ITU-T R 20-1988 TELEGRAPH MODEM FOR SUBSCRIBER LINES《用户线的电报调制调器》.pdf

1、)45G134 2 TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU4%,%2!0(94%,%2!0(G0G042!.3-)33)/.4%,%2!0(G0G0-/$%-amended at Malaga-Torremolinos, 1984 and at Melbourne, 1988)The CCITT,considering(a) that the use of high-level telegraph transmission with single or double current may cause disturbingimpulse no

2、ise in adjacent cable pairs that may be eliminated by applying low-level transmission with telegraphmodems;(b) that telegraph modems would substantially reduce the power consumption in the central office;(c) that where connection to a subscriber has to be achieved over a non-metallic pair (e.g., a v

3、oice-channelfrequency-division multiplex or pulse code modulation system) a telegraph modem has to be utilized;(d) that the frequencies given below are already standardized in Recommendation V.21 1;(e) that suitable inexpensive telegraph modems can be used for full duplex transmission on 2-wire circ

4、uits atmodulation rates up to 300 bauds;unanimously declares the viewthat where low-level telegraph transmission is used, the following method of transmission should berecommended for all modulation rates up to 300 bauds.1 Channel allocationThe method of transmission is based on Recommendation V.21

5、1 with the following frequency designations:Central office to subscriber (channel 1) FA= 1180 Hz,FZ= 980 Hz;Subscriber to central office (channel 2) FA= 1850 Hz,FZ= 1650 Hz.The characteristic frequencies as measured at the telegraph modem line output should not differ by more than 3 Hz from the nomi

6、nal figures.The modem shall continue to operate with a 6 Hz change in receive frequency.It should be noted that there is equipment in use that applies alternative frequencies to those shown in thisRecommendation.2 InterfaceWhere the modem is a separate, self-contained unit, the following interchange

7、 circuits shall be used:Common return (e.g. circuit 102 in Recommendation V.24 2)Transmitted data (e.g. circuit 103 in Recommendation V.24 2)Received data (e.g. circuit 104 in Recommendation V.24 2)Carrier detect (e.g. circuit 109 in Recommendation V.24 2)1)._1)This is considered optional, particula

8、rly at the subscribers location.2 Fascicle VII.1 - Rec. R.203 Electrical characteristicsThe electrical characteristics (for stand alone telegraph modems) of the interchange circuits, should be inaccordance with CCITT Recommendation V.28 3.4 Performance4.1 The modem under test shall be connected to a

9、nother modem (according to this Recommendation or toRecommendation V.21) via an attenuator having a return loss of 4 dB and an insertion loss of 30 dB. The relativegroup-delay characteristics of the transmit filters are subject to further study.4.2 Uniform spectrum Gaussian noise (band limited to 10

10、 kHz) shall be added to give a normalized signal-to-noiseratio of 32 dB. This is defined as:signal energy per bitnoise power per hertssignal powernoise powernoise bandwidthsignal bit rate= 32 dB=4.3 Test signals to Recommendation R.51 bis (QKS) shall be sent at a 13 dBm transmit level in both direct

11、ionssimultaneously. (To ensure incoherence, the rate of the test signals for the direction not under test shall be slightlylower.) The test period shall be 15 seconds.The performance shall be in accordance with Table 1/R.20.TABLE 1/R.204.4 In certain configurations, it is not possible to check the m

12、odem distortion, e.g. telegraph terminals, multiplexand switching equipment with integral telegraph modems on which the d.c. signal output of the telegraph modem isinaccessible for any reason. The performance will normally be in the form of a test for distortion margin before errorsare output from t

13、he equipment.Note The modem needs only be tested at the maximum modulation rate of the equipment into which it is to beincorporated.4.5 The design of the telegraph modem shall be such that the transmission performance is guaranteed withoutadjustment on installation or subsequently.5 Line signal leve

14、ls and terminating impedanceThe line interface of the modem shall be balanced and shall present an impedance of 600 ohms with a returnloss (reference 600 ohms resistive) of not less than 14 dB (reflection coefficient no greater than 20%) over the range300-3400 Hz.Maximum isochronous distortion (%)Tr

15、ansit rate(bit/s)140 or 2600 ohm line 600 ohm lineNo frequency error 6 Hz frequency error No frequency error5075100110134.5150200300567789111578101012131622345566811Fascicle VII.1 - Rec. R.20 35.1 Output signal level5.1.1 When terminated in 600 ohms the output level should be set to 13 dBm.Note In c

16、ertain applications, in particular if carrier circuits for voice channel frequency division multiplex orpulse code modulation systems are used (where the input level should be limited to 13 dBm0) it may be necessary tohave a setting range of output level which could give up to 0 dBm.5.1.2 The differ

17、ence in output levels between the Binary 1 (condition Z) and Binary 0 (condition A) signals shall notbe greater than 1 dB for either of the channels.5.2 Receive signal level5.2.1 When the received signal level is 43 dBm or greater, the equipment should correctly interpret theline condition as either

18、 FAor Fz.5.2.2 When the received signal level remains below a threshold between 45 dBm and 48 dBm, the equipment shallignore incoming information within 300 ms of the signal level dropping below the threshold. During this delay thereceiving equipment may interpret (correctly or incorrectly) characte

19、rs received after the drop in received signal levelbelow the threshold. Thereafter, the equipment should not interpret characters received until the received signal levelhas restored to at least 2 dB above the threshold. If the signal level drops below the threshold for less than 10 ms, theequipment

20、 should take no action.5.2.3 Once the received signal level has fallen below the threshold value, the equipment shall ignore any subsequentincreases in the signal level which are either less than 2 dB above the threshold or of more than 2 dB above thethreshold but less than 10 ms in duration.Note 1

21、The requirements of 5.2.3 need not apply during the first 20 ms from the fall in received signal levelbelow the threshold.Note 2 The signal level detector shall respond to the total power contained within the nominal spectrum of thereceived line signal.6 Maintenance facilitiesMaintenance facilities,

22、 e.g. test loops, are a national matter.7 Protection against high voltagesThe equipment shall withstand residual lightning surges, high voltage line faults, and high leveltelegraphkeying. The protection required is considered a national matter, though in some cases Recommendation K.21could be applie

23、d 4.4 Fascicle VII.1 - Rec. R.208 Line wetting8.1 Line wetting, where required by the type of line plant used, will normally be sourced by the in-stationequipment and looped by the subscribers equipment.8.2 The current has a maximum value of 15 mA with the line short-circuited. The wetting current s

24、hall be aminimum 5 mA on 4000-ohm lines. The open circuit voltage shall be under 80 V.Note In some countries, other values may apply.Where a modem may be required to work in the presence of line wetting, then the modem performance (see 4) and the modem line impedance (see 5) requirements must apply

25、with line wetting current flowing.The noise applied to the line from the wetting power supply shall be under 80 dBm (600 ohms) over a range300 to 3400 Hz (flat).9 Permitted out-of-band energy9.1 The out-of-band energy is a national matter; however the following information is provided to assist equi

26、pmentmanufacturers:9.2 The transmitted signal level (with an output level at 13 dBm) at any one frequency shall not exceed thefollowing limits (with the forward and reverse channel keyed at 300 bit/s with a QKS code):200 Hz to 3200 Hz 13dBm100 Hz to 200 Hz and 3200 Hz to 3400 Hz 23dBmBelow 100 Hz 33

27、dBmAbove 3400 Hz 33dBm and decreasing by 12 dB/octave to 67 dBmFascicle VII.1 - Rec. R.20 5References1 CCITT Recommendation 300 bits per second duplex modem standardized for use in the general switchedtelephone network, Rec. V.21.2 CCITT Recommendation List of definitions for interchange circuits between data terminal equipment and datacircuit terminating equipment, Rec. V.24.3 CCITT Recommendation Electrical characteristics for unbalances double-current interchange circuits,Rec. V.28.4 CCITT Recommendation Resistibility of subscribers terminals to over-voltages and over-currents, Rec. K.21.