OSI/AS Programming Manual

OSI Upper-Layer Concepts
APS Procedures Programming Concepts
056783 Tandem Computers Incorporated 2–9
Tokens The Session Layer standards define the concept of a token. A token is an attribute of a
connection giving the application that holds it the exclusive right to perform a given
function. For example, in half-duplex mode, if you hold the data token, your
application can send normal data. Before the remote application can send data in half-
duplex mode, it must acquire the data token from your application.
Four tokens are defined by the Session Layer. Each is used with either one or two
functional units, as listed in Table 2-5. If your connection uses one or more of the
functional units shown, the corresponding tokens are brought into play and are
available on the connection. The remaining tokens are unavailable.
Table 2-6 lists the APS procedures that affect the ownership of the available tokens.
The table includes those procedures that support basic concatenation, which is
described later in this section under “Basic Concatenation.”
Table 2-7 lists the APS procedures that in certain circumstances return information
about the state of the tokens (that is, their availability and their ownership). For
details on token states, see the procedure call descriptions in Section 3.
Table 2-8 gives the sequence of procedure calls needed to obtain one or more tokens
from the remote application. The procedure calls in square brackets can sometimes be
omitted, depending on the context. The sequence of procedures is explained after the
table.
Table 2-5. Tokens and Functional Units
Token Functional Unit
Data token Half-duplex
Synchronize minor token Minor synchronize
Major/activity token Major synchronize
Activity management
Release token Negotiated release