THE BELL SYSTEM
`TECHNICAL JOURNAL
`
`D E V O T ED TO T HE SCIENTIFIC A ND E N G I N E E R I NG
`ASPECTS OP ELECTRICAL COMMUNICATION
`
`Volume 58
`
`January 1979
`
`Number 1
`
`Copyright
`
`C 1979 American
`
`Tettpùont
`
`and TtLegraph
`
`Company.
`
`Printed
`
`ui
`
`USA.
`
`Advanced Mobile Phone Service:
`Introduction, Background, and Objectives
`
`By W. R. Y O U NG
`
`(Manuscript received June 6, 1978)
`
`in detail
`that describe
`a series of papers
`introduces
`This paper
`Bell System's Advanced Mobile Phone Service
`(AMPS). It presents
`radio, highlighting
`the important
`brief history of mobile
`events
`that preceded
`legal decisions
`development
`of the AMPS system.
`cellular
`system concept
`that has been embodied
`in AMPS makes
`scale mobile-radio
`service affordable
`to a sizable
`segment
`public. This concept calls for dividing
`transmission
`areas
`into
`to handle
`radio
`traffic, and, as traffic grows, subdividing
`those
`into smaller
`segments without
`increasing
`radio spectrum. This
`outlines AMPS objectives
`and sets the stage for more detailed
`on its evolution,
`its design and
`testing, and maintenance
`tions.
`
`the
`a
`and
`The
`large-
`of
`the
`"cells"
`cells
`paper
`articles
`considera(cid:173)
`
`I.
`
`INTRODUCTION
`
`t he
`T he potential for communicating with nonfixed points over
`horizon w i t h o ut t he use of wires was soon recognized following
`t he
`invention of radio in t he late 1800s and its development in t he early
`1900s. T he first major use of this potential was to vessels at sea as an
`aid to navigation and safety. Since those early days, the use of mobile
`radio (as it is now called) h as spread dramatically. T o d ay it is used to
`c o m m u n i c a t e not only with ships at sea b ut with land vehicles, aircraft,
`and even with people using portable e q u i p m e n t.
`T h e expanding need and concomitant growth h a ve led to t he devel(cid:173)
`o p m e n t of t he newest mobile system for common-carrier offering to
`t h e public, t he Advanced Mobile P h o ne Service (A M P S) ,* t he subject
`
`• Known during developmental stages as High-Capacity Mobile Telecommunications
`S y s t e m (HCMTS).
`
`1
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`T h is
`Journal.
`of this special issue of The Bell System
`Technical
`system, in its m a t u re configuration, will h a n d le large quantities of
`mobile t e l e p h o ne traffic. High capacity will be achieved by dividing
`desired service a r e as into m a ny small cells of radio coverage and, m o st
`i m p o r t a n t , by operating with t he s a me radio s p e c t r um utilized m a ny
`t i m es over within t he service area.
`T h i s p a p er surveys t he background and history of mobile radio,
`including governmental regulatory events, t he development of systems
`used up to t he present, and t he emergence of new concepts and
`technology. T he overview of A M PS introduces readers to t he m a k e u p,
`service objectives, and features of t he new system.
`
`II. BACKGROUND AND HISTORY
`
`2.1 Early systems
`
`In 1921 t he D e t r o it police d e p a r t m e nt m a de t he earliest significant
`use of mobile radio in a vehicle.1 T h at system operated at a frequency
`t he
`close to 2 M H z. T he utility of t h is idea was so obvious t h at
`channels in this low-frequency b a nd were soon crowded.
`N e w frequencies between 30 and 40 M Hz were m a de available a b o ut
`1940. A n a t u r al outgrowth of t h at development was t he use of fre­
`quency m o d u l a t i on to improve reception in t he presence of fading of
`t h e signal, electrical noise, and static. Opening t he band encouraged a
`s u b s t a n t i a l buildup of police systems t h at started in t he early 1940s
`a nd continues today.
`Shortly thereafter, o t h er users found a need for this form of com­
`munication. P r i v a te individuals, companies, a nd other public agencies
`p u r c h a s ed a nd operated their own mobile units and land (base) equip­
`m e n t. Over t he years, t he Federal C o m m u n i c a t i o ns Commission (FCC)
`m a de available some 40 M Hz of s p e c t r um in bits and pieces between
`30 a nd 500 M Hz for various recognized and special uses. T o d a y,
`approximately eight million licensed units enjoy this t y pe of private
`service.* T h e se systems are n ot generally connected directly to t he
`t e l e p h o n e network.
`licensed over eight million
`In addition, t he F CC h as currently
`citizens b a nd radio units which a re p e r m i t t ed to operate on 40 chan­
`nels. An equal n u m b er of unlicensed units is also estimated to be
`operating on these channels. T h e se figures graphically show t h at a
`great n u m b er of people w a nt to c o m m u n i c a te while on t he move.
`
`* In the early days. Bell S y s t em companies engineered, furnished, and maintained
`s y s t e m s for private entities and public agencies such as police departments. T h is service
`was eliminated as a result of the 1956 consent decree: Final Judgment of January 24,
`1956, in U.S.A. vs Western Electric et al.
`
`2
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`2.2 Public correspondence systems
`
`Immediately after World W ar II, t he Bell S y s t em e m b a r k ed on a
`p r o g r am of supplying "public correspondence systems." T he
`t e rm
`m e a ns systems provided by a c o m m on carrier to permit communica­
`tion a m o ng a variety of users t h at achieves large-scale economies by
`combining miscellaneous kinds of traffic into larger, more efficiently
`h a n d l ed a m o u n t s. T he FCC's official classification of this service is
`" D o m e s t ic Public Land Mobile Radio Service" (D P L M R S). (See T a b le
`I for a chronology of events in mobile radio history.)
`T h e first of these public correspondence systems was inaugurated in
`1946 to serve t he city of St. Louis" with t h r ee channels n e ar 150 MHz.
`T he FCC h ad originally allocated six channels spaced 60 k Hz apart,
`but t he e q u i p m e nt was not sophisticated enough to prevent interfer­
`ence from adjacent channels being used in t he s a me area. T he St.
`Louis system was called an " u r b a n" system.
`In 1947, a " h i g h w a y" system using frequencies in t he 35- to 44-MHz
`band began operations along the highway between New York and
`Boston. T h e se latter frequencies were t h o u g ht to carry greater dis­
`tances and,
`therefore,
`to be more useful
`in covering stretches of
`highway. However, these frequencies proved troublesome because of
`t h at carried u n w a n t ed
`t h e skip-distance propagation p h e n o m e n on
`conversations across t he country. T o d a y, t he use of t he 35- to 44-MHz
`b a n d is declining.
`
`Table I—History of mobile telecommunications related to common-
`carrier services
`
`FCC Dockets
`
`No. 8658, Bell S y s t em proposal for 40-MHz-
`bandwidth system
`No. 8976, U HF TV, more detailed Bell Sys­
`tem proposal for 40-MHz bandwidth
`system
`
`No. 11997, Bell System proposal for 75-
`MHz system at 800 MHz
`
`No. 18262, Allocation to common carriers
`— 75 MHz, tentative
`— 40 MHz, firm
`Open to "any" common carrier
`Illinois Bell request for developmental
`authorization
`Developmental authorization granted
`
`1946
`
`1947
`
`1949
`
`1956
`1958
`
`1964
`
`1969
`
`1970
`1974
`1975
`1975
`
`1977
`1978
`
`Service Offerings
`
`First Bell S y s t em mobile ser­
`vice (150 MHz)
`Highway mobile service
`MHz)
`
`(35
`
`First manual 450-MHz service
`
`First automatic 150-MHz ser­
`vice — MJ
`First automatic 450-MHz ser­
`vice — Μ Κ
`
`AMPS Developmental System
`trial (850 MHz)
`
`INTRODUCTION, BACKGROUND, AND OBJECTIVES
`
`3
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`B o t h t h e u r b an and highway systems employed push-to-talk oper­
`ation (somewhat unfamiliar to t he ordinary telephone user) and were
`severely limited in t he n u m b er of channels available. Nevertheless,
`m o re systems of both types were installed for cities and highways
`a r o u nd t h e country. In m a ny cases, t he d e m a nd for service was such
`t h at t h e available channels could serve only a fraction of t he d e m a nd
`for traffic a nd prospective customers had to be p ut on backlog lists.
`A r o u nd 1955, t he n u m b er of channels available at 150 M Hz was
`e x p a n d ed from 6 to 11 by t he creation of new channels between old
`ones (i.e., c h a n n el spacing of 30 kHz). T he y e ar 1956 saw t he addition
`of 12 channels n e ar 450 M Hz and t he installation of t he first system in
`this frequency range. AD systems operated in t he " m a n u a l" mode, with
`e a ch call to or from a mobile unit handled by a special mobile operator.
`Mobile service still o p e r a t es on a m a n u al basis in some areas today.
`In 1964, a new system, called t he M J, was developed and installed to
`improve efficiency, to reduce costs, and to achieve trunking advantage
`in cities having multiple channels. T h is system operated at 150 MHz,
`t he
`furnished a u t o m a t i c c h a n n el selection for each call, eliminated
`n e ed for push-to-talk, a nd allowed customers to do their own dialing.
`M o st systems installed since 1964 are a u t o m a t i c, and m a ny of t he
`predecessor m a n u a l systems h a ve b e en replaced.
`In 1969 t he a u t o m a t ic capability was extended to t he 450-MHz
`channels with a system called t he MK. T he MJ and t he MK were p a r ts
`of t he Improved Mobile T e l e p h o ne S y s t em (I M T S) ,3 t he current stan­
`d a rd for mobile service. In some respects, especially in convenience of
`dialing, t h e service given to I M TS customers is c o m m e n s u r a te with t h at
`obtained with land-line telephones.
`P r e s e n t - d ay mobile telephone service requires a single land trans­
`m i t t er station positioned at a high elevation so t h at received signal
`levels at mobile units are substantially above
`t he a m b i e nt noise
`t h r o u g h o ut most of t he desired coverage area. For each channel, t he
`o u t p ut power of t he land t r a n s m i t t er is typically 200 or 250 watts, and
`t r a n s m i t t i ng a n t e n n a gain is s o m e t i m es used to raise t he effective
`r a d i a t ed power to 500 watts. S u ch a system ensures coverage as far as
`20 or 25 miles from t he t r a n s m i t t er site. Although t he signal level on
`a c h a n n el m ay be poor beyond 25 miles, it is still high enough
`to
`interfere significantly with o t h er mobile communications on t he s a me
`frequency within 60 to 100 miles of t he land transmitter. Consequently,
`two land t r a n s m i t t e rs spaced m o re closely t h an this should not use t he
`s a me mobile telephone channel frequency. If land t r a n s m i t t e rs on t he
`s a me frequency are farther apart, each can serve mobile units within
`a b o ut 20 miles with only minor interference, because a ny mobile unit
`is m u ch closer to t he land t r a n s m i t t er serving it t h an to a ny interfering
`t r a n s m i t t e r.
`
`F r o m its inception to t he present, mobile service h as remained a
`
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`scarce luxury. Each m o n t h, mobile telephone customers typically pay
`10 to 20 times as m u ch for mobile service as for residential telephone
`service. Despite t he cost, m a ny telephone companies can cite long lists
`of "held orders"—unfilled requests for service—from people who want
`to become mobile subscribers. Market studies4"' h a ve repeatedly un­
`covered a sizable d e m a nd at lower prices.
`B ut even if the cost of mobile service could be reduced substantially,
`t he primary factor t h at has h a m p e r ed t he spread of mobile service
`t h us far h as been t he unavailability of spectrum. No new customers
`could be a c c o m m o d a t ed
`in m a ny areas because only a few dozen
`channels are available for present-day service, and even these are
`fractured into several frequency b a n ds and partitioned a m o ng different
`classes of service carriers.
`Since 1949, common-carrier entities known as " R a d io C o m m on
`Carriers" (RCC), companies not providing public landline
`telephone
`service, have been given s e p a r a te channels to furnish t he s a me kind of
`mobile services as t he wire-line common carriers (the Bell System and
`other telephone companies). With about t he s a me n u m b er of channels
`available, they serve roughly the s a me n u m b er of customers. T a b le II
`shows t he n u m b er of channels available for each type of carrier and
`t he n u m b e r of two-way mobile units served by each for t he most recent
`year for which figures are available.
`C o m p a re t h e n u m b er (approximately 143,000) of RCC and wire-line
`common-carrier customers with t he estimated 16 million or more
`private units not served by common carriers: the ratio is a b o ut 1:110.*
`T h e re are many, both inside and outside t he Bell System, who believe
`t h at this ratio reflects the n u m b er of available channels allocated to
`t he different uses r a t h er t h an t he inherent d e m a nd for such services.
`T he FCC h as taken this into account in its most recent grant of 40
`M Hz of s p e c t r um for use by common carriers.
`
`2.3 Regulatory history
`
`Since 1946, Bell System planners have been looking forward to t he
`large-scale system they believed necessary to satisfy customer de­
`m a n d s. Proposals for such a system were m a de from time to time, as
`described below. T h e se generally were associated with FCC Dockets,
`as noted in t he left-hand column of T a b le I.
`In 1947, in connection with FCC Docket 8658, t he Bell S y s t em asked
`for 12 more channels to use immediately in t he s a me m a n n er as t he 6
`already granted for urban service. Also requested was sufficient band­
`width for some 150 two-way channels from which large blocks of
`
`' T h is is much smaller than the ratio of frequencies allocated (1:16) but is entirely
`consistent with the fact that the amount of traffic per mobile is much lower and channel
`loading is much higher in the private systems.
`
`INTRODUCTION. BACKGROUND, AND OBJECTIVES
`
`5
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`Table II—Channel allocations, number of mobile units, a nd n u m b er
`of s y s t e ms
`Wireline Common
`Carriers
`
`Bell
`
`Indepen-
`dent
`
`Total
`
`Radio
`Common
`Carriers
`
`Total
`
`54
`2.5
`
`143,000
`(approx.)
`
`2,726
`
`Number of
`
`two-way two-way
`
`channels channels
`M Hz allocated
`Number of
`mobile units
`(December 1977 )
`Number of
`
`systems systems
`
`(December 1977) (December 1977)
`
`23
`1.38
`
`23
`1.38
`
`23*
`1.38
`
`21t
`1.12
`
`44,500
`
`18,200
`
`62,700
`
`80,000t
`80,000t
`
`635
`
`716
`
`1,351
`
`1,375
`
`* Excludes 10 channels in t he 35- to 40-MHz "highway" band, which are of limited
`and declining utility.
`t Excludes the newest shared-with-τν channels in the 470- to 500-MHz band, since
`there has not been time for significant usage to build up.
`φ Projected forward from 1976 and earlier data.
`
`channels could be assembled to achieve spectrum efficiency a nd ca­
`pacity advantages. T he planned 100-kHz spacing plus suitable guard
`b a n ds (between mobile a nd land t r a n s m i t t e rs a nd between mobile and
`o t h e r services in adjacent bands) added up to approximately 40 M H z.
`In 1949, a Bell S y s t em proposal representing a more m a t u re plan for
`a b r o a d b a nd system w as described in connection with F CC Docket
`8976. T h i s docket considered t he disposition of U HF TV (470 to 890
`M H z ). T h e F CC decided at t h at time against providing a broadband
`mobile allocatio n in this band.
`In 1958, t he Bell S y s t em again m a de a b r o a d b a nd proposal, this time
`for a 75-MHz bandwidth (new estimated required spectrum) located
`at 800 MHz. T h is proposal was s u b m i t t ed as a response to an inquiry
`m a de b y t he F CC in its Docket 11997.
`After considerin g t he above proposal a nd t he general pressure for
`m o re radio communications, in 1968 t he F CC started Docket 18262,
`specifically addressed to t he question of alleviating t he large backlog
`of requests for frequencies for mobile use. Deliberating on requests for
`common-carrier service a nd for private-type service led t he F CC to
`tentatively decid e in 1970 to allocate 75 M Hz for wire-line common-
`carrier us e a nd 40 M Hz to supplement private services. It proposed to
`do thi s by eliminating channels 70 t h r o u gh 83 in U HF TV a nd by using
`certain o t h er pieces of spectrum from 806 to 947 M Hz (a total of 115
`M H z ). T he F CC invited
`industry to respond in 18 m o n t hs with
`proposals fo r achieving communication objectives and demonstrating
`feasibility. I n D e c e m b er 1971, t he Bell S y s t em responded with a
`technical report which asserted feasibility by showing in considerable
`detail ho w a system might be composed.8
`
`6
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`In 1974, t he F CC m a de a firm allocation, different from t he above:
`40 M Hz for wire-line common-carrier use and 30 M Hz to s u p p l e m e nt
`private services. T he r e m a i n d er of t he 115 M Hz was to be reserved
`this, t he F CC
`pending further demonstrations of need. In doing
`strongly urged all suppliers to design their systems for greatest utility
`a nd s p e c t r um efficiency.
`Early in 1975, t he F CC m a de some modifications in its 1974 deci­
`sions. One was to open t he 40-MHz allocation for common-carrier
`service to "any qualified c o m m on carrier" r a t h er t h an limit it to t he
`wire-line carriers. In J u ly 1975, t he Illinois Bell T e l e p h o ne C o m p a ny
`filed a r e q u e st to t he F CC for authorization
`to install and test a
`developmental system in Chicago. T h is was granted in M a r ch 1977.
`
`2.4 Emergence of key concepts
`
`F r om our discussion t h us far, it is obvious t h at t he high-capacity
`system h as been t he result of planning a nd key concepts t h at h a ve
`b e e n emerging over a long period of time. P e r h a ps t he first concept to
`be appreciated as necessary to an efficient, large-capacity operation
`was trunking, so m u ch so t h at it was p a rt of t he proposal to t he F CC
`in 1947. T r u n k i n g, as used here, is t he ability to combine several
`channels into a single group so t h at a mobile can be connected to a ny
`u n u s e d c h a n n el in t he group for either an incoming or outgoing call.
`T h is a r r a n g e m e nt reduces blocking probability and greatly increases
`traffic-carrying efficiency relative to t he situation in which a mobile
`unit can utilize only one fixed channel.*
`One problem t h at b o t h e r ed early p l a n n e rs was how to achieve full
`trunking a d v a n t a ge w i t h o ut requiring each mobile unit to be able to
`t u n e to every one of t he channels in use for this service t h r o u g h o ut t he
`country. In those days, each new operating frequency required
`two
`q u a r t z crystals and a position on t he channel selector switch. T he
`solution c a me when it became technologically feasible to construct a
`low-cost frequency synthesizer t h at could be set on a ny of a large
`n u m b er of frequencies but required only a small n u m b er of quartz
`crystals. While t he basic idea is quite old, t he circuit was m a de practical
`a n d economical only in t he early 1970s. It is now taken for granted in
`ongoing planning.
`T he cellular
`concept a nd the realization t h at small cells with spec­
`t r u m re-use could increase traffic capacity substantially seem to have
`materialized from nowhere, although b o th were verbalized in 1947 by
`D. H. Ring of Bell Laboratories in unpublished work. According to t he
`
`* T he IMTS systems employ trunking to advantage, but the small number of channels
`in use in a given system (typically less than the 12 that could be assigned) limits trunking
`efficiency.
`
`INTRODUCTION, BACKGROUND, AND OBJECTIVES
`
`7
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`cellular concept, a desired service area is divided into regions called
`cells, each with its own land radio e q u i p m e nt for transmission to and
`from mobile units within t he cell. It was further recognized t h at if t he
`available channels were distributed a m o ng smaller cells t he
`traffic
`capacity would be greater. T h us a system needing a relatively small
`capacity could use large cells, and, as necessary to achieve
`larger
`capacity, these cells could be divided into smaller ones. E a ch c h a n n el
`frequency can t h en be used for m a ny i n d e p e n d e nt conversations in
`m a ny cells which are spaced far enough from each o t h er to avoid
`u n d u e interference.
`F r om 1947 on, t he t e a ms planning t he eventual system had faith
`t h a t t he m e a ns for administering a nd connecting to m a ny small cells
`would evolve by t he time t h ey were needed. T h o se m e a ns did, in fact,
`b e c o m e a reality with t he advent of electronic switching technology.
`Locating
`and handoffaie
`concepts t h at come directly from t he use
`of small cells. T he act of transferring from one channel to a n o t h er is
`called handoff. "Locating" is a process for determining w h e t h er
`it
`would be b e t t er from t he point of view of signal quality a nd potential
`interference to transfer an active connection with a mobile unit to
`a n o t h er land t r a n s m i t / r e c e i ve equipment, or p e r h a ps to a n o t h er land
`site.* T he process entails sampling t he signal from t he mobile unit to
`d e t e r m i n e if handoff from one voice channel to a n o t h er is required.
`Since a mobile unit will sometimes move beyond t he borders of one
`cell into another, it will be desirable to transfer t he connection to an
`a p p r o p r i a te new cell.
`T h e system, as presently planned, uses omnidirectional a n t e n n as
`w h en t he cells are large. W h en smaller cells are created, directional
`a n t e n n as are used which divide each cell into t h r ee sectors, each
`served by an a p p r o p r i a te directional a n t e n na at t he cell site. T h is
`concept was introduced m a ny years ago.f T h is a d v a n t a g e o us arrange­
`m e nt reduces t he a m o u nt of co-channel interference from surrounding
`cells a nd increases system capacity. It is covered further in Ref. 9.
`T h e plan for increasing traffic capacity, as required, from a sparse
`to a m a t u re system in a given metropolitan service area,
`syste m
`assumes t he division of t he large cells used at first into small cells as
`needed. T he best m e t h od for achieving this is a growth plan developed
`in recent years (see Ref. 9).
`
`III. OVERVIEW
`
`T h i s section gives an overview of t he A M PS system, covering t he
`objectives, t he basic system, services and features, and additional
`problems a nd considerations.
`
`location of
`* T he prime purpose of this process is not to determine the geographic
`the mobile unit, although the geographical location is a statistical factor in performance.
`t Described in Ref. 8.
`
`8
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`3.1 Objectives
`
`T he major A M PS system objectives are discussed in t he following
`paragraphs.
`capacity: T he capability of serving a large
`(i) Large
`subscriber
`a m o u nt of traffic to m a ny t h o u s a n ds of mobile users within a
`local service area, such as a greater metropolitan area, within
`a fixed allocation of several h u n d r ed channels is essential to
`A M P S .
`use of spectrum:* T he scarcity of radio s p e c t r um as
`(ii) Efficient
`a public resource d e m a n ds t h at it will be used responsibly.
`A M P S will use it efficiently, for unless this is achieved, A M PS
`would lack t he ability to t a ke care of t he large anticipated
`traffic within t he allotted band.
`compatibility:
`T he F CC strongly urges nation­
`(Hi) Nationwide
`wide compatibility. T he objective m e a ns t h at mobile systems
`everywhere should provide t he s a me basic service with
`t he
`s a m e s t a n d a r ds of operation to be sure t h at a mobile station
`based in one place will achieve satisfactory service elsewhere.
`availability:
`Studies of existing services show
`(iv) Widespread
`t h at it is i m p o r t a nt to m a ny users to be able to r o am far from
`their normal h o me system and still receive service. N e i t h er
`this characteristic nor nationwide compatibility necessarily
`implies universal coverage. Wide-area
`coverage will be
`achieved gradually as metropolitan systems extend their cov­
`erage into surrounding suburbs, and finally along t he principal
`road and rail routes between metropolitan centers.
`to traffic density: Since t he density will differ
`{v) Adaptability
`from one point in an area to a n o t h er in a city and more r e m o te
`points, and since all of this will change with time, an A M PS
`objective is to be a d a p t a b le to these variable needs.
`to vehicles and to portables: While A M PS is conceived
`(vi) Service
`primarily for use with vehicles, an i m p o r t a nt objective is to
`m a ke it compatible with portables (hand-carried). T h is should
`be possible with little or no compromise in t he design of t he
`land-based network.
`including
`services,
`special
`and
`service
`(vii) Regular
`telephone
`"dispatch":
`In addition to regular telephone service, A M PS
`should provide specialized services, such as dispatch or fleet
`operation, and special features, such as abbreviated dialing.
`quality of service: As for quality of service, t he
`"Telephone"
`capability objective is essentially t he s a me quality as ordinary
`
`(viii)
`
`* A meaningful measure of spectrum efficiency is the number of simultaneous voice-
`communication paths that can be created per megahertz of spectrum and per square
`mile of area. T h is measure is useful where mobile terminals are statistically scattered
`throughout a service area.
`
`INTRODUCTION. BACKGROUND, AND OBJECTIVES
`
`9
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`nonradio telephone service. Since t he t y p es of i m p a i r m e n ts
`encountered are not always t he same, it is sometimes difficult
`t he
`to e n s u re achieving identical quality. T he goal is t h at
`reproduction of voice a nd
`freedom
`audio quality—faithful
`from excessive noise and distortion—will not differ in overall
`effect as perceived by t he user. It also m e a ns t h at service
`quality as m e a s u r ed by occasional blocking of t he p a t hs from
`c u s t o m er to central office will not be noticeably greater t h an
`t h at e n c o u n t e r ed in t he land network. T h is will be a very large
`i m p r o v e m e n t over c u r r e nt radio service, in which t he pressure
`to a c c o m m o d a te m a ny c u s t o m e rs results in c h a n n el loading
`which frequently causes t he probability of blocking to exceed
`50 percent.
`A goal is to m a ke t he service affordable by a
`(ix) Affordability:
`substantial portion of t he public a nd of businesses. Cost econ­
`this
`omies d ue to large production r u ns will t e nd to m a ke
`possible.
`
`3.2 Basic system
`
`Figure 1 shows t he basic structure of t he system as presently
`planned. T he service area to be covered is divided into an a p p r o p r i a te
`n u m b er of cells. E a ch cell site h as radio e q u i p m e nt and associated
`controls t h at can effect t he connection to a ny mobile unit located in
`t he cell. T he cell sites are interconnected to a nd controlled by a central
`Mobile T e l e c o m m u n i c a t i o ns Switching Office
`(M T S O). T he M T SO is
`basically a t e l e p h o ne switching office with substantial capabilities for
`software control. It connects to t he telephone network and also pro­
`vides t he m e a ns to perform m a i n t e n a n ce a nd testing and to record call
`information for billing purposes.*
`All of t he above m a ke up t he land-based p a rt of t he A M PS system.
`T h e mobile units complete t he system.
`T h e frequency layout (channel assignments) plan, t he plan of oper­
`ation for t he system, and t he way objectives cited earlier will be
`achieved a re described in Ref. 9.
`
`3.3 Services and features
`
`T he basic service is a telephone in a vehicle and is analagous to t he
`individual t e l e p h o ne in t he nationwide telephone network.
`B e y o n d this, t he intention is to offer mobile users features ordinarily
`available to t e l e p h o ne users, with e m p h a s is on those of particular
`value in t he mobile environment. One feature not generally available
`
`* In this overview, the MTSO is portrayed as a compact monolithic entity. In future
`practice, however, there may be multiple MTSOS, as required for achieving greatest
`economy and sufficient capacity.
`
`10
`
`THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1979
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`TO T E L E P H O NE N E T W O RK
`
`Fig. I — T he components and layout of the AMPS system.
`
`dialing, will be included. T h is
`in non-mobile phones, preorigination
`allows a c u s t o m er to enter and store t he called destination n u m b er
`before going off-hook. T h e n, when t he user wishes to place t he call, he
`begins t he connection process by going off hook; t he system uses this
`stored n u m b er to complete t he connection. If t he called line is busy or
`doesn't answer, t he user m ay try again later w i t h o ut having to enter
`t h e s a me n u m b er again.
`Eventually, t he following vertical services, Customer Calling
`Ser­
`furnished by E SS offices10 m ay be m a de available for mobile
`vices,
`users:
`(i) " T h r e e - W ay Calling" p e r m i ts a mobile user whose p h o ne is
`already connected to a n o t h er p h o ne to originate a call to a
`third party, to switch back and forth between t he connections,
`to bridge b o th connections as desired, or to connect t he two
`o t h e r parties for continued conversation and t h en disconnect
`himself. W i th this feature, t he mobile user can, for example,
`transfer a connection to a n o t h er party.
`(ii) "Call W a i t i n g" furnishes a signal to alert t he mobile user to an
`incoming call while a conversation is already in progress. By
`making use of t he three-way calling feature, t he customer will
`be able to transfer, accept t he new call, and hold or t e r m i n a te
`t h e former connection in progress.
`(Hi) " S p e ed Calling" p e r m i ts a c u s t o m er to originate a call to any
`of a few frequently called n u m b e rs by pushing one or
`two
`b u t t o n s . T he connection is completed by t he E SS in accordance
`with information stored there. T h is feature should be especially
`useful in a vehicle where a user cannot conveniently consult a
`directory or written notes. T h is feature will be implemented in
`t h e first working system.
`
`INTRODUCTION. BACKGROUND, AND OBJECTIVES
`
`11
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`In addition, it is expected t h at m o re features will be m a de ava