Copyright © 1979 American Telephone and Telegraph Company
`Tun BELL SYSTEM TECHNICAL JOURNAL
`Vol. 58, No. 1, January 1979
`Printed in U.S.A.
`
`Advanced Mobile Phone Service:
`
`Control Architecture
`
`By 2. C. FLUHR and P. T. PORTER
`
`(Manuscript received February 15, 1978)
`
`The cellular concept used in the Advanced Mobile Phone Service
`(AMPS) system to achieve spectrum efficiency requires a complex and
`flexible distributed system control architecture. Three major subwa—
`tems serve as the control elements: the mobile unit, the cell site, and
`the switching office. System control functions are partitioned among
`these subsystems to handle the following AMPS control challenges:
`interfacing with the nationwide switched telephone network, dialing
`from mobile units, supervising calls from mobile subscribers in the
`presence of noise and co-channel interference, performing call setup
`functions includingpaging and access, and locating and handing off
`mobiles between cell sites. This paper explains the techniques used to
`achieve the control functions of the three major subsystems and the
`ways they in turn participate in control of the total AMPS system.
`
`I.
`
`INTRODUCTION
`
`The cellular concept, which achieves radio spectrum efficiency
`through the technique of frequency reuse, requires a grid of control
`elements (cell sites) distributed throughout a mobile covef‘age area to
`serve as the interface between the large numbers of moving customers
`and the nationwide switched telephone network. Meeting these re-
`quirements in a cost-effective manner and providing a framework for
`offering a variety of services to AMPS customers requires a complex yet
`flexible control architecture.
`
`Before proceeding with our description of the system control archi-
`tecture, which draws heavily on earlier investigations,"3 it will be
`helpful to look at some of the important system interfaces with the
`nationwide switched (wire-line) telephone network and with mobile
`users.
`
`43
`
`CLEARWIRE 1005
`
`

`

`II. SYSTEM INTERFACES
`2.1 Network interface
`
`A single AMPS system is designed to serve customers within a given
`geographical area, known as a Mobile Service Area (MSA). This usually
`corresponds to a metropolitan area including a central city, its suburbs,
`and some portion of its rural fringe (see Fig. 1). However, it could
`encompass a portion of an extremely large metropolitan area or per-
`haps two or more cities located relatively close together.
`Mobile customers are expected to subscribe to service in a specific
`MSA. While operating within its boundaries, a customer is termed a
`“home mobile." Outside this area, the customer is termed a “roamer.”
`An objective of AMPS is to provide dial access between home mobiles
`and any other telephone (mobile or land-line) reached through the
`wire-line telephone network. Another objective is to provide access, as
`automatically as possible,
`to and from roamers. These goals are
`achieved by assigning each mobile customer a standard ten-digit
`telephone number composed of a three-digit area code plus a seven-
`digit directory number. This enables an AMPS system to interface with'
`the wire-line telephone network using standard trunking methods (Fig.
`2) and permits calls to be handled with standard telephone routing
`and signaling techniques.
`
`2.2 User interface
`
`AMPS radio links carry call control information in addition to voice
`communication. The customer’s identification and the dialed digits
`(network address) are two call control items that must be supplied in
`a digital mode to the local system on every call from a mobile. Known
`as ”preorigination dialing,” the dialing sequence takes place before the
`mobile unit's first communication with the local system. A mobile
`customer dials the telephone number of the party being contacted into
`a register in the mobile unit, thus recording it in the unit’s memory.
`MOBILE
`SERVICE
`,’”-A%A
`BOUNDARY
`
`
`
`-.\ \\
`
`
`
`
`
`\§\\\\\\\\\ AREAS
`
`
`
`
`
`\n'figg
`
`
`
`
`
`
`
`.\\ \\\\\\\\\
`
`
`
`Fig. l—Typical mobile service area.
`
`44
`
`THE BELL SYSTEM TECHNICAL JOURNAL. JANUARY 1979
`
`

`

`STANDARD
`access \
`TRUNKS \
` I
`
`RADIO LINKS
`LOCAL
`1
`SYSTEM
`
`
`
`
`Paovmmo
`WIRE-LINE
`AMPS
`
`
`TELEPHONE
`
`
`
`
`NETWORK
`OBILE
`
`TELEPHONE
`LANDLINE
`\
`
`EXISTING
`NXX-NXXeXXXX
`TELEPHONE
`\\
`{-
`CUSTOMER
`
`LOOPS
`Nxx—Nxx-xxxx
`
`@M
`
`\‘i
`
`* N = ANY D1GIT 2THROUGH 9
`X = ANY DJGIT 0 THROUGH 9
`
`Fig. 2—AMPS system interfaces.
`
`The customer then initiates the communication with the land portion
`of the system according to procedures outlined in Ref. 4.
`One major advantage of preorigination dialing is that a customer
`can dial at a slow rate without tying up a valuable radio channel. If a
`mistake is made, the Customer can “erase" the dialed digits and redial
`the correct number. Only when the number is completely assembled
`and stored is the radio channel used, and then the number is sent as
`rapidly as possible in coded form along with other call-processing
`information.
`
`The mobile telephone and the land portion of the system also
`exchange other information, such as the unit's supervisory state, the
`cell site being used, and the designated voice channel. These items are
`discussed in later sections.
`
`Ill. SYSTEM CONTROL ELEMENTS
`
`The three major system control elements are the mobile unit, the
`cell site, and the switching office.
`
`3. 1 The mobile unlt
`
`In addition to transmitting network address information, the mobile
`unit performs other control and signaling functions, which are dis-
`cussed in Section IV. As noted in Ref. 5, the mobile unit is tunable on
`system command to any channel in the RF spectrum allocated to AMPS
`at any one of four power levels as pre-programmed. To perform these
`control and signaling functions, its design will most likely include a
`microprocessor.
`
`3.2 The cell site
`
`To achieve the grid of small coverage areas from which the cellular
`concept takes its name, land-based radios are located at cell sites
`throughout the mobile coverage area, as described in Ref. 5. Each cell
`site processes the signals to make them suitable for transmission
`
`CONTROL ARCHITECTURE
`
`45
`
`

`

`between the wire-line network and the radio network for all mobile
`telephones interfacing with it. This requires real-time control, which
`is accomplished with stored-program techniques. In addition, each cell
`site performs other control and signaling functions discussed below.
`
`3.3 The Mobile Telecommunications Switching Office
`
`The Mobile Telecommunications Switching Office (MTSO) serves as
`the central coordinator and controller for AMPS and as the interface
`between the mobile and the wire-line network. As described previously,
`all information exchanged over this interface employs standard tele-
`phone signaling. Hence, standard switching techniques are required
`within the MTSO. In addition, the MTSO must (i) administer radio
`channels allocated to AMPS, (ii) coordinate the grid of cell sites and
`moving subscribers, and (iii) maintain the integrity of the local AMPS
`system as a whole. These new switching functions require extensive
`use of stored-program technology within the MTso.
`
`3.4 Interconnection of subsystems
`
`Interconnection of the three major control elements is shown in Fig.
`3. The mobile telephone communicates with a nearby cell site over a
`radio channel assigned to that cell. The cell site, in turn, is connected
`by land-line facilities to the MTSO, which interfaces with the wire-line
`network. As Fig. 3 also indicates, a considerable amount of data is
`exchanged between pairs of AMPS control elements. For instance, call
`setup data are exchanged between the mobile telephone and the cell
`site over radio channels reserved for this purpose. The voice channels
`also carry data to control and to confirm various mobile telephone
`
`v a
`
`CELL
`SITE
`
`1
`
`“-.
`
`‘-..
`
`\
`
`:fi:
`MOBILE
`
`TELEPHONE
`
`
`T0
`WIHEALINE
`MOBILE
`NETWORK
`
`TELECOMMUNICATIONS
`
`
`
`SWITCHING
`-------------
`
`OFFICE
`
`SITE
`
`RADIO
`LAND—LINE
`COMMUNICATION————.+-—-— COMMUNICATION
`
`Fig. 3—AMPS system control elements.
`
`46
`
`THE BELL SYSTEM TECHNICAL JOURNAL. JANUARY 1979
`
`

`

`actions. Between the cell site and the MTSO, separate facilities carry
`data to handle numerous call-processing and system integrity func-
`tions. All
`these functions are discussed below and in subsequent
`articles. In particular, Section VI of this paper presents scenarios of
`call setup sequences; we choose first to describe some of the general
`techniques and requirements of AMPS control.
`
`IV. CONTROL TECHNIQUES
`
`This section describes several important control techniques required
`by the cellular concept. These techniques relate to the functions of
`supervision, paging and access, and seizure collision* avoidance. Be-
`cause of the distributed nature of the cellular plan, the important
`control function of ensuring system integrity (i.e., reliability and avail-
`ability) is sufficiently broad in scope to require separate treatment (see
`Ref. 6).
`
`4. 1 Supervision
`
`Classical land-line telephony defines supervision as the process of
`detecting changes in the switch-hook state caused by the customer.
`Mobile telephone supervision includes this process but has the addi-
`tional task of ensuring that adequate RF signal strength is maintained
`during a call.
`In a cellular system where intra-system interference is anticipated,
`the older mobile telephone technique of using a combination of RF
`carrier and a burst of tone cannot be used for supervision. As sketched
`in Fig. 4, some interfering signal will exceed typical values of the
`desired signal a significant fraction of the time. This is particularly
`bothersome at the end of a call when the desired mobile unit's
`
`transmitter must be turned off, and a burst of tone sent just prior to
`that time could be missed. Under these conditions, a false supervisory
`indication (caused by a co-channel interferer) would be created. The
`AMPS system uses a combination of a tone burst and a continuous out-
`of—band modulation for supervisory purposes. These are known re-
`spectively as signaling tone (ST) and supervisory audio tone (SAT).
`
`4. 1.1 Supervisory audio tone
`
`Three SATS are set aside at 5970, 6000, and 6030 Hz. Only one of
`these is employed at a given cell site. The concept calls for using a SAT
`much as a land telephone uses dc current/voltage: A mobile unit
`receives a SAT from a cell site and transponds it back (i.e., closes the
`loop). The cell site looks for the specific SAT it sent to be returned; if
`some other SAT is returned, the cell site interprets the incoming RF
`
`‘ Collision. as used here, means the loss of calls because of simultaneous arrival of
`two or more control messages.
`
`CONTROL ARCHITECTURE
`
`47
`
`

`

`
`
`PROBABILITYDENSITY
`
`INDETERMINATE
`
`INTEHFERING
`SIGNAL
`
`DESIRED
`SiGNAL
`
`IHEGION
`
`LOG SIGNAL STRENGTH
`
`Fig. 4—Relative signal-strength distribution of desired and interfering signals.
`
`power as being corrupted by interference, either in the land-to-mobile
`or in the mobile-to-land path (see Ref. 5).
`In Fig. 5, we can see how the use of three SATs effectively multiplies
`the D/R (co-channel reuse) ratio for supervision by [31* For example,
`given a voice channel reuse factor of N = 7, a cell site with both the
`same RF channel set and the same SAT is as far away as if N were 21.
`This three-SAT scheme provides supervision reliability by reducing the
`probability of misinterpreted interference (same SAT and same RF
`channel).
`' The selected SAT frequencies are close together so that one phase-
`locked tracking filter can lock onto any of them. They are distant from
`the voice band by a factor of 2, so that filtering SAT from voice is
`relatively easy and so that intermodulation products are controllable.
`The FM deviation of the SAT is :2 kHz.
`
`4.1.2 Signaling tone
`
`Signaling tone (chosen to be 10 kHz) is present when the user is (i)
`being alerted, (ii) being handed off, (iii) disconnecting, or (iv) flashing
`for mid-call custom services (e.g., hold). Signaling tone is used only in
`the mobile-to-land direction. Figure 6 tabulates the various supervision
`states of the mobile when on the voice channel, as detected by the
`
`land portion of the system.
`
`4. 1.3 Locating
`
`Another aspect of supervision with no counterpart in land line
`telephony is the function of locating. As discussed in Ref. 5, locating
`
`' [t is shown in Ref. 5 that D/R is proportional to NW; thus, if N2/N1 = 21/7. then
`Liz/D] = J3.
`
`48
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`THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1979
`
`

`

`
`
`Fig. 5—SAT spatial allocation.
`
`
`SAT
`SAT
`
`
`
`RECEIVED
`NOT RECEIVED
`
`
`
`MOBILE
`
`ON
`DNiHOOK“ MDBILE
`
`
`IN FADE
`
`on
`MOBILE
`
`TRANSMITTER
`
`OFF
`
`MOBILE
`
`OFFAHOOK
`
`ST
`
`
`
`SAT : SUPERVISOHY AUDIO TONE
`5T: SIGNALING TONE
`
`*NDTE: FIING CONFIRMATION REQUIRES THE
`MOBILE TO DELAY ST UNTILTHE
`RING ORDEFI IS RECEIVED,
`
`Fig. 6—Supervision decisions made at cell site.
`
`and handoff serve to keep the signal strength from a mobile unit at a
`high level during a call so that (i) the mobile's average S/I (signal-to-
`interference) ratio is adequate for its own good communication, and
`
`CONTROL ARCHITECTURE
`
`49
`
`

`

`(ii) other active mobiles do not experience high co-channel or adjacent—
`channel interference.
`The methodology for locating requires two measurements. One is a
`measurement of the RF signal strength on appropriate channels, made
`using a tunable logarithmic receiver located at each cell site. The other
`is a measurement of gross range (based on round-trip delay of SAT),
`made on each active channel using the voice channel radios at the cell
`sites. Analysis of this information at the MTSO determines whether a
`change of channels and/or cell sites (handoff) is required. Additional
`details concerning locating are found in Ref. 7.
`
`4.2 Paging and access (setup channel plan)
`
`Seeking a called mobile unit that is at some unknown position in a
`service area is similar to the function performed vocally by persons
`called pages. Thus, the term “paging” is used in AMPS to describe the
`process of determining a mobile's availability to receive a given incom—
`ing call. The complementary function of beginning a call, performed
`by a mobile unit, is termed access. This involves (i) informing the
`system of the mobile’s presence, (ii) supplying the system with the
`mobile’s identification and the dialed digits, and (iii) waiting for a
`proper channel designation.
`Two techniques are available to perform these paging and access
`tasks: (i) the special calling-charmel method and (ii) the voice—channel
`method. The latter method, employed by older land mobile systems
`presently in service,8 uses an “idle tone" to indicate which of several
`functionally identical channels is available to serve a new call, first for
`signaling, then for voice; the special-channel method, which dedicates
`channels either to the paging and access function or to the voice
`function, is used in the maritime and the air—ground services.9 In a
`cellular system with many thousands of users and hundreds of channels
`and where the mobile unit can be made to scan the dedicated channels
`rapidly, the special calling-channel method is necessary because the
`information needed for the home/roam decision by the mobile cannot
`be handled by the single tone of the voice-channel method. Therefore,
`the AMPS control plan uses a set of special channels called setup
`channels for paging and access functions. These channels are distrib-
`uted among the cell sites in an orderly way based on S/I considerations
`similar to those described in Ref. 5.
`Plans for organizing use of the setup channel, based on the traffic
`assumptions reflected in Table I,
`take into account the differing
`demands placed on the system by paging and accass. Paging infome-
`tion must be spread equally over the entire MSA. The information
`capacity requirements for paging will grow in proportion to the number
`of customers; however, splitting cells as described in Ref. 5 will not
`help to increase the capacity, since each point in the MSA needs all the
`paging information. The access requirements also increase with the
`
`50
`
`THE BELL SYSTEM TECHNICAL JOURNAL. JANUARY 1979
`
`

`

`Table I—Traffic assumptions (based on a limited amount of early
`data from present-day service)
`1 call/subscriber/busy—hour
`Calling rate
`Half of all attempts toward a mobile elicit a response
`Answer rate
`60 percent mobile-originated
`Traffic direction
`40 percent mobile-completed
`4:1
`Home/roam ratio
`
`Mean rate of call arrival I/second in densest cells
`
`traffic, but access capacity increases with cell-splitting since each cell
`in the MSA needs the access information only for mobiles in that cell.
`
`4.2.1 Access requirements
`
`The following are the requirements on the access process:
`(i) The capacity to handle access attempts must relate to the
`number of users. In areas saturated with access traffic, we expect this
`traffic to arrive randomly at about one arrival per second. This
`assumption should hold for cells of both the largest and the smallest
`radii. Furthermore, we expect each user to average 0.6 origination per
`hour, based on present-day usage statistics.
`(ii) It must not place undue demands on the real-time processing
`capabilities of either the MTSO or the cell sites.
`(iii) It must be accurate in the face of (a) co-channel interference
`from other cells and (b) collisions, already defined as the occasional
`arrival of two or more requests for service at the same time.
`(iv) It must be stable (i.e., some rare overload situation must not
`cause the system to enter a state from which it cannot recover quickly).
`Since the setup channels represent an expense both in capital and
`in channel resource (i.e.,
`they subtract from the total reserve of
`channels), it is important to use these carefully and flexibly even
`though future traffic loads for paging and for access are not accurately
`known.‘
`
`4.2.2 Paging requirements
`
`Current assumptions concerning future paging requirements are that
`the process must:
`(1) Be able to handle 0.8 page per user per busy hour, of which half
`go unanswered. (This estimate is based on a sample of present-day
`users.)
`
`(ii) Provide complete number flexibility to permit nationwide roam-
`ing and to accommodate any of the ten-digit telephone numbers
`possible today. (This assumption requires a 34-bit binary number for
`mobile identification.)
`
`" Note, however, that signaling via "idle tone” would also represent an expense in
`that it adds channel holding time to calls in both directions.
`
`CONTROL ARCHITECTURE
`
`51
`
`

`

`(iii) Be capable of serving some unknown future demand (several
`hundred thousand users) While remaining economical in small cities
`with a user population of one thousand or less.
`
`4.2.3 Setup channel plan
`
`The plan that has evolved from these requirements allows paging
`and access functions, for the sake of economy, to be combined on the
`setup channels for the early years of growth when large cells with
`omnidirectional antennas are used. As the system grows, however,
`with cell splitting and the change to cell sites using directional anten-
`nas, more setup channels will be needed to handle the access function.
`The omnidirectional antennas would continue to handle the paging
`functions. Therefore, paging and access become separated when the
`first cell split occurs.
`The paging messages themselves contain the binary equivalent of
`the mobile unit's directory number. Since a large amount of paging
`information has to be sent, efficient design suggests that the data be
`organized into a synchronous format (described later) of fixed-length
`words and synchronizing pulses. When paging is not needed, the cell
`site adds “filler text" in its place, merely to preserve the synchronous
`format.
`Another type of message, called the “overhead word,” is also sent
`periodically as part of the paging data stream to give the mobile certain
`descriptive information about the local system. The use of the over-
`head word permits flexibility in local system parameters (which are a
`function of local subscriber growth rates and traffic characteristics).
`These parameters can then be varied as actual field experience dic—
`tates. The overhead word includes:
`( i) The MSA identification (called the Area Call Sign) to permit the
`automatic roaming feature.
`(ii) The cell site's SAT identification.
`(iii) A parameter (called N) which specifies the number of setup
`channels in the repeating set (the frequency reuse factor“). See item
`(ii) in Section 6.2.
`(iv) A parameter (called CMAX) which specifies the number of setup
`channels to scan when a call is to be made.
`(v) A parameter (called CPA) which tells the mobile units whether
`the paging and the access functions share the same setup channels.
`Figure 7 depicts how the setup channels are assigned as systems
`grow through various sizes. The highest 21 channels are always used
`for setup purposes; these are the channels all mobile units are pre-
`programmed to recognize as those containing the necessary system
`identification (overhead word) information, no matter where the unit
`makes a call.
`
`‘ This frequency reuse factor for setup channels may be different from that for voice
`channels.
`
`52
`
`THE BELL SYSTEM TECHNICAL JOURNAL. JANUARY 1979
`
`

`

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`CONTROL ARCHITECTURE
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`53
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`

`

`During the early period of growth (typically to about 20,000 users in
`a single MSA), paging and access can be combined on the setup
`channels. After that, cell splitting occurs and new access capability is
`needed. The paging capability from the original sites, with omnidirec-
`tional antennas using the original number of paging channels, remains
`adequate. A second cell split, at roughly 60,000 users, requires even
`more access capability but is expected to leave paging on the original
`setup channels from the original sites.
`At some point, the paging capacity of the original setup channels
`becomes saturated. Since each channel is limited to the order of 10,000
`bits/second (b/s), and since high redundancy is required to combat
`the fading problem in areas of low S/I, only 1200 b/s of real information
`can be sent. Some of this is overhead information. Thus, a practical
`limit is about 25 messages per second (90,000 per hour) at 100 percent
`loading. Actual service experience, of course, will dictate the resulting
`customer loading allowed (depending on how many ineffective at-
`tempts there are), but a ratio of 90,000 customers for each paging
`stream is probably an upper bound. Beyond that size, which should be
`exceeded in only a few cities by the end of the century, more setup
`channel sets for paging will be needed at the original omnidirectional
`cell sites. Logic designed into the mobile unit must anticipate the use
`of these additional setup channels. Mobiles will be assigned to a
`specific paging channel set—either the primary (highest N channels)
`or one of the additional sets—for use when at home, much as land
`telephones are assigned to central office codes within an exchange.
`
`4. 2.4 Setup channel use
`
`Mobiles will use setup channels in the following sequence:
`(1') When power is applied to a mobile unit, and about once a minute
`thereafter, it scans the top 21 charmels and picks the strongest one on
`which to read an overhead message. This permits the mobile unit to
`determine if it is “home” and to retrieve the frequency reuse factor N.
`To receive its pages, it then rescans the appropriate“ set of N channels
`to find the strongest channel. Since N can vary from city to city, it is
`read from the overhead message that is periodically being sent on all
`forward (cell site to mobile) setup channels.
`(ii) When a call is to be either originated from or completed to a
`mobile unit, the unit must repeat the scanning process to self-locate
`itself to the best cell site (i.e., the strongest signal) for access. In this
`case, it scans CMAX channels.
`(iii) The unit synchronizes to the word pattern on the chosen setup
`channel and determines if that channel is idle (discussed later). If so,
`it attempts an access by transmitting the necessary information to the
`cell site:
`
`(a) If answering a page, its identification; or
`
`‘ See Section 4.2.3.
`
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`

`(b) If originating a call, its identification and the dialed digits.
`The unit then turns its transmitter off but remains tuned and syn-
`chronized to the chosen setup channel.
`(iv) After the land portion of the system has processed the access
`information, it sends a channel designation message to the mobile unit,
`much as a page would be sent, on the setup channel which the mobile
`unit had used previously. Upon receipt of this message, the mobile
`tunes to the designated channel, and the voice portion of the call can
`proceed.
`
`4.3 Seizure collision avoidance
`
`The initiation of a call by a mobile unit is a random event in both
`space and time, as the land portion of the system perceives it. Since all
`mobiles compete for the same setup channels, methods must be devised
`to minimize collisions and to prevent temporary system disruption if
`collisions do occur. Several techniques are used for this purpose.
`First, the forward (toward the mobile) setup channels set aside every
`11th bit as a “busy/idle” bit. As long as a cell site perceives legitimate
`seizure messages directed toward it, it sees that the “busy/idle” bit is
`set to “busy."
`Second, the mobile sends in its seizure message a “precursor,” which
`tells the land portion of the system with which cell site it is attempting
`to communicate. This is particularly necessary in systems with smaller
`cell sizes. For example, as explained in Ref. 5, in a system with 1_-mile
`cells (R = 1 mile), co-channel interferers are less than five miles (D
`= 4.7 miles) from the serving cell site—well within the mobile’s typical
`range of 5 to 10 miles. The information provided in the precursor is
`the digital-encoded equivalent of the SAT mentioned earlier; the mobile
`unit, having read this digital code message in the forward data stream
`of the setup channel being used, transmits it back to the cell site on
`the reverse half of the channel.
`
`Third, before the mobile attempts to seize (access) a setup channel,
`it waits a random time. This cancels the periodicity introduced into
`the mobile seizures by the format of the setup channel messages.
`Fourth, after a mobile unit sends its precursor, it opens a “window”
`in time in which it expects to see the channel become busy. If the idle-
`to-busy transition does not occur within the time window, the seizure
`attempt is instantly aborted.
`Fifth, if the initial seizure is unsuccessful for any reason, the mobile
`unit will automatically try again and again at random intervals. How-
`ever, to prevent continued collisions and hence system overload, a
`limit is placed on the number of automatic reattempts permitted.
`
`V. DATA REQUIREMENT AND FORMATS
`
`As a result of the control techniques described in the preceding
`section, considerable amounts of data are exchanged between pairs of
`AMPS control elements. The information requirements for the various
`
`CONTROL ARCHITECTURE
`
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`
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`

`Table Il—Requirements for information transfer
`_—_——____—_—_————
`’——4——————~———
`Type of Information
`Number of Bits
`Channel
`Setup:
`Forward
`
`Mobile page
`Channel designation
`Mobile power level
`Overhead (local parameters)
`S stem control
`I entification
`Dialed digits
`System control
`
`24 or 34
`11
`2
`22 to 30
`4
`56 or 66
`64 (16 characters")
`4
`
`Reverse
`
`Voice:
`Forward
`
`Reverse
`
`Orders
`5
`Channel designation
`11
`Mobile power level
`2
`System control
`4
`Order confirmation
`5
`Dialed digits (for custom-
`64 (16 characters)
`calling services)
`
`System control 4E
`' Characters are defined as the digits 0-9, plus #, *, and other symbols reserved for
`future use.
`
`channels are shown in Table II. The radio interface channels (mobile
`unit to cell site) differ from land channels (cell site to MTSO) not only
`in capacity requirements but also in the way they must be handled
`because of the differing nature of the noise impairments. This section
`describes the data requirements and formats“ for the different AMPS
`interfaces: forward setup channel, reverse setup channel, voice channel,
`and land-line data link.
`Before dealing with each of these interfaces in turn, we will describe
`common characteristics of the first three. One is the rapid fading
`experienced by signals as mobiles move through the complex RF
`interference pattern. To combat the burst errors caused by this fading,
`all data words are encoded and repeated several times at the source,
`and a bit-by-bit, 3-out—of-5 majority vote is taken at the receiver to
`determine the best-guess detected word to send to the decoder. The
`coding used on all radio channels is a shortened (63, 51) Bel-I“)- code
`[(40, 28) in the forward direction, and (48, 36) in the reverse direc—
`tion]; this code has the capability of correcting one error while detect-
`ing at least two more, without unreasonable complexity. This type of
`coding scheme, along with the majority-voting technique, provides a
`good balance between a low miss rate (probability of not detecting a
`message when one is sent) and a low falsing rate (probability of
`detecting the wrong message).
`Further description of the data channels is given in Ref. 10. In brief,
`however, the philosophy used is to send the data at the fastest bit rate
`possible over the RF channel, consistent with its bandwidth, thus filling
`
`‘ For the sake of brevity, certain message details are omitted in this section.
`1' Bose—Chandhuri-Hocquenghem originated this linear block systematic error coding
`scheme; (63, 51) indicates that there are 63 bits transmitted, of which 51 are information
`and 12 are parity-check bits.
`
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`

`the channel as evenly as reasonably possible with energy. Channel
`capacity over and above the information needs is used up by redun-
`dancy, i.e., both by encoding and by repeating the message several
`times. Thus, a 10-kbs rate was chosen for the AMPS radio channels, to
`give a total maximum information throughput of 1200 b/s. Mobile unit
`cost is also an important consideration in the choice of formats.
`Another characteristic which these radio channels have in common
`
`is the pair of error requirements for information transfer:
`(1') The miss rate for messages should be in the range of 10‘3 at
`8/1 = 15 dB. Averaged over the entire service area, this implies a miss
`rate of about 10“. This miss rate is very small compared to the
`probability that a call will be missed because a mobile is unattended.
`Furthermore, it is consistent with the requirement placed on mishan-
`dled calls in the wire-line telephone network.
`(ii) The falsing rate (incorrect data interpretation) should be less
`than 10—7 for a given message. This stringent requirement is necessary
`because, for example, in a system where 90,000 users are assigned to a
`given paging data stream, up to 45,000 mobile units might be listening
`to every page;* in this situation, the requirement implies that less than
`one transmitted page in 200 will elicit a false response (which the MTSO
`will be required to screen).
`
`5.1 Forward setup cl'vannrielr
`
`Data on this channel are transmitted continuously in a periodic
`format, so that idle mobile units can synchronize to the format to read
`a large volume of paging and local system information.
`Details on the data format of the forward setup channel are shown
`in Fig. 8. The basic periodicity of the bit stream is 463 bits, summed as
`follows:
`
`200 bits—Word A (40 bits, repeated 5 times)
`200 bits—Word B (40 bits, repeated 5 times)
`10 bits—Bit sync
`11 bits—Word sync
`42 bits—Busy-idle bits
`463 bits
`
`The five repeats of words A and B are interleaved to provide spacing
`in time, which in turn ensures partial decorrelation of bit errors
`between repeats of the same word. Note that a given mobile unit is
`not requ