RECONFIGURABLE COLLISION AVOIDING SYSTEM, STATION AND PROTOCOL FOR A TWO PATH MULTIPLE ACCESS DIGITAL COMMUNICATIONS SYSTEM

Abstract: In order to control the transfer of packets of information among a plurality of stations, e.g., digital computers, the instant communications system, station and protocol contemplate first (10) and second (60) oppositely directed signal paths. At least two stations are coupled to both the first and the second signal paths. A station (70-i) includes arrangements for reading (20-i, 40-i) (and writing (30-i, 50-i)) signals from (and on) each of the two paths. Signals are read from either path by an arrangement which electrically precedes the arrangement for writing signals on the path. If the station has a packet to transmit, it can overwrite a busy control field of a packet on either path. Also having read a packet signal from the path, a logical interpretation may be made within the station as to whether the path is busy or is not busy. If the path is not busy, the packet may be written on the path by over-writing any signal thereon. If the path is busy, the station may defer the writing until the path is detected as not busy, e.g. by reading a busy control field of another packet. Here the two paths are coupled to form two rings structures so the packets flow down the path to a destination station. Eventually all packets will be transmitted and the busy control field loops around to a station which is designated as a control station (70-(1,N)). The control station, responsive to detecting that all packets have been transmitted on one of the paths, may transmit a start cycle packet on the one path, responsive to which event a new cycle may be started by initializing each station on the one path and by permitting each initialized station to transmit. In the event of an electrical open (500), the system, station and protocol are adapted to reconfigure the system so that the station (70-i) electrically preceding the open functions as an end station while the station (70-(i+1)) electrically succeeding the open functions as a head station.

Description: RECONFIGURABLE COLLISION AVOIDING SYSTEM, STATION AND PROTOCOL FOR A TWO PATH MULTIPLE ACCESS DIGITAL COMMUNICATIONS SYSTEM Technical Field This invention relates to a method for controlling digital signals in a communications system, th system including a plurality of stations, at least two of the stations being coupled to both a first and a second signal path, the method including the steps of writing signals on the first path, reading signals from the first path, the first path reading step occurring electrically before the writing on the first path, reading signals from the second path, writing signals on the second path, the second path writing step occurring electrically after the reading from the second path. Background of the Invention In the early days of the telephone art, use of the telephone was often confined to communications among users within a local geographic area. As a result and ove the years, the economies related to accessing a communications system have led to telephones in a local area usually being interconnected through a central controller, often called a local central office in the art. As digital computers came upon the scene, anothe local community of use was discernible. Hence, a central controller is commonly employed for interconnecting variou user terminals. For example, U. S. Patent 3,851,104, discloses a time division, multiple access communications system which, under the control of a central terminal, provides communication links among a plurality of user terminals by way of a single communications signal path. As the digital computer art advanced, parallel advances in the semiconductor art have led to smaller, relatively inexpensive computers. With the advent of such smaller computers, the technique of central computer control is being abandoned in favor of a distributed control technique. Also, because of the usually bursty nature of digital information, the recent trend has also been toward communications systems having a capability for handling packets of digital information. One such distributed control communications system is disclosed in U.S. Patent 4,063,220. Indeed, the '220 patent discloses communications system in which, when a terminal is to star an intended transmission on a communications path, a phase decoder detects the presence of other transmissions on the path and, responsive thereto, delays the intended transmission until no other transmissions are sensed. Onc a transmission has started, if an interference (or collision) therewith is detected, a random number generato is used to select an interval of time at the completion of which the next transmission will be attempted. Collisions being a problem, efforts exist toward providing communication protocols for mitigating the deleterious effects of collisions. There is a communications system, which contemplates first and second oppositely directed signal paths. At least two stations are coupled to both the first and the second signal paths. A station includes arrangements for reading one signal fro the first path and for writing another signal on the first path. The one signal is read from the first path by an arrangement which electrically precedes the arrangement fo writing the other signal on the first path. A similar rea arrangement is electrically preceding a write arrangement on the second path. If the station has a packet to transmit, it can overwrite a busy control field of a signa packet on either path. The station can also read packets on either path. Having read a signal packet on a path, a logical interpretation may be made within the station as t whether the path is busy or is not busy. If a path is not busy, the packet may be written on the path by overwriting any signal thereon. If the path is busy, the station may defer the writing of a packet until the path is detected a not busy, i.e. through the contents of a busy control field. The packets flow down a respective path to a destination station. Eventually all packets will be transmitted and the busy control field may be detected at an end station for' indicating the event that all packets have been transmitted. The end station on one path acts the head station on the other oppositely directed path. Hence, responsive to detecting that all packets have been transmitted, the end station on the one path, which is the head station on the other path, may transmit an end cycle packet on the other path, responsive to the detection of which event a start cycle packet is written by the head station on the one path whereby a new cycle may be started by initializing each station on the one path and by permitting each such initialized station to transmit one o more packets. Still alternative solutions are being sought to further improve the efficiency of such, systems, for example, to mitigate propagation delay time or to adapt to a situation where a signal path becomes broken or electrically open, perhaps because of a natural disaster. Summary of the. Invention The problems are solved in accordance with this invention by a method for controlling digital signals in a communication system in which the method further comprises the steps of reading a busy control field of a packet by a control station which is coupled to both of the signal paths and responsive to the read busy control field indicating that a respective one of the paths is not busy, initiating a start cycle packet on the one path, the start cycle packet being transmitted by the control station on the one path and the start cycle packet including a start cycle field responsive to which a station on the one path may be initialized to transmit a packet. The communications system includes first and second oppositely directed, unidirectional communications signal paths. The respective paths are close connected to form two unidirectional rings. Each of at least two stations such as a digital computer is coupled to both the first and the second paths and hence to the two rings. Each station has an arrangement for reading the signals from the respective paths, which arrangements electrically preceding an arrangement for writing the signals on the respective paths. In response to the read signals, collisions are avoided on the communications paths. For example a busy field of a packet on a signal path indicates that the respective path is either busy or not busy. If the path i not busy, the station may write a packet on the path. If the path is busy, the station may overwrite the busy control field for indicating that the station has a packet to transmit and that it is contending for the path. The electrically first station on a path is called a head station while the electrically last station on the path is called an end station. When an end station detects the path as not busy, the end station need not propagate that information to a distant head station because the head and end station functions are colocated in a control station. The control station may initiate a start cycle, thereby mitigating propagation delay time. Upon detecting an electrical open signal path, each station is adapted such that the station which electrically precedes the open becomes an end station while the station which electricall succeeds the open becomes a head station. Thereby the deleterious effects of an open are mitigated in that the system adaptively is reconfigured to avoid those effects. Brief Description of the Drawing The invention should become fully apparent when taken in connection with the following detailed descriptio and accompanying drawing in which: FIG. 1 illustrates an arrangement including a plurality of stations interconnected with two oppositely directed unidirectional communications paths which arrangement is useful in describing the principles of the invention; FIG. 2 illustrates bipolar digital signals usefu in describing the principles of the invention; FIG. 3 illustrates a synchronizing signal and digital signal pattern useful in describing the principles of the invention; FIG. 4 illustrates a typical packet format including access control field useful in describing the principles of the invention; and FIG. 5 illustrates an arrangement which modifies the arrangement of FIG. 1 to mitigate propagation delay an to reconfigure the system in accord with the principles of the invention. Detailed Description Referring to the illustrative structure shown in FIG. 1, a first unidirectional communications signal path 10 is illustrated as extended through a plurality of cascaded transmission couplers such as passive read coupler 20-i and passive write coupler 30-i. A second unidirectional signal path 60, which path is oppositely directed with respect to first path 10, is also there illustrated as extended through another plurality of cascaded transmission couplers such as passive read coupler 40-i and passive write coupler 50-i. Thus, each o stations 70-1 through 70-N is twice coupled to each of the two oppositely directed communications paths. More particularly, on the one hand, nondirectional read couplers 20-i and 40-i are for electrically coupling signals, which are detected on a respective first 10 or second 60 communications path to station 70-i. In that manner, station 70-i may monitor or read signals on both the first path and the second path. On the other hand, directional write couplers 30-i and 50- are for coupling and inserting or writing signals from station 70-i onto the respective communications path. For example, the inserted signal could, responsive to a signal being priorly read from path 10 through the electrically preceding nondirectional read coupler 20-i, overwrite the first path signal through directional write coupler 30-i, e.g., by the energy of write signal being electrically added to the signal, if any, already on path 10. In parallel fashion, the inserted signal could, responsive to a. signal being priorly read from path 60 through electrically preceding nondirectional read coupler 40-i, overwrite the second path signal through write coupler 50- In that manner, station 70-i may transmit a signal by causing the signal to be inserted or written on the communications path. As should be clear from FIG. 1, ther are actually two transmit sides of the communications system. For example, as to station 70-i transmitting a packet, the packet would be transmitted on communications path 10 if the packet is destined for station 70-k where k is greater than i. On the other hand, if the packet is destined for station 70-j where j is less than i, the packet would be transmitted on communications path 60. Th foregoing constraint would not necessarily apply to the structure of FIG. 5, which will become self-evident shortly. Station 70-i may include station terminal equipment such as a digital computer or a straightforward digital interface unit, the interface unit, for example, for interconnecting paths 10 and/or 60 with another system the other system perhaps being geographically distant from the instant system. As an aside, it is common in the art that, when the stations are electrically relatively close to each other, e.g., within about two miles of each other, such a system is referred to, among other ways, as a local area network. Thus, in line with this advantage of the structure illustrated in FIG. 1 , a plurality of local area networks may be interconnected for forming a still larger communications system. Before more particularly describing the operatio of a system embodying the principles of the instant invention, some ancillary matters are first described. As to electrical signals on the communications paths, assume for illustration and not by way of limitati a bipolar digital signal of the type illustrated in FIG. Each logic zero is assumed to have a first voltage level potential, e.g., a zero level potential. On the other hand, a logic one or a sequence of consecutive logic ones may be transmitted at a second potential level (e.g., +V) until a next logic zero is to be transmitted. The logic zero may be transmitted, as aforedescribed, at the first potential level. After the logic zero or sequence of consecutive logic zeroes is transmitted, a then following logic one or a sequence of consecutive logic ones is transmitted at a third potential level, here the third potential level (e.g., -V) is illustratively the inverse the second potential (+V) level. The logic one potential level may continue such alternations after each logic zero or sequence of consecutive logic zeroes. As to synchronizing digital signals on the communications paths, assume for illustration and not by way of limitation a synchronizing digital signal sequence such as that illustrated in FIG. 3. The there illustrated synchronizing sequence is hereinafter called a sync burst and may be transmitted by any of the stations on a path, the sync burst transmitting station being referred to as the head station. (As will be made clearer hereinafter th head station contemplates certain functions which may also be incorporated in a single control station.) As a furthe aside, each half of the system, e.g., one half including first path 10 and the other half including second path 60, may simultaneously have a respective head station. Accordingly, for brevity of description, we assume for purposes of illustration that station 70-1 is the "head" station on path 10 while station 70-N is the "end" station on path 10. Conversely, station 70-N is the head station on path 60 while station 70-1 is the end station on path 60. Continuing with reference to FIG. 3, although other synchronizing sequences could be used, the illustrative sync burst includes a plurality of bipolar violations followed by a predetermined digital sequence. Bipolar violations may include a succession of alternations between the second and the third potential levels with no intervening logic zero. It may be noted that such a sequence of alternations is contrary to the aforedescribed bipolar signal of FIG. 2, hence the term bipolar violation. The bipolar violations may be transmitted from the respective head stations and may be extended through respective read couplers 20-i and 40-i to synchronizing an timing circuitry of station 70-i for bit synchronizing the station circuitry and the respective path digital signals. Following a prefixed number of bipolar violations, the sync burst may further include a predetermined digital sequence such as the Barker sequence "01011100". The Barker sequence may also be transmitted from the respective head stations and may also be extended through respective read couplers 20-i and 40-i to synchronizing and timing circuitry of station 70-i for synchronizing the station circuitry and the path digital signals. As a result, each of the plurality of stations on each of the two paths includes a straightforward arrangement for both bit synchronizing and data synchronizing the station in response to the detection of the sync burst of bipolar violations and Barker sequence. As to inserting or writing a signal from a station onto a path, subsequent to the transmission of the sync burst, one or more packets of information, e.g., M packets, may be transmitted on the path. Indeed, a packet may, for example, be transmitted in a time slot as time division multiplexed signal. However, absent a control protocol, collisions between the packets could occur. According to the principles of the instant invention, a hereinafter described control protocol may be used for controlling the signals on the system and for avoiding collisions, for mitigating propagation delay and for reconfiguring the system in the event of an electrical open in either communications path 10 or 60 or both. As to the instant control protocol, FIG. 4 depicts an illustrative packet of predetermined size and comprising K bits, where for illustration and not by way limitation K is assumed to equal 160 bits. Also, for illustration, a packet is assumed to include an access control field, which field in turn includes one or more bits. Here, the first eight bits of the illustrative data packet are labelled start cycle, end cycle, don't care, don't care, busy, don't care, don't care and don't care, respectively, and may comprise an access control field for the instant protocol. The packet may, as is common in the art, also include a destination station address field, a source station address field, a data field, and an error control field. Again, each field may include one or more bits. One error control field may also include a well known cyclic redundancy code (CRC) . Also, the packet may be read from or written into a time slot of a time divisio multiplexed signal on the path. Now that the above ancillary matters have been described, the following description focuses on the logica interpretation of the access control field. Firstly, assume, as aforedescribed, that a sync burst has been transmitted from the respective head station on respective paths 10 and 60 for bit and data synchronizing each station. Thereafter, one or more data packets may be transmitted as a time division multiplexed signal. Each data packet is assumed to have the format illustrated in FIG. 4. Secondly, assume for purposes of illustration an not by way of limitation that each of the N stations may transmit a single packet during a time frame, which time frame is hereinafter referred to as a cycle. The first packet in a cycle would have its start cycle bit set to a logic one by the head station. In general, all other packets in the cycle would have their respective start cycle bit set to a logic zero. Responsive to the detectio of a logic one start cycle bit, each respective station on the path is logically alerted that a new cycle is started and accordingly may set a respective "permit register" (hereinafter also called a p-register) to a logic one for indicating that the station may transmit a packet. As hereinafter described, after a station has transmitted a packet, the station resets its permit register to a logic zero for indicating that the station may not transmit a packet. Indeed, each station could have two permit registers, one p-register being related to path 10 and the second p-register being related to path 60. As an aside, station could transmit more than one packet in each cycle. Thirdly, each station which has a packet to transmit may, responsive to -its respective permit register being set to a logic one, extend a logic one through its write coupler 30-i (or 50-i) to path 10 (or 60) for overwriting the busy bit of the read packet, here illustratively bit 5 of the packet shown in FIG. 4. The overwriting of the busy bit with a logic one may occur whether or not the busy bit being overwritten is already a logic one. Fourthly, read circuitry of station 70-i includes circuitry for reading the busy bit. Assume that station 70-i has a packet to transmit and that its permit register is set to a logic one. As mentioned, the busy bit is overwritten with a logic one. However, as is evident from FIG. 1, read coupler 20-i (or 40-i) electrically precedes write coupler 30-i (or 50-i) and the two couplers may be a relatively short electrical distance apart. As should also be evident from FIG. 1, some finite time interval may be needed for determining whether or not the packet may be written onto the path, e.g., the determining action including interpreting the contents, or logic value, of the busy bit of the packet being read. Experimentation has indicated that a relatively few bit time intervals allow adequate time for such a determination. Hence, in the instant embodiment, the "don't care" control bits allow three bit time intervals, e.g., the time interval of don't care bits 6, 7 and 8 of the read packet, for allowing an adequate time interval during which read circuitry of station 70-i may interpret or determine the logic value of the busy bit of the packe being read. On the one hand, if the contents of the busy bit of the packet being read is detected as a logic zero, the packet to be written may be written through write coupler 30-i (or 50-i) onto path 10 (or 60). The station then may reset its permit register to a logic zero for indicating that the station may not transmit a packet. On the other hand, if the contents of the busy bit of the packet being read is detected as a logic one, the packet to be written is not then written. Indeed, if the packet were written, a collision would occur. Instead responsive to the busy bit of the read packet being a logic one, write circuitry of station 70-i makes a transition into a wait state and waits until the next nonbusy packet is detected. Thus, in summary, a station may transmit only when its permit register is set to a logic one and when a read busy bit is a logic zero. After having transmitted a packet, the station sets its permit register to a logic zero. If a station has a packet for transmission and if its permit register is a logic one and if the read busy bi is a logic zero, the station can overwrite the busy bit with a logic one and transmit its packet. Thereafter, as packets flow over respective path 10 or path 60, the packets are read on the receive side of station 70-i through respective read coupler 20-i or 40-i. Each station 70-i can monitor the path for packets having, among other things, a destination address identifying the respective station as the addressed or called station. Upon detection of a destination address for identifying station 70-i as the addressed station, receive side read circuitry in station 70-i can thereafter read the packet and process the packet in whatever manner the station may have been programmed or adapted to do. It should be clear that station 70-N would not usually transmit packets on communications path 10. (However, the foregoing constraint would not necessarily apply to the structure of FIG. 5.) That is, all packets transmitted from station 70-N would be transmitted on communications path 60. Accordingly, responsive to station 70-N detecting a logic zero busy bit on path 10, station 70-N transmits an end cycle packet on communications path 60. The end cycle packet includes a logic one end cycle bit, which may be conveniently located either after the start cycle bit or after the busy bit which location is shown in FIG. 4 to include a don't care bit. Upon detection by station 70-1 of a logic one end cycle bit on path 60, station 70-1, which is the end station on path 60 but the head station on path 10, may initiate a start cycle packet on communications path 10 for commencing a new cycle. It should also be clear that the foregoing description relative to communications path 10 is substantially identical with respect to communications path 60 with the specific head and end station functions being reversed. Of course, it does take some finite time for the end cycle packet to propagate from the end station on one (forward) path to the head station on the one (forward) path with the transmission of the end cycle packet occurring on the other (reverse) path. Recognizing this problem, we propose to merge the head station functions and the end station functions into one collocated station which performs the functions of both the head station and the en station. Inasmuch as the head station and the end station are thusly collocated, the collocated stations can thusly function as, and are hereinafter referred to, as a single control station. The control station, in response to detecting a logic zero busy bit while performing its end station function, can without propagation delay initiate i its head station function a start cycle on the forward path. The foregoing advantage is depicted in the structur illustrated in FIG. 5, which structure makes clear that th resultant structure comprises two unidirectional, but oppositely directed, paths which are interconnected to for two ring networks. Specifically, unidirectional path 10 from FIG. 1 is redrawn to form a unidirectional outer ring 10 of FIG. 5. In like fashion, unidirectional path 60 from FIG. 1 is redrawn to form a unidirectional inner ring 60 o FIG. 5. Further, stations 70-1 and 70-N, the head and end stations from FIG. 1 , are redrawn as merged to form a single control station 70-(1,N) which functions as both head and end stations for the two oppositely directed, unidirectional single paths (or rings) 10 and 60 of FIG. 5. Of course, and as mentioned with the structure of FIG. 1, an end station, e.g. station 70-N for path 10 and station 70-1 for path 60, would not transmit on the path for which it is an end station. Notwithstanding, the ring structure of FIG. 5 is not thusly constrained. In that manner, our improved structure of FIG. 5 mitigates the end cycle packet propagation delay time problem of the structure of FIG. 1. Specifically, since signals on the outer ring are illustrated as propagating in a clockwise direction on path 10, there will come a time when a packet having a logic zero busy bit will be detected at control station 70-(1,N) through one of its read couplers, here coupler 20-(1,N). In like fashion, since signals on the inner ring are illustrated as propagating in a counter- clockwise direction on path 60, there will come a time whe a packet having a logic zero busy bit will be detected at control station 70-(1,N) through the other of its read couplers, here, coupler 40-(1,N). In either case, station 70-(1,N), while performing an end station function, is adapted to transmit a start cycle packet on the respective signal path in response to detecting the logic zero busy bit on that respective path, thusly also performing a head station function. Thereupon a new cycle is initiated responsive to which the stations 70-i t al_ may transmit a packet. Having thus far described an advantage according to an aspect of the principles of this invention, e.g., a mitigation of propagation delay to more quickly start a new cycle, we now turn to another advantage according to another aspect of the principles of this invention. Consider the event in which there is an electrical open in either communications path. According to this aspect of the principles of this invention, each station on the path is adapted to reconfigure itself such that the ring structure of FIG. 5 is adapted to transform to the bus structure of FIG. 1. When in the structure of FIG. 1, the system, stations and protocol can operate. For example, referring to FIG. 5, assume some disaster occurs such that an electrical open occurs either in path 10 or in path 60 or in both paths. The open is assumed to occur at cut 500. An electrical open may be detected by a station in response to detecting the absence of packet signal or synchronizing signals on a path. For example, assume an open occurs at cut 500. As a result, signals on path 10 cannot propagate through cut 500. Hence, station 70-(i+1) through its read coupler 20-(i+1) detects the absence of incoming signals, concludes it is electrically succeeding the open, and adapts itself to be the head station on path 10 as well as the end station on path 60. The ring system of FIG. 5 then functionally reduces to system of FIG. 1 with station 70-(i+1), i.e. the station which electrically succeeds the open, becoming the head station of path 10 and with station 70-i, i.e. the station which electrically precedes the open, becoming the end station of path 10. Of course, the converse in statio occurs on path 60, i.e. station 70-i becomes the head station on path 60 while station 70-(i+1) becomes the end station. Each station is then adapted to function as priorly described, until the electrical open at cut 500 is repaired at which- point in time the ring system of FIG. 5 is resumed. Although the invention has been described and illustrated in detail, it is to be understood that same i by way of illustration and example only. For brevity, so alternative modifications of the principles of the invention have been disclosed. Still other modifications of the principles of the invention will occur to those skilled in the art, e.g., logic values could be inverted o access control field bits could be relocated or the directions of the inner and outer rings could be reversed. Accordingly, the invention is not to be considered limited by the embodiment(s) shown and thus the spirit and scope o the invention are limited only by the appended claims.

Claims: Claims 1. A method for controlling digital signals in a communications system, the system including a plurality of stations, at least two of the stations being coupled to both a first and a second signal path, the method including the steps of: writing signals on the first path; reading signals from the first path, the first path reading step occurring electrically before the writing on the first path; reading signals from the second path; writing signals on the second path, the second path writing step occurring electrically after the reading from the second path; CHARACTERIZED IN THAT the method further comprising the steps of: reading a busy control field of a packet by a control station which is coupled to both of the signal paths; and responsive to the read busy control field indicating that a respective one of the paths is not busy, initiating a start cycle packet on the one path, the start cycle packet being transmitted by the control station on the one path and the start cycle packet including a start cycle field responsive to which a station on the one path may be initialized to transmit a packet. 2. The method defined in claim 1 wherein the first path and the second path are, respectively, unidirectional paths and wherein the paths are oppositely directed. 3. The method defined in claim 1 further comprising the step of overwriting a busy control field of a first packet by a station on the one path with a signal for indicating that the overwriting station has a second packet to transmit on the one path. 4. The method defined in claim 1 further O CO to to __ __. cn o cn o cn o cn 3 Φ Ω Oi F- o XJ rt 3 tr TJ tr Φ rt Φ rt F- 01 F- o F- 3 Hi 3 F- 03 π •-J ft ft rt ta • F- tr φ ft > βl Φ (A tr » F 1 TJ Φ > Fj F- 01 o o tr N F- 3 01 F_ φ Φ 03 01 ft - a 3 F- φ .- 01 βl < TJ Fl J βl F* φ ■ I 01 3 o t_ ft a TJ ft Hi O φ βl 0 3 TJ ft M Φ tr a 2! F- rt 01 Φ 3 3 *» rt F_ F- Φ tα βl rt βl Ω Ω ft a ft F_ Ω fl> βl F- o a TJ 3 rt rt ιa a rt F- 01 o 3 βl 3 03 3 Ω rt φ rt βl φ βl F" s. 3 Hi φ F- 01 F- Ω rt 3 rt ft 3" F- 01 rt ft ft F- Ω Ω F 1 βl 01 βl 01 ft F 1 F- TJ βl 3 01 ft o Ω F- J σ. o φ Φ 3 3 rt • ft F- o 3 the first path and the second path are respective unidirectional paths. 8. The system in accordance with claim 6 CHARACTERIZED IN THAT the system responsive to detecting an electrical open in either of the signal paths, for adapting a first station to function as a head station for adapting a second station to function as an end station. 9. The system in accordance with claim 8 CHARACTERIZED IN THAT the first station electrically succeeds the open and the second station electrically precedes the open.

Inventors: LEVINSON FRANK HOWARD, LIMB JOHN ORMOND, LEVINSON, FRANK, HOWARD, LIMB, JOHN, ORMOND
Applicants: AMERICAN TELEPHONE & TELEGRAPH, AMERICAN TELEPHONE & TELEGRAPH COMPANY
Application year: 1984
Main classification: H04L11
IPC codes: H04L12