Session Layer

The session layer manages sessions between applications, including initiation, maintenance and termination of information transfer sessions. Usually this is visible to the user by having to log on with a password.

The session layer tracks connections, also called sessions. The session layer should keep track of multiple file downloads requested by a particular FTP application, or multiple telnet connections from a single terminal client, or web page retrievals from a web server.

With TCP/IP this functionality is handled by application software addressing a connection to a remote machine and using a different local port number for each connection.

The session layer performs the following functions:

Communication with the Presentation layer above.

Organize and manage one or more connections per application, between hosts.

Communication with the Transport layer below.

Session layer protocols are particularly useful for multimedia applications for which it is necessary to coordinate the timing of two or more types of data, such as voice and moving images, with a high degree of precision. Examples include video conferencing and streaming.

Examples of session layer protocols include DLC (data link control), PAP (printer access control), SMB (server message block), ASP (AppleTalk session protocol), NetBIOS (network basic input/output system) and ZIP (zone information protocol).

TYPES of Session:

Simplex: Transmission from one side only. eg TV, Radio.

Half Duplex: Transmission from both side but onc side at a time. eg Walkie Takie, Chatting.

Full Duplex: Transmission from both side simultaneously. eg Telephoe, Mobile, Video Chat.

SESSION LAYER

The Session Layer is Layer 5 of the seven-layer OSI model of computer networking.

The Session Layer provides the mechanism for opening, closing and managing a session between end-user application processes, i.e. a semi-permanent dialogue. Communication sessions consist of requests and responses that occur between applications. Session Layer services are commonly used in application environments that make use of remote procedure calls (RPCs).

An example of a Session Layer protocol is the OSI protocol suite Session Layer Protocol, also known as X.225 or ISO 8327. In case of a connection loss this protocol may try to recover the connection. If a connection is not used for a long period, the Session Layer Protocol may close it and re-open it. It provides for either full duplex or half-duplex operation and provides synchronization points in the stream of exchanged messages