United States Patent [191
`Chao et al.
`
`[11] Patent Number:
`[45] Date of Patent:
`
`4,893,306
`Jan. 9, 1990
`
`[54] METHOD AND APPARATUS FOR
`MULTIPLEXING CIRCUIT AND PACKET
`TRAFFIC
`[75] Inventors: Hung-Hsiang J. Chao, Madison; Sang
`H. Lee, Bridgewater; Liang T. Wu,
`Gladstone, all of NJ,
`[73] Assignee: Bell Communications Research, Inc.,
`Livingston, NJ.
`[21] Appl. No.: 118,977
`[22] Filed:
`Nov. 10, 1987
`[51] Int. Cl.‘ .......................... .. H04J 25/16; H041 3/26
`[52] us. 01. ................................... .. 370/942; 370/84;
`370/99; 370/112
`[58] Field of Search ..................... .. 370/94, 60, 84, 99,
`370/111, 112, 82, 110.1, 89
`References Cited
`U.S. PATENT DOCUMENTS
`
`[56]
`
`4,321,703 3/ 1982 Schwiiertz et al. ................. .. 370/89
`4,516,240 5/1985 Kume et al. . . . . .
`. . . .. 370/94
`
`4,594,708 6/1986 Servel et a1. . . . . .
`4,685,105 8/ 1987 Shikama et a1. .
`
`. . . .. 370/94
`370/89
`
`4,706,246 11/1987 Kume . . . . . . . . . . .
`
`. . . .. 370/89
`
`4,763,319 8/1988 Rozenblit . . . . .
`
`. . . .. 370/89
`
`370/110.1
`4,764,921 8/ 1988 Graves et a1.
`4,771,425 9/1988 Baran et al. .................... .. 370/1 10.1
`OTHER PUBLICATIONS
`R. W. Muise, et a1., “Experiments in Wideband Packet
`
`Technology”, Proc. 1986, International Zurich Seminar
`on Digital Communications, pp. 136-138.
`W. W. Chu, “A Study of Asynchronous Time Division
`Multiplexing for Time Sharing Computer Systems”,
`Proc. AFIPS, vol. 35, pp. 669-678, 1969.
`A. Thomas, et al., “Asynchronous Time Division Tech
`niques: An Experimental Packet Network Integrating
`Video Communication”, Proc. International Switching
`Symposium, May 1984.
`
`Primary Examiner--Douglas W. Olms
`Assistant Examiner-Min Jung
`Attorney, Agent, or Firm—James W. Falk
`
`ABSTRACT
`[57]
`A data transmission technique referred to herein as
`Dynamic Time Division Multiplexing (DTDM) is dis
`closed along with a set of multiplexers and demultiplex
`ers required to apply DTDM in an actual telecommuni
`cations network. The DTDM technique uses a transmis
`sion format which is compatible with the existing digital
`circuit transmission format and the packet transmission
`format so that DTDM is able to handle the transmission
`of circuit and packet traf?c. Thus, DTDM provides a
`?exible migration strategy between present circuit net
`works and future broadband packet networks.
`
`7 Claims, 10 Drawing Sheets
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`1
`
`'
`
`METHOD AND APPARATUS FOR
`MULTIPLEXING CIRCUIT AND PACKET
`TRAFFIC
`
`10
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`20
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`25
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`4,893,306
`2
`TDM is used, each data stream comprises frames which
`are subdivided into slots. Corresponding slots in each
`frame are allocated to speci?c connections. For exam
`ple, the ?rst slot in each frame is allocated to one spe
`ci?c connection and the second slot in each frame is
`allocated to a second connection, etc. Each frame also
`includes a ?eld which contains transmission overhead
`information including frame synchronization words and
`control words. This traditional circuit transmission for
`mat can be extended to multiple bit rate services by
`allocating multiple slots in each frame to high band
`width services. In such circuit transmission systems, a
`combination of space division switching and time divi
`sion switching is utilized at the network switches to
`swap time slots between various bit streams so that
`connections to and between speci?c subscribers are
`established.
`Historically, the ?rst digital circuit transmission sys
`tems were introduced during the 1960’s. These ?rst
`digital circuit transmission systems were introduced in
`inter-of?ce trlmking applications to carry 24 voice
`channels by a single 1.544 Mb/sec digital stream. This is
`known as the DS-l signal. Subsequently, the wide de
`ployment of digital channel banks in the public tele
`phone network required the multiplexing of several
`DS-l signals into a higher speed bit stream to ef?ciently
`utilize available transmission links. As the network
`grew further, continuing efforts to effectively multiplex
`tributaries having different bit rates into a common bit
`stream resulted in the well-known hierarchical multi
`plexing plan comprising the DS-l (1.544 Mb/sec),
`DS-lC (3.152 Mb/sec), DS-2 (6.312 Mbit/sec), DS-3
`(44.736 Mb/sec) and D84 274.176 Mb/sec signals.
`Conventional circuit transmission systems suffer from
`a number of shortcomings. Perhaps the most important
`problem is the multiplexing hierarchy itself. An impor
`tant result of the hierarchy is an inherent lack of ?exibil
`ity. Since the network can only transmit the set of sig
`nals in the hierarchy, every telecommunications service
`has to meet the stringent interface requirement of given
`hierarchical signal bit rates, instead of the particular
`service being able to transmit at its own natural bit rate.
`Therefore, the packet mode of transmission which is
`inherently bit rate ?exible is favored for future broad
`band networks which are to be adapted to deliver en
`hanced telecommunication services such as high de?ni
`tion video and interactive data communications.
`In contrast with circuit transmission systems which
`transmit data in frames subdivided into slots, packet
`transmission systems transmit data in discrete blocks or
`packets, with each packet having an address header at
`the front thereof. At the network switches, packets are
`routed from a speci?c input line to a speci?c output line,
`based on address information contained in the packet
`header. In this way data packets can be routed from a
`particular subscriber location, through a telecommuni
`cations network, to another subscriber location. Packet
`transmission techniques and especially fast packet trans
`mission techniques (see e.g., R. W. Muise et al., “Exper
`iments in Wideband Packet Technology”, Proc 1986
`International Zurich Seminar on Digital Communica
`tionS, pp. 136-138 are inherently bandwidth ?exible (i.e.
`the number of packets generated by a given service per
`unit time is ?exible) and thus are suitable for wideband
`enhanced communications services. Accordingly, it is
`desirable to introduce packet transmission technology
`
`RELATED APPLICATIONS
`The following applications contain subject matter
`related to the subject matter of the present application,
`are assigned to the assignee hereof and have been ?led
`on the same date as the present application.
`1. J. J. Chao, “DTDM Multiplexer With Cross-Point
`Switc ”, Ser. No. 118,979, now U.S. Pat. No.
`4,855,999, issued Aug. 8, 1989
`2. M. W. Beckner, F. D. Porter, K. Shu, “DTDM
`Multiplexing Circuitry”, Ser. No. 118,897, now
`U.S. Pat. No. 4,833,671, issued May 23, 1989
`3. H. J. Chao, S. H. Lee, “Time Division Multiplexer
`for DTDM Bit Streams”, Ser. No. 118,978, now
`U.S. Pat. No. 4,833,673, issued May 23, 1989
`4. M. W. Beckner, J. J. Chao, T. J. Robe, L. S. Smoot,
`“Framer Circuit”, Ser. No. 118,898, now U.S. Pat.
`No. 4,819,226, issued Apr. 4, 1989.
`FIELD OF THE INVENTION
`This invention relates to the transmission of data in
`telecommunications networks. More particularly, the
`present invention relates to a data transmission tech
`nique referred to herein as Dynamic Time Division
`Multiplexing (DTDM), and a set of multiplexers and
`demultiplexers required to apply DTDM in an actual
`30
`telecommunications network. DTDM is capable of
`effectively handling both circuit and packet traf?c and
`thus provides a migration strategy between the present
`circuit switched telephone network and the future
`broadband packet switched network.
`BACKGROUND OF THE INVENTION
`Presently, there are signi?cant uncertainties when it
`comes to predicting the future demand for broadband
`telecommunications services such as high de?nition
`video and interactive data communications. This uncer
`tainty in the future demand for broadband telecommu
`nications services has a signi?cant impact on the design
`of public telephone networks. First, to satisfy the un
`known growth pattern in future service demands, it is
`desirable to have a robust network design that can be
`easily modi?ed in response to changes in demand for
`particular telecommunications services. Second, the
`network must be able to handle vastly different types of
`traf?c ranging from low speed data and voice to full
`50
`motion video. Third, depending on the demand for
`wideband services, a network design must be capable of
`providing a migration strategy from existing copper
`wires and circuit transmission and switching facilities to
`optical ?bers and'the succeeding generations of high
`speed packet transmission and switching facilities,
`which packet facilities are used in connection with the
`delivery of wideband telecommunications services.
`These three criteria determine the selection of the three
`major components of a network design: network topol
`ogy, transmission systems and switching systems. Here,
`the concern is primarily with transmission systems and
`transmission techniques which meet the foregoing crite
`na.
`Two important types of commercially used transmis
`sion systems are circuit systems and packet systems.
`Typically, circuit systems utilize time division multi
`plexing (TDM) as a transmission technique. When
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`4,893,306
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`into the public telephone network, which up to now is
`based primarily on circuit transmission technology.
`The commonly-held view as to how to introduce
`packet technology into the public network is to deploy
`a packet overlay network because the existing network
`is optimized for circuit transmission and is therefore
`incompatible with packet transmission techniques. Ac
`cordingly, many deployment strategies recommend
`constructing an overlay packet network for a set of
`wideband services and hope that the migration of new
`10
`services to the packet overlay network will allow the
`existing circuit transmission network to be phased out
`slowly. The main advantage of a packet overlay net
`work is the quick realization of an end-to-end network
`for new services. However, the approach requires a
`large initial capital investment and increases operational
`cost by requiring the management of multiple separate
`networks.
`_
`It is an object of the present invention to provide an
`alternate approach for introducing packet transmission
`20
`technology into the public telephone network, which
`approach requires the replacement of existing transmis
`sion components but not the implementation of an en
`tirely new network. Thus, it is an object of the invention
`to provide a digital data transmission system capable of
`25
`handling both existing hierarchical circuit traf?c and
`packet traf?c.
`With regard to the above-identi?ed objects of the
`invention, it should be noted that recent advances in
`network switch designs have blurred the distinction
`between packet networks and circuit networks. A typi‘
`cal switch for use in a telecommunications network has
`three major components: control processor, switch’
`interfaces, and interconnection network. The control
`processor handles call set-up and tear-down, mainte
`nance and administrative functions. The switch inter
`faces convert transmission formats (i.e., the format data
`has when transmitted between switching nodes) to
`switch formats (i.e., the format data has when processed
`within switching nodes). The interconnection network
`40
`routes information blocks from speci?c input lines to
`speci?c output lines of the switch. For the existing
`digital circuit systems used in the public telephone net
`work, the information in a speci?c time slot on an in
`coming line is transferred, via the switch, to a speci?c
`time slot on an outgoing line. Thus, the interconnection
`network serves as a crossconnect for the incoming sig
`nals on a slot-by-slot basis.
`It has recently been shown (see e.g., Day-Giacopelli
`Huang-Wu, US. patent application Ser. No. 021,664
`entitled Time Division Circuit Switch, ?led on Mar. 4,
`1987, now US. Pat. No. 4,782,474, issued Nov. 1, 1988,
`and assigned to the assignee hereof) that a switch for use
`in a circuit network can be built using a self-routing
`packet interconnection network. An example of such a
`self-routing packet network is the Batcher-banyan net
`work. Based on the address headers associated with
`?xed sized packets, the Batcher-banyan network routes
`a plurality of packets in parallel to speci?c destination
`addresses (i.e., speci?c output lines) without internal
`collisions. Thus, to mimic the operation of the conven
`tional time-space-time switches used in circuit net
`works, switch interfaces are provided which perform
`the time slot interchange function and which are able to
`insert headers in front of circuit slots to convert such
`slots into packets for routing through the self-routing
`interconnection network and able to remove headers
`from packets leaving the self-routing interconnection
`
`4
`network to reconvert packets back into conventional
`circuit time-slot format.
`In addition to circuit and packet transmission, an
`other mode of digital transmission is known as Asyn
`chronous Time Division Multiplexing (ATDM). See
`e.g., W. W. Chu “A Study of Asynchronous Time Divi
`sion Multiplexing for Time Sharing Computer Sys
`tems” Proc AFIPS Vol 35, pp. 669-678, 1969 and A.
`Thomas et a1. “Asynchronous Time Division Tech
`niques: An Experimental Packet Network Integrating
`Video Communication” Proc International Switching
`Symposium, May 1984. ATDM is used in connection
`with continuous and bursty data traf?c. ATDM uses
`channel identi?ers with actual data to allow on-demand
`multiplexing of data from subscriber terminals with low
`channel utilization. The channel identi?ers and associ
`ated data form time slots. However, ATDM is bit rate
`?exible since the appearance of packets can be asyn
`chronous. Slot timing is obtained from a special syn
`chronization pattern which is inserted into unused time
`slots. Since the synchronization pattern appears only in
`unused time slots, ATDM cannot be used to carry exist
`ing high speed hierarchical signals wherein the loading
`is close to one hundred percent.
`In short, the situation is that the present public tele
`phone network utilizes circuit transmission technology
`and the associated time division multiplexing transmis
`sion techniques, while future broadband services, the
`demand for which is presently uncertain, are best of
`fered using packet transmission technology. It is there
`fore an object of the invention to provide a transmission
`system which is capable of integrating present circuit
`traf?c with future packet traf?c so as to provide a ?exi
`ble migration strategy from the existing copper wire
`based circuit network to succeeding generations of high
`bandwidth packet transmission networks.
`SUMMARY OF THE INVENTION
`The digital network transport system of the present
`invention, referred to herein as Dynamic Time Division
`Multiplexing (DTDM), is a flexible network transport
`system capable of effectively handling both circuit and
`packet traf?c. By combining conventional time division
`multiplexing techniques and packet transmission tech
`niques, DTDM enables a ?exible transition from the
`existing circuit type networks to future broadband
`packet transmission networks.
`In a network utilizing DTDM, each transmission bit
`stream is divided into frames. These frames are the
`fundamental unit of data transport in DTDM. Each
`such frame comprises two ?xed length ?elds: overhead
`and payload. The overhead ?eld includes, for example,
`a frame alignment word for frame timing and the emp
`ty/full status of the frame. The payload ?eld of each
`frame may be ?lled with a data packet including header
`or a slot from a circuit transmission stream. Before a slot
`from a circuit transmission stream can be inserted into
`the payload ?eld of a DTDM frame, it must ?rst be
`converted into a packet~like form with a header at its
`front. Viewed another way, each occupied DTDM
`frame comprises a transmission overhead ?eld, a header
`?eld, and a data ?eld. Thus, the DTDM transmission
`format is a combination of the circuit transmission for
`mat and the packet transmission‘format.
`In the DTDM system, packet and circuit traf?c can
`be multiplexed through the same multiplexer. Thus,
`such a multiplexer can have continuous circuit type
`tributaries and bursty packet tributaries. To multiplex
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`Details of the assemblers needed to form the basic
`such diverse traf?c, a train of DTDM frames with
`empty payload ?elds is generated. This train has a bit
`DTDM bit streams, the disassemblers needed to disas
`semble the basic DTDM bit streams, and the set of
`rate which de?nes a basic backbone transmission rate
`multiplexers and demultiplexers needed to implement
`for the DTDM transmission system. Data in the form of
`DTDM in an actual network are described in detail
`packets or circuit slots with headers attached are in»
`below along with a framer circuit which plays a signi?
`serted into the empty frames to form the DTDM bit
`cant role in particular implementations of the assem
`stream.
`blers/disassemblers and multiplexers/demultiplexers.
`An appropriate analogy is as follows. The stream of
`empty DTDM frames may be analogized to a train of
`BRIEF DESCRIPTION OF THE DRAWING
`empty freight cars. The empty freight cars are then
`FIG. 1 schematically illustrates the DTDM transmis
`?lled with data from the various tributaries which may
`sion format, in accordance with an illustrative embodi
`have been in circuit or packet format.
`ment of the invention;
`Illustratively, a DTDM multiplexer may be used to
`FIG. 2 schematically illustrates the formation of a
`merge traf?c from three different communications
`backbone DTDM, bit stream, in accordance with an
`sources or tributaries into a single DTDM bit stream.
`illustrative embodiment of the invention;
`These tributaries may be a digital phone generating 64
`FIG. 3 schematically illustrates an end-to-end net
`Kilobits/sec PCM voice, a graphics terminal sending
`work using DTDM, in accordance with an illustrative
`bursty data at l Megabit/sec, and a circuit transmission
`embodiment of the invention;
`stream operating at the DS3 rate of about 45 Megabits?
`FIG. 4 illustrates an assembler for combining diverse
`sec. Illustratively, the bit rate of the backbone DTDM
`tributary data streams into a single DTDM stream, in
`bit stream is 150 Megabits which yields 144,000 frames
`accordance with an illustrative embodiment of the in
`per second given a l30-byte frame size. The available
`vention;
`frames are shared by the three tributaries by giving
`FIG. 5 illustrates a disassembler for separating a
`higher priority to the circuit tributary, and allowing the
`DTDM bit stream into diverse tributary data streams, in
`voice and graphics tributaries to contend on a ?rst»
`accordance with an illustrative embodiment of the in
`come,,?rst-served basis. The circuit tributary seizes one
`vention;
`out of every three empty frames passing by. Thus the
`FIG. 6 illustrates a multiplexer for combining a plu
`regularity of the circuit transmission will be maintained
`rality of DTDM bit streams into a single more densely
`throughout the DTDM transmission link. Illustratively,
`occupied DTDM bit stream having the same bit rate;
`the voice source is packetized by accumulating up to 15
`FIG. 7 illustrates an N:M multiplexer for combining a
`milliseconds worth of voice samples before inserting
`plurality of DTDM bit streams;
`this information into an empty DTDM frame along
`FIG. 8 illustrates how the input lines in the multi
`with a header. In this case the voice tributary will on
`plexer of FIG. 7 are grouped;
`average seize one out of every 2,160 frames. Similarly,
`FIG. 9 illustrates a demultiplexer for separating a
`at a rate of 1 Megabit per second, the graphics tributary
`densely occupied DTDM bit stream into a plurality of
`will ?ll one frame out of 150. In this way, three diverse
`less densely occupied DTDM bit streams;
`data streams are multiplexed into a single bit stream.
`FIG. 10 illustrates a multiplexer for point-to-pointv
`As a second example, DTDM can be used as a re
`transmission.
`placement transmission technology to carry existing
`FIG. 11 illustrates a demultiplexer for use in connec
`inter-of?ce traffic. More speci?cally, consider the need
`tion with point-to-point transmission; and
`to multiplex and transmit three hierarchical signals at
`FIG. 12 illustrates a framer circuit.
`the D81, D82, and D83 rates, respectively, for point-to-'
`point transmission between two of?ces. The traditional
`TDM approach would utilize a step-by-step hierarchi
`cal approach to multiplex and to subsequently demulti
`plex these signals. ‘The conventional hierarchical multi
`plexing scheme requires line conditioning and synchro
`nization circuitry at each level of the hierarchy as well
`as hardware for bit interleaving.
`In contrast, using a DTDM multiplexer, time slots
`from each of the three signals would be inserted into the
`empty frames in a basic DTDM backbone signal. If the
`backbone signal is 150 megabits per second and com
`prises 144,000 frames per second, the DS3 signal would
`require one out of every three DTDM frames, the DS2
`signal would require approximately one out of every
`twenty-one DTDM frames and the DS1 signal would
`require approximately one out of every eighty-four of
`the empty DTDM frames.
`60
`In an actual network, the above-described DTDM
`streams at the basic backbone bit rate generally contain
`empty frames; thus DTDM streams may be multiplexed
`into more densely populated DTDM bit streams at the
`same bit rate. These more densely populated basic back
`bone rate bit streams may then be multiplexed‘ into
`higher bit rate streams for point-to-point inter-of?ce
`transmission.
`
`DETAILED DESCRIPTION
`1. DTDM Transmission Format
`DTDM is an approach to data transport which can
`handle both TDM hierarchical signals and packet traf
`?c in a common integrated structure, while allowing
`complete bit rate ?exibility. As illustrated in FIG. 1, the
`transmission bit stream is divided into frames 1. The
`DTDM frame is the fundamental unit of information
`transport in the DTDM transmission scheme. The
`frames come one after the other so as to form a continu~
`ous chain or train.
`Each frame 1 comprises two ?xed length ?elds desig
`nated transmission overhead (T) and payload in FIG. 1.
`Illustratively, each frame comprises 130 bytes with 10
`bytes being allocated to the transmission overhead ?eld.
`Typically, the bit rate of the DTDM bit stream illus
`trated in FIG. 1 is about 150 Megabits/sec. The follow
`ing information may be available in the overhead ?eld
`of every DTDM frame; frame alignment word for
`frame timing, empty/full status of the frame, and span
`identi?cation.
`As shown in FIG. 1, the payload ?eld of each frame
`may be flled with a data packet including a header (H)
`or a slot from a circuit transmission stream. However,
`
`50
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`20
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`4,893,306
`7
`8
`before a slot from a circuit transmission stream can be
`In the network 20, three multiplexing stages are re
`inserted into the payload ?eld of a DTDM frame, it
`quired to support end-to-end transport. In the user~net
`must ?rst be converted to packet-like form by the inser—
`work interface stage 30, an assembler 32 receives data
`tion of a header (H) at its front. Viewed another way,
`streams on lines 21 from the customer premises equip—
`each occupied DTDM frame comprises a transmission
`meat 22 and combines these streams into a basic back
`overhead ?eld, a header ?eld, and an information ?eld.
`bone DTDM stream of the type discussed in connection
`Thus, the DTDM transmission format is a combination
`with FIGS. 1 and 2. Similarly, disassembler 34 tears
`of the circuit transmission format and the packet trans
`apart a basic DTDM bit stream arriving on line 33 and
`mission format. The packet header provides informa=
`distributes the data to the appropriate customer prem
`tion such as channel number, line number, error detec
`ises equipment 22 via lines 23.
`tion, etc. In general, only the information required in
`As indicated above, the DTDM bit stream formed by
`every frame gets permanent bandwidth allocation in the
`the assembler 32 is not 100% occupied. Thus the multi
`transmission overhead ?eld.
`plexer 36 in the remote electronics stage 38 is used to
`FIG. 2 schematically illustrates the formation of a
`combine several DTDM bit streams arriving on lines 62
`DTDM bit stream. The DTDM bit stream assembler 3
`into a more densely occupied DTDM bit stream of the
`can combine into a single bit stream both continuous
`same bit rate to achieve greater transmission ef?ciency.
`circuit tributaries and bursty packet tributaries. Three
`Similarly, the demultiplexer 39 separates a densely pop
`such tributaries are illustrated in FIG. 2. They are: a
`ulated DTDM bit stream arriving on line 120 into less
`digital phone tributary 5 generating 64 Kilobits/sec
`densely populated DTDM bit streams transmitted via
`PCM voice, a tributary 7 from a graphics terminal send
`lines 33, so that the data contained therein can ulti
`ing bursty data at one megabit per second, and a circuit
`mately be routed to the correct customer premises
`equipment.
`transmission stream 9 operating at the DS3 rate of about
`45 Megabits/sec. Each of the three tributaries has a
`In the point-to-point stage 40, a plurality of DTDM
`characteristic shading in FIG. 2 so that it is possible to
`bit streams arriving via lines 63, 150 are time division
`follow how data from the three tributaries is combined
`multiplexed by means of time division multiplexer 42
`25
`to form the DTDM bit stream.
`for high speed point-to-point transmission via line 165 to
`To multiplex such diverse tra?ic, a train 10 of
`a network switch (not shown). For example, the multi
`DTDM frames with empty payload ?elds is generated.
`plexer 42 receives one DTDM stream via a line 63 from
`This train 10 has a bit rate which de?nes a basic back
`multiplexer 36 and another DTDM stream via line 150.
`bone transmission rate for the DTDM system. Each of
`30
`The DTDM bit stream transmitted via line 150 is
`the frames in the train 10 has an occupied transmission
`formed by DTDM assembler 43 and contains the data
`overhead ?eld (T).
`of three DS3 tributaries 45.
`Illustratively, the train of frames has a bit rate of
`Time division demultiplexer 44 receives a high speed
`about 150 Megabits per second and comprises 144K
`bit stream from a switch (not shown) via line 170 and
`blocks/sec. The assembler 10 serves to insert data from
`demultiplexes this stream into a plurality of DTDM
`35
`the tributaries 5,7,9 into the payload ?elds of the
`streams. One DTDM stream containing data for cus
`DTDM frames in the stream 10. To accomplish this, the
`tomer premises equipment goes to demultiplexer 39 via
`tributaries 5, 7, 9 are ?rst packetized using packetizers
`line 120 and another DTDM stream comprising DS3
`11, 13, 15, respectively to form the packetized streams
`slots goes to disassemblerd47 via line 179.
`17, 19, 21. Each packet comprises a header (H) and an
`40
`3. DTDM Assembler and Disassembler
`information ?eld. In the case of the tributary 5, up to 15
`milliseconds of speech samples are accumulated to form
`The function of the DTDM bit stream assembler 32
`a packet. In the case of the circuit tributary each slot is
`of FIG. 3 is to packetize each incoming data stream
`converted to packet form by placing a header at the
`associated with one particular customer service or
`front thereof.
`transmission channel and then embed these packets into
`To form the DTDM stream 10, the packets compris
`the basic DTDM transmission frames. The assembler 32
`ing the streams 17, 19, 21 are inserted into the empty
`is shown in greater detail in FIG. 4.
`payload ?elds of the empty frames in the stream 10. The
`The assembler 32 comprises a plurality of interface
`empty frames are shared by the three tributaries by
`units 50. Each interface unit 50 serves to interface an
`giving higher priority to the circuit tributary 9 and
`associated data input 21 with the DTDM bit stream. A
`allowing the voice and graphics tributaries 5, 7, to con
`DTDM bit stream comprising empty frames with
`tend for empty frames on a ?rst-come, ?rst-served basis.
`empty payload ?elds is generated by framer unit 52. A
`Thus, the circuit tributary seizes one out of every three
`detailed description of the framer unit is provided be
`frames so that the regularity of the circuit transmission
`low.
`is maintained throughout the DTDM transmission link.
`Each interface unit includes a framer unit 53. The
`Similarly, the voice tributary will seize one out of every
`framer units 52, 53 are connected together in a daisy
`2,160 frames and the graphics tributary will seize on
`chain fashion. The frames comprising the DTDM bit
`average one out of every 150 frames. It should be noted
`stream are passed along the daisy chain from one framer
`that the bit stream 12 is not 100% occupied and that
`unit to the next. More particularly, the DTDM bit
`some frames remain empty. In this way, three diverse
`stream leaves the serial data output (sdo) of the framer
`tributaries are multiplex