OSI/AS Configuration and Management Manual

OSI/AS Subsystem Description
OSI/AS Configuration and Management Manual—424119-001
7-3
Connection Phases
Each layer, as provided by the appropriate process, is described in terms of the following
three categories:
•
Downward information flow. A description of what the layer receives from the layer
above, and what the layer sends to the layer below, as shown in Figure 7-1 on
page 7-2.
•
Upward information flow. A description of what the layer receives from the layer
below, and what the layer sends to the layer above, as shown in Figure 7-1 on
page 7-2.
•
State machine. A description of the key states and events for that layer.
Connection Phases
Before beginning the discussion of each process, it may be helpful to review the
requirements for some of the layers during the three connection phases. In particular, the
Session Layer has requirements during each connection phase that differ from those of
the Presentation Layer and ACSE. This subsection gives a brief description of those
differences.
Before a Session Layer entity can begin the connection-establishment phase with a peer
entity, the Transport Layer must have established its own peer connection, and must
have moved into the data-transfer phase. This is required because session protocol data
units (SPDUs) are transferred as transport user data. A connect request SPDU is passed
down to the Transport Layer as an SPDU, which in turn is sent as a data transport
protocol data unit (DT-TPDU).
Connection-establishment requirements for the Presentation Layer and ACSE are
different. During the connection-establishment phase, presentation and ACSE PDUs are
passed as user data along with the connect SPDUs. They do not require a session
connection to be established before they can be sent.
Requirements for the Network Layer for X.25 networks also differ. Since X.25 is a
connection-oriented protocol, data can be transferred only after a connection is
established.
In the data-transfer phase, presentation protocol data units (PPDUs) flow as user data on
SPDUs, which in turn are sent as user data in transport DT-TPDUs. ACSE is not
involved during the data-transfer phase. On X.25 networks, the X.25 circuit is ready for
data transfer after it receives a valid call accept packet
data is transferred in data packets.
In the connection-release phase, presentation and ACSE PDUs are passed as user data
included in session PDUs. Session-release PDUs travel as user data in transport
DT-TPDUs. After the session connection is released, the session protocol allows the
transport entity to release the connection. In the Network Layer, on X.25 networks,
connection release occurs when a clear packet is sent by X25AM, followed by the
receipt of a clear confirm packet.