1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 3 TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU4%,%2!0(9!,0(!“%4)#!,G0G04%,%2!0(G0G04%2-).!,%15)0-%.453%G0G0/ amended at Mar del Plata, 1968)The CCITT,considering(a) that the telex network is well adapted for the economical transmission of data at fairly
2、slow speeds, forthe equipment required for binary transmission of data by telex stations, over and above the normal equipment, isrelatively simple;(b) but that some limits have to be imposed on data transmission codes used in the telex network becauseof:- the need to make sure that telex calls will
3、not be abruptly released;- exaggerated distortion that may be introduced by amplitude-modulation voice-frequency telegraphsystems when an excessively long-duration start (condition A) modulation element appears in a signal;- the fact that in some networks there is regenerative repetition of start-st
4、op signals, which can be handledonly as if they were constructed like five-unit start-stop information signals;- the possibility that certain long-distance calls may be established over synchronous systems that canhandle only five-unit start-stop signals;(c) that the limitation due to regenerative r
5、epeaters and synchronous systems imposes the use of a five-unitstart-stop code for information, hence 1 of the Recommendation (the more general procedure) deals with datatransmission with a five-unit code on start-stop systems. But in certain circumstances alphabets with more than fiveunits can be u
6、sed for data transmission; hence 2 of the Recommendation,unanimously declares the following view:1 Data transmission with a five-unit code on start-stop systems1.1 Telex calls for data transmission may be set up in the international telex network, subject to the followingprovisions:1.2 The call shal
7、l be set up between the caller and the called subscriber in accordance with the procedurerecommended for the setting-up of a telex call and its supervision by exchange of answer-back codes(Recommendations F.60 1 and U.1 2).1.3 When one of the subscribers concerned wishes to introduce data transmissi
8、on equipment into the connection,he shall transmit the sequence SSSS (or ) of combination No. 19 from International Telegraph Alphabet No. 2(signal for transfer to data). Upon reception of this sequence of combinations, the data transmission or receptionequipment, as the case may be, shall be connec
9、ted to the line. This changeover to the data position may be effected:a) manually at both terminals;b) automatically at both terminals;c) manually at one terminal and automatically at the other.In order to avoid any misunderstanding between the stations concerned, the calling operator should first c
10、heck theequipment of the distant station (whether manual changeover or automatic changeover).2 Fascicle VII.1 - Rec. S.151.3.1 Manual changeover at both terminals1.3.1.1 Once the connection has been set up, the following procedure should be followed.1.3.1.2 The operator of the calling station sends
11、the sequence of four combinations No. 19. This sequence should notconnect the data equipment locally.1.3.1.3 Upon reception of the SSSS (or ) sequence, the operator of the called station likewise sends the sequenceof four combinations No. 19, and then connects his data equipment to the line.1.3.1.4
12、Upon reception of this answer sequence, the calling operator connects his data equipment to the line.1.3.2 Automatic changeover at both terminals1.3.2.1 Once the connection has been set up, the following procedure should be followed:1.3.2.2 The calling station sends the sequence of four combinations
13、 No. 19 and must connect its data equipment tothe line automatically within less than 500 milliseconds, starting from the end of transmission of the last signal of thissequence.1.3.2.3 Reception of the sequence at the other terminal of the connection connects the called station to the dataequipment
14、line automatically within less than 500 milliseconds, starting from the end of reception of the last signal ofthis sequence.1.3.2.4 The data transmission should not commence before the end of the 500-millisecond delay.1.3.3 Calling station with manual changeover and called station with automatic cha
15、ngeover1.3.3.1 Once the connection has been set up, the following procedure is followed:1.3.3.2 The operator of the calling station sends the sequence of four combinations No. 19, and then immediatelyconnects his data equipment to the line.1.3.3.3 Upon reception of the sequence of four combinations
16、No. 19 at the called station, the data equipment must beconnected to the line within less than 500 milliseconds, starting from the end of reception of this sequence.1.3.3.4 The data signals should not be transmitted before the end of the 500-millisecond delay.1.3.4 Calling station with automatic cha
17、ngeover and called station with manual changeover1.3.4.1 Once the connection has been set up, the following procedure should be followed:1.3.4.2 The calling station invites its called correspondent, by a brief preliminary message, to send the sequence offour combinations No. 19. This message must no
18、t include within itself the sequence of four combinations No. 19. If thecalling station is not equipped with a teleprinter attended by an operator, this preliminary message must be sentautomatically.1.3.4.3 The operator of the called station then sends the sequence of four combinations No. 19 and im
19、mediatelyconnects his data equipment to the line.1.3.4.4 Upon reception of this sequence at the calling station, connection of the data equipment to the line must beeffected within less than 500 milliseconds, starting from the end of reception of the last combination No. 19 of thesequence.1.3.4.5 Tr
20、ansmission of the data signals should not begin before the end of the 500-millisecond delay.Note - The arrangements envisaged throughout 1.3 above run counter to the inclusion of the sequence offour combinations No. 19 in the answer-back code of telex lines equipped with a simulator and at the same
21、time in theanswer-back of teleprinters equipped with an automatic device for changeover to data transmission. (This fact shouldbe borne in mind in the further study of this Recommendation.)Fascicle VII.1 - Rec. S.15 31.4 The sequence of four combinations No. 19 will make ineffective, where necessary
22、:- devices that might conceivably emit signals disturbing to data transmissions, in particular the answer-back or, possibly, the delay signal used in connection with error-correcting synchronous radio systems(Recommendation U.22 3);- devices that might be falsely operated by data signals, such as de
23、vices for operator-recall(Recommendation U.21 4).1.5 Data transmission should be made by means of start-stop formed according to the structure of InternationalTelegraph Alphabet No. 2 (ITA2). Users should be left free to decide how combinations should be allocated to thevarious components of the alp
24、habet (of course ITA2 itself may be used).1.6 When error control is necessary, one of the following methods of error control may be used:- return of information to the transmitting station (information feedback system);- block transmission with check characters at the end of the block;- character-by
25、-character transmission with check bits (in the case of five-unit signals with redundancy).1.7 Unless the exception stated in 1.8 below is employed at the end of the data transmission, the telex clearingsignal described in Recommendation U.1 2 shall be emitted. This will cause the call to be cleared
26、 down and theterminal equipment to return to the telex position, and will cause the devices that might have been rendered inoperativeon certain special circuits (see 1.4 above) to go back to normal. This clearing signal must set off the clear-confirmation (see Recommendation U.1 2).Note - Users may
27、expect that some combinations No. 32, possibly followed by other combinations, may bereceived before the connection is cleared.1.8 As soon as the telex connection has been transferred to the data transmission equipment, the transmissionmust be controlled by the data equipment at each terminal. If it
28、 is useful, for some reason, to return to telex operation,the data terminal equipment must control the transfer back to telex. This possibility of returning to the telex conditionis used by a subscriber who considers it useful, after a data transmission, to return to teleprinter operation for a tele
29、xconnection, instead of sending the clearing signal as mentioned in 1.7 above. This return should be accompanied bythe re-entry into service of the answer-back device. This control may be caused:a) by the transmission of a special data signal over the line, causing the receiving installation to retu
30、rn tothe telex position. The received data terminal equipment must send the same signal in the reversedirection to the opposite terminal before it causes transfer to the telex condition. This mutual signallingidentifies the situations at the two terminals;b) by a local control causing return to the
31、telex situation, set off if no data or supervisory signal istransmitted or received during a given time interval agreed upon by the users.Note - Telex connections that include error-correcting synchronous radio systems often insert long pausesinto the message and due attention should be paid to this
32、 in selecting the agreed interval.For these control operations, a special circuit should be set aside in the interface connecting the data terminalequipment to the transfer device.Note - The provisions of 1.8 above could be applied with advantage to the case of telex lines not equippedwith teleprint
33、er equipment but simply with answer-back unit simulators.1.9 The signals transmitted by the data transmission devices must meet the requirements of 1.1, 1.2, 1.3 and2.1 in Recommendation S.3. The receiving equipment of the data reception devices must meet the requirements of 1.1, 1.2, 1.6 and 3.1 in
34、 Recommendation S.3.4 Fascicle VII.1 - Rec. S.152 Data transmission with codes different from the start-stop code of ITA22.1 The attention of Administrations is drawn to the fact that it is impossible to send signals other than those of afive-unit start-stop code over international connections via t
35、ime-division multiplex sections specially designed for afive-unit code. However, telex connections for data transmission may be set up over such relations in the conditions setout in 1 of this Recommendation for the transmission of messages composed of signals different from those of thefive-unit st
36、art-stop code. A service of this nature may be obtained by regrouping the units of these signals in the formof five-unit signals. Such regrouping calls for the use of additional code converters at the sending and receivingterminals.2.2 Between telex networks that can take signals different from thos
37、e of the five-unit start-stop code (that is tosay, when telex calls between such networks do not call for regenerative repeaters, or for certain synchronous systemsthat would clash with them), by agreement between the Administrations concerned, data transmission with datatransmission alphabets using
38、 these signals may be made, subject to the following:a) Application of the procedure described under 1.2;b) Application of the procedure described under 1.3;c) Application of the procedure described under 1.4;d) Use of a code with a modulation rate of 50 bauds should avoid composition of signals hav
39、ing more thanseven consecutive elements of start polarity. (This limit is imposed to avoid clearing the connectionunexpectedly in the exchanges as well as not to introduce excessive distortion on AMVFT channels.)Data may be transmitted by start-stop, or isochronously;e) When error control is necessa
40、ry, one of the following methods of error control may be used:- return of information to the transmission station (information feedback system);- block transmission with check characters at the end of the block;- character-by-character protection by means of a parity check or a constant ratio code,
41、for examplethe seven-unit code standardized in Recommendation S.13 (International Telegraph AlphabetNo. 3).In all cases item d) above should be taken into consideration;f) Application of the procedure described under 1.7;g) Application of the procedure described under 1.8.References1 CCITT Recommend
42、ation Operational provisions for the international telex service, Rec. F.60.2 CCITT Recommendation Signalling conditions to be applied in the international telex service, Rec. U.1.3 CCITT Recommendation Signals indicating delay in transmission on calls set up by means of synchronoussystems with automatic error correction by repetition, Rec. U.22.4 CCITT Recommendation Operator recall on a telex call set up on a radiotelegraph circuit, Rec. U.21.
