concepts with two—wire and four-wire operation, discussed in Chapter 4,
`Section 4.4. '
`
`
`
`(c) Hybrid Operation
`
`Figure 1.8 Two-way and one-way circuits: two-way operation (a), one-way operation (b). and a hybrid
`scheme, a combination of one-way and twouway operation (0).
`
`1.3.7 Network Topologies
`
`The IEEE (Ref. 2) defines topology as “the interconnection pattern of nodes on a net-
`work.” We can say that a telecommunication network consists of a group of intercon—
`nected nodes or switching centers. There are a number of different ways we can inter—
`connect switches in a telecommunication network.
`
`If every switch in a network is connected to all other switches (or nodes) in the
`network, we call this “pattern” a full-mesh network. Such a network is shown in Figure
`1.9a. This figure has 8 nodes.9
`
`9The reader is challenged to redraw the figure adding just one node for a total of nine nodes. Then add a
`
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`11
`
`Figure 1.93 A full-mesh network connecting eight nodes.
`
`In the 19705, Madrid (Spain) had 82 switching centers connected in a full-mesh net—
`work. A full—mesh network is very survivable because of a plethora of possible alter-
`native routes.
`
`mAlbany is the capital of the state of New York.
`
`Figure 1.9b shows a star network. It is probably the least survivable. However, it
`is one of the most economic nodal patterns both to install and to administer. Figure
`1.9c shows a multiple star network. Of course we are free to modify such networks by
`adding direct routes. Usually we can apply the 20 erlang rule in such situations. If a
`certain traffic relation has 20 erlangs or more of BH traffic, a direct route is usually
`justified. The term trafiic relation simply means the traffic intensity (usually the BH
`traffic intensity) that can be expected between two known points. For instance, between
`Albany, NY, and New York City there is a traffic relation.10 On that relation we would
`probably expect thousands of erlangs during the busy hour.
`‘
`Figure 1.9d shows a hierarchical network. It is a natural outgrowth of the multiple star
`network shown in Figure 1.9c. The PSTNs of the world universally used a hierarchical
`network; CCITI‘ recommended such a network for international application. Today there
`is a trend away from this structure or, at least, there will be a reduction of the number
`of levels. In Figure 1.9d there are five levels. The highest rank or order in the hierarchy
`
`B
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`C
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`F
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`E
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`Figure 1.9!) A star network.
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`Figure 1.91:! A typical hierarchical network. This was the AT&T network around 1988. The CCITT-
`
`Figure 1.9:: A higher-order or multiple star network. Note the direct route between 231 and 232. There
`is another direct route between 3A5 and 3A5.
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`recommended network was very similar.
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`is the class 1 center (shown as 1 in the figure), and the lowest rank is the class 5 office
`(shown as 5 in the figure). The class 5 office (switch), often called an end ofifice, is the
`local serving switch, which was discussed previously. Remember that the term ofiice is
`a North American term meaning switching center, node, or switch.
`In a typical hierarchical network, high-usage (HU) routes may be established, regard-
`less of rank in the hierarchy, if the traffic intensity justifies. A high-usage route or con-
`nectivity is the same as a direct route. We tend to use direct route when discussing the
`local area and we use high-usage routes when discussing a long-distance or toll net-
`work.
`
`1.3.7.1. Rules of Conventional Hierarchical Networks. One will note the back»
`
`'bone structure of Figure 1.9d. If we remove the high-usage routes (dashed lines in the
`figure), the backbone structure remains. This backbone is illustrated in Figure 1.10. In
`the terminology of hierarchical networks, the backbone represents the final route from
`which no overflow is permitted.
`Let us digress and explain what we mean by overflow. It is defined as that part of the
`offered traffic that cannot be carried by a switch over a selected trunk group. It is that
`traffic that met congestion, what was called blockage earlier. We also can have overflow
`of a buffer (a digital memory), where overflow just spills, and is lost.
`In the caSe of a hierarchical network, the overflow can be routed over a different
`route. It may overflow on to another HU route or to the final route on the backbone
`(see Figure 1.10).
`A hierarchical system of routing leads to simplified switch design. A common expres-
`sion used-when discussing hierarchical routing and multiple star configurations is that
`lower-rank exchanges home on higher-rank exchanges. If a call
`is destined for an
`exchange of lower rank in its chain, the call proceeds down the chain. In a similar
`
`Figure 1.10 The backbone of a hierarchical network. The backbone traces the final route.
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`Subscriber
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`Subscriber
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`manner, if a call is destined for another exchange outside the chain (the opposite side of
`Figure 1.9d), it proceeds up the chain and across. When high-usage routes exist, a call
`may be routed on a route additional or supplementary to the pure hierarchy, proceeding
`to the distant transit center and then descending to the destination.” Of course, at the
`highest level in a pure hierarchy, the call crosses from one chain over to the other. In
`hierarchical networks only the order of each switch in the hierarchy and those additional
`high usage links (routes) that provide access need be known. In such networks admin-
`istration is simplified, and storage or routing information i