OSI/AS Programming Manual

Glossary
056783 Tandem Computers Incorporated Glossary-3
Basic concatenation. A feature of the Session Layer protocol that allows two SPDUs to
be contained in one TSDU. This feature is available through the APS procedures,
which allows applications to invoke a give token primitive or a please token primitive
to be concatenated with the requested primitive. See also PDU.
C-series system. A system running any Cxx version of the Guardian 90 operating
system, such as C21 or C30.
Called address. The address to which a connect request is addressed. See also initiator
or responder.
Called user. The application to which a calling user requests a connection. For OSI/AS,
an application calls the APS_ASSOC_ATTACH_ procedure to indicate that it is ready
to be a called user for a connection. Contrast calling user or responding user.
Calling address. The address from which a connect request is initiated. See also
initiator. Contrast responding address.
Calling user. The application that initiates a request to establish a connection. For
OSI/AS, an application is the calling user when it calls the
APS_ASSOC_CONNECTREQ_ procedure. The calling user can, at the same time, be a
called user for other connections. Contrast called user or responding user.
Capability data. In the Session Layer, data sent and received between session service
end points while activity services are available but no activity is in progress. Only a
limited amount of such data can be exchanged. Capability data exchange requires the
selection of the activity-management functional unit and the capability-data-exchange
functional unit.
CCITT (International Telegraph and Telephone Consultative Committee). A division of the
United Nations International Telecommunications Union that coordinates standards-
setting activities.
CEPI (connection endpoint identifier). An identifier of a connection endpoint at a given
OSI layer, used to identify the corresponding connection at a service access point for
that layer. The CEPI plays the same role in OSI as a Guardian 90 file ID does for disk
I/O, but CEPI is not a Guardian 90 file number; it is a negative integer rather than a
positive integer. It is like a file ID in that APS procedures read and write data using a
CEPI to identify the connection. When a connection is established, the application
decides at which layer the APS procedures are going to go in. The CEPI that is
returned as a result of the connection being established will force all subsequent APS
read and write operations to that layer: either ACSE (in the Application Layer), the
Presentation Layer, or the Session Layer.
Checksum. A technique used to detect certain degrees of corruption of a message. It
could be implemented, for example, as an algorithm that treats a message as a
sequence of small integers and adds their values to test for random bit errors.
Class of protocol. See Transport Classes.