1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 5 TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU4%,%2!0(G0G037)4#().3).!,).G0G0/6%2G0G02!$)/G0G0!.$G0G0-5,4)0,%8%$#(!%,34%,%8G0G0!.$G0G0%.4%8G0G03).!,).G0G0/.2!$)/G0G0#(!%,3G0G0 39.#(2/./53G0G0 G135.)43934%-3G0G0! amended at New Delhi, 1960, Geneva, 1964,
2、Mar del Plata, 1968, and Geneva, 1972)The CCITT,considering(a) that numerous radiotelegraph circuits working in association with 5-unit start-stop apparatus make use oferror-correcting synchronous systems having a special error-detecting 7-unit code enabling errors to be corrected by arequest for a
3、repetition (ARQ system);(b) that when they are usable for switched communications, on the radio section these synchronous systemsuse two combinations and , which characterize the permanent conditions of start polarity and stop polarityrespectively in the start-stop part of the connection (see Recomm
4、endation S.13 1);(c) that the special make-up of these systems is such that a change in the significant condition at the input tothe system is not reproduced at the output with a constant delay;(d) that the experience acquired with telex and gentex switching through these radiotelegraph systems seem
5、ssufficient to justify the laying down of general rules specifying signalling arrangements for manual, semi-automaticand automatic working in such international radio channels,unanimously declares the viewthat the signals, enumerated in Recommendation U.1, to be used in setting up international tele
6、x and gentexcalls over radio channels comprising synchronous systems with error correction by automatic repetition should becharacterized as follows:1 Free line condition1.1 Successive combinations on the forward and backward paths.2 Call2.1 Transition from combination to combination on the forward
7、signalling path. Reception of two consecutive signals over the forward signalling path shall be interpreted as a calling signal.2.2 On circuits automatically operated in both directions, reception of a single signal at the end of the circuitremote from the calling subscriber must cause the outgoing
8、equipment on this circuit at that end to be marked busyimmediately. This busy condition must be applied until two signals are received.2.3 If the motor of the FRXD (fully automatic reperforator transmitter distributor) or equivalent motor-drivenstorage device is not already running, it must be start
9、ed without delay, in order to accept the subsequent selectionsignals. Furthermore, if the motor of the storage device at the called end is not already working, it must be started.2.4 It is desirable that, in the busy hour at least, the starting of the motor of the storage device should not bedepende
10、nt on the calling signal for each call. One simple method of meeting this requirement is to provide a device thatdelays the switching off of the motor until about 5 minutes after the call has been cleared.3 Call-confirmation signal3.1 Transition from combination to combination on the backward signal
11、ling path. The reception of twoconsecutive signals over the backward signalling path shall be interpreted as a call-confirmation signal.2 Fascicle VII.2 - Rec. U.203.2 The return of this signal can be initiated either by the switching equipment or by the radio equipment. Not morethan one second shal
12、l elapse at the incoming end between the moment when two signals have been received and thereturn of the first signal of the call-confirmation signal.3.3 With manual switching the call-confirmation signal shall be returned independently of the operators answer.3.4 For retest purposes radio circuits
13、may be considered faulty when the call-confirmation signal is not receivedwithin three seconds.4 Signals preceding selection4.1 Proceed-to-select signal4.1.1 Semi-automatic working4.1.1.1 If the automatic switching equipment at the receiving end can receive the selection information immediatelyafter
14、 the sending of the call-confirmation signal, the call-confirmation signal shall constitute the proceed-to-selectsignal.4.1.1.2 If the automatic switching equipment at the receiving end cannot receive the selection information immediatelyafter the sending of the call-confirmation signal, a distinct
15、proceed-to-select signal, combination No. 22, shall bereturned over the backward signalling path after the call-confirmation signal. For 99% of calls in the busy hour, thissignal must be returned not more than 3 seconds after the transmission of the call-confirmation signal begins. (Forsome existing
16、 systems this delay will be 4 seconds.)4.1.2 Fully-automatic working4.1.2.1 The proceed-to-select signal, combination No. 22, returned over the backward signalling path shall always bedistinct from the call-confirmation signal and should be returned within the limits specified under semi-automaticwo
17、rking.4.2 Proceed-to-transmit signal4.2.1 On the backward signalling path teleprinter signals indicating the called operators position.4.2.2 The sending of the proceed-to-select or the proceed-to-transmit signal should be delayed until two consecutive signals have been correctly received over the ba
18、ckward signalling path. Two consecutive signals can be presumedto have been or to be received when four signals have been accepted by the storage of the error-correcting equipmentat the sending end. (This allows for the loss of one signal as an undetected error.)4.2.3 The receiving equipment shall b
19、e arranged so that when two signals are received and followed immediatelyby teleprinter signals representing the call-confirmation and proceed-to-select (or proceed-to-transmit) signals in rapidsuccession the recognition of the two signals as the call-confirmation signal will allow the teleprinter s
20、ignals to bepreceded by 140 ms (minimum) of stop polarity.4.2.4 Measures should be taken so that, if proceed-to-select or proceed-to-transmit signals are relayed by the FRXD(or equivalent storage device), the switching equipment does not return these signals until the motor has reached its fullspeed
21、.5 Selection signals5.1 For manual working, teleprinter signals over the forward signalling path.5.2 For semi-automatic working, teleprinter signals over the forward signalling path, as follows: the prepare-for-digits signal shall be combination No. 30 (figure-shift); digits of the called subscriber
22、s number (preceded by transit access codes, if required) in InternationalTelegraph Alphabet No. 2;Fascicle VII.2 - Rec. U.20 3 end-of-selection signal, combination No. 26. This may be followed by another combination characterizingthe class of traffic in the incoming country5.3 For fully-automatic wo
23、rking: teleprinter signals over the forward signalling path, as follows: the prepare-for-digits signal shall be combination No. 30 (figure-shift); digits of the called subscribers number (preceded by transit access codes, if required) in InternationalTelegraph Alphabet No. 2; if an end-of-selection
24、signal is required, this should be combination No. 26. This may be followed byanother combination characterizing the class of traffic in the incoming country.5.4 The transmission of the selection signals should be delayed if the motor of the FRXD has not yet gained speed.5.5 Where the incoming syste
25、m uses a uniform numbering plan so that the number of digits in the number can bedetermined from the initial digit, the outgoing Administration must transmit an end-of-selection signal if this isrequired by the incoming country. Where the incoming system has a non-uniform numbering scheme the end-of
26、-selection signal cannot be made obligatory. However, for such a system it may be advantageous to use this signalsubject to the agreement of the outgoing Administration, in those cases where the outgoing system can readily insertthe signal. To avoid undue occupation of trunks and equipment, Administ
27、rations should take all reasonable steps toensure that selection signals are transmitted over radio circuits without undue delay.6 Call-connected signal6.1 Manual working: the code DF over the backward signalling path.6.2 Semi-automatic working: either answerback signals or the signals defined for f
28、ully-automatic working below6.3 Fully-automatic working: combination No. 32, followed by 11 to 13 combinations No. 29 (letter-shift) followedby the obtained subscribers answerback code. The insertion of the combinations No. 29 must not cause mutilation ofthe subsequent signals in the sequence.6.4 In
29、 the case of transit operation where the first circuit in the connection is an ARQ radio circuit and the secondcircuit in the connection uses Type A or B signalling to a country that returns the answerback automatically, thenumber of combinations No. 29 of the radio call-connected signal may be redu
30、ced to eight to avoid mutilating theanswerback.7 Idle circuit condition7.1 Combinations on the forward and backward signalling paths.8 Clearing8.1 Clearing signal8.1.1 The appearance of combinations in the direction in which the clearing signal is sent. Reception of twoconsecutive signals will have
31、to be interpreted as a clearing signal.8.1.2 On recognition of the clearing signal received over the radio circuit any text remaining in the store, at the pointwhere the clearing signal is recognized, must be destroyed.8.1.3 On recognition of the clearing signal received over the land line, any text
32、 remaining in store, at the point wherethe clearing signal is recognized, must be transmitted before the signals are sent over the radio path.8.2 Clear-confirmation signal8.2.1 The appearance of combinations in the direction opposite to that from which the clearing signal was sent.Reception of two c
33、onsecutive signals will be interpreted as a clear-confirmation signal when a clearing signal of 7 signals has been accepted by the storage of the radio equipment without a request for repetition. The transmission of7 signals in this way ensures that, allowing for the loss of one signal as an undetec
34、ted error, the clearing signal canbe presumed to have been received and recognized at the distant end.4 Fascicle VII.2 - Rec. U.208.2.2 For radio circuits using an eight-character cycle with four characters stored, a sequence of 8 signals shall beused in place of the above sequence of 7 signals. For
35、 radio circuits using an eight-character cycle with sevencharacters stored, a sequence of 11 signals shall be used in place of the above sequence of 7 signals.8.2.3 It is desirable that the equipment shall be arranged so that the clearing and clear-confirmation signals do notcause spurious character
36、s (including combinations No. 32) to be transmitted over the radio path. Where electronicstorage devices are used it is possible to arrange for these spurious characters to be suppressed in the storage device.Where electro-mechanical storage devices are used, the generation of spurious characters by
37、 the clear-confirmationsignal can be minimized by arranging that when the clearing signal is received over the radio circuit, the input to thestorage device is blocked.8.2.4 In order to ensure that, on transit calls, switching equipment and possibly the subscribers teleprinter set are notunnecessari
38、ly held because of delay in transmitting the clearing and clear-confirmation signals over the radio path, theradiotelegraph equipment should return the clear-confirmation signal to the switching equipment without waiting forthe exchange of clearing and clear confirmation signals over the radio path.
39、8.3 Guard delay8.3.1 The circuit shall be guarded on release as specified in Recommendation U.1 except that the delay shall bemeasured from the moment when the equipment has both:a) transmitted 7 signals over the radio path without request for repetition;b) has received two consecutive signals over
40、the other signalling path.8.3.2 During the guard period the free line condition shall be maintained on both signalling paths of the internationalcircuit.8.3.3 Because it is possible for the circuit to be opened for traffic at one end before the equipment at the other endhas completed the transmissio
41、n of the 7 signals, it is possible that an incoming call may be received before the 7 signals have been transmitted. Where this occurs, the call should be accepted but the call-confirmation signal shouldnot be returned until the transmission of the 7 signals has been completed. (See 8.2.2 above.)9 R
42、egister congestion9.1 Semi-automatic working: the return of a signal indicating congestion may be allowed; the NC sequence withthe standard form of service signal should be used to indicate the situation.9.2 Fully-automatic working: the return of a signal indicating congestion is prohibited.10 Servi
43、ce signals10.1 Teleprinter signals (OCC, NC, NCH, NA, NP, DER, ABS) preceded by the carriage-return, line-feed andletter-shift signals and followed by line-feed (preferably together with carriage-return) and then immediately by theclearing signal in all cases.11 Both-way working11.1 For both-way ARQ
44、 radio circuits used in the fully-automatic telex and gentex services, the following action tominimize the incidence of head-on collision is recommended:a) that inverse order testing, or a close approximation to it by testing the route in small groups of circuits infixed order, always starting the s
45、earch from the same initial position, should be adopted at opposite ends ofa both-way group of trunk circuits.b) that calls should be offered in such a way that each circuit is tested once only for the minimum period oftime necessary to ascertain whether it is free or busy, and the outgoing selector
46、s should not have facilitiesfor delayed hunting.Fascicle VII.2 - Rec. U.20 511.2 The absence of the proceed-to-select signal will serve to detect a head-on collision when the group of circuits istotally occupied or very nearly totally occupied. The two calls will then be cleared down unless there ar
47、e still freecircuits in the route.Note The recognition of the calling, call-confirmation, clearing and clear-confirmation signals requires thedetection of two consecutive signals or as specified. The detection device should, in new equipment, be arranged torecognize two consecutive signals even thou
48、gh these may be separated by a period of automatic correction, i.e. thediscrimination involves counting. In some existing equipments the detection device requires that the two signals to berecognized shall occur in consecutive character periods, i.e. the discrimination involves timing. The transmiss
49、ion of thecall-confirmation, clearing and clear-confirmation signals requires that the appropriate number of or signals shallbe offered to the storage of the radio equipment without a request for repetition, i.e. the control should be by a timingdevice that is reset when automatic correction occurs.6 Fascicle VII.2 - Rec. U.20Reference1 CCITT Recommendation Use on radio circuits of 7-unit synchronous systems giving error correction byautomatic repetition, Rec. S.13.
