OSI/TS Configuration and Management Manual

Background Information About Addressing
OSI/TS Configuration and Management Manual—424831-001
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What Are Subdevices?
What Are Subdevices?
Each OSI/TS subdevice corresponds to a single transport connection, and can be thought
of as part of a transport service access point (TSAP). Subdevices configured with the
same local TSAP address (in other words, they have the same X.25 port, the same local
NSAP address, and the same local TSEL (which is the TSAP ID)) together constitute
the TSAP (the transport service access point) that is represented by that configured
TSAP address. Remote applications can use that TSAP address to reach a particular
type of local application residing on the Compaq host.
Subdevices greatly facilitate the task of configuring an OSI/TS subsystem. For
example, different applications might require different connection attributes. Each
subdevice, therefore, can be configured to suit those various needs (for example, with a
different transport protocol class, or multiplexing on or off). Once a connection is
established, you can obtain dynamic information about the connection via the subdevice.
Address Sharing vs. Multiplexing
An application can provide multiple parallel access by configuring multiple subdevices
with the same local TSAP address. The transport connections mapped to these
subdevices might or might not use the same network connection (X.25 virtual circuit).
This configuration is called address sharing and relates only to attaches (incoming, or
passive, connect requests).
In contrast, when a TSP process initiates a transport connection, and the subsystem is
configured to allow multiplexing, and the TSP process has already established a network
connection having the same local network address and the same remote network address
(the same X.25 virtual circuit and the same NSAP) as requested by the new connect
request, it can multiplex the new transport connection on the existing network
connection. This configuration is called connection multiplexing and relates only to
connect requests (outgoing, or active, requests to initiate connections).
Connection multiplexing, as defined in the ISO standards, occurs only when multiple
transport connections share a single network connection (also as defined in the ISO
standards). Therefore, connection multiplexing is valid only for connection-oriented
network service (WAN connections). (Since LANs use connectionless-mode network
service, this concept does not apply to LANs—all transport connections map to a single
set of TLAM/PAM ports.) Multiplexing is discussed in more detail in Section 5, OSI
Transport Services.
Although each subdevice can support at most one transport connection at a time, it is
possible for several applications to share the same subdevice. This is accomplished by
one or more applications issuing multiple opens to the same subdevice (at the same
time). The applications must coordinate the usage of the subdevice (for example, a
duplicate connection request must not be issued).