DE1186494B - Data transmission system - Google Patents

Description

FEDERAL REPUBLIC OF GERMANY

GERMAN

PATENT OFFICE

EDITORIAL

Boarding school Class: H 04 b

German class: 21 al -25/01

Number: 1186 494

File number: J 19315 VIII a / 21 al

Filing date: January 20, 1961

Opening day: February 4, 1965

The invention relates to a method for data transmission for simultaneous bidirectional operation on a single channel of limited bandwidth, in particular on a telephone channel.

In the context of the present description, "data" refers to coded messages, which are suitable to control data processing systems or to be entered into them. To such Data also includes the messages normally transmitted over telegraph systems.

The invention is based on the object of the transmission paths, in particular the telephone channels, more economical to use.

For more than 20 years, both the domestic and the international telephone network has been using Success used to transmit messages in a telegraph code, in shape of audio frequency telegraphy. A large number of different telegraphy channels can be created through modulation of this type can be accommodated in a single telephone channel of normal bandwidth. The terminal equipment consist in these cases of automatic sending and receiving teleprinters, which in general work at speeds up to 100 baud, but not higher, mostly at 50 to 75 baud. Compared to those achieved with other, more modern types of data facilities these speeds are relatively slow. The economic use of data processing High-speed machines (computers) will be used in the future in connection with the transfer of data from distant places a network of transmission paths to a data center must be given. There are reasons too the assumption that such a line transfer of data (the word "line" is intended here in broadest meaning to include any type of transmission channel) not in normal teletype operation can be done because this will introduce a serious delay would. However, data transfer can be done much faster when used for this purpose Instead of a narrow-band telegraphic channel, a telephone channel is used. Actually are for data transfer because of the more convenient handling medium speeds as they are about a telephone channel can be achieved, instead of providing larger bandwidths, that would allow higher speeds.

The expected flow of data for a distance and for 1 day should be around 0.25 million Data transmission system

Applicant:

International Standard Electric Corporation,
New York, NY (V. St. A.)

Representative:

Dipl.-Ing. H. Ciaessen, patent attorney,

Stuttgart W, Rotebühlstr. 70

Named as inventor:

Esmond Philip Goodwin Wright,

Victor John Terry,

William Francis Sidney Chittleburgh, London

Claimed priority:

Great Britain January 21, 1960 (2178)

binary characters (this is the equivalent of a thousand punch cards of two hundred and fifty binary Characters), and this means a daily transmission of a thousand binary characters per second Occupancy of 4 minutes.

This immediately leads to the following conclusions:

a) The daily flow of data does not represent a remarkable increase in the occupancy of a Transmission path;

b) the interruption is for normal use of the transmission path for telephony negligible;

c) It would be a very costly proposition just for data transmission over the telephone lines to rent.

The advantage of leased lines is that they are more quiet than the lines chosen in the general network. This advantage leads to greater utilization of the lines for data transmission and thus to fewer errors. In principle, it would not be necessary to use an error correction technique.
In addition to the possibility of operating a number of 50 baud or 75 baud telegraphy channels of conventional size in parallel within a telephone channel, there are obviously numerous ways in which

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A medium-speed data transmission system can be operated over a telephone channel can. The following four important procedures to be considered are briefly outlined below will: ;

1. The parallel method, in which a number of channels are used, each one significant have greater bandwidth than provided within a telegraphic switching system will. This is the fastest system that can be used for a given bandwidth of a telephone channel can be created.

2. The serial process, in which the data is in continuous flow through a system with alternating Directional letters are transmitted. In this stream can be used to carry out a Correction of errors in the receiving station a "redundancy" can be provided. The bug fix is based on the assumption that the places with disturbed characters are undisturbed by sequences Characters are separated from each other.

3. The serial process, in which the data is sent via a system for alternating letters (half-duplex) are transmitted in blocks containing the redundancy. The receiving station can with Error detection or work with error correction or both. Errors that are only recognized but not corrected immediately, will be corrected on repetition. The block length and the error correction facilities must be automatic or from for each message Hand adjusted.

4. The serial procedure, in which the data is sent via a system for counter-writing with monitoring (Duplex) can be transmitted in blocks without redundancy. Both stations speak to the Redundancy, which is transmitted in the opposite direction and checked at the sending station to determine if a repetition is necessary. This method proves itself in a system with echo cancellers is difficult.

For the methods outlined under 2, 3 and 4 "the drawing or step speed is approximately a thousand steps per second, while the step speed in the method mentioned under 1 depends on the number of channels used. Typical speeds in this case are 5 x 350 or 6 300 steps per second.

The sound of the block depends on the method used and with the method described under 3 (half-duplex) a certain freedom of movement is necessary. A block length of no more than three thousand steps can be chosen, with the There is a possibility to reduce this length in cases where the data is normally in smaller Groups are offered. It is favorable that the use of blocks with a length that is below the specified maximum, does not require any special setting in the receivers. in the In the case of the second method (simplex), the continuous flow of data does not require any restriction to one maximum block length. In the fourth case (duplex) the optimal block length is very small and lies in the A hundred paces. Under these circumstances, the block length should preferably be kept constant. The parallel method, which was described under 1, could with regard to the block length with all three methods just described can be carried out.

At the end of a message, an indication is expediently derived from the end of the transmission, the can be used to draw the attention of an operator or to trigger a signal that is on the sending station is retransmitted. For the simplex method, it is used for repetition or acknowledgment no further signaling required.

With the duplex method, the only reverse signals required are the one to be transmitted backwards Redundancy, however, some signal, such as the end of modulation, must be provided in order to to indicate that no more need to be repeated.

With the half-duplex method, each block can end be regarded as the end of the message for the simplex case. Signals must be in the reverse direction be transmitted as a block acknowledgment or for block repetition. These signals are allowed do not occur intolerably delayed, and care must be taken that they are not caused by any existing ones Echo locks are swallowed. The same conditions can apply in the case of parallel proceedings are present.

The presence of echo suppressors in a network makes the simplex method particularly favorable appear. With the half-duplex method, the recovery time of the echo suppressors must be taken into account can be comparable in length to the data block length. Echo blocking on a selected connection make the application of the duplex method difficult. However, it is possible to set up in to make the broadcasting station so that it interrupts the transmission if no reverse signals are recorded will. The interruption of reception would, however, now result in the redundancy being sent backwards trigger. These are not very advantageous circumstances, but they are likely to result in only one small percentage of connections are echo cancellers.

The object of the invention described at the beginning is achieved in that switching means known per se are provided that divide the channel width into two sub-channels of different bandwidth, and that the transmission device in which the larger message flow is present, the larger the sub-channel Bandwidth is allocated.

An exemplary embodiment of a method for data transmission will now be described below with reference to the drawings described according to the invention.

F i g. Fig. 1 shows a simplified block diagram arm of the overall arrangement;

F i g. Fig. 2 is a block diagram for explaining the function of the arrangement in more detail.

With »redundancy«, one already used above and in data transmission and processing technology The term used becomes additional information linked to the actual message which, however, does not form part of the message itself or is independent message-like Has meaning. It is derived from the content of the actual message by an arithmetic operation and is used to detect and / or correct a certain number of errors that may have occurred while handling the message. The redundancy can take many forms

and transmitted and reproduced in various ways. In practice, this must

has no other meaning than that faults in counters, for example, precautionary measures are taken,

to recognize the actual message and / or to ensure that no local loops arise,

correct. In the present case, the data equipment in station A includes one

5 Data memory 14 (DS), the output of which at a

correct length and an error detection by a modulator (M) 15 lies: The resulting modulated

Check of checksum (paraty check) - ie a signal is transmitted to the line via a high-pass filter (FH)

Check by counting redundancy steps 16 a, a path of the hybrid circuit (H) and YIa

in certain groups of data steps within a contact of the key 13 α supplied. By means of a

of the block at both ends of the transmission path, relay 18 can replace the data memory 14 with a

and comparison of the locally derived special character generator 19 (FG) at one end, which is via a

ten redundancy steps that are controlled by the »Start« button 20 from the other end can be set and

form transmitted to match between the input for the modulator,

them - as well as an error correction by the How- The backward path over the fork 17 a leads over

repetition of the data blocks recognized as faulty. ₁₅ a low-pass filter (FL) 21a to a demodulator (D)

The system is designed for use on the 22, from which the demodulated reverse signal

borrowed, leased telephone channels or applied to the error checking circuit (EC) 23,

homely, selected telephone channels. Stations A and B can use the same equipment

and does not require four-wire connections to the part-

subscriber station. ao serve the directions. In the present

Apart from the special signals (preamble) writing, it is assumed that only one

and the final signals (signal for the end of the post-data transmission from A to B is required, see above

richt), the forward transmission consists of data - that only the equipment required for this

and from monitoring signals. The latter are shown in the respective station,

to determine the data blocks as correct or 25 station B is thus shown as a pure receiving station.

delete them. The reverse transmission is set. The data path branches from the surrounding

only from the redundancy information. The forward switch button 13 6 off in a way of the fork 176 and

and the reverse transmission takes place simultaneously via a high-pass filter 166 to a demodulator 24.

instead of. The backward path is made up of a modulator 25 in order to enable a duplex procedure

borrowed, the telephone band is fed into two sub-bands 30 via a low-pass filter 216. The modulator 25

divided, of which the upper of the transmission in receives its input signal alternately from one

Forward direction, the lower of the transmission in the start signal generator (SG) 26 or from a redundancy

Reverse direction is used. dance generator (RG) 27 via a changeover relay

The modulation measure for the signal transmission contact 28. The output of the demodulator 24 is located

via the line is adjustable. For the 35 on a data memory 29, a monitoring

Forward direction and for leased channels nine lamp 30 6 and at the redundancy generator 27.

hundred steps per second, for selected channels After switching in station B by the

five hundred steps per second. For the back key 136, the start signal generator 126 sends at JS

in the downward direction, in which only the small amount of Re- a Kerin signal that is demodulated in station A

redundancy information is to be transmitted, the 40 is modulated, whereby a signal is generated that the

lation measure somewhat lower, e.g. B. a third desjeni- monitoring lamp 30 a can light up. this shows

gen for the forward direction. to the participant in station A that the station

With reference to Fig. 1 it should first be said that tion B is ready to receive. Meanwhile the relay

the reference numerals used both the function contact 28 shifted at B , so that now the re-

a device per se should be referred to as 45 redundancy generator 27 is located on the modulator 25 in order to

the establishment itself. In the case of the more important the first part of the message (the special characters

Function blocks ι will work with reference letters or the preamble) to address, which is currently from the

in order to facilitate the recognition of their significance - station A is received,

tern. ; . The participant closes ^ as the next function

In Fig. 1 are two subscriber stations A and B 5 ° its start button 20, whereby the preamble or with a normal J subscriber set 10 α or special character generator 19 (via the high-frequency 106 via a telephone network by means of the switch), sends out the appropriate message preamble, ter or selector 11 connected to one another, from around the station B the type of expected over-which is usually a number in series. To indicate the borrowing. The preamble also contains a connection between the participants is in the 55 Synchronisierüngssignal. (The details of the usual way through the function of a voter bearing and function will be explained in more detail later disk 12 a produced (in this description will be,) This preamble is assumed in station B that the subscriber, 4 modulates the caller and causes the redundancy generator 27, and the sending participant is). Then select the appropriate "redundancy", which is answered by the call at 106, and the subscriber 60 the lower-frequency path back to station ^ mer / 1 then informs subscriber B that a transmission is taking place at. The data received from station A are available and are now transmitted to B. Redundancy is to be included in the error checking circuit 23. At an agreed time, the value of the corresponding speeches stored there will be the keys 13a and 136 in each station redundancy, which is there during or before the transmission, so that now a connection between the 65 data stores has been determined according to the outgoing message the sending and receiving station: about, is, compared. The comparison result shows arises. It should be noted that this switchover function neither causes the correct reception of the preamble through the keys 13a and 136, only symbolically station B to or causes the repetition through

the station A, if received incorrectly. If the preamble has been received correctly, the relay 18 now switches the data memory 14 to the input of the modulator 15 instead of the preamble generator 19.

The data are then transferred block by block from memory 14 of station A to memory 29 of station B. The redundancy is determined for each block one after the other and stored in station .4 in order to then compare it with the corresponding received redundancy sent back by the station.

A code for the end of the message may be recognized by which the monitoring lamps both stations are brought into a state in which they only glow and thus indicate to both participants that the message is being transmitted is finished.

As a result of the already mentioned relationship between the amount of the message sent in the forward direction To be transmitted is to the one in the reverse direction, one can use the bandwidth of the two Select channels differently. So may the forward channel - that is, the higher-frequency channel - the band from 900 to 1900 Hz, while the reverse channel is between 350 and 500 Hz.

Let us now consider the data transmission system from a theoretical point of view.

At the transmitting station A , the data to be transmitted are present in the memory 14 in any division into blocks of a certain fixed length, which division does not necessarily have to match the division of the data arriving in the memory. Except for the insertion of a monitoring signal (correct or incorrect), these blocks are

ίο transmitted between them unchanged.

The blocks are, however, treated as if they were divided into a number ("") of nested "checksum words" ("Parity" words), for each of which a checksum check step correct and stored together with the temporary storage of the data block in a data memory 31 has a capacity for two complete blocks including the associated checksum test steps having.

Table A shows the transmission sequence of the preamble or the special character block, the monitoring and data blocks for a typical hundred data block message. the two preamble blocks have the same shape as the data blocks, and on each preamble and The data block is followed by a monitoring signal.

Table A.

Forward channel monitoring Reverse channel block redundancy (n £ steps) (n steps) (n steps) for Preamble 1 Receipt Preamble 2 Preamble 1 Acknowledgment preamble 1 Data block 1 Preamble 2 Acknowledgment preamble 2 Data block 2 Data block 1 Acknowledgment of data block 1 Data block 3 Data block 2 Data block 2 delete signal and data block 3 Data block 2 Data block 3, not used Receipt Data block 3 Data block 2 * Acknowledgment of data block 2 Data block 4 Data block 3 * Acknowledgment of data block 3 : Data block 99 Data block 98 Acknowledgment of data block 98 Data block 100 Data block 99 (Incomplete block: χ message steps, y filling steps) Acknowledgment of data block 99 Dummy block Data block 100 (x "1" step, y »0« steps) Acknowledgment of data block 100 End of forward signals

In the receiving station, the received checksum check steps assigned to each data block are performed not saved. Rather, they are transmitted over the reverse channel. There is one here too Temporary storage of the received data in a data memory 32 is provided, the two complete Can accommodate blocks.

In the transmitting station, the checksum test steps recorded from the reverse channel are compared in the circuit 23 with those which are stored in the memory 31. A “correct” or “false” monitoring signal emerges from this comparison and is stored in a corresponding memory 33 (for “correct”, CFM) or 34 (for “incorrect”, CAN) . As a result of transit time delays in the transmission channel and in the terminal equipment, the monitoring signal does not immediately follow the block to which it relates, but is transmitted at the end of the next block. A »CorrectK monitoring signal therefore relates in the receiving station to the penultimate, stored data block and transfers this data from the temporary storage in memory 32 to the processing device.

The first block of a message always redeems False correct signal that follows it and that relates to a hypothetical data block, which is not to be transferred and therefore has no fence. can. At the end of a message is; add a dummy data block - which thus precedes the monitoring signal that is on refers to the last actual data block. When the end of message signal is recognized, the dummy data block must be deleted.

A "false" signal or cancel signal sent by received by the receiving station also refers to the last but one saved Data block, but in this case deletes the data. To prepare for the repetition of a faulty Block, the last stored data block is also deleted without the statement whether it is correct or incorrect is awaited. Repeated after sending a clear signal the transmitting station receives the data block that was found to be incorrectly received on the basis of the test. This is followed by a repeat signal and the next data block. Then the normal follows "Correct" or "Incorrect" signal relating to the first of the repeated blocks, and so on.

The checksum check steps received during the repetition of the faulty block remain without paying attention. According to Table A, it is assumed that data block 2 was transmitted incorrectly became.

Some functional features of the arrangement will now be explained in more detail. In the transmitting station A , each block fed to the modulator 15 either from the preamble generator 19 or from the memory 14 for the purpose of transmission is simultaneously fed into the stage A of the memory 31 for temporary storage provided for two blocks. The block that previously occupied level A has now been moved to level B. It is assumed that memory 31 also determines the checksum checking steps for each block and stores them at P.

In the same way, in receiving station B, the incoming block is stored in stage A of memory 32 for temporary storage. Here, too, the block previously stored there has been transferred to level B. Simultaneously with the transmission of a block, the redundancy supplied by the redundancy generator 27 of station B is transmitted for the preceding block in the reverse channel. After the demodulation in station A , the redundancy signals in the test circuit 23 with those test steps

ίο compared, which are stored there for the corresponding block in stage B of memory 31. If the two sequences of test steps match, a "correct" signal is generated and stored in its memory 33 for application to the modulator 15 and finally fed to the line after the end of the transmission of the current block. Finally, at the same time, a free signal is fed to the memory 14 (or, if necessary, to the preamble generator 19) in order to bring the next block to be transmitted.

If the check of the redundancy steps shows that a transmission error has occurred, then the erase signal output of circuit 23 is energized. As a result, a cancel signal for the transmission is fed from the memory 34 to the modulator, furthermore a free signal to the memory 31 in order to correctly transfer the two blocks stored there Order to bring. During the transfer, these blocks are re-entered into this memory enrolled to enable a possible further repetition.

In the receiving station B , each of the successive blocks is first stored in the stage of the memory 32 and then shifted to stage B. The arrival of a "correct" signal, which relates to the penultimate block to be received, has the effect that this block is output from stage B of memory 32 into final memory 29. An erase signal erases the blocks in both levels of the memory, preparing it for receiving the repeated blocks.

It is clear that the units in the drawings' - forming arrangement shown, are only represented by their function. Their construction and mode of operation are familiar to those skilled in this technical field. The present invention consists primarily in the operation of the overall system and not in structural or functional details of the various functional units.

During the transmission, the checksum check step for each checksum check word is recorded by binary counters in both the transmitting and receiving stations. These test steps become progressively available in the sending station when the last η data steps of the block have been transmitted and in the receiving station when the same steps have been received. The number of steps (k) within a word determines (vice versa) the ratio of redundancy to the data, but it should not be made too large because this increases the risk of undetected errors. The number of words (n) within a data block determines the separation between the steps within a word. If the separation or the gap is too small, the risk arises that a single disturbance will cause a combination of errors which is not recognized.

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ί 186 494

II 12

The number of steps per data block (nk) can be provided, which indicates when the

must be as small as possible to accommodate the cost of receiving station is ready.

the memory and other furnishings low to the F i g. Figure 2 shows a modified type of one

keep. On the other hand, the portable minimum length is the receiving station B, which is suitable for unmanned operation of the blocks due to the requirement, waiting 5. In this figure is the two-wire line

times to avoid those with the "recovery" times represented by a single conductor 40 running through

in the transmission terminal equipment and with the switching and the contact kl of the relay K,

Transit times of the signals on the telephone line in the toggle key 13 b of FIG. 1 corresponds to

related to both directions. It will run over it. If someone calls on the line, so later In a closer look at the block, the alternating call current via the loop stimulates that

length to be able to speak. It also results in relay A. The contact al closes the hold circuit

like the preferred values for the block length for relay A.

and the number of test steps for a rented The contact al sets a machine C for literal

Telephone line, whose running times are in each Rieh- borrowed announcement in progress, in which it is a processing should not exceed 20 milliseconds with a 15 headed magnetic tape or around a device

(Forward) modulation measure of nine hundred one film and one light cell of a known type

Steps per second are to be selected. This is your can. A cam or a rib on top of it

from Table B below. Wheel that carries the tape or film operated

a contact set el when the wheel is turning

Table B ₂ o begins. As a result, the hold circuit of relay A

_n _ g η g pni ^ orte opened, and the relay drops out. Still that remains

'Open machine circuit independent of contact al

k = 9 8 8 steps per check word, keep right. The relay B works via electrical. Its

nk = 54 56 64 steps per data block. When closing, contact b \ puts a direct current off over the loop in the receiving station as

Corresponding block lengths can also be short-circuited for the winding of the relay A and

selected connections with a (forward) mod- thus gives an answer. The new way contains one

lation measure of five hundred steps per second winding of the transformer 41, over which the literal

be applied. However, there may be circumstances in which the message is returned to the sending station. Has

occur, under which the greater economy 30 the machine ends its response, so goes

a smaller block length is desirable. Spring contact c 1 in its normal position, so that the

If no error detection is necessary, the machine drive is interrupted. The relay K redundancy device in the transmitting station by hand responds, and the input loop is switched off to the. This automatically changes the preamble that is sent before the fork 17 b and the two filters 16 b and 16 lib appropriate data transmission, 35 sets. The contact k 3 actuates the modulator for which thus indicates to the receiving station that the return 25, to which two oscillators belong, transmission of a redundancy is not necessary that the two signal frequencies X1 and Yl for the and that the received data is immediately sent Provide reverse direction, as well as a third oscilloscope processing can be supplied. lator, which is dimensioned so that it works with the one in baud

The monitoring signals can each work six 40 measured speed of the channel.

Include steps that are encoded in a suitable manner. If the signal relay L is at rest, it transmits

are, and a preamble signal can be a block from the modulator using the X2 frequency as a start signal

01010101 ... 01 steps to which a block the line.

from 00110011 ... 11 steps follows which blocks operate during normal transmission

are separated by the "correct" signal. The pre- 45 signals running in the forward direction affect the reception

Ambelsignal can first synchronize the relays XlR and FIi? after another. Both relays

Dlfcsen the reception and, if the relay ZIi? with dropout delay,

catcher is correctly recognized, the return of the corresponding contact ζIrI the hold circuit for the relay B

arrange suitable redundancy. If the latter is in, it now falls off. The response state

the transmitting station has been correctly received, then triggers 50 of the switch relay K is now dependent on that

they have a display (e.g. a green lamp) and the contact zlrl is closed. The end of

causes another "correct" message to be transmitted - the absence of prior

Signal and the start of data transmission. signals are signaled, which after a short time

The end of the message is triggered by the special delay in which the relay Z Ii? and K er monitoring signal are displayed and followed by the sequence. If the relay K drops out and the synchronization of the current loop of the receiving station transmitted in the forward direction is interrupted, inTones is acknowledged by the line. This state- if his contact A1 opens, the change on the receiving side, after a delayed state, corresponds to that determined by a warning that is sufficient to cause an incorrect operator to display the state at the end of the message instead of this receiving device could,
to avoid line noise. It has already been explained that the

The receiving station can be manned or in an inbound direction transmitted monitoring signal, the

be manned. In any case, a verbatim announcement should be made as to whether there is redundancy for a data block

termination take place before the data setup is correct, not immediately on the concerned

is connected, and this announcement should follow 65 data block, but only to the next

by a signal consisting of approximately fifty zero data blocks, so that approximately the transmission time.

steps, beesdet. A block of data is left in the send to ensure redundancy

station can determine a lamp or other signal, as well as for the running times.

If you consider the available time in detail, it should be noted that a monitoring signal is sent out after each data block, which may contain six steps, so that the period of a data block and a monitoring signal corresponds to the transmission time for {nk + 6) steps . It should also be noted that the definition of the redundancy in the receiving station cannot begin before the last step of the first check word has been received. This step is received «steps before the end of the data block. The comparison of the redundancies must end on the sending side

before the following data block is completely transmitted, and the important time interval to be taken into account in the transmitting station is therefore between the last step (or its transmission) of the first check word and the completion of the next data block. This interval corresponds to the sending time of {nk + 6) steps plus the sending time of «steps, which represent the last step of each check word. Hence the entire interval is equal to the time it takes to send out [« {k + 1) + 6] steps.

During this time, the functions listed in Table C must be expected:

Table C.

Run time on the line in the forward direction - typical

Total recovery times of the forward signal in transmitting and receiving devices -

Transmission time for the "steps of redundancy with a third of the speed of the modulation measure for the data ... -

Running time on the line for redundancy in the reverse direction -

Total of the recovery times of the redundancy in the reverse direction for the transmitting and receiving devices .... -

20 milliseconds

3 steps

3 «steps

20 milliseconds

9 steps

Total time .... (3 «+ 12) steps + 40 milliseconds

Hence η {k + 1) + 6 steps 3 η + 12 steps + 40 milliseconds η {k - 2) - 6 steps 40 milliseconds

At nine hundred steps per second, each step takes ¹⁰ / e milliseconds, while at five hundred steps per second, each step takes 2 milliseconds.

If the values 6, 7 and 8 are taken for η, the values given in Table D for the two step speeds for k result.

Table D.

At 900 steps per second ... At 500 steps per second ...

«= 678 Jt = 9 8 8

k = 7 6 6

There is one point worth considering here. The incoming data are random, i. H. independent divided by their natural division into blocks of about fifty steps each. In many In some cases, however, the data to be transmitted will not fill an integer number of blocks, so that the last block is incomplete. This block is therefore padded with "dummy" data, and it raises the difficulty of determining the end of the actual data and thus the beginning of the bogus data.

It is suggested to take advantage of the view that the message brings to end a dummy block, which is the monitoring signal for the last, actual data block precedes. This dummy block can be designed so that it contains a number pattern at where the steps occupied by ones indicate where data is in the last block while the steps occupied by zeros indicate the position of the filling steps. In Table A this is Procedure indicated.

An economical data transmission system has been described above, which is based on the duplex method and works at medium speed and for use on rented or chosen Two-wire circles is suitable. The system includes automatic error correction by repetition of the data blocks concerned.

The special features of the system are the use of different speeds for the transmission of the data on the one hand and the redundancy on the other hand as well as the utilization of the Preamble and the monitoring signals for the synchronization.

If an error detection or correction is not necessary, the transmission conditions are easier. On the other hand, the transmission conditions become much more difficult when the data transmission must be passed over a network in which echo cancellers are included. The receiving station will normally be manned so that the incoming lines for telephony and for Telegraphy can be used. However, it is possible to operate without manning, especially at night.

Claims (16)

M 86 If there is no automatic error detection or Correction is required, a special preamble signal is used which, after decoding the Receiving station indicates that no redundancy needs to be determined and that none Repetitions are required. The data transfer is thus · faster, because there are intermediate storage omitted. ■■ - In general, it is not possible to determine whether the selected connection, which runs over a conventional telephone network, contains an echo suppressor, neither at the transmitting nor at the receiving station. An echo suppressor, however, causes an additional delay for the arrival of the redundancy at the transmitting station. As a result, the sending station can ^: be set up to start the data transmission again using a special preamble signal that instructs the receiving ^{station to delay the retransmission of the redundancy '1} SO' length so that the delay caused by the echo suppressor ao is called, is covered. This considerably reduces the actual speed of data transmission. As already stated, selected connections should only contain echo suppressors in rare cases. The data receiving means may be either a signal hm, indicating the end of the message!, After two or more acknowledgment signals, or the absence of the signal tone via a festgesfetete ^"1 overtime be shut down. When the receiving means is turned off, sound for-., a short ₍ time ijefn: warning tone, and-after this the DC loop is opened. Another advantage of the one described above System is iß the 'possibility of use of different types of preambles in order to pass between different types of reception to choose automatic control change. 40 ... . ... '■ -. Claims; '1'. Method of Baien transfer for simultaneous Bidirectional operation on a single ICannel with limited bandwidth, in particular on emem telephone channel, marked äaduTCh, that per se, known switching means are provided that subdivide the channel bandwidth into two sub-channels of different bandwidths, 'and (leave the transmission direction, in which the larger message fifipß is present, to the subchannel larger bandwidth is assigned. 2. The method according to claim 1, characterized in that that the messages are in the two directions at different speeds be transmitted, 'in the direction for which the sub-channel uses the larger bandwidth becomes, with the greater speed. 3. Method according to claim 2, characterized in that that the transmission speed and the bandwidth of each sub-channel such are coordinated so that a complete Utilization results. 4. The method according to claim 1 to 3, characterized in that in 'one direction, the . The “forward” direction to which the subchannel with larger bandwidth is assigned is the useful messages (Data) are transmitted, in the »backwards« direction, on the other hand, only monitoring signals. 5. The method according to claim '4, - characterized in that in the terminal equipment (A, B in F i g. 1) means are provided to correct the messages transmitted in the forward direction due to the backward direction transmitted monitoring signals of a lesser extent Check receipt. 6. The method according to claim 4 and 5, characterized in that in the transmitting station (A) means are provided to subdivide the incoming messages, for transmission into blocks of any desired, but the same length, and that the check block by block one after the other he follows. 7. The method according to claim 4 to 6; through this characterized in that checking for transmission errors for a particular block during the transfer of the next block takes place. 8. The method according to claim 4 to 7, characterized in that the end station (A) is equipped for the transmission in the forward direction with a memory (31) for the last two blocks transmitted and that the transmission of both blocks is repeated if in the first of these two blocks a distance : is set. . · ... , - 9. The method according to claim 4 to 7, characterized in that the end station (B) is equipped for receiving from the forward direction with 'a memory (32) for the last two received blocks, both of which are deleted when in the first of these an error is detected in both blocks. 10. The method according to claim 4 to 9, characterized in that the check of a specific block on the end station (A) for the transmission in the forward direction, which sends a monitoring signal after the next block, either the specific block from the memory on ctas (Level B of 32) of the receiving station for the forward direction is stored or the specific block and the next in this memory (level B or level A of 32) are deleted. 11. The method according to one or more of claims 6 to 10, characterized in that the forward transmission end station has means for determining whether the last block of a message is not completely filled by its signals, and around this block to be filled up with fill signals if necessary. 12. The method according to any one of claims 6 to 11, characterized in that according to the a further block (dummy block) is transmitted to the last block containing useful messages, whose content indicates which part of the last block containing useful messages from useful 'news exists. 13. The method according to claim 1, characterized in that the end stations (A or B) switching means (13 a or 13 b) are assigned, with the help of which either these data transmission end stations or telephone subscriber sets (10 a or Wb) to the Transmission path are connectable. ^; 14. The method according to claim 13, characterized in that the switching means (13 a, 13 b) automatically control the data transmission terminal then switch to the transmission path when a signal announcing a data transmission is over the transmission path has been received. 15. The method according to claim 13 and 14, characterized in that the connection of a of the terminal equipment in this triggers a signal sent via the transmission path that indicates their readiness to start the data broadcast. 16. The method according to claim 15, characterized in that the readiness to receive the signal indicating an end station triggers the start of the data transmission in the opposite station. 1 sheet of drawings