LANs and the OSI Reference Model

The ISO (International Standards Organization) has described an architecture which defines how devices can communicate over a network. The Architecture, referred to as the OSI (Open Systems Interconnection) the Reference Model, divides the communication functions into seven distinct and independent layers. That is display in the given Figure:

OSI reference model

Every layer receives services from the underlying layer, the service provider, by SAPs (service access points). A layer might have various SAPs with every SAP having its own address. The Software or hardware entities implement the services defined through each layer. Essentially, communication takes place among peer layers on every machine. Protocols specify how the communication takes place among the peer layers.

Application

The application layer describes how a user or procedure accesses the network. File transfer and mail services are the classical instance of application-layer processes.

Presentation

The presentation layer describes the format, involving semantics and syntax of the data exchanged among devices. The application layer might deliver the data in a particular format ASCII, for instance) to the presentation layer, where it is transformed into a format appropriate for transmission over the network.

Session

The session layer describes how communication (known as a session) is established among machines. The services in this layer also maintain various aspects of the session like as synchronization and traffic direction.

Transport

The transport layer describes the connection among the source and destination machines (end-to- end). That ensures  in which  the  data  from  upper-layer  processes,  commonly  encapsulated  in  packets,  arrives  at  its destination error-free and in the correct order.

The given paragraphs describe the services provided through each layer of the OSI model:

OS Reference Model

Network

The network layer describes how the packets are routed by the network to their last destination. Network layer isolates the upper layers from the underlying aspects of the network.

Data Link Layer

The data link layer describes how blocks of data (packets) are reliably transferred among adjacent network nodes. Data link layer is also responsible for flow control and error detection.

The Physical Layer

The physical layer describes how the raw bits of upper-level data are physically transferred over the network medium (Physical cable). Physical layer concerns itself with the mechanical and electrical aspects of data transmission.

As display in Figure, LANs implement the services describe through the physical and data link layers of the OSI reference model. Further communications standards, like as the BSD/DOD application utilities and the TCP/IP protocols, have to be implemented in a few manner to achieve the services defined through the upper layers of the OSI reference model.