NZ198044A - Concatenating data from remote terminals and line concentrator therefor - Google Patents

Description

I 98044 Date: 14 August 19 81 COMPLETE SPECIFICATION "DATA COMMUNICATION CONCENTRATOR AND PROTOCOL FOR GROUP POLLING" 1/We, THE TOTALISATOR AGENCY BOARD, of 30 4 Lambton Quay, Wellington, New Zealand, a body corporate established under the Gaming Amendment Act 19 49 hereby declare the invention for which ^ / we pray that a patent may be granted toJSOSK/us, and the method by which it is to be performed, to be particularly described in and by the following statement:- ' 98044 This invention relates to data "coimunication"devices and systems. It particularly relates to line concentrators and a communication protocol for synchronous networks to allow group polling.

The present invention is concerned with data communication networks where typically communication of data takes place between a large number of terminals and a central processing unit. Communication paths in such in a network will frequently be too long for baseband communication and modems are necessary to enable data transfer to take place over low bandwidth lines such are used in the common carrier telephone system.

There are many well known techniques used to improve the efficiency of transmission, or to reduce network costs or both. For example, synchronous transmission is usually preferred to asynchronous transmission even though many computer systems may be basically asychronous in operation. in synchronous transmission a block or frame of data is assembled and sent as a continuous stream of characters rather than the stop-start one-character at-a-time asynchronous transmission. A further technique is the use of "multi-drop" lines which lower network costs and improve network efficiency by obviating the need for separate lines connecting the primary to each secondary. A multi-drop line is a single line connecting a primary and a number of '98044 secondaries so that communication takes place over a common ' transmission line.

Yet another common technique is to use a line concentrator which may link a group of intermittently used lines to a line on which transmission is organised as efficiently as possible. In this context the term "concentrator" is to be distinguished from "multiplexer"/ the latter being taken to mean a device which allows transmissions from a group of low bandwidth lines to be 10 transferred over a single line of higher bandwidth.

A concentrator may be of the "hold and forward" type which receives transmissions in asynchronous form and builds up blocks or frames of data from each incoming line and inserts them efficiently on a single outgoing line. Alternatively, a concentrator may simply allocate a common transmission line to a group of sources of synchronous transmissions indicating to each source when to commence transmission. The line concentrator of the present invention falls within this latter category. 20 A concentrator may concentrate transmissions received directly from terminals or from lines which themselves may carry transmissions from other concentrators or terminal controllers. For this reason in the present specification all these sources will be grouped generically as "secondaries" and the central processing unit with which the secondaries are communicating will be referred to as a "primary".

A portion of a typical constant carrier multi-drop data f98044 conmiunication network is shown in figure 2. In such neta work, since all terminals 2 cannot be communicating with central processing unit 1 at the same time, each terminal is given a unique address which enables the central processing unit to direct messages to a selected terminal. To allow for an orderly transmission of messages from terminals to the central processing unit, the central processing unit "polls" each terminal in turn asking if it has a message to transmit. If the polled terminal has nothing to send a 10 negative reply will be received by the central processing unit whereupon the next terminal is polled. A considerable problem arises with this standard technique of line discipline when there are large numbers of terminals in the network. Many polls may occur before a particular terminal is polled and this means there may be a considerable degradation in terminal response time. In addition the utilisation of the network line by polling signals becomes high in proportion to utilisation by message transmission.

It is therefore an object of the present invention to go 20 at least some way towards overcoming the abovementioned problem by providing a communication protocol and a line concentrator which make group polling possible.

By "group polling" it is meant a system of polling whereby only one poll is required per group of secondaries connected to a line concentrator. The group of secondaries are arranged to respond to the single group poll in an orderly sequential manner.

Accordingly the invention in one aspect consists in a 198044 method of achieving group polling in a synchronous data communication network which includes at least one line concentrator which links a group of secondaries to a common transmission line polled toy a primary, said method comprising: establishing a communications protocol for said network whereby (1) each secondary is indentified by a two-part address consisting of a group address and a secondary identifier, (2) each secondary may be polled by its own 10 unique address or the group part of that address, and (3) one control bit in each data frame is allocated for indicating whether a transmitted frame is the last frame to be transmitted from a secondary, said control bit setting being capable of being inhibited; causing each line concentrator in the network to perform the following functions, (a) during transmission from one secondary in the group to monitor the inputs from other secondaries in that group to ascertain if such secondaries have messages to transmit and if so (b) command the 20 transmitting secondary to inhibit the setting of the last frame control bit, (c) subsequently connect the next waiting secondary to the transmission line, and (d) cyclically repeat functions (a), (b) and (c) until no further secondaries of the group indicate they have messages to send whereupon, (e) the command to inhibit the setting of the last frame control bit by the currently transmitting secondary is suppressed, and no further action taken until a secondary again indicates it has a message to send in '98044 response to a poll.

In a further aspect the invention consists in a line concentrator which allows group polling in a synchronous data communication network wherein said line concentrator links a group of secondaries to a common transmission line polled by a primary, and wherein a communications protocol is used whereby (1) each secondary is identified by a two-part address consisting of a group address and a secondary identifier, (2) each secondary may be polled by its own 10 unique address or the group part of that address, and (3) one control bit in each frame is allocated for indicating whether a transmitted frame is the last frame to be transmitted from a secondary, said control bit setting being capable of being inhibited, said line concentrator comprising: means for continuously monitoring inputs from secondaries connected to it to ascertain if there are secondaries which have messages to transmit in response to polling -from the primary, means for sequentially connecting 20 the secondaries in the group which have messages to send to said common transmission line, and means for commanding the currently transmitting secondary to inhibit the setting of the last frame control bit if said monitoring means detects that other secondaries in the group have messages to transmit.

A preferred embodiment of the invention will now be described with reference to the accompanying drawings in f98044 which Figure 1 shows the synchronous frame structure used in this invention, Figure 2 is a diagrammatic representation of a data communications network, Figure 3 is a block circuit diagram of a line concentrator, and Figure 4 is a logic diagram of the inhibit set final control logic 25 shown in figure 3.

The protocol which is an aspect of the present invention is a variation of the International Standards Organisation (ISO) High Level Data Link Control (HDLC) protocol. The basic data unit, the frame, is that defined in ISO Standard 3309 1976 (E) and is shown in figure 1. Each frame is defined by a start bit pattern flag F. The first eight bits following the start flag comprise the address field A and the first bit of the address field is always 1. The next three bits of this field constitute the group address and the final four bits constitute the terminal identifier, the 20 whole address field being unique to each terminal in the network.

The second eight bits following the start flag constitute the control field C and are used to signal control functions independent of the message information. The third group of bits following the start flag are the information bits I and this sequence may be of any length provided that it is a multiple of 8 and a minimum of 24 bits. Following the information bits are a series of 16 1 9804 bits FCS which are used for frame checking sequence and error detection. The frame is completed with a finish flag F.

The protocol establishes the fifth bit of the control bit field C as the "poll/final" bit or "p/f" bit which has two functions. First, in command frames, that is frames transmitted from the central processing unit, this bit is called the poll bit and is used to solicit a response from the addressed terminal (or any terminal in the group if the poll frame is an "unnumbered poll") by the setting of this poll bit to 1. Second, in response frames, that is frames transmitted by the terminals, this bit is known as the final bit and may be set by the terminal to 1 in the last transmitted frame to be transmitted to indicate that all queued frames have been transmitted. The protocol of the present invention provides that the setting of the final bit, as mentioned above, may be inhibited for reasons which will become apparent from the description below.

In the preferred embodiment of this invention a line concentrator is provided with up to 8 ports to which may be connected up to 8 terminals as shown with the line concentrator 5 in figure 2. Alternatively, a line concentrator port may be fed from the output of another line concentrator as shown with line concentrator 6 in figure 2. Each terminal 2 connected to a line concentrator is "intelligent" and among other things assembles data to be transmitted into frames which it queues according to its buffer size and sets, or does not set, the final bit in the f98044 last queued frame according to commands received from the line concentrator to which it is connected. Each line concentrator and the secondaries connected to its ports constitute a "group" and are addressable as such by the central processing unit 1.

The network shown in figure 2 includes a central processing unit modem 3 and modems 7, 4, ... at each drop point Dl, D2, ... The secondary modems, for example modems 7 and 4, are each connected to a respective line 10 concentrator using one of the standard interfaces CCITT V24/V28 or CCITT Vll/(X27). The following terminal/line concentrator/modem control signals are specifically explained to facilitate an understanding of description which follows: DTR - Data Terminal Ready. This is a signal from the terminal and is logically true whenever the terminal is powered on and indicates that infor mation transfer is possible.

RTS - Request To Send. This is a signal from the terminal and is made logically true when the terminal is polled and it has queued frames to be transmitted. This signal will remain true until the last queued frame from the terminal has been transmitted.

CTS - Clear To Send. This is a signal from the modem, but issued under the control of the 198044 line concentrator to a selected terminal and is made logically true to indicate to that terminal that it may commence transmission of queued frames.

ISF - Inhibit Set Final. This is a signal from the line concentrator to a terminal and is made logically true whenever the line concentrator requires that a connected terminal should not 10 set the final bit on the last queued frame from that terminal.

It should be understood that the partial network shown in figure 2 includes only two configurations in which the line concentrator of the present invention may be used. There are others, a typical one being where the output of a terminal controller receiving the inputs from a number of terminals is connected to one input port of a line concentrator.

Referring to the block circuit diagram of the line concentrator in figure 3, the signal connections are shown grouped such that the terminal connections are on the left-hand side and the modem connections are on the right-hand side. Where input/output lines fan out or where there are a plurality of lines to form a bus, these are indicated by broad bands in the figure.

If a terminal connected to the line concentrator shown is in a valid condition and is powered on its DTR line will I 98044 be logically true. Upon such a terminal receiving from the modem via line RXDATA (which by-passes the line concentrator circuitry) either a poll or any other command frame having the appropriate address (for example that terminals group address) its RTS line will go true indicating to the line concentrator that it has a response to send. This response will be a message in the form of queued frames, a receive ready signal or other appropriate response. These two signal terms are logically ANDed by validation circuit 20 10 and the existence of a true RTS. DTR term will cause an address counter 22, which drives a scanner and port select circuit 21, to cease counting when it reaches the port address (0 to 7) of the terminal with RTS true. This action "freezes" that three bit port address on the address bus 23. The address counter 22 is clocked by signal TXCK from the modem and causes scanner 21 to cyclically scan the RTS.DTR line from each terminal seeking one where that term is true.

When address counter 22 is stopped by the occurrence mentioned above the CTS and TXDATA control circuit 24 sets 20 the CTS (Clear To Send) line on the selected terminal true and connects that terminal's TXDATA line to the TXDATA line to the modem so that that terminal is in a position to transmit messages to the central processing unit via the modem and network line.

On the terminal receiving CTS true, it commences transmission of queued message frames. Concurrent with this, the inhibit set final (ISF) logic 25 monitors the states of the RTS.DTR lines from all other terminals in the 7 98044 group as described later. If this logic detects that one or more other terminals of the group have RTS.DTR true, that is, that the said terminal or terminals have messages to send, logic 25 sets the ISF lines true thereby commanding the terminal currently transmitting not to set the final bit of its last frame to be transmitted. As already mentioned the terminals have built-in intelligence and their action in assembling data frames can be altered.

It will be appreciated that if a terminal has queued 10 data frames to send, its RTS line will have been set true upon receipt of the poll from the central processing unit to that group.

Upon transmission of the last frame from the first terminal to transmit, or the originally addressed terminal if it was addressed specificially, its RTS line will become not true indicating to scanner 21 by RTS.DTR not true that the terminal has finished transmission. This causes the address counter to resume counting until scanner 21 scans a terminal. RTS.DTR line which is true. At this stage the 20 address counter 22 is halted and the current address on address bus 23 is again frozen. A delay counter 26 counts a predetermined number of clock pulses TXCK (selectable at either 7 or 15) to establish a break between message frames transmitted from different terminals in the group. During this inter-terminal delay period CTS for the previously transmitting terminal has been set not true and a logical mark set on the TXDATA line to the modem by the CTS and TXDATA control circuit 24. Once the delay time has elapsed, 198044 the terminal whose port address is now frozen on the address bus 23 will have its CTS line set true and its TXDATA line allocated the TXDATA line to the modem. The process repeats as previously described.

If ISF logic 25 does not detect any RTS.DTR lines true then upon the terminal currently addressed assembling its last frame the ISF control 25 will not set ISF true and thus the terminal is able to set the final bit in that frame as programmed. This will indicate to the central processing 10 unit that transmission from the group has finished. It is then free to poll the next group.

Referring to figure 4 there is shown the logic circuit for part of the ISF control logic 25. The inputs to this logic are the RTS.DTS terms from each terminal port on the line concentrator. That is, the inputs are the signals from validation circuit 20. The output of this logic circuit 25 is an ISF term fanned out to all terminals. The RTS.DTS terms from each port are each applied to one input of a respective two-input exclusive OR gate 30a to 30h. The 20 second input of each gate 30 is fed by a respective output of a l-out-of-8 decoder 31 connected to the address bus 23. The address bus will hold the address of the currently selected port and accordingly the decoder 31 will produce a unique output on its output line corresponding to the currently selected port. This will inhibit the corresponding gate 30 so that the RTS.DTS term from this port will not influence the output of OR gate 32 to which the outputs of gate 30 are connected. The output of gate 32 1 98044 constitutes the ISF term which is fanned to the individual terminal ISF lines. The ISF function is now determined only by those terminals not currently transmitting (if any).

It is obvious that any suitable logic circuitry could perform the above described ISF function. The requirement of such circuitry is simply to receive as inputs the RTS.DTR terms from each port and to provide a logically true output only if one or more of its input terms, other than that from the currently selected port, are true.

From the prior description it will be understood that the communications protocol and line concentrator provided by the present invention enable group polling to be achieved on a synchronous data communication network with the line concentrator organising the responses of the individual terminals in the group.

Claims (9)

198044 MATf/WE CLAIM ISr

1. A method of achieving group polling in a synchronous data communication network which includes at least one line concentrator which links a group of secondaries to a common transmission line polled toy a primary, said method comprising: establishing a communications protocol for said network wheretoy (1) each secondary is indentified by a two-part address consisting of a group address and a secondary identifier, (2) each secondary may be polled by its own unique address or the group part of that address, and (3) one control bit in each data frame is allocated for indicating whether a transmitted frame is the last frame to be transmitted from a secondary, said control bit setting being capable of being inhibited; causing each line concentrator, in the network to perform the following functions, (a) during transmission from one secondary in the group to monitor the inputs from other secondaries in that group to ascertain if such secondaries have messages to transmit and if so, (b) command the transmitting secondary to inhibit the setting of the last frame control bit, (c) subsequently connect the next waiting secondary to the transmission line, and (d) cyclically repeat functions (a), (b) and (c) until no further secondaries of the group indicate they have messages to send whereupon, (e) the command to inhibit the setting of the last frame control bit by the currently transmitting - 15 - 198044 secondary is suppressed, and no further action taken until a secondary again indicates it has a message to send in response to a poll.

2. A line concentrator which allows group polling in a synchronous data communication network wherein said line concentrator links a group of secondaries to a common transmission line polled by a primary, and wherein a / " communications protocol is used whereby (1) each secondary is identified by a two-part address consisting of a group address and a secondary identifier, (2) each secondary may be polled by its own unique address or the group part of that address, and (3) one control bit in each frame is allocated for indicating whether a transmitted frame is the last frame to be transmitted from a secondary, said control bit setting being capable of being inhibited, said line concentrator comprising: means for continuously monitoring inputs from secondaries connected to it to ascertain if there are secondaries which have messages to transmit in response to polling from the primary, means for sequentially connecting the secondaries in the group which have messages to send to said common transmission line, and means for commanding the currently transmitting secondary to inhibit the setting of the last frame control bit if said monitoring means detects that other secondaries in the group have messages to transmit.

3. A line concentrator according to claim 2 wherein said monitoring means comprise a counter with a module equal to - 16 - 19 the number of secondary inputs clocked by clock pulses supplied by the network modem to which the line concentrator is connected, scanning means driven by said counter and having as inputs control signals derived from said secondaries which are logically true when a secondary has a message to send, said scanning means halting said counter when an input which is logically true is scanned, and commanding said connecting means to connect the message line of the terminal whose address count is currently stored in the counter to said common transmission line.

4. A line concentrator according to claim 3 including means for generating a clear to send control signal, said means being activated by said scanning means substantially simultaneously with the command to said connecting means, and the output of said clear to send generating means being connected to a prescribed connector way of each and every secondary input.

5. A line concentrator according to either of claims 3 or 4 wherein -the input from each secondary is multi-way, one such way receives a control signal from the secondary which is made logically true when that secondary has a message to send, another such way receives a control signal from the secondary which is logically true when that secondary is in an active state, and including validation circuits for each secondary which receive as inputs said control signals and true when both the respective control signal inputs are logically true. - 17 -

6. A line concentrator according to any one of claims 3 to 5 wherein said inhibit command means comprises a logic circuit which receives as inputs said control signals derived from said secondaries and provides an output which is logically true only when one or more said control signals other than the control signal from the currently transmitting secondary is true, said output being the command to the transmitting secondary to inhibit the setting of the last frame control bit. 10

7. A line concentrator according to claim 6 when dependent on claim 5 wherein said control signals are the signals produced by said validation circuits.

8. A method of achieving group polling in a synchronous data communication network when performed substantially as hereinbefore described.

9. A line concentrator, which allows group polling in a synchronous data communication network, substantially as hereinbefore described with reference to the accompanying drawings. 20 . .flgjsaof • Sxxnk.... &y irR/their A. J. PAR^^SON. per: