United States Patent
`
`[19]
`
`Bella et a1.
`
`[11] Patent Number:
`
`4,542,499
`
`[45] Date of Patent:
`
`Sep. 17, 1985
`
`[54] DISTRIBUTED CONTROL SYSTEM FOR
`MULTIPLE RANDOM ACCESS TO THE
`TRANSMISSION LINE OF A LOCAL
`NETWORK FOR SPEECH AND DATA
`
`[75]
`
`Inventors:
`
`[73] Assignee:
`
`Luigi Bella, Turin; Roberto Brignolo,
`Settimo Torinese; Giulio Barberis;
`Maurizio Sposini, both of Turin, all
`of Italy
`cselt eentro studi e Lahoratori
`Telecomunieazioni SPA, Turin, Italy
`
`[21] App1.No.: 425,858
`
`[22] Filed:
`
`Sep. 28, 1982
`
`[30]
`
`Foreign Application Priority Data
`
`Oct. 20, 1981 [IT]
`
`Italy ............................... 68352 A/81
`
`[51]
`
`[52]
`
`[58]
`
`[56]
`
`Int.cnn ........................... iro4J 3/02;11041 eztxx
`1104: 3/00
`tLS.(n. ........................................ 370/85;370/94;
`370/91
`Fhfld offikauch ....................... 370/85,89,95,94
`370/93,91
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`3,846,587 11/1974 Schenkel et al.
`..................... 370/85
`3,851,104 11/1974 Willard et a1.
`..... 370/85
`
`.
`3,859,465
`1/1975 Schenkel et a1.
`.. 370/85
`
`3,959,594
`5/1976 Srivastava
`370/95
`
`3,963,870 6/1976 Couder et al.
`370/85
`4,154,983
`5/1979 Pedersen .......
`370/89
`4,234,952 11/1980 Gable et al.
`..... 370/85
`
`4,375,097
`2/1983 Ulug ..............
`370/94
`4,402,073 8/ 1983 Hammond
`..... 370/85
`
`4,432,088
`2/1984 Frankel ................................. 370/85
`
`OTHER PUBLICATIONS
`
`B. Liibben et al., “Signalling in a Decentralized Sub-
`scriber Stage”, ISS 1979, May 1979, pp. 1240-1245.
`Tobagi, “Multiaccess Protocols in Packet Communica-
`tion Systems”, IEEE Trans. of Com., vol. Com—28,
`No. 4, Apr. 1980, pp. 468-488.
`L. West, “Carrier Sense Subset Multiple Access Sys-
`tem", IBM Tech. Disc, vol. 22, No. 11, Apr. 1980, pp.
`4811—4812.
`D. Bantz et a1., “Decentralized Bus Architecture with
`
`Collision Retry", IBM Tech. Disc, vol. 23, No. 2, Jul.
`1980, pp. 858—861.
`H. Miyahara et al., “Flexible Multiplexing Technique
`with Two Types of Packet for Circuit and Packet
`Switched Traffic", Trans. of IECE of Japan, vol. E64,
`No. 6, Jun. 1981, pp. 390-397.
`K. Tomaru et a1., “A Worldwide Integrated Service
`Communication System for Voice and Non-Voice",
`ITE and Symposium, Sep. 1981, pp. 4547.
`National Telecommunications Conference, vol. 2 of 4,
`Nov. 30—Dec. 4, 80, NY (US) D. L. Giovachino: “A
`High Speed Packet Switched Local Networ ", pp.
`41.1.1 to 41.1,5.
`Proceedings of the Fifth International Conference on
`Computer Communication, Atlanta, Oct. 27-30, 1980.
`G. C. O’Leary: “Local Access Area Facilities for
`Packet Voice", pp. 281 to 286.
`European Search Report App. No. 0077563, EP 82 10
`9637, dated Jan. 18, 1983, The Hague, Examiner: R. J.
`Wanzeele.
`
`"A Carrier Sense Multiple Access Protocol for Local
`Networks”, (Author: Simon S. Lam, Computer Net-
`works, 4 (1980) 21—32).
`“Performance Analysis of Carrier Sense Multiple Ac-
`cess with Collision Detection" (Fouad A. Tobagi and
`V. Bruce Hunt, Computer Networks, 4 (1980) 245—259).
`
`Primary Examiner—Douglas W. Olms
`Assistant Examiner—Wellington Chin
`Attorney, Agent, or Firm—Karl F. Ross; Herbert Dubno
`
`[5n
`
`ABSIIuAcr
`
`The system allows the implementation of packet-
`switched synchronous spwch communications in the
`network without requiring any centralized device syn-
`chronizing all the network stations. To obtain that, the
`active periods of each station are subdivided into frames
`each having a duration equal to the time elapsing be-
`tween the generation to two subsequent speech packets
`relating to the same communication; once a station has
`accessed the line for the speech communications, these
`communications are allotted time slots which are main-
`tained for the whole communication duration. The slots
`not allotted to speech communications are allotted to
`data transmission. The access to the line for a speech
`communication is obtained by the same contention tech-
`niques used also for data transmission.
`
`5 Claims, 12 Drawing Figures
`
`
`
`112?
`
`ST. IUDE 1026
`
`

`

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`
`US. Patent
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`
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`Sep. 17,1985
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`-
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`Sheet 1.0f10
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`4,542,499
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`IRP
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`Fig.4
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`2
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`US. Patent
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`Sep. 17,1985
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`SheetZoflO
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`4,542,499
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`3
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`

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`Ch.58am3.:.Emumag;2.5$318
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`61
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`RC
`_..._J:.’
`“" '1
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`|J
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`_ -19.--- ___
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`
`F193
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`

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`US. Patent
`
`sep. 17,1985
`
`Sheet4of10
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`4542,499
`
` CHANNEL' SLOT
`ASSOCIATION
`
`PROGRESS ?
`
` USED FOR
`
`CONTQOL?
`CHECK
`
`SPEECH
`
`
`PACKET
`
` CONTROL YES
`
`PACKET?
`
`
`
` FURTHER
`
`ACTIVE
`
`CHANNELS?
`
`
`T
`
`
`PACKETS ?
`FUR-HER
` SLOT
`
`BOOKING IN
`
`
`
`
`
`
` WAIT FOR
`
`
` WAIT FOR
`PVCU
`
`CHANNEL NAP
`
`a
`
`c
`
`o
`
`F{9.5A
`
` A
`
`TEMPORARY-
`ASSOCIATlON
`CHANNEL/SLOT
`
`SET FLAG
`0F"BOOKING
`IN PROGRESS”
`
`

`

`US. Patent
`
`Sep. 17, 1985
`
`Sheets oflO
`
`4,542,499
`
`
`BOOKING
`
`YES
`
`STORE CHANNEL
`SLOT
`ASSOCIATION
`
`
`
`
`
`
`
`
`
`
`
`
`
`RESET FLAG
`OF BOOKING
`IN PROGRESS
`
`
`
`0UT GOIN
`
`
`
`
`CONTRO
`
`
`
`
`READ CORRESPOII
`-ING SLOT m
`MEMORY
`
`
`
`CONTROL
`
`'A CKEIS?
`
`
`
`PREPARE TO
`READ SPEECH
`
`
`CHANNELS
`
`
`
`Fig.58
`
`

`

`U.S. Patent
`
`Sep. 17,1985
`
`Sheet 6oflO
`
`4,542,499
`
`PVCE
`
`WAIT FOR QQCU
`FEOM CALLING
`SUBSCQBER
`
`
`
`
`
`wm FOR RRCU
`FROM CALLED
`
`
`
`
`
`suescmam
`
`SEND DIALLNG
`TONE
`
`
`
`WAIT :0? P‘fCE
`FROM CALEB
`EUBSCR‘BER
`
`
`
`
`
`
`
`PERA’E
`QINGER
`
`
`
` WAIT FOR
`SUBSCRfiBER 'S
`
`ANSWER
`
`Fig.6A
`
`

`

`
`US. Patent
`
`
`
`
`
`Sep. 17, 1985
`
`
`
`Sheet7of10
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`4,542,499
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`F T N
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`O
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`BUSY TONE
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`p.955
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`
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`(RELEASE) ®
`
`
`
`
`
`
`ANSWER
`
`
`CALLED ,
`SUBSCRLBERS
`
`R LEASE
`
`
` CALUNG ,
`
`SUBSCRBERS
`'E EAS
`
`8
`
`

`

`
`
`US. Patent
`
`
`
`Sep. 17,1985
`
`
`
`
`Sheet 8 of 10
`
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`
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`4,542,499
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`WAIT FOR DATA
`FROM LEVEL
`
`
`
`'
`
`WAIT FOR
`RECEPTION
`
`FROM LEVEL’Z
`
`STORE
`PACKET
`
`
`
`
`FROMLEVELZ
`
`WAIT FOR DATA
`F ROM LEVEL Q
`
`
`
`WAIT FOR
`PACKETS
`I
`
`
`
`
`Fig.7-
`
`PROCESSING AND
`
`RECOGNITION
`
`
`
`
`
`AIT FOR
`
`RECEPTION
`
`E1910
`
`9
`
`

`

`U.S. Patent
`
`Sep. 17, 1985
`
`Sheet9 of 10
`
`4,542,499
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`AIT FOR Zb
`
`OR SYNC
`
`
`PREPARE PACKETS RECENEb
`III PRECEbIIIG FRAME
`
`FOR TRANSMISSION
`WAIT FOR 20
`
`I:
`YNC
`
` AIT FOR TRAN
`ISSIOII WSTAFI
`
`
`' I-l
`
`ICEORNIIE T0 CSMA
`
`OR SYNC
`
`‘
`
`Ho
`
`YES
`
`RESCHEDULE THE
`PACKET
`
`
`ISSION INSTANT
`
`AIT FOR TRAHS'
`
`WAIT FOR CI)
`
`
`
`RESCHEBULE TIIE
`PACKET
`
`
`
`" KET TRAHSHI
`VER THE CHAIIII
`
`.
`
`AIT FOR TRAHS'
`
`IS ION INSTAH
`
`
`
`
`
`F193
`
`
`
`
`
`HA 0F PACKET'
`
`RAHSMISSIOH
`
`AIT FOR Zb
`0R SYNC
`
`10
`
`

`

`US. Patent
`
`Sep. 17, 1985
`
`Sheet 10 oflO 4,542,499
`
`WA IT FOR
`TRANSHISSION
`
`'
`
`
`
`
`
`PROCESflNG
`
`T
`TRANSNISON
`YN
`
`
`
`
`
` YES
`TR14?NVSIGISSION
`
`\T F0 ’
`
`FURTHER
`PACKETS
`
`TRANSMISSIO
`
`Fig.9
`
`11
`
`

`

`4,542,499
`
`1
`
`
`DISTRIBUTED CONTROL SYSTEM FOR
`
`
`
`
`MULTIPLE RANDOM ACCESS TO THE
`
`
`
`
`
`TRANSMISSION LINE OF A LOCAL NETWORK
`
`
`
`
`
`FOR SPEECH AND DATA
`
`
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`10
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`15
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`20
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`25
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`30
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`35
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`4O
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`OBJECT OF THE INVENTION
`
`
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`It is an object of the invention to provide an access
`system for speech and data transmission which both
`
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`offers the advantages typical of the contention access
`
`
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`systems and satisfies the speech synchronism require-
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`
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`
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`ments.
`
`Our present invention provides a process for multiple
`
`
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`
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`random access to a transmission line in a local network
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`for data and speech transmission, comprising a plurality
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`of stations which mutually exchange information
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`through said line in packet form, characterized in that at
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`each station, under the exclusive control of a local tim-
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`ing system, the station octivity is organized into frames
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`whose duration is equal to the time elapsing between
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`the generation of two consecutive packets relating to
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`the same communication of a kind requiring mainte-
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`_ nance of synchronous characteristics (speech communi-
`cations) and, during each frame, the speech communica-
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`tions are allotted time slots equal to the duration of
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`transmission of a packet which time slots are reserved to
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`said communications for their whole duration,
`thus
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`implementing synchronous communication channels;
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`characterized also in that a timing map of the line occu-
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`pation by the synchronous channels is built up at every
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`frame at each station independently, only monitoring
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`the transmission line and in that the frame periods not
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`allotted to active synchronous channels are utilized
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`both for communications which do not require the syn-
`chronous characteristics to be maintained (data commu-
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`nications) and for the booking of further synchronous
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`channels by each station, the time slots which may be
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`allotted to synchronous channels forming a predeter-
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`mined and not exceedable fraction of a frame duration.
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`BRIEF DESCRIPTION OF THE DRAWING
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`The characteristics of the invention will be better
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`understood from the following description of a pre-
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`ferred embodiment, given by way of example and not in
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`a limiting sense, depicted in the annexed drawings, in
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`which:
`
`FIG. 1 is a schematic representation of a local net-
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`work;
`
`FIG. 2 is a block diagram of the access device in a
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`terminal;
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`FIG. 3 is a circuit diagram in block form of the first
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`hierarchic level of the access device;
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`FIG. 4 is a block diagram of the interface of the first
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`hierarchic level; and
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`FIGS. 5a, 5b, 6:1, 6b, and 7 to 10 are flow-charts of the
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`operations carried out by the device.
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`SPECIFIC DESCRIPTION
`
`
`In FIG. 1 a local network RL comprises a high-speed
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`bus 1 to which a plurality of stations or terminals T1, T2
`
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`. Tn are connected through lines 2-1 .
`. 2-11. Each
`.
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`station comprises the actual speech and data subscriber
`
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`sets AUl, AU2 .
`.
`. AUn and the device DAl, DA2 .
`.
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`. DAn for the access to the network according to con-
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`tention techniques. An interface IRP connects the local
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`network to the public network (not shown).
`
`
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`
`. Tn
`Subscriber sets AUl .
`. AUn of stations Tl .
`.
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`can consist e.g. of the whole of the computing resources
`
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`and telephones belonging to the served area.
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`Bus 1 can be realized by any physical transmission
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`media, e.g. a copper or optical fiber cable, and its nature
`
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`has no importance to the purposes of the invention.
`
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`However the invention finds its most advantageous
`
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`2
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`.
`
`BACKGROUND OF THE INVENTION
`
`
`
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`The present invention relates to the so~called “local—
`
`
`
`
`
`
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`networks", i.e. communications networks able to inter—
`
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`connect a large number of stations or terminals in a
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`restricted area. More particularly the invention relates
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`to a system for the accessing of a plurality of stations or
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`terminals, capable of transmitting both data and speech,
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`to a transmission line in one of such networks in which
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`the information is transmitted in packet form.
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`FIELD OF THE INVENTION
`
`
`
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`Packet-switched local networks were originally de-
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`signed for data transmission only; however the present
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`technique is oriented to use such networks also for
`
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`speech transmission and therefore network topologies,
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`access systems and transmission procedures suitable for
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`both types of traffic are necessary.
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`It is known that for packet-switched data transmis-
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`sion networks and for distances less than 1 km, the most
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`widely used topology comprises a wide—band bus (con-
`
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`sisting of an optical fiber or a coaxial cable), which
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`different terminals can access according to contention
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`
`
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`
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`(or random multiple access) techniques, such as the one
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`
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`known as CSMA (carrier-sense multiple access).
`
`
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`In these systems each terminal can send its message to
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`the network at any instant, provided the line is not busy;
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`in that case the transmission is rescheduled for a subse-
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`quent instant. As the line occupation by a terminal be—
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`comes known to the other terminals with a certain delay
`
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`due to the propagation time, collisions, i.e. substantially
`
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`simultaneous access requests, can arise; the collisions
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`are resolved by suitable procedures, well known in the
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`art, owing to which the packets involved in the collision
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`are retransmitted after a suitable and random delay.
`
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`Said systems are preferred because they do not re-
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`
`
`
`
`
`quire any centralized device synchronizing all the net-
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`work stations. Consequently their construction is sim-
`
`
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`pler and their operation more reliable, as they do not
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`present the reliability node formed by the synchronism
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`system.
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`Should the network be used also for speech transmis-
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`sion, speech packets would be subjected to collisions
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`and retransmissions like data packets. Therefore, they
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`would undergo transmission delays varying from
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`packet to packet, which delays would cause at the re-
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`ception side a degradation of the voice quality which
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`strongly depends on the synchronism characteristics.
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`This degradation could be tolerated inside a private
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`network but cannot be accepted when the communica-
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`tion is to be routed onto the public network.
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`To meet these speech synchronism requirements the
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`solutions so far proposed to allow the use of local net-
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`works also for speech transmission are based on meth-
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`ods requiring a central synchronism,
`such as
`the
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`TDMA (Time-Division Multiple Access), wherein
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`frames are built up in which each terminal is allotted its
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`own time slot. In this way the advantages of circuit
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`simplicity and reliability typical of contention access
`systems are sacrificed to speech transmission quality
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`requirements.
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`45
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`50
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`55
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`60
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`65
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`12
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`12
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`4,542,499
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`25
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`3
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`utilization in the cases in which bus 1 is unidirectional;
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`the reasons for this will be expounded later on.
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`Devices DAl, DAZ .
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`. DAn, which form the object
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`of the invention, should allow the transmission of
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`speech and data packets on the bus so that the synchro-
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`nous characteristics of the speech are maintained with-
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`out requiring a centralized synchronization of the termi-
`nals; in this way the advantages present in the conten—
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`tion access techniques are maintained.
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`To obtain that, according to the present invention a 10
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`timing map of the line occupation by speech communi-
`cations is built up at each terminal, which map is contin-
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`uously updated and has a duration equal to the time
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`elapsing between the generation of two consecutive
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`speech packets relating to the same communication.
`Such time, which owing to speech characteristics is to
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`be constant, will be hereinafter referred to as a “frame”.
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`Since speech packets must have a fixed duration the
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`map can be easily implemented by memorizing the in—
`stants of beginning and end of the speech packet trans—
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`mission. Such a map will be used for each new speech
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`communication or for data packet transmission, thereby
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`controlling the access of the station to the bus. Data
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`packet transmission or new telephone calls will take
`place at instants not allotted to the packets of ongoing
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`speech communications. In this way,
`if the network
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`operates correctly, only data packets or packets de-
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`signed to book the line for a speech communication can
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`undergo collisions, i.e. packets which do not require the
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`maintenance of synchronous characteristics.
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`A correct memorization of the line state is to be re-
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`ferred to the point of transmission on the bus.
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`In case of a bidirectional bus, the transmission and
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`reception points coincide and therefore the map built up
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`at the reception coincides with that available for the
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`transmission. However, the bus being bidirectional, the
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`actual propagation delays depend on the relative termi-
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`nal position, which is generally unknown; therefore, the
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`end of the transmission by a terminal must precede the
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`actual instant of detection of the activity on the bus by
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`a time equal to twice the maximum propagation delay in
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`the bus, in order to take into account the most unfavor-
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`able condition (packet starting from one end and ad-
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`dressed to the other end); these guard times can be of 45
`the same order of magnitude as the time required to
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`transmit a packet at the line rate and then they can be
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`very heavy. A delay equal to twice the actual propaga—
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`tion time between the involved stations could be possi-
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`bly introduced, if such time was known, but this would 50
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`entail, at each terminal, reading the starting address of
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`each packet and,
`in case of network modifications,
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`would require reinitialization of the whole system.
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`In the case of unidirectional bus the transmission and
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`reception points of a terminal are separate; in this case
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`however the propagation delay between transmitting
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`and receiving points is fixed and can be determined
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`while initializing the terminal, so that it is easy to re—
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`cover the transmission map from the map built up at the
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`receiving point by simply “anticipating” the instants
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`memorized by a time equal to said delay; consequently
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`there is no necessity to introduce guard times to take
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`into account propagation delays, and therefore the bus
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`exploitation is greatly improved.
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`Booking packets for a telephone call must be as long
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`as the actual speech packets, so as to provide a slot for
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`that call in the frame. Once the booking packet has been
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`transmitted over bus 1, the speech packets will be trans-
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`4
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`mitted in the subsequent frames during the same time
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`slot previously occupied by the booking packet.
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`Of course no problems arise if that time slot was
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`allotted to data transmissions in the previous frames,
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`since, as already stated, such transmissions do not re-
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`quire maintenance of time correlations. If the booking
`packets collide,
`the retransmission procedures com-
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`monly used in the contention access techniques are
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`utilized.
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`From what is stated above, it comes out that tele-
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`phone transmissions have a certain priority with respect
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`to data transmissions. However, considering frames
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`with a duration of some ten milliseconds (say, 20 ms),
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`packets built up of a thousand bits and transmission rates
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`on the bus of some tens Mbit/s (e.g. 30 of Mbit/s), the
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`transmission of a packet takes some ten microseconds
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`(about 30 us, with the data considered), so that even in
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`case of heavy speech traffic, the 20 ms frame has space
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`enough for data transmissions; when necessary, an
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`upper limit can be established for the number of tele-
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`phone calls.
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`In FIG. 2, in conformity with the usual representa-
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`tions of the access systems implementing internationally
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`the
`(e.g.
`standardized data transmission protocals
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`CCITT X25 protocol), access device DAI has been
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`subdivided into three hierarchical levels, schematically
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`represented by blocks L1, L2, L3. The first level, shown
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`in detail as it is the most concerned by the present inven—
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`tion, performs as a logic and physical interface with the
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`bus; the second level is entrusted with the control and
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`supervision of the connection, while the third manages
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`the calls, as to speech, and the packets, as far as data are
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`concerned.
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`More particularly, the tasks of block L1 are:
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`(a) serial digital
`transmission and reception on the
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`wide-band bus;
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`(b) local synchronization;
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`(c) realotime reconstruction of the line occupation by
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`speech communications;
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`(d) reception and temporary storage of the packets
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`(speech and data) addressed to the station;
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`(e) speech packet transmission management, so that
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`the transmission takes place at the instants estab—
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`lished by the higher hierarchical level (block L2);
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`(f) data packet transmission management, so that the
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`transmission takes place in the zones not allotted to
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`speech packets and according to the access system
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`and protocal chosen; by way of example reference
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`will be made to CSMA-CD system (carrier sense
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`multiple access with collision detection).
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`It is to be mentioned that operations a, d, f (as to the
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`part relating to CSMA) are identical to those provided
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`at level 1 of a standard data transmission protocol.
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`To perform these tasks, block L1 comprises, from a
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`functional standpoint, an interface IB for connection to
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`bus 1, the circuits managing data and speech transmis-
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`sions (blocks TD, TV), a receiver RC for data and
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`speech packets, a memory of the line state, MSC, a local
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`oscillator 0L and a time base BT timing the operations
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`inside the terminal.
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`Interface 18, which is connected to bus 1 through line
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`2-1, receives from blocks TD and TV, through connec~
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`tions 3, 4, data and speech packets to be sent to bus 1
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`upon a suitable modulation, and through connection 5 it
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`transfers to receiver RC, upon suitable demodulation,
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`the packets addressed to the terminal
`it belongs to.
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`Besides, IB sends to data transmitter TD and to memory
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`MSC, through connection 6, control signals relating to
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`5
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`15
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`20
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`'
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`30
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`35
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`4O
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`55
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`60
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`65
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`13
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`13
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`

`

`4,542,499
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`10
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`15
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`20
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`25
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`30
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`35
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`4O
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`5
`the line state (in particular beginning and end of carrier
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`and collision detection). The operations of IB are timed
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`by local oscillator 0L (connection 7).
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`Data transmitter TD receives from block L2, through
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`connections 8, 9, and stores the data packets to transmit
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`and the indications of the time slots in which such trans-
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`mission is possible, and manages data transmission with
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`the modalities required by a CSMA-CD system.
`In
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`addition TD sends to block L2, through a wire of con-
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`nection 8, a signal indicating that it is ready to receive
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`the packets.
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`For its operations TD receives suitable timing signals
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`from time base BT (connection 10).
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`Speech packet transmitter TV receives from block
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`L2, through connection 11, and stores speech packets to
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`be transmitted the related indications of the beginning
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`time instants of the periods allotted to such transmis-
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`sions, and effects in such periods IB. Also TV is con-
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`trolled by time base BT through signals present on
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`connection 10.
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`
`Receiver RC receives in real time from interface IB
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`through connection 5,
`the packets addressed to the
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`terminal it belongs to and transfers them to block L2
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`through connection 12. This transfer occurs upon re-
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`quest by the higher hierarchical level.
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`Memory MSC builds up in real time on a frame by
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`frame basis the busy period map necessary for the
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`higher levels to receover in each frame the time slots
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`allotted to speech packets. To this aim MSC receives
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`from interface IB the signals emitted on connection 6
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`and relating to the events occurred on the line, and from
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`ET the information on the time instants relating to the
`events communicated by interface IB, and sends to L2,
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`through connection 13, the busy-state map relating to
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`the preceding frame. Also the information stored in
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`MSC is transferred upon request of the higher hierar-
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`
`chical level.
`
`
`Time base BT receives from local oscillator OL,
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`through wire 7, the fundamental clock signal and gener-
`ates the local time references which are supplied to the
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`other blocks of L1 or to the higher levels through con-
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`nection 10. In particular time base ET is to supply a
`pattern which indicates, with the precision of the signal
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`generated by OL, the current time instant within the
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`frame, and to supply the frame synchronism and the
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`synchronism requested by the speech on the input chan-
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`nels.
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`The circuit implementation of a number of functional
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`blocks of L1 will be described with reference to FIGS.
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`3, 4.
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`As to the higher hierarchical levels L2, L3 of the
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`access device, they can be functionally subdivided into
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`a first part implementing the higher levels of the data
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`protocol chosen (blocks GDl, GD2,
`in L2, L3) and
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`another part managing the telephone calls (blocks GV1,
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`GV2). L2 comprises also an interface ID connecting
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`block DGl and the data transmission devices in L1.
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`Interface ID is to recognize data packets in the packet
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`flow arriving from RC and to send them to GDl
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`through connection 14. In the opposite direction ID
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`receives from GDl and transfers to L1 the data packets.
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`The operations of ID are synchronized by time base BT
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`through signals sent over connection 10.
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`Block GDl operates in a way similar to what estab-
`lished for the systems which do not employ common-
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`bus topologies, therefore its operation requires no de-
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`tailed description. Line 15 schematizes the connection
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`between GDl and GD2.
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`14
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`6
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`As to the ongoing telephone calls, at each frame GV1
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`recognizes in the packet flow coming from RC the
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`packets relating to such calls,
`forwards the actual
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`speech packets to the various “channels” of AUl (i.e. to
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`the various subscribers connected to T1) and sends
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`GV2 the control packets relating to the same calls. The
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`opposite operations will be carried out for the packets
`to transmit.
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`To this purpose block GV1 comprises a memory
`storing the association between the time-slots allotted to
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`speech in the frame and the subscribers.
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`As to new calls, GV1 is to ask MSC the line state and
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`on the basis of such state is to book a time slot for the
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`transmission; the association of such slot with the sub-
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`scriber will be carried out in the memory only upon
`detection that no collisions occurred.
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`The connections of GV1 to the lst level devices have
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`already been examined. The connections of GV1 to the
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`subscriber sets are schematized by line 16, comprising as
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`many bidirectional “channels” as are the sets in AU1.
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`The connections of GV1 to the control devices of the
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`speech transmission in L3 are schematized by line 17,
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`which conveys the control packets relating to incoming
`or outgoing calls, signals relating to the beginning and
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`end of the outgoing calls as well as the possible control
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`signals. A wire 18 used by L3 to signal the end of an
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`outgoing call has been separately represented.
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`As to the third level, block GD2 carries out the tasks
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`of the 3rd level of the data protocol, while block GV2
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`sets up and releases the calls and has the signalling tasks
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`well known in telephony.
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`Blocks ID, GV1, GV2, GDl, GD2 can be imple-
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`mented by a conventional computer.
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`The tasks of GDl, GD2 will not be described in
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`further detail, as they are identical to those of the higher
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`levels in the access protocols for data only.
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`In FIG. 3, block TD comprises a buffer BUl, e.g. of
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`the FIFO type, which receives through wires 80 of
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`connection 8 the data packets coming from ID (FIG. 2).
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`These packets are then serially transferred to IE
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`through connection 31 in cable 3 which comprises a
`further wire 30 for the control signals.
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`The packets are transferred to 1B under the control of
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`a logic network LC1, which controls the access to the
`bus according to the CSMA technique. More particu-
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`larly LCl causes the reading of BUl (and therefore the
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`transmission) in the absence of collisions and resch-
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`edules the transmission in case of collisions; in addition
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`LC1 signals to the higher levels that the buffer is full
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`(wire 81). LC1 is preset to operation by a signal indicat—
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`ing the presence of data packets to be transmitted
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`(which signal is sent by BUl through wires 21) and is
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`enabled,
`in the time intervals not allotted to speech
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`transmissions, by a signal ZD coming through a wire 20
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`by a coincidence circuit CCl, which will be described
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`hereinbelow. In order to operate corre