Networking - OSI Layers


  • The OSI Model is Not Tangible, it is a concept.
  • The Open System Interconnection (OSI) model defines a networking framework to implement protocols in seven layers. 
  • In the OSI model, control is passed from one layer to the next, starting at the application layer in one station, and proceeding to the bottom layer, over the channel to the next station and back up the hierarchy.
  • The OSI model takes the task of internetworking and divides that up into what is referred to as a vertical stack that consists of the following 7 layers

Physical Layer 

  • Sends the bit stream - electrical impulse, light or radio signal , through the network at the electrical and mechanical level.
  • It provides the hardware means of sending and receiving data on a carrier, 
  • Carrier includes cables, cards etc. like Fast EthernetRS232, and ATM are protocols with physical layer components.

Data Link Layer

  • The data link layer is divided into two sub layers: The Media Access Control (MAC) layer and the Logical Link Control (LLC) layer. 
  • The MAC sub layer controls how a computer on the network gains access to the data and permission to transmit it. The LLC layer controls frame synchronization, flow control and error checking.
  • Layer 2 Data Link examples include PPP, FDDI, ATM, IEEE 802.5/ 802.2, IEEE 802.3/802.2, HDLC, Frame 

Network Layer

  • Switching and routing technologies work in this layer
  • Logical paths, known as virtual circuits, for transmitting data from node to node 
  • Addressing and packet sequencing.
  • Network examples include AppleTalk DDP, IP, IPX.

Transport Layer

  • Provides transparent transfer of data between end systems, or hosts.
  • Is responsible for end-to-end error recovery and flow control. 
  • It takes care of data transfer completely.
  • Transport examples include SPX, TCP, UDP.

Session Layer

  • This layer establishes, manages and terminates connections between applications. 
  • It sets up, coordinates, and terminates conversations, exchanges, and dialogues between the applications at each end. 
  • It deals with session and connection coordination.
  • Session examples - NFS, NetBios names, RPC, SQL.

Presentation Layer

  • This layer provides independence from differences in data representation (e.g., encryption) by translating from application to network format, and vice versa. 
  • It transforms data into the form that the application layer can accept. 
  • This layer formats and encrypts data to be sent across a network, providing freedom from compatibility problems. 
  • It is also called the syntax layer.
  • Presentation examples - encryption, ASCII, EBCDIC, TIFF, GIF, PICT, JPEG, MPEG, MIDI.

Application Layer

  • It facilitates application and end-user processes. 
  • Communication partners are identified, quality of service is identified, user authentication and privacy are considered, and any constraints on datasyntax are identified. 
  • Everything at this layer is application-specific. 
  • This layer provides application services forfile transfers, e-mail, and other network software services. 
  • Telnet and FTP are applications that exist entirely in the application level. 
  • Tiered application architectures are part of this layer.
  • Application examples include WWW browsers, NFS, SNMP, Telnet, HTTP, FTP