United States Patent (19)
`Goodman
`
`54) TELECOMMUNICATIONS SWITCHING
`SYSTEM
`75 Inventor: David J. Goodman, Aberdeen
`Township, Monmouth County, N.J.
`73) Assignees: American Telephone and Telegraph
`Company, New York, N.Y.; AT&T
`Bell Laboratories, Murray Hill, N.J.
`21) Appl. No.: 263,928
`22 Filed:
`Oct. 28, 1988
`
`56)
`
`51) Int. Cl." .................................oves to a o a on a H04Q 11/04
`52 U.S. Cl. .................................... 370/60; 370/85.12
`58) Field of Search ....................... 370/60,94, 95,88;
`455/33, 34,53,54, 56; 379/59, 60
`References Cited
`U.S. PATENT DOCUMENTS
`4,679,189 7/1987 Olson et al. ........................... 370/60
`4,797,882 1/1989 Maxemchuk.
`... 370/60
`4,811,334 3/1989 Matt ..................
`... 370/60
`4,823,111 4/1989 Tsuchiya et al. ...
`... 370/60
`OTHER PUBLICATIONS
`"Packet Reservation Multiple Access for Local Wire
`less Communications', Proc. 38th IEEE Vehicular Tech
`nology Conference, Phil., Jun. 1988, D. J. Goodman, R.
`A. Valenzuela, K.T. Gayliard, and B. Ramamurthi, pp.
`701-706.
`"Cellular Access Digital Network (CADN): Wireless
`Access to Networks of the Future', IEEE Communica
`
`11
`45
`
`Patent Number:
`Date of Patent:
`
`4,916,691
`Apr. 10, 1990
`
`tions Magazine, vol. 25, No. 6, Jun. 1987, E. S. K. Chien,
`D. J. Goodman and J. E. Russell, Sr., pp. 22-31.
`"Standards for Metropolitan Area Networks', IEEE
`Communications Magazine, vol. 26, No. 4, Apr. 1988, J.
`F. Mollenauer, pp. 15-19.
`"The QPSX Man", IEEE Communications Magazine,
`vol. 26, No. 4, Apr. 1988, R. M. Newman, Z. L. Budri
`kis and J. L. Hullett, pp. 20-28.
`"A Narrowband TDMA System for a New Generation
`Cellular Radio', Proc. 37th IEEE Vehicular Technology
`Conference, Tampa, Jun. 1987, J. Uddenfeldt and B.
`Persson, pp. 286-292.
`Primary Examiner-Benedict V. Safourek
`Assistant Examiner-Wellington Chin
`Attorney, Agent, or Firm-Samuel H. Dworetsky
`57
`ABSTRACT
`A switching system is described in which routing infor
`mation, which may be associated with packets originat
`ing in cellular calls, is divided into information which
`does not change as the subscriber crosses a cell bound
`ary, and information which does change as the sub
`scriber crosses a cell boundary. Routing procedures
`associated with the information which does not change
`as the subscriber crosses a cell boundary are established
`and stored in the memory of the switch at the beginning
`of the call. However, routing procedures associated
`with the information which does change as the sub
`scriber crosses a cell boundary may be derived from the
`header information of each packet as it arrives at the
`appropriate portion of the switch.
`
`4 Claims, 11 Drawing Sheets
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`U.S. Patent
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`Apr. 10, 1990
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`Apr. 10, 1990
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`U.S. Patent
`US. Patent
`
`Apr. 10, 1990
`Apr. 10, 1990
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`Sheet 11 of 11
`Sheet 11 of 11
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`4,916,691
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`

`1.
`
`TELECOMMUNICATIONS SWITCHING SYSTEM
`
`5
`
`O
`
`5
`
`FIELD OF THE INVENTION
`This invention involves telecommunications switch
`ing systems, and has particularly useful application to
`cellular radio telecommunications.
`BACKGROUND OF THE INVENTION
`The commercial introduction of cellular radio tele
`communications approximately five years ago is revolu
`tionizing the telecommunications industry. The com
`mercial interest in the cellular technique stems from its
`ability to enable high volume traffic to operate over the
`limited number of available radio channels. This is ac
`complished by dividing large geographical areas into
`Smaller geographical areas, or cells. This permits the
`reuse of the same radio channels in different cells which
`are sufficiently separated spatially so as to avoid inter
`ference. Consequently, a large geographical area which
`20
`previously had been limited to, for example, 7000 chan
`nels, and consequently, to 700 telephone calls, could
`now be divided into, for example, 70 cells, each one of
`which could use a channel set comprising 100 channels,
`without interference from adjacent cells which are
`25
`using different 100-channel sets. Consequently, 7,000
`telephone calls can now be made from the large geo
`graphical area which previously had been limited to 700
`telephone calls. This new architecture is effected by
`introducing appropriate telephone switches which en
`30
`able the system to maintain the integrity of each tele
`phone call as the source of the telephone call moves
`from one of the cells to another of the cells. To accom
`plish this, the system assigns a new frequency to a mo
`bile telephone as it moves from one call to another, and
`35
`assigns appropriate resources to route the signal from
`the new cell to the switch itself. Clearly, sophisticated
`routing and Switching equipment, including appropriate
`software, had to be designed and developed to imple
`ment this architecture.
`As cellular radio becomes more popular, the cells
`become saturated due to the presence of more active
`subscribers within a given cell than there are available
`frequencies. However, cellular radio has within it an
`inherent technique for dealing with such an increase in
`45
`subscribers. This technique is called cell splitting. In
`implementing cell splitting, the size of the cell is re
`duced, thereby once again bringing the number of ac
`tive subscribers within each cell to a number less than or
`equal to the number of available frequencies. However,
`50
`the explosive growth of demand for cellular radio
`makes it apparent that the cell splitting technique will
`soon become ineffective. This ineffectiveness is not
`associated with some inherent limitation on the size of
`the cells, but rather is associated with limitations of the
`55
`switching machines, which, as the cells become smaller
`and smaller, have a greater demand placed on them
`because of the increased frequency with which active
`subscribers cross cell boundaries. Clearly, every time an
`active subscriber crosses a cell boundary the switching
`maching must hand over the call, i.e., assign a new
`frequency to the mobile terminal and assign resources to
`connect the signal from the new cell to the switch.
`Larger switching machines may be able to handle this
`increased traffic volume. However, they are much
`65
`more expensive and may not be readily available be
`cause of the explosive demand for cellular radio appara
`tus. This invention is a new cellular radio architecture
`
`4,916,691
`2
`and infra-structure which permits rapid growth by dis
`tributing many of the swtiching and control functions
`into small modulator units which can be easily added to
`the system as it grows.
`To understand the basic philosophy and operation of
`the inventive architecture, it is helpful to understand the
`basic underlying principles of the classic circuit switch,
`the packet switch, and the virtual circuit packet switch.
`The classic circuit switch involves an architecture
`which dedicates resources, including transmission re
`sources, to each call, for the duration of the call. In
`circuit switching architecture, as it is applied to cellular
`radio, when the caller crosses a call boundary the cellu
`lar switch must release the resources that had previ
`ously been assigned to the given call, and must dedicate
`new resources to the call, thereby performing many of
`the functions characteristic of terminating a call and
`establishing a new call. Essentially all of the comercial
`ized cellular radio architectures are circuit switch archi
`tectures, and, because of the heavy burden that such
`architectures place on the switch, these architectures
`rapidly saturate the switches as the cells are split time
`and again to meet increasing demand. Cell splitting
`increases the burden on the switch because as the cells
`are split the frequency of boundary crossing increases.
`(It is understood that the term “call' as used here, and
`the invention in general, is not limited to transmissions
`representing audio communications, but rather includes
`any type of communication including the transmission
`of data, facsimile, audio, video, etc.)
`The pure packet switch architecture in some sense is
`the direct opposite of the circuit switch architecture, in
`that the pure packet switch architecture never perma
`nently assigns transmission resources to a given call.
`Rather, the information being transmitted in the call is
`divided into packets of information, each one of which
`is assigned transmission resources based on "header'
`information associated with each packet, and is routed
`independently of the other packets of information.
`Clearly, the advantage of the packet switch architecture
`is that since transmission resources are not dedicated to
`any given call, such resources may be used for other
`calls when the information transmitted by a call is
`"bursty' rather than steady. The transmission resources
`can be used for other calls during the idle time periods
`between the bursts of information. The disadvantage of
`the packet switch architecture is that it places a heavy
`burden on the switch, since the switch must establish
`routing for each packet, unlike the circuit switch archi
`tecture which establishes "permanent' routing once for
`each call. The advantage of packet switching comes at
`the expense of bandwidth since each packet must con
`tain the necessary "header' information which conveys
`to the switch the routing requirements of each packet.
`In a sense, the advantages of the packet switch archi
`tecture and the circuit switch architecture are combined
`in the virtual circuit packet switch. In the virtual circuit
`packet switch architecture, the virtual circuit packet
`switch, like the circuit switch, establishes a route for
`each call at the beginning of the call. This route is stored
`in the memory of the switch. Thereafter, each packet
`contains in its header a call identification number, rather
`than more complete header information, which enables
`the switch to find in its memory the correct route for
`that call. In this manner, the switch does not have to go
`through the entire routing procedure in order to trans
`mit each packet of a given call, but rather can rely on
`
`

`

`25
`
`4,916,691
`3.
`4.
`the previous routing procedure that had been estab
`portions of the header, in a manner representative of the
`lished at the inception of the call. Although virtual
`second segment of the route.
`circuit packet switching may be applied to cellular ra
`Current cellular switches establish both the initial
`dio, switching must still occur as the active subscriber
`route, and the new routes required every time a cell
`crosses each cell boundary. Consequently, in a very real
`boundary is crossed. However, in contradistinction to
`sense, conventional virtual circuit packet switching
`these current cellular switches, the inventive system
`offers little relief to the switch from the burdens associ
`may have two separate units, one of which establishes
`ated with the increasing frequency of boundary cross
`the initial route, and another one of which is used to
`ings as the cells become smaller.
`vary only that portion of the header which changes as a
`cell boundary is crossed
`SUMMARY OF THE INVENTION
`While the discussion has been in terms of cells and
`This invention is a new switching system and archi
`variations which occur as cell boundaries are crossed, it
`tecture which is particularly effective in addressing the
`is clear that the invention may be applied to other appli
`problem of increased switching burden due to the in
`cations which also require the varying of only a portion
`creasing frequency of boundary crossings as cellular
`15
`of the route during the transmission of a number of
`radio cells become smaller. An aspect of the invention
`packets. Such applications may include, for example,
`lies in the realization that routing information, associ
`the varying of a portion of the route in response to
`ated with packets originating, for example, in an em
`malfunctions.
`bodiment of the invention involving cellular calls, can
`As the public switched network evolves from a voice
`be divided into information which does not change as
`telephone network to a generalized information net
`the active subscriber crosses a cell boundary, and infor
`work, attention is increasingly focused on packet
`mation which does change as the active subscriber
`switching technologies, including pure packet switch
`crosses a cell boundary. Routing procedure associated
`ing and virtual circuit packet switching. This evolution
`with the information which does not change as the
`of the public network, as well as the ability of packet
`active subscriber crosses a cell boundary are established
`switching to deal with the increasing burdens of an
`and may be stored in the memory of the switch, only
`expanding cellular network, makes the application of
`once, at the beginning of the call, as in the circuit switch
`packet-type switching, as in this invention, even more
`architecture. However, the routing procedures associ
`attractive to cellular technology.
`ated with the information which does changes as the
`30
`active subscriber crosses a cell boundary may be de
`BRIEF DESCRIPTION OF THE DRAWING
`rived from the header information of each packet that
`FIG. 1 is a schematic representation of a prior art
`arrives at the appropriate portion of the switch. In this
`cellular telecommunication system;
`way the invention has the dynamic characteristics of a
`FIG. 2 is a schematic representation of an embodi
`packet switch, thereby enabling the inventive architec
`ment of the inventive telecommunication switching
`35
`ture to adapt to very frequent crossings of cell bound
`system involving cellular switching;
`aries by active subscribers, but at minimum increased
`FIG. 3 is a schematic representation of an exemplary
`burden to the switch. In an embodiment of the inven
`cellular trunk interface unit (TIU) which may be used in
`tion interface units derive routing information from
`the inventive cellular switching system;
`packet address fields and control the flow of informa
`FIG. 4 is a schematic representation of an exemplary
`tion without the intervention of a central controller.
`cellular wireless terminal interface unit (WIU) which
`Specific embodiments may also involve storage of this
`may be used in the inventive cellular switching system;
`information, as well, until the cell boundary is crossed.
`FIGS. 5 and 6 are schematic representations of a
`An embodiment of the invention includes means for
`conversation using the inventive cellular switching
`establishing an initial route for transmitting a number of 45
`system;
`packets of information between a first party and a sec
`FIGS. 7 and 8 are schematic representations of a
`ond party. Each of the packets comprises a header
`handover protocol using the inventive cellular switch
`which contains both information associated with a ter
`ing system;
`minal of the first party, and information associated with
`FIGS. 9 and 10 are schematic representations of a call
`a terminal of the second party.
`setup sequence using the inventive cellular switching
`As indicated above, the established route comprises
`system; and
`at least two segments. One of the segments of the route
`FIG. 11 is a schematic representation of a dual bus
`has associated with it constant routing information
`protocol embodiment of the inventive cellular switch
`which is stored in a memory portion of the system. This
`ing system.
`routing information remains constant during transmis
`55
`sion of the packets, and the headers of the packets have
`DETAILED DESCRIPTION
`portions associated with this constant routing informa
`I. Introduction To A Specific Embodiment of the
`tion. These portions of the headers also remain constant
`Invention Involving Cellular Telecommunications
`during the transmission of the packets.
`However, as also indicated above, a second segment
`In order to provide wireless access to public net
`of the route may vary during transmission of the pack
`works, current cellular systems contain wireless termi
`ets. This segment therefore has associated with it vary
`nals, base stations, and at least one cellular switch. As
`ing routing information. The headers of the packets
`shown schematically in FIG. 1, the switch is connected
`have portions associated with this varying routing in
`to an ensemble of base stations. Each base station is in
`formation as well. These portions also may vary from
`radio contact with many wireless terminals. The system
`65
`one packet to another during transmission of the pack
`infrastructure is the combination of hardware and soft
`ets, corresponding to the varying routing information.
`ware that links base stations with cellullar switches, and
`Finally, the system has means for varying these latter
`cellular switches with one another.
`
`10
`
`50
`
`

`

`10
`
`4,916,691
`5
`6
`The complexity of a cellular system is due in large
`tions. In this Section we describe the capabilities of the
`part to the mobility of the wireless terminals. Unlike
`interface units.
`fixed networks, cellular systems require frequent
`II.a Cellular Trunk Interface Unit (TIU)
`changes in configuration. To establish each call, the
`The TIU accepts and delivers information in the
`system has to learn the location of a wireless terminal;
`standard format of the public network. The speech
`and, it must be able to adapt itself to one or more
`changes in terminal location during the call.
`format, for example, is 64 kb/s companded pulse code
`Many network control functions are unique to cellu
`modulation. As indicated in FIG. 3, the TIU converts
`lar networks. Some are not necessary in fixed networks,
`this information to and from the format of the wireless
`and others are performed differently in cellular and
`access physical layer by means of trandcoders and chan
`fixed networks. Examples include: authentication, loca
`nel coders matched to the wireless access environment
`tion updating, paging, call set-up, call release, power
`of the inventive cellular switching system. Each inven
`control, and handover. In present cellular systems,
`tive cellular switching system can be customized for its
`these functions are primarily tasks of the cellular
`own transmission environment (for example, urban mo
`15
`bile, indoor, or mobile satellite) by means of the trans
`switches.
`It is anticipated that future systems will serve a much
`coders and channel coders installed in the TIU's. An
`higher population of users than present systems. As a
`architecture that admits many terminal-base transmis
`consequence, cells will be considerably smaller than at
`sion technologies, each matched to a specific environ
`present and the volume of network rearrangements,
`ment, may be important to the successful operation of
`20
`necessary each time the subscriber crosses a cell bound
`future wireless access systems (E. S. K. Chien, D. J.
`ary, (location updating, power control, and handover)
`Goodman, and J. E. Russell, Sr., Cellular Access Digi
`will grow by orders of magnitude. This volume will
`tal Network (CADN): Wireless Access to Networks of
`overwhelm the control capacity of present cellular
`the Future, "IEEE Connunications Magazine'', Vol. 25,
`switches.
`No. 6, June 1987, pp. 22-31).
`25
`To provide the necessary control, an embodiment of
`In addition to transforming user information between
`the invention, as applied to cellular switching systems,
`the formats of the fixed network and the wireless access
`exploits packet communication technology to distribute
`channels, the TIU may contain a packet assembler and
`network information among small processors (interface
`disassembler (PAD). Each PAD combines user infor
`units) residing in all network elements. It uses the ad
`mation or network control information, with a packet
`30
`dress field of each packet to provide routing informa
`header. The header may contain flags, an error control
`tion corresponding to the changing location of the wire
`field, a packet control field and an address field. The
`address of the TIU can be the permanent trunk identi
`less terminal.
`fier, or it can be a call control number assigned by the
`As indicated in FIG. 2, I view the infrastructure of
`the inventive cellular switching system as a wide area
`cellular control unit. Packets sent from the TIU are
`35
`network (WAN) linking base stations, public switches,
`routed to a base station by means of a portion of the
`address containing a permanent base station identifier.
`and a cellular control unit. Information may enter and
`leaves the WAN through cellular interface units includ
`During a call this portion of the address changes to the
`ing base station interface units (BIU); trunk interface
`permanent base station identifier of the new cell as the
`units (TIU), each connected to a central office trunk of
`wireless terminal moves from the service area of one
`40
`the public network; and a controller interface unit, CIU,
`base station to another.
`connected to the cellular control unit.
`In FIG. 3, the packet disassembler reads the destina
`The packet switching capability of a WAN works
`tion address of all packets arriving on the WAN. When
`well with the Packet Reservation Multiple Access tech
`this address matches either the permanent trunk identi
`nique for information transfer between base stations and
`fier (during call set up), or the call control number
`45
`(during a call), the packet disassembler processes the
`wireless terminals (D. J. Goodman, R. A. Valenzuela,
`K. T. Gayliard, and B. Ramamurthi, Packet Reserva
`arriving packet. If the packet has arrived from a base
`tion Multiple Access for Local Wireless Communica
`station, the TIU records the source address of the
`tions, "Proc. 38th IEE Vehicular Technology Confer
`packet in its base station identifier register. This identi
`ence', Philadelphia June 1988, pp. 701-706). As a statis
`fier then becomes the destination address for packets
`50
`tical multiplexer, PRMA offers an attractive combina
`launched into the WAN from the TIU.
`tion of simple control, efficient bandwidth utilization,
`II.b Wireless Terminal Interface Unit (WIU)
`and robustness in the presence of wireless access chan
`nel impairments. To marry PRMA to the inventive
`In generating packets, the WIU is similar to the TIU.
`cellular switching system, we may introduce to each
`One difference is that the TIU receives user information
`terminal a wireless terminal interface unit (WIU).
`from the public network, while the WIU generates its
`own user information, with, for example, a 64 kb/s
`II Interface Units
`analog-to-digital converter for speech signals. As indi
`cated in FIG. 4, the packet assembler of the WIU deliv
`The WIU, BIU, TIU, and CIU of this embodiment
`organize information transfer among wireless terminals,
`ers packets to the radio transmitter by way of an exem
`plary PRMA protocol processor.
`base stations, central office trunks, and the cellular con
`trol unit, respectively. Each packet contains a source
`As in the TIU, the packet disassembler compares the
`destination address of received packets with the either
`address and a destination address. Sometimes the ad
`dress is the permanent identifier of an interface unit. At
`the permanent terminal identifier (during call set up), or
`other times, the address is a call control number associ
`the call control number (during a call). It extracts the
`65
`ated with a particular communication session. The ad
`information fields of speech packets destined for this
`dressing procedures are discussed in Section III in the
`terminal and converts them to a continuous 64 kb/s
`signal stream.
`context of specific communication and control func
`
`55
`
`

`

`10
`
`20
`
`30
`
`4,916,691
`7
`8
`In order to implement terminal initiated handover,
`"postcard", the return or "source" address is shown in
`the WIU refers to a channel quality monitor to deter
`the upper lefthand portion of the "postcard' and the
`mine a base station identifier. This monitor indicates the
`'information field' is shown in the center lefthand por
`identity of the base station best able to serve the termi
`tion of the postcard. In these Figures, the horizontal
`direction represents location, with each interface unit
`nal in its current location. This base station becomes the
`destination of packets sent from the wireless terminal.
`represented by a column on the page, and the vertical
`direction represents time. In FIGS. 6, 8 and 10, as in
`II.c Base Station Interface Unit, BIU
`FIGS. 5, 7 and 9, the horizontal direction represents
`This unit relays information between the TIU's and
`location and each interface unit is represented by a
`the wireless terminals. It also broadcasts, over its radio
`column on the page. The vertical direction represents
`channel, the acknowledgement packets called for by the
`time. The sequence of packet transfers is from top to
`exemplary PRMA protocol (D. J. Goodman, R. A.
`bottom on the page. Each packet is indicated by a rect
`Valenzuela, K. T. Gayliard, and B. Ramamurthi,
`angle. The left side of the rectangle contains the source
`Packet Reservation Multiple Access for Local Wireless
`address of the packet. The right side contains the desti
`Communications, 'Proc. 38th IEEE Vehicular Tech
`nation.)
`15
`nology Conference', Phil., June 1988, pp. 701-708).
`In FIGS. 9 and 10 there is shown a sequence of con
`The BIU multiplexes information packets that are sent
`trol packets leading to the transmission of the first
`to the radio transmitter. It also queues upstream packets
`speech packet ("Hello') from the public network to the
`for transmission over the WAN.
`wireless terminal. In addition to the addresses of each
`The BIU may always be addressed by its permanent
`packet, we also indicate, over the transmission arrow,
`identifier. If an incoming packet arrives with a call
`the information content of the packet.
`control number in its address field, this number becomes
`First the terminal sends an "off hook' message to the
`the destination address when the packet is relayed, ei
`nearest base station. The base station relays the message,
`ther to a TIU (upstream packet), or to a WIU (down
`and the identity of the wireless terminal, to the cellular
`stream packet). Certain network control packets arrive
`control unit. The controller uses the information in this
`25
`without call control numbers. These packets are either
`message to authenticate the calling party. If the caller is
`relayed to the cellular control unit; or, they are relayed
`authorized to place a call, the controller returns a "dial
`to a WIU by means of the permanent identifier of the
`tone' message to the base station. The base station ex
`WIU. This identifier is extracted from the information
`tracts the terminal identifier from the information field
`field of the control packet. Examples of these routing
`of the dial tone message and uses their identifier to relay
`procedures appear in Section III.
`dial tone to the WIU. The response to this message is a
`packet containing the called party's number. This ena
`II.d Cellular control unit interface, CIU
`bles the controller to attempt to establish a connection,
`The cellular control unit receives, processes, and
`through the local central office, to the called party.
`generates network control packets. It may always be
`When the connection is established, the controller
`35
`addressed by its permanent identifier. It assigns a con
`issues a call control number to the WIU that initiated
`trol number to each cellular call, and sends this number
`the call and to the TIU assigned to this call by the con
`to the TIU selected for the call, and to the relevant
`troller. With the call control number recorded in both
`WIU. To distribute the call control number to the TIU,
`the WIU and TIU, the conversation proceeds as in
`the CIU may use the permanent identifier of the TIU.
`FIGS. 5 and 6. Handovers, as necessary, take place as
`To send the call control number to the WIU, the CIU
`shown in FIGS. 7 and 8.
`may place the base station identifier in the destination
`III.b Conversation (no handover)
`address field of a control packet, and the permanent
`terminal identifier in the information field of the control
`As long as the wireless terminal remains in a single
`packet.
`cell, packets move from terminal to base station to cen
`tral office trunk (and in the opposite direction) in a
`straightforward manner. FIGS. 5 and 6 illustrate such
`bidirectional flow of speech information. Generally a
`conversation consists of a sequence of talkspurts alter
`nating in direction, with each talkspurt containing sev
`eral packets. The average number is about 60, but there
`is a wide variability in the number of packets per talk
`spurt. Because the TIU and the WIU contain speech
`activity detectors, in this embodiment no packets need
`be generated in the silent gaps between talkspurts.
`III.c Handover
`The Inventive Cellular Switching System hands a
`call from one base station to another when the wireless
`terminal determines that the call can best be handled by
`the new base station. FIGS. 7 and 8 are schematic repre
`sentations of a handover protocol. As indicated in
`FIGS. 6 and 7, the terminal initiates the handover by
`sending a packet to the new base station, "base 2', in
`stead of "base 1", which received earlier packets. The
`speech packet is relayed to the central office trunk.
`There, the TIU learns the identity of the new base sta
`tion and sends new packets to the terminal through base
`
`III. Network Control Examples
`By referring to three examples: conversational
`speech, handover, and terminal initiated call set up, we
`show how the inventive cellular switching system orga
`50
`nizes the flow of user information and system control
`information. The source and destination addresses of
`each packet control the routing of the packet to the
`correct interface unit. Prior to call set up, terminals and
`trunks are addressed by their permanent identifiers. In
`55
`setting up a call, the cellular control unit assigns to the
`call a call control number. This number may then be
`come the address of both the TIU and the WIU in
`volved in the call. Base stations and the cellular control
`unit may always be addressed by their permanent identi
`fiers.
`
`45
`
`III.a Terminal initiated call set-up
`FIGS. 9 and 10 show one possible scenario for a