Telecommunications
`Essentials
`The Complete Global Source
`for Communications Fundamentals,
`Data Networking and the Internet,
`and Next-Generation Networks
`
`Lillian Goleniewski
`
`."T Addison-Wesley
`Boston • San Francisco • New York • Toronto • Montreal
`London • Munich • Paris • Madrid
`Capetown • Sydney • Tokyo • Singapore • Mexico City
`
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`Many of the designations used by manufacturers and sellers to distinguish their products are claimed as trade(cid:173)
`marks. Wh ere those designations appear in this book, and Addison-Wesley, Inc. was aware of a trademark claim,
`the designations have been printed with initial capital letters or in all capitals.
`
`Lido Telecommunications Essentials® is the registered trademark of The Lido Organization, Inc.
`
`The author and publisher have taken care in the preparation of this book, but make no expressed or implied war(cid:173)
`ranty of any kind and assume no responsibility for errors or omissions. o liability is assumed for incidental or con(cid:173)
`sequential damages in connection with or arising out of the use of the information or programs contained herein.
`
`The publisher offers disco unts on this book when ordered in quantity for special sales. For more information,
`please contact:
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`
`Library of Congress Cataloging-in-Publication Data
`
`Golen iewski, Lillian .
`Telecommunications essentials : the complete global source for communications
`fundamentals, data networking and the Internet, and next-generation networks I Lillian Goleniewski.
`p. em.
`Includes bibliographical references and index.
`ISBN 0-201-76032-0
`1. Telecommunication . 1. Title.
`
`TK5l01 G598 2002
`621.382-dc21
`
`Copyrigh t© 2002 by Pearson Education, Inc.
`
`2001053752
`
`All rights reserved. No part of this publication may be reproduced , stared in a retrieval system, or transmitted, in
`any form , or by any means, electronic, mechani cal, photocopying, recording, or otherwise, without the prior con(cid:173)
`sent of the publisher. Printed in the United States of America. Published simultaneously in Canada.
`
`For information on obtaining permission for use of material from this work, please submit a written request to:
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`Pearson Education, Inc.
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`
`ISBN 0-201-76032-0
`Text printed on recycled paper
`
`1 2 3 4 56 7 8 9 10-CRS-0504030201
`f irst printing, December 2001
`
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`Chapter
`
`The PSTN
`
`This chapter talks about the public switched telephone network (PSTN). It talks
`about what comprises the PSTN , what sorts of technologies have been used to com(cid:173)
`plete the connections, how the signaling systems operate, and what the basic back(cid:173)
`bone architectures entail in terms of components and transmission capacities. This
`chapter also discusses intelligent networks (INs) and what they promise in terms of
`service logic and feature availability. Finally, this chapter describes some of the trends
`in the evolution of the PSTN that will support the new generation of applications.
`
`• The PSTN Infrastructure
`
`Our views about what a network should be designed to support and what the infra(cid:173)
`structure should be comprised of have changed quite a bit over the years, as appli(cid:173)
`cations and technology have changed. Before discussing what is needed in a
`network today, this chapter takes a look at how the PSTN infrastructure evolved
`and where it is today.
`The traditional PSTN infrastructure was specifically designed to support only
`voice communications. At the time this infrastructure was being designed, we had
`n o notion of data communications. Initially the traffic type the PSTN was designed
`to support was continuous real-time voice.
`Another variable that's important to the design of the PSTN has to do with the
`length of calls. Most voice calls are quite short, so the circuit switches in the PSTN
`are engineered for call durations of three minutes or less. The average Internet
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`Chapte r 5
`
`• The PSTN
`
`session, on the other hand, lasts around an hour. This means that increased Inter(cid:173)
`net access through the PSTN has, in some locales, put a strain on the local
`exchanges. If a circuit switch is blocked because it is carrying a long Internet ses(cid:173)
`sion, people may not be able to get a dial tone. There are several solutions to this
`problem. For example, as discussed in Chapter 10, "Next-Generation Networks,"
`we can apply intelligence in front of some exchanges so that calls destined for
`ISPs can be diverted over a packet-switched network to the lSP rather than being
`completed on a circuit-switched basis through the local exchange.
`Yet another variable that's important to the design of the PSTN has to do with
`what it was designed to support. The capacities of the channels in the PSTN are of
`the narrowband generation-they are based on 64Kbps channels. The worldwide
`infrastructure to accommodate voice communications evolved to include a series
`of circuit switches. Different switches are used based on the locations to which
`they're connecting. The switches have a high degree of intelligence built into them,
`both for establishing the communications channels and fo'r delivering the service
`logic to activate a growing array of features. ln the traditional framework , the
`monolithic s"vi.tches in the network had all the smarts. The switch manufacturer
`and the carrier worked together very closely, and the carrier was not able to intro(cid:173)
`duce new features and services into a particular area until a software release was
`available for the switch platform through which the neighborhood was being ser(cid:173)
`viced. Thus, carriers were often unable to roll out new services and features
`because they hadn't yet received the new software releases from the switch manu(cid:173)
`facturers. Over time, we have separated the functions of switching and connection
`establishment from the functions involved in the intelligence that enables various
`services and features to be activated.
`The traditional PSTN is associated with highly developed, although not neces(cid:173)
`sarily integrated, operational suppor-t systems (such as billing systems, provision(cid:173)
`ing systems, network management systems, customer contact systems, and
`security systems). These systems have very well-developed business processes and
`techniques for managing their environments . But the various systems' databases
`cannot yet all speak to one another to give one comprehensive view. (But at least
`those systems exist, unlike in the public Internet, where the operational support
`systems are only n ow beginning to emerge to help manage that environment. )
`Th e backbone of the traditional PSTN was largely based on a generation that
`we call the Plesiochronous Digital Hierarchy (PDH), which includes the T-carrier,
`E-carrier, and ]-carrier standards. The local loop of the PSTN was provisioned as a
`twisted-copper-pair analog subscriber line.
`
`Service Providers
`Many abbreviations and acronyms are used to define the various players and the
`parts of the network in which they play. Some telcos can and do fulfill more than
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`The PSTN Infrastructure
`
`one of these functions; the extent to which they can or do fulfill more than one of
`these functions partly depends on the policy, regulatory, and licensing conditions
`that prevail in different countries. The following terms are largely used in the
`United States, but they are important to the discussion in this chapter because they
`illustrate the functions service providers are addressing:
`
`•
`
`• PTO-PTO stands for pLtblic telecommunications operator, which is the
`name for an incumbent carrier in places other than the United States.
`• VAN- VAN stands for value-added network provider. This term originated
`around 1970 and was applied to companies that were competing to provide
`telecommunications services, specifically with offerings focused on data
`communications and data networking. VANs provided more than a simple
`pipe from Point A to Point B. They provided some additional intelligence in
`the network, to, for example, perform error detection and correction, or to
`convert protocols or languages that different computers speak so that you
`could have interoperability across the network.
`• LEC-In the local environment we use the acronym LEC for local exchange
`carrier. There was originally no competition among LECs, but as soon as
`competition in the local loop picked up , LECs were segmented into lLECs,
`CLECs, and DCLECs.
`ILEC- The ILEC is the incumbent local exchange carrier, the original com(cid:173)
`mon carrier that either once had, or in some countries still has, monopoly
`rights in tl1e local loop. For most residents in the United States, this would
`be one of the four "baby Bells"-Qwest Communications International, SBC
`Communications, BellSouth Corporation, and Verizon Communications.
`• CLEC- The CLEC is the competitive local exchange carrier. CLECs came
`about as a result of the Telecommunications Act of 1996, which opened up
`competition in the local loop. The CLEC is the competitor to the ILEC.
`Although the decline of the telecommunications economy in 2000 and
`2001 forced several CLECs out of business, there are still some CLECs in
`the United States, and they currently focus on delivering dial tone to busi(cid:173)
`ness customers.
`• DCLEC (or DLEC)-DCLEC stands for data competitive local exchange car(cid:173)
`rier. The DCLEC is a company that is specifically focused on supporting
`data services (for example, providers that offer DSL services to end users).
`• ELEC- ELEC stands for Ethernet local exchange carrier. The ELEC special(cid:173)
`izes in providing Ethernet solutions in the local loop and metro area.
`IXC-The interexchange carrier (IXC) is the carrier for long-distance and
`international communications. AT&T Corporation, WorldCom, Sprint,
`Qwest, and Verizon are the primary IXCs in the United States. Unless certain
`
`•
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`Chapter 5
`
`• The PSTN
`
`stringent requirements imposed by the Federal Communications Commis(cid:173)
`sion are met, an lXC cannot offer long-distance services in the areas where it
`is also the ILEC.
`• SP-Because so many lines are being blurred today by bundled services and
`bundled territories of operation, the basic term service provider (SP) is com(cid:173)
`monly used to refer generically to providers of different types of services.
`
`Network Access
`Figure 5.1 is a simple diagram of network access. On the left-hand side is the cus(cid:173)
`tomer environment, which includes residences (single-line instruments being served
`by an access line) and business premises (with onsite telephone systems such as pri(cid:173)
`vate branch exchange [PBXs] or key telephone systems-smaller site systems for
`installations where there are 50 or fewer employees). Those in the customer environ(cid:173)
`ment are connected to the PSTN via access lines. The access network, or the local loop
`we so often talk about, includes whatever equipment resides at the customer premise
`(that is, the customer premises equipment [ CPE]), the access line leading to the local
`exchange, the components at the local exchange on which those access lines termi(cid:173)
`nate (that is, the distribution cross-connects), and the logic used to help control the
`flow of traffic over the access lines. ln the United States, competition is allowed in
`the local loop , and a myriad of players are interested in owning the local loop (for
`example, Internet service providers [ISPs], wireless operators, cable TV companies,
`power utilities). However, worldwide, the incumbent local providers continue to
`dominate the local loop, and, as usual, politics and economics are principal factors in
`delaying the mass deployment of high-speed residential access.
`The local exchange, in the c_enter of Figure 5 .l , is the backbone, or the core, of
`the network. From the local exchange , we can establish connections into the other
`providers, such as IXCs for long distance, international carriers for overseas calls,
`cellular providers, and lSPs.
`
`Services Beyond the Local Loop
`Traditionally, we have thought of the local loop as leading to the home or to the busi(cid:173)
`ness and ending there. But the need for additional bandwidth and capability is now
`shifting: We need these things within the premise, as well as on the local loop. It is
`therefore a logical extension for the service provider to not only give you access lines
`and termination, but also to provide you with the home area networking facilities you
`need in order to have an end-to-end broadband package. Chapter 15, "The Broad(cid:173)
`band Home and HANs," talks more about this.
`
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`The PSTN Infrastructure
`
`Local Exchange Customers
`
`Local
`Exchange
`Network
`
`ILEC or CLEC (for example, I
`PBX~
`
`ISP, wireless provider, cable TV
`operator, power utility)
`
`.
`
`CPE
`
`. Transport Network
`
`Figure 5.1 Network access
`
`lnterexchange
`Carriers
`
`International
`Carriers
`
`Cellular
`Providers
`
`Internet
`Service
`Providers
`
`The underlying network access facilities can be either analog or digital loops,
`and they connect the exchanges to the customer premises. At the customer pre(cid:173)
`mises there are the network interfaces, CPE, premises distribution systems where
`wiring is cross-connected, and network interfaces. The equipment for providing
`switch access services includes line-termination cards, carrier and multiplexer
`equipment, and local exchange switching capabilities that support addressing,
`supervisory alerting, call progress, and other signaling functions.
`
`Access Services
`The main categories of access services are trunks, business lines for key telephone
`systems, centrex service, leased lines, and residential subscriber lines.
`Trunks are used to provide connections into the PBX environment. There are
`three subcategories of trunks:
`
`• Two-way local exchange trunks-On these trunks, traffic flows in both the
`incoming and outgoing directions.
`• DID trunks- Direct inward dialing (DID) trunks are designed for only
`incoming calls. A benefit of DID trunks is that they enable the dialed num(cid:173)
`ber to ring directly on a user's phone rather than having to go through a
`centralized attendant. If the population knows whom they want to call
`directly, and if you want to ease the process of connecting the call, this can
`be a very useful feature. Another benefit of DID trunks is that they make it
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`Chapter 5
`
`• The PSTN
`
`The PM
`
`seem like a private line goes directly to the u ser, bu t with DID you can sup(cid:173)
`port perhaps 100 different numbers with a group of only 25 to 35 trunks
`(traffic engineering is used to determine the proper number of trunks).
`• DOD trunks-Direct outward dialing (DOD) trunks are used specifically
`for outgoing calls. DOD trunks are used when you dial an access code such
`as the number 9 or the number 8 to get an outside-line dial tone before you
`can dial the actual number that you want to reach.
`
`To service the key telephone systems, business lines connect the network ter(cid:173)
`mination at the user to the local exchange. Users that want to use the local
`exchange as if it were their PBX rent centrex trunks on a monthly basis. Large com(cid:173)
`panies often access the network via leased lines, which can be a very expensive
`solution , and home users access the network via residential subscriber lines.
`Access lines can either be in analog facilities or they can be digital carrier ser(cid:173)
`vices. Analog transmission is often called plain old telephone s'ervice (POTS for
`short). Three main types of digital services are offered by using twisted-pair cable.
`The first type of digital services involves T-l access (at 1.5Mbps), E-1 access (at 2.
`048Mbps), and J-1 access (at 1.544Mbps). The second type of digital services is
`narrowband ISDN (N-tSDN) services, including Basic Rate Interface (BRI) for resi(cid:173)
`dences and small businesses and Primary Rate Interface (PRl) for larger businesses.
`The third type of digital services is the xDSL subscriber lines and high-speed digi(cid:173)
`tal subscriber lines that enable the all-important applications of Internet access and
`multimedia exploration. (Chapter 3, "Transmission Media: Characteristics and
`Applications," describes th e digital senrices in more detaiL)
`
`Transport Se rvices
`Transport services are the network switching, transmission, and related services
`that support information transfer between the originating and terminating access
`facilities. The underlying facilities include local exchanges and tandem switches,
`toll and transit switches, international gateways, and interoffice transmission
`equipment. Transport services include switched services, nonswitched services,
`and virtual private networks (VPNs).
`
`Switched Services
`There are two main types of switched services: public and private.
`Switched public services include local calling, long-distance calling, toll-free
`calling, international calling, directory assistance, operator assistance, and emer(cid:173)
`gency services.
`Switched private services can be switchable either because they are deployed
`within the CPE or because they are deployed on a carrier basis. With CPE-based ser-
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`The PSTN Infrastructure
`
`vices, you can add capabilities to the telephone systems onsite in the PBXs- a feature
`called electronic tandem networhing. For example, you can use electronic tandem net(cid:173)
`working to gain some flexibility in routing around congestion points: If the preferred
`leased line from Switch A to Switch B is occupied or not available, the switch can
`decide how to reroute that traffic to still reach Switch B, but through a different series
`of leased lines. However, because leased lines (also referred to as tie trunks) are mile(cid:173)
`age sensitive and dedicated to individual customers, they are very expensive; thus,
`not much private voice networking is done over Lie trunks because there are several
`more attractive solutions, such as VPNs, which are discussed shortly.
`With carrier-based switched private services, a centrex customer could parti(cid:173)
`tion and implement extensions across multiple local exchanges and in this way be
`able to switch traffic between those locations.
`
`Nonswitched Services
`Non switched services include leased lines, foreign exchange (FX) lines, and off(cid:173)
`premises exchanges (OPXs). With leased lines, two locations or two devices are
`always on, using the same transmission path.
`FX lines allow you to make a toll call appear to be a local call. For example,
`you might have a dedicated leased line that runs from your customer premise to a
`local exchange in a distant area where you call large numbers of customers. When
`anyone behind your PBX dials a number associated with that foreign local
`exchange, the PBX automatically selects the FX line. The dial tone the caller
`receives is actually coming from the distant local exchange, and the call proceeds
`as if it were a local call. The tradeoff with FX lines is that although you are not
`charged per call for your long-distance calls to the specified exchange, you pay a
`flat monthly fee for the. leased line and you have to apply some traffic engineering
`to ensure that you're not making people wait for the FX line to become available.
`So with FX lines, you need to find the right balance point between reducing costs
`and ensuring a high level of service.
`OPXs are used in distributed environments, such as a city government. Say that
`the city governmen t has public works stations, libraries, fire stations, and parks and
`recreation facilities that are too far from the PBX to be served by the normal cabling.
`The city uses an OPX setup: It leases a circuit from the PBX to the off-premise loca(cid:173)
`tion and ties it in as if it were part of that PBX. City government employees can then
`call one another, using their norn1al extension plan, their call accounting informa(cid:173)
`tion can be accumulated so that cost allocations can be perfonned, and the employ(cid:173)
`ees can have access to the full suite of features that a business PBX offers.
`
`VPNs
`Although you might think that VPNs are related to the Internet or to Internet Proto(cid:173)
`col (lP) and are a somewhat new development, they actually originated in the circuit-
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`IXCPOP~
`
`Chapter 5
`
`The PSTN
`
`IXC POP Switch
`
`SMS
`
`Figure 5.2 An example of a VPN
`
`switched network environment, with AT&Ts software-defined network (SDN) in the
`early 1980s. A VPN is a concept, not a technology platform or a set of networking
`techniques. A VPN defines a network in which customer traffic is isolated over
`shared-service provider facilities, so as more customers share the same facilities, their
`costs go down. The purpose of a VPN, then, is to reduce the high cost of leased lines,
`while still providing high quality of service and guaranteeing that private traffic has
`capacity between locations. Figure 5.2 shows an example of a VPN.
`The underlying facilities of a VPN indude the carrier public network, aug(cid:173)
`mented by network control points and service management systems. Under com(cid:173)
`puter control, the traffic is then routed through the public network in a manner
`that makes the VPN service seem like a facilities-based private network. Access to
`the VPN can occur via dedicated access, leased lines, or carrier-switched access,
`using either an analog or a digital carrier.
`The network control point represents a centralized database that stores a sub(cid:173)
`scriber's unique VPN information. The network control point screens every call
`and then applies call processing in accordance with the customer-defined require(cid:173)
`ments. A common-channel signaling network connects the various network ele(cid:173)
`ments so that they can exchange information with each other in real-time.
`(Common-channel signaling is discussed later in this chapter, in the section "Sig(cid:173)
`naling Systems.")
`A service management system is used to build and maintain th e VPN database.
`It allows customers to program specific functions to accommodate their particular
`
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`The PSTN Infrastructure
`
`business applications. It transmits information to the network control points, with
`important instructions on a customer-by-customer basis. Thus, VPNs introduce to
`the realm of the PSTN a lower-cost alternative to building a private voice network.
`
`PSTN Architecture
`The PSTN includes a number of transmission links and nodes. There are basi(cid:173)
`cally four types of nodes: CPE nodes, switching nodes , transmission nodes, and
`service nodes.
`
`CPE Nodes
`CPE nodes generally refer to the equipment that's located at the customer site. The
`main function of CPE nodes is to transmit and receive user information. The other
`key function is to exchange control information with the network. In the tradi(cid:173)
`tional realm, this equipment includes PBXs, key telephone systems, and single-line
`telephones.
`
`Switching Nodes
`Switching nodes interconnect transmission facilities at various locations and route
`traffic through a network. They set up the circuit connections for a signal path, based
`on the number dialed. To facilitate this type of switching, the ITU standardized a
`worldwide numbering plan (based on ITU E.l64) that essentially acts as the routing
`instructions for how to complete a call through the PSTN. The switching nodes
`include the local exchanges, tandem exchanges (for routing calls between local
`exchanges within a city), toll offices (for routing calls to or from other cities), and
`international gateways (for routing calls to or from other countries). Primary net(cid:173)
`work intelligence is contained in the Class 4 s•vitches (that is, toll offices switches)
`and Class 5 switches (that is, local exchange switches) . The Class 4 toll switches pro(cid:173)
`vide long-distance switching and network features , and the Class 5 switches provide
`the local switching and telephony features that subscribers subscribe to. Figure 5.3
`shows where the types of telephone exchanges are located.
`
`The Local Exchange The local exchange (also called the Class 5 office or central
`office) is where communications common carriers terminate customer lines and
`locate the switching equipment that interconnects those lines. This class office rep(cid:173)
`resents the local network. Every subscriber line location in a local exchange is
`assigned a number, generally seven or eight digits. The first three (or four) digits
`represent the exchange and identify the local exchange switch that serves a partic(cid:173)
`ular telephone. The last four digits identify the individual line number, which is a
`circuit that is physically connected from the local exchange to the subscriber. The
`traditional local exchange switch can handle one or more exchanges, with each
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`Chapter 5
`
`• The PSTN
`
`CPE
`
`Local loop
`
`Backbone, or
`Core Network
`
`Individual
`User Station
`Lines, or
`Extensions
`
`Residential
`Service
`
`Toll/Transit Switch
`(Class 4): for
`routing calls to or
`from other cities
`
`Tandem/Junction
`Exchange: for
`routing calls
`between local
`exchanges within
`the city.
`
`Local Exchange,
`Class 5 Switch,
`Central Office
`
`Tandem Trunks
`
`Figure 5.3 Types of tel ephone exchanges
`
`exchange capable of handling up to 10,000 subscriber lines, numbered 0000 to
`9999. In large metropolitan areas, it is common to find one local exchange building
`housing more than one local exchange switch and for each switch to handle five or
`more exchanges. These offices are sometimes referred to as multi-entity buildings.
`
`The Tan dem Office The tandem office, or junction network, is an exchange that
`is used primarily as a switching point for traffic between local exchanges in a met(cid:173)
`ropolitan area. It is an office that is used to interconnect the local end offices over
`tandem trunks in a densely settled exchange area where it is not economical for a
`telephone company to provide direct interconn ection between all end offices. The
`tandem office completes all calls between the end offices but is not directly co n(cid:173)
`nected to subscribers.
`
`The Toll Office The toll office (also called the trunk exchange or transit switch )
`is a telephone company switching center where channels and toll message circuits
`terminate-in other words, where national long-distance connections are made.
`This is usually one particular exchange in a city, but larger cities may have several
`exchanges where toll message circuits terminate.
`
`The International Gateway An international gateway is the point to and from
`which international services are available in each country. Protocol conversion may
`take place in the gateway; in lTU terminology, this is called a centre de transit (CT) .
`Cl and C2 international exchanges connect only international circuits. CT2
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`The Transport Network Infrastructure
`
`123
`
`exchanges switch traffic between regional groups of countries, and CT1 exchanges
`switch traffic between continents. CT3 exchanges connect switch traffic between
`the national PSTN and the international gateway.
`
`Transmission Nodes
`Transmission nodes are part of the transport infrastructure, and they provide com(cid:173)
`munication paths that carry user traffic and network control information between
`the nodes in a network. The transmission nodes include the transmission media
`discussed in Chapter 3, as well as transport equipment, including amplifiers and/or
`repeaters, mu ltiplexers, digital cross-connects, and digital loop carriers.
`
`Service Nodes
`Service nodes handle signaling, which is the transmission of information to control
`the setup, holding, charging, and releasing of connections, as well as the transmis(cid:173)
`sion of information to control network operations and billing. A very important
`area related to service nodes is the ITU standard specification Signaling System 7
`(SS7), which is covered later in this chapter.
`
`• The Transport Network Infrastructure
`
`The transport network includes two main infrastructures. The first is the PDH, also
`known as T-carrier, E-canier, and J-carrier vvideband transmission standards. This
`infrastructure was first introduced in the early 1960s. The second infrastructure of
`the transport network is the Synchronous Digital Hierarchy (SDH; lTU terminology),
`also known as Synchronous Optical Network (SONET; ANSI terminology), which
`was first formalized and standardized in 1988. SDHJSONET is the second generation
`of digital hierarchy, and it is based on a physical infrastructure of optical fibers.
`PDH and SDHISONET are voice-centric circuit-switched network models that
`switch millions of 64Kbps circuits between various switching points. Each circuit
`is multiplexed numerous times for aggregation onto transmission facilities. Aggre(cid:173)
`gation occurs at many points in the network: in the access network, within the
`local exchange, and throughou t th e interexchanges. Hence, a significant portion of
`the cost of a network goes to the equipment that performs this aggregation-the
`multiplexers and cross-connects in both the PDH and SDHJSONET environments.
`
`The PDH Infrastructure
`it's
`The term Plesiochronous makes PDH sound like a dinosaur, and in a way, it is-
`an ou tdated architecture from the standpoint of the data rates it offers. But the word
`Plesiochronous means "minute variations in timing," and that refers to the fact that
`
`AT&T Exhibit 1013
`AT&T v. VoIP, IPR 2017-01382
`Page 13
`
`

`

`Chapter 5
`
`• The PSTN
`
`the PDH is an asynchronous infrastructure. Each network element-that is, each
`exchange, multiplexer, cross-connect, repeater, and so on-gets its clocking pulse
`from a different clocking source, and even though those clocking sources are syn(cid:173)
`chronized, there are minute fluctuations in timing. To differentiate the beginning
`and the end of a conversation, we have to channelize conversations.
`PDH. was the first system designed to use digitized voice transmission. It was
`born of the telcos' desire to better use their cable facilities and to enhance the quality
`of calls. PDH was first used by telcos as a means of aggregating multiple voice chan(cid:173)
`nels into a single high-speed digital backbone. Standards that are used today for all(cid:173)
`digital switching and transmission come from the original PDH. specifications.
`PDH defines several things: First, it's an integrated digital network, so it can cany
`a range of traffic, as long as that traffic is being presented in a digital manner. There(cid:173)
`fore, PDH represented the first opportunity for users and carriers to combine voice
`and data traffic over the same pipes. Second, it specifies the different transmission lev(cid:173)
`els or data rates, some of which are available for customers to subscribe to and others
`of which are used by operators internally within the backbones. Third, it defines
`within each of the transmission levels how many channels can be made available.
`
`The T-, E-, and }-Carrier Standards
`T-carrier, E-carrier, and ]-carrier are PDH standards that are followed in different
`regions of the world: ]-carrier is followed throughout Japan; T-carrier is followed
`throughout North America; and E-carrier is followed throughout Europe and the
`majority of other locations throughout the world, including large parts of Asia,
`Latin America, and Africa. Figure 5.4 compares these three standards. They all
`share one increment as a common denominator: 64Kbps. But each of the three
`standards multiplexes together a different number of these 64Kbps channels to
`derive higher transmission rates.
`Having three separate standards-T-, E-, andj-carrier- means that we have to
`cross between systems that use differen