1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 6 TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU$!4!G0G0#/-5.)#!4)/.G0G0/6%2G0G04(%G0G04%,%0(/.%.%47/2+-/$%-3G0G0 amended at Malaga-Torremolinos, 1984)Systems for parallel data transmission can be used economically when the transmitting sets (outstations)
2、usethe signalling frequencies of push-button telephone sets to transmit data to a central receiving set (instation) via theswitched telephone network.1 ScopeIn many networks, the introduction of keyboard telephone sets allows simple, one-way data transmission atspeeds up to about 10 characters per s
3、econd to be made from a large number of push-button telephone sets serving asoutstations to a common instation, via the general switched telephone network. Transmissions in the instation-to-outstation direction are generally confined to simple acoustic signals and voice replies.The CCITT thereforeun
4、animously recommendsthat the modems to be used for stations operating in the general switched telephone network should meet thespecifications shown below.2 General characteristics2.1 Data channelThe transmission system uses two sets of frequencies in accordance with Recommendation Q.23 1. Eachcharac
5、ter is transmitted in the form of two simultaneously transmitted frequencies. These two frequencies belong totwo separate sub-assemblies. Each of these two assemblies consists of four frequencies “2 (1/4)“ code. This codingcan thus be used to transmit 16 different character combinations and perhaps
6、more (see Note).The actual transmission consists in sending a frequency pair for a time greater than 30 ms, followed by asilence period of not less than 25 ms.Note - In order to stretch the set of characters, several frequency pairs may be transmitted before the silenceperiod. It should be noted tha
7、t in this case character coding and decoding will not be effected by the DCEbut by the DTE.2.2 Backward channelThe following possibilities might be considered:a) a telephone channel not simultaneous with data transmission in the forward direction;b) a backward channel for audible signalling;c) a bac
8、kward channel for electrical signalling.Possibilities b) and c) are provided on a basis of non-simultaneity or, optionally, simultaneity with the datatransmission channels in the forward direction.A loudspeaker will be built into the outstation modem. Optionally, a continuous signalling output may b
9、eprovided. If the national regulations permit, an output for response to the channel may also be provided as an option.2 Fascicle VIII.1 - Rec. V.193 Frequency assignments3.1 Data transmission channelThe 2 groups of 4 frequencies specified in Recommendation Q.23 1 are defined as follows:- low group
10、frequencies: 697, 770, 852, 941 Hz;- high group frequencies: 1209, 1336, 1477, 1633 Hz.The frequency pairs are assigned to the different digits as shown in Table 1/V.19.TABLE 1/V.19B1= 1209 Hz B2= 1336 Hz B3= 1477 Hz B4= 1633 HzA1= 697 HzA2= 770 HzA3= 852 HzA4= 941 Hz147*2580369#ABCD3.2 Backward cha
11、nnelFor audible signals and electrical signalling, the backward channel frequency will be 420 Hz. This frequencymay be amplitude-modulated at a rate of up to 5 bauds.Use may also be made of an FM backward channel similar to that of the Recommendation V.23 type modem,or of the No. 2 transmission chan
12、nel of a Recommendation V.21 type modem (if the frequency 1633 Hz is not used).These two types of backward channel may be used at the same time as the data frequencies in the forward direction; theuse of these backward channels is optional.4 Tolerances4.1 Data frequency tolerancesThe data frequency
13、tolerances are defined in Recommendation Q.23 1; the difference between eachfrequency and its nominal frequency must not exceed 1.8% of the nominal frequency. Apart from this tolerance of 1.8% on transmission, the instation receiver should be able to accept a difference of 6 Hz due to the carrier sy
14、stems.4.2 Frequency tolerance on backward channelThe tolerance of 420 Hz on the backward channel should be 4 Hz; the receiver of the outstation should alsobe able to accept a difference of 6 Hz due to the carrier systems.5 Line power levelsOn the basis of Recommendation V.2, the following maximum po
15、wer levels are recommended for eachfrequency transmitted, measured at the relative zero point:-13 dBm0 for the data transmission channel without the simultaneous backward channel;-16 dBm0 for the data transmission channel with the simultaneous backward channel;-10 dBm0 for the non-simultaneous backw
16、ard channel;-16 dBm0 for the simultaneous backward channel.Fascicle VIII.1 - Rec. V.19 36 Power levels on receptionIn view of the provision of Recommendation V.2 and the statistical values of the maximum transmission lossbetween subscribers, it is recommended that the instation receiver should be ab
17、le to detect frequency pairs received at-45 dBm.Note - Studies should be continued with a view to permitting levels on reception below -45 dBm.7 Character receptionA character will be detected and delivered to the DTE interface if, and only if, the two frequenciescorresponding to the character are d
18、etected and are stable for at least 10 ms.The silent period will be detected and delivered to the DTE interface if no frequency belonging to the codeappears for at least 10 ms.Note - During silent periods, the microphone of the telephone set is connected to the telephone line, so thatinterfering sig
19、nals (ambient noise, speech) may be received. The receiver must be fitted with devices capable ofdistinguishing between these interfering signals and data signals (speech protection). It would be advisable to studyfurther the method of assessing receiver response to the simulation of data signals by
20、 interfering signals. Areproducible test signal should be defined, so that comparable measurements can be made.8 Detection of line signal received on the data channelCircuit 109 must be in the ON position when a character is received, the circuit may be switched from ON toOFF:1) on detection of the
21、silent period;2) after a time-out of 60 10 ms following detection of the silent period.9 Timing for characters receivedBy its very principle, the system is asynchronous; however, it may be useful to provide the DTE, on anoptional basis, with a signal which indicates the sampling times of the data wi
22、res. In this case, it is advisable to usecircuit 131, which will switch from OFF to ON when the character reaches the interface, and then back to OFF after atime T. This time will be chosen in such a way that the data are stable at the DTE interface.The value T = 15 ms may be recommended by way of e
23、xample.This clock may optionally be disabled on reception of a silent period.10 Interface of instation modemThe functional characteristics of the interchange circuits concerned are as defined in Recommendation V.24(see Note 1).10.1 List of interchange circuits concerned102 Signal ground or common re
24、turn104 Received data 8 circuits. These circuits are designated A1, A2. . . B4according to theircorrespondence with the relevant frequency in Table 1/V.19 (see Note 2 below)105 Request to send (see Note 3 below)107 Data set ready108/1 Connect data set to line (see Note 4 below)108/2 Data terminal re
25、ady (see Note 4 below)109 Data channel received line signal detector4 Fascicle VIII.1 - Rec. V.19125 Calling indicator130 Transmit backward tone191 Transmitted voice answer (see Note 3 below)The following interchange circuits are optional:110 Data signal quality detector131 Received character timing
26、Note 1 - Manufacturers who marketed a modem of this type prior to the publication of this Recommendationmay regard the interface defined in this paragraph as optional.Note 2 - To make the interface compatible with the relevant specifications of Recommendation V.20, thecombination A4, B4may be transm
27、itted on circuit 104 instead of a pause (“1“ on all circuits), provided circuit 107 is inthe ON position and circuit 105 is in the OFF position. This simulated idle combination is optional.Note 3 - These circuits are required if the “telephone channel“ facility is provided in the modem. The electric
28、alcharacteristics of interchange circuit 191 are still under study.Note 4 - Circuit 108 must be available either as circuit 108/1 - Connect data set to line, or as circuit 108/2 -Data terminal ready.10.2 Electrical characteristics of interchange circuitsUse of electrical characteristics conforming t
29、o Recommendation V.28 is recommended together with theconnector and pin assignment plan specified by ISO 2110.Note - Manufacturers may wish to note that the long-term objective is to replace electrical characteristicsspecified in Recommendation V.28, and that Study Group XVII has agreed that the wor
30、k shall proceed to develop amore efficient, all balanced, interface for the V-Series application which minimizes the number of interchange circuits.11 Interface of outstation modemsIn view of the purpose of these modems, which are or will be more or less integrated in economic terminals,the specification of the interface is liable to result in a much higher equipment cost. Hence no interface isrecommended.Reference1 CCITT Recommendation Technical features of push-button telephone sets, Rec. Q.23.
