The M&T Networking Technology Series
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`COMMUNICATIONS
`SYSTEMS & NETWORKS
`
`SECOND EDITION
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`An incisive overview of network convergence,
`
`
`
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`from voice and data to video and multimedia, from wired to wireless
`
`Now completely
`
`revised ancl
`upclatecl
`
`Ray Horak
`
`
`
`Mark A Miller, P.E., Consulting Editor
`
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`Forewords by Mark A Miller and Harry Newton
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`Communications
`Systems
`and Networks,
`
`Second Edition
`
`Ray Horak
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`M&T Books
`M&T =N
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`ally, the regula­
`ally available at
`ave been set well
`ss-subsidies
`sup­
`ocal loops)
`gen­
`anced services
`considered
`to be
`if basic services.
`ard competition
`minant carriers
`n in 1968, car-
`. cations
`Act of
`. The intentions
`providing the
`, AT&T and the
`ans, the regula
`-
`to prevail.
`This
`FCC gradually
`e as a result
`of
`Chapter
`15.
`the traditional
`basic and nee­
`d, particularly
`y has become
`speed modem,
`as a threat
`to
`of interested
`to regulate
`or
`are not parties
`temet service
`oFR), is possi-
`is available.
`Frame Relay
`nication
`gen­
`rove quality,
`Frame Relay
`
`carriers such
`ata networks
`communica­
`orks-inher­
`trently, these
`·e. Therefore,
`
`(PSTN)
`Network
`Telephone
`Switched
`5: Public
`Chapter
`159
`
`under heavy load.
`Yet,
`especially
`is yet to be determined,
`the level of voice quality
`voice networks;
`than the traditional
`more efficient
`are inherently
`these networks
`to be much less.
`are likely
`costs to the consumer
`therefore,
`Voice over Frame Relay,
`voice over the Internet,
`competition,
`Local exchange
`service, which has
`been a
`of universal
`the concept
`and Voice over IP all threaten
`of the PSTN since the formation of the FCC in 1934. In order to ensure
`cornerstone
`cost to the subscriber,
`a
`at affordable
`of voice service
`availability
`the universal
`incumbent
`between
`(cross-subsidies)
`developed
`structure
`of settlements
`complex
`area such as Hackberry,
`in a high-cost
`a subscriber
`IXCs and LECs. Thereby,
`in New York,
`as could a subscriber
`access,
`network
`could gain affordable
`Arizona
`Unless
`the
`service.
`in the carriers'
`cost differences
`the obvious
`New York, despite
`contributing,
`with all carriers
`fund is maintained,
`service
`integrity of the universal
`footnote.
`to a historical
`may be relegated
`service
`of universal
`the concept
`
`and Network Topology
`Carrier Domains
`of the Bell Operating
`prior to AI&T's divestiture
`Some years ago, and certainly
`in terms of its ownership
`and
`simple
`was relatively
`in 1984, the network
`Companies
`service in its franchised
`company provided
`Each operating telephone
`topology.
`on a fairly
`network
`to the AT&T long distance
`and gained access
`serving areas,
`on a lay­
`organized
`straightforward basis. Beginning
`the network
`in the late 1920s,
`[5-3].
`known as classes
`of hierarchy,
`with five levels
`ered basis,
`which
`Offices (COs),
`offices, or Central
`Class 5 offices are the local exchange
`Class 5
`The approximately
`19,000
`local loop connections.
`serve end users through
`to address
`a Carrier
`positioned
`are geographically
`offices in the United States
`of approxi­
`5-1. The CSA has a radius
`in Figure
`Area ( CSA), as illustrated
`Serving
`of a local loop without
`maximum length
`which is the typical
`feet,
`mately 18,000
`boosters)
`or
`signal
`(analog
`by either amplifiers
`conditioning provided
`special
`of the CSA
`the radius
`can extend
`The carrier
`regenerators).
`signal
`(digital
`repeaters
`remote COs, or unintelligent
`remote
`intelligent
`of either
`the deployment
`through
`high-capac­
`CO through
`to the centralized
`are connected
`The remotes
`line shelves.
`ity circuits.
`between
`directly
`of traffic be exchanged
`Should significant volumes
`More commonly, they are interconnected
`interconnected.
`COs, they may be directly
`tandem switches.
`through
`which serve to interconnect
`Class 5 of­
`offices are tandem toll centers,
`Class 4
`these also serve as the
`toll center,
`class of
`As the lowest
`directly.
`fices not connected
`Class 4 offices are inter­
`network.
`or toll,
`first point of entry to the long distance,
`to higher-order
`access
`and provide
`local toll network
`a relatively
`within
`connected
`as a Class 5 office; in
`a Class 4 office also serves
`In many instances,
`toll centers.
`as both a Central Office and a tandem toll of­
`serving
`a hybrid switch
`other words,
`partitioning
`and physical
`logical
`through
`provided
`functions
`fice, with the separate
`in· North
`existed
`1,500 tandem toll centers
`within the switch. Approximately
`of the BOCs.
`to ATB:T's divestiture
`America prior
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`
`
`160 Communications Systems and Networks, Second Edition
`
`coast tand
`down an a
`Note that e
`sic scenari
`the connec
`lower in th
`
`finally s
`
`switches w
`wards the
`to end. Th
`dsed, and
`ing trans
`
`Class 1
`
`Class 2
`
`Class 3
`
`n.nJl
`
`18Kft ..--�����­
`
`
`
`Figure 5-1 Class 5 office with Carrier Serving Area (CSA)
`
`
`
`
`
`toll centers, generally Class 3 offices, or primary toll centers, are higher-order
`
`
`
`
`serving to connect
`
`
`
`Class 4 offices for intrastate toll calling. Class 4 offices typically
`
`
`
`
`serve to interconnect independent telcos and BOCs. Approximately 200 such offices
`existed
`prior to divestiture.
`primary toll cen­Class 2 offices, or sectional serve to interconnect toll centers,
`
`
`
`
`
`
`
`
`
`
`
`ters, largely for interstate calling within a geographic region such as the Northeast
`
`
`toll centers existed in the ATEtT net­
`
`
`or the Southwest. Approximately 67 sectional
`work prior to divestiture.
`sectional toll cen­serve to interconnect Class 1 offices, or regional toll centers,
`
`
`
`
`
`
`
`
`
`
`ters in support of interregional calling. There were 10 regional toll centers in place
`
`
`
`
`
`in the United States prior to divestiture; seven currently exist in the United States,
`and two in Canada.
`As illustrated in Figure 5-2, the offices traditionally were interconnected on a
`
`
`
`hierarchical
`
`
`
`basis, with end offices residing at the bottom of the network food
`
`
`
`
`
`
`chain. As a user places a long distance call, the Class 5 switch examines and ana­
`c area it
`
`
`
`lyzes the destination telephone number in the context of the geographi
`
`
`
`
`
`serves. Based on that information and relying on program logic, the CO processes
`
`
`
`
`and routes the call. Local long distance calls {e.g., within the San Francisco Bay
`
`
`
`
`
`Area) either are handled by directly connected Class 5 offices, or through a Class 4
`
`Class 5 offices. A coast-to-coast
`
`tandem toll office which interconnects multiple
`
`
`
`
`
`call, on the other hand, might involve all 5 classes of the hierarchy. For instance, a
`
`
`
`call from Turlock, California to New York, New York originates in the Class 5 switch
`
`
`
`
`of Evans Telephone Company, an independent telco, and is handed to a nearby
`
`
`AT&T tandem toll center. The call then works its way up the hierarchy until it
`
`
`
`
`
`reaches the Class 1 regional toll center in San Francisco. High-capacity, coast-co-
`
`Class 4
`
`Class 5
`
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`
`Chapter
`
`
`
`
`5: Public Switched Telephone Network (PSTN) 161
`
`coast tandem toll trunks carry the call to New York Oty, where it works its way
`
`
`
`
`down an abbreviated hierarchy and is delivered to the target number in Manhattan.
`
`
`Note that each switch acts on the call more or less independently. In the above clas­
`
`
`
`
`
`sic scenario, each switch looks at the call, determines whether or not it can serve
`
`
`
`
`
`the connection request, and hands the call off to another switch, either higher or
`
`
`
`
`lower in the hierarchy. The sole exception is the terminating Class 5 switch, which
`
`
`
`
`
`finally serves to establish the connection to the target telephone number.
`While the
`
`
`
`
`switches work together to set up the connection, each acts independently and for­
`wards the connection
`
`
`
`
`request blindly along, never knowing what will happen end
`
`
`
`to end. The same set of processes takes place in each switch, the same logic is exer­
`
`
`
`
`cised, and a talk path is set up through each switch and across each interconnect­
`
`ing transmission link.
`
`rally
`1cally
`fices
`
`cen­
`east
`net-
`
`en­
`face
`tes,
`
`na
`ood
`
`
`
`Figure 5-2 Traditional network hierarchy
`
`
`
`
`
`This approach was quite sensible in the days when AT&T dominated the local and
`
`
`
`
`
`
`
`long distance networks. As calls worked their way through the network, ever larger
`
`
`
`
`volumes of traffic were aggregated by ever more capable switches, and shipped over
`
`
`
`
`trunks of ever greater capacity, taking advantage of the economies of scale.
`
`
`
`This network topology has flattened over the years as the cost of transport over
`
`
`
`
`fiber optic facilities has dropped, the cost of switches has decreased, and competition
`
`
`
`has increased. As a result, there exist fewer more intelligent and multifunctional
`
`
`
`
`
`
`switches, interconnected by higher capacity transmission facilities in a sparse
`net­
`work configuration.
`
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