Data transmission method and apparatus

What is claimed is:

1. A method for transmitting data from a first location to at least one receiver location comprising the steps of:

receiving a plurality of data messages of varying bit rates at said first location;

organizing said data messages into a multiframe matrix, said matrix including a plurality of columns, each column representing a period of time, and a plurality of rows, each row representing a frame of said matrix, individual characters of said data messages being selectively inserted into said columns of said matrix in real time;

transmitting over a transmission medium, for each successive frame of said matrix, the characters in successive columns at a defined bit rate greater than the bit rates of any of said data messages; and

decoding at each receiver location the transmitted matrix, messages which are addressed to a receiver location being available for display at said receiver location.

2. The method recited in claim 1 wherein the first row of said matrix contains information associated with the bit rates and the addresses of said data messages.

3. The method recited in claim 1 wherein said step of receiving comprises the step of receiving data messages having a bit rate of up to approximately 300 bits/sec and data messages having a bit rate between approximately 300 and approximately 4800 bits/sec.

4. The method recited in claim 1 wherein said defined bit rate is 9600 bits/sec.

5. The method recited in claim 1 wherein the first column of said matrix includes synchronizing information.

6. The method recited in claim 5 wherein said second column includes synchronizing information.

7. The method recited in claim 2 wherein said second row further includes addressing information.

8. The method recited in claim 1 wherein said plurality of data messages include control information for controlling associated equipment at said receiver location and subscriber information for display at said receiver location.

9. The method recited in claim 1 further including the step of relaying said transmitted matrix at an intermediate location between said first location and said receiver location.

10. The method recited in claim 9, further lncluding the step of inserting further messages into said matrix at said intermediate location.

11. The method recited in claim 10 wherein said plurality of data messages includes a first data communication having a first bit rate and a second data communication having a second higher bit rate.

12. The method recited in claim 11 wherein said first data communication is constrained to a single column of said matrix and said second data communication may occupy a plurality of columns.

13. The method recited in claim 11 wherein said first data communication may originate at said first location or at said intermediate location.

14. The method recited in claim 13 wherein said first data communication may be addressed to an individual one of said receiver locations or may be addressed to a group of said receiver locations.

15. The method recited in claim 11 wherein said plurality of data messages further includes additional data communications having said first bit rate, said data communications having said first bit rate being inserted into selected columns of said matrix, said data communications having said second bit rate being inserted into remaining columns of said matrix.

16. The method recited in claim 11 wherein said first bit rate is up to 300 bits/sec and said second bit rate is between approximately 300 and 4800 bits/sec.

17. The method recited in claim 1 wherein said step of transmitting comprises the step of modulating said matrix onto an F.M. broadcast radio station subcarrier.

18. A system for transmitting data from a first location to at least one receiver location comprising:

means for receiving a plurality of data messages having varying bit rates at said first location;

means for organizing said data messages into a mutltiframe matrix, said matrix including a plurality of columns, each column representing a period of time, and a plurality of rows, each row representing a frame of said matrix, individual characters of said data messages being selectively inserted into said columns of said matrix in real time;

means for transmitting over a transmission medium, for each successive frame of said matrix, the characters in successive columns at a defined bit rate greater than the bit rate of any of said data messages; and

means for decoding at each receiver location the transmitted matrix, messages which are addressed to a receiver location being available for display at said receiver location.

19. The system recited in claim 18 wherein the first row of said matrix contains information associated with the bit rates and the addresses of said data messages.

20. The system recited in claim 18 wherein said means for receiving comprises means for receiving data messages having a bit rate of up to approximately 300 bits/sec and data messages having a bit rate between approximately 300 and approximately 4800 bits/sec.

21. The system recited in claim 18 wherein said defined bit rate is 9600 bits/sec.

22. The system recited in claim 18 wherein the first column of said matrix includes synchronizing information.

23. The system recited in claim 22 wherein said second column includes synchronizing information.

24. The system recited in claim 19 wherein said second row further includes addressing information.

25. The system recited in claim 18 wherein said plurality of data messages include control information for controlling associated equipment at said receiver location and subscriber information for display at said receiver location.

26. The system recited in claim 18 further including means for relaying said transmitted matrix at an intermediate location between said first location and said receiver location.

27. The system recited in claim 26, further including means for inserting further messages into said matrix at said intermediate location.

28. The system recited in claim 27 wherein said plurality of data messages includes a first data communication having a first bit rate and a second data communication having a second higher bit rate.

29. The system recited in claim 28 wherein said first data communication is constrained to a single column of said matrix and said second data communication may occupy a plurality of coluumns.

30. The system recited in claim 28 wherein said first data communication may originate at said first location or at said intermediate location.

31. The system recited in claim 30 wherein said first data communication may be addressed to an individual one of said receiver locations or may be addressed to a group of said receiver locations.

32. The system recited in claim 28 wherein said plurality of data messages further includes additional data communications having said first bit rate, said data communications having said first bit rate being inserted into selected columns of said matrix.

33. The system reacted in claim 28 wherein said first bit rate is up to 300 bits/sec. and said second bit rate is between approximately 300 and 4800 bits/sec.

34. The system recited in claim 18 wherein said means for transmitting comprises means for modulating said matrix onto an F.M. broadcast radio station subcarrier.

35. A method for transmitting data from a first location to at least one receiver location comprising the steps of:

receiving a plurality of data messages of varying bit rates at said first location;

organizing said data messages into a multiframe matrix, said matrix including a plurality of columns, each column representing a period of time, and a plurality of rows, each row representing a frame of said matrix, individual characters of said data messages being selectively inserted into said columns of said matrix in real time, the first row of said matrix containing information associated with the bit rates and the addresses of said data messages;

transmitting over a transmission medium, for each successive frame of said matrix, the characters in successive columns at a defined bit rate greater than the bit rates of any of said data messages; and

decoding at each receiver location the transmitted matrix, messages which are addressed to a receiver location being available for display at said receiver location.

36. The method recited in claim 35 wherein said step of receiving comprises the step of receiving data messages having a bit rate of up to approximately 300 bits/sec and data messages having a bit rate between approximately 300 and approximately 4800 bits/sec.

37. The method recited in claim 35 wherein said defined bit rate is 9600 bits/sec.

38. The method recited in claim 35 wherein the first column of said matrix includes synchronizing information.

39. The method recited in claim 38 wherein said second column includes synchronizing information.

40. The method recited in claim 35 wherein said second row further includes addressing information.

41. The method recited in claim 35 wherein said plurality of data messages include control information for controlling associated equipment at said receiver location and subscriber information for display at said receiver location.

42. The method recited in claim 35 further including the step of relaying said transmitted matrix at an intermediate location between said first location and said receiver location.

43. The method recited in claim 42, further including the step of inserting further messages into said matrix at said intermediate location.

44. The method recited in claim 43 wherein said plurality of data messages includes a first data communication having a first bit rate and a second data communication having a second higher bit rate.

45. The method recited in claim 44 wherein said first data communication is constrained to a single column of said matrix and said second data communication may occupy a plurality of columns.

46. The method recited in claim 44 wherein said first data communication may originate at said first location or at said intermediate location.

47. The method recited in claim 46 wherein said first data communication may be addressed to an individual one of said receiver locations or may be addressed to a group of said receiver locations.

48. The method recited in claim 44 wherein said plurality of data messages further includes additional data communications having said first bit rate, said data communications having said first bit rate being inserted into selected columns of said matrix, said data communications having said second bit rate being inserted into remaining columns of said matrix.

49. The method recited in claim 44 wherein said first bit rate is up to 300 bits/sec and said second bit rate is between approximately 300 and 4800 bits/sec.

50. The method recited in claim 35 wherein said step of transmitting comprises the step of modulating said matrix onto an F.M. broadcast radio station subcarrier.

51. A system for transmitting data from a first location to at least one receiver location comprising:

means for receiving a plurality of data messages having varying bit rates at said first location;

means for organizing said data messages into a multiframe matrix, said matrix including a plurality of columns, each column representing a period of time, and a plurality of rows, each row representing a frame of said matrix, individual characters of said data messages being selectively inserted into said columns of said matrix in real time, the first row of said matrix containing information associated with the bit rates and the addresses of said data messages;

means for transmitting over a transmission medium, for each successive frame of said matrix, the characters in successive columns at a defined bit rate greater than the bit rate of any of said data messages; and

means for decoding at each receiver location the transmitted matrix, messages which are addressed to a receiver location being available for display at said receiver location.

52. The system recited in claim 51 wherein said means for receiving comprises means for receiving data messages having a bit rate of up to approximately 300 bits/sec and data messages having a bit rate between approximately 300 and approximately 4800 bits/sec.

53. The system recited in claim 51 wherein said defined bit rate is 9600 bits/sec.

54. The system recited in claim 51 wherein the first column of said matrix includes synchronizing information.

55. The system recited in claim 54 wherein said second column includes synchronizing information.

56. The system recited in claim 51 wherein said second row further includes addressing inforamtion.

57. The system recited in claim 51 wherein said plurality of data messages include control information for controlling associated equipment at said receiver location and subscriber information for display at said receiver location.

58. The system recited in claim 51 further including means for relaying said transmitted matrix at an intermediate location between said first location and said receiver location.

59. The system recited in claim 58, further including means for inserting further messages into said matrix at said intermediate location.

60. The system recited in claim 59 wherein said plurality of data messages includes a first data communication having a first bit rate and a second data communication having a second higher bit rate.

61. The system recited in claim 60 wherein said first data communication is constrained to a single column of said matrix and said second data communication may occupy a plurality of columns.

62. The system recited in claim 60 wherein said first data communication may originate at said first location or at said intermediate location.

63. The system recited in claim 62 wherein said first data communication may be addressed to an individual one of said receiver locations or may be addressed to a group of said receiver locations.

64. The system recited in claim 60 wherein said plurality of data messages further includes additional data communications having said first bit rate, said data communications having said first bit rate being inserted into selected columns of said matrix.

65. The system recited in claim 60 wherein said first bit rate is up to 300 bits/sec. and said second bit rate is between approximately 300 and 4800 bits/sec.

66. The system recited in claim 51 wherein said means for transmitting comprises means for modulating said matrix onto an F.M. broadcast radio station subcarrier.

Description Submit all comments and votes

BACKGROUND OF THE INVENTION

The present invention relates to data transmission systems and methods and more particularly, to a digital data transmission system and method which groups and organizes incoming data communications having different baud rates in real time and transmits the data at a defined baud rate for reception by a large number of subscribers. The present invention has particular application, for example, to systems for transmitting digital data via an FM broadcast station's subcarrier, or as it is known, by a Subsidiary Communications Authorization (SCA) subcarrier or other forms of one-way broadcast systems.

The need exists for a method and apparatus for transmitting digital data via an FM subcarrier which has the ability to transmit multiple communications at varying speeds, for example, at both low speed (below 300 bits/sec) and high speed (up to 4800 bits/sec). The present invention provides a means for fulfilling this need.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide a method and apparatus for transmitting both low and high speed messages via a single communications link, such as an FM broadcast subcarrier communications link.

It is a further object of the present invention to provide such a method and apparatus which allows the nearly simultaneous transmission of communications which are normally transmitted at different bit rates, e.g., low speed, below 300 bits/sec. and high speed, up to 4800 bits/sec.

It is still a further object of the present invention to provide a data transmission method and apparatus which groups incoming data messages in real time and transmits the data with minimal throughput delay.

It is still a further object to provide a data transmission method and apparatus which is highly tolerant of burst errors.

It is yet a further object to provide a data transmission method and apparatus which provides for the insertion of data communications into a data transmission at a local level.

It is yet another object to provide a data transmission method and apparatus which operates both in synchronous and asynchronous modes.

These and other objects of the present invention are achieved by a method for transmitting data from a first location to at least one receiver location comprising the steps of receiving a plurality of data messages of varying bit rates at the first location, organizing the data messages into a multiframe matrix, the matrix including a plurality of columns, each column representing a period of time, and a plurality of rows, each row representing a frame of the matrix, individual characters of the data messages being selectively inserted into the columns of the matrix in real time, transmitting over a transmission medium, for each successive frame of the matrix, the characters in successive columns at a defined bit rate greater than the bit rates of any of the data messages and decoding at each receiver location the transmitted matrix, messages which are addressed to a receiver location being available for display at the receiver location.

The invention also includes within its scope a system for carrying out the method, as will be clear from the description which follows.

Other objects, features and advantages of the present invention will be apparent from a reading of the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in greater detail in the following detailed description with reference to the drawings, in which:

FIG. 1 is a block diagram of the basic system employing the present invention including satellite transmission from a central location to a local FM radio station prior to FM SCA subcarrier data transmission to individual receivers;

FIG. 2 shows the matrix framing structure for synchronous transmission of data communications according to the present invention;

FIG. 3 shows the matrix framing structure for asynchronous transmission of data communications according to the present invention; and

FIGS. 4a-m, when viewed in combined form, are a flow chart illustrating the operation of a receiver which decodes the received data transmitted via the framing structure of FIG. 3.

DETAILED DESCRIPTION

With reference now to the drawings, FIG. 1 illustrates one arrangment of the basic system. A central computer 10 directs messages from phone lines 12 and other messages generated by the computer, either automatically or manually, to a microwave transmitter and antenna 14. A satellite 18 in geosynchronous orbit receives the transmissions via up-link 16 and returns them to a local broadcast station's antenna 22 via down-link 20. The local station 25 includes a receiver 27 and local telephone messages can optionally be directed from local telephone lines 24 into the system if the local station is equipped with a local computer 26. A special transmission protocol, to be discussed below, is used by both the central and local computers in order to allocate messages to particular time slots. The received signals are then coupled to F.M. transmitter 28 which includes spectral shaping circuitry 33, subcarrier generator and modulator 34 and F.M. modulator 35. Transmission between local station 25 and F.M. transmitter 28 is typically via a land line of several miles. In transmitter 28, the received data is spectrally shaped by circuitry 33, modulates the SCA subcarrier in circuit 34 prior to insertion into the multiplexed composite signal 31 and then modulates the station's main carrier frequency prior to transmission. An antenna 30 radiates the modulated F.M. carrier to local users. Each user is equipped with a special receiver 32 for recovering the signal modulating the subcarrier including a microprocessor for determining when messages are addressed to itself and for processing and displaying the recovered data. The operation of one embodiment of such a receiver will be discussed in detail below.

Components of the F.M. transmitter 28 which can be used to practice the invention include the following: 33, the Digital Partial Response Filter described in copending application Ser. No. 483,738, filed Apr. 11, 1983, of Michael T. Hills and Raymond L. Heinrich; 34, the Digital Subcarrier Generator and Modulator described in copending application Ser. No. 483,737, filed Apr. 11, 1983, of Raymond L. Heinrich, Michael T. Hills and David W. Brown.

An important feature of the present invention is the ability to maintain the security of transmitted messages through system control of who may receive the messages. Those receivers 32 that do not include circuitry capable of decoding the transmitted data will be unaware of data messages transmitted on the subcarrier, although still able to receive normal F.M. programming. Those equipped with the required microprocessor circuits will need to respond to specific addresses in order to receive data, and the addresses may be changed at will at the central computer or at the local station.

The central, local and receiver computers process data messages in accordance with a framing structure or matrix to be discussed below. There are two sources of data messages at central computer 10: Those received from one or more telephone lines 12 and those generated by the central computer, either automatically or through manual entry. All data messages are merged into a single data stream which operates at a faster rate than any of the sources. This new data stream is inserted into the main transmission 16 destined towards the satellite 18.

At the local station 25, a local computer 26 is required only if a local data message capability is needed. If the capability is not needed, then the down-side transmission 20 is forwarded to FM transmitter 28 without changing the content.

Local computer 26 has three sources of data messages: Those received from one or more local telephone lines 24, those generated by the local computer either automatically or through manual entry and those received in the down-side transmission 20. The local computer will extract the composite data messages from the main transmission 20, verify accuracy, remove any messages addressed to itself and locate available portions so that local messages can be inserted to form a modified composite data message stream. This stream will be re-inserted into the main transmission destined for FM transmitter 28.

At the individual receivers 32, the composite data message will be extracted from the main transmission. A computer, for example, a microprocessor, will verify accuracy and process only those data messages it recognizes to be addressed to itself. Processing includes the ability to change internal group address codes internally at the receivers 32.

The present invention can be used to transmit data both synchronously and asynchronously. Both user generated messages and control directives may be transmitted. Such messages may originate centrally or at the national level, entering the network before transmission to the satellite 18, or may enter at the local level and join the stream received from the satellite. Also, the content of the data may be addressed individually or to a group of receiver terminals. Group addressing allows several receiver terminals to accept a single message. Further, the speed of the processed data messages may be low, that is, at or below 300 baud asynchronous or it may be a high speed transmission from 300 baud up to 4800 baud asynchronous.

The following five types of messages as shown in Table I may be transmitted using the method of the present invention:

TABLE I ______________________________________ TYPE 0 National Origination, Individually Addressed. TYPE 1 National Origination, Group Addressed. TYPE 2 Local Origination, Individually Addressed. TYPE 3 Local Origination, Group Addressed. TYPE 4 High Speed Message. ______________________________________

In Table I, TYPE 0-3 messages are low speed messages, and can be either control or subscriber messages. TYPE 4 messages are high speed subscriber messages only.

These messages are transmitted via a transmission protocol having a multiframe format comprising a number of frames organized into a matrix. The framing structure for synchronous transmission is shown in FIG. 2, while the structure for asynchronous transmission is shown in FIG. 3. The framing structure shown in FIGS. 2 and 3 will hereinafter often be referred to as a "multiframe." Each row of the multiframe is a frame. Each frame is divided into a number of cells, the cells forming columns. The first two frames (Rows 0 and 1) generally contains message classification and group addressing information.

The framing structure for synchronous data transmission will be discussed first. As shown in FIG. 2, the multiframe contains 10 frames of 31 cells, where each cell contains, for example, an 8 bit character. The data in the cells is transmitted serially, from left to right and top to bottom, and preferably at 9600 bits/sec. Each cell therefore represents a time slot. To illustrate the operation of multiframe transmission, a low speed 300 baud asynchronous message to be inserted into the multiframe is received by central computer 10 or local computer 26 at a rate of 30 characters per second, for example. The multiframes are transmitted at a rate of 9600 baud, or 1200 characters per second. As the incoming message is received, it is placed in a column of the matrix in real time, i.e., the characters of each message are immediately inserted into the multiframe when received, and then transmitted, in contrast to a delayed or "packet" type transmission where transmission is not performed until an entire "packet" is formed. Excluding message classification and addressing information in frames 0 and 1, each multiframe will transmit 8 characters of the incoming low speed message at 9600 baud. Each multiframe occupies 0.2583 seconds ##EQU1## will arrive on the low speed channel. Thus, the multiframe size is chosen to be slightly faster than the incoming channel so as to stay ahead of the sources. This allows real time organization of the multiframe. Accordingly, throughput delay is kept to a minimum.

Each cell of the multiframe contains a single 7 bit character, for example, an ASCII character as shown in Table II, plus 1 bit of parity, for example, odd parity. Odd parity is where the total count of logical ones in a character of 8 bits is always an odd number. The central computer 10 generates the multiframe in real time as the data is received, inserts the data and immediately scans the data out for transmission. A scan is started from left to right, beginning with row zero and then continuing with row one through row nine. The local computer 26 scans the received multiframe, extracts messages for itself, inserts local messages and forwards the multiframe to the FM transmitter 28 for transmission to the individual receivers 32. Each receiver selects those characters belonging to messages addressed to itself.

In order to synchronize the local stations and the individual receivers, a synchronization column is provided. In the synchronous version shown in FIG. 2, a "frame lock" is provided in order to allow the local stations and the individual receivers to recognize where each character is located in the multiframe matrix. In FIG. 2, a frame lock is created by placing only the sync character S in column 0 of the multiframe. In order to determine where a multiframe begins, another character, other than the sync character, shown in FIG. 2 as S, is placed in column 0, row 0. If ASCII code is employed, the character decimal 22 is used, as can be determined from a review of Table II, which shows the ASCII character set.

TABLE II ______________________________________ ASCII ASCII Char. Oct Nov Dec Char. Oct Nov Dec ______________________________________ NULL 000 00 0 @ 100 40 64 SOH 001 01 1 A 101 41 65 STX 002 02 2 B 102 42 66 ETX 003 03 3 C 103 43 67 EOT 004 04 4 D 104 44 68 ENQ 005 05 5 E 105 45 69 ACK 006 06 6 F 106 46 70 BELL 007 07 7 G 107 47 71 BS 010 08 8 H 110 48 72 HT 011 09 9 I 111 49 73 LF 012 0A 10 J 112 4A 74 VT 013 0B 11 K 113 4B 75 FF 014 0C 12 L 114 4C 76 CR 015 0D 13 M 115 4D 77 SO 016 0E 14 N 116 4E 78 SI 017 0F 15 O 117 4F 79 DLE 020 10 16 P 120 50 80 DC1 021 11 17 Q 121 51 81 DC2 022 12 18 R 122 52 82 DC3 023 13 19 S 123 53 83 DC4 024 14 20 T 124 54 84 NAK 025 15 21 U 125 55 85 SYNC 026 16 22 V 126 56 86 ETB 027 17 23 W 127 57 87 CAN 030 18 24 X 130 58 88 EM 031 19 25 Y 131 59 89 SUB 032 1A 26 Z 132 5A 90 ESC 033 1B 27 [ 133 5B 91 FS 034 1C 28 134 5C 92 GS 035 1D 29 ] 135 5D 93 RS 036 1E 30 * 136 5E 94 OS 037 1F 31 -- 137 5F 95 space 040 20 32 ' 140 60 96 041 21 33 a 141 61 97 " 042 22 34 b 142 62 98 # 043 23 35 c 143 63 99 $ 044 24 36 d 144 64 100 % 045 25 37 e 145 65 101 & 046 26 38 f 146 66 102 ' 047 27 39 g 147 67 103 ( 050 28 40 h 150 68 104 ) 051 29 41 i 151 69 105 = 052 2A 42 j 152 6A 106 + 053 2B 43 k 153 6B 107 ' 054 2C 44 l 154 6C 108 -- 055 2D 45 m 155 6D 109 .multidot. 056 2E 46 n 156 6E 110 / 057 2F 47 o 157 6F 111 0 060 30 48 p 160 70 112 1 061 31 49 q 161 71 113 2 062 32 50 r 162 72 114 3 063 33 51 s 163 73 115 4 064 34 52 t 164 74 116 5 065 35 53 u 165 75 117 6 066 36 54 v 166 76 118 7 067 37 55 w 167 77 119 8 070 38 56 x 170 78 120 9 071 39 57 y 171 79 121 : 072 3A 58 z 172 7A 122 ; 073 3B 59 [ 173 7B 123 < 074 3C 60 .vertline. 174 7C 124 -- 075 3D 61 ] 175 7D 125 > 076 3D 62 - 176 7E 126 ? 077 3F 63 DCL 177 7F 127 ______________________________________

Message group addressing information is sent in row zero and row one, except for column zero which, as noted, is used for synchronization purposes. Type of message information is contained in row zero and is depicted in FIG. 2 by the symbols a0 (TYPE 0), a1 (TYPE 1), a2 (TYPE 2), a3 (TYPE 3) and a4 (TYPE 4). Thus, the multiframe shown in FIG. 2 illustrates all five types of messages. Of course, a transmitted multiframe need not include all five types of messages if they were not received for insertion into the multiframe. In addition to message type information these symbols also contain partial group addressing information. If ASCII characters are used, the characters used in row zero for message type information and group addressing are chosen from the decimal characters, starting at 33 up through 126 (see Table II). Characters below decimal 33 and the character decimal 127 are used for special purposes, to be described below. The exact allocation of the number of usable ASCII characters for a0, a1, a2, a3 and a4 is furthermore dynamically controlled. For example, if the 79 (decimal 48 to decimal 126 inclusive) usable ASCII characters were allocated so that 30 were for partial group addressing of message TYPE 3, then there will be 49 available for message TYPE 1. Thus for:

TYPE 0 Messages, the range of a0 is 33 only

TYPE 1 Messages, the range of a1 is 48 to 96

TYPE 2 Messages, the range of a2 is 34 only

TYPE 3 Messages, the range of a3 is 97 to 126

TYPE 4 Messages, the range of a4 is 35 only

Row one is the second half of the group address (b0, b1, b2 and b3) and these symbols are chosen from the decimal ASCII characters from Table II, which yields 79 usable characters between 48 and 126 inclusive.

TYPE 0 through 3 messages are constrained in a single column because they are all low speed. The individual subscriber address for TYPE 0, 1, 2 and 3 messages is sent as part of the message, thereby permitting unlimited address flexibility. The message style is called block mode and is shown in Table III. The message is in three parts: A header, the text and an error check. If ASCII characters are employed, the key characters used to punctuate the three message parts are: Start of Header--SOH (Decimal 1), Start of Text--STX (Decimal 2), End of Text--ETX (Decimal 3) and End of Transmission--EOT (Decimal 4). The address for block mode messages is sent in the header and may be several safe characters long. Safe characters are defined as those between Decimal 33 and Decimal 126 inclusive of the ASCII character set shown in Table II.

TABLE III ______________________________________ SOH address . . . STX . . . text . . . ETX . error check . ETB EOT ______________________________________

Once a TYPE 0 message has been assigned a column (as close to column one as possible) it continues to occupy the same column until the next frame after the message completes. TYPE 0 messages may be assigned into columns 1 though 5, for example, as shown in FIG. 2.

TYPE 1 messages are also assigned one column and only then if there are characters waiting to be sent. They may appear in any column between TYPE 0 and TYPE 2 messages. One message may not be inserted twice in the same frame. In FIG. 2, TYPE 1 messages are shown in columns 6-9.

TYPE 2 local messages are treated the same way as TYPE 0. The transmission from the central site simply reserves at least three columns after the last TYPE 1 message. TYPE 2 messages are shown in columns 10-19.

TYPE 3 local messages are treated the same way as TYPE 1 messages. They are shown in columns 20-23.

TYPE 2 and 3 messages are inserted into the multiframe by local computer 26 in spaces reserved by central computer 10.

TYPE 4 messages are sent one at a time. They occupy all unused columns, at least three. In FIG. 2, TYPE 4 messages are shown in columns 24-30. No addressing information will be found in row one in those columns assigned to TYPE 4 messages. The message is of the block type as shown in Table III, which means the message includes the individual subscriber address.

Characters taken sequentially from a message TYPE 0, 1, 2 or 3 are placed in a single column of the frame beginning at row two until row nine. More than one TYPE 0, 1, 2 or 3 messages can appear, but each such message can only occupy a single column. If the message has less than eight characters, then fill characters are placed after the message characters through to row nine. In a preferred version using ASCII code, the fill character is ASCII decimal 21 (NAK) but this may be changed for best spectral response.

Characters taken sequentially from a TYPE 4 message are placed along a row, beginning with row one and continuing through row nine. Accordingly, TYPE 4 messages are inserted from left to right, top to bottom. If the message does not have enough characters to fill the space, then the fill character is employed.

Certain ASCII characters may be reserved for system use and may not be used in the message, its address or for control. These characters are as shown in Table IV:

TABLE IV ______________________________________ ASCII Decimal Character Value Description ______________________________________ NULL 0 DLE 16 DC1 17 Replication DC2 18 Replication DC3 19 Replication DC4 20 Replication NAK 21 Fill Character SYNC 22 Frame Lock CAN 24 Void Frame and Repeat SUB 26 Triple Numeral or Space Compressions DEL 127 ______________________________________

Certain ASCII characters are furthermore reserved for block message use and may not be used in the message or for addressing, as shown in Table V:

TABLE V ______________________________________ ASCII Decimal Character Value ______________________________________ SOH 1 STX 2 ETX 3 EOT 4 ETB 23 ESC 27 ______________________________________

The replication and compression functions specified in Table IV are as follows. If an ASCII character in a message is a number (Decimal 48 through Decimal 57), the system will send the number three times but remove the repetition after transmission, so that at best a two out of three decision may be employed to guard from error.

If the message has several consecutive space characters (ASCII Decimal 32), then a coding scheme is used to reduce the total number of space characters sent. Use is made of the characters:

DC1: Weight 2 Spaces

DC2: Weight 3 Spaces

DC3: Weight 4 Spaces

DC4: Weight 6 Spaces

The above describes the transmission of data in synchronous format, i.e., where data transmission is synchronized with an external clock signal.

A simpler receiver design results if asynchronous, or start-stop transmission is used, however. This, of course, results in a lower number of characters per second throughput. In FIG. 3, the asynchronous multiframe format is shown. Features of the asynchronous system will be described below where they are different than in the above description.

Synchronization in the asynchronous system is achieved by sending one or more SYNC characters at the start of each frame in column 0 followed by the frame number in Column 1. A multiframe may contain 10 frames or rows, as shown.

In a preferred embodiment, a maximum of 24 columns is allowed, although less than 24 columns may be transmitted during low density traffic periods. The form of the data messages in the asynchronous embodiment is whown in Table VI. In Table VI, Cs is a separator character which separates the portions of the address and "Type" is two ASCII characters which indicate if numerals will be repeated three times and if spaces are compressed or transmitted as found in the text, as discussed above.

TABLE VI ______________________________________ SOH Address 1 Cs Address 2 Cs Type STX . . Text . . ETX Error Check ETB EOT ______________________________________

If ASCII code is employed, thirty-three ASCII characters are never used in the message text but are reserved for signalling only. These characters are sometimes referred to as illegal characters. In the described system, the purpose of these characters is described in Table VII.

TABLE VII ______________________________________ ASCII Character (Decimal) Name Purpose ______________________________________ 0 NULL Unused 1 SOH First character in a Block Message 2 STX Text of message follows 3 ETX Text has finished 4 EOT Last character in a Block Message 5 ENQ Unused 6 ACK Unused 7 Bell Unused 8 BS Unused 9 HT Unused 10 LF Unused 11 VT Unused 12 FF Unused 13 CR Unused 14 SO Unused 15 SI Unused 16 DLE Unused 17 DC1 Two spaces, used with text space compression 18 DC2 Three spaces, used with text space compression 19 DC3 Four spaces, used with text space compression 20 DC4 Six spaces, used with text space compression 21 NAK Fill character, used anywhere in Frame for empty 22 SYNC Synchronization, sent in column zero of every Frame 23 ETB Last character of block check code 24 CAN Cancel whatever is in progress 25 EM Clears `triple numeral` or `space compression` 26 SUB Sets up `triple numeral` or ` space compression` 27 ESC Unused 30 RS Unused 31 US Unused 127 DEL Unused ______________________________________

Fifteen ASCII characters are reserved for special signalling purposes. These characters are in addition to the special characters ASCII decimal 0 through 31 and 127, shown in Table VII. The special signalling characters are described in Table VIII.

TABLE VIII ______________________________________ ASCII/ Character (Decimal) Purpose ______________________________________ 33 If found in row zero, Low Speed Block