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ND Computer Systems

INTRODUCTION

X.25 is a procedure used to ease the interconnection of computers. It may be used to join computers of the same or different types. An ND computer system may be connected to X.25 based packet-switched networks, e.g., PSS (GB), TRANSPAC (F), DATEX-P (D).

The V.24 physical specification may be used on dial-up or leased lines. Optionally, SINTRAN III's X.21 facility may be used, i.e. it is possible to access circuit-switched networks, e.g. the Nordic Public Data Network.

All software implemented conforms to the X.25 provisional recommendation. The current package is intended for use by people with an understanding of data communications in general and the X.25 recommendation in particular.

The X.25 recommendation consists of a set of standards for communication between data equipment for 3 levels of communication:

LEVEL 1 – Physical Level

Defines the physical, electrical, functional and procedural characteristics necessary for establishing, maintaining and disconnecting a physical link.

LEVEL 2 – Link Level

Defines the link access procedure for data interchange across the physical link. The purpose of the access procedure is to offer a reliable transmission medium and it therefore contains such functions as error checking and sequence control.

LEVEL 3 – Packet Level

Defines the packet format and control procedure for exchange of packets containing control information and/or user data. This level makes it possible to establish one or more logical connections using one physical link.

PRODUCT DESCRIPTION

NORSK DATA's X.25 Packet and Link level software conforms to the CCITT X.25 recommendation of 1977 and consists of the following parts:

  1. Link Level software able to run unbalanced (LAP) or balanced (LAP-B) link access procedures.
  2. X.25 Packet Level offering Switched Virtual Calls and Permanent Virtual Circuits.

In addition, NORSK DATA's HDLC-DMA interface (V24/X.21) satisfies the requirements for the physical level (level 1).

FEATURES

  • DTE operation for public networks
  • Adaptive operation for private networks, circuit switching or leased lines
  • V.24/X.21 line interface
  • LAP/LAP-B Link Level
  • Switched Virtual Calls (SVC)
  • Permanent Virtual Circuits (PVC)
  • Test system

10199-A2-6000-0582

Scanned by Jonny Oddene for Sintran Data © 2010

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IMPLEMENTATION

Physical Level

The HDLC-interface takes care of the physical level, the generation of flags, transparency and the frame checking sequence. If X.21 is to be used to dial up other computers in a circuit-switched network, the X.21 driver in SIN-TRAN III is used in the connection and disconnection phases. The electrical characteristics currently in use are given in the V.24 and X.21 standards.

The link level is implemented as a real-time program which uses the HDLC driver. The link level user can choose between the balanced and the unbalanced link access procedures (LAP-B and LAP). Currently the link level can handle up to 8 physical lines.

The link level collects statistics over its own activity (i.e., the number of messages sent and received). These statistics may be obtained by an operator command. A logging facility capable of logging data being exchanged with lower and higher levels is also present. The logging can be turned on and off during a session of data exchange. The log command must be used to control the logging, and logged data is written on a log file which may be printed on a terminal, printer or file.

Packet Level

The packet level is implemented as a real-time program. A user program can send and receive commands to and from the packet level using the general task-task message system, XMSG, to communicate between background and RT-programs.

USER INTERFACE TO PACKET LEVEL (LEVEL 3)

Commands to the packet level are divided into 5 groups:

  • Routing
  • Setup and clearing of switched virtual calls
  • Reservation and release of permanent virtual circuits
  • Data transfer
  • Supervisor functions

The purpose of the routing commands is to allow each user to specify which INCOMING CALLS he wishes to receive.

For switched virtual calls (SVCs), it is necessary to establish a connection before data can be transmitted. After the data transfer the connection must be terminated.

For permanent virtual circuits (PVCs), no connection/disconnection is necessary. A user program need only GUARD and RESERVE the permanent channel and release it after use.

The data transfer commands are those for transmission of data, acknowledgment of data, and transmission of interrupt and reset.

The SUPERVISOR commands are the restart, error and the status commands. The restart command can only be sent by the network supervisor. The error command is sent from the packet level to a user when the packet level receives a data packet that is considered illegal. With the status command the user can obtain the status of a given connection.

DOCUMENTATION

Document Reference
X.25 Reference Manual ND-60.138
X.25 Supervisor Guide ND-30.023 
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       NORSK DATA                COMTEC

  [Contact Information and Locations]
  Oslo, tel.02-309030, tkx.18661 nd n
  ... [additional contact information] ...
  Dusseldorf, tel. 0211-660388, tkx. 658277 comt d

Note: NORSK DATA reserves the right to change specifications without notice.