1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 6 TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU$!4!G0G0#/-5.)#!4)/.G0G0/6%2G0G04(%G0G04%,%0(/.%.%47/2+%.%2!,G0G03425#452%G0G0/ amended at Geneva, 1976 and 1980, and at Melbourne, 1988)The CCITT,I. considering, firstly,the agreement between the Internation
2、al Organization for Standardization (ISO) and the CCITT on the maincharacteristics of a seven-unit alphabet International Alphabet No. 5 (IA5) to be used for data transmission and fortelecommunications requirements that cannot be met by the existing five-unit International Telegraph Alphabet No. 2(I
3、TA2);the interest, both to the users and to the telecommunication services, of an agreement concerning thechronological order of transmission of bits in serial working;declares the viewthat the agreed rank number of the unit in the alphabetical table of combinations should correspond to thechronolog
4、ical order of transmission in serial working on telecommunication circuits;that, when this rank in the combination represents the order of the bit in binary numbering, the bits should betransmitted in serial working with the low order bit first;that the numerical meaning corresponding to each inform
5、ation unit considered in isolation is that of the digit:0 for a unit corresponding to condition A (travail = space), and1 for a unit corresponding to condition Z (repos = mark),in accordance with the definitions of these conditions for a two-condition transmission system;II. considering, moreover,th
6、at it is often desirable, in character oriented data transmission, to add an extra “parity“ unit to allow for thedetection of errors in received signals;the possibility offered by this addition for the detection of faults in data terminal equipment;the need to reserve the possibility of making this
7、addition during the transmission itself, after the seveninformation units proper have been sent;declares the viewthat signals of the International Alphabet No. 5 code for data transmission should, in general, include anadditional “parity“ unit;that the rank of this unit and, hence, the chronological
8、 order of the transmission in serial working should bethe eighth of the combination thus completed;_1)See Recommendation X.4 1 for data transmission over public data networks.2 Fascicle VIII.1 - Rec. V.4III. consideringthat, in start-stop systems working with electromechanical equipment, the margin
9、of such equipment and thereliability of the connection are considerably increased by the use of a stop element corresponding to the duration oftwo unit intervals of the modulation;that for transmissions over telephone circuits via modems installed on the users premises, the latter must beable to use
10、 the connections at the highest possible practical rate in characters per second, and that in such a case asingle-unit stop element leads to a gain of about 10% as regards this practical rate;that, however, it does not appear that the production of electronic devices capable of working at will withs
11、tart-stop signals having a stop element equal to one or two unit intervals should lead to costly complications and thatsuch an arrangement can have the advantage of appreciably limiting the error rate without greatly reducing the practicalefficiency of the connection;declares the viewthat in start-s
12、top systems using combinations of the seven-unit alphabet normally followed by a parity unit, thefirst information unit of the transmitted combination should be preceded by a start element corresponding to conditionA (space);that the duration of this start element should be a one-unit interval for t
13、he modulation rate under consideration,at transmitter output;that the combination of seven information units, normally completed by its parity unit, should be followed bya stop clement corresponding to condition Z (mark);that for start-stop systems using the seven-unit code on switched telephone net
14、works, a two-unit stop elementshould be used with electromechanical data terminal equipments operating at modulation rates up to and including 200bauds. In other cases, the use of a one-unit stop element is preferable. However, this is subject to a mutual agreementbetween Administrations concerned;t
15、hat similar situations when a one-unit stop element can be used may apply to leased circuits;that the start-stop receivers should be capable of correctly receiving start-stop signals comprising a single-unitstop element, whose duration will be reduced by a time interval equal to the deviation corres
16、ponding to the degree ofgross start-stop distortion permitted at receiver input. However, for electromechanical equipment which must use atwo-unit stop element (eleven-unit code signal) with a modulation rate of 200 bauds or less, receivers should becapable of correctly receiving signals with a stop
17、 element reduced to one unit;IV. considering, finally,that the direction of the parity unit can only be that of the even parity on the perforated tapes, particularlyowing to the possibility of deletion (combination 7/15 of the alphabet) which causes a hole to appear in all tracks;that, on the other
18、hand, the odd parity is considered essential in the equipment which depends on transitions inthe signals to maintain synchronism in cases where combination 1/6 (SYNC) of the alphabet does not permit of aneconomical solution;declares the viewthat the parity unit of the signal should correspond to the
19、 even parity in links or connections operated on theprinciple of the start-stop system;that this parity should be odd on links or connections using end-to-end character oriented synchronousoperation;that arrangements should be made when necessary to reverse the direction of the parity unit at the in
20、put andoutput of the synchronous equipment connected either to apparatus working on the start-stop principle or receivingFascicle VIII.1 - Rec. V.4 3characters on perforated tape;that the detection of a character out-of-parity may be represented by:a) reverse question mark graphic character or a rep
21、resentation of the capital letters SB (see ISO 2047)provided that these letters occupy a single character position on the screen or printer, and could havebeen entered by a single key stroke, recognizing it may be difficult to achieve a legible “SB“ characterfrom some matrix printers or displays whe
22、re the characters are printed; orb) a recording of the 1/10 (SUB) character on the tape or other storage medium, where providedand that, where a SUB character occurs in a received transmission, or is presented to a DTE via a storagemedium, e.g. paper tape, then the reaction should be as in a) and b) above.Reference1 CCITT Recommendation General structure of signals of International Alphabet No. 5 code for characteroriented data transmission over public data networks, Vol. VIII, Rec. X.4.
