C:\WINWORD\CCITTREC.DOT_______________ Recommendation X.3 PACKET ASSEMBLY DISASSEMBLY FACILITY (PAD) IN A PUBLIC DATA NETWORK (provisional, Geneva, 1977; amended at Geneva, 1980, Malaga–Torremolinos, 1984 and Melbourne 1988) Preface The establishment in various countries of public data networks pro- viding packet switched data transmission services creates a need to produce standards to facilitate access from the public telephone network, circuit switched public data networks and leased circuits. The CCITT, considering (a) that Recommendations X.1 and X.2 define the user classes of ser- vice and user facilities in public data networks, Recommendation X.96 defines call progress signals, Recommendation X.29 defines the procedures between a packet assembly/disassembly facility (PAD) and a packet mode DTE or another PAD, Recommendation X.28 defines the DTE/DCE inter- face for a start–stop mode DTE accessing the PAD; (b) that the logical control links for packet switched data transmission ser- vices are defined in Recommendation X.92, and that in particular Recom- mendation X.92 allows for the incorporation of a PAD; (c) the urgent need to allow interworking between a start–stop mode DTE on a public switched telephone network, a public switched data network or leased circuit, and a packet mode DTE or another start–stop mode DTE using the virtual call facility of the packet switched data service; (d) that DTEs operating in the start–stop mode will send and receive net- work control information and user information in the form of characters or the break signals; (e) that DTEs operating in the packet mode will send and receive network control information and user information in the form of packets in accor- dance with Recommendation X.25; (f) that the packet mode DTE shall not be obliged to use the control pro- cedures for PAD functions, but that some packet mode DTEs may wish to control specific functions of the PAD; unanimously declares (1) that the functions performed by, and operational characteristics of, the PAD for the start–stop mode DTE are described below in § 1, Descrip- tion of the basic functions and user selectable functions of the PAD; (2) that the operation of the PAD for the start–stop mode DTE should depend on the possible values of internal variables known as PAD parame- ters which are described below in § 2, Characteristics of PAD parameters; (3) that the PAD parameters for the start–stop mode DTE and their possi- ble values should be those which are listed below in § 3, List of PAD param- eter and possible values; (4) that the PAD features described in §§ 1, 2 and 3 below could be expanded by future studies to allow interworking with non packet mode DTEs other than start–stop mode DTEs. 1 Description of the basic functions and user selectable functions of the PAD 1.1 The PAD performs a number of functions and exhibits operational characteristics. Some of the functions allow either or both the start–stop mode DTE and the packet mode DTE (or remote PAD) to configure the PAD so that its operation is adapted to the start–stop mode DTE characteris- tics, and possible to the application. 1.2 The operation of the PAD depends on the value of the set of internal variables called PAD parameters. This set of parameters exists for each start–stop mode DTE independently. The current value of each PAD param- eter defines the operational characteristics of its related function. 1.3 Functions of the PAD 1.3.1 Basic functions include: – assembly of characters into packets; – disassembly of the user data field of packets; – handling of virtual call set–up and clearing, resetting and interrupt procedures; – generation of service signals; – a mechanism for forwarding packets when the proper conditions exist, e.g., when a packet is full or an idle timer expires; – a mechanism for transmitting data characters, including start, stop and parity elements as appropriate to the start–stop mode DTE; – a mechanism for handling a break signal from the start–stop mode DTE; – editing of PAD command signals; – a mechanism for setting and reading the current value of PAD parameters. 1.3.2 Optional functions include: – a mechanism for the selection of a standard profile; – automatic detection of data rate, code, parity and operational charac- teristics; – a mechanism for the remote DTE to request a virtual call between the start–stop mode DTE and another DTE. 1.4 User selectable functions which may be provided by the PAD A number of packet–switched data network facilities may be available either on a subscription basis or on a per call basis, to start–stop mode DTEs as described in Recommendation X.2 for user classes of service 20–22. In addition, the following features may be available on a subscription basis: – selection of an initial profile; – modem type, speed, code and parity to be used by the PAD; – other operational characteristics of the DTE. As defined in this Recommendation, parameters provide for functions which concern: – management of the procedure between the start–stop mode DTE and the PAD; – management of the assembly and disassembly of packets; – a number of additional functions related to the operational character- istics of the start–stop mode DTE. The method for the control of these functions is specified in Recom- mendation X.28 for the start–stop mode DTE and in Recommendation X.29 for the packet mode DTE or for another PAD. Table 1/X.3 shows details of the valid values and combination of val- ues of PAD parameters standardized by CCITT. Other values and combina- tions of values are for further study. 1.4.1 PAD recall using a character This function allows the start–stop mode DTE to initiate an escape from the data transfer state or the connection in progress state in order to send PAD command signals. 1.4.2 Echo This function provides for characters received from the start–stop mode DTE to be transmitted back to the start–stop mode DTE as well as being interpreted by the PAD. 1.4.3 Selection of the data forwarding characters This function allows the selection of defined sets of character(s) received from the start–stop mode DTE to be recognized by the PAD as an indication to complete the assembly and forward a complete packet sequence as defined in Recommendation X.25. 1.4.4 Selection of idle timer delay This function allows the selection of the duration of an interval between successive characters received from the start–stop mode DTE which, when exceeded, will cause the PAD to terminate the assembly of a packet and to forward it as defined in Recommendation X.255. 1.4.5 Ancillary device control This function allows for flow control between the PAD and the start– stop mode DTE. The PAD indicates whether it is ready or not to accept characters from the start–stop mode DTE by transmitting special characters. These characters are those which in International Alphabet No. 5 (IA5) are used to switch an ancillary transmitting device on and off. 1.4.6 Control of PAD service signals This function provides the start–stop mode DTE with the ability to decide whether or not and in what format PAD service signals are transmit- ted. 1.4.7 Selection of operation of the PAD on receipt of the break signal This function allows the selection of the operation of the PAD after the receipt of a break signal from the start–stop mode DTE. 1.4.8 Discard output This function provides for a PAD to discard the content of user sequences in packets upon request rather than disassembling and transmit- ting these to the start–stop mode DTE. 1.4.9 Padding after carriage return This function provides for the automatic insertion by the PAD of pad- ding characters in the character stream transmitted to the start–stop mode DTE after the occurrence of a carriage return character. This allows for the printing mechanism for the start–stop mode DTE to perform the carriage return function correctly. 1.4.10 Line folding This function provides for the automatic insertion by the PAD of appropriate format effectors in the character stream transmitted to the start– stop mode DTE. The predetermined maximum number of graphic charac- ters per line may be set. 1.4.11 Flow control of the PAD by the start–stop mode DTE This function allows for flow control between the start–stop mode DTE and the PAD. The start–stop mode DTE indicates whether it is ready or not to accept characters from the PAD by transmitting special characters. These characters are those which in IA5 are used to switch an ancillary transmitting device on and off. 1.4.12 Linefeed insertion after carriage return This function provides for the automatic insertion by the PAD of a linefeed character in the character stream to or from the start–stop mode DTE or after each of each carriage return character. This function applies only in the data transfer state. 1.4.13 Padding after linefeed This function provides for the automatic insertion by the PAD of pad- ding characters in the character stream transmitted to the start–stop DTE after the occurrence of a linefeed character. This allows for the printing mechanism of the start–stop mode DTE to perform the linefeed operation correctly. This function applies only in the data transfer state. 1.4.14 Editing This function provides for character delete, line delete and line dis- play editing capabilities in the PAD command state and the data transfer state for the start–stop mode DTE. During the PAD command state the edit- ing function is always available. 1.4.15 Editing PAD service signals This function provides the start–stop mode DTE with the ability to decide whether or not editing PAD service signals are transmitted and which format should be used. 1.4.16 Echo mask When echo is enabled (§ 1.4.2), this function allows selected defined sets of character(s) received from the start–stop mode DTE not to be trans- mitted back to the start–stop mode DTE. 1.4.17 Parity treatment This function allows the PAD to check parity in the data stream from the start–stop mode DTE and/or generate parity in the data stream to the start–stop mode DTE. 1.4.18 Page wait This function allows the PAD to suspend transmission of additional characters to the start–stop mode DTE after a specified number of linefeed characters have been transmitted by the PAD. 2 Characteristics of PAD parameters 2.1 In this Recommendation parameters are identified by decimal refer- ence numbers. 2.2 In this Recommendation the possible values of the parameters are rep- resented by decimal numbers. 2.3 Specific procedures, described in Recommendations X.28 and X.29 are available for initializing, reading and changing values of PAD parame- ters. 2.4 Determination of the values of PAD parameters 2.4.1 Initial values of PAD parameters On initialization, the initial value of each PAD parameter is set according to a predetermined set of values called an initial standard profile. Table 1/X.28 gives details of the initial values of parameters for transparent and simple standard profiles which have been agreed by CCITT. Networks may offer other standard profiles that provide different, pre- determined sets of PAD parameter values. 2.4.2 Current values of PAD parameters The current values of PAD parameters are the values resulting from possible modifications by the PAD, the start–stop mode DTE and/or the packet mode DTE (or remote PAD). 3 List of PAD parameters and possible values Restrictions on the permissible relationships between the values of the various parameters is a subject for further study. 3.1 PAD recall using a character Reference 1 The Parameter will have the following selectable values: not possible – represented by decimal 0; possible by character 1/0 (DLE) – represented by decimal 1; possible by graphic character defined by the user – represented by decimal 32–126. A graphic character, defined by the user to escape from the data transfer state and to recall the PAD, is the binary representation of the decimal value in accordance with Recommendation T.50. 3.2 Echo Reference 2 The parameter will have the following selectable values: no echo – represented by decimal 0; echo – represented by dec- imal 1. Note – If parameter 20 is implemented, the selection of the characters to be echoed is dependent on the value of parameter 20. 3.3 Selection of data forwarding character(s) Reference 3 This parameter is respresented by the following encoding of basic functions, each having a decimal value as shown below: no data forwarding character – represented by decimal 0; alphanumeric characters (A–Z, a–z, 0–9) – represented by decimal 1; character CR – represented by decimal 2; characters ESC, BEL, ENQ, ACK – represented by decimal 4; characters DEL, CAN, DC2 – represented by decimal 8; characters EXT, EOT – represented by decimal 16; characters HT, LF, VT, FF – represented by decimal 32; all other characters in columns 0 and 1 of IA5 not included in above – represented by decimal 64. Note – The decimal representation of individual values of this parameter allows coding to represent a single function or combination of functions, see Table 1/X.3. 3.4 Selection of idle timer delay Reference 4 The parameter will have the following selectable values: any number from 0 to 255 – represented by the respective decimal number. The value 0 will indicate that no data forwarding on time–out is required; a value between 1 and 255 will indicate the value of the delay in twentieths of a second. Note 1 – Some PAD implementations may not offer all possible values of idle timer delay within the selectable range. In such cases where the value selected is not available, the PAD will assume the next higher available value in the range. Note 2 – The effect of the idle timer delay on data forwarding may be sub- ject to flow control constraints. 3.5 Ancillary device control Reference 5 The parameter will have the following selectable values: no use of X–ON (DC1) and X–OFF (DC3) – represented by decimal 0; use of X–ON and X–OFF (data transfer) – represented by decimal 1; use of X–ON and X–OFF (data transfer and command) – represented by dec- imal 2. 3.6 Control of PAD service signals and PAD command signals Reference 6 The parameter is represented by the following encoding of basic func- tions, each having a decimal value as shown below: no service signals are transmitted to the start– stop mode DTE – represented by decimal 0; service signals other than the prompt PAD service signal are transmitted in the standard format – represented by decimal 1; prompt PAD service signal is transmitted in the standard format – represented by decimal 4; PAD service signals are transmitted in a network–dependent format – represented by decimal 8–15. PAD service signals and PAD command signals in the extended dialogue mode format: extended dialogue mode handling, with PAD service signals in English – represented by decimal 16; extended dialogue mode handling, with PAD service signals in French – represented by decimal 32; extended dialogue mode handling, with PAD service signals in Spanish – represented by decimal 48. Note 1 – The decimal representation of individual values of this parameter allows coding to represent a single function or combination of functions, see Table 1/X.3. The transmission of service signals is controlled by the values 0, 1, 4 and 5. In addition, the actual text of the service signal is controlled by the values 16 and above. Note 2 – Values of 64 to 240 in multiples of 16 represent additional lan- guages provided on a network dependent basis. Note 3 – Values of 16 to 240 in multiples of 16 may be combined with val- ues 8–15 to provide a network–dependent extended dialogue mode (e.g. a value of 41 is equivalent to 32+9, network–dependent French). Note 4 – To enable or disable service signals in the extended dialogue mode, the values 16–240 in multiples of 16 can be combined with; 0 for no service signals, 1 for service signals other than the prompt, and 5 (4+1) for service signals and the prompt. 3.7 Selection of operation of PAD on receipt of break signal from the start–stop mode DTE Reference 7 This parameter is represented by the following encoding of basic functions, each having a decimal value as shown below: nothing – represented by decimal 0; send to packet mode DTE or other PAD an interrupt packet – represented by decimal 1; reset – represented by dec- imal 2; send to packet mode DTE or other PAD an indication of break PAD message – represented by decimal 4; escape from data transfer state – represented by decimal 8; discard output to start–stop mode DTE – represented by decimal 16. Note – The decimal reprsentation of individual values of this parameter allows coding to represent a single function or combination of functions, see Table 1/X.3. 3.8 Discard output Reference 8 The parameter will have the following selectable values: normal data delivery to the start–stop mode DTE – represented by decimal 0; discard output to start–stop mode DTE – represented by decimal 1. 3.9 Padding after carriage return Reference 9 The parameter will have the following selectable values: any number from 0 to 255 – represented by the respective decimal number A value between 0 and 255 will indicate the number of padding characters to be generated by the PAD after a carriage return character is transmitted to the start–stop mode DTE. When parameter 9 is 0, there will be no padding except that PAD service signals will contain a number of padding characters according to the data rate of the start–stop mode DTE. 3.10 Line folding Reference 10 The parameter will have the following selectable values: no line folding – represented by decimal 0; any value between 1 and 255 – represented by the respective decimal num- ber. A value between 1 and 255 will indicate the number of graphic characters per line that will be transmitted by the PAD without inserting appropriate format effectors. 3.11 Binary speed This parameter is a read–only parameter and cannot be changed by either of the DTEs. It enables the packet–mode DTE to access a characteris- tic of the start–stop mode DTE which is known by the PAD. Reference 11 The parameter will have the following values: 50 bit/s – represented by decimal 10; 75 bit/s – represented by decimal 5; 100 bit/s – represented by decimal 9; 110 bit/s – represented by decimal 0; 134.5 bit/s – represented by decimal 1; 150 bit/s – represented by decimal 6; 200 bit/s – represented by decimal 8; 300 bit/s – represented by decimal 2; 600 bit/s – represented by decimal 4; 1 200 bit/s – represented by decimal 3; 1 800 bit/s – represented by decimal 7; 75/1 200 bit/s – represented by decimal 11; 2 400 bit/s – represented by decimal 12; 4 800 bit/s – represented by decimal 13; 9 600 bit/s – represented by decimal 14; 19 200 bit/s – represented by decimal 15; 48 000 bit/s – represented by decimal 16; 56 000 bit/s – represented by decimal 17; 64 000 bit/s – represented by decimal 18; Note – The values implemented in individual PADs depend on the range of DTE data transmission rates which are supported. The allocation of decimal values to all known rates is to avoid revision of the Recommenda- tion in the future. 3.12 Flow control of the PAD by the start–stop mode DTE Reference 12 The parameter will have the following selectable values: no use of X–ON (DC1) and X–OFF (DC3) for flow control – represented by decimal 0; use of X–ON and X–OFF for flow control – represented by decimal 1. 3.13 Linefeed insertion after carriage return Reference 13 This parameter is represented by the following encoding of basic functions, each having a decimal value as shown below: no linefeed insertion – represented by decimal 0; insert linefeed after each carriage return in the data stream to the start–stop DTE – represented by decimal 1; insert linefeed after each carriage return in the data stream from the start–stop mode DTE – represented by decimal 2; insert linefeed after each carriage return in the echo stream to the start–stop mode DTE – represented by decimal 4. Note 1 – The decimal representation of individual values of this parameter allows coding to represent a single function or combination of functions, see Table 1/X.3. Note 2 – This function applies only in the data transfer state. 3.14 Linefeed padding Reference 14 The parameter will have the following selectable values: any number from 0 to 255 – represented by the respective decimal number. A value between 0 and 255 will indicate the number of padding characters to be generated by the PAD after a linefeed character is transmitted to the start–stop mode DTE during the data transfer state. 3.15 Editing Reference 15 The parameter will have the following selectable values: no use of editing in the data transfer state – represented by decimal 0; use of editing in the data transfer state – represented by decimal 1. The use of value 1 suspends the following operations of the PAD: a) data forwarding on full packet until the editing buffer is full, b) data forwarding on idle timer period expiry. Note – The value of parameter 4 remains unchanged. 3.16 Character delete Reference 16 The parameter will have the following selectable values: one character from IA5 – represented by decimal 0–127. The character defined by the user for character delete is the binary represen- tation of the decimal value in accordance with Recommendation T.50. 3.17 Line delete Reference 17 The parameter will have the following selectable values: one character from IA5 – represented by decimal 0–127. The character defined by the user for line delete is the binary representation of the decimal value in accordance with Recommendation T.50. 3.18 Line display Reference 18 The parameter will have the following selectable values: one character from IA5 – represented by decimal 0–127. The character defined by the user for line display is the binary representa- tion of the decimal value in accordance with Recommendation T.50. 3.19 Editing PAD service signals Reference 19 The parameter will have the following selectable values: no editing PAD service signals – represented by decimal 0; editing PAD service signals for printing terminals – represented by decimal 1; editing PAD service signals for display terminals – represented by decimal 2; editing PAD service signals using one character from the range of IA5 – represented by decimal 8 and 32–126. Note – This parameter does not apply if the value of parameter 6 is set to 0. 3.20 Echo mask Reference 20 This parameter is represented by the following encoding of basic functions, each having a decimal value as shown below: no echo mask (all characters echoed) – represented by decimal 0; no echo of character CR – represented by decimal 1; no echo of character LF – represented by decimal 2; no echo of characters VT, HT, FF – represented by decimal 4; no echo of characters BEL, BS – represented by decimal 8; no echo of characters ESC, ENQ – represented by decimal 16; no echo of characters ACK, NAK, STX, SOH, EOT, ETB, ETX – represented by decimal 32; no echo of editing characters as designated by parameters 16, 17 and 18 – represented by decimal 64; no echo of all other characters in columns 0 and 1 of IA5 not mentioned above, and the character DEL – represented by deci- mal 128. Note 1 – The decimal representation of individual values of this parameter allows coding to represent a single function or combination of functions, see Table 1/X.3. Note 2 – If parameter 5, 12 or 22 is set to a non–zero value, then X–ON and X–OFF are not echoed. Note 3 – The PAD recall character specified by the value of parameter 1 is not echoed. Note 4 – A character need only be specified by a single value of parameter 20 in order for it not to be echoed. Note 5 – This parameter applies only when parameter 2 is set to 1. Note 6 – Parameter 20 value 64 (in combination) does not apply if editing is not enabled. 3.21 Parity treatment Reference 21 This parameter will have the following selectable values: no parity checking or generation – represented by decimal 0; parity checking – represented by decimal 1; parity generation – represented by decimal 2. Note 1 – The decimal representation of individual values of this parameter allows coding to represent a single function or a combination of functions, see Table 1/X.3. Note 2 – Characters generated by the PAD itself (e.g. PAD service signals) have even parity when parameter 21 is set to zero unless the parity is deter- mined by alternate means. 3.22 Page wait Reference 22 This parameter will have the following selectable values: page wait disabled – represented by decimal 0; page wait condition after n linefeed characters are sent by the PAD to the start–stop mode DTE – represented by decimal 1– 255. TABLE 1/X.3 Possible values and combination of values of PAD parameters (Note 1) Parameter reference Parameter description Selectable pos- sible values PAD parameter meaning Remark s number Manda tory Option al (Note 2) 1 PAD recall using a character (E) 0 1 32 to 126 Not possible Character DLE Possible; using one graphic character defined by user 2 Echo (E) 0 1 No echo Echo 3 Selection of data for- warding character(s ) (E) 0 No data forward- ing character(s) 1 Alphanumeric characters (A–Z, a–z, 0–9) Character CR 2 4 Characters ESC, BEL, ENQ, ACK 6 Characters CR, ESC, BEL, ENQ, ACK Value formed by combin ation (2+4) 8 Characters DEL, CAN, DC2 16 Characters ETX, EOT 18 Characters CR, EOT, ETX Value formed by combin ation (2+16) 32 Characters HT, LF, VT, FF 64 All characters in columns 0 and 1 of IA5 except those shown above for values 2, 4, 8, 16, 32 Values 3, 5–7, 9– 15, 17–31, 33–63, 65–125 127 may be formed by combi- nations of values 1, 2, 4, 8, 16, 32, 64 126 All characters in columns 0 and 1 and character DEL Value formed by combin ation (2+4+8 +16 +32+64 ) 4 Selection of idle timer delay (E) 0 20 255 1 to 19 21 to 254 Value of idle timer in twentieths of a second (Note 3) 5 Ancillary device control (E) 0 No use of X–ON (DC1) and X–OFF (DC3) 1 Use of X–ON and X–OFF (data trans- fer) 2 Use of X–ON and X–OFF (data trans- fer and command) TABLE 1/X.3 (cont.) Parameter reference Parameter description Selectable pos- sible values PAD parameter meaning Remark s number Manda tory Option al (Note 2) 6 Control of PAD ser- vice signals and PAD command signals (E) 0 1 No PAD service signals are trans- mitted to the start– stop mode DTE PAD service sig- nals are transmit- ted in the standard format 5 PAD service sig- nals and the prompt PAD service signal are transmitted in the standard format Value formed by com- bination (1+4) 8 to 15 PAD service sig- nals are transmit- ted in a network dependent format 16 Extended dialog mode, PAD service signals are in English 32 Extended dialog mode, PAD service signals are in French 48 Extended dialog mode, PAD service signals are in Span- ish 7 Selection of opera- tion of the PAD on receipt of break sig- nal from the start– stop mode DTE (E) 0 2 1 Nothing Interrupt Reset 4 Send to DTE an indication of break PAD message 5 Interrupt and indi- cation of break Value formed by com- bination (1+4) 8 Escape from data transfer state 16 Discard output, to start–stop mode DTE 21 Discard output, interrupt and indi- cation of break Values 3, 4, 6, 7, 9– 20, 22–31 may be formed by combi- nation of values 1, 2, 4, 8, 16 Value formed by com- bination (1+4+1 6) 8 Discard output (E) 0 1 Normal data deliv- ery Discard output 9 Padding after car- riage return (CR) (E) 0 1 to 7 No padding after CR (Note 4) 8 to 255 Number of pad- ding characters inserted after CR TABLE 1/X.3 (cont.) Parameter reference Parameter description Selectable pos- sible values PAD parameter meaning Remark s number Manda tory Option al (Note 2) 10 Line fold- ing (E) 0 No line folding 1 to 255 Number of graphic characters per line 11 (read only) Binary speed of start–stop mode DTE (E) 0 2 1 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 110 bit/s 134.5 bit/s 300 bit/s 1 200 bit/s 600 bit/s 75 bit/s 50 bit/s 800 bit/s 200 bit/s 100 bit/s 50 bit/s 75/1 200 bit/s 2 400 bit/s 4 800 bit/s 9 600 bit/s 19 200 bit/s 48 000 bit/s 56 000 bit/s 64 000 bit/s The values implem ented in indi- vidual PADs depend on the range of DTE data transmi ssion rates which are sup- port- ed. The allocati on of decimal values to all known rates is to avoid revision of the Recom- mendati on in the future 12 Flow con- trol of the PAD (E) 0 No use of X–ON (DC1) and X–OFF (DC3) for flow control 1 Use of X–ON (DC1) and X–OFF (DC3) for flow control 13 Linefeed insertion after car- riage return (A) 0 No linefeed inser- tion 1 Insert linefeed after transmission of CR to the start– stop mode DTE 2 Insert linefeed after each carriage return in the data stream from the start–stop mode DTE 4 Insert linefeed after echo of CR to start–stop mode DTE 5 Insert linefeed after transmission to the start–stop mode DTE and after echo of CR Combin ation (1+4) 6 Insert linefeed in data stream after CR from the start– stop mode DTE and after echo of a CR to the start–stop mode DTE Combin ation (2+4) 7 Insert linefeed in the data stream to and from the start– stop mode DTE and after echo of a CR to the start–stop mode DTE Combin ation (1+2+4 ) Note – Applies only to data transfer state TABLE 1/X.3 (cont.) Parameter reference Parameter description Selectable pos- sible values PAD parameter meaning Remark s number Manda tory Option al (Note 2) 14 Padding after line- feed (A) 0 1 to 7 No padding after linefeed 8 to 255 Number of pad- ding characters inserted after line- feed Note – Applies only to data transfer state 15 (Note 5) Editing (A) 0 No use of editing in the data transfer state 1 Use of editing in the data transfer state 16 (Note 5) 0 to 126 Character delete (A) 127 Character 7/15 (DEL) One character from IA5 17 (Note 5) Line delete (A) 0 to 23 One character from range of IA5 24 Character 1/8 (CAN) 25 to 127 One character from range of IA5 18 (Note 5) Line dis- play (A) 18 0 to 17 One character from IA5 Character 1/2 (DC2) 19 to 127 One character from IA5 19 (Note 5) Editing PAD ser- vice signals (A) 1 0 No editing PAD service signals Editing PAD ser- vice signals for printing terminals 2 Editing PAD ser- vice signals for dis- play terminals 8 32 to 126 Editing PAD ser- vice signals using one character from the range of IA5 20 (Notes 6 and 7) Echo mask (A) 0 No echo mask (all characters echoed) 1 No echo of CR 2 No echo of LF 4 No echo of VT, HT, FF 8 No echo of BEL, BS 16 No echo of ESC, ENQ 32 No echo of ACK, NAK, STX, SOH, EOT, ETB, ETX Values may be formed by com- bination of basic values 64 No echo of editing characters as desig- nated by Parame- ters 16, 17, 18 (Note 8) 128 No echo of all other characters in columns 0 and 1 not mentioned above and DEL TABLE 1/X.3 (end) Parameter reference Parameter description Selectable pos- sible values PAD parameter meaning Remark s number Manda tory Option al (Note 2) 21 (Note 9) Parity treatment (A) 0 1 No parity checking or generation Parity checking 2 Parity generation 3 Parity checking and parity generation Value formed by com- bination (1+2) 22 Page wait (A) 0 Page wait disabled 23 1 to 22 24 to 255 Number of line feed characters considered by the PAD for the page wait function E An essential parameter to be made available internationally. A An additional parameter which may be available on certain data net- works and may also be available internationally. Note 1 – Other values and possible combination of values are for further study. Note 2 – These parameter values provide additional user facilities which are not necessarily provided in all PADs. Note 3 – Some PAD implementations may not offer all possible values of idle timer delay within the selectable range. In such cases where the value selected is not available, the PAD will assume the next higher value avail- able. Note 4 – There is no padding after CR except that PAD service signals will contain a number of padding characters according to the data signalling rate of the start–stop mode DTE. Note 5 – When parameter 15 is implemented, the values of parameters 16, 17, 18 and 19 are either default values or are selectable from the optional range shown. The editing function is provided during the PAD command state whether parameter 15 is implemented or not. If parameters 16, 17, 18 and 19 are implemented, the editing characters and editing PAD service sig- nals during the PAD command state are defined by the appropriate values of these parameters. If parameters 16, 17, 18 and 19 are not implemented, the default values for the functions of these parameters are applicable to the PAD command state. Note 6 – This parameter does not apply if parameter 2 is set to zero. Note 7 – If parameter 5, 12 or 22 is set to a nonzero value, then the X–ON and X–OFF characters are not echoed. Note 8 – Parameter 20 value 64 (in combination) does not apply if editing is not enabled. Note 9 – Characters generated by the PAD itself (e.g., PAD service signals) have even parity when parameter 21 is set to zero unless the parity is deter- mined by alternate means.