Friday 17 August 2012

OSI Reference Model














Application Layer


‘Closest’ layer to the user
Works with the applications you use to communicate over the network
    E.g.. Services include SMTP, HTTP and FTP
Clicking on a link on a web page issues a command for the browser to retrieve the relevant information from the Internet
As an example say your computer is the source,
   and the host of the web site information is the
   destination
 The application completes your request and
   delivers the information to your computer




This layer has three fundamental functions 
(1) Data presentation
(2) Data compression
(3) Data encryption     
Data Presentation
Enables receiving device to understand the information sent from the source
Converts data from native format (abstract syntax) to a common format (transfer syntax), e.g. ASCII 
Data Compression
By reducing the volume of data, transfers can take place in less time
Packets are examined and such things as spaces in text removed
The destination device returns the data to its original format before passing to the Application Layer
Data Encryption
Allows data to be converted to a form which hides its meaning, apart from those you wish to see it
Not all data is encrypted on its journey across the network
In order for decryption to occur at the destination device a ‘key’ is required





Primarily responsible for handling the session between devices (beginning, maintaining and finishing)
Enforces order in the communication between devices
Regulates the flow of data
It takes responsibility for the following
   Session services
The following services are provided by Session Layer 
Establishing a connection
Maintaining the session
Ending the connection
Dialogue control
Dialogue separation
Handshaking – SYN and ACK packets
‘Keep alive messages’
Session must be terminated (otherwise one device will be still transmitting without any device actually listening)
Dialogue control (simplex, half-duplex, full-duplex)
 Dialogue separation – checkpoints within the 
  transmission which allow the detection of lost packets,     
  and subsequent re-transmission

Application Services
File services
Electronic-mail services
Network-printing services
Application services
Database services





Functions of Transport Layer
Ensures reliable transport of packets from source to destination
It performs multiplexing, segmentation at sending end
It performs de-multiplexing and reassembly at the receiving end
Manages the speed of transmission – flow control
There are two types of transmission
       - Connection-oriented transmissions
            - Connectionless transmissions
Connection-Oriented Transmissions
Also known as ‘ Reliable Transport Method’
    – uses acknowledgement (ack) packets on 
       successful receipt of data
Extra packets slows down communication
Features are
 Reliability
 Slower communication
 Packets are re-transmitted if corrupted or not             
    received
 Once all the data is received successfully, the
    packet is re-assembled and   passed to the  Session    
    Layer
Connectionless Transmissions
In this mode, the transmitting device does not require 
    acknowledgements from the receiver, and continues
    to transmit on the assumption that the data was
    received
Features are
Little or no reliability
Faster transmission
Packets are not re-transmitted
Flow Control
Establishes the maximum speed at which both sender and receiver can communicate at
Transport Layer determines largest packet size which can be sent
Packets are numbered – to allow re-assembly in the correct order





Network Layer
Network layer packets are known as datagrams 
Uses the network address ( this is a logical address – and does not depend upon any hardware in the device, or the device’s physical location)
Functions of Network Layer
The Network Layer performs the following functions
Adds the address to the packet (encapsulation)
Responsible for the correct addressing and delivery of packets of data
Maps the network address to the device physical address
Determines the best path for the packet (routing)
Divides outgoing messages into datagrams and  assembles incoming datagrams into messages for higher layer
Ensures that the packet is in the correct format for the destination
How does it Work ?
If a packet must move to another network, a routing 
   protocol is required
 If different packet lengths are used on the different 
   networks, the Network Layer formats the data accordingly
 The primary piece of hardware which works on this layer is 
    the router. (covered in detail, later in course)




Data Link Layer
Has two sub layers of its own
Logical Link Control (LLC)
Media Access Control (MAC)
LLC acts between protocols such as Internet Protocol (IP) and the MAC method
MAC is responsible for the connection to the physical media (e.g. cable)
Medium Access Control (MAC)
Each NIC has a unique number hard coded into the card  called its physical address
The first 6 digits denote the manufacturer, the next six are unique) – type “winipcfg” on your PC to get physical address
When the MAC address is added to the packet it is now known as a frame
It now has all the information required to travel from the source to the destination

Functions of Data Link Layer
Synchronization of data frames
To take the packets from the upper layer (i.e., network layer)  converts them into frames 
To take bits from the lower layer (i.e., physical layer)  converts them into packets
Error detection and control
Framing
Ensures link level reliability
Quiz
3. Data Link layer deals data in terms of 
       (a) packets         (b) Frames       (c) Messages              (d) Stream of bits
4. How many sub layers does a Data Link layer  
      consist of 
        (a) 3         (b) 2        (c) 4        (d) 5 










To activate, maintain and deactivate the physical connection
To define voltages and data rates needed for transmission
To convert the digital bits into electrical signal and vice versa
To decide whether the transmission is simplex, half duplex or full duplex
To take the data from upper layer (data link layer) and sends it in the form of digital pulses through the media
To take digital pulses from cable and convert them into bits and send them to data link layer


l







Physical layer





















                                                                 Quiz

1. The top most layer in OSI model is  _____________ 
                       (a) Transport              (b) Network layer     (c) Application     (d) Session

2.   Data encryption will be done at 
a) Session  (B) Data link (c)Presentation (d)Transport

3. Dialogue control will be done at ____________________
         (a) Presentation          (b) Session         (c) Transport         (d) Physical

4. SMTP, FTP services are provided by ______________
 (a) Application layer          (b) Presentation layer          (c) Session layer          (d) Transport layer



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