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Software Communications¶

ND 10573 COSMOS X.25 Option (using HDLC DMA)¶

flowchart LR
    subgraph COSMOS X.25 Option
        direction LR
        Application1[Application] --> Transport1[Transport Layer]
        Transport1 --> Network1[Network Layer]
        Network1 --> X25NS1[X.25NS]
        X25NS1 --> Packet1[Packet level]
        Packet1 --> Link1[Link level (LAP-B)]

        Application2[Application] --> Transport2[Transport Layer]
        Transport2 --> Network2[Network Layer]
        Network2 --> X25NS2[X.25NS]
        X25NS2 --> Packet2[Packet level]
        Packet2 --> Link2[Link level (LAP-B)]
    end
    X25[X.25 Network]
    Link1 --> X25
    Link2 --> X25

HDLC interface handles the V.24/X.21 bis standard.

INTRODUCTION¶

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.25 Option is a subsystem used to interconnect COSMOS Local Area Networks (LANs) through an X.25 packet switched network, normally provided by a common carrier (in the US) or by the governmental agency. The «carriers» are generally called PTT (Postal Telephone and Telegraph). The X.25 protocol is an internationally accepted recommendation developed by the CCITT (the International Telegraph and Telephone Consultative Committee).

FEATURES¶

Some of the most important features provided by the COSMOS X.25 Option are listed below:

Translation of COSMOS logical addresses to X.25 DTE numbers, and vice versa. COSMOS Network users need not know where a computer is located. They just have to remember the logical name of the destination. The routing is defined by the system supervisor during system startup.
Cost optimization. The cost of using a packet switched network system depends on several factors: destination, connection-time, dial-up cost and quantity of data. The system supervisor will initially define the different cost parameters according to national price level. The system will use tables based on these parameters to decide whether to make/break connections automatically. The user will not recognize that the link will in fact be disconnected. The logical channel will be kept open.
Multiple calls. The network server can handle 10 calls to different DTE numbers at any time. Different calls to the same computer/LAN (with the same DTE number) are multiplexed on higher levels. More than 10 logical channels may thus be obtained. At the «lower level», packet level, 50 simultaneous connections may be established. This figure may be of interest, in spite of the restricted capacity of the network server, due to the fact that the X.29 communication is placed directly on top of this level.
COSMOS X.25 Option supports the following types of calls:
- Virtual Call. Requires call set-up and clearance. Through a virtual call you have the opportunity to choose a «communication partner» among the predefined computers in the network. Node to node error checking, sequence control and flow control are provided by the network.
- Permanent Virtual Circuit. Almost similar to Virtual Call. It gives a constant route through an X.25 network. Through one PVC you may only access one predetermined host. No call setup and clearance is required.
- Checking the identity of the Calling DTE. If the calling DTE is not registered in a table (a part of the network server) inside the called DTE, the request for «conversation» will be refused.
- Powerful tools to operate the X.25 connection are available.
  - X25-Command Program.
    - Examples: Start/Stop and configure an X.25 module.
  - XMSG-Command Program. Some commands concern X.25 connection. The remainder concerns general COSMOS communication.
  - X25-Printlog Program. Converts the output of a loop to a readable format. It is possible to get a display/hardcopy either of all traffic, or just parts of it, for example link or packet level.
It is possible to vary the «window size» (the number of outstanding packets without reception of an acknowledgement) in order to improve the throughput. This is done from the X25-Command Program.

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Product Description¶

X.25 is a communication protocol that defines the interfaces between computer vendors' equipment and the Public Data Network.

The module COSMOS X.25 Option consists of two main parts:

X.25 Network Server
X.25 Software (PL, LL (Packet Level, Link Level))

The bridge between COSMOS Basic Module (IS-XMSG) and the X.25 software is built by the X.25 Network Server, providing the end user with a total, transparent solution. This means that all the functions available within a COSMOS Local Area Network (LAN), are available through a public X.25 network system. No changes are required in user procedures/ND applications subsystem procedures when the remote host is an ND system.

To control the X.25 Network Server, the system supervisor has a powerful tool in the XMSG-Command program.

X.25 is a network-dependent protocol divided into 3 functionally separate layers.

At level (layer) 1 COSMOS X.25 Option will use the X.21 bis standard which concerning interface operation is essentially similar to V.24/28 and V.35.

Level 2, the frame level, is divided into 2 parts, physical framing and logical frame control. The physical framing uses the bit orientated system defined by ISO (International Standards Organisation) as part of HDLC (High Level Data Link Control) procedures. The logical frame procedure regulates the setting up of the frame level, resource management and error correction by means of retransmission. COSMOS X.25 Option supports LAP-B, (Link Access Procedure Balanced), a subset of ISO HDLC.

In packet switching, the transmission path is not allocated on a per call basis, but only when a packet is transmitted. This is the logical channel concept. Since the network must deliver packet for a virtual call in the same sequence as the calling DTE delivers packets to the network, the virtual call is said to be set up on a logical channel. The packet level of COSMOS X.25 Option supports 50 virtual calls simultaneously. Among the tasks handled in packet level are control of virtual calls including flow control.

The X.25 software is controlled by the powerful X25-Command program.

The COSMOS X.25 Network Option is a prerequisite for running X.29 (Triple-X) communication through an X.25 based network. (See data sheets: ND 10405 COSMOS X.29 PAD and ND 10578 COSMOS X.29 SERVER)

Requirements¶

Hardware¶

HDLC DMA interface card, ND 730.

Software¶

SINTRAN III version H or later, generated for IS-XMSG and HDLC DMA and configured with buffer pool monitor calls (TPS).

IS XMSG version H or later, generated with gateway software and a minimum of 12 XMSG ports.

COSMOS BASIC MODULE, ND 10374

Documentation¶

COSMOS X.25 Option Operator's Guide ..... ND-30.034

Manual	Number
SINTRAN III Reference Manual	ND-60.128
SINTRAN III System Supervisor	ND-60.103
COSMOS Operator's Guide	ND-30.025
COSMOS User's Guide	ND-60.163