To reduce design complexity, most networks are organized as a stack of layers or levels, each one built upon the one below it. Each layer is a kind of virtual machine, offering certain services to the layer above it.

A service is a set of operations that a layer provides to the layer above it. The service defines what operations the layer is prepared to perform on behalf of its users. A protocol, in contrast, is a set of rules governing the format and meaning of the frames, packets, or messages that are exchanged by the peer entities within a layer.

the osi reference model:

This model is based on a proposal developed by the International Standards Organization (ISO). The model is called the ISO OSI (Open Systems Interconnection) Reference Model. This Model divides the complete process of communication into 7 parts/layers.

  1. Application Layer
  2. Presentation Layer
  3. Session Layer
  4. Transport Layer
  5. Network Layer
  6. Data Link Layer
  7. Physical Layer

function of osi layers:

Layer # 1 (Physical Layer)

  • Physical characteristics of interfaces and media
  • Representation of bits (types of encoding)
  • Date rate (transmission rate (duration of bits))
  • Synchronization of bits
  • Line configuration (point-to-point configuration, multipoint configuration)
  • Physical topology (mesh, ring, bus topology etc)

Layer # 2 (Data Link Layer)

  • Frames
  • Physical addressing (source/destination address)
  • Flow control
  • Error control

Layer # 3 (Network Layer)

  • Datagram Packets
  • Source to destination delivery of packets
  • Logical addressing
  • Routing

Layer # 4 (Transport Layer)

  • Segment
  • Source to destination (end-to-end) delivery of entire message
  • Segmentation and reassembly
  • Connection control (connection oriented / connection less)

Layer # 5 (Session Layer)

  • Establish, maintain, synchronize and terminates session between two Communication systems
  • Synchronization
  • Provides for full-duplex, half-duplex, or simplex operation

Layer # 6 (Presentation Layer)

  • Translation
  • Encryption / decryption
  • Compression

Layer # 7 (Application Layer)

  • File transfer, access and management (FTAM)
  • Mail services