Abstract:
An unslotted radio frequency multiple access bus communications system employing a contention listen-while-talk (LWT) protocol. The system includes a plurality of remotely located terminals coupled to a communications bus. The bus includes a pair of oppositely directed, unidirectional signal paths which are connected at one end so that one path is an inbound path to that end and the second is an outbound path from that end. Each of the remote terminals includes a bus interface unit (BIU) coupled to both the inbound and outbound paths at the remote location. Each BIU is adapted to transmit and receive modulated carrier signals on the inbound and outbound paths, respectively. 
     When a terminal wishes to send a message to another terminal, the terminal&#39;s BIU initially performs an rf carrier sensing operation to make an initial determination as to whether or not any other subscriber is transmitting on the inbound path. In the event no carrier is detected, the BIU confirms this determination by first beginning to transmit a message packet modulated on a carrier signal and addressed to the desired receiving terminal on the inbound path, and then monitoring the outbound path during a collision window. If the transmitted message is received intact by the transmitting terminal within this collision window, the transmitting terminal&#39;s BIU determines that no collision has taken place between its message and a message sent by another terminal or noise, and that the transmitting terminal has gained access to the bus. Thereafter, the BIU may transmit any remaining portion of its message packet on the inbound path. However, in the event a collision is detected, the transmitting terminal&#39;s BIU aborts its transmission and then backs off for a random time period before again attempting to gain access to the bus and transmit its message.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to communications systems, and more particularly, to time division, multiple access digital bus communications systems. 
     The need for the efficient interconnection of computers and computer-related devices has become increasingly apparent with the development of computer technology. In particular, multi-processor systems typically include a number of computers, often widely distributed, which are interconnected by a data bus to remote subscriber terminals and associated peripheral devices. With the developing technology, such terminals and devices have become diverse and capable of wide distribution. The interconnection between such devices has become critical to the system use and performance, particularly in view of the need for adequate digital communication bandwidth, fast response time, high reliability, and the requirement for full access to all coupled but physically distributed devices. Conventional systems typically utilized circuit and message switching techniques. In addition, packet switching techniques have been utilized, although such systems have required relatively expensive store and forward nodes, which raise cost and reliability problems. 
     Primarily, the conventional time division multiple access (TDMA) bus communications systems have relied on time slotting and assignments of these slots to transmitting subscribers, either permanently or at sign-on to the TDMA system. For example, see U.S. Pat. No. 3,851,104, Willard, et al. While such systems are efficient for a population of high duty cycle traffic, large numbers of low duty cycle users, such as intercommunicating computers, quickly overburden many of the prior art TDMA systems having slots uniquely assigned to various subscribers. 
     In view of this problem, bus contention protocols have been developed to more efficiently utilize the available bandwidth for computer and terminal communications. Generally, contention systems permit a user to gain access to the entire channel bandwidth for his message burst. In operation, a user having a message to send is permitted to transmit his message whenever he is ready. In the event another user is presently transmitting over the channel, or does so during the first user&#39;s transmission, then a message collision occurs. The system users must sense these collisions and, in response, retransmit their respective messages. Some systems provide random delays before retransmissions to avoid &#34;butting&#34;, i.e. continual collisions between users. 
     The contention protocols take advantage of the low duty cycle or &#34;bursty&#34; nature of data being transmitted from terminals and computers. For a large subscriber population of bursty users, the law of large numbers ensures that the channel bandwidth is only required to match the average aggregate data transmission rate of the entire population, rather than matching the sum of the peak rates for bursty subscribers as in the noncontention systems. 
     Several approaches to the contention protocol have been developed in the prior art. For example, the ALOHA system (see N. Abrahamson &#34;The ALOHA System--Another Alternative For Computer Communications&#34;, 1970 Fall Joint Computer Conference, AFIPS Conf. Proc. Vol. 37, AFIPS Press, 1970), describes an unslotted system wherein a plurality of remote stations are connected to a central station by way of a single radio channel. The various remote stations contend for the channel and transmit complete packets of data. In this unslotted configuration, the data packets are not synchronized between stations. When a collision is detected by failure to receive an acknowledgement from the central station, each of the transmitting terminals then retransmits its packet. With this configuration, the ALOHA system is relatively efficient for bursty terminals (e.g. a terminal duty cycle on the order of 1%), compared with a system using separate channels for each station. However, a collision detected during a packet transmission results in the waste of the entire overlapping transmission intervals for the terminals (i.e. as much as two packet lengths), limiting maximum efficiency. 
     Slotted (i.e. systems where the transmitted packets are synchronized from terminal to terminal) ALOHA systems have been suggested to provide an improvement in efficiency compared with unslotted ALOHA systems. In the latter configuration, the channel is divided into time slots of duration equal to the fixed data packet length. Each user is constrained to begin transmission of a data packet at the start of any time slot. When a collision occurs during a slot, the wasted time is equal to a single packet length, resulting in a substantially higher efficiency than the unslotted system. 
     Another form of slotted, contention protocol communications system is disclosed in U.S. patent application Ser. No. 881,704, assigned to the assignee of the present application. 
     An unslotted, listen-while-talk (LWT), protocol, baseband communications system is disclosed in U.S. Pat. No. 4,063,220. This system includes a plurality of remote terminals coupled to a two-way communications medium. Prior to transmission, a terminal monitors the medium to detect activity (i.e. to sense whether the medium is currently busy). If the medium is busy, the terminal must defer at least until the current user is finished and the medium is no longer busy. If any terminal determines that the medium is not presently busy, that terminal may attempt to transmit on the medium. 
     As a user transmits his message, he also monitors the medium. If the user does not detect any errors in his own transmission for the duration of the end-to-end propagation delay along the medium, then he determines that he has gained sole access to the medium and will successfully complete his packet transmission (unless due to noise interference other than from other users competing for the medium). Upon detection of a collision within the propagation delay interval, each colliding user aborts his transmission, and subsequently retransmits his message at a time when he determines that the medium is no longer busy. With this configuration, the time wasted during a collision is only the end-to-end propagation delay interval for the medium, in contrast to an interval equal to a single packet length as in the slotted ALOHA system, and an interval greater than a packet length as in the unslotted ALOHA system. 
     Although relatively efficient compared with contention systems of the unslotted and slotted ALOHA types, the prior art LWT systems incorporating baseband transmission over a two-way medium generally require bi-directional repeaters located with minimum separations along the medium. This requirement is particularly important for relatively large systems where a terminal must perform the collision detection packet comparison operation with his own very high signal-to-noise ratio transmission with the substantially lower signal-to-noise ratio transmission which might come from a distant subscriber. In order to perform this operation, repeaters are provided to digitally regenerate the baseband signals on the medium, including reclocking of the signals to obtain full period bits with satisfactory rise and fall times. In addition, the repeaters must be spaced relatively close together on a multi-drop medium in order to offset delay distortion. These requirements for close repeater spacing are costly in terms of equipment and reliability. Furthermore, the prior art baseband LWT systems are highly susceptible to low frequency noise often found in industrial and computer environments. 
     It is an object of the present invention to provide an unslotted radio frequency, multiple access bus communications system employing a listen-while-talk contention protocol. 
     It is a further object to provide a radio frequency, multiple access bus communications system with listen-while-talk contention protocol over a system having unidirectional inbound and outbound signal paths connecting subscriber terminals. 
     SUMMARY OF THE INVENTION 
     Briefly, the present invention is directed to an unslotted radio frequency multiple access bus communications system employing a contention listen-while-talk (LWT) protocol. The system includes a plurality of remotely located terminals coupled to a communications bus. The bus includes a pair of oppositely directed, unidirectional signal paths which are connected at one end so that one path is an inbound path to that end and the second is an outbound path from that end. Each of the remote terminals includes a bus interface unit (BIU) coupled to both the inbound and outbound paths at the remote location. By way of example, the communications bus may comprise a pair of coaxial cables and conventional CATV components connected in a two-way cable distribution network. 
     Each BIU is adapted to transmit and receive modulated carrier signals on the inbound and outbound paths, respectively. The bus interface unit transmits on the inbound cable and receives on the outbound cable. No repeater or other head end equipment is necessary since the system is unslotted and each BIU transmits and receives on the same frequency. In alternative embodiments, frequency conversion may be utilized at the junction of the inbound and outbound paths. 
     In operation, when a terminal wishes to send a message to another terminal, the terminal&#39;s BIU initially performs an rf carrier sensing operation on the outbound path to make an initial determination as to whether or not any other subscriber is transmitting on the inbound path. In the event no carrier is detected, indicating that no other subscriber is using the inbound path, the BIU confirms this initial determination by performing its listen-while-talk function by first transmitting a message addressed to the desired receiving terminal on the inbound path, and then monitoring (i.e. &#34;listening-while-talking&#34;) the outbound path for at least the maximum (end-to-end) propagation delay of the entire bus. If the transmitted message is received intact by the transmitting terminal within this period, the transmitting terminal&#39;s BIU determines that no collision has taken place between its message and a message sent by another terminal, and that the transmitting terminal has gained access to the bus. Thereafter, the BIU may transmit any remaining portion of its message packet on the inbound path. However, in the event a collision is detected, the transmitting terminal&#39;s BIU aborts its transmission and then backs off for a random time period before again attempting to gain access to the bus and transmit its message. 
     Thus, with this multiple access configuration, all subscribers (i.e. terminals) share the digital channel and have access to all information on the channel and are capable of transmitting all other subscribers on the channel. In addition, no subscriber starts a transmission unless, to the best of the subscriber&#39;s knowledge, the channel is not in use. Generally, this determination is made by sensing carrier on the outbound path. 
     With this configuration, a large number of low duty cycle, or &#34;bursty&#34;, subscribers may be accommodated by determining a bandwidth appropriate to match the average aggregate data transmission rate of the entire population, rather than match the sum of the peak rates for bursty subscribers, as in conventional systems. Through the use of message transmission by modulated carrier signal on a unidirectional signal path, the present invention provides a more efficient system than the presently known listen-while-talk protocol systems, since the transmitted data is effected relatively little by signal-to-noise degradation on the line, and no repeaters are required. Furthermore, by utilizing the unidirectional signal paths aspects of the present invention, relatively large bandwidth can be used and thereby increasing the data transfer capability. As a further advantage of the present system, the modulated rf signals are relatively immune to error generation in the data due to interference from external noise sources compared with comparable data signals used in baseband systems. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects of this invention, the various features thereof, as well as the invention itself, may be more fully understood from the following description, when read together with the accompanying drawings in which: 
     FIG. 1 shows, in block diagram form, an exemplary communications system embodying the present invention; 
     FIG. 2 shows, in block diagram form, the bus interface unit (BIU) of the system of FIG. 1; 
     FIG. 3 shows, in schematic form, the bus interface unit (BIU) of the system of FIG. 1; 
     FIG. 4 shows, in schematic form, the modulator of the BIU of FIG. 2; 
     FIG. 5 shows, in schematic form, the demodulator of the BIU of FIG. 2; 
     FIGS. 6A-6C show, in block form, the functional operation of the BIU of FIG. 2; 
     FIGS. 7-14 show, in detailed flow chart form, the functional operation blocks of FIG. 6; and 
     FIG. 15 shows the packet format for the system of FIG. 1. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 shows a communications system 5 embodying the present invention. This embodiment includes a communications bus comprising an inbound unidirectional signal path 10 and an outbound unidirectional signal path 12 and a unidirectional path coupler 14 for transferring signals from the inbound path to the outbound path at the system head-end. The inbound and outbound paths 10 and 12, respectively each pass through a plurality of remote locations at which subscribers may couple to the communications bus. In the present embodiment, signal paths 10, 12 and coupler 14 include a single coaxial cable and associated conventional unidirectional coupling devices (at each subscriber location). 
     At each subscriber location, a subscriber terminal having a bus interface unit (BIU) and an associated subscriber device may be coupled to paths 10 and 12. Each BIU has an associated subscriber address. In FIG. 1, each BIU is denoted by reference designation 20 and a suffix representative of one of the exemplary locations A,B,C, and D. Similarly, the subscriber devices are denoted by reference designation 22 and a location-keyed suffix. Each BIU 20 receives data from its associated subscriber device 22, formats that data into a standard packet with header information (including originating and destination terminal addresses) and buffers that data packet until transmission time. At that time, the BIU 20 transmits the addressed data packet on the inbound path 10. 
     In addition, each BIU 20 also scans each packet of data on the outbound path for its own address. If a packet with the address for that BIU is detected, then the BIU reads the complete packet from the outbound path 12 into a buffer, error checks the packet, acknowledges the transmission, and then clocks the data to the subscriber device 22 at a predetermined data rate. 
     FIGS. 2 and 3 show a BIU 20 in block diagram form and detailed schematic form, respectively. BIU 20 includes a clock/bit rate generator 28, microprocessor 30, random access memory (RAM) 32, programmable read-only memory (PROM) 34, program interval timer 36, modem interface 40 and subscriber device interface 42. The microprocessor 30 is an MOS Technology type 6502A NMOS microprocessor. In Intel type 8205 decoder is configured to select the various units peripheral to microprocessor 30 based on the state of address line A 8  -A 10 . RAM 32 is a 2048 byte memory configured from four Intel type 2114 units to provide message buffering, program stack, and temporary storage. PROM 34 is a pair of 1024 byte Intel type 2708 units for providing storage of the BIU program, as shown in the Appendix. The program interval timer 36 is an MOS Technology type 6522 unit. The clock/bit rate generator 28 is a Motorola type 14411 unit, 1.8432 MHz crystal, and switch network arranged to provide a 1.8432 MHz clock signal to microprocessor 30, a 307.2 Kbps clock signal to the modem interface 40, and eight switch selectable clock signals for the device interface 42 (9600 bps, 4800 bps, 2400 bps, 1200 bps, 600 bps, 300 bps, 110 bps, 75 bps). 
     The modem interface 40 includes a bus universal asynchronous receiver transmitter (UART) 50 and rf modulator 52 and demodulator 54. Bus UART 50 is a Motorola type 6850  asynchronous communications interface adapter (ACIA). The modulator 52 and demodulator 54 are shown in detailed schematic form in FIGS. 4 and 5, respectively. Tables I and II show the component values for the circuit elements in modulator 52 and demodulator 54, respectively. 
     
                       TABLE I______________________________________R1 = 10K     Z1 = 7406     Cl = 10 pfR2 = 10K                   C2 = 18 pfR3 = 10K                   C3 = 18 pfR4 = 10K     Q1 = 40673    C4 = 10 pfR5 = 5K,     Q2 = 40673    C5 = 10 pfR6 = 5.6K    Q3 = 2N3904   C6 = 1 ufdR7 = 10K                   C7 = 1 ufdR8 = 10K                   C8 = 36 pfR9 = 1.2K    D1 = MV2105   C9 = 36 pfR10 = 560    D2 = 1N914    C10 = 200 pfR11 = 100K   D3 = 1N914    C11 = 1 ufdR12 = None                 C12 = 82 pfR13 = 820                  C13 = 370 pfR14 = 1K,    L1 = 51/2 T   C14 = 2 pfR15 = 5.1K   L2 = 1 μHy C15 = 36 pfR16 = 2K     L3 = 4 1/4 T  C16 =  2 pfR17 = 75     L4 = 5 1/2 T  C17 = 36 pfR18 = 1K     L5 = 5 1/2 T  C18 = 2 pfR19 = 1K     L6 = 4 1/4 T  C19 = 82 pf                      C20 = 370 pf                      C21 = 1 ufd                      C22 = 1 ufd______________________________________ 
    
     
                       TABLE II______________________________________R1 = 75  Z1 = NE592  C1 = 270 pf C19 = 1 ufdR2 = 820 Z2 = LM2111 C2 = 82 pf  C20 = 1 ufdR3 = 1K  Z3 = MC1330 C3 = 2 pf   C21 = 62 pfR4 = 100 Z4 = RC3302 C4 = 36 pf  C22 = 390 pfR5 = 1K  Z5 = 74122  C5 = 2 pf   C23 = 1 ufdR6 = 100 Z6 = 7486   C6 = 36 pf  C24 = 1500 pfR7 = 5K  Z7 = 7438   C7 = 2 pf   C25 = 1 ufdR8 = 100             C8 = 82 pf  C26 = 1 ufdR9 = 200             C9 = 270 pf C27 = 500 pfR10 = 1K D1 = IN961  C10 = 1500 pf                            C28 = 680 pfR11 = 100    D2 = IN914  C11 = 1 ufd C29 = 750 pfR12 =  100           C12 = 1500 pf                            C30 = 1500 pfR13 = 5K             C13 = 22 pf C31 = 130 pfR14 = 1K L1 = 4 1/4 T                C14= 1 ufd  C32 = 1 ufdR15 = 1K L2 = 5 1/2 T                C15 = 1 ufd C33 = 1 ufdR16 = 1K L3 = 5 1/2 T                C16 = 5 pf  C34 = 1000 pfR17 = 2K L4 = 4 1/4 T                C17 = 36 pfR18 = 100    L5 = 5 1/2 T                C18 = 1 ufdR19 = 2.7K    L6 = 1.5 μHyR20 = 560R21 = 13KR22 = 6.2KR23 = 1K,R24 = 1KR25 = 5.1K______________________________________ 
    
     As shown in FIG. 2, the device interface 42 includes an RS-232C serial data port 56 and an 8 bit full duplex parallel data port 58, although only one of these ports may be selected for operation at a time. In alternative embodiments, both or other conventional device interfaces may be used. The port 56 includes a device UART 60 and signal conversion unit 62, provided by a Motorola type 6850 ACIA unit and associated Signetics type 1488 and 1489 RS-232-C quad driver and quad receiver units, respectively. With this configuration, an Infoton type terminal, for example, may readily be coupled to the BIU. The port 58 includes a parallel port interface 64 which is provided by the type 6522 unit used for the program interval timer 36. 
     In operation, on power-up or when reset, each BIU 20 asks its associated subscriber device 22 for the destination address (DA) of subsequent transmissions. Where the device 22 is an attended data terminal in the present embodiment, the inquiry is accomplished by printing an inquiry code at the terminal and then waiting for a keyboard entry. BIU 20 transforms the operators response to a 16-bit DA word. The operator may change the entered DA at any time. Where the device 22 is a computer, either a similar dialogue occurs in software or the BIU is programmed to accept and supply completely formatted packets to/from the computer. 
     All subsequent data from the subscriber device is then buffered in the BIU until an end-of-message, e.g. carriage return, is received or the buffer is filled. The destination address and the BIU&#39;s own 16-bit address (OA-originator address) are then appended to the message, together with other header data, to form a packet. 
     Once a packet has been assembled for transmission, the BIU reads the &#34;receive key&#34; line from the RF demodulator. When the receive key is &#34;off&#34; (i.e. no rf carrier is detected on outbound path 12) no other subscriber is currently transmitting. An &#34;on&#34; condition (i.e. rf carrier is detected) for the receive key indicates that the bus is in use. This is the basic carrier sense mechanism. With the receive key off, the BIU lowers the &#34;transmit key&#34; line and the BIU begins to transmit the message. Multiple transmit buffers for the subscriber device inputs are used so that data may still be accepted from the device while the bus transmission is in progress. 
     With its receiver enabled, the transmitting BIU sees the receive key go on after the round trip propagation delay of the inbound and outbound paths, and then begins to receive its own transmission. The four bytes (32 bits) are compared against those transmitted and, if identical, the transmitting BIU disables its receiver, and then transmits the remainder of the packet. If the test fails due to noise or another subscriber keying on during the propagation delay window, the BIU disables the transmitter, raises the transmit key signal (the receive key will then go high) and waits a pseudorandom time interval whose mean is approximately 400 microseconds. This random back-up feature is required to eliminate the possibility of two subscribers continually attempting simultaneous retransmissions. 
     With this configuration, the transmitting BIU listens for only 4 bytes (32 bits), i.e. the destination address and originator address. The four byte collision window (i.e. listening period) is longer than the maximum end-to-end propagation delay of the bus, which defines the limits of the time window in which other subscribers can possibly start transmissions without hearing, and therefore, deferring from transmitting their packets. This is the basic feature of the &#34;listen-while-talk&#34; protocol. Detection of collisions is insured since the first two characters of each message are the destination address and originator&#39;s address. This is a unique combination for each transmitting BIU. 
     In alternative configurations, different length collision windows may be utilized depending on the system geometry. The collision window must commence at least t 1  from the packet start transmit time and extend at least t 2  from that transmit time, where t 1  equals the signal propagation time from the location of the transmitting BIU along the inbound path, the head end coupler, and along the outbound path back to the transmitting BIU, and where t 2  equals t 1  plus the signal propagation time along the inbound path between the transmitting BIU and the BIU located furthest from the head end along that path. 
     Computer subscribers to the bus insert the destination address into each packet transmitted since in the present embodiment no dialogue with the computer itself is conducted to establish addresses. Computer subscribers also supply the byte count of the packet. The BIU then accepts the proper number of bytes from the computer and then the packet is transmitted. 
     Packet reception from the outbound path which originate from other subscribers occurs in the following manner. At power-up the receiver of each BIU is enabled. The BIU compares the first 16 bits of each received packet with its own address. If the address comparison is successful, (i.e. detects a match), subsequent characters from the network are buffered until the received byte count is satisfied or the receive buffer is full. If the comparison fails the BIU disables its receiver. During any bus transmission, the receive key signal is on. When the receive key returns to the off condition, i.e., the previous packet transmission is complete, the BIU re-enables the receiver and awaits reception of the next packet. 
     Other receive filtering criteria are possible. Most systems require special message types (status, assignment) or filtering on originator&#39;s address as opposed to or in addition to the destination address in the packet. These features are readily programmed within the BIU. 
     Once a complete packet has been received, the BIU clocks the data to the subscriber at the subscriber data rate. Again, multiple buffering of received packets is employed so that subsequent packets may be received from the network while the first is being clocked to the subscriber. 
     For data received from the outbound path 12, the bus UART 50 performs serial-to-parallel conversion of data from the demodulator 52, checks each character for the proper number of start/stop bits, and generates an interrupt to the microprocessor 30 for each character received when the bus UART 50&#39;s receiver is enabled. Since the bus UART 50 is a full duplex device, data transmission can occur simultaneously. For data to be transmitted on inbound path 10, the microprocessor 30 first strobes each data word onto the bus UART 50 data bus lines. Then the bus UART 50 inserts start/stop bits, parity bits, performs the parallel-to-serial conversion, and clocks the data to the modulator at the supplied transmit clock rate. Since the bus UART 50 is double buffered on receive and transmit, a second character can be received while the first is being read by the microprocessor 30 or a second character can be loaded for transmission by the microprocessor 30 while the first is being clocked out to the modulator 52. 
     The bus UART 50 also provides a status register which can be read by the microprocessor 30 to indicate: 
     (1) the receiving holding register is full (this also causes an interrupt); 
     (2) the transmitter holding register is empty; 
     (3) the status of the &#34;receive key&#34; signal (logic &#34;1&#34; indicates the network is available, logic &#34;0&#34; indicates the network is busy); 
     (4) a false start bit was detected; 
     (5) the receiver holding has been overrun by a second character since the first was not read by a peripheral device; 
     (6) a parity error was detected; 
     (7) an interrupt was generated. 
     The device UART 60 used to interface the subscriber device 22 functions in conjunction with microprocessor 30 in a manner similar to UART 50. Serial data input by the device 22 is received by RS-232-C signal conversion hardware and then input to the device UART 60. 
     In the present configuration, each BIU performs the following functions in support of the basic packet transfer mechanism of &#34;listen-while-talk&#34;: 
     (1) Initialize the microprocessor and its peripheral units at power-on and when the BIU is reset; 
     (2) move packets from the buffer memory 32 to the bus UART 50; 
     (3) check each packet transmitted to insure that collisions have not occurred. If a collision occurs, randomly back-off and then automatically retransmit the packet; 
     (4) receive properly addressed packets from the inbound path 10 and deposit them in memory 32. 
     In general, the device software is specific to the particular subscriber device 22 being interfaced to a location along the bus. However, each BIU 20 performs the following functions to support its associated subscriber device: 
     (1) Move received packets from the memory 32 to the connected device 22; 
     (2) accept data from the connected device 22 and format complete subscriber messages into the network packet; 
     (3) support a dialogue with terminal subscribers to establish destination address of subsequent data. 
     In addition, the present embodiment performs packet error checking and acknowledgements, retransmission of packets not acknowledged, and generation of status messages. 
     The above functions are depicted generally in the function block diagrams of FIGS. 6A-C, and in detail in the flow-charts of FIGS. 7-14, where each of FIGS. 7-14 corresponds to one of the blocks of FIGS. 6A-C. 
     The present embodiment provides an error correcting, packet communications protocol which includes the parts described in the following sections: 
     (1) Packet Transfer--This part of the protocol provides the listen-while-talk (LWT) mechanism for sharing the digital channel among a large number of subscribers. 
     (2) Error Correction--This part of the protocol provides end-to-end (transmitting BIU to receiving BIU) error detection and automatic retransmission of packets in error. An 8-bit check-sum is used for error detection. 
     (3) Flow Control--Receiving subscribers to the bus may control the rate at which buffers are transmitted in point-to-point conversations. Through the use of large buffers in the BIU&#39;s and this flow control mechanism, receiver overrun is avoided. 
     (4) Status--The BIU 20 automatically provides unsolicited status messages at approximately one minute intervals. Status messages contain the results of BIU self tests (e.g. memory and peripheral status register checks) and the current state of the connected device 22. 
     (5) Networking Addressing--Each subscriber device 22 may establish and change destination addresses. 
     Packet Transfer 
     The LWT protocol requires each subscriber to hear his own packet transmissions as well as those of all other subscribers. When a BIU is ready to transmit, the BIU first checks the outbound channel to see if it is busy. If busy, the BIU simply waits until the line is free. When free, the first words of the packet are transmitted on the inbound channel and the BIU begins to see its own transmission on the outbound channel but delayed by the propagation time. The BIU then checks to see if the transmitted data is identical to that being received. If so, the BIU transmits the remainder of the packet knowing that all other subscribers are now waiting. If an error (either caused by noise or another subscriber&#39;s transmission), is detected, the BIU determines that a &#34;collision&#34; exists. In response to this determination, the BIU stops transmitting, freeing the channel for other subscribers, and then backs off a random amount of time before retransmitting. Thus, the transmitting subscriber listens to his own transmission for at least the maximum proagation delay of the cable system, since this is the time window in which another subscriber could start before hearing the original transmission. 
     In the present embodiment, the choice of the random back-off time uses a random number generator in the BIU with a fixed means of about 400 microseconds. In heavily loaded systems, collisions between packets and subsequent random retransmissions of the packet occur frequently. In other words, as the system becomes busier, the probability that two subscribers start a packet transmission within the end-to-end propagation delay window increases. 
     FIG. 15 shows the basic packet format for the LWT bus. Four 16 bits form the header portion, with the words being representative of the destination address (DA), originator address (OA), message type (MT), and byte (8 bits) count (BC). The header portion is followed by as many as 960 bits of message data and an 8 bit check-sum word used for error control. 
     In addition to receiving its own transmission, each BIU examines the outbound channel for address matches or special message types. Subject to processing time limitations within the BIU, this message type and address filtering may readily be varied. 
     Error Correction 
     Although the LWT packet transfer protocol provides some error detection on the front end of each packet and subsequent retransmission, an error correcting end-to-end technique is used in addition. More particularly, an acknowledgement for each packet transmitted without error. Automatic retransmission is provided for each packet not acknowledged. To provide this error correction, each packet includes a sequence bit (1 or 0) and an 8 bit check-sum (CS). When the packet is received at the destination address, the check-sum is compared with received packet. If the check-sum is correct for the received packet, the receiver transmits an ACK message to the originating terminal containing the same sequence bit as that received. Thus, a transmitter whose packet is not acknowledged must retransmit the original packet after waiting a fixed time interval. A receiver who acknowledges a packet but subsequently receives a packet with the same sequence bit must assume that the ACK packet was lost and must retransmit the ACK. If eight retries are unsuccessful, the transmitter deletes the packet from the transmit queue, and increments a &#34;failed packet transmission&#34; count. Each retry also causes a &#34;retry&#34; count to be incremented. 
     Flow Control 
     Receivers may control the flow of data transmitted to them by selectively failing to acknowledge packets. The data flow is effectively controlled since the transmitter must retransmit the same packet. This aspect of the present invention is particularly useful in aiding transmissions from a computer subscriber device to an I/O terminal subscriber device. In this case, the large buffers contained in the terminal BIU can free the computer from a relatively slow I/O terminal. 
     Status 
     The programmable communications interface provides an opportunity to perform BIU 20 self-tests and report this information to a central status monitor and/or the connected subscriber device 22. To this end, the BIU performs self-tests in the background when it is not involved in the transmission or reception of data from either the network or the connected device. Specifically, the following tests are performed and results thereof maintained in memory: 
     (1) The BIU performs a complete memory test of all random access memory (RAM) 32 locations. 
     (2) The BIU examines the status registers contained in the parallel port 58, serial port 56, and interval timer 36. The comparison is performed to determine differences between the expected values and the actual values of each register. 
     In addition, the following status data is maintained between status messages: 
     (1) A count of the number of unacknowledged packets. 
     (2) A count of the number of packets that were discarded because eight retransmissions were attempted but not acknowledged. 
     The results of each of these tests, the status data, the current value of the device status register in either parallel port 58 or serial port 56, the number of packets queued for transmission, and the number of queued received packets are reported in each transmitted status message. Each BIU provides an unsolicited status message approximately every minute based on interval timer 36 within the BIU. 
     The status message is a unique message type on the LWT bus, and is characterized by an associated code word in the message type (MT) portion of the packet format. 
     Network Addressing 
     The BIU permits its connected subscriber to quickly change the destination address of packets it wishes to transmit. Computer subscribers may be in conversation with a number of other network subscribers. Thus, successive packets may contain a variety of destination addresses. Therefore, computer subscribers must completely format each packet before sending it to the BIU. The BIU simply accepts this formatted packet and faithfully transmits it onto the network. 
     For attended terminal devices, power-on reset or the manual reset switch causes the BIU 20 to explicitly inquire as to the desired receiver. Valid responses are converted into destination addresses, and the BIU transmits a sign-on request message to the specified destination address. If no response is received within approximately fifteen seconds, a &#34;SYSTEM NOT AVAILABLE&#34; message is transferred to the device 22. If a response is received, the BIU 20 assumes a proper connection and a &#34;LINK ACCEPTED&#34; message is transferred to device 22. 
     For unattended devices, i.e. printers, cassette recorders, discs, etc., sign-on requests are immediately accepted if the device is not currently logically connected to another subscriber. Otherwise, a response to the sign-on request is not transmitted, and a &#34;SYSTEM NOT AVAILABLE&#34; message is transferred to the device that originated the sign-on request. 
     The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein. ##SPC1## ##SPC2##