C:\WINWORD\CCITTREC.DOT_______________ ANNEX B (to Recommendation X.21) Interface signalling sequence diagrams and time–out operations Definition of symbol used to illustrate time–out operation in the signalling sequence diagrams: CCITT-25052 FIGURE B-1/X.21 CCITT-25064 FIGURE B-2/X.21 CCITT-25074 FIGURE B-3/X.21 CCITT-61411 ANNEX C (to Recommendation X.21) DTE time–limits and DCE time–outs C.1 DTE time–limits Under certain circumstances this Recommendation requires the DCE to respond to a signal from the DTE within a stated maximum time. If any of these maximum times is exceeded, the DTE should initiate the action indi- cated in Table C–1/X.21. To maximize efficiency, the DTE should incorpo- rate time–limits to send the appropriate signal under the defined circumstances summarized in Table C–1/X.21. The time–limits given in the first column are the maximum times allowed for the DCE to respond and are consequently the lower limits of the times a DTE must allow for proper net- work operation. A time–limit longer than the time shown may be optionally used in the DTE; for example, all DTE time–limits could have one single value equal to or greater than the longest time–limit shown in this table. However, the use of a longer time–limit will result in reduced efficiency of network utilization. The actual DCE response time should be as short as is consistent with the implementing technology and in normal operation should be well within the specified time–limit. The rare situation where a time–limit is exceeded should only occur when there is a failure in DCE operation. The time–limits and actions for loop testing are given in Table C–3/ X.21. C.2 DCE Time–outs Under certain circumstances this Recommendation requires the DTE to respond to a signal from the DCE within a stated maximum time. If any of these maximum times is exceeded, a time–out in the DCE will initiate the actions summarized in Table C–2/X.21. These constraints must be taken into account in the DTE design. The time–outs given in the first column of the table are the minimum time–out values used in the DCE for the appro- priate DTE response and are consequently the maximum times available to the DTE for response to the indicated DCE action. The actual DTE response time should be as short as is consistent with the implementing technology and in normal operation should be within the specified time–out. The rare situation where a time–out is exceeded should only occur when there is a failure in the DTE operation. The time–outs and actions for loop testing are given in Table C–4/ X.21. TABLE C–1/X.21 DTE time–limits Time– limit Time– limit number Started by Normally terminated by Preferred action to be taken when time–limit exceeded 3 s T1 Signalling of call request (state 2) Reception of pro- ceed–to–select (state 3) DTE signals DTE ready (state 1) 20 s T2 Signalling end– of–selection or DTE waiting (direct call) (state 5) Reception of call progress signals, DCE provided information, ready for data or DCE clear indica- tion (states 7, 10A, 12 or 19) DTE signals DTE clear request (state 16) 6 s T3A Reception of call progress signals or DCE–provided information (states 7 or 10A). Restarted by additional call progress signals or DCE–pro- vided informa- tion (states 7 or 10A) (see Note 2) Reception of ready for data or DCE clear indica- tion (states 12 or 19) DTE signals DTE clear request (state 16) 60 s T3B (see Note 1) Reception of applicable call progress signals (state 7). Restarted by additional call progress signals or DCE pro- vided informa- tion (states 7 or 10A) (see Note 2) Reception of ready for data or DCE clear indica- tion (states 12 or 19) DTE signals DTE clear request (state 16) 2 s T4A Signalling of proceed with call information (state 9B). Restarted by reception of call information (state 10C) (see Note 2) Reception of end of call informa- tion character or DCE clear indica- tion (state 19) or DCE waiting (state 6C) DTE signals DTE clear request (state 16) 6 s T4B Signalling of call accepted (states 9 and 9C). Restarted by reception of DCE–provided information (state 10B) (see Note 2) Reception of ready for data or DCE clear indica- tion (states 12 or 19) DTE signals DTE clear request (state 16) TABLE C–1/X.21 (cont.) Time– limit Time– limit number Started by Normally termi- nated by Preferred action to be taken when time–limit exceeded 2 s T5 Change of state to DTE clear request (state 16) Change of state to DCE read11y (state 21) DTE regards the DCE as DCE not ready and signals 2 s T6 Change of state to DTE clear confirmation (state 20) Reception of DCE ready (state 21) DTE ready (state 18) 0,2 s T7 Change of state to ready (state 1) when charge information (state 10B) has been requested Reception of incoming call (state 8) DTE returns to nor- mal operation and may note absence of charge information (state 10B) Note 1 – T38 shall be used when receiving CPS 01 and may also be used when receiving other CPS in code group 0. Note 2 – Restart means time–out set to ZERO and restarted. TABLE C–2/X.21 DCE time–outs Time– out Time– out number Started by Normally termi- nated by Action to be taken when time–out expires 36 s T11 DCE signalling of proceed–to– select (state 3) DCE reception of end–of–selection signal or in the case of direct call, DTE waiting (state 5) DCE will signal DCE clear indica- tion (state 19) or 6 s T12 DCE signalling of proceed–to– select (state 3) DCE reception of first selection character or in the case of direct call, DTE waiting (state 5) transmit appropri- ate call progress signals (state 7) followed by DCE clear indication (state 19) 6 s T13 DCE reception of nth selection character (state 4) DCE reception of (n + 1)th selection character or end– of–selection sig- nal 0,5 s T14A DCE signalling of incoming call (state 8) Signalling of pro- ceed with call information (state 9B) or call The DTE is noted as not answering. The DCE will signal 60 s T14B (see Note 1) accepted (state 9) or call collision (state 15) ready (state 1) 20 s T14C (see Note 2) DCE transmit- ting end of call information character. Restarted by state 25 DTE– provided infor- mation (see Note 3) Change of state to call accepted (state 9C) DCE will signal DCE clear indica- tion (state 19) or transmit appropri- ate call progress sig- nals followed by DCE clear indica- tion (state 19) 0,5 s T15 Change of state to DCE clear indication (state 19) Change of state to DTE clear con- firmation (state 20) DCE will signal DCE ready and mark DTE uncon- trolled not ready (state 24) 100 ms T16 Change of state to DCE ready (state 21) Change of state to ready (state 1) DCE will mark DTE uncontrolled not ready (state 24) Note 1 – T14B will be provided when manual answering DTEs are allowed. It is not envisaged that manual answering DTEs will use the enhanced sub– addressing procedure (see § 4.1.6.2.2). Note 2 – T14C applies only to enhanced sub–addressing. Note 3 – Restart means time–out set to ZERO and restarted. TABLE C–3/X.21 DTE time–limits for loop testing Time– limit Time– limit number Started by Normally termi- nated by Preferred action to be taken when time–limit expires 2 s T20A DTE sending the loop com- mand (state L21) DCE signalling loop confirma- tion (state L22) DTE stops sending loop command and enters any state 6 s T20B DTE sending the loop com- mand (state L31) DCE signalling loop confirma- tion (state L32) or signals DTE clear request (state 16) 2 s T21 DTE sending loop 2 clear request (state L24A or L24B) DCE signalling loop 2 clear con- firmation (state L25) DTE signals DTE clear request (state 16) TABLE C–4/X.21 DCE time–outs for loop testing Time–out Time– out number Started by Normally ter- minated by Action to be taken when time–out expires Duration is a national option T22 DCE signalling receive loop 2 command (state L26) DCE reception of loop released (state L28) or test data (state L23) or DTE clear confirma- tion (state 20) DCE at the tested side causes the con- nection to be cleared Duration is a national option T23 Test data (state L23) being received by the tested DCE Loop released (state L28) or DTE clear con- firmation (state 20) being received by the tested DCE DCE at the tested side causes the con- nection to be cleared ANNEX D (to Recommendation X.21) Formats of selection, call progress, and DCE–provided information signals The following description uses Backus Normal Form as the formalism for syntactic description. A vertical line “ | ” separates alternatives. <*> : : = IA 5 character 2/10 <+> : : = IA 5 character 2/11 < , > : : = IA 5 character 2/12 <–> : : = IA 5 character 2/13 < . > : : = IA 5 character 2/14 : : = IA 5 character 2/15 : : = IA 5 characters 3/0–3/9 <:> : : = IA 5 character 3/10 : : = See Annex G : : = See Annex G : : = See Recommendation X.121 : : = National option : : = See Annex H : : = See Annex H : : = See Annexes F and G : : = See Annex G : : = See Annex G : : = See Annex F : : = See Annex H : : = See Annex F The above signals are combined as follows:
: : = | < . >
: : =
|
< , >
: : =
: : = | < , > : : = | : : = | < , > : : = <–>
<+> | <–> <+> |
<+> | <–> <+> : : = | <–> : : = <+> | < , > : : = <*> <+> : : = <**> <+> : : = | < , > : : = <*> <+> : : = <**> <+> : : = <*> <+> : : = <+> | <,> (Note) : : = <+> | <,> Note – For DCE–provided information signals and blocks other than calling or called line identification signals and blocks. ANNEX E (to Recommendation X.21) Interworking between DTEs conforming to Recommendations X.21 and X.21 bis It is recognized that interworking between V–Series DTEs connected to a public data network according to Recommendation X.21 bis at one end and Recommendation X.21 at the other end should always be possible for DTEs not using half–duplex operation. Certain Administrations may provide facilities allowing interworking between DTEs interworking between DTEs operating in accordance with Recommendations X.21 and X.21 bis using half–duplex operation by switching circuit C, I and circuit 109, 105 during the data transfer phase in accordance with Figure E–1/X.21. Those Administrations not providing this facility shall cause the Recom- mendation X.21 DCE to signal r = 1, i = ON when the Recommendation X.21 bis DTE signals circuit 105 OFF. This will permit half–duplex opera- tion for those DTEs that do not require circuit 109 to be OFF before signal- ling circuit 105 ON. FIGURE E-1/X.21 CCITT-25080 ANNEX F (to Recommendation X.21) TABLE F–1/X.21 Coding of call progress signals and DTE provided information Code group (see Note 1) Code Indicator Significance Category 0 00 01 02 03 04 05 – – – – – – Note 2 Terminal called Redirected call Connect when free Private net- work reached (see Note 3) Public network reached (see Note 4) Without clearing 2 20 21 22 23 – – – – No connection Number busy Selection sig- nals procedure error Selection signals trans- mission error With clear- ing due to short–term conditions 3 Unassigned 4 and 5 41 42 43 44 45 45 46 47 48 49 51 52 – – – – – YY–MM–DD– hh:mm – – – – – – Access barred Changed num- ber Not obtainable Out of order Controlled not ready DTE inactive until . . . Uncontrolled not ready DCE power off Invalid facility request Net- work fault in local loop Call information service Incom- patible user class of service With clearing due to long– term condi- tions 6 61 – Network con- gestion With clear- ing due to network short–term conditions 7 71 72 – – Long–term net- work conges- tion RPOA out of order With clear- ing due to network long–term conditions 8 81 82 83 – – – Registration/ cancellation confirmed Redirection activated Redi- rection deacti- vated With clear- ing due to network pro- cedure 9 Reserved for national purposes Notes to Table F–1/X.21: Note 1 – From the DTE point of view group 0 means “wait”, groups 2 and 6 mean “try again, next try may result in a call set–up”, groups 4 and 5, and 7 mean “there is no reason for the DTE to try again because the answer will be the same for a longer period of time”. Since group 8 results from a proce- dure between the DTE and the network, no special action is expected to be taken by the DTE. Some Administrations may specify by regulation both the delay between and the maximum number of call re–attempts permitted by a DTE in these circumstances (see Recommendation X.96). Note 2 – Reserved for use in case of point–to–multipoint calls. This coding is used for a remote DTE to indicate that the call can be established with this one (i.e. no clearing due to the remote DTE or to the network), so that the same order of sequence of call progress signals and called lines identifica- tion can be kept. Note 3 – In the case of sub–addressing, the DTE may see call progress sig- nals which have originated in the public network and/or the private network. In such cases the call progress signal private network reached shall be used to discriminate between the different origins. Note 4 – In the case of a DTE being connected to a private network which also provides for access to a public network, the DTE may see call progress signals which have originated in the private network and/or the public net- work. In such cases the call progress signal public network reached should be used by the private network in order to discriminate between the different origins. ANNEX G (to Recommendation X.21) Facility request, indicator and parameter coding (for use as appropriate in facility request signals and facility registration/ cancellation signals) TABLE G–1/X.21 (see Annex D for formats and Note 1 below) Facility request code Facil- ity paramet er Indic ator Registration parameter Addr ess signa l Facility 0 – – – – Reserved for future use (may be com- bined with second character) 1 XX (see Note 2) – – – Closed user group (other than preferen- tial) 2 – – – – Unassigned 3 – – – – Unassigned 45 – 1 YY–MM– DD–hh:mm – DTE inactive regis- tration 45 – 2 – – DTE inactive cancel- lation 50 – – – – Reserved 51 – – – – Reserved 53 – – – – Reserved 60 0, 1, 2, 3, 4, 5 – – – Multiple address calling 61 – – – – Charging informa- tion 62 – – – – Called line identifi- cation 63 – 1 – – Redirection of call activation 63 – 2 – – Redirection of call cancellation 63 – 3 – – Redirection of call status 64 – – – – Reverse charging 65 – 1 – AS Direct call registra- tion 65 – 2 – – Direct call cancella- tion 66 – 1 AAS AS Abbreviated address registration 66 – 2 AAS – Abbreviated address cancellation 68 – – – – Reserved 7 – – – – Reserved 8 – – – – Reserved 9 Reserved for national purposes AAS abbreviated address signal. AS address signal. Note 1 – For an interim period, the 2/15 “/” separator in the formats will not be used in some networks. Note 2 – XX is an index number, i.e., a key code for closed user group other than the preferential group. The index number shall be used to distinguish between parts or groups within one facility. The index number shall further- more be chosen from IA5, column 3, positions 3/0–3/9, giving a range of possible numbers from 00 to 99. G.1 Multiple address calling This facility provides the calling DTE with the capability to request a category of point to multipoint service. The coding is as follows: <60> <–>
<+> where h is a numerical character with the following signficance: 0 Reserved 1 Reserved 2 Reserved 3 Centralized multipoint 4 Reserved 5 Reserved. G.2 Charging information This facility enables the calling DTE to request at the call establish- ment phase that charging information for the call be provided at the end of the call. <61> <–>
<+> G.3 Redirection of call This facility enables the DTE to request the network to route its incoming calls towards another address. The use of this facility is assigned for an agreed contractual period. Activation of redirection of call – The activation of this facility is coded as follows: <63> <1> <–> <+> Cancellation of redirection of call – The cancellation of this facility is coded as follows: <63> <2> <–> <+> Status of redirection of call – The DTE has the capability to ask the network for the status of its redirection. The coding is as follows: <63> <3> <–> <+> G.4 Reverse charging This facility enables the calling DTE to request that reverse charging be applied for the call. The coding is as follows: <64> <–>
<+> G.5 Direct call on a per call basis This facility enables the DTE to designate the address to which all calls will be established when the selection signals phase (state 4) is bypassed during call establishment. Registration – The coding of this registration of the address is as fol- lows: <65> <1> <0>
<–> <+> where
is the designated address. The DTE is able to amend the designated address by performing a cancellation procedure followed by the registration procedure. The coding of the cancellation procedure is as follows: <65> <2> <–> <+> G.6 Abbreviated address calling This facility enables the DTE to define a full address by an abbrevi- ated address. The registration coding of an abbreviated address is as follows: <66> <1>
<–> <+> where = abbreviated address corresponding to the full address,
= full address. Cancellation – The coding of the cancellation of an abbreviated address is as follows: <66> <2> <–> <+> where is the abbreviated address. G.7 Closed user group selection This facility provides the calling DTE with the possibility to commu- nicate within more than one closed user group. The coding is as follows: <1> <–>
<+> where is the closed user group index number, i.e. the key code for the closed user group other than the preferential group. The index number shall be used to distinguish between parts or groups within one facility. The index numbers are numerical characters chosen from column 3 of IA5. G.8 DTE inactive registration/cancellation This facility enables the DTE to inform the network about a period of time during which the DTE is unable to accept incoming calls for circuit– switched service. DTE inactive registration – The activation of this facility is as follows: <45> <1> <–> <+> where YY: Year, MM: Month, DD: Day, hh: Hour, mm: Minute IA5 characters are used for “YY”, “MM”, “DD”, “hh”, “mm”, “–”, and “:”. DTE inactive cancellation is as follows: <45> <2> <–> <+> ANNEX H (to Recommendation X.21) Information content of DCE provided information H.0 General Except for the calling and called line identification, the general format for DCE–provided information, as defined in § 4.6.3 should apply. The coding of numerical character used to distinguish between differ- ent types of DCE–provided information is indicated in Table H–1./X.21. H.1 Information content of calling and called line identification Two formats are defined: i) Calling and called line identification consist of the international data number as defined in Recommendation X.121 preceded by two prefixes 2/10 (“ ** ”). In the case where the originating network does not provide calling line identification, only the data network identification code (DNIC) part of the International Data Number preceeded by two prefixes 2/10 (“ ** ”) may be sent in place of the dummy line identification. ii) Calling and called line identification consist of the national number (NN) or network terminal number (NTN) preceded by the prefix 2/ 10 (“ * ”). H.2 General coding of the DCE–provided information TABLE H–1/X.21 Coding of DCE provided information Identifier Meaning Remarks 0 Reserved 1 Charging information See details in § H.3 2 Charging information 3 Charging information 4 Sub–addressing See details in § H.4 5 Date and time indication See details in § H.5 6 Characteristics of the call See details in § H.6 7 Type of call indication See details in § H.7 81 Closed user group indi- cation See details in § H.8 82 Closed user group outgo- ing access indication See details in § H.8.1 9 Reserved H.3 Information content of charging information The charging information will inform the subscriber of either the monetary charges for a call, the duration of the call, or the number of units used during the call. When charging information is given in monetary charges for the call, n = 1 and the information shall consist of x number of integer digits option- ally followed by a colon and two digits representing the fraction. The format applied is as follows: <1> <1> <:> When the charging information is presented as the duration of a call, n = 2 and the information shall consist of x number of integer digits represent- ing seconds, the format applied is as follows: <2> When the charging information is presented as the number of units used, n = 3, and the information shall consist of x number of integer digits representing the units, the format applied is as follows: <3> H.4 Sub–addressing information The sub–addressing information will inform the called DTE of the sub–address sent by the calling DTE. The format of the sub–addressing information is as follows: <4> The format of the dummy information is as follows: <4> H.5 Date and time indication The date and time indication will inform the subscriber of the date and time the call is established. The format for the date and time indication is as follows: <5> where YY: Year, MM: Month, DD: day, hh: hour and mm: minute IA5 characters are used for “YY”, “MM”, “DD”, “hh”, “mm”, “–” and “:”. H.6 Characteristics of the call The characteristics of the call will inform the called DTE of the differ- ent facilities that have been requested by the calling DTE. The format of the characteristic of the call is as follows: <6> where X and Y are two numerical characters. Table H–2/X.21 indicates the allocation of values of these two charac- ters to facilities. TABLE H–2/X.21 00 Reserved 01 Reverse charging 02 Reserved 03 Reserved H.7 Type of call indication The type of call indication will inform the called DTE of the configu- ration of the incoming call. The format of the type of call indication is as follows: <7> where X and Y are two numerical characters. Table H–3/X.21 indicates the allocation of values of those two charac- ters to different configurations of calls. TABLE H–3/X.21 00 Reserved 01 Reserved 02 Reserved 03 Centralized mul- tipoint 04 Reserved H.8 Closed user group indication The closed user group indication will inform the called DTE to which closed user group the incoming call belongs. The format of the closed user group indication is as follows: <81> where : closed user group index number. H.8.1 Closed user group outgoing access indication The closed user group outgoing access indication will inform the called DTE from a DTE belonging to a closed user group with outgoing access facility. If the called DTE belongs to the same closed user group, the local closed user group index number will be indicated. In other cases, no indication will be given. The format of the closed user group outgoing access indication is as follows: <82> where is the closed user group index number. ANNEX I (of Recommendation X.21) Reference and transition tables TABLE I–1/X.21 Cross reference of interchange circuit signals, states, and reference section T, C R, I State No. Reference in the Recom- mendation (§) 1, OFF 1, OFF 1 2.5.3.1 1, OFF 0, OFF 18 2.5.3.3 0, OFF 1, OFF 21, 24 2.5.3.6 0, OFF 0, OFF 17, 20, 22 2.5.3.4 1, OFF BEL, OFF 8 4.1.5 01, OFF 1, OFF 14 2.5.3.2 01, OFF 0, OFF 23 2.5.3.5 X, X 01, OFF L27 7.3.2.8 0011, OFF D, ON L21 7.3.2.1 0011, OFF 0011, OFF L22 7.3.2.2 *, OFF BEL, OFF 9B 4.1.6.2.2.1 *, OFF IA5, OFF 10C 4.1.6.2.2.2 IA5, OFF SYN, OFF 25 4.1.6.2.2.4 *, OFF SYN, OFF 6C 4.1.6.2.2.3 1, OFF D, ON 13R 5.2.2, 5.3.1.2, 5.3.2.2 0, OFF 1, ON 16 6.1 0, OFF 0, ON 16 6.1 0, OFF D, ON 16 6.1 1, ON 1, OFF 11 4.1.10 1, ON 0, OFF 19 6.2 0, ON 1, OFF 2 4.1.1 0, ON 0, OFF 19 6.2 1, ON BEL, OFF 9 4.1.6 1, ON +, OFF 5 4.1.4 1, ON SYN, OFF 6A, 6B, 9C 4.1.7 1, ON IA5, a) OFF 7, 10A, 10B 4.1.8, 4.1.9 0, ON BEL, OFF 15 4.3 0, ON +, OFF 3 4.1.2 IA5, ON +, OFF 4 4.1.3 D, ON 1, OFF 13S 5.2.1, 5.3.1.1, 5.3.2.1 D, ON 0, OFF 19 6.2, Fig. A– 3/X.21 1, ON 1, ON 12 4.1.1.1 D, ON D, ON 13 5.1, 5.2.3, 5.3.3 a) An IA5 character other than BEL.