OSI/AS SCF Reference Manual

Glossary
OSI/AS SCF Reference Manual—424121-001
Glossary-8
service
service. A set of primitives (operations) that a layer provides to the layer above it. The
service defines what operations the layer can perform on behalf of its users, but not how
these operations are implemented. A service relates to an interface between two layers,
with the lower layer being the service provider and the upper layer being the service
user.
service access point (SAP). The access means by which a pair of communicating entities in
adjacent layers use or provide services.
service data unit (SDU). Information passed across the network to a peer entity and then up
to the next layer.
Service primitive. An abstract, implementation-independent interaction between a service
user and a service provider. Service primitives describe the sequences of events between
adjacent layers that occur through the service access point (SAP)
.
session. (1) In the context of Open Systems Interconnection (OSI)
data transmission, the
period during which two entities can exchange data. (2) In the context of a management
application, the period during which an application can issue commands to a subsystem.
Session Layer. Layer 5 in the Open Systems Interconnection (OSI) Reference Model
. It
allows the setup and termination of and controls communication sessions between nodes
on the network.
session selector (SSEL). A logical address in the Session Layer
through which session
services are made available. A single SSEL can service one or more connections
simultaneously.
session service access point (SSAP). A network-unique address in the Session Layer
through
which connections are established and maintained.
SPI. See Subsystem Programmatic Interface (SPI)
.
SSEL. See session selector (SSEL)
.
SSID. See subsystem ID (SSID)
.
station group address. A media access control (MAC) address, common to the same type of
system (that is, intermediate systems or end systems), that is used by a system to
broadcast to all systems of the same type.
subdevice. A logical entity providing access to a physical entity (a communications line).
There may be one or many subdevices associated with a given line or process. Exactly
what constitutes a subdevice differs depending on the subsystem. A X.25 Access
Method (X25AM) subdevice corresponds to an X.25 network virtual circuit. Tandem
LAN Access Method (TLAM) subdevices are of several types: FSAPs, ports, services,
domains, workstations, windows, and gateways [for G06 and later releases, Port Access
Method (PAM) subdevices correspond to PAM ports].