1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 8 TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU05“,)#G0G0$!4!G0G0.%47/2+3).4%2 amended at Geneva, 1980, Malaga-Torremolinos, 1984,and Melbourne, 1988)The CCITT,considering that(a) the interface between DTE and DCE on public data networks requires, in addi
2、tion to the electrical andfunctional characteristics of the interchange circuits, the definition of procedural characteristics for call controlfunctions and selection of the facilities according to Recommendation X.2;(b) the functions of the circuits defined in Recommendation V.24 are based on the r
3、equirements of datatransmission over the general telephone network and are not appropriate for use at DTE/DCE interfaces in public datanetworks;unanimously declaresa Recommendation to include the list of definitions of interchange circuits for use in public data networks isrequired.1 Scope1.1 This R
4、ecommendation applies to the functions of the interchange circuits provided at the interface betweenDTE and DCE of data networks for the transfer of binary data, call control signals and timing signals.For any type of practical equipment, a selection will be made from the range of interchange circui
5、ts defined inthis Recommendation, as appropriate. The actual interchange circuits to be used in a particular DCE for a user class ofservice according to Recommendation X.1 and defined user facilities according to Recommendation X.2, are thoseindicated in the relevant Recommendation for the procedura
6、l characteristics of the interface, e.g.,Recommendation X.20 or X.21.To enable a standard DTE to be developed, the use and termination by the DTE of certain circuits even whenimplemented in the DCE are not mandatory. This is covered by the individual interface Recommendations.The interchange circuit
7、s defined for the transfer of binary data are also used for the exchange of call controlsignals.The electrical characteristics of the interchange circuits are detailed in the appropriate Recommendation forelectrical characteristics of interchange circuits. The application of those characteristics fo
8、r a particular DCE isspecified in the Recommendation for the procedural characteristics of the interface.1.2 The range of interchange circuits defined in this Recommendation is applicable to the range of serviceswhich could be offered on a public data network, e.g., circuit switching services (synch
9、ronous and start/stop), telexservice, packet switching services, message registration and retransmission service and facsimile service.2 Line of demarcationThe interface between DTE and DCE is located at a connector which is the interchange point between thesetwo classes of equipment shown in Figure
10、 1/X.24.2 Fascicle VIII.2 - Rec. X.242.1 The connector will not necessarily be physically attached to the DCE and may be mounted in a fixed positionnear the DTE. The female part of the connector belongs to the DCE.2.2 An interconnecting cable will normally be provided together with the DTE. The cabl
11、e length is limited byelectrical parameters specified in the appropriate Recommendations for the electrical characteristics of the interchangecircuits.3 Definition of interchange circuitsA list of the data network series interchange circuits is presented in tabular form in Table 1/X.24.TABLE 1/X.24D
12、ata network interchange circuitsInterchange circuit Interchange circuit nameData Control TimingdesignationFromDCEToDCEFromDCEToDCEFromDCEToDCEGGaGbTRCISBFXSignal ground or common returnDTE common returnDCE common returnTransmitReceiveControlIndicationSignal element timingByte timingFrame start ident
13、ificationDTE signal element timingXXXXXXXXXXXX3.1 Circuit G - Signal ground or common returnThis conductor establishes the signal common reference potential for unbalanced double-current interchangecircuits with electrical characteristics according to Recommendation V.28. In the case of interchange
14、circuits accordingto Recommendations V.10 and V.11, it interconnects the zero volt reference points of a generator and a receiver toreduce environmental signal interference, if required.Fascicle VIII.2 - Rec. X.24 3Within the DCE, this conductor shall be brought to one point, protective ground or ea
15、rth, by means of ametallic strap within the equipment. This metallic strap can be connected or removed at installation, as may berequired, to minimize the introduction of noise into electronic circuitry or to meet applicable regulations.Note - Where a shielded interconnecting cable is used at the in
16、terface, the shield may be connected either tocircuit G, or to protective ground in accordance with national regulations. Protective ground may be further connectedto external grounds as required by applicable electrical safety regulations.For unbalanced interchange circuits with electrical characte
17、ristics in accordance with Recommendation V.10,two common-return conductors are required, one for each direction of signalling, each conductor being connected toground only on the generator side of the interface. Where used, these shall be designated circuits Ga and Gb, and theyare defined as follow
18、s:Circuit Ga - DTE common returnThis conductor is connected to the DTE circuit common and is used as the reference potential for theunbalanced X.26 type interchange circuit receivers within the DCE.Circuit Gb - DCE common returnThis conductor is connected to the DCE circuit common and is used as the
19、 reference potential for theunbalanced X.26 type interchange circuit receivers within the DTE.3.2 Circuit T - TransmitDirection: To DCEThe binary signals originated by the DTE to be transmitted during the data transfer phase via the data circuit toone or more remote DTEs are transferred on this circ
20、uit to the DCE.This circuit also transfers the call control signals originated by the DTE, to be transmitted to the DCE in thecall establishment and other call control phases as specified by the relevant Recommendations for the proceduralcharacteristic of the interface.The DCE monitors this circuit
21、for detection of electrical circuit fault conditions, according to thespecifications of the electrical characteristics of the interface. A circuit fault is to be interpreted by the DCE as definedin the Recommendation for the procedural characteristics of the interface.3.3 Circuit R - ReceiveDirectio
22、n: From DCEThe binary signals sent by the DCE as received during the data transfer phase from a remote DTE, aretransferred on this circuit to the DTE.This circuit also transfers the call control signals sent by the DCE as received during the call establishmentand other call control phases as specifi
23、ed by the relevant Recommendations for the procedural characteristics of theinterface.The DTE monitors this circuit for detection of electrical circuit fault conditions, according to the specificationsof the electrical characteristics of the interface. A circuit fault is to be interpreted by the DTE
24、 as defined in theRecommendation for the procedural characteristics of the interface.4 Fascicle VIII.2 - Rec. X.243.4 Circuit C - ControlDirection: To DCESignals on this circuit control the DCE for a particular signalling process.Representation of a control signal requires additional coding of circu
25、it T-Transmit as specified in the relevantRecommendation for the procedural characteristics of the interface. During the data phase, this circuit shall remain ON.During the call control phases, the condition of this circuit shall be as specified in the relevant Recommendation for theprocedural chara
26、cteristics of the interface.Note - After appropriate selection of special user facilities (not yet defined), it might be required to change theON condition after entering the data phase in accordance with the regulations for the use of these facilities. This subjectis for further study.The DCE monit
27、ors this circuit for detection of electrical circuit fault conditions, according to thespecifications of the electrical characteristics of the interface. A circuit fault is to be interpreted by the DCE as definedin the Recommendation for the procedural characteristics of the interface.3.5 Circuit I
28、- IndicationDirection: From DCESignals on this circuit indicate to the DTE the state of the call control process.Representation of a control signal requires additional coding of circuit R-Receive, as specified in the relevantRecommendation for the procedural characteristics of the interface. The ON
29、condition of this circuit signifies thatsignals on circuit R contain information from the distant DTE. The OFF condition signifies a control signallingcondition which is defined by the bit sequence on circuit R as specified by the procedural characteristics of theinterface.The DTE monitors this circ
30、uit for detection of electrical circuit fault conditions, according to the specificationsof the electrical characteristics of the interface. A circuit fault is to be interpreted by the DTE as defined in theRecommendation for the procedural characteristics of the interface.Note - For use with special
31、 user facilities (not yet defined) it might be required to use the OFF condition afterentering the data transfer phase in accordance with the regulations for the use of these facilities. This subject is forfurther study.3.6 Circuit S - Signal element timingDirection: From DCESignals on this circuit
32、provide the DTE with signal element timing information. The condition of this circuitshall be ON and OFF for nominally equal periods of time. However, for burst isochronous operations, longer periodsof OFF condition may be permitted equal to an integer odd number of the nominal period of the ON cond
33、ition asspecified by the relevant procedural characteristics of the interface.The DTE shall present a binary signal on circuit T-Transmit and a condition on circuit C-Control, in which thetransitions nominally occur at the time of the transitions from OFF to ON condition of this circuit.The DCE pres
34、ents a binary signal on circuit R-Receive and a condition on circuit I-Indication in which thetransitions nominally occur at the time of the transitions from OFF to ON condition of this circuit.The transition from ON to OFF condition shall nominally indicate the centre of each signal element on circ
35、uitR.The DCE shall transfer signal element timing information on this circuit across the interface at all times thatthe timing source is capable of generating this information.Fascicle VIII.2 - Rec. X.24 53.7 Circuit B - Byte timing (see Note 2)Direction: From DCESignals on this circuit provide the
36、DTE with 8-bit byte timing information. The condition of this circuit shallbe OFF for nominally the period of the ON condition of circuit S-Signal element timing which indicates the last bit ofan8-bit byte and shall be ON at all other times within the period of the 8-bit byte.During the call control
37、 phases, the call control characters and steady state conditions used for all informationtransfers between the DCE and the DTE, in either direction, shall be correctly aligned to the signals of circuit B.The DTE shall present the beginning of the first bit of each call control character on circuit T
38、-Transmitnominally at the time of the OFF to ON transition of circuit S which follows the OFF to ON transition of circuit B.A change of condition of circuit C-Control may occur at any OFF to ON transition of circuit S, but it will besampled in the DCE at the time of the OFF to ON transition of circu
39、it B, i.e., for evaluation of the following callcontrol character on circuit T.The centre of the last bit of each call control character will be presented by the DCE on circuit R-Receivenominally at the time of the OFF to ON transition of circuit B.A change of condition of circuit I-Indication will
40、occur nominally at the OFF to ON transition of circuit Swhich follows the OFF to ON transition of circuit B.The DCE shall transfer byte timing information on this circuit across the interface at all times that the timingsource is capable of generating this information.Note 1 - During the data transf
41、er phase, DTEs communicating by means of an 8-bit code may utilize the bytetiming information for mutual character alignment.It is a prerequisite for the provision of this feature that character alignment is preserved after the call has entered thedata transfer phase and that the alignment obtained
42、at one interface is synchronized to the alignment at the otherinterface. (This is only possible on some connections.)Furthermore, where this feature is available, a change of condition on circuit C as defined above may result in anequivalent change in the relative alignment on circuit I at the dista
43、nt interface.Note 2 - In some Recommendations for the procedural characteristics of the interface (e.g., X.21), the use andtermination of this circuit by the DTE is not mandatory even when implemented in the DCE.3.8 Circuit F - Frame start identificationDirection: From DCESignals on this circuit con
44、tinuously provide the DTE with a multiplex frame start indication when connectedto a multiplexed DTE/DCE interface.The condition on this circuit shall be OFF for the nominal period of one bit, indicating the last bit of themultiplex frame. At other times the circuit shall remain ON.The first data bi
45、t on subscriber channel 1 shall be transmitted or received beginning nominally at the OFF toON transition of circuit F.6 Fascicle VIII.2 - Rec. X.243.9 Circuit X - DTE transmit signal element timing (see Note)Direction: To DCESignals on this circuit provide signal element timing information for the
46、transmit direction in cases wherecircuit S only provides signal element timing for the receive direction. The condition of this circuit shall be ON andOFF for nominally equal periods of time. However, for burst isochronous operations, longer periods of OFF conditionmay be permitted equal to an integ
47、er odd number of the nominal period of the ON condition as specified by therelevant procedural characteristics of the interface.The DTE shall present a binary signal on the circuit T-Transmit and a condition on circuit C-Control, in whichthe transitions nominally occur at the time of the transitions from OFF to ON condition of this circuit.The transition from ON to OFF condition shall nominally indicate the centre of each signal element on circuitT.Note - The use and termination of this circuit by the DCE is a national matter.
