1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 5 TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU4%,%2!0(G0G037)4#().3).!,).G0G0/6%2G0G02!$)/G0G0!.$G0G0-5,4)0,%8%$#(!%,353%G0G0/ amended at Geneva, 1972)1 Charging on the basis of elapsed timeWhere a radiotelegraph circuit equipped with ARQ equipment forms
2、 part of an international telex network andcan be engaged in a telex connection established by fully automatic switching, the Administrations are faced with adifficult problem regarding automatic charging of the calls. The difficulty arises from the fact that in case of badtransmission conditions on
3、 the radiotelegraph circuit, signals recognized as erroneous are repeated. These repetitionscan be numerous at certain times. For manual or semi-automatic operation, in order to establish the basis for charging,the Administrations or recognized private operating agencies (RPOA) deduct the time durin
4、g which the circuit has beentransmitting repetitions from the elapsed duration of the connection.The application of this method to fully automatic calls although desirable is made difficult by the fact thatthe charge for these calls is made in the originating country and by automatic methods. When t
5、he call is not establishedthrough the intermediary of radiotelegraph circuits incorporating ARQ equipment, the charge is made according to theelapsed time of the communication. It would then be necessary to advise the originating country that the call hasinvolved a radiotelegraph circuit that incorp
6、orates ARQ equipment, and to advise what correction should be applied tothe elapsed time of the communication in order to account for the periods of inefficiency of the radiotelegraph circuit.Some study has been made for finding a solution that is both technically and economically acceptable for the
7、transmission and use of information necessary for corrected charging as a function of the inefficiency of theradiotelegraph circuit. However, due to the declining importance of radio circuits incorporating ARQ equipment forfully automatic traffic in the telex network and the tendency for them to be
8、relegated to the role of standby circuits,further study of the method of charging based upon efficient time has been abandoned.The alternative solution of charges based upon elapsed time has now been adopted as the standard to beapplied. It will then be necessary before incorporating a circuit with
9、ARQ equipment in the fully automatic telexservice to ensure that it meets with certain stability requirements. Safeguard measures designed to avoid, in certaincases, an excessive overcharge of the calling subscriber, as indicated in the present Recommendation, will benecessary.2 Safeguard measuresWh
10、en charges are to be based on elapsed time, the methods of safeguard are:i) busying of an unoccupied radiotelegraph channel whenever transmission conditions on this channel areinadequate;ii) forced release of an established connection on such a channel whenever transmission conditions are bad.In the
11、 application of the latter type of safeguard (forced release of an established connection), there are twoconflicting requirements:i) the need to avoid substantial differences between the charged time and the time during which theconnection was efficient;ii) the need to avoid, as much as possible, fo
12、rced release of established connections.A reasonable compromise solution should achieve the following main objectives:i) the percentage of forced releases must not exceed three;ii) the average overcharge for a call must not exceed five per cent;iii) the maximum overcharge for a call must not exceed
13、twenty-five per cent.2 Fascicle VII.2 - Rec. U.233 Control of forced releaseAdministrations employing radiotelegraph circuits incorporating ARQ equipment should use the efficiencyfactor for controlling the forced release of an established connection. With this arrangement, an established connectionw
14、ill be cut whenever the efficiency factor, averaged over 60 consecutive seconds, falls below 80%. This form ofcontrol, especially if it is applied to circuits that conform to the stability requirements specified in 9 below, ought notto result in more than two or three per cent of connections being i
15、nterrupted; this figure is quite comparable with thenumber of fortuitous releases recorded in the use of cable circuits.4 Control of busyingAt those times when its efficiency factor is too low, a circuit that is not carrying traffic should be busied at bothends so that it cannot be seized by a call
16、until such time as the efficiency factor reverts to an acceptable value. Thecircuit will be busied if the mean value of the efficiency factor, measured over an interval of 20 consecutive seconds, isless than 80%.5 Practical application of busyingFor a radiotelegraph system corresponding to 50 bauds
17、(see Recommendation S.13 1), the maximum numberof transmissible elements in a 20-second period is 20 48 and the corresponding number of characters is (20 48)/7i.e. 137. If r is the number of repetition cycles during 20 seconds, the efficiency factor is (137 - 41)r)/137. Hence, it issufficient to cou
18、nt the number of repetition cycles because if, in a period of 20 consecutive seconds, there are 72)repetition cycles or more, then the mean efficiency factor is below 80% during that period.The two most practical methods of dividing the time up into intervals of 20 seconds are the procedure ofsplitt
19、ing the time into 20-second blocks and the method of using sliding periods of 20 seconds.In the procedure of splitting the time into blocks, the time is divided into fixed intervals of 20 seconds. Therepetition cycles are counted during each of these intervals and the count is recommenced for each i
20、nterval, no accountbeing taken of the result of the count for the preceding interval. In the sliding period method, the earliest count iseliminated and a new count added.The block method uses simpler equipment than the sliding period method; it is a little less exact because of thefact that the infl
21、uence of a bundle of repetitions arriving at about the same time as the division between successiveblocks is spread over two successive and independent blocks.After very close consideration of the discrepancies between the results given by the two methods, it wasconcluded that the effect of these di
22、screpancies is small and of no practical importance as far as subscribers areconcerned. Administrations may therefore select either method.If, during a counting period, the number of repetition cycles has already reached a figure corresponding to amean efficiency factor of lower than 80% over the 20
23、-second period, the decision to order busying of the circuit will bemade immediately, without waiting for the end of the current 20-second period.The manner in which the order to busy the circuit is sent from the ARQ equipment to the switching centre is amatter that interests only the Administration
24、 that operates the centre and the ARQ equipment to issue an internationalrecommendation on this matter.The timing of intervals at the two ends of the same circuit is not synchronized, so that instants of busying ordebusying a circuit at one end may differ from the corresponding instants at the other
25、 end by several seconds. As aresult, while one end of the circuit is marked busy, a call can seize the circuit at the other end. This situation isconsidered as admissible, and the incoming call is accepted.After a circuit is marked busy, the measurement of the efficiency factor proceeds in accordanc
26、e with the sametime-division process. If, during a 20-second period, the mean efficiency factor reaches or exceeds 80%, the busymarking is removed. It follows that, whenever the efficiency factor is varying at about 80%, periods of busying and ofreturn to service can succeed one another at intervals
27、 of about 20 seconds. This effect was considered to be permissible._1)This figure is 8 in the case of an 8-character-repetition cycle.2)3.5 with an 8-character-repetition cycle.Fascicle VII.2 - Rec. U.23 36 Application of forced releaseA call can seize the radiotelegraph circuit only during a period
28、 when the circuit is not marked busy. In the caseof a calls arriving on the radiotelegraph circuit after the occurrence of the first marker denoting the termination of a 20-second period, the time division will proceed on the basis of 60-second intervals (instead of 20-second ones), andeverything th
29、at has been said about 20-second periods applies equally to 60-second periods. In particular if, during a60-second period, it is already evident that the efficiency factor cannot reach an average value of at least 80%, forcedrelease of the connection shall be ordered without waiting for the end of t
30、he period.If the efficiency falls so far that the connection must be cut at the calling end of the ARQ circuit, a long timecould elapse, in the event of very adverse transmission conditions, before the release signal could be sent to the calledsubscriber. Consequently, the called subscriber (especia
31、lly in stations not supervised by a receiving operator) remainsengaged and cannot be reached by other subscribers. Also, the re-establishment of the call by way of another channelbecomes impossible. Therefore, it is desirable to be able to effect a release at the receiving end in unfavourableconditi
32、ons. The method of release employed at the receiving end, however, should not initiate release more easily thanat the calling end. It is proposed for this purpose that, once there is evidence at the receiving end that the meanefficiency factor has remained lower than 80% for two successive 60-second
33、 periods, release at the receiving endshould follow.7 Elimination of signals still registered in the memoryOnce the decision has been made to break the established connection at either end, the signals that are stillrecorded in the ARQ equipment memory must be destroyed. It must be pointed out that
34、in this case the forced releasesignal has been due to the bad transmission conditions; it is very probable that the subscriber, at the receiving end, willbe released by the auxiliary safeguard measures (two successive periods of 60 seconds with the efficiency factor below80%); the signals that the m
35、emory would continue to dispose of in the forward direction will probably not reach thecalled subscriber. For this reason the elimination of the signals still registered in the memory has been decided.8 Advising the calling subscriberIt has been proposed that the calling subscriber should be advised
36、 by a special service signal preceding theforced release signal; in this way the calling subscriber would know that he must reforward his whole message. Thisservice signal would above all have the advantage of enabling the automatic charging device to recognize that it isdealing with a connection th
37、at has been interrupted as a result of operation of the safeguard feature of an ARQequipment and that the call must not be charged.Although the principle of this solution may have escaped critiscism, its application has provoked objections.The first would be the cost and complexity of equipment that
38、 would ultimately be used for only a very small proportionof calls. Another objection would be the fact that, in certain types of apparatus, automatic transmission could not beinterrupted by the reception of signals; the only result would be mutilation on the local copy of the transmitted text andof
39、 the service code; the meaning of these mutilations could be obscure to the subscriber. The aspect of the other end ofthe communication, which could also have a message in the process of transmission to the calling subscriber, must alsobe taken into account. Finally, the use of the clearing signal o
40、nly, without the use of a preliminary service signal, wasproposed.9 Precautions to be taken before incorporating circuits with ARQ equipment in automatic switchingnetworksIn spite of these precautions, fully-automatic operation on a radiotelegraph circuit incorporating ARQequipment can be considered
41、 only if this circuit possesses adequate stability.Before incorporating a circuit with ARQ equipment in the fully-automatic switching network, theAdministrations must carry out extended trials. These trials should be made under normal traffic conditions, over aminimum period of three consecutive hou
42、rs chosen from the busy period (or periods), when heavy traffic is foreseen tooccur on the route under consideration (allowing for the traffic, whether terminal or transit, that prevails on the routeaccording to the season). The condition that must be fulfilled before a circuit can be accepted for u
43、se in the fully-automatic network is that its mean efficiency factor, measured over periods of 20 consecutive seconds each, shall notfall below 80% for more than 10% of the total time involved in the measurements. The measurements must be repeatedas often as will be necessary for the Administration
44、to have an assessment of the suitability of the circuit.The attention of Administrations is drawn to the fact that, before offering fully-automatic transit working on aradio route incorporating ARQ equipment, the grade of service on the route under consideration must be in accordancewith that propos
45、ed in Recommendation F.68 2, i.e. only one call lost in 50.If these conditions are not complied with, it would be better to retain semi-automatic operation.4 Fascicle VII.2 - Rec. U.23For these reasons, the CCITTunanimously declares the following view(1) Administrations operating radiotelegraph circ
46、uits equipped with ARQ systems that may be engaged in afully-automatic telex call, such that the charging of the subscriber is made automatically in the originating countryaccording to the elapsed time of the connection, must take precautions to avoid too great a difference between thecharged time a
47、nd the time during which the radiotelegraph circuit was efficient.(2) If, in the course of an established connection, the mean value of the efficiency factor3) is lower than 80%over a period of 60 consecutive seconds, the connection will be released and the clearing signal will be sent to thecalling
48、 subscriber under the control of the ARQ equipment.(3) For a circuit involved in a fully automatic telex network, measurements will be made, at those times whenthe circuit is not held by a call, in order to determine the mean efficiency factor based on periods of 20 consecutiveseconds. If, during su
49、ch a period, the mean efficiency factor falls below 80%, the circuit shall be marked busy on thefirst switching centre located backward of the ARQ equipment that assessed this situation. If, during a period of 20consecutive seconds, the mean efficiency factor rises above 80%, the busy marking shall be removed and the circuitwill be able to be seized by a call.(4) Interruption of an established connection will occur, at the calling side when, during a 60-second period, itbecomes apparent, without waiting until the end of the period, that the mean efficiency f
