Layer 5 of the OSI model is the Session Layer, which establishes, manages, and terminates sessions between applications.

Following are some of the communication tasks performed at this layer:

• Establishing connections
• Maintaining connections
• Synchronizing communications
• Controlling dialogues
• Terminating connections

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.

Some facts and requirements to know about full-duplex operations:

Requires full-duplex NIC cards

Loopback and collision detection must be disabled in the NIC card

The NIC card’s device driver must support simultaneous transmission and receiving.

Full-duplex circuits are capable of 10 Mbps, 100 Mbps, and Gigabit Ethernet data speeds.

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)

X Window System and X Terminal

ZIP (AppleTalk Zone Information Protocol)

Tags: session layer, communication protocol layers, layers of communication protocol, communications protocols image, layer, Layer 5

Related posts:

1. The Transport Layer – Layer 4
2. The Data Link Layer – Layer 2