1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 8 TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU05“,)#G0G0$!4!G0G0.%47/2+3).4%2 amended at Geneva, 1976 and 1980,Malaga-Torremolinos, 1984 and Melbourne, 1988)The CCITT,considering(a) that Recommendations X.1 and X.2 define the services and facilities to b
2、e provided by a public datanetwork;(b) that Recommendation X.92 defines the hypothetical reference connections for public synchronous datanetworks;(c) that Recommendation X.96 defines call progress signals;(d) that the necessary elements for an interface Recommendation should be defined in architect
3、ural levels;(e) that it is desirable for characteristics of the interface between the DTE and DCE of a public data networkto be standardized,unanimously recommendsthat the interface between the DTE and DCE in public data networks for user classes of service employingstart-stop transmission should be
4、 as defined in this Recommendation.1 Scope1.1 This Recommendation defines the physical characteristics and call control procedures for a general purposeinterface between DTE and DCE for user classes of service, as defined in Recommendation X.1, employing start-stoptransmission.1.2 The formats and pr
5、ocedures for selection, call progress and DCE provided information are included in thisRecommendation.1.3 The provision for duplex operation is covered.2 DTE/DCE physical interface elements2.1 Interchange circuitsA list of the interchange circuits concerned is presented in Table 1/X.20. Definitions
6、of these interchangecircuits are given in Recommendation X.24.2 Fascicle VIII.2 - Rec. X.20TABLE 1/X.20Interchange circuit Interchange circuit Directionnameto DCE from DCEG(see Note)GaGbTRSignal ground orcommon returnDTE common returnDCE common returnTransmitReceiveXXXXNote - This conductor may be u
7、sed to reduce environmental signal interference at the interface. Incase of shielded interconnecting cable, the additional connection considerations are part ofRecommendations X.24 and ISO 4903.2.2 Electrical characteristicsThe electrical characteristics of the interchange circuits at the DCE side o
8、f the interface will comply withRecommendation X.26.The electrical characteristics at the DTE side of the interface may be applied according toRecommendations X.26, X.27 (without cable termination in the load), or Recommendation V.28.For interworking between a V.28-DTE and a X.26-DCE refer to Recomm
9、endation X.26 and ISO 4903.2.3 Mechanical characteristicsRefer to ISO 4903 (15-pole DTE/DCE interface connector and contact number assignments) for mechanicalarrangements.2.4 Fault conditions of interchange circuitsFor the association of the receiver circuit-failure detection to particular interchan
10、ge circuits in accordance withthe type of failure detection, see Recommendation X.26, 11 and Recommendation X.27, 9.2.4.1 Circuit R in failure stateThe DTE should interpret a fault condition on circuit R as r = 0, using failure detection type 2. When theelectrical characteristics are applied accordi
11、ng to Recommendation V.28, the DTE should interpret generator in power-off condition or open-circuited interconnecting cable as a binary 0.2.4.2 Circuit T in failure stateThe DCE will interpret a fault condition on circuit T as t = 0, using failure detection type 2.3 Call control characters and erro
12、r checkingAll characters for call control purposes are selected from International Alphabet No. 5 according toRecommendation T.50.Even parity according to Recommendation X.4 applies for IA5 characters interchanged for call controlpurposes.Fascicle VIII.2 - Rec. X.20 34 Elements of the call control p
13、hase for circuit switched serviceThe state diagram provided in Figure A-1/X.20 shows the relationship between the various call control phasestates as defined below, together with the recognized transactions between these states under normal operatingconditions. Illustrated examples of the time seque
14、nce relationships between these states and associated time-outoperation are provided in Figure B-1/X.20.The call control phase can be terminated by either the DTE or the DCE by clearing as defined in 6 below.4.1 Events of the call control procedures(See Figure A-1/X.20.)4.1.1 Ready (state 1)Circuits
15、 T and R show binary 0.4.1.2 Call request (state 2)The calling DTE shall indicate a request for a call by signalling steady binary condition t = 1 provided that itwas previously signalling DTE ready (t = 0).4.1.3 Proceed-to-select (state 3)When the network is prepared to receive selection informatio
16、n, the DCE will signal steady binary conditionr = 1.The proceed-to-select signal will start within 6 seconds of the call request being sent.4.1.4 Selection signal sequence (state 4)The selection signal sequence shall be transmitted by the DTE on circuit T.The format of selection signal sequence is d
17、efined in 4.6.1 below.The information content and coding of the selection signal sequence is contained in Annex G andRecommendation X.121.The selection signal sequence shall start within 6 seconds of the proceed-to-select being received and shall becompleted within 36 seconds.The maximum permissible
18、 interval between individual selection characters is 6 seconds.4.1.5 DTE waiting (state 5)During DTE waiting, the DTE signals steady binary condition t = 1.4.1.6 Incoming call (state 8)The DCE will indicate an incoming call by signalling steady binary condition r = 1.4.1.7 Call accepted (state 9)The
19、 DTE shall accept the incoming call not later than 600 ms by signalling the steady state binary conditiont = 1.10-100 ms thereafter, the DTE transmits the call control character 0/6 (ACK).4 Fascicle VIII.2 - Rec. X.204.1.8 Call not accepted (state 18)If the DTE does not wish to accept the incoming c
20、all it shall signal this not later than 600 ms by changingcircuit T to steady binary condition 1.10-100 ms thereafter, the DTE shall transmit the call control character 1/5 (NAK) followed by DTE clearrequest (state 13).4.1.9 Call progress signal sequence (state 6)The call progress signal sequence wi
21、ll be transmitted by the DCE to the calling DTE on the R circuit when anappropriate condition is encountered by the network.A call progress signal sequence will consist of one or more call progress signal blocks. A call progress signalblock will consist of one or more call progress signals.The forma
22、t of the call progress signal sequence is defined in 4.6.2 below.The coding of call progress signals is provided in Annex E.The description of call progress signals is provided in Recommendation X.96.A call progress signal sequence will be transmitted by the DCE within 60 seconds of: 1) the end-of-s
23、electionsignal or 2) in case of direct call, the proceed-to-select signal being sent by the DTE. The call progress signalsequence, however, will not be sent by the DCE before the reception of the end-of-selection signal except in the case ofexpiration of time-outs described in 4.1.4 where there may
24、be a call progress signal sequence followed by the clearindication.Note - When an error is detected in a received call progress signal sequence, the DTE may choose to either ignore thesignal or attempt a new call after clearing.4.1.10 DCE provided information sequence (states 7A and 7B)The DCE provi
25、ded information sequences will be transmitted by a DCE to the calling DTE (state 7A) or acalled DTE (state 7B) on circuit R.A DCE provided information sequence will consist of 1 or more DCE provided information blocks. Each DCEprovided information block will be limited to a maximum length of 128 cha
26、racters.The format of the DCE provided information sequences is defined in 4.6.3 below.The information content of DCE provided information is given in Annex G.A DCE provided information sequence (state 7B) will be sent to the called DTE within 60 seconds of the callaccepted signal being sent.4.1.10.
27、1 Line identificationCalling and called line identification is an optional additional facility.The information content of calling and called line identification is provided in Annex G.Calling and called line identification will be transmitted by the DCE on the R circuit during states 7B or 7Arespect
28、ively.When provided, called line identification (state 7A) will be transmitted by the DCE to the calling DTE after allcall progress signals, if any.When provided, calling line identification (state 7B) will be transmitted by the DCE to the called DTE aftercall accepted has been sent by the DTE.Fasci
29、cle VIII.2 - Rec. X.20 5In the case where the calling line identification facility is not provided by the originating network, or thecalled line identification facility is not provided by the destination network, a dummy line identification shall beprovided by the DCE to the DTE.4.1.10.2 Charging in
30、formationCharging information is an optional additional facility provided during state 7B.Upon completion of clearing the call for which charging information has been requested, the DCE will, within200 ms after entering ready (state 1), establish an incoming call to the DTE for the purpose of giving
31、 charginginformation.Charging information will be transmitted by the DCE on circuit R.The DCE will send clear indication (state 16) upon sending the last charging information block. The DTEshould send clear request (state 13) when it has correctly received the charging information signal, if the DCE
32、 has notpreviously signalled the clear indication.The format of charging information is defined in 4.6.3 below.4.1.11 Connected (state 10)The DCE signals that the connection is being established by the transmission of the call control character 0/6(ACK) on circuit R. Because of possible switching de
33、lays in the network, the DTE must keep circuit T in steady binarycondition 1 during this state.4.1.12 Ready for data (state 11)20 ms after the reception of the call control character 0/6 (ACK) in state 10, the connection is available fordata transfer between both DTEs.4.1.13 Events of the call contr
34、ol procedures for multipoint circuit-switched service4.1.13.1 Ready (state 1)See 4.1.1.4.1.13.2 Call request (state 2)See 4.1.2.4.1.13.3 Proceed to select (state 3)See 4.1.3.4.1.13.4 Selection signal sequence (state 4)See 4.1.4.A facility request signal is used to indicate the category of the point
35、to multipoint service which is required.The coding is defined in Annex F.4.1.13.5 DTE waiting (state 5)See 4.1.5.4.1.13.6 Incoming call (state 8)See 4.1.6.6 Fascicle VIII.2 - Rec. X.204.1.13.7 Call accepted (state 9)See 4.1.7.4.1.13.8 Call not accepted (state 18)See 4.1.8.4.1.13.9 Call progress sign
36、al sequence (state 6)See 4.1.9.The call progress signals related to each of the called DTEs are transmitted and then, in the same order, thecalled line identification signals of the different called DTEs.When no specific call progress signals are necessary for a given called DTE, then the call progr
37、ess signal “00“is used for this DTE so that the order could be kept.4.1.13.10 DCE provided information sequence (states 7A and 7B)The DCE provided information sequences will be transmitted by a DCE to the calling DTE (state 7A) or acalled DTE (state 7B) on circuit R.A DCE provided information sequen
38、ce will consist of one or more DCE provided information blocks. EachDCE provided information block will be limited to a maximum length of 128 characters, except for called lineidentification in case of multipoint calls where the maximum length is 512 characters.The format of the DCE provided informa
39、tion sequences is defined in 4.6.3 below.The information content of DCE provided information is given in Annex G.A DCE provided information sequence will be sent to the called DTE within 60 seconds of the call acceptedsignal being sent.4.1.13.10.1 Line identificationSee 4.1.10.1.The called line iden
40、tification related to the different called DTEs are provided in sequence.4.1.13.10.2 Charging informationSee 4.1.10.2.4.1.13.11 Connected (state 10)See 4.1.11.4.1.13.12 Ready for data (state 11)See 4.1.12.4.2 Unsuccessful callIf the required connection cannot be established, the DCE will indicate th
41、is and the reason to the calling DTEby means of a call progress signal. Afterwards the DCE will signal DCE clear indication (state 16).4.3 Call collision (state 19)A call collision is detected by the DCE when it receives call request in response to an incoming call. The DCEmay either accept the call
42、 request or may perform DCE clearing.Fascicle VIII.2 - Rec. X.20 74.4 Direct callFor the direct call facility, selection signals (state 4) are always bypassed.Note - The direct call facility can only be provided on a subscription basis and not on a per-call basis.4.5 Facility registration/cancellati
43、on procedureRegistration/cancellation of optional user facilities shall be carried out by a DTE in accordance with normalcall establishment procedures using the selection sequence, which is defined in 4.6.1 below.The format of the facility registration/cancellation signal is defined in 4.6.1.3 below
44、.The facility registration/cancellation procedure shall not be combined with establishment of a normallyaddressed call, but shall be taken as an independent procedure.In response to acceptance or rejection of the facility registration/cancellation procedure, the network willprovide the appropriate c
45、all progress signal followed by clear indication.4.6 Selection, call progress and DCE provided information formats(See also Annex D for a syntactic description of the formats.)4.6.1 Format of selection sequenceA selection sequence shall consist of a facility request block, or an address block, or a
46、facility request blockfollowed by an address block, or a facility registration/cancellation block.4.6.1.1 Facility request blockA facility request block shall consist of one or more facility request signals.Multiple facility request signals shall be separated by character 2/12 (“,“).A facility reque
47、st signal shall consist of a facility request code and may contain one or more facilityparameters. The facility request code, facility parameter and subsequent facility parameters shall be separated bycharacter 2/15 (“ / “). For an interim period the 2/15 (“ / “) separator will not be used in some n
48、etworks.The end of a facility request block shall be indicated by character 2/13 (“ - “).The coding of facility request, indicator and parameter is provided in Annex F.4.6.1.2 Address blockAn address block shall consist of one or more address signals.An address signal shall consist of either a full
49、address signal or an abbreviated address signal.Start of an abbreviated address signal shall be indicated by a prefix character 2/14 (“ . “).Multiple address signals shall be separated by character 2/12 (“ , “).4.6.1.3 Facility registration/cancellation blockA facility registration/cancellation block shall consist of one or more facility registration/cancellation signals.A facility registration/cancellation signal shall consist of up to four elements in order: facility request code,indicator, registration parameter, address signal.8 Fascicle VII
