USOO6973067B1
`
`(12) United States Patent
`US 6,973,067 B1
`(10) Patent N0.:
`Haartsen
`
`(45) Date of Patent:
`Dec. 6, 2005
`
`(54)
`
`MULTI-MEDIA PROTOCOL FOR
`SLOT-BASED COMMUNICATION SYSTEMS
`
`EP
`EP
`
`0495600 A
`0827308 A2
`
`7/1992
`3/1998
`
`(75)
`
`Inventor:
`
`Jacobus Cornelis Haartsen, Borne
`(NL)
`
`(73)
`
`Assignee: Telefonaktiebolaget L M Ericsson
`(publ), Stockholm (SE)
`
`* cited by examiner
`
`Primary Examiner—Ricky Ngo
`Assistant Examiner—Soon D. Hyun
`(74) Attorney, Agent, or Firm—Potomac Patent Group
`PLLC
`
`(*)
`
`Notice:
`
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 0 days.
`
`(21)
`
`Appl. No.: 09/348,495
`
`(22)
`
`Filed:
`
`Jul. 7, 1999
`
`(60)
`
`(51)
`
`(52)
`
`(58)
`
`(56)
`
`Related US. Application Data
`
`Provisional application No. 60/109,692, filed on Nov.
`24, 1998.
`
`Int. Cl.7 ........................... H04B 7/212; H04] 3/00;
`H04L 12/66
`370/337; 370/347; 370/349;
`370/352; 370/442; 370/493
`Field of Search ................................ 370/471, 352,
`370/356, 355, 337, 347, 468, 346, 348, 349,
`370/336, 442, 493
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`5,483,524 A *
`5,875,184 A *
`6,011,784 A *
`6,055,242 A *
`6,393,013 B1 *
`
`................... 370/355
`1/1996 Lev et al.
`2/1999 Altvater et a1.
`............. 370/330
`1/2000 Brown et a1.
`............... 370/329
`
`4/2000 Doshi et al.
`.....
`370/468
`5/2002 Masui et al.
`................ 370/346
`
`FOREIGN PATENT DOCUMENTS
`
`EP
`
`0584820
`
`3/1984
`
`(57)
`
`ABSTRACT
`
`A method and apparatus are described establishing multi-
`media communications on a shared communications chan-
`nel. A first and a second communication unit, such as a
`master and slave unit, establish a synchronous communica-
`tions link. Additional synchronous communications links
`may be established. A first data packet associated With the
`synchronous communication link is communicated to the
`second communication unit by including an address. Time
`slots reserved for the synchronous channel by the first unit
`are separated by a fixed time interval. One or more addi-
`tional communications units may communicate over an
`asynchronous link established between the master and addi-
`tional units using remaining time slots. Data packets may be
`communicated to additional units by including addresses
`associated With each additional units The synchronous link
`may be interrupted With the asynchronous link by commu-
`nicating an asynchronous data packet on a time slot reserved
`for the synchronous communications link. The asynchro-
`nous link may be a Time-Division duplex link for alternately
`transmitting and receiving on different ones of the remaining
`time slots. Asynchronous data packets communicated to
`additional units on remaining time slots. The master unit
`may poll each additional units for a response packet to the
`asynchronous data packet. On a Time-Division duplex link,
`additional units alternately receive the poll from the first
`communication unit and transmit the response packet on
`different ones of the remaining time slots.
`
`26 Claims, 4 Drawing Sheets
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`Dec. 6, 2005
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`US. Patent
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`Samsung EX. 1006 p. 4
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`

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`US. Patent
`
`Dec. 6, 2005
`
`Sheet 4 0f 4
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`US 6,973,067 B1
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`

`

`US 6,973,067 B1
`
`1
`MULTI-MEDIA PROTOCOL FOR
`SLOT-BASED COMMUNICATION SYSTEMS
`
`CROSS-REFERENCE TO COPENDING
`APPLICATIONS
`
`This application claims the benefit of US. Provisional
`Application No. 60/109,692, filed Nov. 24, 1998.
`
`BACKGROUND
`
`The present invention relates to communication systems.
`In particular the present invention is related to communica-
`tion systems which use time-slot based protocols and which
`support both asynchronous data services and synchronous
`and/or isochronous data services.
`The term “Multimedia” generally refers to the integration
`of data, voice and video services which share common
`platforms and data channels. Service providers (SPs) around
`the world continue to develop more advanced systems for
`delivering a wide array of services on a common channel
`including Internet data services,
`telephone services, and
`television services to a subscriber base. For obvious eco-
`
`nomic reasons, SPs prefer that all services they provide be
`carried by a common medium, such medium being, for
`example, television cable, telephone cable, or air interface in
`the case of wireless systems. Particularly with new global
`standards emerging within the wireless communications
`community that provide for
`the allocation of physical
`resources between circuit
`switched services
`and,
`for
`example, General Packet Radio Services (GPRS) as outlined
`in “Digital Cellular Telecommunications System (Phase 2+);
`General Packet Radio Services (GPRS); Overall Description
`of the GPRS Radio Interface; Stage 2 (GSM 03.64 version
`6.0.1 Release 1997” ETSI TS 101 350 V6.0.1(1998—08), for
`example, at chapter 6, the need to integrate both types of
`services on a physical resource or common channel is great.
`Problems arise however with the integration and provi-
`sion of services on the same medium since data associated
`with different services, such as voice, video, and non-real
`time (NRT) data transfer, have different characteristics and
`requirements. Data associated with NRT data transfer, for
`example, is typically communicated in bursts, and requires
`a high degree of integrity, leading to bit error probability
`requirements in the realm of 10'12 to 10'“. Variable delay
`associated with the communication of NRT data however is
`
`generally well tolerated. In sharp contrast, live voice and
`live video data or voice and video on-line playback data, for
`example, have real-time requirements and may be charac-
`terized by constant data streams which, if interrupted or
`delayed may have severe quality degrading consequences.
`Individual bit errors in the data stream however may be
`tolerated but may lead to distortion or minor degradation.
`Human beings may normally tolerate certain levels of
`distortion in an audio or video or combined audiovisual
`
`presentation before the distortion causes continued listening
`or viewing to become annoying or tiresome. In some cases
`the threshold for distortion tolerance is high, at least for
`short intervals, with the important factor most likely being
`the ability to continue to discern information content. Clicks,
`pops, noise, distortion and related audio anomalies may
`make audio communication less pleasant and “snow” or
`other visual anomalies in a video stream may make a video
`presentation less appealing, but, in most cases of distortion
`due to bit errors in the data stream of audio and video, the
`information content is generally preserved. However, since
`audio and video data streams may be tightly synchronized in
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`2
`relatively highly correlated, particularly with
`time, e.g.
`regard to combined audio and video streams, variable delay
`associated with incoming data packets is not permitted,
`since delay would cause, for example, talker and listener to
`become out of synch with each other in a conversation,
`words to be unintelligible, and the like. Variable delay
`introduced in a voice stream would be noticeable to the
`
`listener since it affects the timing and shape of the acoustical
`waveform. Likewise, delay introduced in a video stream
`would cause, for example, variations or interruptions in the
`speed of motion of the video stream, and, worse for
`example, a loss of frame synchronization in the receiver. In
`these cases, information content is seriously compromised.
`To avoid delay related anomalies, synchronous services
`such as voice and video may typically be carried in a
`communications system over circuit-switched connections.
`Circuit switched connections may be established in a time-
`slot environment by reserving a portion of the communica-
`tion medium exclusively for a particular link between a
`source and destination. Circuit switching is attractive for
`links which are constantly in use but may be inefficient for
`asynchronous data traffic typically transferred in bursts
`without regard to delay. Asynchronous communications
`conducted on a circuit switched connection may result in an
`unnecessarily idle channel during intervals when no data is
`being transferred, and consequently channel capacity and
`ultimately system capacity is wasted. This condition may be
`illustrated by example with reference to a user of Internet
`services, who during the interval, for example, when waiting
`for a request to be processed, is receiving no data, or during
`the interval when data has been delivered and displayed and
`a user is reviewing the information, is receiving no data.
`Therefore, data of this kind, (e.g., asynchronous data) is
`typically carried over packet-switched connections.
`It is important to note that in prior art circuit switching,
`once a circuit is set up for circuit switched communications,
`it is presumed that all subsequent data packets on the circuit
`switched connection are destined for the party at the other
`end of the connection. Thus, in prior art circuit switched
`connections, addresses are not used.
`In the packet switched environment, one or more channels
`of the medium may be shared among a large number of
`packet users in a more efficient manner. Apacket data source
`such as an Internet server, for example, may seize the
`medium or a portion of it when it becomes available and
`may use it for relatively short duration of time sufficient to
`send its packet or packets whereupon the medium is
`released. Other packet data sources may wait until
`the
`medium is idle to seize the medium and send their packets.
`Due to the bursty nature of traffic associated with packet
`data, packet switching is much more effective and leads to
`greater efficiency of use of the communications medium.
`Packet switching in communications systems may further
`provide an overall communications channel capacity gain
`due to the advantages provided by statistical multiplexing.
`Statistical multiplexing allows existing logical packet-
`switched channels to seize any free slot space. Packets of
`different
`logical channels are concatenated on the same
`slotted physical channel driven by availability and capacity
`need. Systems using statistical multiplexing may employ a
`buffer memory which may temporarily store packet data
`during periods of peak traffic. Statistical multiplexing mini-
`mizes channel waste due to inactive channels. For more
`
`information related to statistical multiplexing, see “Data
`Communications, Computer Networks and Open Systems”,
`Halsall & Fred, Addison Wesley, p160—161, 1995. Non-real
`time data transfer typically involves the transmission of
`
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`

`US 6,973,067 B1
`
`3
`files, documents, drawings, photo’s, still video and other
`text- or picture-based material Recently, downloading
`webpages over the Internet has become an important NRT
`traffic service. NRT data has no strict delivery requirements.
`The transmission of a file can take seconds or minutes
`
`depending on the file size and data speed. Variations in the
`delivery time are unimportant. In addition, the file can be
`sent in chunks (or packets), and the delivery of each chunk
`can be handled separately. The only requirement at
`the
`recipient is that finally all chunks have arrived, and that there
`is means in the recipient to place the chunks in the proper
`order to reconstruct the original file.
`As previously described, integrating synchronous real-
`time data and asynchronous packet data services on the same
`medium gives rise to a problem: circuit switching is ineffi-
`cient for asynchronous packet data services; and packet
`switching is detrimental for synchronous real-time data
`services, which cannot tolerate delay.
`It would therefore be appreciated in the art for a method
`and apparatus for combining the delivery of synchronous
`and asynchronous data on the same medium at the same time
`such that
`the medium may continue to support packet-
`switched connections and circuit-switched connections con-
`
`currently.
`
`SUMMARY
`
`It is therefore an object of the present invention to provide
`a communication system having a communications channel
`which is capable of supplying both synchronous and asyn-
`chronous data within a communications system.
`It is a further object of the present invention to provide
`such a communications channel
`in a TDMA, CDMA,
`FDMA, and related wireless communications systems.
`In accordance with one aspect of the present invention,
`the foregoing and other objects are achieved in a method and
`apparatus involving a communication system where multi-
`media communications may be established on a shared
`communications channel using a first and second commu-
`nication unit, such as a master and slave unit. The master
`communication unit, for example, may be configured to
`establish a synchronous communications link with the sec-
`ond, or slave, communication unit, communicating a first
`data packet on first time slot of a first set of time slots
`associated with the synchronous communication link to the
`second communication unit by including the address asso-
`ciated with the second communication unit
`in the data
`
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`packet. Moreover, additional synchronous links may be
`established by reserving addition “sets” of time slots. The
`terms “sets” herein refers to all of the timeslots associated
`
`50
`
`with a group of timeslots which appear at regular intervals
`conventionally referred to collectively as a “timeslot”. The
`term timeslot herein refers to a single instance of a slot
`within the set.
`
`In establishing the synchronous link, the master commu-
`nication unit may reserve a set of time slots for use by the
`synchronous link. To effect time division on what is, for
`example in a cellular system, an otherwise unrestricted
`channel, the master communications unit may separate each
`one of the time slots associated with the set by a fixed time
`interval.
`
`It may be desirable to add one or more additional com-
`munications units including the second communications unit
`to the communication system and accordingly an asynchro-
`nous communications link may be established between the
`master or first communications unit and the one or more
`
`additional communications units using one or more of the
`
`4
`remaining time slots. Data packets may be communicated on
`a first of the one or more of the remaining time slots
`associated with the asynchronous communications link from
`the first communication unit to the one or more additional
`
`communication units by including one or more addresses
`associated with each of the one or more additional commu-
`nications units.
`
`In another embodiment of the present invention, the first
`communication unit may be further configured to interrupt
`the synchronous communications link with the asynchro-
`nous communications link by communicating an asynchro-
`nous data packet on a time slot reserved for the synchronous
`communications link. Further, the asynchronous link may be
`a Time-Division duplex link where, for example, the master
`communication unit alternately transmits and receives on
`different ones of the remaining time slots. Asynchronous
`data packets may further be communicated from the first
`communication unit to additional units on remaining time
`slots. On the Time-Division duplex link, for example, the
`master or first communication unit may poll each of the one
`or more additional units for a response packet to the asyn-
`chronous data packet. Accordingly,
`the additional units
`alternately receive the poll from the first communication unit
`and transmit the response packet on different ones of the
`remaining time slots.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`The objects and advantages of the invention will be
`understood by reading the following detailed description in
`conjunction with the drawings in which:
`FIG. 1 is a diagram illustrating an exemplary slot-based
`communication channel;
`FIG. 2A is a diagram illustrating an exemplary circuit-
`switched connection over a slot-based channel in accordance
`
`with the present invention;
`FIG. 2B is a diagram illustrating an additional exemplary
`circuit-switched connection over a slot-based channel in
`
`accordance with the present invention;
`FIG. 2C is a diagram illustrating an exemplary packet in
`accordance with the present invention;
`FIG. 3 is a diagram illustrating an exemplary packet-
`switched connection over slot-based channel in accordance
`
`with the present invention;
`FIG. 4A is a diagram illustrating an exemplary multime-
`dia connection in accordance with the present invention
`having synchronous and asynchronous links; and
`FIG. 4B is a diagram illustrating an exemplary multime-
`dia connection in accordance with the present invention
`having additional synchronous links.
`
`DETAILED DESCRIPTION
`
`55
`
`60
`
`65
`
`The various features of the invention will now be
`
`described with respect to the figures, in which like parts are
`identified with the same reference characters.
`
`The present invention provides a flexible communication
`channel in the context of a wireless communication system
`using time slots separated by intervals of fixed length. It
`should be noted that
`in an embodiment of the present
`invention, data associated with each time slot, in accordance
`with the present invention, may be sent using a different
`frequency. An exemplary system in which such an embodi-
`ment could be implemented may be found in a technology
`known as “Bluetooth” for providing low-cost, robust, effi-
`cient, high capacity, ad hoc voice and data connectivity (see,
`“Bluetooth, the Universal Radio Interface for Ad Hoc wire-
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`US 6,973,067 B1
`
`5
`less connectivity”, J. C. Haartsen, Ericsson Review, Tele-
`communications Technology Journal, No. 3, 1998.)
`With reference to the present invention, communications
`channel 100 carrying synchronous and asynchronous data
`services may be divided into time slots of equal length in
`accordance with the present invention as illustrated in FIG.
`1 to provide a flexible communications channel. Generally,
`on each recurrence of time slot 110, data packet 120 may be
`sent which may further include an address associated with
`one or more recipients. A circuit-switched connection may
`be established by reserving time slot 110 at a fixed interval.
`Time slots not reserved by a circuit-switched connection
`may be used freely by one or more packet-switched con-
`nections which may be set-up and taken down periodically
`over time, which decision may be taken on a per packet
`basis. Connected units on a packet switched connection may
`typically send packets to any other connected units. As
`mentioned above each of timeslots 110 may be transmitted
`on a separate frequency in a frequency hopping embodi-
`ment, such as may be found within the Bluetooth technol-
`ogy.
`Many digital wired and wireless communication systems
`make use of a slot-based protocol over a physical interface
`whether it be a fiber optic, wire, air interface, and the like.
`Accordingly, communications channel 100 may be divided
`into fixed-length time slots, such as time slot 110 as
`described; and in time slot 110, data packet 120 may be
`transmitted as either a part of a synchronous data stream or
`an asynchronous data packet. In the case of a voice or video
`data stream on a synchronous channel, data packet 120 may
`represent one of a stream of data packets and may include
`voice or video information to be transferred which may first
`be digitized and then loaded into packets according to, for
`example, a particular link layer protocol which specifies the
`packet size, and the like which packets may then be indi-
`vidually transmitted over communications channel 100 in a
`corresponding time slot 110. In the case of an asynchronous
`data transfer, a stored data record, for example, a record
`from an Internet server may be transferred in bursts, depend-
`ing on traffic, block size, etc until the record is completely
`transmitted. It is important to note that in accordance with
`most packet protocols, asynchronous packet data has a much
`lower tolerance for errors based on the type of data trans-
`ferred. Thus depending on the error correction, and data
`acknowledgment protocols certain packets may require
`retransmission suggesting first that the acknowledge process
`requires additional
`time and that packets which are re-
`transmitted may be out of sequence. It will be appreciated by
`those skilled in the art that out of sequence reception of
`packets associated with a synchronous real time data stream
`would be highly disruptive if not fatal to the information
`content of the stream. However if data associated with a non
`
`real time data stream is transferred asynchronously to be
`played back in a real time mode off line, such data may be
`transferred on an asynchronous link as described.
`As described, data packet 120 may be associated with a
`synchronous data stream like that associated, for example,
`with a voice connection. Data may be transmitted continu-
`ously throughout the duration of the connection even during
`silent intervals. Therefore, the capacity of communications
`channel 100 is usually much larger than what is required for
`the synchronous connection. Accordingly, only a certain
`number of time slots 110 need be used to sustain a synchro-
`nous link on communications channel 100. Referring now to
`FIG. 2A, where an exemplary circuit switched connection
`between master 250 and Slave A 210 is shown having six
`repeating time slots, although more or less could be used,
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`only one out of six time slots 110 is used for a single
`synchronous link on communication channel portions 100a
`and 100b. It should be noted that communication channel
`
`portions 100a and 100b are portions of a single communi-
`cations channel 100. It may be further appreciated that in, for
`example, a frameless protocol, a link may be established
`using one or more time slots. Several individual time slots,
`often referred to collectively as a “time slot” in a framed
`protocol, may be established by separating such individual
`time slots by a fixed time interval, such as fixed time interval
`T 230. A synchronous two way data stream, such as a voice
`conversation, may be compressed into packets 251—254
`from master 250 and packets 211a—214a from slave A 210
`each of which are sent at regular times separated by fixed
`time interval T 230.
`
`The reservation of a particular one of time slots 110 for
`establishing a link for synchronous information can be
`accomplished in different ways. In case of a communication
`system with decentralized control, the reservation of a time
`slot 110 may be accomplished by agreement of all units on
`communication link 100 involved. In a more conventional
`
`manner not illustrated, units wanting to establish a synchro-
`nous link, for example, may broadcast the reservation to all
`participants on communications channel 100. In the exem-
`plary decentralized case, reservations are established on a
`first come first served basis and each unit knows exactly
`which time slot is reserved for the synchronous link. Accord-
`ingly, since new units accessing communication channel 100
`will not know of previously existing slot reservations, a
`problem may arise. Based on the reservation of, for example,
`one or more time slots 110 for one or more synchronous
`links, units on communications channel 100 may be cogni-
`zant of which of time slots 110 are left for other services
`
`such as asynchronous packet data links. In a conventional
`reservation based system, data packets sent on the synchro-
`nous links need not carry an address or identity of the
`recipient since, for example, time slots 110 are exclusively
`allocated to the recipient. However, in accordance with an
`embodiment of the present invention, centralized control is
`used. Master 250 may be a unit connected to communica-
`tions channel 100 over, for example,
`link portion 100a,
`while all other participants such as slave A 210 are desig-
`nated as slaves.
`Master 250 controls the traffic over communications
`
`channel 100 by, for example, scheduling transmission on the
`synchronous links established over one of time slots 110
`separated by fixed interval T 230 as described. In the case of
`centralized control, packet address A251a—A 254a may be
`required to be included when sent from master 250, since
`otherwise, in the absence of a reservation system where all
`participants know which of time slots 110 are allocated, a
`recipient such as slave A 210 cannot associate a particular
`time slot 110 with a particular slave. Centralized control may
`be advantageous in that master 250 need only agree with a
`single slave, such as slave A210 about which time slot 110
`the synchronous link will be established upon. Moreover,
`general agreement between participants on communication
`channel 100 is not needed and no broadcasting is required.
`If address A251a—A 254a in packets 251—254, for example,
`do not match the address of slave A 250, slave A 250 is not
`interested whether packets 251—254 concern a synchronous
`connection or an asynchronous connection. It will be appre-
`ciated that
`in addition to the foregoing advantages,
`the
`synchronous link established on communications channel
`100 between master 250 and slave A210 may be interrupted
`at any time by sending an asynchronous data packet with
`address of slave A in the time slot intended for the synchro-
`
`Samsung EX. 1006 p. 8
`
`Samsung Ex. 1006 p. 8
`
`

`

`US 6,973,067 B1
`
`7
`nous link. Master 250 may further interrupt communications
`with any slave to communicate with any other slave unit.
`Such an interrupt capability may provide for enhanced
`services, or may allow asynchronous communications to
`occur as a “background” process between master 250 and
`slave A210 over the established synchronous link or to any
`other slave on communications channel 100.
`
`As previously described, asynchronous data, for example
`Internet data traffic may have a bursty character. On time
`slots 110 not used for establishing synchronous links, asyn-
`chronous links may be set up on communications channel
`100 for the exchange of asynchronous data. For a more
`conventional communication system using decentralized
`control not shown, some kind of listen-before-talk (e.g.
`collision avoidance) must be used to avoid multiple units
`seizing communication channel 100 simultaneously.
`In
`accordance with another embodiment of the present inven-
`tion, centralized control may be used with master 250
`assuring that no collisions take place. Such centralized
`control may be accomplished, for example, using a Time-
`Division duplex scheme where master 250 alternatively
`transmits and receives. A more thorough description of the
`use of master and slave units in a communication system
`using centralized control may be found in US. patent
`application Ser. No. 09/210,594 by J. C. Haartsen et al,
`entitled “CENTRAL MULTIPLE ACCESS CONTROL
`FOR FREQUENCY HOPPING RADIO NETWORK ”,
`filed Dec. 15, 1998 and incorporated herein by reference.
`It
`is important to note that additional circuit-switched
`connections may be established in the manner described, as
`illustrated in FIG. 2B. Master 250 may reserve additional
`time slots 110 to establish a second synchronous link with,
`for example, slave B 310. In a manner similar to establishing
`a synchronous link with Slave A210, data packets 281—284
`may be communicated to Slave B 310 using addresses
`281a—284a. In response, Slave B 310 may respond alter-
`nately with data packets 211b—214b. Preferably, as illus-
`trated, additional synchronous links use the same slot inter-
`val T 230 in order to avoid interference. If the interval T 230
`is not chosen identical on all links, then periodically, time
`slots in the sets of two or more links coincide. However,
`since this interference happens in a determined manner and
`can be predicted, appropriate countermeasures can be taken
`by the master and/or the slave to prevent distortion of the
`synchronous information. Each additional synchronous link
`is staggered by an even number of time slots to allow for the
`return link. Additional time slots 110 not used for synchro-
`nous links may be used for asynchronous links as described
`in greater detail hereinafter. In addition, any synchronous
`links may be interrupted in the manner previously described.
`It should be noted that with reference to “addresses”, in an
`exemplary embodiment of the present invention, packets
`used on the synchronous and asynchronous links may all
`have the same appearance, an example of which is shown in
`FIG. 2C. Packet 540 may be assembled using, for example,
`three elements: preamble 510, header 520, and payload 530.
`Preamble 510 may be used by a recipient for timing syn-
`chronization and frequency or DC compensation and may
`also include an identification label identifying the channel,
`for example, communication channel 100. Packet header
`520 may include supervisory control information and may
`further include a slave address. It is important to note that
`although, for example, addresses B 251a and B 252a asso-
`ciated with packets 251 and 252 are described using different
`reference numerals, addresses B 251a and address B 252a
`may more appropriately refer to the individual packet header
`portions of the corresponding packets and preferably contain
`the same address. Hereinafter, the address portion of the
`individual packet header and the “address” of the destination
`will be used interchangeably and though separately desig-
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`nated, will preferably be the same value for each different
`slave unit. Payload 530, carries user information or data
`which can be real-time as in the case of an exemplary
`synchronous link, or non-real time as in the case of an
`exemplary asynchronous link.
`In the exemplary packet switched embodiment illustrated
`in FIG. 3, for example, when master 250 transmits data
`packet A 255 to slave A 210, slave A 210 is configured to
`listen. After receiving data packet 255, slave A 210 may
`return data packet 215. Only one slave at a time can return
`a packet to master 250. A polling scheme may be used to
`prevent multiple slaves from sending packets to master 250.
`Such an embodiment may be illustrated in greater detail in
`FIG. 3 where an exemplary packet-switched connection
`between master 250 and two slaves, slave A 210 and slave
`B 310 on communication channel 100 is shown on, for
`example, link portions 100a, 100b, and 100C. Data packets
`255—263 may carry addresses of slave A210 and slave B 310
`accordingly such that each of slave A 210 and slave B 310
`knows which of data packets 255—263 is addressed thereto.
`By further example, master 250 may address data packet 256
`to slave B 310 with address B 256b. Only the slave
`addressed in master-to-slave slot 250b, slave B 310 in this
`example, is allowed to respond on the Time-Division duplex
`link in the following slave-to-master slot 310a. It is apparent
`