Session Layer
The OSI Session Layer Protocol (ISO-SP) provides the session management, e.g. opening and closing of sessions. In case of a connection loss it tries to recover the connection. If a connection is not used for a longer period, the session layer may close it down and re-open it for next use. This happens transparently to the higher layers. The Session layer provides synchronization points in the stream of exchanged packets.
Following are some of the communication tasks performed at this layer: Establishing connections, Maintaining connections, Synchronizing communications, Controlling dialogues, Terminating connections.
The Session Protocol Machine (SPM), an abstract machine that carries out the procedures specified in the session layer protocol, communicates with the session service user (SS-user) through an session-service-access-point (SSAP) by means of the service primitives. Service primitives will cause or be the result of session protocol data unit exchanges between the peer SPMs using a transport connection. These protocol exchanges are effected using the services of the transport layer.
When you create a connection, you authenticate the user account at the sending and receiving computers. Connection creation also involves determining the type of communication that will take place and the protocols that will be used by the lower layers.
Data transfer and dialogue control are used to determine which computer is making requests and which computer is making responses. This also determines whether acknowledgments are required for data transmission.
A session is a series of related connection-oriented transmissions between network nodes. Another way to look at it is that a session is the interrelated communications between two or more presentation entities, which emphasizes that the Session layer provides services to the Presentation layer.
For the CCNA exam, one of the Session Layer's transmission modes that you need to focus on is the Ethernet half-duplex and its design and operation. The key element to understanding half-duplex is that each of the two stations in a communications session can use only one circuit each at a time.
The three basic transmission modes are Simplex Communications: only allows data to flow in one direction, Half-duplex Communications: Two way data flow, only one way at a time, Full-duplex Communications: Two way data flow simultaneously.
The following services and protocols are defined on the Sessions layer:
- ASP (AppleTalk Session Protocol),
- NFS (Network File Services),
- RPC (Remote Procedure Call),
- SCP(Serial Communications Protocol),
- SQL (Structured Query Language),
- ZIP (AppleTalk Zone Information Protocol).
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