GB2193862A - Digital data transmission systems - Google Patents

Abstract

A data transmission system uses a control means (10) to allow a secondary data stream from a secondary DTE (14) to be transmitted over a data link (2A, 2B) when the data stream from a primary DTE (6) contains repeated characters. The control means (10) imposes its own protocol on the primary data stream by inserting an additional "0" bit to prevent a sequence of more than five consecutive "1" bits. The primary data stream is passed to a buffer (18) in which sequences of at least a predetermined length of repeated characters are recognised and replaced by control characters including the repeated character, a flag character containing six consecutive "1" bits, and a status and count character which indicates the number of secondary data characters being transmitted. When the data stream is received, flag characters are recognised by a device (38) and this causes the identified repeated character to be sent to the input channel of the associated primary DTE and the secondary data characters to be redirected to a secondary DTE (14). In this way additional secondary data has been transmitted over the data link without affecting the overall length of transmission or the transmission rate used. Monitor 52 ensures that the secondary data mode is not triggered by power failure. <IMAGE>

Description

SPECIFICATION Digital data transmission systems The present invention relates to synchronous digital data transmission systems and, more particularly, to such systems which enable a secondary channel to be incorporated into the main transmission.

In many data transmission systems which use a link between two terminals in order to transmit digital data, there will almost certainly be periods during which only redundant data is being transmitted along the line. In most cases this will take the form of a continually repeated character. A character is defined as a sequence of bits of a predetermined length, for example eight bits. Transmitted characters may represent letters of the alphabet, numerals or control codes.

The present invention is directed towards solving the technical problem of utilising these redundant characters in order to provide a secondary channel over the data link without affecting the overall transmission speed on that link.

The present invention accordingly provides a digital data transmission system comprising a data link for transmission of data between two stations, each station comprising a primary and a secondary terminal each adapted to transmit and receive data represented as digital characters, and control means connecting said primary and secondary terminals to said data link, said control means comprising means adapted to receive and temporarily store characters of a primary data stream from said primary terminal, and detect the presence therein of at least a predetermined number of repeated characters, means connected to receive and temporarily store characters of a secondary data stream from said secondary terminal and, in response to the detection of a predetermined number of repeated characters in the primary data stream, substitute for some of the repetitions of said repeated character a unique flag character, a number of characters from the secondary data stream and a character representing a count of the number of said secondary characters being substituted, said substituted data stream being transmitted over the data link, means connected to the data link for receiving the data stream transmitted therealong and for recognising said flag chara acter, means for passing the data characters of the received data stream to said primary terminal until a flag character is recognised by said recognising means, and thereafter passing the repeated character to said primary terminal for the number of times represented by the count character and passing the received secondary characters to said secondary terminal.

Such a system is advantageous in that the length of the transmission and the data rate over the data link are independent of the secondary terminal which is only allowed to transmit characters from its data stream when there are repeating characters in the primary data stream. For example, if the primary terminal is a computer transmitting data over a private leased telephone line as the data link, it may transmit a repeated "idle" character when there is no data available for transmission or transmit nothing which is perceived as a repeated character composed, for instance, entirely of "0" bits.The secondary terminal can then transmit characters from its data stream during such periods and make full use of the leased line without impeding the flow of information between the primary terminals which take priority over the data link as soon as a different character is output in the primary data stream.

Another possible application for such a system is where the primary terminals are transmitting text information between them. In this case the secondary characters are transmitted when there are repeated text characters such as a repeated series of "underline" or "space" characters.

In order that the data transmission system should be independent of the communications protocol used by the primary terminals, the control means further comprises means for monitoring the data stream transmitted to the date link and inserting a "0" bit after any series of a fixed number of consecutive "1" bits, where said flag character is. a character containing an unbroken series of said fixed.

number plus one of "1" bits, said flag character being generated after said "0" bit insertion, and means for restoring the timing of the data stream output from said control means to the data link after said "0" bit insertion means has inserted a predetermined number of "0" bits. The use of such "0" bit insertion allows the described system to be independent of the protocol used between the primary terminals in a relatively simple and straightforward manner.

Preferably the system further comprises switch means for bypassing said control means in response to the absence of a power supply to said control means or an indication of the presence of a fault in said control means. The provision of such a switch allows the control means to be bypassed in case of power failure or a fault within the control means so that the link between the primary terminals is not affected by such conditions.

In a preferred embodiment, such a switch may comprise a field effect transistor (FET).

An embodiment of a digital data transmission system in accordance with the invention will now be described, by way of example only, with reference to the accompanying diagrammatic drawing which represents onelsta- tion of a pair of stations within such a system.

The figure illustrates one station that forms part of a data transmission system communicating with another similar station over a data link 2 shown as an output channel 2A and an input channel 2B connected to data-circuit terminating equipment (DCE) 4. This data link may be provided by any known means. For example, it may be a telephone line or a satellite data link. A primary data terminal or data terminal equipment (DTE) 6 is connected to the data-circuit terminating equipment 4 by means of an output channel 8A and an input channel 8B. The connection between the primary DTE 6 and the data link 2 may use any conventional digital transmission system for the transmission and receipt of digital data in the form of a series of bits. The protocol and coding employed in this part of the system is immaterial to the present invention and may be of any appropriate design.The present invention concerns the provision of a control means 10 which, together with the insertion of a switch 12, in the channels 8A, 8B allows a secondary data terminal equipment (DTE) 14 to transmit and receive data over the data link 2. It is intended that a unit containing the control means 10 and switch 12 should be provided separately for incorporation in existing data transmission systems so that these can be used with greater efficiency to allow a secondary DTE 14 to be used without requiring an additional data link.

For the purposes of the present description, the data stream output on channel 8A by the primary terminal 6 will be considered to be a series of characters where each character is a fixed number, for example 8, of bits of the data stream. Such characters may represent actual information. For example, if the data stream output by the primary terminal 6 represents text, then the characters may be standard ASCII characters. In other applications, for example where the information to be transmitted has been encoded before output on the channel 8A, the characters may not directly relate to real information. However, this has no effect on the operation of the control means now to be described.

Because the data stream received from the primary DTE 6 may use any protocol, the control means 10 must impose its own protocol so that a unique flag is available so that the control means 10 can signal whether primary or secondary data is being transmitted. For the purposes of illustration, a "0" bit is inserted after any consecutive series of five "1" bits so that the data stream after "0" bit insertion will not normally include more than five "1" bits in a series. This type of zero bit insertion is a well-known technique. In order to send a "flag" event to the data link for signalling purposes, a character consisting of "01111110", which is unique in that it contains six "1" bits consecutively, is placed on the output line.

When the switch 12 which is interposed between the primary DTE 6 and the DCE 4 is open, the primary data stream output from the DTE 6 on channel 8A is directed to control means 10 via channel 16A, where it is passed to a primary transmit data buffer 18.

The secondary data stream output from the secondary DTE 14 is passed via channel 32A to a separate secondary transmit data buffer 34. Both buffers comprise a memory in which the characters from the respective input data streams are temporarily stored. A repeated character detector 20 monitors the stored characters in the primary buffer 18 and identifies any series of more than a fixed number of identical characters. Until such a series of repeated characters is detected by the device 20, the primary data stream is fed out from the buffer 18 via data path 22 and a switch 26 to a zero bit inserter 28 which inserts a "0" bit after any consecutive series of five "1" bits as previously described. When a sufficiently long series of repeated characters is detected by the device 20, data is further processed before being output via an output line 30A to the output channel 8A.For the purposes of illustration, the repeated character detector device 20 controls a switch 26 and a flag generator 29 by means of control line 24.

When a series of repeated characters of the appropriate length is detected, the repeated character detector 20 controls switch 26 and flag generator 29 in such a way that some of the repeated characters from the buffer 20 are overwritten by characters from the secondary transmit data buffer 34, and that the data stream output on data line 30A contains the "flag" character described above. At other times the data stream from buffer 20 is passed unaffected by the switch 26, and is subject only to processing by the zero bit inserter 28 before being passed to the output channel 8A. It will be appreciated that this is a mere illustration and does not necessarily reflect the actual hardware used to implement the operation.

The switch 26 receives inputs from the primary data stream via channel 22, and also from the secondary transmit buffer 34 via channel 36. The secondary data stream is received from the secondary DTE 14 via data path 32A and held in buffer 34 until being accepted by switch 26.

Due to the action of the zero bit inserter 28, a discrepancy exists between the number of bits transmitted by the primary DTE 6 and the number presented by the control means 10 to the DCE 4. In order to correct for this, a correction mechanism is introduced which causes more repeating characters to be deleted from the data stream than are replaced.

Assuming characters of 8 bits, each character so deleted will compensate for the insertion of 8 "0" bits. in order that information about this correction, the number of secondary data characters passed, and other information may be communicated to the other station of the pair, a control character is inserted into the data stream with the secondary data characters. Other control characters may also be inserted. The flag character is also inserted into the transmitted data stream although it is generated after zero bit insertion.

The data stream received by the DCE 4 on data channel 2B is processed by the control means 10 in a manner complementary to that described for transmitted data. In this case the data stream received on the channel 2B of the data link 2 is passed via the DCE 4 to the control means 10 provided the switch 12 is open. If the switch 12 is closed, data is passed unaffected to the primary DTE 6. The received data stream is passed via data channel 30B to a device 38 which removes those "0" bits added into the original data stream to prevent a sequence of more than five "1" bits appearing in the data stream as sent to the data link 2. The device 38 also searches the data stream for the flag character "01111110".Detection of this flag character in the received data stream initiates a process which identifies the repeating character in the data stream and controls the repeated character restoration and secondary data extraction device 46 in such a way that all repeating characters replaced by other information by switch 26 and flag generator 29 in the transmitting station are restored to the data stream output on data channel 16B. At the same time the secondary characters inserted into the data stream at the transmitting station by switch 26 are extracted and sent via channel 48 to a secondary receive buffer 50 from where they are output via data channel 32B to the secondary DTE 14.

It will be appreciated that the control means 10 has had no effect whatsoever on any protocol being employed by either the primary or secondary DTEs 6, 14 because of the imposition of a separate protocol by means of the "0" bit insertion devices 28 and 38 of the control means itself which ensure that a unique flag character is available for flagging the fact that secondary data has been inserted over repeated characters in the primary data stream. Although a method of imposing this protocol by "0" bit insertion has been described, it will be appreciated that other techniques for protocol imposition can be employed.

The described control means can readily be implemented by the use of random access memory to provide the various buffer stores, together with a suitably programmed microprocesor to manipulate the data in the manner described above. The block diagram shown in the drawing illustrates the function of the device and is not intended to limit its hardware implementation.

Another function of the microprocessor programme in the control means 10 is to establish on power up whether there is a corresponding control means at the other station of the pair. This may be done by sending a control message instead of secondary data. Only if a corresponding control message is received in reply can the secondary DTE 14 proceed to transmit data as described above.

In order to ensure that the control means 10 does not interfere with the operation of the data transmission link of the primary DTE 6 if it contains a fault, the control means 10 is provided with a circuit monitor device 52 which monitors the operation of all components of the control means 10, and particularly the power supply (not shown) and produces an output on a control line 54 to the switch 12 if the power supply of the control means 10 is switched off or a fault is detected in any of the components of the control means 10 with which the circuit monitor is associated by means not shown. When a control signal is output to the switch 12 on the line 54, the switch is closed so that the primary DTE 6 is connected directly to the data-circuit terminating equipment 4 and the control means 10 itself is bypassed. In this way a fault or power failure in the control means 10 does not affect the operation of the transmission between the primary data terminals at the stations. The switch 12 may be implemented as an FET where the presence of a control signal on the control line 54 is sufficient to switch the FET between a non conducting state (switch open) to a conducting state (switch closed). Circuits for implementing such FET switches are well known in the art and will not be described in detail herein.

Claims (8)

1. A digital data transmission system comprising a data link for transmission of data between two stations, each station comprising a primary and a secondary terminal each adapted to transmit and receive data represented as digital characters, and control means connecting said primary and secondary terminals to said data link, said control means comprising means adapted to receive and temporarily store characters of a primary data stream from said primary terminal, and detect the presence therein of at least a predetermined number of repeated characters, means connected to receive and temporarily store characters of a secondary data stream from said secondary terminal and, in response to the detection of a predetermined number of repeated characters in the primary data stream, substitute for some of the repetitions of said repeated character a unique flag character, a number of characters from the secondary data stream and a character representing a count of the number of said secondary characters being substituted, said substituted data stream being transmitted over the data link, means connected to the data link for receiving the data stream transmitted therealong and for recognising said flag character, means for passing the data characters of the received data stream to said primary terminal until a flag character is recognised by said recognising means, and thereafter passing the repeated character to said primary terminal for the number of times represented by the count character and passing the received secondary characters to said secondary terminal.

2. A system as claimed in claim 1, wherein the control means further comprises means for monitoring the data stream transmitted to the data link and inserting a "0" bit after any series of a fixed number of consecutive "1" bits, where said flag character is a character containing an unbroken series of one more than said fixed number of "1" bits, said flag character being generated after said "0" bit insertion, and means for restoring the timing of the data stream output from said control means to the data link after said "0" bit insertion means has inserted a predetermined number of "0" bits.

3. A system as claimed in claim 1 or 2, further comprising switch means for bypassing said control means in response to the absence of a power supply to said control means or an indication of the presence of a fault in said control means.

4. A system as claimed in any one of the preceding claims, wherein the switch means comprises a field effect transistor.

5. A data transmission system substantially as herein described with reference to the accompanying drawing.

6. A method of transmitting a secondary stream of digital data characters over a data link carrying a primary data stream of digital characters, comprising the steps of modifying the primary and secondary data streams to prevent the occurence of a distinctive bit pattern, monitoring the primary data stream for a series of repeated characters of at least a predetermined length, and, on detection of such a series, overwriting some of the characters of said series with a flag character having said distinctive bit pattern, and characters from said secondary data stream.

7. A method of receiving data transmitted by the method of the preceding claim, comprising the steps of monitoring the received data stream and, on detection of the flag character in the received data, repeatedly outputing the repeated character to a receiver of the primary data stream, and outputing the secondary data characters separately to a receiver of the secondary data stream.

8. A method of transmitting and receiving data substantially as herein described with reference to the accompanying drawing.