1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 8 G0BISTELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU05“,)#G0G0$!4!G0G0.%47/2+3).4%2 amended at Geneva, 1980,Malaga-Torremolinos, 1984 and Melbourne, 1988)The CCITT,considering(a) that the interface between Data Terminal Equipment (DTE) and Data Circuit Te
2、rminating Equipment(DCE) for start-stop transmission on public data networks is specified in Recommendation X.20,(b) that several Administrations are planning to provide as an interim measure the connection to public datanetworks of start-stop DTEs which are designed for interfacing to V-Series mode
3、ms for start-stop transmission,unanimously declaresthat the interface between a V-Series type DTE and a DCE in public data networks for user classes of serviceemploying start-stop transmission should be as defined in this Recommendation.1 ScopeThis Recommendation applies to the interface between a D
4、TE designed for interfacing to duplex V-Seriesmodems for start-stop transmission and a DCE on public data networks.The operation is limited to start-stop transmission at data signalling rates and character structures specified forstart:-stop transmission in Recommendation X.1.The application compris
5、es:a) circuit switched service,b) leased circuit service (point-to-point and centralized multipoint).2 Fascicle VIII.2 - Rec. X.20 bis2 Interchange circuits2.1 Functional characteristicsThe functional characteristics of the interchange circuits concerned (see Table 1/X.20 bis) comply withRecommendat
6、ion V.24.TABLE 1/X.20 bisInterchange circuitNumber Designation102103104106107108/1a)108/2b)109125c)141d)142Signal ground or common returnTransmitted dataReceived dataReady for sendingData set readyConnect data set to lineData terminal readyData channel received line signal detectorCalling indicatorL
7、ocal loop backTest indicatora)Used in case of automatic control of the direct call facility.b)Used in case of switched data network service.c)Not provided in leased circuit service.d)Not provided in those networks which do not provide automatic activation of the test loops.2.2 Electrical characteris
8、ticsThe electrical characteristics of the interchange circuits comply with Recommendation V.28, using the 25-poleDTE/DCE interface connector and contact number assignments in ISO Standard 2110.3 Use of interchange circuits3.1 Operation of interchange circuit 107 - Data set readyThis circuit is used
9、to indicate the operational functions given in Table 2/X.20 bis.Fascicle VIII.2 - Rec. X.20 bis 3TABLE 2/X.20 bisCondition of circuit 107 Meaning in the data networkONOFFOFFReady for data (see Note)DCE clear indicationDCE clear confirmationNote - Since no circuit 105 is operated, the ON condition on
10、 circuit 106 is applied 0 to 20 milli-seconds after circuit 107 is turned ON.3.2 Use of interchange circuits 108/1 and 108/23.2.1 Circuit 108/1 - Connect data set to lineThis circuit is used alternatively to circuit 108/2. The operational functions given in Table 3/X.20 bis shouldbe indicated.TABLE
11、3/X.20 bisCondition of circuit 108/1 Meaning in the data networkONONOFFOFFCall request for direct call (see 3.4.1)Call acceptedDTE clear requestDTE clear confirmation (see 3.4.4)3.2.2 Circuit 108/2 - Data terminal readyThis circuit is used alternatively to circuit 108/1. The operational functions gi
12、ven in Table 4/X.20 bis shouldbe indicated.TABLE 4/X.20 bisCondition of circuit 108/2 Meaning in the data networkONOFFOFFCall acceptedDTE clear requestDTE clear confirmation (see 3.4.4)4 Fascicle VIII.2 - Rec. X.20 bis3.3 Circuit 125 - Calling indicatorThe ON condition indicates incoming call. The c
13、ircuit will be turned OFF as follows:- in conjunction with circuit 107 turned ON, or- DCE ready is received from the network, or- DCE clear indication is received from the network.3.4 Operational requirements for circuits 106, 107, 108/1, 108/2 and 1093.4.1 Call request for direct callFor a direct c
14、all facility the DTE indicates a call request by turning circuit 108/1 ON. Circuit 108/2 cannot beused for this purpose.3.4.2 Call acceptedA DTE receiving an incoming call should turn circuit 108/1 or 108/2 from OFF to ON within 500 ms toindicate call accepted, otherwise the call will be cleared. A
15、DCE presenting an incoming call to a DTE which alreadyhas circuit 108/2 ON will regard the ON condition on circuit 108/2 as an indication of call accepted.Optionally when a DTE does not provide circuit 108/1 or 108/2, the call accepted signal to the network wouldbe generated within the DCE as an ans
16、wer to the incoming call signal received from the network. However, it may alsobe possible to signal to the network a DTE controlled not ready by a manual action on the DCE.3.4.3 Operation of interchange circuits 109 and 106The DCE switches circuit 109 to ON together with circuit 107. Circuit 106 is
17、 put to ON 0 to 20 ms after theappearance of the ON condition on circuit 107.The circuits 109 and 106 are switched to OFF either when circuit 108 is switched to OFF or when circuit 108is ON and the DCE signals DCE clear indication (see 3.4.4).3.4.4 DCE clear indication and DTE clear confirmationDCE
18、clear indication is signalled to the DTE by turning circuit 107 OFF. The DTE clear confirmation, whenimplemented, should be given by the DTE turning OFF circuit 108/1 or 108/2 within 500 ms after the DCE clearindication is signalled on circuit 107. Otherwise, the DCE may consider the DTE as being un
19、controlled not ready untilcircuit 108/1 or 108/2 is turned OFF or a ready signal is generated by a manual action on the DCE.Circuit 108/1 should always be able to give DTE clear confirmation.Optionally, when a DTE does not turn circuit 108/2 OFF for DTE clear confirmation this would beautomatically
20、generated within the DCE as an answer to the clear indication received from the network and the DTEwill be considered in the ready condition.In the case when the DTE expects to have circuit 107 OFF only as a response to circuit 108/1 or 108/2 OFF,the DCE will not turn circuit 107 OFF as a DCE clear
21、indication and in this case the DCE indication will not besignalled to the DTE across the interface. The necessary DTE clear confirmation signal will then be automaticallygenerated within the DCE as an answer to the clear indication signal received from the network. The DTE may beregarded as uncontr
22、olled not ready until circuit 108/1 or 108/2 is turned OFF.3.4.5 Centralized multipoint operationAs the circuits 106 and 109 are always in the ON condition, the transmission disciplines must be determinedby end-to-end control procedures of the DTEs.4 Call progress signals and DCE provided informatio
23、nCall progress signals and DCE provided information cannot be handled by V-Series DTEs.Fascicle VIII.2 - Rec. X.20 bis 55 Failure detection and isolation5.1 Fault conditions of interchange circuitsIf the DTE or DCE is unable to determine the condition of circuits 107, 108/1 or 108/2 and possibly cir
24、cuits103 and 104, it shall interpret this as an OFF condition or binary 1 (circuits 103 and 104) as specified in the relevantelectrical interface specifications.5.2 DCE fault conditionsIf the DCE is unable to provide service (e.g. loss of incoming line signal) for a period longer than a fixedduratio
25、n it will turn circuit 107 to the OFF condition. The value of this duration is network dependent.5.3 Test loopsThe definitions of the test loops and the principles of maintenance testing using the test loops are provided inRecommendation X.150.5.3.1 DTE test loop - type 1 loopThis loop is used as a
26、basic test of the operation of the DTE, by looping back the transmitted signals inside theDTE for checking. The loop should be set up inside the DTE as close as possible to, the DTE/DCE interface.Except as noted below, while the DTE is in loop 1 test condition:- circuit 103 is connected to circuit 1
27、04 inside of the DTE;- circuit 103 as presented to the DCE must be in the binary 1 condition;- circuit 108/1 or 108/2 may be in the same condition as it was before the test;- circuits 140 and 141, if implemented, must be in the OFF condition.The conditions of the other interchange circuits are not s
28、pecified but they should if possible permit normalworking.Loop 1 may be established from either the data transfer phase or the idle phase.If the loop is established from the data transfer phase, the DCE may continue to deliver data to the DTEduring the test as though the DTE were in normal operation
29、. It will be the responsibility of the DTE to recover fromany errors that might occur while the test loop is activated.If the loop is established from the idle phase, the DTE should continue to monitor circuit 125 so that anincoming call may be given priority over a routine test.5.3.2 Local test loo
30、p - type 3 loopLocal test loops (type 3 loops) are used to test the operation of the DTE, the interconnecting cable and eitherall or parts of the local DCE, as discussed below.Where allowed by national testing principles loop 3 may be established from any state.For testing on leased circuits and for
31、 short duration testing, on circuit-switched connections the DCE shouldcontinue to present toward the line the conditions that existed before the test (e.g. either data transfer or ready state).Where this is not practical (e.g. in some cases for loop 3a) or desirable (e.g. for long duration testing
32、in circuit-switchedapplications) the DCE should terminate an existing call.Manual control should be provided on the DCE for activation of the test loop.The automatic activation on this loop, if provided, should be controlled by circuit 141.The precise implementation on the test loop within the DCE i
33、s a national option. At least one of the followinglocal test loops should be implemented.6 Fascicle VIII.2 - Rec. X.20 bis5.3.2.1 Loop 3dThis loop is used to test the operation of the DTE, including the interconnecting cable, by returningtransmitted signals to the DTE for checking. The loop is set u
34、p inside the local DCE and does not include interchangecircuit generators and loads.While the DCE is in the loop 3d test condition:- circuit 103 is connected to the circuit 104;- circuits 107 and 142 are placed in the ON condition.Note - While test loop 3d is operated, the effective length of the in
35、terconnecting cable is doubled. Therefore,to insure proper operation of loop 3d, the maximum DTE/DCE interface cable length should be one-half the lengthnormally appropriate for the data signalling rate in use.5.3.2.2 Loop 3cThis loop is used to test the operation of the DTE, including the interconn
36、ecting cable and DCE interchangecircuit generators and loads.The configuration is identical to that given for loop 3d in 5.3.2.1 with the exception that the looping ofcircuit 103 to circuit 104 includes the interchange circuit generators and loads. The notes concerning restriction ofinterface cable
37、length and load input impedance are not applicable.5.3.2.3 Loop 3bThis loop is used as a test of the operation of the DTE and the line coding and control logic and circuitry of theDCE. It includes all the circuitry of the DCE with the exclusion of the line signal conditioning circuitry (e.g.,impedan
38、ce matching transformers, amplifiers, equalizers, etc.). The delay between transmitted and received test data isa few octets. (See Note).The configuration is identical to that given for loop 3c in 5.3.2.2 except for the location of the point ofloopback.Note - In some networks the setting of loop 3b
39、will cause clearing of existing connections.5.3.2.4 Loop 3aThis loop is used to test the operation of the DTE and the DCE. The loop should include the maximum amountof circuitry used in DCE working including, in particular, the line signal conditioning circuitry. It is recognized that, insome cases,
40、 the inclusion of devices (e.g., attenuators, equalizers or test loop translators) may be necessary in theloopback path. The subscriber line is suitably terminated during a loop 3a test condition.The configuration is identical to that given for test loop 3b in 5.3.2.3 except for the location of the
41、point ofloopback.Note - In some networks the setting of loop 3a will cause clearing of existing connections.5.3.3 Network test loop - type 2 loopNetwork test loops (test 2 loops) are used by the Administrations test centre to test the operation of the leasedline or subscriber line and either all or
42、part of the DCE, as discussed below.Where, allowed by national testing principles loop 2 may be used by a DTE, as follows:a) In the case of switched circuit networks in the data transfer phase to test the operation of the networkconnection including the remote DCE. It should be possible to reenter t
43、he data transfer phase aftercompletion of the network loop test.b) In the case of leased lines in the idle phase to test the operation of the line including the remote DCE.When the test is in progress the DCE will return circuits 107 and 109 in the OFF condition, circuit 104 inthe binary 1 condition
44、 and circuit 142 in the ON condition.Fascicle VIII.2 - Rec. X.20 bis 7The loop may be controlled manually by a switch on the DCE or automatically by the network. The control ofthe loop and the method used for automatic control, when implemented, is a national option.In case of a collision between a
45、call request and the activation of the loop, the loop activation command willhave priority and the call request is ignored.The precise implementation of the test loop within the DCE is a national option. One of the following networktest loops should be implemented:5.3.3.1 Loop 2bThis loop is used by
46、 the Administrations test centre(s) and/or the remote DTE to test the operation of thesubscriber line and all the circuitry of the DCE with the exception of interchange circuit generators and loads.While the DCE is in the loop 2b test condition:- circuit 104 is connected to circuit 103 inside of the
47、 DCE,- at the interface, the DCE places circuit 104 in the binary 1 condition and circuit 109 in the OFFcondition, or alternatively, may present an open circuit or power off condition on circuits 104 and 109,- circuits 106, 107 and 125 to the DTE are placed in the OFF condition,- circuit 142 to the
48、DTE is placed in the ON condition.5.3.3.2 Loop 2aThis loop is used by either the Administrations test centre(s) or the remote DTE to test the operation of thesubscriber line and the entire DCE.The configuration is identical to that given for loop 2b in 5.3.3.1 except for the location of the point of
49、loopback.5.3.4 Subscriber-line test loop - type 4 loopSubscriber-line test loops (type 4 loops) are provided for the maintenance of lines by the Administrations.5.3.4.1 Loop 4aThis loop is only provided in the case of 4-wire subscriber lines. Loop 4a is for the maintenance of lines byAdministrations. When receiving and transmitting pairs are connected together, the resulting circuit cannot beconsidered normal. Loop 4a may be established inside the DCE or in a separate device.While the DCE is in the loop 4a test condition:- circuit 104 to the DTE is placed in the binary 1 co
