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ND Networking System

      +------------+                                             +------------+
      | Application|                                             | Application|
      +------------+                                             +------------+
            |                                                         |
      +-------------+                                           +-------------+
      | Transport   |                                           | Transport   |
      | Layer       |                                           | Layer       |
      +-------------+                                           +-------------+
            |<-------------------------- X.21 Option ------------------------>|
      +-------------------+                                    +-------------------+
      | Network           |                                    | Network           |
      | Layer             |                                    | Layer             |
      | (IS-XMSG)         |                                    | (IS-XMSG)         |
      +-------------------+                                    +-------------------+
            |                                                         |
      +------------------------+                            +------------------------+
      | COSMOS X.21           |                            | COSMOS X.21           |
      | NETWORK SERVER        |                            | NETWORK SERVER        |
      +------------------------+                            +------------------------+
            | LAP-B         | X.21 Interface                           | LAP-B         | X.21 Interface
            +---------------+                                           +---------------+
                           \                                        /
                            \                                      /
                             ------ X.21 NETWORK --------

ND 10403 COSMOS X.21 OPTION (using HDLC-DMA)

COSMOS is Norsk Data's distributed communications environment. It provides communication and application services to augment the services offered by SINTRAN-III to its users.

The COSMOS X.21 option allows use of an X.21 based circuit switching network as a transmission medium under COSMOS. The X.21 Network Server (X21NS) functions as a bridge between two IS-XMSG entities in the COSMOS Network Layer.

The X21NS is invisible for all COSMOS users/user programs. This means that no changes are required in user procedures when the X.21 Option is added to an already operational COSMOS Network.

The X21NS performs a number of functions as listed below:

  • Translation of COSMOS Network Addresses to X.21 DTE numbers and vice versa. COSMOS Network users/user programs need not know anything about DTE numbers; they are defined by the System Supervisor during system startup.
  • Cost optimization. The cost of using a circuit-switched network depends on the duration of the established connections. The X21NS minimizes costs by clearing connections when there is no data to send, and automatically re-establishes the connection(s) when needed.
  • Multiple connections. The COSMOS X.21 Network Server (X21NS) can handle multiple connections to the X.21 network (group-number).
  • Check the identity of the Calling DTE to ensure authorized access only.
  • Accumulation of accounting information to keep track of the costs of using the X.21 network.

Some X.21 network options offered do not need any support from the X21NS. I.e. they are invisible to the X21NS and the user is free to utilise these options. For example:

  • Closed user group(s)
  • Incoming (outgoing) (international) call barred
  • Connect when free
  • Redirection of calls

10403-A1-6000-0982

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X.21 Network Protocols

The X21NS uses a simple network protocol to control the establishment and orderly release of connections. The protocol also detects and rectifies possible duplicates that may occur during link reset at the link level.

At the link level standard LAP-B from X.25 is used. First the X.21 connection is established. When this phase is completed the logical link at the link level is connected. From then on full duplex data exchange can take place. The data exchanged consists of IS-XMSG Network Protocol Data Units (NPDU's). When the connection is no longer needed (long or short term), the logical link is disconnected at the link level and the X.21 connection is cleared.

Specifications

  • Interface hardware must be HDLC-DMA
  • X.21 Data rate up to 9600 bps
  • Standard configuration for 1, 2, 3 or 4 connections to the X.21 network

Prerequisites

  • COSMOS Basic System.
  • SINTRAN-III generated with the proper number of HDLC-X.21 device drivers.
  • Proper number of HDLC-DMA interfaces.
  • X.21 DCE-X (full X.21) on NPDN.
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Note: NORSK DATA reserves the right to change specifications without notice.