Copyright © 1979 American Telephone and Telegraph Company
`THE BELL SYSTEM TECHNICAL JOURNAL
`Vol. 58, No. 1, January 1979
`Printed in U.SA.
`
`Advanced Mobile Phone Service:
`
`Mobile Telephone Switching Office
`
`By K. J. S. CHADHA, C. F. HUNNICUTI, S. A. PECK,
`and J. TEBES, Jr.
`
`(Manuscript received August 30, 1978)
`
`The Mobile Telephone Switching Office provides centralized con(cid:173)
`trol of the Advanced Mobile Phone Service. Using a No. 1/lA
`Electronic Switching System, the Mobile Telephone Switching Office
`coordinates and controls the activities of the cell sites, interconnects
`the mobile telephones with the land telephone network, and main(cid:173)
`tains system integrity through automated maintenance. This paper
`gives an overview of the Mobile Telephone Switching Office and
`addresses the unique call processing and maintenance aspects of
`mobile telephony.
`
`I. INTRODUCTION
`The Advanced Mobile Phone Service (AMPS) provides high-capacity,
`high-quality mobile telephone service to a large number and variety of
`customers. The system blends two major communication disciplines:
`radio transmission and switching. The radio subsystem is based on
`cellular FM technologi·2 operating in the 850-MHz band. The switching
`subsystem is implemented on the No. l / lA family of the Electronic
`Switching Systems (Ess).
`As described in Ref. 3, the central coordinating element for AMPS is
`the Mobile Telephone Switching Office (MTSO). It controls the AMPS
`system and interfaces it with the land telephone network. The MTSO
`provides mobile customers with services that are similar to those
`available for land telephones. Basic mobile service includes direct
`dialed mobile-to-mobile, mobile-to-land, and land-to-mobile calling.
`An MTSO serves a large geographic coverage area, and all AMPS mobile
`calls are switched through it.
`The AMPS radio equipment is located in remote cell sites.2 Each cell
`site also contains duplicated stored program controllers, data link
`interface equipment, and auxiliary maintenance equipment. Cell sites
`
`71
`
`AT&T, Exh. 1004, p. 1
`
`

`
`are connected to the MTSO via voice trunks (referred to as cell-site
`trunks) and data links.
`A high degree of system reliability is attained through automated
`maintenance. The MTSO controls a number of automated hardware
`and software maintenance facilities that provide cell-site fault recog(cid:173)
`nition, recovery, and diagnostic capabilities.
`
`II. OVERVIEW OF NO. 1/1A ESS
`The No. 1 ESS is described in detail in Ref. 4; the 1A Processor is
`described in Ref. 5. For completeness, however, an overview of the
`system is given here. A No. 1 and No. lA ESS differ in the processor
`and memory complex.
`A No. 1/lA ESS consists of processors, memory, switching network,
`trunk circuits, and miscellaneous service circuits. This is illustrated in
`Fig. 1. It is organized as a common control system. Programs that are
`stored in the switching system's memory provide the logic to control
`telephone calls. The processors and memory are duplicated for relia(cid:173)
`bility.
`The switching network provides a means of interconnecting the lines
`and trunks. It consists of a matrix of reed switches. Lines from local
`subscribers terminate on the switching network. Likewise, trunks
`interconnecting with other switching offices terminate on the network
`through trunk interface circuits. The reeds are switched under the
`control of the central processor to produce a metallic voice connection
`or path between them. The switching network is configured to connect
`
`LINES TO{ ••
`• • • --+-1,__---1
`LOCAL
`SUBSCRIBERS ••• --+-1-+---1
`
`SWITCHING
`NETWORK
`
`• · } T RUNKS
`TO OTHER
`• • • OFFICES
`
`SERVICE
`CIRCUITS !TONES.
`ANNOUNCEMENT S,
`TRANSMITTERS.
`RECEIVERS)
`
`}
`
`TO ALL UNITS
`
`• F ROM TRUNK AND
`: SERVICE CIRCUITS
`
`TELETYPEWRITER
`
`AUTOMATIC
`MESSAGE
`ACCOUNTING
`TAPE
`
`DUPLICATED I DUPLICATED
`PROGRAM
`I
`CALL
`STORE
`I
`STORE
`
`Fig. 1- No. l / lA ESS block diagram.
`
`72
`
`THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1979
`
`AT&T, Exh. 1004, p. 2
`
`

`
`any two lines and/or trunks together with an engineered probability
`of blocking.
`As mentioned above, trunks interface to the switching network
`through trunk circuits which contain interface elements, sensing fer(cid:173)
`rods for call supervision, and relays to control trunk states for talking,
`signaling, and testing. A variety of trunk circuits are available, and
`each application is engineered to meet a particular need, e.g., trunk
`circuits connected to 2-wire or 4-wire transmission facilities.
`The supervisory state of a call is indicated by the presence or
`absence of a direct-current flow. At least one, and often two, ferrods
`are employed in every connection (usually in the trunk circuit). Su(cid:173)
`pervisory changes are detected by scanners which are read under the
`control of the central processor. Stored program control logic then
`interprets the scanner results. Scanners, then, are the real-time data
`input devices to the processor.
`The central processor controls the switching network and trunk and
`service circuits by sending orders to them through pulse distributors.
`The pulse distributors perform an inverse function to the scanners.
`Craftpeople interact with the processor through one or more tele(cid:173)
`typewriters. Through these, the processor prints its output messages
`and craftpeople input specific commands to the system. Another form
`of output is the Automatic Message Accounting (AMA) tapes that are
`used for charge recording data collection. These tape drives only record
`billing data.
`The No. l/lA ESS operates under the control of its stored program.
`Three types of programs are resident in the system: call processing,
`hardware maintenance, and administration. Call processing programs
`provide the logic that controls call setup and disconnect actions for
`the wide variety of call types. The maintenance programs provide the
`means of recognizing hardware failure conditions and reconfiguring
`the active/standby units to achieve a working system. The mainte(cid:173)
`nance programs also provide diagnosis of suspected failed units to aid
`in the repair. Administrative programs provide mechanism for chang(cid:173)
`ing the system data base. The data base includes customer records,
`trunk records, billing data, and traffic counts.
`An MTSO is built upon standard No. l/lA ESB hardware. As explained
`in Section III, the switching operations are all trunk-to-trunk. The
`logic to control mobile telephone calls and to maintain the cell-site
`hardware is implemented as an addition to the ESB stored program.
`
`Ill. MTSO INTERCONNECTION
`3. 1 MTSO to wire-line network
`In the Chicago Equipment Test, the MTSO occupies a position in the
`switching hierarchy below a class 5, or local, office. This is illustrated
`in Fig. 2. The MTSO can be interconnected with one or more local
`offices over standard trunk facilities. Directory numbers for mobile
`
`MOBILE TELEPHONE SWITCHING OFFICE
`
`73
`
`AT&T, Exh. 1004, p. 3
`
`

`
`TRAFFIC
`SERVICE
`POSITION
`SYSTEM
`
`INTERTO L L TRUNKS
`
`TRAFF IC
`SERVICE
`POSITION
`SYSTEM
`
`TSPSTRUNK,
`
`' '
`
`TOLL
`SWITCHING
`OFFICE
`
`DIRECTORY
`ASSISTANCE
`
`-
`
`TOLL
`-CONNECTING
`TRUNKS
`
`TANDEM
`.....__TOLL
`TRUNKS
`
`DIRECTORY
`ASSISTANCE
`
`TANDEM
`SWITCHING
`OFFICE
`
`LOCAL TRUNKS
`
`MOBILE
`TELEPHONE
`SWITCHING
`OFFICE
`
`LOCAL
`(CLASS SI
`SWITCHING
`OFFICE
`
`CELL-SITETRUNKS-,
`
`DATA LINKS-,,
`
`LOCAL
`(CLASS 5)
`SWITCHING
`OFFICE
`
`\
`
`'-.... MOBILE
`'
`INTERCONNECTING
`TRUNKS
`
`CELL
`SITE
`
`Fig. 2-Position in hierarchy.
`
`telephones are assigned from within the local exchanges that are
`served by those interconnected offices. The MTSO interconection ar(cid:173)
`rangement is similar to that used with a Private Branch Exchange
`(PBX), and it makes use of existing capabilities in ESS local offices.
`Mobile-originated calls into the land telephone network are out(cid:173)
`pulsed from the MTSO using TOUCH-TONE* signaling. Dial-pulse
`signaling can also be used. The MTSO selects and seizes the outgoing
`trunk to the local office. It begins outpulsing the called digits after the
`local office sends a start pulsing, or wink, signal, The wink is a
`momentary battery reversal on the trunk. Answer and disconnect
`supervision signals are returned from the local office to the MTSO
`allowing charging records to be made.
`On land-to-mobile calls, the local office outpulses the called mobile's
`telephone number to the MTSO using either multifrequency, dial pulse,
`or TOUCH-TONE signaling. The MTSO returns answer and disconnect
`supervision signals back to the local office.
`The MTSO routes calls within the AMPS system and into the wire-line
`network. The simplest call routing is the mobile-to-mobile call. The
`
`*Registered service mark of AT&T Co.
`
`74
`
`THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1979
`
`AT&T, Exh. 1004, p. 4
`
`

`
`MTSO receives the dialed digits from the calling mobile, determines
`that the called number is another mobile, and completes the connec(cid:173)
`tion to that called mobile. None of the interconnected offices are
`involved. This case is illustrated in Fig. 3a.
`On direct-dialed, mobile-to-land calls, the MTSO routes the call into
`the land telephone network through one of the local offices. Routing
`tables stored in the MTSO provide the association between the called
`number and the proper local office to be used. For standard calls, the
`directory number of the calling mobile does not influence the routing.
`Exceptions to this include operator assistance, emergency service, and
`repair service. The mobile-to-land connection is shown in Fig. 3b.
`Land subscribers can directly dial calls to mobiles. Since mobile
`directory numbers are assigned from those available in local exchanges,
`there is a correspondence between each mobile and a particular local
`office. The land telephone network directs calls to the local office
`serving the exchange of the called number without knowing the call is
`to a mobile. Upon receiving such a call, the local office connects that
`call to a direct trunk to the MTSO which, in turn, completes the
`connection to the mobile. A land-to-mobile connection is identical to
`that shown in Fig. 3b.
`Operator-assisted and service calls (e.g., repair service) can also be
`dialed from a mobile. The MTSO does not have direct trunks to operator
`or service bureau positions. Instead, it makes use of those services
`already available in the local offices. The MTSO routes these calls to a
`local office which connects the calls to operator and service position
`trunks. The call routing is shown in Fig. 3c.
`
`3.2 MTSO to cell site
`Figure 4 illustrates the interconnection of the MTSO to several cell
`sites. Two types of facilities are used. First, cell-site trunks provide a
`voice communication path. The number of trunks is engineered on the
`basis of traffic and desired blocking probability. Each trunk is physi(cid:173)
`cally connected to a cell-site voice radio. The type of trunk is dependent
`on the overall system transmission plan. This selection determines the
`appropriate ESS trunk circuit.
`A cell site acts in the radio frequency domain as a traffic concentrator
`for the MTSO. Assuming an average busy-hour mobile unit occupancy
`of a few percent and a grade-of-service objective comparable with land
`service, an average busy-hour radio channel occupancy of at least 60
`to 70 percent results for higher traffic cells.
`The MTSO also connects with the cell sites through two 2400-baud
`data links operating in a full duplex mode. Figure 4 illustrates the
`connection of the data link equipment to the ESS and to the cell site.
`The data link hardware at the MTSO is not unique to AMPS; it is also
`used in No. 1 ESS Toll Common Channel Interoffice Signaling (CCIS).
`A Terminal Access Circuit (TAC) provides the interface between the
`internal ESS buses and the data link terminals (see Fig. 10). The TACS
`
`MOBILE TELEPHONE SWITCHING OFFICE
`
`75
`
`AT&T, Exh. 1004, p. 5
`
`

`
`Al-
`
`~+£(?
`MOBILE
`TEL EPHONE
`
`DATA LINK
`
`CELL-SITE TRUNK
`
`MOBI L E
`TELEPHONE
`SWITCHING
`OFFICE
`
`(a)
`
`DATA LIN K
`
`CEL L- SITE TRUNK
`
`--..:._
`
`~
`MOBILE
`TE LEPHONE
`
`LOCAL
`CLASS 5
`SWITCHING
`OFFICE
`
`TANDEM
`TRUNK I TANDEM
`SWITCHING
`OFFICE
`
`TANDEM
`TRUNK
`
`LOCAL
`CLASS 5
`SWITCHING
`OFFICE
`
`(b)
`
`LAND
`TELEPHONE
`
`MOBILE
`-INTERCONNECTING
`/TRUNK
`I
`I
`I
`
`DATA LINK
`
`MOBI LE
`TELEPHONE
`SWITCHING ~------I
`CELL-SITE
`OFFICE
`TRUNK
`
`--..:._
`
`~ ;;g:;o-i
`MOBILE
`TE LEPHONE
`
`at
`
`-4
`:I:
`m
`aJ
`m
`,....
`,....
`
`"' ~ m
`
`~
`-4 m
`0
`:I: z
`0 > ,....
`c...
`0 c
`::c z
`)>
`!
`c...
`)> z
`c
`)>
`::c
`-<
`
`(0
`--I
`(0
`
`TOLL
`CONNECTING,
`TRUNK
`\
`I
`I
`
`I \
`
`LOCAL
`CLASS 5
`SWITCHING
`OFFICE
`
`LAND
`TE LEPHONE
`
`TRAFFIC SERVICE
`POSITION SYSTEM
`
`TOLL
`SWITCHING
`OFFICE
`
`TSPS
`TRUNK
`
`LOCAL
`CLASS 5
`SWITCHING
`I OFFICE
`
`(c)
`
`MOBILE
`,. INTERCONNECTING
`/ TRUNK
`I
`I
`I
`
`DATA LINK
`
`I
`
`CELL- SITE
`TRUNK
`
`--..:._
`~
`MOBILE
`TELEPHONE
`
`MOBI LE
`TELEPHONE
`
`SWITCHING I
`
`OFFICE
`
`Fig. 3-(a ) Mobile-to-mobile call. (b) Typical mobile-to-land call. (c) Typical mobile-originated, operator-assisted call.
`
`AT&T, Exh. 1004, p. 6
`
`

`
`SWITCHING
`NETWORK
`
`\
`
`,,.
`
`' >CELL-SITE TRUNKS
`
`VOICE
`RADIO
`
`VOICE
`RADIO
`
`r -C"EU.:SiTE!.1 - 1
`I
`I
`I
`I
`I
`I
`I
`I
`I
`I
`I
`
`l
`I
`L _______ J
`
`DUPLICAT ED
`CONTROLLER
`
`DATA LINK
`CONTROL
`EQUIPMENT
`
`I
`L ___ _ _ _ _ _ _J
`
`VOICE
`RADIO
`
`DUPLICATED
`CONTROLLER
`
`L _ __ __ _ _
`
`Fig. 4- MTSO-Cell-site interconnection.
`
`are duplicated for reliability. A duplicated pair ofTACs controls 16 data
`link terminals, each of which drives two data modems. The data links
`to each of the cell sites are controlled by different terminals to provide
`high reliability. In the event of a data link failure, communication can
`continue between the MTSO and a cell site by reconfiguring either the
`TAC or terminal, as appropriate.
`
`IV. MTSO CALL PROCESSING
`The MTSO is the central controller for processing mobile telephone
`calls. Reference 3 describes the basic system plan and outlines some
`representative call sequences. In carrying out these sequences, the
`MTSO performs a number of functions that differ from a conventional
`wire-line switch either in the nature of the function itself or in the
`implementation. Table I illustrates these differences.
`The following activities are representative of the MTSO call process(cid:173)
`ing:
`(i) Providing switched interconnection with the land telephone
`network.
`(ii) Providing switched connections between mobile subscribers
`served by the MTSO.
`(iii) Administering the usage of the radio voice channels.
`
`MOBILE TELEPHONE SWITCHING OFFICE
`
`77
`
`AT&T, Exh. 1004, p. 7
`
`

`
`Table I-Land switching/mobile switching functional comparison
`Function
`Local ESS
`MTSO
`1. Connection
`Space division net(cid:173)
`Space and distributed frequency divi-
`work
`sion network
`Path reconfiguration (handoffs)
`Central control
`Remote control (cell sites)
`Setup channel location/ identification
`
`2. Control
`
`Central control
`
`3. Attending
`(origination)
`4. Information re(cid:173)
`ceiving and
`transmitting
`
`5. Busy testing
`6. Availability
`testing
`7. Alerting (ring(cid:173)
`ing)
`8. Supervising
`(call in prog(cid:173)
`ress)
`9. Monitoring for
`transmission
`quality
`
`Scanners
`
`Dial pulse
`TOUCH-TONE®
`Standard interoffice
`signaling
`Memory function
`Continuity tests
`
`Preorigination digital dialing
`TOUCH- TONE interoffice signaling
`
`Memory function
`Paging
`
`Subset ringing from
`co (90 v, 20 Hz)
`Line scanners
`
`Activated in mobile by a digital mes(cid:173)
`sage from MTSO
`Trunk scanners
`
`Location function
`
`(iv) Providing control over signaling with the mobile units.
`(v) Providing control of the intercell location process and the
`resulting handoffs.
`(vi) Recording charge information.
`(vii) Providing custom services to mobile users.
`The following examples illustrate the nature of mobile call processing
`within the MTSO.
`
`4. 1 Moblle-or/g/nated calls
`The MTSO receives a request for a mobile-originated call as a data
`message from a cell site. Each origination message contains the calling
`mobile's identification, the complete called number as dialed, and the
`serving cell-site identification. The MTSO analyzes the called number.
`If the origination attempt is correct and allowed, the MTSO selects an
`outgoing trunk. The MTSO may deny an origination attempt from a
`restricted subscriber. If an attempt is incorrect or incomplete, the
`MTSO sends a reorder or intercept data message to the subscriber.
`For the successful origination attempts, the MTSO selects an idle cell(cid:173)
`site trunk (and associated voice radio). The MTSO sends a data message
`to the cell site serving the mobile, instructing the mobile to tune to the
`assigned voice frequency. The MTSO then scans the ferrod associated
`with the cell-site trunk for an on-hook to off-hook state transition
`which indicates that the mobile did indeed tune to the new channel. If
`a voice channel assignment confirmation is not received, the MTSO
`attempts a single retry. The call is terminated if the retry is not
`confirmed.
`For those calls where the voice channel assignment is successful, the
`MTSO seizes an outgoing trunk to the local office as determined by digit
`
`78
`
`THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1979
`
`AT&T, Exh. 1004, p. 8
`
`

`
`analysis. A transmitter is connected, and the called number is out(cid:173)
`pulsed. At the completion of outpulsing, the cell-site trunk is connected
`to the outgoing trunk, thereby establishing a talking path. Figure 5
`illustrates the mobile-originated call setup switching process and as(cid:173)
`sociated signaling.
`On each mobile-originated call, the MTSO makes an AMA entry for
`charging purposes. Section 5.1 elaborates on the contents of the data
`entry.
`
`4.2 Mob/le-completed calls
`On a call to a mobile, the MTSO receives the completion attempt on
`an incoming trunk. It connects a digit receiver and collects the called
`digits. The MTSO analyzes the digits and identifies the called mobile if
`the dialed number is valid. Calls to invalid numbers are routed to an
`intercept announcement. The MTSO then initiates a paging process to
`locate the mobile within a particular cell. The MTSO does this by
`sending a data message to each active cell site. Only one cell site
`should respond, thereby identifying the called mobile's location. Three
`possible situations can occur. First, only one cell site responds and the
`call processing proceeds as described below. Second, no response is
`received. In this case, the MTSO retries the page process one time. A
`second no-response condition is taken to mean the mobile is not there,
`
`SWITCH
`CONFIGURATION
`
`ORIG INATION
`
`NETWORK
`
`NETWORK D LJ
`
`CHANNE L
`ASSIGNMENT
`
`OUTPULSING
`
`TALKING
`
`NETWORK
`
`NETWORK LJ
`
`DATA TO MTSO
`
`ORIGINATION
`_JUUl MESSAGE
`
`DATA FROM MTSO
`
`CHANNEL
`-------~nfln ASSIGNMENT
`
`OFF - HOOK
`
`ARR IVEO
`AT DESIGNATED
`ON HOOK
`CELL- SITE TRUNK
`STATE AT MTSO - - - - - - - - - - - - - ' CHANNEL
`
`TIME__.
`
`CST : CEL L- SITE TRUNK
`OGT : OUTGOING TRUNK
`XMTR : T RANSMITTER FO R
`TOUCH- TONE SIGNALING
`
`Fig. 6-Mobile-originated call.
`
`MOBILE TELEPHONE SWITCHING OFFICE
`
`79
`
`AT&T, Exh. 1004, p. 9
`
`

`
`and the MTSO connects the calling party to a recorded announcement.
`Third, more than one response may be received. This is an error
`condition, and the MTSO attempts to set up the call based on the first
`response. Subsequent responses are ignored.
`For a successful page response, the MTSO directs the mobile to tune
`to a voice channel in the same manner as described in the previous
`section. Once the mobile has tuned successfully, the MTSO begins the
`alerting, or ringing, process. The MTSO alerts the mobile customer by
`sending a data message which activates an alert facility (ringer) in the
`mobile unit. The MTSO connects audible ringing tone to the calling
`party. Confirmation of the mobile's receiving the alert message ~ seen
`at the MTSO as an off-hook to on-hook state transition on the cell-site
`trunk. Next, the MTSO detects the mobile customer answer by scanning
`for an on-hook to off-hook transition on the cell-site trunk.
`When an answer is received, the MTSO removes the audible ringing
`tone from the incoming trunk. It then connects the incoming trunk
`with the cell-site trunk, establishing the talking path between the two
`parties. Figure 6 illustrates the signaling and MTSO switching actions.
`
`4.3 Locating and handoff
`Calls in a talking state are supervised for adequate voice channel
`signal quality by the MTSO through a coordinated effort with the cell
`
`SWITCH
`CONFIG (cid:173)
`URATION
`
`DATA FROM
`MTSO
`
`DATA TO
`MTSO
`
`CELL- SITE
`TRUNK STATE
`AT MTSO
`
`DIGIT
`COLLECTION
`
`PAGING
`
`CHANNEL
`ASSIGNMENT
`
`ALE RTING
`
`TALKING
`
`NETWORK
`
`NETWORK
`
`NETWORK
`
`NETWORK
`
`~ TLJ
`
`PAGE nnn
`
`CHANNEL
`ASSIGNMENT nnn
`
`ALERT nllfl..._ __
`
`PAGE
`RESPONSEnllfl~--------------~
`
`OFF - HOOK
`
`ALERT
`ARRIVEO AT CONFIRM-
`DESIGNATED ATION
`ON- HOOK
`=...;..;.:~---------...J CHANNEL
`
`ANSWER
`
`TIME-
`
`ICT: INCOMING TRUNK
`CST: CELL- SITE TRUNK
`RCVR : TRUNK OIGIT RECEIVER
`AUD: AUDIBLE RINGING TONE
`
`Fig. 6-Mobile-completed call.
`
`80
`
`THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1979
`
`AT&T, Exh. 1004, p. 10
`
`

`
`TALKING
`
`LOCATING
`
`HANOOFF
`INITIALIZ ATION
`
`HANDOFF
`
`TALKING
`
`NETWORK
`
`NETWORK
`
`NETWORK
`
`NETWORK
`
`NETWORK
`
`SWITCH
`CONFIGURATION
`
`~
`
`~
`~
`
`OATATO
`MTSO
`
`OATA F ROM
`MTSO
`
`LOCATION
`MEASUREMENT
`
`...
`MEASUREMENT fl Jl
`_______________ .....
`
`LOCATION
`
`REQUESTS
`
`HANDOFF nnn
`------------------------------
`
`MESSAGE
`
`O LO CELL-SITE
`TRUNK STATE
`
`OFF- HOOK
`
`ON- HOOK
`
`I LEFT OLD
`
`CHANNEL
`
`NEW CELL-SITE
`TRUNK STATE
`
`OFF-HOOK
`
`ON- HOOK
`
`;;::;""";:;;:;:~-------------------------__J' ARR IVED AT
`
`NEW CHANNEL
`
`TIME -
`
`Fig. 7-Handoff.
`
`ICT: INCOMING TRUNK
`CSToLo' OLD CELL- SITE TRUNK
`CST NEW' NEW CELL-SITE TRUNK
`
`s::
`0
`CJ r m
`-I m
`r m
`-0
`::c
`0 z m
`:E
`:::j
`(")
`
`CJ)
`
`::c z G)
`
`0
`"Tl
`"Tl
`0 m
`()I) ...
`
`AT&T, Exh. 1004, p. 11
`
`

`
`sites. Under their internal program control, cell sites monitor the
`received mobile signal quality on all voice channels that are in use.
`The MTSO collects signal quality information from serving and adjacent
`cells and determines when a handoff process should be initiated.
`However, if no cell site can adequately serve the mobile, the MTSO
`leaves the call undisturbed on the original cell site.
`The handoff process involves four operations:
`(i) Selecting a cell-site trunk to the new serving cell site.
`(ii) Instructing the mobile to tune from its present voice channel
`to the one corresponding to the new cell-site trunk.
`(iii) Setting up a talking path in the switching network from the
`new cell-site trunk to the incoming or outgoing trunk.
`(iv) Idling the talking path in the switching network between the
`old cell-site trunk and the incoming or outgoing trunk.
`As shown in Fig. 7, the MTSO uses the states of the cell-site trunks to
`infer the state of the mobile. After the MTSO sends a data message
`instructing the mobile to tune, it scans the old cell-site trunk for an
`off-hook to on-hook transition as a confirmation that the mobile left
`the old voice channel. The MTSO also scans the new cell-site trunk for
`the opposite transition as a confirmation that the mobile arrived at
`the new channel. No AMA record is made for handoffs.
`
`4.4 Disconnect
`The MTSO is responsible for controlling the states of all equipment
`in a speech path. This equipment consists of an incoming or outgoing
`trunk, a switching network path, a cell-site trunk, a cell-site radio
`(transmitter-receiver pair). and a mobile unit transceiver. During both
`normal disconnect and failure actions, the MTSO deals with the radio
`components in addition to the switching network and trunks.
`The MTSO turns on a cell-site transmitter when a voice path is set up
`and turns it off when a voice path is torn down. Thus, cell-site
`transmitters radiate power only while their associated channels are in
`use. The MTSO does this by sending data messages to the cell site.
`During a normal disconnect where the land party goes on-hook first,
`the MTSO instructs the mobile via a data message to tune back to the
`setup channel. This clears the associated voice channel for the next
`call and puts the mobile in the correct state to initiate or receive its
`next call. If the mobile disconnects first, it autonomously retunes to
`the setup channel, and the MTSO turns off the associated cell-site
`transmitter by sending a data message.
`Several ambiguous situations can occur during disconnect processing
`where the MTSO does not know the true state of the mobile. An
`example is when no carrier signal from a cell site is seen by a mobile.
`Here, the mobile autonomously times for several seconds and retunes
`to the setup channel. Thus, for some time, a mobile may be on a voice
`channel in an autonomous timing state during a possible disconnect.
`The MTSO holds the cell-site trunk associated with such a mobile for
`
`82
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`THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1979
`
`AT&T, Exh. 1004, p. 12
`
`

`
`a 5- to 6-second guard timing interval. This ensures that the trunk will
`not be reassigned to a new call before the old mobile leaves the
`channel.
`In addition, mobiles exhibit another characteristic called fading. A
`mobile may be in a talking situation and drive into a radio path fade
`due to a poor propagation situation. The MTSO is notified of such an
`event by the cell site. If the length of a fade exceeds about 5 seconds,
`the cell site signals this to the MTSO by placing the associated trunk
`on-hook. The MTSO initiates normal disconnect processing, and it
`discovers that the disconnect was due to a fade when the cell-site voice
`transmitter is turned off. The cell site sends a message to the MTSO
`saying that the call was involved with fade timing. The MTSO records
`a fade indicator in the AMA record so that billing adjustments can be
`applied.
`
`4.5 Unsuccessful calls
`Calls may be unsuccessful for a variety of reasons including:
`(i) Dialing errors.
`(ii) Equipment malfunctions.
`(iii) Busy conditions.
`(iv) Traffic blocking.
`(v) Signaling errors.
`The MTSO has a set of informative recorded announcements and tones
`which are used to provide indications about call failures to the origi(cid:173)
`nating party. For example, traffic-busy conditions are indicated by
`reorder tone.
`Tones given to mobile users may come from either the· MTSO tone
`sources or the mobile unit itself. The MTSO controls the application of
`the tones. For failures encountered after a mobile has successfully
`tuned to a voice channel, the MTSO connects the cell-site trunk to the
`appropriate tone source or announcement. If the failure occurs before
`voice channel assignment, the MTSO sends a data message instructing
`the mobile unit to activate an internal tone.
`
`V. MTSO ADMINISTRATIVE PROCESSING
`5.1 811//ng
`The MTSO records all charge-related data for the AMPS system on its
`AMA tape. In contrast to conventional wire-line switching offices, the
`MTSO records a billing entry for all calls to a mobile as well as those
`from a mobile. An entry is made on all calls that successfully tune to
`a radio voice channel.
`For mobile-originated calls, the data that are recorded include the
`conventional called and calling numbers, answer time, and disconnect
`time. This portion of the record deals with the message unit and toll
`charges associated with the wire-line network. Also recorded in the
`AMA entry is the radio voice channel seizure and release times and the
`initial cell-site identification. These items pertain to the usage of the
`AMPS radio facilities.
`
`MOBILE TELEPHONE SWITCHING OFFICE
`
`83
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`AT&T, Exh. 1004, p. 13
`
`

`
`For mobile-completed calls, the office in which the call originated
`creates a billing record covering the usage of the land telephone
`network. The MTSO records the radio usage data (the voice channel
`seizure and release times and initial cell site).
`
`5.2 Service orders
`The MTSO translation data base contains the records associated with
`each mobile customer. Typical data include:
`(i) Mobile directory number.
`(ii) Billing classification data.
`(iii) Custom services subscribed to.
`Each time a customer is added or deleted or changes service options,
`the MTSO data base is updated accordingly. The records associated
`with this activity are called service orders.
`Clerks receive written orders detailing service changes. They input
`these orders into the MTSO data base via a teletypewriter.
`Messages changing the MTSO data base are called recent change
`messages. They are processed by an extensive set of MTSO programs
`which check them for format and data errors. Error conditions are
`flagged back to the clerk on the teletypewriter. Error-free messages
`cause the appropriate translation data records to be altered.
`
`5.3 Trunk changes
`The number of trunks interconnecting the MTSO with each local
`office and cell site is engineered based on the expected traffic. Given
`a traffic load and desired blocking probability, the required number of
`trunks can be determined. As a system grows, the number of trunks
`changes. Recent change messages are available in the MTSO to add or
`delete trunk records in the translation data base. Again, administrative
`programs process these messages for validity and update the data base
`accordingly.
`One unique aspect of trunk changes in AMPS is a correspondence
`between a cell-site trunk and a radio voice frequency. The translation
`data in the MTSO memory associate a cell-site trunk with a specific
`frequency. When voice-channel frequencies are changed at a cell site,
`the corresponding trunk translations are altered with recent change
`messages.
`
`5.4 Traffic measurements
`As previously mentioned, the amount of traffic the system carries
`determines the amount of equipment in the MTSO. The MTSO collects
`traffic data on call attempts and equipment usage to aid office engi(cid:173)
`neering. The traffic-engineered items include:
`(i) Number of trunks.
`(ii) Number of service circuits.
`(iii) Amount of memory.
`(iv) Amount of switching networks.
`
`84
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`THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1979
`
`AT&T, Exh. 1004, p. 14
`
`

`
`(u) Number of data links.
`(ui) Number of cell sites.
`Generally, three counts are maintained on each traffic-engineered
`component. These counts are total seizure attempts, total blocked
`attempts, and usage. Seizure attempts and blocked attempts are self(cid:173)
`explanatory peg counts. Usage counts are taken at 10 or 100-second
`intervals. The usage counts indicate how many pieces of equipment in
`the group are busy at the sample time. The traffic measurements are
`summarized and printed on the MTSO teletypewriter at various times
`of the day.
`
`VI. AMPS SYSTEM MAINTENANCE
`6. 1 System maintenance concepts
`The measure of the success of a commercial venture is directly
`related to customer satisfaction and economic considerations. Tele(cid:173)
`phone customers gauge their satisfaction by low-cost, continuous,
`accurate service with minimal service delay and/ or interruption. Sys(cid:173)
`tem availability is defined as a measure of service continuity and
`accuracy, while maintainability can be defined as the ease with which
`failures can be detected, isolated, and corrected.4
`Common measures of functional system reliability include the fol(cid:173)
`lowing measures, which are graphically illustrated in Fig. 8:
`
`A= Availabilit = MUT =
`MEAN-UP-TIME
`y MTBF MEAN-TIME-BETWEEN-FAILURES
`A= l _ MEAN-RECOVERY-TIME( MRT)
`MTBF
`DT = Total Down Time Over System Life(T) = (l - A)T.
`While the overall AMPS system objectives are comparable to those
`for the No. 1 ESS central office, the downtime objectives for a single
`cell site are somewhat less stringent. This is in part due to the
`geographic redundancy afforded by the cellular concept. Mobile cov(cid:173)
`erage, albeit sometimes degraded, can be adequately provided by
`neighboring cells in the mature system.
`
`•
`
`i -AT-i-- - - - ----MUT- - - - -
`
`TIME -
`
`- - - - i
`
`t - - - - - - - - - - - MTBF- - - - - - - - - - -1
`
`FAI LURE
`
`FAI LURE
`
`•
`
`MAT : MEAN AECOVEA Y-T IME
`MUT: MEAN UP- TIME
`MTBF: MEAN T IME BETWEEN FAI L URES
`
`Fig. 8-Equipment availability.
`
`MOBILE TELEPHONE SWITCHING OFFICE
`
`85
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`AT&T, Exh. 1004, p. 15
`
`

`
`To increase the MTBF of a system, the design must
`(i) Reduce component count.
`(ii) Increase component reliability.
`(iii) Make the system immune to human error.
`However, in practice, the aforementioned items may be difficult to
`achieve. Equiv