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DISCUSSION CCITT Recommendation X.3, X.28, and X.29 define how an asynchronous (start-stop) terminal connects to a synchronous packet switched network. A Packet Assembler/Disassembler (PAD) is employed to perform the X.25 functions that the asynchronous terminal cannot manage by itself. The PAD can reside in the network or at the DTE end. X.3 describes the parameters that the PAD uses and can be modified by the terminal and host. There are 22 parameters defined in the CCITT Red Book: 1) Escape from Data Transfer - if enabled, tells the PAD what key is used to toggle between the data transfer mode and command mode. 2) Echo - decides whether characters sent from the asynchronous terminal should be echoed. 3) Data Forwarding Signal - tells the PAD which char(s) mean that it is time to assemble and send a packet. 4) Idle Timer Delay - if enabled, tells the PAD that if no char(s) have been sent from the asynchronous terminal after the specified time, packetize and send the data in the input buffer. 5) Ancillary Time Control - decides whether flow control is used for data sent from a terminal with an attached floppy disk or data storage medium to the PAD. 6) PAD Service Signals - decides whether Service signals are to be sent to the terminal. Service signals are sent by the PAD to the operator in response to the commands sent by the operator. 7) Procedure on Break - defines the resulting action when the operator has hit the break key. 8) Discard Output - used together with parameter 7. Set by the X.25 computer to tell the PAD whether any data in the output buffer should be discarded or sent to the terminal. 9) Padding after CR - if enabled, specifies the number of padding characters to insert after a CR when transmitting to the terminal. 10) Line Folding - if enabled, inserts a CR after a predefined number of chars in the input buffer. 11) Terminal Data Rate - read-only parameter for the operator to view the speed of the asynchronous terminal. 12) Flow control of the PAD - If enabled, allows the terminal to control the data flow from the PAD. 13) Linefeed Insertion - if enabled, tells the PAD when to insert a linefeed after CR. 14) Linefeed Padding - if enabled, specifies the number of padding character to insert after an LF is sent to the terminal. 15) Editing - defines whether editing of the data in the PAD input buffer, while in data transfer mode, is permitted. Parameter 16, 17, and 18 define which keys are to be used for editing. 16) Character Delete - defines which character is used to mean char delete. 17) Line Delete - defines which character is used to mean line delete. 18) Line Display - defines which character is used to display a line of text. Useful when the user is editing text. 19) Editing, PAD Service Signals - defines the character the PAD will respond with when it receives a char delete or line delete from the terminal. 20) Echo mask - specifies which characters are not to be echoed. 21) Parity Treatment - defines the manner in which the PAD treats the parity bit, bit 8. 22) Page Wait - if enabled, defines the number of LFs the text can contain before the PAD waits for a signal from the terminal to proceed with data transmission. Allows time for a new sheet of paper to be fed into a printer or to store received chars on a diskette. X.28 defines the interface, mainly the command language, between the asynchronous terminal and PAD. These commands enable the following functions: * Set up and clear of a virtual call * Select a profile that suits the terminal * Modify the X.3 values * Read the values of the X.3 parameters * Send an Interrupt packet * Query the status of a virtual call * Request reset of a virtual call X.29 is the protocol between the PAD and the X.25 terminal/host. PAD messages are used by the X.25 host to set or read X.3 parameters in the PAD and to receive an indication that the terminal has sent a BREAK and an INVITATION TO CLEAR. Acronym Glossary ---------------- ACK - Acknowledgement ANSI - American National Standards Institute ATDM - Asynchronous time division multiplexing BER - Bit error rate BOP - Bit-oriented protocol BPS - Bits per second BSC - Binary synchronous communication CCITT - International Telegraph and Telephone Consultative Committee CNS - Communication network systems CS - Circuit switching DCE - Data Circuit-Terminating Equipment DDCMP - Digital data communication message protocol DNIC - Data network identification code DSU - Data service unit DTE - Data Terminating Equipment FDM - Frequency divsion multiplexing FDX - Full duplex FIFO - First-in-first-out FM - Frequency modulation FSK - Frequency shift keying HDLC - High-level Data link control HDX - Half-duplex IPSS - International packet switching service ISO - International Standards Organization LAPB - Link access procedure, balanced LCN - Logical channel number LIFO - Last-in-first-out MS - Message switching MTBF - Mean time between failures NAK - No acknowledgement NCC - Network Control Center NMC - Network management center NPA - Numbering plan area PABX - Private automatic branch exchange PDN - Public data network PPS - Private packet switching PS - Packet switching PSK - Phase shift keying PTT - Postal, Telephone, and Telegraph RJE - Remote job entry SDLC - Synchronous data link control S/F - Store and forward SNA - System network architecture TCO - Telenet central office TDM - Time division multiplexing TELCO - Telephone company TP - Telenet processor VAN - Value-added network VC - Virtual Connection WATS - Wide-Area Telecommunication Service Article Change History: 02 Sep 1994 - Reviewed. 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| Document Information | |
| Product Area: | Communications-Networking |
| Category: | MacX25 |
| Sub Category: | General Topics |
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