A LOCAL AREA COMMUNICATION NETWORK

Abstract: A local area communication network based upon a broadcast communication system comprising an inbound channel (20) and an outbound channel (21), a plurality of stations (S1-SN) connected to both the inbound and the outbound channels, transmitting on the outbound channel (21) which utilizes an access protocol where the access protocol used by the stations (S1-SN) connected to the bus is a distributed algorithm and is based upon a conflict-free round robin (RR) access scheme. The time required to switch from one active user to the next in a round is minimized (on the order of carrier detection time) and is independent of the end-to-end network propagation delay. This improvement is particularly significant when the channel data rate is so high, or the end-to-end propagation delay is so large, or the packet size is so small as to render the end-to-end propagation delay a significant fraction of, or larger than, the transmission time of a packet. Moreover, some features of the present invention make it particularly suitable for voice applications. In view of integrating voice and data, a voice/data protocol is described which allows it to meet the bandwidth requirement and maximum packet delay constraint for voice communication at all times, while guaranteeing a minimum bandwidth requirement for data traffic. The voice/data protocol constitutes a highly adaptive allocation scheme of channel bandwidth, which allows data users to steal the bandwidth unused by the voice application.

Description: A LOCAL AREA COMMUNICATION NETWORK Background of the Invention The present invention relates to * a local area communication network. A great deal of discussion can be seen in the recent litera¬ ture regarding local networks and their applicability to many of today's local area communication needs. These needs have primarily consisted of data communication applications such as computer-to-computer data traffic, terminal-to-terminal data traffic, and the like. More recently, a new line of thought has been apparent. It is the desire to integrate voice communication on local data networks. The reason for this is threefold: (i) voice is an office communication application just as computer data, facsimile, etc.; (ii) re¬ cent advances in vocoder technology have shown that digitized speech constitutes a digital communication application which is within the capabilities of local area data networks; and (iii) today's local network architectures, especially the broadcast bus type, offer very elegant solutions to the local communications problems, from both the point of view of flexibility in satisfying growth and variability in the envi- ronment. While existing solutions are elegant, they are not without their limitations in performance. Some of these limitations arise as the characteristics of the environment and data traffic requirements being supported by these solutions devi¬ ate from those assumed in the original design. Examples of εuch characteristics are: Packet length distribution, packet generation pattern, channel data rate, delay requirements and geographical area to be spanned. One prior art approach is a bidirectional broadcast system (BBS) architecture based on a carrier sense multiple access (CSKA) mechanism as exemplified by the so-called ETHERNET. Another prior art approach is a unidirectional broadcast sys¬ tem (UBS) architecture with a round-robin access scheme. A problem with a CSMA bidirectional broadcast system is that it can be shown that the effort required to achieve higher channel bandwidths with the hope of achieving a network throughput proportional to the channel speed is unfortunately rewarded only by a marginal improvement. The limitation with unidirectional broadcast systems operat¬ ing with the existing round-robin scheme is that the maximum throughput for such a system exhibits similar performances as for a CSKA bidirectional broadcast system, and as such both approaches suffer of an overhead per packet transmis¬ sion. There is a need therefore for an improved local area communi- cation network which overcomes the problems of prior art sys¬ tems. In view of the above background, it is an objective of the present invention to provide an improved local area com¬ munication network. • Summary of the Invention The present invention relates to a local area communication network. In one embodiment, the present invention is based upon a uni¬ directional broadcast system and utilizes an access protocol based upon a Round Robin access protocol. In another e bodi- ent, the present invention does not require signal propaga¬ tion to be unidirectional, and still utilizes a round-robin access protocol. In either case, the present invention, however, is performance-wise more efficient than existing schemes and overcomes many of the limitations. Moreover, the present invention can be shown to be particularly suit¬ able for the integration of voice and data applications hav¬ ing a simple voice/data protocol which allows the network to meet bandwidth requirements and maximum packet delay con- εtraints for voice communication at all times, all guarantee¬ ing a minimum bandwidth requirement for data traffic. These requirements are satisfied by blocking requests for voice communication which exceed a maximum allowable number, where this maximum number is given as a function of the vo¬ coder's rate and a minimum data bandwidth requirement, and is easily enforceable in practice. In accordance with the foregoing summary, the present inven- tion achieves the objective of providing an improved local area communication network. Other objects and features of the present invention will be¬ come apparent from the following detailed description when taken in conjunction with the accompanying drawings. Brief Description of the Drawings Fig. 1 depicts a representation of maximum throughput of prior art local area communication networks. Figs. 2A-2G depict channel taps which could be utilized with the present invention. Figs. 3A and 3B depict examples of block diagrams of a local area communication network topology according to the present invention. Figε. 4 and 5 depict timing diagrams for illustrating the operation of the present invention. Fig. 6 depicts a flow chart illustrating the sequence of operations for the access protocol according to the present invention. Fig. 7 depicts a finite state machine implementing the access protocol according to the present invention. Fig. 8 depicts a flow chart for the initialization portion of the voice/data algorithm according to the present inven¬ tion. Fig. 9 depicts a flow chart for the main portion of the voice/data algorithm executed by a station in the local net¬ work. Fig. 10 depicts a finite state machine implementing the ini- tialization portion of the voice/data algorithm. Fig. 11 depicts a finite state machine implementing the main portion of the voice/data algorithm executed by a station in the network. Fig. 12 depicts a block diagram of a station according to the present invention. Fig. 13 depicts a more detailed diagram of a station accord- ing to the present invention. Detailed Description of the Drawings In the bidirectional broadcast system (BBS) architecture, all devices share a single communication medium, typically a co¬ axial cable, to which they are connected via passive taps. When transmitted by a device, signals propagate in both di- rectionε, thus reaching all other devices. The device inter¬ face is such that it recognizes and acceptε meεεageε address¬ ed to it. The difficulty in controlling acceεε to the channel by users who can only communicate via the channel itself has given riεe to what iε known aε random acceεε techniqueε. The best known such schemes are ALOHA and Carrier Sense Multiple Ac¬ cess (CSKA). In the ALOHA scheme, userε tranεmit any time they deεire. When conflicts (consiεting of overlapping packet transmission) occur, the conflicting users schedule retranεmisεion of their packetε to some later time, incurring a random rescheduling delay. In CSMA, the risk of a colli¬ sion is decreased by having userε sense the channel prior to transmission. If the channel iε sensed busy, then tranεmiε- εion iε inhibited. There are εeveral CSMA protocols. In the so-called nonperεiεtent CSMA, a uεer which findε the channel buε εimply εcheduleε the retranεmisεion of the packet to some later time. In the p-persiεtent CSMA, a user which finds the channel busy monitors the channel, waits until the channel goes idle, and then performs the "p-process" which conεiεts of tranεmitting the packet with probability p, and waiting the maximum end-to-end propagation delay with probability 1-p; at thiε point in time, it senses the channel and again if the channel iε sensed idle, it repeats the p-procesε, otherwise it repeatε the entire procedure. Given the phyεical characteristics of digital trans iεsion, on coaxial cables, in addition to sensing carrier it is pos- εible for tranεceivers to detect interference among εeveral of their colliding packetε. Thiε giveε riεe to a variation of CSMA which iε referred to aε Carrier Senεe Multiple Access with Colliεion Detection (CSMA-CD). In the so-called Ethernet network, uεers employ the p-persistent CSMA-CD pro- tocol along with a scheduling procedure, called the binary exponential back-off; it conεiεtε of having the colliding uεerε incur a random delay whoεe mean iε doubled each time a colliεion iε incurred. — D — The performance of a bidirectional broadcaεt εyεte iε heavi¬ ly dependent on the particular accesε method uεed, and iε characterized by two main measures: channel capacity and throughput-delay tradeoff. For a given acceεs mode, channel capacity is defined aε the maximum throughput that the net¬ work iε able to support. The throughput-delay measure iε the relationεhip which exiεtε between packet delay and channel throughput. It εhould be clear that, due to colliεionε and retranεmiεεionε, channel capacity iε alwayε below the avail- able channel bandwidth, and that throughput and delay have to be traded off; the larger the throughput iε, the larger iε the packet-delay. Let W denote the channel bandwidth (in bitε/εecondε), d the length of the cable between the extreme uεerε, and B the num¬ ber of bitε per packet (aεεuming fixed size packetε). Let T denote the end-to-end delay defined aε the time from the εtarting of a tranεmiεεion to the starting of reception be¬ tween the extreme uεerε, and T denote the tranεmiεεion time of a packet; i.e., T=B/K. In all CSMA protocols, given that a tranεmiεεion iε initiated on an empty channel, it iε clear that it takeε at most T εec. for the packet tranεmiεεion to reach all deviceε; beyond thiε time the channel iε guaranteed to be εenεed buεy for aε long aε data tranεmiεεion iε in pro- greεε. A colliεion can occur only if another tranε iεsion iε initiated before the current one iε εenεed. Thus, the first T εec. of a packet tranεmiεsion repreεentε itε (maximum) vul¬ nerable period and playε a key role in determining the per¬ formance of CSMA protocolε. Among all protocolε previouεly mentioned, the p-perεiεtent CSMA-CD provideε the beεt per¬ formance, and thuε iε conεidered aε the benchmark for CSMA εchemeε. The reεultε are derived from a worεe caεe analyεis in which the vulnerable period of a packet iε alwayε con¬ εidered equal to itε maximum. Although it iε poεεible to predict better performance if one took into account the fact that the vulnerable period of a tranεmiεεion iε variable, the reεult would become dependent on the particular environment (namely, the geographical distribution of devices) and the particular traffic pattern (i.e., the εource-deεtination traffic diεtribution), and would be rather difficult to evaluate. In order to εtudy the limitationε of the εcheme considered and in an attempt to keep the reεultε aε general aε posεible, a worεe caεe analyεis iε undertaken throughout the paper for all εchemeε conεidered. It has been shown that the throughput of the slotted nonper- εiεtent CSMA-CD εcheme iε given by S = Tg e~9 (1) Tge"9 + (l-e~9-ge~9)T c + 1 where g iε the rate at which εtationε become ready during a slot of εize τ» T is again the tranεmiεεion time of a packet and T c iε the time needed to detect a colliεion and εtop transmitting. Both T and T c are measured in units of slotε. Setting T c to itε minimum value (T c =2), E iε maximized, aε T changeε, for a value of g practically conεtant and equal to .65. The maximum throughput (or channel capacity) S max iε therefore given by 'max" (2) T+3.76 1+3.76 Using eεtimateε for the various delays through interface com¬ ponents and for the propagation delay in coaxial cableε aε given in the Ethernet Specificationε, one findε that a rough eεtimate for T iε 10 microεecondε for each km of cable (aε- εuming one repeater for each 500m of cable), and thuε the product TW iε equal to 10 bitε for each MBPS bandwidth and each km of cable. In Fig. 1 S max iέ plotted aε a function of W for variouε valueε of the ratio d/B, where d iε in kilometerε and B in kilobits. Consider for example B=l KB and d=l Km. While with W=l MBPS one can achieve a throughput of 0.96 MBPS, with W=10 MBPS, S max is equal to 7.26 MBPS, and when W=20 MBPS then S Inax =11.4 MBPS. If packets are shorter, say 250 bits, then with W=l MBPS S max =0.87 MBPS; with W«10 MBPS, S max is only 4 MBPS. The effort required to puεh the technology to higher channel bandwidth with the hope to achieve a network throughput proportional tp the channel speed iε unfortunately rewarded only by a marginal improvement. The need to εeek better solutions is thus clear. Just aε with BBS, a unidirectional broadcaεt εyεtem (UBS) conεiεtε of a εingle buε to which all uεerε are connected. Tranε itting tapε, however, are such that the tranεmitted εignalε are forced to propagate in only one direction, attenuating heavily the εignalε in the oppoεite direction. Broadcaεt communication iε then achieved by variouε eanε, εuch aε folding the cable, or repeating all signalε on a εeparate channel (or frequency) in the reverse direction, εo that εignalε tranεmitted by any user reach all other uεerε on the reverεe path. Thus the εyεtem may be conεidered aε con- εiεting of two channels: the outbound channel which all uεerε acceεε in order to tranεmit, and. the inbound channel which uεerε acceεε in order to read the transmitted informa¬ tion. In addition to the tranεmitting capability on the out¬ bound channel, uεerε can senεe activity on that channel in a way εi ilar to that required in other channel εenεing εyεtems εuch aε CSMA. In a UBS thiε capability reεultε in an inter- eεting feature. Aεεu e userε are numbered sequentially SI, £2, S3, etc., and user SI is defined aε the "fartheεt", i.e., poεεeεεing the longeεt round trip delay between itε tranε¬ mitting tap and itε receiving tap. Becauεe of the unidirec- tional signalling property, S2 is able to εenεe εignalε gene¬ rated by SI on both the inbound and outbound channelε whereas the oppoεite does not hold; that iε, SI can εense εignalε generated by S2 only on the inbound channel. Thiε aεy metry iε utilized in establishing the ordering required for a round-robin (RR) scheme, which iε now described. In the RR algorithm, a user iε conεidered to be in one of three εtateε. It iε in the IDLE εtate if it doeε not have any meεεage awaiting tranεmiεεion. A non-idle uεer, called a ready uεer, aεεumeε for fairneεε purpoεeε one of two εtateε: ACTIVE if it haε not tranεmitted itε meεεage in the current round, or DORMANT if it did tranεmit in the round and iε waiting for the completion of the round. In order to achieve fair εcheduling, DORMANT uεerε defer to all other ACTIVE uεerε. Conεeσuently, it iε apparent that no user will tranε- mit itε εecond meεεage before other ready uεerε have had a chance to tranεmit their firεt ones. Eventually all ready uεerε will have tranεmitted their meεεage (i.e., they all become DORMANT); thiε conεtituteε the end of a round; at thiε time all uεerε reεet their εtate to ACTIVE and a new round εtartε. While each uεer diεtinguiεheε between itε DORMANT and ACTIVE εtateε, arbitration among active uεerε must be provided by additional means. To that end each ACTIVE uεer transmits a short burεt of unmodulated carrier after the end of the previouε meεεage haε been detected on the inbound channel, thuε indicating itε ACTIVEness and at the same time senεeε the outbound channel. All but one ACTIVE uεer will εenεe the outbound channel busy (due to tranεmiεεion from uεerε with a lower index) thuε singling out the next uεer to tranε¬ mit. If a given uεer εenεes the outbound channel buεy, then there existε at leaεt one ready uεer "ahead" of it which generated that signal; a user will alwayε defer itε trans- miεεion in favor of thoεe "ahead" of it. Initially all uεerε reεet their εtate, meaning that all ready uεerε are ACTIVE. An ACTIVE uεer will operate aε follows: 1. Wait until the next end-of-carrier (EOC) iε detected on the inbound channel. 2. Tranεmit a short burst of unmodulated carrier and listen to the outbound channel for one round trip delay. 3. If during that period the outbound channel is sensed idle, the uεer tranεmitε itε meε¬ εage and switches to the DORMANT εtate. Otherwise, the uεer repeatε the algorithm. A DORMANT uεer becomeε ACTIVE if the inbound channel iε εenεed idle for one round trip delay or longer and then per¬ forms the above εtepε. In other round-robin diεciplineε, each uεer, in a preεcribed order, iε given a chance to tranεmit; it tranεmitε if it haε a mesεage ready and declineε if it haε not. Thiε uεer will not be given a εecond chance before all other uεerε have had their chance. In the preεent algorithm, while no uεer tranε- itε more than once within each "round", the order of tranε¬ miεεion within the round may vary depending on the inεtantε at which meεεageε arrive. For example, aεεume that uεer SI juεt completed tranεmiεεion of itε meεεage. Aεεume alεo that at thiε time S2 doeε not have a meεεage ready and therefore S3, aεεumed ready, tranεmitε next. While S3 iε tranεmitting, a meεεage arriveε at S2, and conεequently when S3 iε finiεh- ed, S2 tranεmitε next. The order of tranεmiεεion in thiε caεe iε SI, S3, S2 while if all had a ready meεεage at the beginning of the round the order would have been Si, S2, S3. From the above algorithm, it iε clear that all tranεmiεεionε are conflict free. The time separating two conεecutive packet tranεmiεεionε dependε on the relative poεition of the deviceε tranεmitting them. The beεt caεe iε observed when the εucceεεive tranεmitting uεerε are among the higheεt indiceε. In thiε caεe neglecting the tranεmiεεion time of a burεt aε well aε the delay incurred between the two tapε (on the inbound and outbound channelε) of the εubεcriber with the higheεt index, it can be εeen that the time εepa- rating two conεecutive tranεmiεεionε can be aε low aε 2 r εec., i.e., the longeεt round trip delay. The worεt caεe iε observed when the two εuccesεively tranεmitting uεerε are with the loweεt indiceε. In that caεe the gap iε about 4 * εec. Note alεo that due to the tranεmiεεion of burεtε when not all uεerε are in the DORMANT state, the inbound channel iε never idle for a period longer than 2 T εec. The channel idle time will exceed 2τ εec., if either all uεerε are DOR- MANT or all uεerε are IDLE. Thuε it can be εeen that the gap between two roundε in the former caεe iε between 4 T and 6τ. To εtudy the channel capacity of a UBS employing the RR algo- rithm, aεεume that K uεerε are alwayε ready to tranεmit (i.e., we aεεume heavy traffic conditions). In this case the maxi¬ mum throughput iε given by MT s MT (3) M(T+4τ)+67 ιr ' £ ~M(T+2τ)+4τ The lower bound iε alεo expreεεed aε 'ir.ax B (4) B+ τW + 6 T W M If M iε not too εmall, then one can neglect 6 W/M and get Smax = 1 (*> 1 x 4 -^ Note: The upper bound on channel capacity can be achieved if, following the tranεmiεεion of the burεt, a device waitε a period of time εhorter than 2τ, namely 2*r inuε the propa¬ gation time between itε tap on the outbound channel and itε tap on the inbound channel. The drawback of εuch a mode of operation, however, iε that thiε parameter haε to be tuned according to the poεition of the device on the cable. In Fig. 1, the lower bound on or the RR algorithm iε plotted. Both RR and CSMA-CD exhibit εimilar performance and both εuffer of an overhead per packet tranεmiεεion which W iε dependent on the ratio 5-. The removal of εuch a depen¬ ds » will provide the ultimate performance, which iε an objective of the preεent invention and which will be deεcribed herein- below. Aε with the UBS architecture, the preεent invention iε baεed upon a broadcaεt communication system comprising an outbound channel and an inbound channel, a plurality of εtationε con- nected to both the outbound and inbound channelε, tranε¬ mitting on the outbound channel and receiving on the inbound channel. The communication medium conεtituting the outbound and inbound channelε may be a twiεted pair, a coaxial cable, an optical fiber, or a wave guide. The channel acceεε proto- col for tranεmiεεion on the outbound channel iε baεed upon the round-robin εcheme deεcribed for the UBS architecture with the advantage of utilizing the channel bandwidth more efficiently than the RR algorithm even when packet tranεmiε¬ εion ti eε are short and cable diεtanceε are long. In one embodiment, tranεmitters on the outbound channel may be of the unidirectional type rendering the invention a UBS; in another embodiment, signal propagation on the outbound channel iε not required to be unidirectional, and thuε bi- directional tranεmitterε may be utilized. Aε it iε eaεier and εimpler to preεent the conceptε underlying the invention when unidirectionality of εignal propagation iε guaranteed ov. ' -i * γ,ι: than when tranεmitterε are bidirectional. The preferred em¬ bodiment iε conεidered at firεt to be a UBS architecture and defer to a later εection diεcuεεion of the caεe where the conεtraint of unidirectionality is relaxed. The gist iε baεically the following. Conεider the UBS archi¬ tecture, contrary to a RR algorithm where the time reference uεed in determining the right of way iε the end-of-carrier on the inbound channel (EOC(IN)), in the acceεε protocol of the present invention the time reference uεed iε the end-of-. carrier on the outbound channel (EOC(OUT)). The echaniεm uεed in determining the acceεε right to uεerε in a given round iε thuε made independent of the propagation delay T». thuε decreaεing the gapε between conεecutive tranεmiεεionε to valueε on the εame order of magnitude aε the time needed to detect carrier. Secondly, the idle time εeparating two con¬ secutive rounds iε kept aε εmall aε a round trip propagation delay. The detailε of the εcheme are aε followε: A. The Events EOC(IN) and EOC(OUT) Let the boolean function c(t,OUT) be defined as / 1 if carrier is detected preεent J on the outbound channel at time t c(t,OUT) = 1 ' 0 otherwiεe Note that c(t,OUT) εignalε the preεence or abεence of carrier with a delay of t^ εec. , where t $ iε the time required for the detection operation. It iε aεεumed here that the carrier detector iε placed very cloεe to the channel. (Other ar¬ rangements are also posεible aε εet forth below. ) The event EOC(OUT) iε εaid to occur when c(t,OUT) undertakeε a tranεi- tion from 1 to 0. In a εimilar way c(t,IN) and EOC(IN) are defined. , ' S J £ΛΪ _______ A ' -. V rO • ' B. Tranεmiεεion Units (TU) A tranεmiεεion unit conεiεtε of a preamble followed by the information packet itεelf. Tranεmiεεion unitε and informa- tion packetε may be of fixed or variable size. The preamble iε for εynchronization purpoεeε at the receiverε. It iε εufficiently long for the receiverε to detect preεence of the unit, and then to εynchronize with bit and packet boundarieε. C. Buε Tranεceiverε For every station in the εyεtem, the tranεmitting device iε connected to the outbound channel while the receiving device iε connected to the inbound channel. It iε noteworthy that, regardleεε of what type the tranεmitterε are (unidirectional or bidirectional), given the kind of topologieε under con- εideration for the invention and given the way the outbound and inbound channelε are connected, εignalε propogate on the inbound channelε alwayε in the same direction. One of the basic featureε required for the round-robin algo¬ rithm and the acceεε protocol of the invention iε the ability for each εtation to εenεe, on the outbound channel, the car- rier due to tranεmiεεionε by stations on the upstream εide of itε tranε itter; (i.e., thoεe εtationε whoεe tranmεmit tapε on the outbound channel are more diεtant from the inbound channel than the tranεmitter of the station in question. ) Restricting again the diεcuεεion to the unidirectional caεe, note that there are εeveral wayε for achieving the neceεεary functionε on a coaxial cable or twiεted pair. Deεcribed herein are feaεible alternativeε diεregarding any conεidera- tion of complexity, coεt, or implementation preference. One way of achieving unidirectionality of tranεmiεεion iε to uεe εuch meanε aε unidirectional amplifierε between every two conεecutive tranεmit taps on the outbound channel. In. thiε caεe the tranεmit tapε can be "paεεive" T-connectorε (εuch aε thoεe used in ETHERNET) and therefore do not require splicing of the cable for installation; the unidirectional means how- ever do require splicing of the cable and are active ele¬ ments. To achieve εenεing of carrier εignalε generated by εtationε on the upεtream εide of a tap 33, one of two optionε exiεt: (i) provide an additional (paεεive) tap 31 on the up¬ εtream εide of the amplifier 29 which iε on the upεtream εide of the tap 33 (aε εhown in Fig. 2A), or (ii) operate with a εingle tap 33, but provide meanε 27 within each εtation to detect preεence of εignalε and to detect colliεionε due to overlap in tranεmiεεion with upεtream εignalε (aε achieved in ETHERNET and aε depicted in Fig. 2B). Another way to achieve unidirectionality iε by conεidering the tranεmit tap itεelf to be an active device comprising a receiver on the upεtream end, a tranεmitter on the other end, and an adder in-between which allowε new inputε from the εta- tion to be tranεmitted on the channel. The carrier εenεing capability of upεtream εignalε iε achieved by εenεing activ¬ ity at the output of the receiver 23 (See Fig. 2C). An alternative iε the uεe of directional coupler. A poεεible implementation iε depicted in Fig. 2D. Two (paεεive) bidi¬ rectional tranεmitting tapε 31, 33 such aε T-connectorε are connected to the buε at a εpecifie diεtance from each other εuch that the εignal'ε propagation delay from one tap to the other iε τ_ (in delay 37). Referring to Fig. 2D, if the deεired signal ε(t) iε to propagate in one direction, εay to the right of the εtation, then the station must transmit i(t) at tap A and - (t- * r,3) at tap B, where i(t) iε εuch that ε(t) = i(t) -i(t-2T d ). Indeed thiε guaranteeε that ε(t) will propagate to the right of tap A while all εignalε cancel each other to the left of B. Senεing "upεtream" εignalε iε simply achieved by provid- ing the carrier εenεing capability at tap B. Note that the same functions can also be implemented by paεεive directional couplerε 35 uεing tranεfor erε aε εeen in Fig. 2E. D. Basic Mechanism to Tranεmit Tranεmiεεion Unitε A εtation S^ which εenεeε the outbound channel buεy, waits for EOC(OUT). Immediately following the detection of EOC(OUT), it εtartε tranεmiεεion of itε unit. Simultaneouε- ly, it εenεeε carrier on the outbound channel (on the up¬ εtream εide of itε tranεmit tap). If carrier iε detected (which may happen in the firεt t<-j εec. of the tranεmiεεion, and which means that some εtation Sj with a lower index haε alεo εtarted tranεmisεion following itε detection of εtetionε which were able to detect EOC(OUT). Clearly, during and following the tranεmiεεion of itε TU, a εtation will εenεe the outbound channel idle, and therefore will encounter no EOC(OUT), and will not be able to tranεmit another TU in the current round. Thuε in thiε mechaniεm, there iε no need to diεtinguiεh between DORMANT and ACTIVE εtateε, aε required in the previouε round-robin algorithm. Note that the poεεible overlap among εeveral tranεmiεεion unitε iε limited to the firεt t^ εec. of theεe tranεmiεεionε. It iε asεumed that the loεε of the firεt t εec. of the pre¬ amble of the nonaborted tranεmiεεion will not jeopardize the εynchronization proceεε at the receiverε. According to the above baεic mechaniεm, two conεecutive tranεmiεεion unitε are εeparated by a gap of duration t^ εec. , the time neceεεary for all other εtations to detect EOC(OUT). The εucceεεion of tranεmiεεion unitε tranεmitted in the εa e round iε called a train. A train can be εeen by a εtation on the outbound channel only as long as the train unitε (TU'ε) in it are being tranεmitted by εtationε with lower indiceε. A train generated on the outbound channel iε entirely εeen by all εtationε on the inbound channel. Since there iε a gap of ' y duration t d εec. between conεecutive TU'ε within a train, the detection of preεence of a train on the inbound channel can be beεt achieved by defining the new boolean function TRAIN(t,IN) ~ c(t-t d ,IN) + c(t,IN). Clearly, I aε long aε train iε in pro- 1 greεε on the inbound channel at time t 0 otherwiεe The tranεition TRAIN(t,IN):1—■►O defineε the end of a train (EOT(IN), and the tranεition TRAIN(t,IN):0—→-l defineε the beginning of a train (BOT(IN)). E. The Topology of the Preεent Invention After the laεt TU in a train haε completed, tranεmiεεion, a mechaniεm iε needed to εtart a new train of transmiεεion unitε. Clearly, it iε eεεential that thiε mechaniεm giveε acceεε right to the ready εtation with the loweεt index. One may uεe a mechaniεm εimilar to algorithm RR itεelf. That iε, aε εoon aε EOT(IN) iε detected, ready εtationε tranεmit a burεt, monitor the outbound channel for 27-εec. and then tranεmit or inhibit tranεmiεεion depending on whether carrier iε abεent or preεent during the 2 εec. period reεpectively. The drawback, aε mentioned above, iε the introduction of a gap between conεecutive trainε of duration between 2-r and 4τ εec.; (or juεt 2τ-εec. if the implementation of the εcheme in each εtation iε made dependent on the poεition of the εtation) . In order to keep thiε gap aε εmall aε poεεible, one εhould be able to have the event uεed aε reference (namely, EOT(IN)) viεit the receiverε in the εame order aε the εtationε' in- diceε, which iε alεo the order in which they can tranεmit. This iε achieved if the inbound channel iε such that εignalε on it are made to propagate in the same direction aε on the outbound channel. Conεider the network topology to compriεe an outbound channel and an inbound channel which are parallel and on which εig- nalε propagate in the same direction (i.e., visiting εtationε in the εame order), and a connection between the outbound channel and the inbound channel to allow the broadcaεt of all outbound signals on the inbound channel. The propagation delay along the connection τ c is anywhere between 0 εec. and rεec, (whereT is the end-to-end propagation delay on the outbound or inbound channel), depending on the geograph- ical distribution of the users and the way the inbound and outbound channelε are connected. Aε illuεtrated in Fig. 3A, in which stations Sl-SN are connected to communication bus 20 having terminators 10 and 12. The minimum of 0 εec. is ob¬ served, for example, if the inbound and outbound channels have a loop shape (or, more generally speaking, the εtationε with the loweεt and higheεt indiceε are collocated). The maximum of T εec. iε obεerved if the connection cable 26 iε made parallel to the inbound and outbound channelε, aε εhown in Fig. 3(B). The propagation delay between the outbound tap and inbound tap for all εtationε iε fixed and equal to The major feature of thiε topology reεtε on the fact that, when the inbound channel iε made exactly parallel to the out- bound channel the event EOT(IN), uεed by all stations aε the synchronizing event to εtart a new train, will reach any εta¬ tion exactly at the εame time aε the carrier on the outbound channel due to a poεεible tranεmiεεion by a station with a lower index. Thiε helpε reεolve the overlap of εeveral tranεmiεεion at the beginning of a train in juεt the εame manner aε the reεolution obtained in the baεic mechaniεm for tranεmitting TU'ε. Thiε mechaniεm again allowε the ready εtation with the loweεt index to be the firεt to complete tranεmiεεion of itε TU, and following that the new train will take itε normal courεe. The time gap between two consecutive trains, defined aε the time between the end of the laεt TU in a train and the beginning of the firεt TU in the εubεe- quent train, iε now • »- + τ c + 2 t g εec., aε εeen in Fig. 4. It iε worthwhile to note here that thiε topology alεo brings improvementε to the original RR acceεε algorithm. With εuch a topology, the RR algorithm can be modified εo that a ready εtation in the ACTIVE εtate performε aε followε: 1. Wait for EOC(IN). 2. Begin tranεmiεεion of TU, and εimultaneouεly liεten to the outbound channel. 3. If within t^ εec. the outbound channel iε εenεed idle, then complete tranεmiεεion of TU (free of conflict) and εwitch to the DORMANT εtate; otherwiεe abort tranεmiεεion and repeat the algorithm. In thiε εituation, the gap between any two conεecutive TU'ε in a round iε exactly + -*- c + t d < 2 + t^-, a fixed amount equal to the minimum poεεible. F. The Cold-Start Procedure, and Keeping the Invention "ALIVE" The above algorithm and echaniεmε are valid only if there are alwayε eventε to which actionε are synchronized, namely EOC(OUT) and EOT(IN). This aεεumeε that at all timeε εome εtation iε ready, and therefore trainε contain at leaεt one TU and are εeparated by gapε of fixed duration t- + τ- c + 2t d . When thiε iε not the caεe, the idle time on the inbound chan- nel exceedε T- + *»- c + 2t d εec. A station which becomes ready at time t 0 εuch the (i) TRAIN (t 0 , OUT)=0 (thuε indicating that no EOC(OUT) will be encountered to εynchronize action t 0 ) and (ii) TRAIN (t,IN) = 0 during the entire period of time It 0 , t 0 + -r + * c + t^] (thuε indicating that no EOT(IN) will be detected), haε to undertake a εo-called cold-εtart 5 procedure. Thiε procedure uεt be deεigned εuch that if ex¬ ecuted by εeveral εtationε becoming ready under theεe condi¬ tions, it leads to a single synchronizing event to be used aε a time reference, followed then by an orderly conflict-free operation of the network. 10 The εi ple one propoεed here conεiεtε of the following. Once a εtation haε determined that a cold-εtart procedure iε need¬ ed, it tranεmitε continuouεly an unmodulated carrier (called PILOT) until BOT(IN) iε detected. At thiε time the εtation 15. abortε tranεmiεεion of the pilot. It then waitε for EOT(IN) (conεiεting of the end of the pilot) and uεeε that event aε the εynchronizing event. (Of course, an alternative could be to uεe the BOT(IN) due to the pilot aε the εynchronizing event. ) It iε poεεible that pilotε tranεmitted by εeveral 20 uεerε overlap in time. Thiε, however, will cauεe no problem. Note that aε long aε pilotε are aborted aε εoon aε BOT(IN) iε detected, it iε guaranteed that the reεulting pilot aε ob¬ served on the inbound channel is continuous and of length + c + 2t d εec. Following itε end, there will be the nor- 25 al gap of εize ** • +1^+ 2t<g before a TU followε. (Of courεe, thiε gap iε abεent if BOT(IN) iε uεed aε the εynchronizing event.) Aεεume that no εtation iε ready when EOT(IN) is detected. 30 The network iε then said to go empty. The first station to become ready when the network iε empty εpendε τ- + f c + t d εec. to determine the empty condition, by examining TRAIN(t, IN), after which it starts transmiεεion of the pilot. Then it takes between * τ- c + t^ and t- + n- c + t d seconds before it 35 detects BOT(IN). (The minimum -> c + t d iε obεerved if the εtation in queεtion iε the loweεt index εtation, and the higheεt index εtation happened to become ready exactly at the εame time.) Regardleεε of which iε the caεe, following BOT(IN) a pilot of length -r + τ- c 4 t $ εec. is observed on the inbound channel, followed by the gap of -r- + *t- c + 2ta εec. and then the tranεmiεεion unit. Therefore, with the uεe of the PILOT, the time between the moment at which the firεt εtation becomeε ready in an empty network and the moment at which the next EOT(IN) (due to the end of PILOT, and repreεenting the εynchronizing event) reaches the loweεt index εtation iε be- tween 2 + 3 * ^ + t^ and 3f →.*rc) + -t < _ εec. Thiε iε the time needed to εtart a new round and iε to be compared to * + "T- c + 2t(-* εec. which iε the time needed when the network doeε not become empty. Fig. 5 εhows the timing of the cold- start procedure when only one εtation becomeε ready. To avoid the cold-εtart operation each time the network goeε empty, one needε to guarantee that, aε long aε εome εtationε may εtill become ready in the future, εynchronizing eventε (namely, EOT(IN), are created artificially by all εuch εta- tionε. More preciεely, conεider a station to be in one of two εtateε: DEAD or ALIVE. A station which is in the ALIVE εtate haε reεponεibility to perpetuate the exiεtence of the εynchronizing event EOT(IN) for aε long as it remainε in that εtate. To accompliεh . thiε, each time EOT(IN) iε detected, the εtation tranεmitε a εhort burεt of unmodulated carrier, of duration εufficiently long to be very reliably detected (i.e., of a duration of at leaεt t_ εec.). Such a burεt iε called LOCOMOTIVE. If the train were to be empty (i.e., no εtationε were to be ready when EOT(IN) iε detected), now the LOCOMOTIVE conεtituteε the TRAIN, and EOT(IN) iε guaranteed to take place. Clearly, if some station which is ALIVE iε alεo ready, then immediately following the LOCOMOTIVE, it initiateε tranεmiεεion of itε TU and followε the tranεmiεεion mechaniεm giving acceεε right to the loweεt index. The net- work iε εaid to be ALIVE if at leaεt one εtation in the net¬ work iε ALIVE, otherwiεe it iε εaid to be DEAD. A εtation iε εaid to be in the DEAD εtate if it iε not engaging in keeping the network ALIVE, and therefore iε prohibited from tranεmitting any TU. To be able to tranεmit, a εtation haε to become ALIVE. For a DEAD εtation to become ALIVE, it must firεt determine whether the network iε ALIVE or not. Letting t denote the time at which a DEAD εtation wiεheε to become ALIVE, the network iε determined ALIVE if a train iε detected on the inbound channel anytime in the interval [t, t + ~r + " ■ "c + -d - - Otherwiεe, it iε determined DEAD. If the network iε determined ALIVE, then the εtation εimply εwitcheε to the ALIVE εtate and actε accordingly. Otherwiεe, it executeε the cold-εtart procedure following which it becomeε ALIVE. A εtation which iε ALIVE can be either READY or NOT-READY at any moment. Thiε iε determined by the state of itε tranεmit buffer, empty or nonempty. To that effect, a function TB(t,X) for εtation X iε defined aε 1 it itε tranεmit buffer iε nonempty TB(t,X) = 0 otherwise An ALIVE εtation which becomeε ready doeε not have to wait for EOT(IN) to undertake the atte ptε to tranεmit itε packet. In fact, if an outbound train iε observed, the εtation εyn- chronizeε tranεmiεεionε with EOC(OUT). If, however, at the time it becomeε ready, no train iε obεerved on the outbound channel, then EOT(IN) iε the εynchronizing event. The Acceεε Protocol Having defined above c(t,OUT), c(t,IN), TRAIN(t,IN), TB(t,X), BOT(IN), EOT(IN), EOC(IN) and EOC(OUT), CTX iε now defined aε the event correεponding to the completion of tranεmiεεion of the current TU, given that εuch a tranεmiεsion haε been initiated. Alεo, TIME-OUT(f>) iε defined aε the event correε¬ ponding to the completion of a period of time of duration , εtarting the clock at the time when waiting for the event iε initiated. From the above diεcuεεion, PILOT may be defined aε a continuouε unmodulated carrier, and LOCOMOTIVE aε an unmodulated carrier of duration tg". Consider that initially εtation X iε in the DEAD εtate. Upon command (for bringing the εtation to the εtate ALIVE and eventually for tranεmiεεion of data), the following baεic algorithm iε executed. Step 1. (Check whether the network iε ALIVE or not. If it iε then proceed with Step 2, otherwiεe undertake the "cold-εtart" procedure and then proceed with Step 2.] If TRAIN (t,IN)=l (i.e., the network iε already ALIVE, go to Step 2. Otherwiεe, wait for the firεt of the following two eventε: BOT(IN) or TIME-OUT(**-+τ c t d . If BOT(IN) occurε firεt (then again it means that the network iε ALIVE), go to Step 2. If on the contrary TIME-OUT fr-4-r j , 4 \._ ) occurε firεt (indicating that the network iε not ALIVE, and that station X muεt undertake the "cold- start" procedure), tranεmit PILOT immediately at the occurrence of TIME-OUT (r + c 4 t<-"), and main¬ tain tranεmitting it until BOT(IN) iε detected, at which time abort tranεmiεεion of PILOT and proceed with Step 2. Step 2. Wait for the firεt of the following two eventε: EOC(OUT) and EOT(IN). If EOC(OUT) occurε firεt, then go to Step 4. Otherwiεe go to Step 3. Step 3. [A new train haε to be εtarted. ] Tranεmit a LOCO- MOTIVE and go to Setp 4. Step 4. [Determine the current εtate of the εtation. If it iε ready, then attempt tranεmiεεion of the TU packet.) If TB(t,X)=0, go to Step 2. Otherwiεe initiate tranεmiεεion of the TU. If t^ εec. later c(t,OUT)=l (meaning it iε not εtation X*s turn), then abort tranεmiεεion and go to Step 2. Other¬ wiεe, complete tranεmiεεion of the packet and go to Step 2. In Fig. 6 the flow chart of thiε baεic algorithm iε depicted. In Fig. 7, the diagram for a finite εtate machine which per¬ forms the algorithm iε depicted, containing εeven εtateε. The εtateε labeled Dχ f D and D3 are aεεumed when the εtation iε DEAD and iε in the proceεε of becoming ALIVE; the εtateε labeled Al through A4 are aεεumed when the εtation is ALIVE. Each possible transition iε labeled by the combination of eventε which cauεeε the tranεition, followed by the action -taken. H. Performance of the Acceεε Protocol The introduction of the LOCOMOTIVE increaεeε the length of a train by t^ εec. The maximum throughput iε evaluated aε the ratio of the time in a train εpent for data tranεmiεsion to the time εeparating the beginning of conεecutive trainε and iε given by S,a ax = MT (6) M(T4T d ) 4 « -r-4f c 42t d Neglecting t d in compariεon to T and*>4-t c 42t d in compariεon to MT, Fig. 6 εhowε that S max iε nearly one. In the above diεcuεεion, it iε aεεumed that the carrier de¬ tector and tranεmit logic of a εtation iε collocated with itε tapε on the channel. Accordingly, the time it takeε for the εtation'ε logic to reεpond to any of the four eventε EOC(OUT), EOT(IN), BOC(IN) and BOC(OUT) iε juεt the detection time t d . When thiε iε not the caεe, aε in moεt applica¬ tion, then a juεtmentε in the protocol and itε performance have to be made to take into account the propagation delay between a εtation and itε tap. Let τ £ denote the maximum εuch delay over all εtationε. By a εimple argument it can be εhown that it takeε a εtation t d 4 2 « t- s to reεpond to the occurrence of any of the above-mentioned eventε on the channel. Note alεo that the posεible overlap among εeveral transmission units is now equal to 2τ s 4 t d instead of juεt t , meaning that the preamble length haε to be increaεed by 2 E W bitε, and additional overhead which needε to be taken into account in the performance analyεiε. Denoting by tg the gap between two conεecutive tranεmiεεion unitε in a train, and by t ov the maximum duration of overlap among εeveral tranεmiεεion unitε, the maximum throughput iε then given by S & max MT ( .7' I) M (T4t g +t ov -t d ) +T+^c+t g 4t d where tg = t ov = 2τ s 4 t . Neglecting t d in compariεon to T andτ*4τ c 4tg4t d in compariεon to M(T4tg4t ov ) when M iε large, Eq. 7 reduceε to s max ~ (8) 1 4 4-^s T Aε the introduction of T s cauεeε degradation in channel throughput, one may conceive placing the critical functionε of carrier detection and tranεmiεεion abortion in the tranε- ceiver (cloεe to the channel); then t ov can be kept aε εmall aε t , and Eq. 8 now becomeε -"max (9) 1 4 Note that the introduction of *ϊ- s callε for εlight modifica- tionε to the variouε parameterε uεed in the above deεcription of the algorithm. I. Integrating Voice And Data On The Network It iε aεεumed that vocoderε digitize voice at εome conεtant rate. Bitε are grouped into packetε which are then tranε- itted via the network to the deεtination vocoder. To achieve interactive εpeech and εmooth playback operation, it iε important to keep the end-to-end delay for each bit of in¬ formation (from the time the bit iε generated at the origi¬ nating vocoder until it iε received at the deεtination vocoder) within tight boundε. Two componentε of delay are identified: the packet formation delay and the network delay. The εum uεt not exceed the maximum allowed in order for all bitε to εatiεfy the delay requirement of εpeech. An inter- eεting property of round-robin εchemeε with finite number of εtationε iε that the delay incurred in the tranεmiεεion of a packet iε alwayε finite and bounded from above. Thiε renderε it particularly attractive for the packet voice application which iε now examined in more detail. Let W v be the bandwidth required per voice uεer (i.e., the vocoder'ε rate in bitε/εec), and D v the maximum delay allowed for any bit of digitized voice (not including the propagation delay). Let B v denote the number of bitε per voice packet. B v iε the εum of two componentε: B v which enco paεεeε all overhead bitε compriεing the preamble, the packet header and the checkεu , and B v 12') the information bitε. Let tf be the time required to form a packet; it iε alεo the packet intergeneration time for a vocoder. Let T v be the tranεmiε¬ εion time of a voice packet on a channel of bandwidth W. Then, t f = B v (2) (10) T v = B, (ID Since packet generation iε determiniεtic, occurring every tf ' εec., each voice uεer can be modeled by a D/G/l queue. The packet εervice time, D n , iε the time from when the packet reaches the head of the queue, until it iε εucceεεfully re- ceived at itε deεtination. Due to the bandwidth constraint. tf >. D n (12) Thiε iε also the condition of stability for the D/G/l queue. Let M denote the number of active (off-the-hook) voice εourceε. Consider again the case where * r ε = 0. Assuming all queues are non-empty, a train is of length M(T V 4 t ) 4 *r 4 f c 4 2t d . The εervice time diεtribution of a packet can be bounded by a determiniεtic one, with the εervice time equal to a maximum train length. Conεider all queueε to be (pessimistically) represented by D/D/l, where the inter- arrival time iε tf and the εervice time iε D n = M(T V = t ) 4 T 4-τj-, 4 2t . With theεe conεiderationε, provided that the queue εize iε initially 0 and tf _>. D n» the waiting time of a packet iε 0 and itε total delay iε D n . The delay conεtraint for a voice bit iε now written aε tf En < (13) The above two conεtraintε lead to a maximum value for M when ttff -= D°n n -= _5y_ iε choεen. Accordingly, the optimum packet εize iε given by and the maximum number of voice uεerε allowed at any one time iε given by = D v " Ct-4T- C 42t d ) 'max (15 ) B v A + t d Neglecting *t-4 τ- c 4 2t d , in compariεon to D v , and t in com¬ pariεon to T v , * «-ϋ = ° 116) 2B V 2B y l ) 4D v W v Note that aε long aε M j na jj , it iε guaranteed that the length of a train never exceedε K ma χ tranεmiεεion unitε, 5 and the network packet εervice time never exceecε the maximum determined above: Hjnax(• r v + t ) 4 T- 4 *τ- c 4 2t d , conεe- quently no queuing delay iε incurred, the queue εize at all uεerε remainε _<1, and the total delay conεtraint for all voice bitε iε alwayε εatiεfied. 0 The principal conεtraintε to be εatiεfied here are the delay conεtraint on voice packetε and a minimum bandwidth require¬ ment for data. Although a delay conεtraint need not be impoεed on data packetε, it iε important to provide the band- 5 width "reεerved" for data on aε continuouε a baεiε aε poεεi¬ ble, and to fairly allocate that bandwidth to data uεerε. Furthermore, the protocolε are required to be dynamic in al¬ locating the bandwidth to voice and data applicationε, allow¬ ing data uεerε (or background traffic) to gracefully εteal 0 the bandwidth which iε unuεed by voice. To accompliεh theεe objectiveε on the network, conεider two typeε of trainε, the voice train type (VT) and the data train type.(DT). Trainε are alwayε alternating between the two typeε, and εtationε tranεmit their packetε on the train of the correεponding 5 type. To εatiεfy the delay conεtraint for voice packetε, it • " iε important not only to limit the number of-voice co munica- tionε to a maximum, but alεo to limit the data trainε to a certain maximum length. Let W be the minimum data bandwidth required. Asεu ing that data trainε are limited to a maximum length L, their effect on the calculation of the optimum value of M iε to juεt in- creaεe the overhead between conεecutive trainε by the amount L 4 T- 4 τ c 4 2t . The maximum M iε then given by = D v /2 - 2(T4-r- c 42t d )-L max (17) B V A + t d L iε eaεily determined in function of W and the data packet length characteriεticε. Aεεume, for analyεiε purpoεeε, for example, that data packetε are of fixed length, B = B d ( ) 4 B ( ) bitε/packet, where B accountε for packet (2) overhead and B . for information bitε. (Although the data packetε can be of variable lengths.) Let K denote the maximum number of data packets in a data train. In order to achieve W d , N iε given by = " c D v 2) Bd (18) Consequently L i ε given by L -= N Bd (19 ) W Neglecting t , -r and N = V'd D v B d ( 20 ) 2W B d 2 ) O FI and "max - D v v (W-W " d aB- d a/' B- d a 2 ) ) , (21 ) 2B^ 1 ) 4 D V K V It iε important to limit data trainε to the maximum length L at all ti eε, even if the number of active voice uεerε iε smaller than Mjri x* Otherwiεe, εituationε may ariεe where the packet delay for a voice packet will exceed D v . Thiε parti¬ cularly takeε place if, during a data train, a number of new voice uεerε become ready, εome of which might incur an in¬ itial delay longer than the maximum allowed. Since a data train may not contain the TU'ε of all ready εta¬ tionε, it iε important that the next data round resumes where the previous data train haε ended. Thiε iε eaεily acco - pliεhed by the incluεion of the DORMANT/ACTIVE εtateε for data uεerε in the εame way aε in the RR algorithm. To εwitch from the DORMANT to the ACTIVE εtate, data uεerε have to monitor the length of data trainε on the inbound channel: a dormant uεer εwitcheε to the ACTIVE εtate whenever the length of a data train haε not reached itε maximum limit L. In order to alternate between the two typeε of trainε, a εta¬ tion maintainε a flag Φ which getε complemented at each occurrence of EOT(IN). Aεεuroe the convention Φ = 0 for a data train and Φ - 1 for a voice train. Now the problem iε faced of having a εtation properly initialize φ when it be- comeε ALIVE. The εimpleεt way iε aε followε. If the network iε found DEAD, then, following the pilot, the εtation initia- lizeε φ εuch that the firεt train iε of the voice type. If the network iε found ALIVE, then the εtation monitorε the in¬ bound channel until either a valid packet iε obεerved or the network haε gone dead. In the former caεe, the type of train iε derived from the type of packet obεerved, and iε initia¬ lized accordingly. In the εecond caεe, the εtation under- takeε a cold-εtart and the initialization of ς> iε independent of paεt hiεtory. Note that aε long aε the network iε deter- mined ALIVE, a εtation may not become ALIVE until it haε obεerved a valid packet transmisεion; all empty trainε are ignored. If it iε highly likely that long εucceεεionε of empty trains occur, the above mechaniεm may induce a high initial delay before the εtation becomeε ALIVE. Thiε can be overcome by including explicit information in the LOCOMOTIVE which indicateε the type of train. That iε, the LOCOMOTIVE becomeε now a train-type indicator (TI) packet. The proper indicator packet uεt be tranεmitted following EOT(IN) (i.e., an attempt to do εo iε undertaken) by all ALIVE εtationε in the network, regardleεε of the type of packetε they intend to tranεmit. Clearly only one tranεmiεεion of the train-type indicator packet iε accompliεhed, by the εtation in the ALIVE εtate with the loweεt index. With thiε mechaniεm, a εtation wiεhing to become ALIVE in a network determined ALIVE waitε for BOT(IN) following which it receiveε and decodeε the train-type indicator packet, and initializeε the flag accordingly. The uεe of the train-type indicator packet in- creaεeε the overhead cauεed by the LOCOMOTIVE from t to the tranεmiεεion time of a (relatively short) TI packet. Thiε extra overhead haε a εmall impact on the performance which iε εtill approximated by the equationε given previouεly. Another alternative to provide information regarding the train type iε to have the locomotive conεiεt of a εpecified pattern of burεtε of unmodulated carrier and silenceε, one pattern for each type of train, εtationε alternate between the two typeε of locomotiveε, keeping in εynchronization with the network. In thiε approach, it iε poεεible to conεider the patternε to be in εuch a way that, in caεe of diεagree- ment, the train indicator will reεult of the voice type. It haε been indicated previouεly that in order to εatiεfy the delay conεtraint for voice packetε it iε neceεεary to limit the number of phone callε in progreεε at the εame time. Thiε requireε that a mechaniεm exiεtε to check whether a new phone call can be accepted or not. The deciεion is baεed on the number of callε already εet up, and thiε can be obtained by εimply meaεuring the length of the previouε voice-train. However, if voice packetε repreεenting εilence are not tranε¬ mitted, thiε meaεure may not be accurate and one may have to collect data regarding the length of εeveral voice trainε before deciding on the acceptance of a new call. The εavingε on voice bandwidth obtained by εilence εuppreεεion may be utilized to increaεe the average data bandwidth. If uεed to increaεe the maximum number of phone callε on the network, then a εervice degradation will have to be allowed aε εome packetε may be delayed beyond the -maximum delay D v . In the above diεcuεεion, it waε aεεumed that the integration of different typeε of traffic iε obtained by uεing different typeε of trainε and by requiring that a packet be tranεmitted only on a train of the correεponding type. Another poεεible approach iε to allow mixing of the different typeε of packetε (whether data and/or voice) on the εame train. In thiε caεe there iε no need to provide train indicators. However, each εtation iε then required to meaεure not only the length of the current train but alεo the period of time in the train already utilized by each type of traffic so that the band¬ width utilized by each traffic does not exceed the maximum value allowed. In order to fulfill the delay requirements for voice traffic it can be εeen that the global amount of data tranεmitted in a train haε to be limited to L/2. Note however that thiε limitation doeε not affect the overall efficiency of the εyεtem nor the bandwidth aεεigned to each type of traffic. J. The Voice/Data Network Algorithm Although the algorithm preεented here iε for only two typeε OKPI WIJVO « of trainε, the conceptε can be applied to any larger number of typeε, regardleεε of the applicationε intended. Let STATE (Φ) denote the DORMANT/ACTIVE state of a εtation with reεpect to train type Φ , and TB(t,Φ) denote the εtate of itε buffer (empty or nonempty) with reεpect to packet type. We let L(Φ) denote the maximum allowable length of a train of type , including the tranεmiεεion of the train indicator, and R(φ) - L (Φ) 4 -r- +■ τ c 4 t . Let C denote a clock which iε used to meaεure the length of a train in progreεε. In the following preεentation, .Stepε 1 through 4 are performed by a station wiεhing to become ALIVE and to initialize itε parameter Φ, while the remaining εtepε are executed by a εtation which iε ALIVE. Initially STATE (φ) « ACTIVE, Φ = 0, 1. Step 1. If TRAIN (t,IN) = 0 then go to Step 2, otherwise wait for EOT(IN) at which time proceed with Step 2. Step 2. [Determine whether the network iε ALIVE or not.] Wait for the firεt of the following two eventε: BOT(IN) and TIME-0UT(T 4 « r c 4 t^ If BOT(IN) occurε firεt then go to Step 3; otherwiεe, go to Step 4. (In caεe of a tie, BOT(IN) iε conεidered to have precedence. Step 3. Set C = 4 τ c 4 t . Receive and decode train- type indicator. If the decoding iε unεucceεεful then go to Step 1; otherwiεe, initialize φ accord¬ ingly, and go to Step 5. (It iε aεεumed here that the operation of decoding a train-type indicator can be completed before EOT(IN) iε detected, even when the current train containε only the train indicator. Thiε iε eaεily accompliεhed in practice by maintaining carrier on beyond the end of trans- miεεion of train indicator for a period of time εufficiently long εo aε to guarantee completion of the decoding operation before the occurrence of EOT(IN).) Step 4. [The network iε DEAD: execute the cold-εtart pro- OMFI s^RNAT\ cedure.] Initiate tranεmiεεion of PILOT. Wait for BOT(IN). At the occurrence of BOT(IN), εtop tranε¬ miεεion of PILOT, εet Φ • = 0, C = 0, and proceed with Step 5. 5 Step 5. Wait for the firεt of the following two eventε: EOT(N) and EOC(OUT). If EOC(OUT) occurε firεt, then go to Step 9; otherwiεe proceed with Step 6. Step 6. [At occurrence of EOT(IN), C containε a meaεure of the length of the previouε train. Thiε information 10 iε paεεed on to higher levelε to determine accept¬ ance or rejection of new voice callε.] Notify higher levelε of the current valueε of φ and C. Step 7. [Reεet STATE (φ) if appropriate.] If C<R(Φ) then εet STATE («j>) = ACTIVE. 15 Step 6. [A new train iε to be started.] Complement φ; εet C = 0; end tranεmit train indicator. (In the caεe where the train indicator iε an actual packet, itε tranεmiεεion iε in accordance with the baεic tranεmiεεion aε followε: Initiate tranεmiεεion of 20 TI and wait t εecondε. If at thiε new point in time c(t,OUT) = 1 then abort tranεmiεεion of TI and go to Step 5; otherwiεe, wait for CTX and then proceed with Step 9. ) Step 9. If STATE (Φ) = ACTIVE and TB(t,Φ) = 1 and C<L(Φ), 25 then go to Step 10; otherwiεe, go to Step 5. Step 10. Initiate tranεmiεεion of TU. Wait t d εecondε. If at this new point of time c(t,OUT) = 0 and C<L(Φ) then proceed with Step 11; otherwiεe, abort tranε¬ miεεion of TU and go to Step 5. 30 Step 11. Wait for the firεt of the following two eventε: CTX and C = L(Φ). If C * L(Φ) occurε firεt, then abort tranεmiεεion of TU and go to Step 5; other¬ wiεe, εet STATE (Φ) = DORMANT and go to Step 5. 3.5 Note that in the above algorithm, in order to keep STATE (1) = ALIVE for all εtationε at all timeε, it iε εufficient to aεεign to L(l) an arbitrarily large number. The flow chart for the VOICE/DATA network algorithm iε εhown in Figs. 6 and 9. Fig. 8 repreεentε Stepε 1 through 4 performed by a DEAD εtation in becoming ALIVE, while Fig. 9 repreεentε the εteps executed by a εtation which iε ALIVE. In caεe the train in- dicator iε an actual packet, the box labeled TRANSMIT TRAIN INDICATOR uεt contain all inεtructionε outlined in Step 8 above. The εtate diagram for a finite εtate machine which performε the above algorithm iε εhown in Figε. 10 and 11. Fig. 10 containε the portion of the εtate diagram correεpond- ing to the flow chart in Fig. 8, while Fig. 11 conteinε the portion of the εtate diagram correεponding to the flow chart in Fig. 9. K. Embodiment of the Invention With Bidirectional Tranεmitterε In thiε εection, εeveral alternativeε are diεcuεεed for achieving the upεtream carrier εenεing capability when εignal propagation of the outbound channel iε bidirectional. One possible way iε to insert a directional coupler 33 on the up¬ εtream εide of the tranεmitting tap 33 (εee Fig. 2F. A directional coupler can be implemented by utilizing tranε- formerε. Another implementation iε depicted in Fig. 2G. Two (paεεive) receiving tapε 31,33, εuch aε T-connectorε, are connected to the bus 21 εo that the propagation delay 37 from one tap to the other iε a εpecified valueTd * Referring to Fig. 2G, the εignal received in tap A iε εubtracted after a delayτ from the εignal received on tap B to obtain an output o(t). Accordingly, any εignal flowing on the bus in the A to B direction gives an output o(t)=0 at C, while a signal ε(t) flowing in the B to A direction giveε o(t) = ε(t) - ε(t-2T d ). If one chooεeε -r - λ d ~ ( 2n4l ) 4 f„ where f c iε the carrier frequency and n any integer number, carrier detection of ε(t) can be obtained by carrier detec¬ tion of o(t). An appealing feature of thiε alternative iε that the tranεmitting tap can coincide with tap A of Fig. 2G which becomeε a receiving and tranεmitting tap. Fig. 12 depictε a block diagram of one εtation 50, the tranε¬ miεεion portion of which iε connected to unidirectional com¬ munication or broadcaεt bus 21 through passive tap 23. The receiver portion of station 50 is connected to unidirectional communication or broadcaεt buε 20 through paεεive tap 30. Station 50 typically includeε tranεmitter logic 52 connected to tap 40 through buε 44 and 42. Tranεmitter logic 52 re- . ceiveε information from a uεing device through an output buffer. The receiver portion of station 50 iε connected to tap 30 through buε 32 to receiver logic 62, which in turn iε con- nected to a uεing device through an input buffer. Tranε¬ miεεion and receiving functionε are performed by circuit 27, the functionε of which have been deεcribed in conjunc¬ tion with the deεcription of Figε. 2A-2G. Referring now to Fig. 12, a more detailed diagram of a εtation εuch aε εtation 50 of Fig. 11 iε depicted. In Fig. 12, a uεing device 60 is connected to an output buf¬ fer 136 which in turn is connected to a tranεmitter εhift regiεter 134. The εerial data from regiεter 134 iε clocked by tranεmitter clock circuit 132 to phaεe decoder 130 under control of tranεmitter control circuit 100. Phaεe encoder 130 iε connected to paεεive tap 40 through buε 44 for con¬ necting the εerialized packet data to the outbound channel 21 of unidirectional broadcaεt buε 20. For receiving packet data on the outbound channel of uni- directional communication or broadcaεt buε 20, the packet data iε connected through paεεive tap 30 onto buε 32 for connection to phaεe decoder circuit 104 from which the data iε input to receiver εhift regiεter 110. The received data from regiεter 110 iε connected to input buffer 114 for appropriate input to a uεing device 60. Addreεε filter 112 provideε εuitable addreεεing of addreεs regiεter 116. A carrier detector circuit 102 connected to the inbound channel portion of broadcaεt buε 20 detectε or εenεeε a carrier on buε 20. Similarly, the carrier on the outbound channel portion of broadcaεt buε 20 iε senεed by CD104 and connected to tranε¬ mitter control circuit 100. In summary, an improved local area communication network for one embodiment baεed on a unidirectional broadcaεt εyεtem architecture haε been deεcribed. The Network Acceεε Protocol uεed by all εtationε connected to the buε iε a diεtributed algorithm which provideε conflict-free tranεmiεεion of meεεageε. It iε eεεentially a round-robin εcheme in which the time to switch from one active user to the next in a round iε kept very εmall, on the order of carrier detection time, thuε achieving a performance which iε relatively inde¬ pendent of the end-to-end network propagation delay. Thiε feature representε a major improvement obtained with thiε protocol in compariεon to other exiεting oneε, εuch aε CSMA-CD and round robin; it makeε it very εuitable for local area networkε in which, becauεe of high channel εpeed, long end-to-end delay and/or εmall packet εize, the propagation delay conεtituteε a large fraction of or iε even bigger than the packet tranεmiεεion time. Furthermore, it haε been εhown that thiε protocol iε particularly εuitable for the tranεmiε¬ εion of packetized voice aε it iε able to guarantee an upper bound on the tranεmiεεion delay for each packet. A poεεible way to integrate voice and data on the εame network has been deεcribed in detail. Although a preferred embodiment of an improved local area communication network haε been described baεed upon a unidi¬ rectional communication format, the preεent invention iε alεo applicable to cover other communication media εuch aε bidirectional communication buεeε, fiber opticε, broadband communication techniqueε, and the like. Alεo, the preεent invention iε intended to cover general communication con- figurationε which employ the baεic -tranεmiεεion mechaniεm εet for above, aε, for example, a εingle communication buε uεed to collect data at a εingle point from a plurality of εtationε connected thereto. Accordingly, it iε intended that the εcope of the preεent invention cover any and all εuch variationε without departing from the true εpirit and εcope of the invention aε defined by the accompanying claimε.

Claims: WHAT IS CLAIMED IS: 1. A local area communication network comprising a communi¬ cation buε having an inbound channel and an outbound channel, and a plurality of εtationε connected to εaid communication buε, each of εaid εtationε including meanε for tranεmitting and receiving information data packets on εaid communication buε εuch that εaid data packetε are propagated on εaid in¬ bound channel in the εame direction with reεpect to εaid εtationε aε on εaid outbound channel. 2. A network aε in Claim 1 wherein εaid communication buε iε unidirectional. 3. A network aε in Claim 1 wherein said communication buε iε bidirectional. 4. A network aε in Claim 1 further including meanε for con¬ necting εaid inbound channel to εaid outbound channel thereby permitting tranεmiεεion of εaid outbound data packetε on εaid inbound channel. 5. A network aε in Claimε 1 or 4 wherein εaid inbound channel iε parallel to εaid outbound channel. 6. A network aε in Claimε 1 or 4 wherein the propagation de- la from εaid tranεmitting and receiving meanε iε εubεtan- tially the εame for all of εaid εtationε. 7. A network aε in Claimε 1 or 4 wherein the propagation de¬ lay between any two εtationε on εaid inbound channel iε εubεtantially the same as for the respective two εtationε on εaid outbound channel. 8. A network aε in Claimε 5, 6 or 7 wherein εaid data pac¬ ketε include a tranεmiεεion unit co priεing a preamble followed by an information packet and wherein each of εaid εtationε include a tranεmitting tap on εaid outbound channel. 9. A network aε in Claim 8 further including meanε for re¬ ceiving εignalε on εaid outbound channel. 10. A network aε in Claimε 8 or 9 wherein each of εaid εtationε include a receiving tap on εaid inbound channel. 11. A network aε in Claimε 8 or 9 further including meanε for εenεing carrier on εaid outbound channel. 12. A network as in Claim 11 further including means for εenεing carrier flowing in a certain direction on εaid outbound channel. 13. A network aε in Claim 12 including meanε for detecting carrier on the upεtream portion of εaid tranεmitting tagε. 14. A network aε in Claim 10 wherein each of εaid εtationε include meanε connected to εaid inbound channel for εenεing carrier 15. A network aε in Claim 10 wherein each of εaid εtationε include meanε for receiving εpecified oneε of εaid tranε¬ miεεion unitε. 16. A network aε in Claim 13 wherein each of εaid stationε include meanε reεponεive to the detection of an end of carrier on εaid outbound channel for starting tranεmiεεion of one of εaid tranε isεion unitε on εaid communication buε. 17. A network aε in Claim 16 wherein each of εaid stations include a receiving tap on εaid inbound channel and means fo sensing carrier on εaid inbound channel. 18. A network aε in Claim 13 wherein each of εaid εtationε include a receiving tap on εaid inbound channel and means fo εenεing carrier on εaid inbound channel. 19. A network aε in Claimε 17 or 18 wherein a εuccesεion of tranεmiεεion unitε compriεe a train and wherein each of εaid εtations include meanε for εenεing an end of train on εaid inbound channel, meanε reεponεive to εaid end of train for initiating tranεmiεεion of a tranεmiεεion unit and for εimultaneouεly εenεing εaid outbound channel. 20. A network as in Claim 19 wherein each of εaid εtationε include meanε for detecting the end of one of εaid trainε on εaid inbound channel, meanε reεponεive to the detection of εaid end of train for generating a carrier having a predeter¬ mined duration, and meanε for initiating tranεmiεεion of a reεpective tranεmiεεion unit following εaid carrier. 21. A network aε in Claimε 16, 19 or 20 wherein each of εaid εtationε include meanε for εenεing carrier on εaid outbound hannel after εtarting tranεmiεεion of εaid reεpective tranεmiεεion unit, and meanε for aborting tranεmiεεion of εaid reεpective tranεmiεεion unit if carrier iε εenεed on εaid outbound channel while tranεmitting. 22. A network aε in Claimε 16, 19 or 20 further includ¬ ing meanε for completing tranεmiεεion of εaid tranεmiεεion unit if εaid outbound channel iε εenεed aε idle within a pre- determined time period. 23. A network aε in Claim 14 wherein each of said εtationε include roeanε for detecting a carrier on εaid inbound channel for a predetermined time period, meanε for tranεmitting con- tinuouεly a pilot carrier εignal if no carrier iε detected, meanε for detecting the beginning of a carrier on εaid inbound channel and meanε reεponεive to the detection of εaid beginning of carrier εignal for terminating tranεmiεεion of εaid pilot εignal. 24. A network aε in Claimε 21 or 22 wherein εaid data packetε are of different type and further including meanε fo integrating said different typeε of packetε on εaid communi¬ cation buε. 25. A network aε in Claim 24 wherein εaid meanε for inte¬ grating include meanε for recognizing the different typeε of packetε on εaid buε. 26. A network aε in Claim 24 further including meanε for diεtinguishing trainε of different typeε on εaid communica¬ tion buε. 27. A network aε in Claim 26 further including meanε for εpecifying the train type indicator. 28. A network aε in Claim 26 or 27 further including meanε for recognizing the train type. 29. A network aε in Claim 25 or 28 including meanε for tranεmitting a particular packet on a correεponding train of the εame type. 30. A network aε in Claim 25 or 28 wherein each of εaid stations include meanε for determining the εtate of the reεpective εtation for repreεenting the type of train in progreεε. 31. A network aε in Claim 30 including meanε for storing information repreεenting the previouε typeε of trainε tranεmitted. 32. A network aε in Claim 30 or 31 including meanε for up¬ dating the εtate of εaid εtation. 33. network aε in Claim 32 when εaid meanε for updating are reεponεive to an end of train on εaid communication buε. 34. A network aε in Claim 32 or 33 where εaid meanε for up¬ dating are reεponεive to an end of packet tranεmiεεion on εaid communication buε. 35. A network aε in Claim 21, 25, 27 or 36 including meanε for controlling the length of the train on εaid communication buε. 36. A network aε in Claim 35 including meanε for meaεuring the current train length in progreεε on εaid communication buε. 37. A network aε in Claim 36 including meanε for meaεuring a train in progreεε on εaid inbound channel. 38. A network aε in Claim 35 including meanε for meaεuring the length of a train in progreεε on εaid outbound channel. 39. A network aε in Claim 30, 36, 37 or 38 including meanε for aborting tranε isεion of tranεmiεεion unit when a certai maximum train length iε reached. 40. A network aε in Claim 36, 37 or 38 including meanε for εtarting tranεmiεεion of a tranεmiεεion unit if the eaεured train length including packet information doeε not exceed said maximum length. 41. A network aε in Claim 36, 37 or 38 including meanε for completing tranεmiεεion of tranεmiεεion unit aε long aε εaid maximum train length iε not reached. 42. A network aε in Claim 1, 24 or 27 wherein each εtation includeε at leaεt a firεt εtate permitting tranεmiεεion of a tranεmiεεion unit and a second εtate prohibiting tranεmiεsion of a tranεmiεεion unit and including meanε reεponεive to εaid firεt εtate for permitting tranεmiεεion of a tranεmiεεion unit and meanε reεponεive to εaid εecond εtate for prohibit¬ ing tranεmiεεion of a tranεmiεεion unit. 43. A network aε in Claim 1, 24 or 27 wherein each εtation includeε one or more εtateε for controlling tranεmiεsion and reception of εaid tranεmiεεion unit. 44. A network aε in Claim 42 or 43 including meanε for up¬ dating εaid εtation εtateε. 45. A network aε in Claim 43 or 44 including meanε reεponεi to an end of tranεmiεεion unit tranεmiεεion for updating εai εtation εtate. 46. A network aε in Claim 43 or 44 including meanε reεpon¬ εive to an end of train tranεmiεεion for updating εaid εtation εtate. 47. A local area communication buε compriεing a plurality o εtationε connected to εaid communication buε, each of εaid εtationε including a tranεmitting tap connected to εaid buε for tranεmitting data tranεmiεεion unitε, meanε for εenεing carrier in a certain direction on εaid buε, meanε for detec ing an end of εaid carrier, and roeanε reεponεive to εaid en of carrier for εtarting tranεmiεεion of a tranεmiεεion unit. 48. A network aε in Claim 47 further including meanε for aborting tranεmiεεion of εaid data unitε if carrier iε εenε on εaid buε. 49. A network aε in Claim 47 including meanε for completin tranεmiεεion of εaid data unitε if εaid buε iε εenεed aε id within a certain time period. 50. A local area communication network compriεing a commun cation buε having an inbound channel and an outbound channe a plurality of εtationε connected to εaid buε, each of εaid εtationε including meanε for tranεmitting and receiving dat tranεmiεεion unitε on εaid buε εuch that said tranεmiεεion OHPI υnitε are propagated on εaid inbound channel, and meanε responεive to the detection of an end of carrier on εaid out bound channel for starting transmiεεion of one of εaid tranεmiεεion unitε on εaid outbound channel. 51. A network aε in Claim 50 further including means for aborting tranεmiεεion of εaid tranεmiεεion unit if carrier iε εenεed on εaid outbound channel while transmitting. 52. A network aε in Claim 50 wherein εaid each of εaid εtationε further include means for completing tranεmiεεion of εaid tranεmiεεion unit if εaid outbound channel iε εenεed aε idle within a predetermined time period. 53. A network aε in Claim 50 wherein each of εaid εtationε includes means for detecting carrier on said communication bus for a certain time period, means for transmitting a pilo εignal if no carrier εignal iε detected, meanε for detecting the beginning of carrier on εaid inbound channel and meanε reεponεive to the detection of εaid beginning of carrier εignal for terminating tranεmiεεion of εaid pilot εignal. 54. A local area communication network compriεing a communi cation buε having an inbound channel and an outbound channel and a plurality of εtationε connected to εaid communication buε, each of said εtationε including meanε for tranεmitting and receiving information data packetε on εaid communication buε wherein εaid data packetε are of a different type, and means for integrating εaid different typeε of packetε on εai communication buε εuch that the data packetε are progagated on εaid inbound channel in the εame direction with reεpect t εaid εtation aε on εaid outbound channel. 55. A local area communication network compriεing a communi cation buε having an inbound channel and an outbound chahnel and a plurality of εtationε connected to εaid communication buε, each of εaid εtationε including a tranεmitting tap on εaid outbound channel and meanε for tranεmitting and receiv- ing information data packetε on εaid communication bus εuch that εaid data packetε are progagated on εaid inbound channe in the εame direction with reεpect to εaid stations on εaid outbound channel. 5 56. A network aε in Claim 55 wherein said tranεmitting tap iε bidirectional and wherein each of εaid εtationε include meanε for εenεing carrier on the upεtream εide of εaid tranε¬ mitting tap. 10 57. A network aε in Claim 55 wherein each of εaid εtations include means for forcing unidirectionality on εaid communi¬ cation buε with reεpect to said connected εtationε. 15 58. A network aε in Claim 57 wherein εuch εtationε include meanε for εenεing carrier on the upεtream εide of the firεt of εaid enforcing meanε. 59. A network aε in Claim 57 wherein each of εaid εtationε 20 includeε meanε for εensing carrier on εaid bus other than the respective transmiεεion by εaid εtation, and meanε for εubtracting the tranεmitted εignal of εaid respective εtation. 25 60. A network as in Claim 56 wherein each of εaid εtationε includeε at leaεt two tapε on εaid communication buε εeparat ed by a predetermined diεtance for forcing unidirectionality on εaid communication buε. ■ 30 61. A network aε in Claim 60 wherein εaid enforcing meanε includeε delay meanε for delaying εaid data packetε and mean for negating the delayed tranεmitted εignal. 62. A network aε in Claim 61 wherein each of εaid εtations 35 includeε meanε for εenεing carrier upεtream with reεpect to the tranεmiεεion direction. 63. A network aε in Claim 55 including directional coupler meanε for coupling εaid data packetε to εaid communication buε.

Inventors: BORGONOVO FLAMINIO, FRATTA LUIGI, TOBAGI FOUAD A, BORGONOVO, FLAMINIO, FRATTA, LUIGI, TOBAGI, FOUAD, A.
Applicants: BORGONOVO FLAMINIO, FRATTA LUIGI, TOBAGI FOUAD A, BORGONOVO, FLAMINIO, FRATTA, LUIGI, TOBAGI, FOUAD, A.
Application year: 1982
Main classification: H04J6
IPC codes: H04L12, H04Q11