1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 8 TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU05“,)#G0G0$!4!G0G0.%47/2+3-!).4%.!.#%02).#)0,%3G0G0/ amended at Malaga-Torremolinos, 1984)1 IntroductionThe CCITT,considering(a) the increasing use being made of data transmission systems;(b) the volume of th
2、e information circulating on data transmission networks;(c) the savings to be made by reducing interruption time on such data circuits;(d) the importance of being able to determine responsibilities (of necessity involving several parties) inmaintenance questions for networks; and(e) the advantages o
3、f standardization in this field,unanimously declaresthat the locating of faults can be facilitated in many cases by data circuit loop testing procedures in DTEs andDCEs.2 ScopeThis Recommendation specifies the principles of maintenance testing for public data networks using theDTE/DCE test loops. Th
4、e definition of the loops, the principles for the control of the loops and the principles for theindications to be presented when the loops are activated are described herein for general application with all DTE/DCEinterfaces. Specific details concerning the implementation of these principles are co
5、ntained in the individual DTE/DCEinterface Recommendations. The maintenance philosophy, definitions and general principles are applicable to interfaceRecommendations, such as X.20, X.20 bis, X.21, X.21 bis, and X.22.2.1 Maintenance philosophyThe provision of DTE and DCE test loops in public data net
6、works is based upon the following maintenancephilosophy:a) test loops may be used by an Administrations test centre(s) to test the operation of either leased lines orcircuit-switched network subscriber lines, including either all or part of the DCE, without necessarilyrequiring the dispatch of netwo
7、rk maintenance personnel to the subscribers premises;b) where, allowed by national testing principles, DCE test loops may also be used by a DTE to test theoperation of network connection or leased lines. Where provided, the intent is for the DTE to make initialtests to isolate a trouble condition ei
8、ther in the DTEs or in the data circuit.2.2 Loop testing principlesThe provision of loop testing capabilities should be based on the following principles:a) the loops should be transparent, e.g. they should be bit sequence independent;b) loop testing is a disruptive type of testing, e.g. when a loop
9、 has been established transmission of data is notpossible;c) test loops may be established from any state; however, when loop testing is commenced from the datatransfer state, the same test procedures may be used for both leased lines and circuit-switched connections;2 Fascicle VIII.3 - Rec. X.150d)
10、 when allowed, the preferred DTE test sequence is: loop 1 - loop 2 - loop 3, sequentially from both ends ofthe circuit.3 Definition of the loopsNine loops are defined as shown in Figure 1/X.150. For clarity these nine loops have been grouped asfollows:a) DTE test loop - type 1 loop ( 3.1)Loop 1 ( 3.
11、1.1)b) Local test loops - type 3 loops ( 3.2)Loop 3d ( 3.2.1)Loop 3c ( 3.2.2)Loop 3b ( 3.2.3)Loop 3a ( 3.2.4)c) Subscriber-line test loops - type 4 loops ( 3.3)Loop 4a ( 3.3.1)Loop 4b ( 3.3.2)d) Network test loops - type 2 loops ( 3.4)Loop 2b ( 3.4.1)Loop 2a ( 3.4.2)3.1 DTE test loop - type 1 loop3.
12、1.1 Loop 1This loop is used as a 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.Loop 1 may be established from either the data transfer or ready state.I
13、n some networks, for short routine tests during the data transfer state, the DTE should either maintain the samestatus on the interchange circuits as before the test or where possible send the DTE controlled not ready signal.Fascicle VIII.3 - Rec. X.150 3If the loop is established from the data tran
14、sfer state, the DCE may continue to deliver data to the DTE duringthe test as though the DTE were in normal operation. It will be the responsibility of the DTEs to recover from any errorsthat might occur while the test loop is activated.If the loop is established from the ready state, the DTE should
15、 continue to monitor so that an incoming call maybe given priority over a routine test. Alternatively, in cases where the DTE cannot accept incoming calls, DTEs shallsignal one of the not ready states.The DCE continues to present signal element timing and, if implemented, byte timing. The DTE need n
16、ot makeuse of the timing information during a loop test.3.2 Local test loops - type 3 loopsLocal test loops (type 3 loops) are used to test the operation of the DTE, the interconnecting cable and either allor parts of the local DCE, as discussed below.Where allowed by national testing principles, lo
17、op 3 may be established from any state.4 Fascicle VIII.3 - Rec. X.150For testing on leased circuits and for short duration testing on circuit-switched connections, the DCE shouldeither continue to present toward the line the conditions that existed before the test (e.g. either data transfer or ready
18、state) or send the controlled not ready state to the remote DTE. Where this is not practical (e.g. in some cases for loop3a) or desirable (e.g. for long duration testing in circuit-switched applications), the DCE should terminate an existing calland, if possible, signal toward the subscriber-line on
19、e of the not ready states.The DCE continues to present signal element timing, and, if implemented, byte timing. The DTE must make useof the timing information.3.2.1 Loop 3dThis loop is used to test the operation of the DTE, including the inter-connecting cable, by returning transmittedsignals to the
20、 DTE for checking. The loop is set up inside the local DCE and does not include interchange circuitgenerators and loads. The DCE may make either type 2 or type 4 loop tests during the loop 3d test condition.Note - While test loop 3d is operated, the effective length of the interface cable is doubled
21、. Therefore, to insureproper operation of loop 3d, the maximum DTE/DCE interface cable length should be one-half the length normallyappropriate for the data signalling rate in use.3.2.2 Loop 3cThis loop is used to test the operation of the DTE, including the inter-connecting cable and DCE interchang
22、ecircuit generators and loads. In this case, the Note in 3.2.1 concerning restrictions of interconnecting cable length doesnot apply.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 th
23、e DCE, with the exclusion of the line signal conditioning circuitry (e.g. impedancematching transformers, amplifiers, equalizers, etc.). The delay between transmitted and received test data is a few octets.Note - In some DCEs, the setting of loop 3b will result in momentary loss of envelope alignmen
24、t causingspurious signals to appear on receiving interchange circuits for a period of time. This may impact upon the DTE testprocedure. Refer to the DTE/DCE Recommendations for information regarding the signal element timing. In somenetworks, the setting of loop 3b will cause clearing of existing co
25、nnections.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, the inclusion of devices (e.g. atte
26、nuators, equalizers or test loop translators) may be necessary in theloopback path. The subscriber line is suitably terminated during a loop 3a test condition. The delay between transmittedand received test data is a few octets.Note - In some DCEs, the setting of loop 3a will result in momentary los
27、s of envelope alignment causingspurious signals to appear on the receiving interchange circuits for a period of time. This may impact upon the DTE testprocedure. Refer to the DTE/DCE interface Recommendations for information regarding the signal element timing. Insome networks, the setting of loop 3
28、a will cause clearing of existing connections.3.3 Subscriber line test loops - type 4 loopsSubscriber line test loops (type 4 loops) are provided for the maintenance of lines by the Administrations. TheDCE signals to the local DTE a not ready or test mode state, as appropriate for the specific DTE/D
29、CE interface. TheDCE continues to present signal element timing and, if implemented, byte timing.Note - In the case of loops 4 and 2 (see below) the DCE may signal the local DTE in such a manner that theDTE can distinguish a test mode from a network failure.3.3.1 Loop 4aThis loop is only provided in
30、 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 circuit under test may not bemeasured as a data circuit. Loop 4a may be established inside the DCE or in a separate device.Fascicle VIII.
31、3 - Rec. X.150 53.3.2 Loop 4bThis loop is used by Administrations to test the operation of the subscriber line including the line signalconditioning circuitry in the DCE. When the receiving and transmitting circuits are connected at this point, loop 4bprovides a connection that can be considered as
32、a data circuit.Note - The implementation may be such that some impairment of the performance is expected since the DCE, inthis case, does not perform a complete signal regeneration/conversion.3.4 Network test loops - type 2 loopsNetwork test loops (type 2 loops) are used by the Administrations test
33、centre(s) to test the operation of theleased line or subscriber line and either all or part of the DCE as discussed below.Where allowed by national testing principles, loop 2 may also be used by a DTE, as follows:- in case of switched circuit networks when the DTEs are in the data transfer state, to
34、 test the operation ofthe network connection including the remote DCE;- in case of leased lines in the ready state, to test the operation of the line including the remote DCE.The DCE signals the local DTE a not ready or test mode state as appropriate for the specific DTE/DCE interface(see Note to 3.
35、3). The DCE continues to present signal element timing and, if implemented, byte timing.3.4.1 Loop 2bThis loop is used by either 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 ci
36、rcuit generators and loads.3.4.2 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.Note - While in the loop 2a condition, the DCE may present an open circuit condition to the DTE on certaininterchang
37、e circuits. It is assumed that the DCE detects electrical signal faults condition as not ready as appropriate forthe specific electrical characteristics.4 Minimum implementation of test loops4.1 DCE test loopsSufficient test loops should be provided in the DCE to allow the customer and/or the Admini
38、strationsmaintenance personnel to positively distinguish between DTE and DCE/line faults.The DCE will implement at least one of the four local test loops (type 3). The DCE will also implement at leastone of the two network test loops (type 2). The implementation of the loops within the DCE is a nati
39、onal matter.Implementation of test loops beyond the minimum set specified above may be provided by some Administrations.4.2 DTE test loopsIt is suggested that all new DTEs provide loop 1.5 Loop control5.1 GeneralWhere available, the means for remotely controlling a loop in one country from a locatio
40、n in another country aredescribed in appropriate DTE/DCE interface Recommendations.In leased circuit services, subscriber-line and network test loops should not be activated before the customer hasbeen informed. However, some Administrations may activate these loops when abnormal conditions are dete
41、cted in thenetwork without first informing the customer.In circuit-switched services, subscriber-line and network test loops should not be activated when the DTE isengaged in a call. In case of a collision between call request and the activation of these loops, the loop activationcommand will have p
42、riority and the call request will be cancelled. These loops may be activated without the priorknowledge and agreement of the customer for periods which normally do not exceed one second.6 Fascicle VIII.3 - Rec. X.1505.2 Control of the local test loopsTo facilitate the testing of the DTE by the custo
43、mer, manual activation (by means of a switch of the DCE) willbe provided for at least one of the four local test loops (type 3). The precise implementation is a national matter.However, customer-controlled automatic activation of the local test loops via the DTE/DCE interface should beconsidered.Whe
44、re available, the means for DTE control of these loops via signals in the DTE/DCE interface are describedin the appropriate DTE/DCE interface Recommendations.Note - With the introduction of the electrical interchange circuits as defined in Recommendations X.26/X.27,some Administrations may locate th
45、e DCE in a location that is remote (up to 1000 metres) from the DTE. Therefore,manual loop activation by the customer may be either difficult or impossible. Thus, some form of an automatic activationof these loops should be considered. Also the limitations in the Note of 3.2.1, loop 3d, should be co
46、nsidered.5.3 Control of the network test loops5.3.1 GeneralEach network test loop implemented in the DCE will be activated either by a manual switch on the DCE orremotely from the Administrations test centre(s) or both. The means for loop activation, the method for achieving remotecontrol and the me
47、thod for notifying the network of manual activation are national matters. Random signals may bedelivered to the DTE prior to closing of loops.Where loop 2a or 2b is provided for customer use, the procedure for its use is subject to the relevant interfaceRecommendations.5.3.2 Leased circuits5.3.2.1 P
48、oint-to-point leased linesIn case of point-to-point line circuits, the Administrations will provide one or more of the following:a) customer control of the network test loop in the local DCE via a manually operated switch on the DCE;b) customer control of the network test loop via the remote DTE/DCE
49、 interface.Note - The provision for the remote control of a loop in one country from a location in another country issubject to bilateral agreement between the affected Administrations.c) remote control from the Administrations data test centre.When available, the method for activation of the network test loop in a DCE by a command signal from aremote DTE/DCE interface will be as described in the appropriate DTE/DCE interface Recommendation.5.3.2.2 Centralized multipoint leased circuitsThe principle of maintenance testing for centralized multipoint circuits are for
