How OTS/9000 Uses the X.25 Network

OTS/9000 supports the Connection-Oriented Network Service (CONS) and Connectionless Network Service (CLNS) over the X.25 network. The main objectives of X.25 are:

OTS/9000 is primarily concerned with the virtual circuit (VC) objective, leaving the other objectives to the X.25 product.

When a connection is to be established, a virtual circuit is established at the network layers. "Virtual"' implies that it is not necessary for a dedicated connection to exist between systems, but that a logical association be made through a network based on the source and destination addresses of the two systems. This does not mean that there cannot be dedicated circuits. X.25 allows both permanent virtual circuits (PVC) and switched virtual circuits (SVC), with the distinction based on how long the association is valid. A PVC maintains a permanent association. An SVC maintains the association only for the current connection call.

	NOTE: OTS/9000 supports only SVCs.

System addresses are based on the X.121 standard which uses a sequence of 15 deecimal digits (0-9) that includes a subaddress of 0 to 5 digits. When configuring local system addresses )in ots_subnets), the system address is constructed by concatenating the X.121 address from the X.25 configuration with the subaddress specified by the snet_x25_subaddress parameter. The physical address specified by the dest_phys_address parameter in X.25 ots_dests entries is a system address and contains both the X.25 address and the subaddress.

Null subaddresses can be used for both CONS and CLNS X.25 subnetwork switches that do not support subaddressing. Subaddresses are unique on a particular link and cannot be shared by different X.25 applications. (See the HP OTS Addendum for information on the snet_x25_subaddress parameter for defining null subaddresses.)

OTS/9000 uses the X.25 address to configure a logical mapping of the NSAP. For X.25 implementations using the 1984 specification or later, the NSAP is passed in the Extended Address (EA) facilities field in the X.25 call packets, if configured; the NSAP is the calling address. If the X.25 switch or X.25 network does not support the 1984 X.25 protocol, or the extended address facility is not configured, OTS/9000 will use the X.25 address as the calling address.

X.25 offers several connection management facilities. OTS/9000 can be configured to support the Reverse Charge (through osiadmin) and Closed User Group (through customization) facilities. Refer to the HP X.25 documentation for a description of these facilities.

Using Protocol Identifiers (PIDs) and Subaddresses

A single X.25 link is capable of supporting many applications, including OTS. Different applications, including multiple attachments to the same link by OTS, are differentiated from each other by the use of subaddresses and Protocol Identifiers (PIDs).

If the underlying X.25 subnetwork supports subaddressing, the easiest way to differentiate applications is to assign each one a unique subaddress without regard to PIDs. However, if subaddressing is not supported or not desired, the NULL subaddress must be used. If NULL subaddressing is used, then unique PIDs must be assigned to differentiate each application.

By default, OTS does not use a PID for X.25 subnetworks (it uses a NULL PID). This allows for the greatest degree of interoperability with remote OSI systems. If differentiation by PID is required, then OTS must be configured to use the standard OSI PIDs.

To configure OTS to use the standard OSI PID, use an editor to modify the active ots_subnets file. Change the snet_bind_by_pid parameter for the affected subnetwork(s) to yes (1). This change will take effect the next time the stack is started. Other related parameters are dest_pid in the ots_dests file and tpcons_null_pid in the ots_parms file.