Download Chapter 3 OSI Model

Open System
Interconnection
ISO – ORG FOR STANDARDISATION
• The International Organisation for Standardisation (ISO) is an
International standards organisation responsible for a wide
range of standards, including many that are relevant to
networking
• In 1984 in order to aid network interconnection without
necessarily requiring complete redesign, the Open Systems
Interconnection (OSI) reference model was approved as an
international standard for communications architecture
OSI Model
• An ISO standard that covers all aspects of
Network Communication is the Open System
Interconnection
• An open system allows two different systems to
communicate with each other regardless of their
underlying architecture
• ISO is an Organization
• OSI is a model
THE NEED FOR STANDARDS
• Previously Networks used different Hardware &
Software implementations
– Resulting in incompatibilities
– With different specifications nodes were unable to
communicate with each other
• To address problem ISO researched various network
schemes
– Need to create a NETWORK MODEL
– Help vendors create interoperable network
implementations
THE OSI REFERENCE MODEL
• OSI is short for Open System Interconnection
• Task : Moving info between computers over a network
– Divide the task into SEVEN smaller / more
manageable tasks
– Separation into smaller more manageable functions
is known as layering
• WHAT needs to be done to send data from one
computer to another and not HOW it should be done
OSI Model
A LAYERED NETWORK MODEL
• Each of 7 layers has particular Network functions
• Each layer provides a service to the layer above it
• Each layer correspond with the layers above and
below it and also the Peer layer.
– Through agreed upon rules called Protocols
• Peer to Peer process
• Each layer add info to message it receives from layer
above it called Header & Trailer (Data link layer only)
• By breaking up tasks into layers reduces complexity
• Enhance professionalism
Organization of Layers
• Lower 3 layers are Network Support Layers
• Concerned with flow of data from end to end through Network
– Combination of HW & SW
– Physical layer always implemented in Hardware
• Upper 3 layers are User Support Layers
– Always implemented in Software.
• Layer 4 ensures end to end reliable data transmission
• Layer 2 ensures reliable transmission on a single link
• Layers glued together by interfaces
• Each interface defines what info & services it must provide for
the above layer
• Headers & Trailers are added as data moves from Application
to Physical and removed vice versa
• Data is Encapsulated with the necessary protocol information
as it moves down the layers before network transit.
OSI Layers
Only last 3 layers involved as data moved through Intermediate nodes
Exchange Using the OSI Model
Header & Trailer being added & removed
LAYER 7: APPLICATION
• The application layer is responsible for providing services to the
user
• Closest to the user and provides user interface
• Establishes the availability of intended communication partners
• Does not provide services to any other OSI layer
• Examples: Spreadsheet programs, word processing programs,
electronic mail, remote file access & transfer and share
database management and other distributed information
services
• Examples of Application layer protocols are: Telnet, SMTP,
FTP, SNMP
LAYER 6: PRESENTATION
• Presentation layer is concerned with the syntax and
semantics of the information exchanged between two
systems
• This layer is primarily responsible for the translation,
encryption and compression of data
• It ensures that the information that the application layer of
one system sends out is readable by the application layer of
another system. Defines coding and conversion functions
• This layer also manages security issues by providing
services such as data encryption and data compression
• Examples of these formats and schemes are: MPEG,
QuickTime, ASCII, EBCDIC, GIF, TIFF, JPEG
LAYER 5: SESSION
• The session layer defines how to start, control and end
conversations (called sessions) between applications
• Establishes dialog control between the two computers in a
session, regulating which side transmits, plus when and how
long it transmits (Full duplex)
• Synchronization: Allows processes to add check points. E.g.
Insert check point at every 100 page of 2000 page file to ensure
that each 100-page unit is received & acknowledged
• Transmits Data
LAYER 4: TRANSPORT
• It regulates information flow to ensure process-to- process
connectivity between host applications reliably and accurately
• Adds service point address or Port address
• Segmentation & Re-assembly: SEGMENTS data from sending
node and reassembles data on receiving node
• Flow control / Error control at Source to destination level
• Connection oriented transport service ensures that data is
delivered error free, in sequence with no losses or duplications
• Establishes, maintains and terminates virtual circuits
• Connection oriented / Connectionless: TCP (Reliable, provides
guaranteed delivery), UDP (Unreliable, less overhead, reliability
can be provided by the Application layer)
• Provides multiplexing; the support of different flows of data to
different applications on the same host
LAYER 3: NETWORK
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Defines source to destination delivery of packets across NWs
Defines logical addressing
No need if 2 systems on same Link
Treat each packet independently
Defines how routing works and how routes are learned
Converts frames to packets
Routed protocols ( encapsulate data into packets) and Routing
protocols (create routing tables) work on this layer
Examples of Routed protocols are: IP, IPX, AppleTalk and
Routing protocols are OSPF, IGRP/EIGRP, RIP, BGP
Relieves higher layers from knowing about underlying data
transmission and switching technologies used to interconnect
systems
May use packet switched / circuit switched networks
Routers operate at Layer 3.
LAYER 2: DATA LINK
• Packages raw bits from the physical layer into FRAMES
• The data link layer provides reliable transit of data across a
physical link by using the Media Access Control (MAC)
addresses
– Source & Destination ( address of device that connects one
Network to next) address
• Flow Control: Prevent overwhelming of Receiving Node
• Error Control: Through Trailer
• Access Control: Which device to have control
• Data Link LAN specifications: Fast Ethernet, Token Ring, FDDI
• Data Link WAN specifications are: Frame Relay, PPP, X.25
• Bridges and Switches operate at this layer
LAYER 1: PHYSICAL
• Deals with the physical characteristics of the Transmission
medium
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Transmits bits from one computer to another
Electrical: Voltage levels / timing of voltage changes
Mechanical: Physical connectors (DB-25)
Procedural: Sequence of events by which bit stream is
exchanged across Physical medium
• Data rates / Maximum transmission distances
• Physical Topology
• Synchronization of bits: Tx & Rx sync
• Line configuration: Pt –to –Pt / Multi Point
• Transmission modes & Repeaters operate at this layer
Data Encapsulation
• Data Encapsulation is the process of adding a header to wrap
the data that flows down the OSI model.
• The 5 Steps of Data Encapsulation are:
1. The Application, Presentation and Session layers create
DATA from users' input.
2. The Transport layer converts the DATA to SEGMENTS
3. The NW layer converts the Segments to Packets (datagram)
4. The Data Link layer converts the PACKETS to FRAMES
5. The Physical layer converts the FRAMES to BITS.
Important
• Data-Link: Communication between nodes on same network
• Network: Comm between nodes on possibly different NWs
• Transport: Communication between processes (running on
machines on possibly different networks)
• Connecting Networks Devices
– Repeater:
Physical layer
– Bridge:
Data link layer
– Router:
Network layer
– Gateway:
Transport layer and above
SUMMARY
• No standard for networks in the early days
• Difficult for Networks to communicate with each other
• ISO recognised this and researched various network schemes,
and in 1984 introduced OSI reference model
• The OSI ensure greater compatibility and interoperability
between various types of network technologies
• Organizes network functions into seven numbered layers
• Each layer provides a service to the layer above it and
communicates with the same layer on other computers
• Layers 1-4 are concerned with the flow of data from end to end
through the network and Layers 5-7 are concerned with
services to the applications