OSI LAYER (Open Systems Interconnection)

                                          What is OSI Layer Model 

The Open Systems Interconnection (OSI) model describes how networks can be broken into 7 different layers, each layer playing a different role.  Identifying these layers helps us to fault-find, analyse and design networks.  Although it can apply to any kind of network, we have used examples from Ethernet LANs and the Internet.

7.  Application Layer

6.  Presentation Layer

5.  Session Layer

4. Transport Layer

3. Network Layer

2. Data Link Layer

1. Physical Layer

Layer 1 — Physical    {BIT}

The physical layer carries signals along a bus.

• Copper and fibre-optic cables, radio/microwave links

• Repeaters

• Hubs

This layer is concerned with moving signals from place to place.

 Examples of this include cables, repeaters (used to boost signals, extending cabling length) and hubs (used to join cables together).      

Layer 2 — Data Link    {FRAME}

The data link layer moves frames of data between devices (via the physical layer).

• Network cards

• Bridges

• Switches (switched hubs)

{Using MAC addresses to identify devices }

This layer moves data from device to device along a single network.  The data frame is broadcast to all devices, but it contains the MAC address of the intended recipient.  Unique MAC addresses are burned into network cards when they are manufactured. 

                                

Layer 3 — Network   {PACKET}

The network layer moves packets of data between devices that may be on different networks (via the data link layer).

• Routers

• ping

{Using IP addresses to identify networks and devices }

This layer moves data packets across networks.  Every device on a network is assigned an IP address in addition to its MAC address.  The difference is that IP addresses are assigned geographically:

Layer 4 — Transport   {DATA SEGMENT}

The transport layer breaks data into pieces and delivers it to the destination in order (via the network layer).

• TCP — checks that pieces have arrived, sorted into order, checked for errors

• UDP — sends pieces in order; used for streaming video & audio

The transport layer also decides upon the optimum size of data transmissions, using a technique called “sliding windows”.

Layer 5 — Session 

The session layer initiates connections between devices (via the transport layer).

• Simplex — one-way communication

• Half-duplex — both ways, one-at-a-time

• Full duplex — both ways, both at the same time

The session layer is responsible for signalling the start and finish of communication between devices. It used for signalling the start and finishing of communication between devices.

Layer 6 — Presentation

Presentation layer encodes or translates data before passing to the session layer.

• MIME attachment encoding for emails

• SSL webpage encryption for credit card details

This is typically used for encryption for security purposes, or encoding to convert data into an appropriate format (e.g. MIME represents data as ASCII text, because email was only designed to handle ASCII text).

       

Layer 7 — Application

The application layer forms an interface for programs to use when communicating across a network. This later is very closest to user.

• HTTP for web pages

• FTP for file transfer

• SMTP / POP3 / IMAP for email

• Telnet for command-driven remote control of programs

• NetBIOS for network file and printer sharing

Summary Of OSI Model :

             OSI Reference Model – Example Devices & Protocols