COMPUTER NETWORK | TOPIC 1 | ACTIVITY

OSI MODEL

Open Systems Interconnection model or OSI Model is a conceptual framework for understanding relationships. Its purpose is to guide vendors and developers so that the digital communication products and software programs they create will be completely understood and also to facilitate clear comparisons among communications tools.

7 LAYERS OF OSI MODEL

1. Physical layer

The lowest layer of the OSI model, is concerned with the transmission and reception of the unstructured raw bit stream over a physical medium. It describes the electrical/optical, mechanical, and functional interfaces to the physical medium, and carries the signals for all of the higher layers. 

Purpose	Protocols
Provides the hardware means of sending and receiving data on a carrier, including defining cables, cards and physical aspects.	·      Bluetooth ·      Ethernet ·      DSL ·      ISDN ·      Wi-Fi

2. Data Link Layer

The data link layer provides error-free transfer of data frames from one node to another over the physical layer, allowing layers above it to assume virtually error-free transmission over the link.

Purpose	Protocols
It furnishes transmission protocol knowledge and management and handles errors in the physical layer, flow control and frame synchronization.	·      ARP ·      ATM ·      CDP ·      FDDI ·      STP

3. Network Layer

It controls the operation of the subnet, deciding which physical path the data should take based on network conditions, priority of service, and other factors.

Purpose	Protocols
Provides switching and routing technologies, creating logical paths known as virtual circuits, for transmitting data from node to node. Routing and forwarding are functions of this layer, as well as addressing, internetworking, error handling, congestion control and packet sequencing	·      ICMP ·      IGMP ·      IPsec ·      IPv4 ·      IPv6 ·      IPX ·      RIP

4. Transport Layer

The transport layer ensures that messages are delivered error-free, in sequence, and with no losses or duplication. It relieves the higher layer protocols from any concern with the transfer of data between them and their peers.

Purpose	Protocols
Provides transparent transfer of data between end systems or hosts and is responsible for end to end error recovery and flow control. It ensures complete data transfer.	·      TCP ·      UDP

5. Session Layer

Allows session establishment between processes running on different stations.

Purpose	Protocols
It 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	·      NetBIOS ·      NFS ·      PAP ·      SCP ·      SQL ·      ZIP

6. Presentation Layer

The presentation layer formats the data to be presented to the application layer. It can be viewed as the translator for the network. This layer may translate data from a format used by the application layer into a common format at the sending station, then translate the common format to a format known to the application layer at the receiving station.

Purpose	Protocols
Provides independence from differences in data presentation by translating from application to  network format, and vice versa. It transforms data into the form that the application layer can accept. It formats and encrypts data to be sent across a network providing freedom from compatibility problems.	·      JPEG ·      MIDI ·      MPEG ·      PICT ·      TIFF

7. Application Layer
It serves as the window for users and application processes to access network services.

Purpose	Protocols
Supports application and end-user processes. It provides application services for file transfers, e-mail and other network software services	·      DNS ·      DHCP ·      FTP ·      HTTPS ·      NTP ·      TFTP