Open System Interconnection Reference Model
Introduction
The Open System Interconnection (OSI) reference model describes how information from a software application in one computer moves through a network medium to a software application in another computer. The OSI reference model is composed of seven layers, each specifying particular network functions.
The model was developed by the International Organization for Standardization (ISO) in 1984.The OSI model serves as a standard for intercomputer communications.
The OSI model divides the tasks involved with moving information between networked computers into seven layers. In some communication systems two or three OSI upper layers may be condenced into one.
Below are listed the seven layers of the OSI model as defined
by the ISO:
Application Layer
Presentation Layer
Session layer
Transport layer
Network layer
Data link layer
Physical layer
The OSI layers
The seven OSI layers can be divided in to two mayors categories:
- the upper layers
- the lower layers
The upper layers includes the application, presentation, session, and transport layers.
They deal mostly with application issues. Implementation is mainly
done using software.
The lower layers consist of the internet, network, and physical layers.
Their pupose is to ensure the communication of messages from one
source host to a destination host though one or different networks.
For the purpose of simplifying the task of understanding the OSI model, we will condense the upper layers in to two: Application and Transport.
Application
|
Transport
|
Internet
|
Data Link
|
Physical
|
The application layer deals with compatibilty issues of applications/programs on different hosts.
The transport layer ensures communication between two different hosts through protocol.
The internet layer connects different networks though routers.
The data link layer uses switches to move messages withing a
single network
The physical layer transmits the messages in bits through a physical
connection (copper wire, fiber optic...)
OSI model in action :
source: lecture 2 ISM4220
OSI layers serve as a medium between information being transferred from a software application in one computer system to a software application in another .
Lets use for example a software application in System A that has information to transmit to a software application in System B. Communication is first vertical (downward) between the layers of
System A , vertical at the physical layers of both System A
and B, and lastly vertical(upward) between the layers of System B.
This process is explained in more details below.
The application program in System A will pass its information to the application layer (Layer 7) of System A. The application layer then passes the information to the presentation layer (Layer 6), which relays the data to the session layer (Layer 5), and so on down to the physical layer (Layer 1). At the physical layer, the information is placed on the physical network medium and is sent across the medium to System B. The physical layer of System
B removes the information from the physical medium, and then its physical layer passes the information up to the data link layer (Layer 2), which passes it to the network layer (Layer 3), and so on, until it reaches the application layer (Layer 7) of System B. Finally, the application layer of System B passes the information to the recipient application program to complete the communication
process.
Source http://www2.rad.com/networks/1994/osi/layers.htm
Encapsulation/Decapsulation:
Messages at the internet layer are called packets. As a packet travels down through the data link layer, it is placed in placed into a frame based on standard specification and network type.
An Ethernet frame would be used for an ethernet network, PPP frames for direct connections.
At the physical layer the frame is transmitted in bits accross multiple swicthes and routers to its destination host depending on the route.
When the frame arrives to a switch, the physical layer of the switch
puts the frame back together from bits signals and passes it
to the data link layer process.
The frame address field is determines which outgoing port it should
be sent to.
The frame destination address is either that of the next switch if it is still within the same network or that of the next router if traveling accross different networks.
The physical layer of the outgoing port transmits the frame again
in bits accross the physical connection (copper, fiber optic, radio...)
Assuming that the frame must travel through different networks then the frame must pass though a router (which is used to connect networks) .
The recieving port of the router converts the bits signal into frame's bits at the physical layer and then passes the frame up to the data link layer where it is decapsulated after being checked for errors.
The IP packet is then passed up to the router's internet layer process which determines the outgoing port port based on the IP packet's destination address.
The data link layer of the outgoing port encapsulates the IP packet based on the new network type(ethernet frame for ethernet, PPP frame for direct connection...) and then passes it down to the physical layer of the outgoing port which transmits the frame in bits accross the physical connection (copper, fiber optic, radio...)
When the frame finally arrives to the destination host, it goes through the physical layer of the NIC(Network Interface Card) up to the data link layer process where it is decapsulated after being checked for errors. The IP packet is then passed up to the internet layer process which passes it to the upper layers. As mentionned earlier, the upper layers resolves compability issues between applications on System A (source host) and System B (destination host).
Websources :
www.isi.salford.ac.uk//books/osi/osi.html
www.howstuffworks.com/osi.htm
www.webopedia.com/TERM/O/OSI.html
www.prenhall.com/panko
