`
`
`
`Taking over when no one’s talking
`
`
`
`Mobile end system. A CDPD user
`communicates with the CDPD net-
`work by using the mobile end system.
`The physical location of the mobile
`end systems may change from time to
`time, but continuous network access is
`still maintained. A mobile end system
`may be a credit card verification unit
`installed in a taxi, a personal commu-
`nicator or a personal data assistant, or
`a CDPD modemfor a personal com-
`puter, a palmtop or a laptop. (The lap-
`
`The CDPDnetwork
`To other intermodiate Systems
`Jeferencemodel
`
`
`System
`
`
`lular Digital Packet Data
`
`(CDPD) offers on-the-go
`users access to a low-cost,
`
`/ widespread, wireless data
`network. CDPD can be overlaid on
`existing analog cellular systems, and
`share their infrastructure equipment
`on a non-interfering basis.
`Basically, CDPD transmits packet
`data over idle cellular channels. It
`autonomously switches to another
`channel when the current channel is
`about to be assigned for
`voice usage. CDPD does
`not communicate with the
`underlining cellular net-
`work. However, CDPD
`does use its knowledge
`about the channel assign-
`ment algorithm of the cel-
`lular cell to predict the
`channels available for
`CDPDuse.
`CDPD mayserve as the
`wireless extension to other
`data networks (e.g., Inter-
`net) or public switched tele-
`supports
`phone
`networks.
`It
`connectionless network services which
`have packets called network protocol
`data units (NPDU). Every packetis
`routed individually based on the desti-
`nation address of the packet and
`knowledge of the current network
`topology.Initially, CDPD provides two
`connectionless network services: the
`standard open systems interconnection
`connectionless network protocol and
`the Internet protocol.
`
`The architecture
`There are two basic classes ofnet-
`work entities in the CDPD network:
`end systems and intermediate systerns.
`For our purpose, we only considerthe
`mobile parts of the network; that is, we
`only will consider the mobile end sys-
`tems and the mobile data-intermediate
`systems. Figure | ulustrates the CDPD
`network reference model.
`
`to be colocated with the voice equip-
`ment that provide cellular telephone
`service. Furthermore, mobile data
`base stations may share cellular equip-
`ment (such as antennas for transmit-
`ters and receivers) to communicate
`with the mobile end systems. Mobile
`end systems can only communicate
`with the outside world through mobile
`data base stations. Also, there is no
`direct communication path between
`mobile end systems. Because of the
`short synchronization delay
`(i.e., two bit delay) between
`the mobile data base station
`and the mobile end system, the
`radius of a CDPD is typically
`limited to less than 10 miles.
`Mobile data-intermediate
`system. A mobile data-inter-
`mediate system connects to
`-sevcral mobile data base sta-
`tions via wire links (e.g., mul-
`tiple digital signal 0 trunks) or
`microwaves. The mobile data-
`intermediate system consists
`of a frame relay switch, a
`packet router and a workstation. It
`receives data from one correspondent
`network entity and forwards it to
`another correspondent network entity.
`The mobile data-intermediate system
`supports user mobility by operating a
`CDPD-specitic mobile network loca-
`tion protocol to exchange location
`information of CDPDusers.
`A mobile end system communi-
`cates with the corresponding mobile
`data base station by a 19.2 kb/s raw
`duplex wireless link. (This is referred
`to as a CDPD channel stream.) A
`CDPD channel stream can be accessed
`by several mobile end systems.
`CDPD channel streams use idle
`cellular radios to transmit data to and
`trom mobile end systems. However,
`CDPD systems are designed to use
`idle cellular capacity without direct
`communication with the cellular sys-
`tem. Also, a strict requirementis that
`
`
`
`| Mobile Data:
`Intermediate
`
`top may incorporate a cellular
`transceiver and rechargeable batteries
`in a palm-sized, serially-connected
`unit to a personal computer.)
`The mobile end system has three
`parts: the subscriber unit, used to
`access the radio interface; the sub-
`scriber identity module, which con-
`tains information to identify a
`subscriber, and the mobile application
`subsystem, which provides the mobile
`end system application functionality. It
`can be a personal computer or a simple
`data gathering/telemetric tool.
`Mobile data base station. The
`mobile data base station is responsible
`for detailed control of the radio inter-
`face such as radio channel allocation,
`interoperation with cellular voice
`channel usage and radio media access
`control. In order to share radio
`resources with the cellular system,
`mobile data base stations are expected
`
`
`
`AUGUST/SEPTEMBER 1997
`
`0278-6648/97/$10.00 © 1997 IEEE
`
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`
`CDPDtransmissions must not interfere
`with cellular voice calls. We will
`describe how this goal is achieved in the
`following sections.
`
`Al inferface and protocols
`The link from the mobile data base
`station to the mobile end system is
`called the forward link. The link from
`the mobile end system to the mobile
`data base station is called the reverse
`link. The raw data rate on both linksis
`19.2 kb/s. The mobile data-intermediate
`system queues all frames, and sends
`them to the corresponding mobile data
`base station for transmission on the for-
`ward link. The mobile data base station
`broadcasts the frames in its radio cover-
`age arca. Only mobile end systems that
`have valid network entity identifiers can
`decode the data received. It is clear that
`the transmission on the forward link ts
`contentionless.
`CDPDfollowsthe traditional slotted,
`non-persistent digital sense multiple
`access protocol for the reverse link
`access. The protocol works as follows.
`The mobile data base station broadcasts
`(on the forward link) the availability of
`the reverse link bythe idle/busy control
`flags. If there are no transmissions on the
`reverse channel, the mobile data base
`station sets the control flag on “idle.”
`Upon detecting the idle status, a
`mobile end system may transmit the
`data on the reverse link. If the mobile
`end system detects the “busy” status,
`it waits for a random period of time,
`and rechecksthe status of the control
`flag. Two mobile end systems may
`detect the “idle” status and try to
`access the reverse link at the same
`ume. In this case. a collision occurs
`and the mobile end systems follow
`an exponential back off procedure
`for retransmission.
`A mobile end system may repeatedly
`transmit long bursts. To prevent this chan-
`nel hogging situation, a minimum time
`period is defined between two bursts of a
`mobile end system. This ensuresthat other
`mobile end systems have the chance to
`access the reverse link.
`A sleep mode is provided in CDPD
`to allow an idle mobile end system to
`shut off power for a defined period. To
`“wake up” the mobile end system, the
`mobile data-intermediate system period-
`ically broadcasts a notification message
`that lists mobile end systems that are
`recipients of the frames it has in queue.
`The mobile end system periodically
`
`12
`
`anism, the mobile data base station
`
`activates its receiver to listen to the
`broadcast notification message. If it
`hears its name, the mobile end system
`leaves the sleep mode by sending a mes-
`sage to the mobile data-intermediate sys-
`tem. With this mechanism, the battcry
`life of the mobile end system can last
`longer. (Twelve hour battery life using
`the CDPD power-saving mode has been
`reported in commercial products.)
`
`Radio resource dliocation
`During the monitoring phase, the
`mobile data base station can determine
`the availability of channels in two ways.
`If a communication link between the
`cellular system and the CDPD system
`exists, then the information of the voice
`channel usage can be obtained directly
`from the cellular system. If such a com-
`munication link does not exist, then the
`mobile data base station detects the
`channel usage through the cellular
`transmit path by using a forward power
`monitor called a sniffer. Since every cell
`may contain as many as 60 voice chan-
`nels, it is critical that the mobile data
`base station finds the available channels
`in real-time.
`With the channel monitoring mech-
`
`should be able to change channels
`before a voice assignment is made on
`the current CDPD channel. This action
`should be completed in 40 msec. When
`the mobile data base station detects
`that the CDPD channel is about to be
`assigned to a voice call, it performs an
`emergency or forced hop byswitching
`the channel without informing its
`mobile end systems. When a mobilc
`end system loses contact with the for-
`ward link, it searches the “likely
`hopped” channel list (broadcasted by
`the mobile data base stalion) to
`reestablish the radio link.
`With channel sniffing and channel
`hopping, CDPD allows mobile end sys-
`
`tems to use idle voice channels without
`interrupting the cellular system. A
`mobile data base station periodically
`scans the channels within its radio cov-
`crage area and generates a candidate list
`of available channels for CDPD traffic.
`Thislist is then forwarded to the mobile
`data-intermediate system. The mobile
`data-intermediate systemcollects all
`channel lists from the associated mobile
`dala base stations, and determines the
`CDPD channel streams based on its
`knowledge of the voice channel alloca-
`tion algorithm.
`The mobile data base station may
`periodically perform channel switching
`(referred to as timed or planned hop) to
`avoid channel sealing or channel steal-
`ing. When the cellular system notices
`interference on a channel, the channel is
`sealed and becomes unavailable to a
`voice user, Since the cellular system
`cannot recognize CDPD, it may seal a
`channel used by CDPD. Then the chan-
`nel cannot be used bythe cellular users.
`If so, CDPDis said to have stolen the ©
`channel from the cellular network. This
`violates the rule that CDPD should not
`affect the voice system.
`To avoid the sealing of a CDPD chan-
`nel, the mobile data base station uscs a
`timed hop to switch a CDPD channel
`stream periodically. (The hop period
`is on the order of 10 seconds.) In a
`timed hop, the mobile data base sta-
`tion broadcasts a control message to
`all mobile end systems using the
`channel, and instructs them to move
`to a new channel (if any). Channel
`hopping may not be performed if ded-
`icated channels are assigned for
`CDPD use.
`When a mobile end system in
`communication moves from one cell
`to another, a radio link transfer
`process is required to reconnect the
`mobile end system in the newcell. The
`CDPD transfer process is controlled by-
`the mobile end system.
`The mobile end system monitors the
`radio link quality. If the quality falls
`below the defined thresholds for the
`
`radio signal strength and errorrates, the
`mobile end system initiates the link
`transfer process by scarching a better
`quality channel. (A list of available
`channels are broadcast by the mobile
`data base stations.)
`The mobile end system tunes to the
`new channel and informs the new
`mobile data base station that it has
`entered the cell. The new mobile data-
`
`
`
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`
` the mobile end sys-
`
`forwarding —
`procedure.
`
`intermediate system updates its regis-
`trationdirectory so that future data for
`the mobile end system are directed to
`the currentcell. If the old and the new
`mobile data base stations connect to
`different mobile data-intermediate sys-
`tems, the transport layer retransmission
`is required to reestablish the end-to-end
`connection.
`
`Roaming management
`The CDPD roaming managementis
`achieved by two functions: the mobile
`home function and the mobile serving
`function. A mobile end system is identi-
`fied bya distinct nctwork entity identifier.
`Every network entity identifier is associat-
`ed with a home mobile data-intermediate
`system. The home mobile data-intermedi-
`ate system maintains a location directory
`to record the address of the current serv-
`ing mobile data-intermediate system for
`cach of its home mobile end systems.
`(This procedure is referred to as the loca-
`tion directory service.)
`The mobile end system may roam
`away from its home mobile data-inter-
`mediate system and visit another mobile
`data-intermediate system. The visited
`mobile data-intermediate system main-
`tains a registration directory to keep
`track of all visiting mobile end systems.
`(This procedure is referred to as the reg-
`istration directory service in the mobile
`home function.)
`Location registration. From the chan-
`nel stream, a mobile end system identi-
`fies its location (the address of the
`serving mobile data-intermediate sys-
`tem). When the mobile end system
`moves to another serving area, it regis-
`ters at the registration directory of the
`current serving mobile data-intermediate
`system. (The registration service that is
`in the mobile serving function).
`This mobile data-intermediate system
`then notifies the home mobile data-inter-
`mediate system of the currentlocation of
`
`tem. Then the location
`directory of the home
`mobile data-interme-
`diate system is updat-
`ed by the registration
`service in the mobile
`home function. The
`home mobile data-
`intermediate system
`then sends a message
`to the old serving
`mobile data-intermedi-
`ate system todelete the
`mobile end system record in its registra-
`tion directory. The location registration
`procedureis illustrated in Fig. 2.
`Network Protocol Data Unit (NPDU)
`forwarding. To route an NPDUto a
`mobile end system, the NPDUis first
`touted to the home mobile data-interme-
`diate system according to the network
`entity identifier of the mobile end sys-
`lem (see Step | in Fig. 3). If the mobile
`end system is not in the homearea, then
`the address ofthe current serving mobile
`data-intermediate system of the mobile
`end system is identified in the location
`directory. The NPDUis then encapsulat-
`ed and tunneled to the current serving
`mobile data-intermediate system (see
`Step 2 in Fig. 3). This process is referred
`to as the redirection and forwarding ser-
`vice in the mobile home function at the
`home mobile data-intermediate system.
`(Encapsulation is the process of
`enclosing the data within an NPDU
`inside another NPDU-header. This
`process is also known as “tunneling,”
`since it can be used to hide the original
`NPDU-header information during deliv-
`ery to the new NPDUdestination speci-
`fied in the encapsulated NPDU.)
`Upon the receipt of the forwarded
`NPDU,the current serving mobile data-
`intermediate system decapsulates the
`NPDUandroutes it to the cell where the
`mobile end system resides (see Step 3 in
`Fig. 3). This processis referred to as the
`readdress service in the mobile serving
`function at the serving mobile data-inter-
`mediate system.
`The reader may query CDPD Forum
`Headquarters (info@forum.cdpd.net) to
`receive up-to-date CDPD information
`(such as existing CDPD systems, future
`applications expected and the money
`factor).
`
`Read more aboutit
`° Cellular Digital Packet Data System
`Specification: Release 1.1. Technical
`
`report, CDPD Forum,Inc., January 1995.
`¢ Budka, K., Cellular Digital Packet
`Data: Channel Availability. Proc. IEEE
`Personal Indoor and Mobile Radio Con-
`ference, 1995.
`* Jain, N., and Basu, K., Queuing
`Model of CDPD Implementation on
`American Cellular Systems. ITC Mini
`Seminar: Mobility and Intelligent Net-
`works, 1994,
`¢ Mason, C., CDPD: The Promises
`and the Challenges. Wireless, 3(6),
`November/December 1994.
`° Nanda, S., Chawla, K. and Budka,
`K., CDPD over Shared AMPS Chan-
`nels: Interference Analysis. Proc. [EEE
`Personal Indoor and Mobile Radio Con-
`Jerence, 1995.
`e O’Shea, D., Vendors: CDPD
`Needs Marketing Push. Telephony, 10
`October 1995.
`¢ Pawlowski, C., and McConnell, P.,
`CDPD Air Interface Basics. Telephony,
`5 December 1994.
`* Quick, R.F., Jr., An Overview of
`the Cellular Digital Packet Data (CDPD)
`System. Proc. IEEE Personal Indoor
`and Mobile Radio Conference, pages
`338-343, 1993.
`° Surace, K.J., A Comparison of
`CDPDand Circuit Switched Data. Wire-
`less Newsgroup (comp.std.wireless),
`March 1995.
`
`Abouthe author
`Yi-Bing Lin received his BSEE
`degree from National Cheng Kung Uni-
`versity in 1983, and his Ph.D. degree in
`Computer Science from the University
`of Washington in 1990. Between 1990
`and 1995, he was with the Applied
`Research Area at Bell Communications
`Research (Bellcore), Morristown, NJ. In
`1995, he was appointed full professor of
`Department of Computer Science and
`Information Engineering, National
`
`Chiao Tung University.
`
`AUGUST/SEPTEMBER 1997
`
`13
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