Osi model

OSI (Open System Interconnection) Model
The Open Systems InterConnection (OSI) model is a reference model developed by
International Organization for Standardization (ISO) in 1984. Which describes how
communication should take place between two nodes in a network.
Description
 Framework design to communicate across all types of computer systems.
 Consists of seven separate but related layers.
 Each layer performs specific functions and then passes result to another layer.
 Each layer defines the process of moving information across a network.
 Request of data moves from top application layer to bottom physical layer and reverse
happens at the receiving end.
 Remember its not a protocol but its a model.
Diagram

Layers
 1. Physical Layer
 2. Data Link layer
 3. Network Layer
 4. Transport Layer
 5. Session Layer
 6. Presentation Layer
 7. Application Layer
1. Physical layer is the first lowest layer in the OSI reference model, which defines the
procedures and functions of communicating network devices over a physical medium.
Description
Following are the functions and procedures performed by physical layer:
 Defines the electrical and physical characteristics of the interface between the network
devices and the transmission medium like voltage, current, frequency, type of cable, layout of
pins and more.
Transmission data: Data consists of stream of bits i.e. sequence of 0's and 1's, calculated in
terms of number of bits per second. This data is transmitted by encoding into signals, which
is defined by physical layer.
Defines the functions and services to be performed by physical devices:
 Modulation - conversion of signals to bits and vice versa.
 Multiplexing - establishment and termination of connection.
 Synchronization of bits.
 Line coding etc.
Defines type of line configuration to be use:
 Point-to-point - dedicated link
 Point-to-multipoint - shared link
Defines type of transmission mode to be use:
 Simplex - one way communication
 Half-duplex - both way, but one at a time communication
 Full duplex - both way, at the same time communication

Physical topology: Ring, Bus, Star, Mesh etc.
Protocols used: Ethernet, ISDN, and other IEEE 802.
Network devices used: Network card, Modem, Repeater, Oscilloscope, Amplifier.
2. Data link layer is the second lowest layer in the OSI reference model, which is responsible for
node-to-node error free data delivery. To achieve this data link layer is divided into two sub
layers Logical Link Control (LLC) and Media Access Control (MAC).
Description
Following are functions and tasks performed by the data link layer:
 Data framing - Encapsulation of the packet or message received from the network layer to
form frames.
 Logical Link Control (LLC) sub layer - Defines the functions to be needed for establishment
and control of logical links between local devices in the network.
 Flow control - Imposes a flow control to prevent overwhelming of the receiver.
 Error control - Adds a method to detect and retransmit damaged or lost frames and also
method to prevent duplication of frames.
 Media Access Control (MAC) sub layer - It is related to procedures used by network devices
to control access of network medium. As number of devices and networks shares a medium
for communication, it is essential to specify rules for managing the medium that who has
access at particular time to avoid conflicts.
 Physical addressing - To distribute frames in its own network of device it adds header to
the frame which contains MAC address of sender and receiver.
And to forward the frame to other network of device then the receiver address is the
address of the device that connects to adjacent network.
 Access control - Uses multiple access protocol to specify, who has control over the link at
any given time.
Protocols used: Ethernet, CSMA/CD, Token bus, Token ring.
Network devices used: Switch, ISDN router, Hub, NIC.

3. Network Layer is the third lowest layer in the OSI reference model, which is responsible for
packet delivery from source to destination through intermediate routers across multiple
networks.
Description
Network layer functions:
 Logical Addressing - Adds a header to the packet received from the transport layer, which
consists of logical (network) address to distinguish sender and receiver address over other
network, as physical addresses are limited to Local Area Network (LAN).
 Routing - Network layer provides the mechanism of moving packets across a series of
interconnected networks. And connecting devices like router and gateway routes the packet
to their final destination.
 Fragmentation and Reassembling - When packets are too large that can be processed by
data link layer, network layer splits the packet into small pieces and sends each piece one by
one to data link layer, while at the receiving end these pieces are reassembled to form a
packet.
 Controlling packet switching and data congestion.
Protocols used: IP, ARP, RARP, ICMP, IGMP, IPX, DDP etc.
Network devices used: router, gateway, frame relay device.
4. Transport layer is fourth and middle layer in the OSI reference model, which is responsible
entire message delivery from source to destination and insures data integrity of the message.
Description
Functions and responsibilities of transport layer:
 Fragmentation and reassembling - Splits the message received from session layer into
segments (small packets) and assigns each segment a sequence number and sends it to the
lower layer.
At receiving end these segments of sequence number allows transport layer to reassembly
the message properly and to check and replace lost and damaged segments in the
transmission.
 Connection Control - Transport layer can be connectionless or connection-oriented. A
connectionless is unreliable with less overhead where each packet is treated as independent
packet.
Connection-oriented is reliable with high overhead, where connection should be established
before transmitting a packet.
 Flow and Error Control - Performs at the level of source and destination i.e. end-to-end,
instead of across a single link like data link layer.
Protocols used: TCP, UDP, ARP, RARP, ATP ,NetBIOS/NetBEUI etc.

Network devices used: Gateway, Router.
5. Session layer is the fifth layer in the OSI reference model, which allows communicating
systems to establish and manage sessions between them. In general a session is a lengthy
time duration logical link between two applications processes to exchange information.
Description
Following are the functions and responsibilities of session layer:
 Allows communication between local and remote application process in either half duplex or
full duplex transmission mode.
 It sets up, manages and terminates the connection between two application processes.
 Synchronizes the communication by adding checkpoints into a stream of data.
 Informs errors of upper layer to the user.
Protocols used: NetBIOS, RPC.
Devices used: Gateway.
6. Presentation layer is the sixth layer in the OSI reference model, which deals with the syntax
(structure format) and semantics (meaning) of the data exchanged and how data is presented
to the user.
Description
Following are functions performed by the presentation layer:
 Translation - As data types, formats and encoding methods are different for various operating
systems, presentation layer translates it into common dependent format.
 Compression - Reduces the size of data to be transferred and increases the throughput of
the data.
 Encryption and Decryption - Secret or confidential or important data is first encrypted using
algorithm (protocol) and then transferred over the network. Decryption is done at the
receiving end to get back the original form of data.
Protocol used: LPP (Lightweight Presentation Protocol)
7. Application layer is the seventh top layer in the OSI reference model which provides user
interfaces and service utilities to the end user.
Description
Following are the services provided by the application layer:

 Provides interfaces to user processes.
 Identifies the communicating users and their authentication.
 Provides services like browsing, e-mail, file transfer, chatting, directory etc.
Protocols used: HTTP, FTP, SMTP, DHCP, Telnet, SNMP, POP3 etc.

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