WPCI 2%.B pz W"S^11>bbu"::Dg1:11bbbbbbbbbb11gggbuuuk1Xubuukuuuk111Rb:bbXbb1bb''X'bbbb:X1bXXXX;.;g:=::m:::mmmmm::::::mm:k1mubububububXubububub11111111bbbbbbbbbuXubbkbuXmmmmumububXXXXbububububbmbbbbbb:k:k::=kmmX:uXb'b:b:b:b'bmbbbb:::uXuXuXuXk:k:k:mbbbmbuXkXkXKQmmmm^b:kbbbbmbA@mmbmmbmmmmmmm:b:mmmbbmmmmmmmmmmmmXXmmmmmmmmmmmmmmmmmmcm`m`mm`m:mmmmmm}}}mjjmmmmmmmmmmmmmmm0mm}mmmmmmmmmmmmmmmmmmmmmmm}Mmmmmmmmmmmmmjmmmtmmmmmmmmm`'mmm`mmjmlWmmmmmmmmmmmmmmmmmmmW`mmmmjmM-lvetica#|x`H1`D4PkCQMS PS Jet Plus /800 II QPJPII.PRSPl`D4PkCg2W_qr|xHelveticaCourier@ ,`H1`D4PkCmQrrr r  @C2 KLG@ ,`H1`D4PkCmQrrr r  @C;,>>> >  @C ` X` hp x (#%'H   x|@  3'3'Standard@8'@8'StandardC6QMS $=R- Recommendation R.20 HReplace by the following: <TELEGRAPH MODEM FOR SUBSCRIBER LINES <(Geneva, 1980; amended at MalagaTorremolinos, 1984 <and at Melbourne, 1988) HThe CCITT, considering  ` H H(a)  that the use of highlevel telegraph transmission with single or double current may cause disturbing impulse noise in adjacent cable pairs that may be eliminated by applying lowlevel transmission with telegraph modems; H(b)  that telegraph modems would substantially reduce the power consumption in the central office; H(c)  that where connection to a subscriber has to be achieved over a nonmetallic pair (e.g., a voicechannel frequencydivision multiplex or pulse code modulation system) a telegraph modem has to be utilized; H(d)  that the frequencies given below are already standardized in Recommendation V.21 [1]; H(e)  that suitable inexpensive telegraph modems can be used for full duplex transmission on 2wire circuits at modulation rates up to 300 bauds; unanimously declares the view Hthat where lowlevel telegraph transmission is used, the following method of transmission should be recommended for all modulation rates up to 300bauds. 1.HChannel allocation HThe method of transmission is based on Recommendation V.21 [1] with the following frequency designations: HCentral office to subscriber (channel 1) FA = 1180 Hz, FZ = 980 Hz; HSubscriber to central office (channel 2) FA = 1850 Hz, FZ = 1650 Hz. HThe characteristic frequencies as measured at the telegraph modem line output should not differ by more than + 3 Hz from the nominal figures. HThe modem shall continue to operate with a + 6 Hz change in receive frequency. HIt should be noted that there is equipment in use that applies alternative frequencies to those shown in this Recommendation. 2.HInterface HWhere the modem is a separate, selfcontained unit, the following interchange circuits shall be used: HCommon return (e.g. circuit 102 in Recommendation V.24 [2]) HTransmitted data (e.g. circuit 103 in Recommendation V.24 [2]) .Ԍ HReceived data (e.g. circuit 104 in Recommendation V.24 [2]) HCarrier detect (e.g. circuit 109 in Recommendation V.24 [2]).1 3.HElectrical characteristics HThe electrical characteristics (for stand alone telegraph modems) of the interchange circuits, should be in accordance with CCITT RecommendationV.28 [3]. 4.HPerformance 4.1HThe modem under test shall be connected to another modem (according to this Recommendation or to Recommendation V.21) via an attenuator having a return loss of 4 dB and an insertion loss of 30 dB. The relative groupdelay characteristics of the transmit filters are subject to further study. 4.2HUniform spectrum Gaussian noise (band limited to 10 kHz) shall be added to give a normalized signaltonoise ratio of 32 dB. HThis is defined as: SIGNAL ENERGY PER BITĠ=ĠSIGNAL POWERĠxĠNOISE BANDWIDTHĠ= 32 dB NOISE POWER PER HERTZNOISE POWERSIGNAL BIT RATE à 1 This is considered optional, particularly at the subscriber's location. 4.3HTest signals to Recommendation R.51bis (QKS) shall be sent at a13dBm transmit level in both directions simultaneously. (To ensure incoherence, the rate of the test signals for the direction not under test shall be slightly lower.) The test period shall be 15 seconds. HThe performance shall be in accordance with Table 1/R.20. H!TABLE 1/R.20 4.4HIn certain configurations, it is not possible to check the modem distortion, e.g. telegraph terminals, multiplex and switching equipment with integral telegraph modems on which the d.c. signal output of the telegraph modem is inaccessible for any reason. The performance will normally be in the form of a test for distortion margin before errors are output from the equipment. Note The modem need only be tested at the maximum modulation rate of the equipment into which it is to be incorporated. 4.5HThe design of the telegraph modem shall be such that the transmission . performance is guaranteed without adjustment on installation or subsequently. 5.HLine signal levels and terminating impedance HThe line interface of the modem shall be balanced and shall present an impedance of 600 ohms with a return loss (reference 600 ohms resistive) of not less than 14 dB (reflection coefficient no greater than 20%) over the range 3003 400 Hz. 5.1HOutput signal level 5.1.1HWhen terminated in 600 ohms the output level should be set to 13dBm Note In certain applications, in particular if carrier circuits for voice channel frequency division multiplex or pulse code modulation systems are used (where the input level should be limited to 13 dBmO) it may be necessary to have a setting range of output level which could give up to 0 dBm. 5.1.2HThe difference in output levels between the Binary 1 (condition Z) and Binary 0 (condition A) signals shall not be greater than 1 dB for either of the channels. 5.2HReceive signal level 5.2.1HWhen the received signal level is 43 dBm or greater, the equipment should correctly interpret the line condition as either Fa or Fz. 5.2.2HWhen the received signal level remains below a threshold between 45dBm and 48dBm, the equipment shall ignore incoming information within 300ms of the signal level dropping below the threshold. During this delay the receiving equipment may interpret (correctly or incorrectly) characters received after the drop in received signal level below the threshold. Thereafter, the equipment should not interpret characters received until the received signal level has restored to at least 2 dB above the threshold. If the signal level drops below the threshold for less than 10 ms, the equipment should take no action. 5.2.3HOnce the received signal level has fallen below the threshold value, the equipment shall ignore any subsequent increases in the signal level which are either less than 2 dB above the threshold or of more than 2dB above the threshold but less than 10 ms in duration.  Hh Note 1ĩ The requirements of 5.2.3 need not apply during the first 20ms from the fall in received signal level below the threshold. Note 2 The signal level detector shall respond to the total power contained within the nominal spectrum of the received line signal. 6.HMaintenance facilities HMaintenance facilities, e.g. test loops, are a national matter. 7.HProtection against high voltages HThe equipment shall withstand residual lightning surges, high voltage line faults, and high level telegraph keying. The protection required is considered a national matter, though in some cases Recommendation K.y could be applied [3]. 8.HLine wetting 8.1HLine wetting, where required by the type of line plant used, will normally be sourced by the instation equipment and looped by the subscriber's equipment. .Ԍ 8.2HThe current has a maximum value of 15 mA with the line shortcircuited. The wetting current shall be a minimum of 5 mA on 4000 ohms lines. The open circuit voltage shall be under 80 V. Note In some countries, other values may apply. HWhere a modem may be required to work in the presence of line wetting, then the modem performance ( 4) and the modem line impedance ( 5) requirements must apply with line wetting current flowing. HThe noise applied to the line from the wetting power supply shall be under 80 dBm (600 ohms) over a range 300 to 3400 Hz (flat). 9.HPermitted outof band energy 9.1HThe outofband energy is a national matter; however, the following information is provided to assist equipment manufacturers. 9.2HThe transmitted signal level (with an output level at 13dBm) at any one frequency shall not exceed the following limits (with the forward and reverse channel keyed at 300 bit/s with a QKS code): HH200 Hz to 3 200 Hz13 dBmp&H HH100 Hz to 200 Hz and 3 200 Hz to 3 400 Hz23 dBmp&H HHBelow 100 Hz33 dBmp&H  h HAbove 3 400 Hz33 dBm and decreasing by12dB/octave to 67dBm.  REFERENCESă  X [1]CCITT Recommendation 300 bit/s per second duplex modem standardized for use in the general switched telephone network, Recommendation V.21. [2]CCITT Recommendation List of definitions for interchange circuits between data terminal equipment adn data circuit terminating equipment, Recommendation V.24. [3] CCITT Recommendation Electrical characteristics for unbalances double current interchange circuits, Recommendation V.28, [4]CCITT Recommendation Resistibility of subscribers' terminals to over voltages and overcurrents, Recommendation K.y.