1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 6 TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU$!4!G0G0#/-5.)#!4)/./6%2G0G04(%G0G04%,%0(/.%G0G0.%47/2+%15)6!,%.#%G0G0“%47%.G0G0“).!29G0G0./4!4)/.39-“/,3G0G0!.$G0G04(%G0G03).) amended at Geneva, 1964 and I972)Binary numbering expresses numbers by means of
2、two digits normally represented by the symbols 0 and 1.Transmission channels are especially well suited to the transmission of signals by a modulation having two significantconditions (two-condition modulation). These two significant conditions are sometimes called “space“ and “mark“ or“start“ and “
3、stop“, or they may be called condition A or condition Z 1.It is very useful to make the two conditions of a two-condition modulation correspond to the binary digits 0and 1. Such equivalence will facilitate the transmission of numbers resulting from binary calculation, the conversion ofcodes for bina
4、ry numbers and of codes for decimal numbers, maintenance operations and relations betweentransmission personnel and the personnel in charge of data-processing machines.At first sight, it does not seem to matter whether the symbol 0 corresponds in transmission to condition A orcondition Z, the symbol
5、 1 then corresponding to condition Z or condition A or vice versa.In telegraphy, however, when a telegraphic communication is set up and the sending of signals is stopped(called the idle condition of the line), the signal sent over the line consists of condition Z throughout the suspension oftransmi
6、ssion.It is logical (and for certain VF telegraph systems also essential) to use the same rule in data transmission.During the “idle periods“ of transmission, condition Z should be applied to the circuit input.Data transmission on a circuit is often controlled by perforated tape. On perforated tapes
7、 used for telegraphy,condition Z is represented by perforation. When binary numbers are represented by means of perforations, it iscustomary to represent the symbol 1 by a perforation. It is therefore logical to make this symbol 1 correspond tocondition Z.For these reasons, the CCITTunanimously decl
8、ares the following view:1 In transmitting data by two-condition code, in which the digits are formed using binary notation, the symbol1 of the binary notation will be equivalent to condition Z of the modulation, and the symbol 0 of the binary notationwill be equivalent to condition A of the modulati
9、on.2 During periods when there is no signal sent to the input of the circuit, the circuit input condition is conditionZ.3 If perforation is used, one perforation corresponds to one unit interval under condition Z.4 In accordance with Recommendation R.31, the sending of symbol 1 (condition Z) corresp
10、onds to the tonebeing sent on a channel using amplitude modulation.5 In accordance with Recommendation R.35, when frequency modulation is used, the sending of symbol 0corresponds to the higher frequency, while the sending of symbol 1 corresponds to the lower frequency.6 a) For phase modulation with
11、reference phase:the symbol 1 corresponds to a phase equal to the reference phase;the symbol 0 corresponds to a phase opposed to the reference phase.2 Fascicle VIII.1 - Rec. V.1b) For differential two-phase modulation where the alternative phase changes are 0 degree or 180 degrees:the symbol 1 corres
12、ponds to a phase inversion from the previous element;the symbol 0 corresponds to a no-phase inversion from the previous element.7 A summary of equivalence is shown in Table 1/V.1.TABLE 1/V.1Summary of equivalence (see Note 1)Digit 0“Start“ signal in start-stop codeLine available conditionin telex sw
13、itching“Space“ element of start-stop codeCondition ADigit 1“Stop“ signal in start-stop codeLine idle conditionin telex switching (Note 2)“Mark“ element of start-stop codeCondition ZAmplitude modulation Tone-off Tone-onFrequency modulation High frequency Low frequencyPhase modulation withreference ph
14、aseOpposite phase to the reference phase Reference phaseDifferential two-phasemodulation where thealternative phase changesare 0 degree or 180 degreesNo phase inversion Inversion of the phasePerforations No perforation PerforationNote 1 - The standardization described in this Recommendation is general, whether over telegraph-type circuits or overcircuits of the telephone type, making use of electromechanical or electronic devices.Note 2 - It primarily applies to anisochronous use.Reference1 CCITT Definition: Position A; position Z, Vol. X, (Terms and Definitions).
