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DATA
COMMUNICATION
Lecture-8 & 9
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Recap of Lecture 7
Roles of Computer in Network Peer
Client
Server
Network Models Peer to Peer (Workgroup)
Server Based (Domain Environment)
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Overview of Lecture 8 & 9
The OSI Model
Function of Layers
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The layered model that dominated data communications and
networking literature before 1990 was the Open SystemsInterconnection (OSI) model.
Everyone believed that the OSI model would become the ultimate
standard for data communications, but this did not happen.
The TCPIIP protocol suite became the dominant commercial
architecture because it was used and tested extensively in theInternet; the OSI model was never fully implemented.
Established in 1947 by International Standards Organization (ISO)
An ISO standard, that covers all aspects of network
communications is the Open Systems Interconnection (OSI) model.
It was first introduced in the late 1970s.
No physical existence, but it is conceptual that helps to
understand the complex methods of data transmission among
various devices.
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How physically computers communicate and connect.
How bits are presented in a network medium.
How this ensure that data will reach on the target
error free and it will be received by correct computer. How network devices communicate, and how data is
converted from one format to another.
How computer comes to know that when it should
transmit the data and when not. How network devices maintain proper rate of data flow.
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Layered Tasks
Sender, Receiver and Carrier
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Layered Architecture
The OSI model is composed of seven layers ;
Physical (layer1), Data link (layer2), Network (layer3)
Transport (layer4), Session (layer5), Presentation (layer6)
Application (layer7) Layer
Designer identified which networking functions had relateduses and collected those functions into discrete groups thatbecame the layers.
The OSI model allows complete interoperability betweenotherwise incompatible systems.
Each layer uses the services of the layer immediately below it.
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Seven layers of the OSI model
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Peer-to-peer Processes
Layer x on one machine communicates with layer x onanother machine - called Peer-to-Peer Processes.
Interfaces between Layers
Each interface defines what information and
services a layer must provide for the layer aboveit.Well defined interfaces and layer functions
provide modularity to a network Organizations of the layers
Network support layers : Layers 1, 2, 3User support layer : Layer 5, 6, 7It allows interoperability among unrelated software systems
Transport layer (Layer 4) : links the twosubgroups
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The interaction between layers in the OSI model
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Figure 2.4An exchange using the OSI model
The data portion of a packet at level N-1 carries the wholepacket from level N.The concept is called encapsulation.
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Physical Layer
Physical layer coordinates the functions required to transmit
a bit stream over a physical medium.
The physical layer is responsible for movements of individual
bits from one hop (node) to the next.
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Physical Layer
Physical layer is concerned with the following:
(deal with the mechanical and electrical specification of theprimary connections: cable, connector) Physical characteristics of interfaces and medium
Representation of bits / Encoding Data rate : transmission rate
Synchronization of bits
Line configuration
Physical topology
Transmission mode baseband and broadband transmissionsMultiplexing /De multiplexing
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Data Link Layer
The data link layer is responsible for moving
frames from one hop (node) to the next.
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Data Link Layer
Sends data frames from the Network layer to the Physicallayer.
On the receiving end, it packages raw bits from the physicallayer into the data frames.
A data frames is an organized, logical structures in whichdata can placed.
The data link layer is responsible for providing the error freetransfer of these frames from one computer to anotherthrough the physical layer.
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Data Link Layer
waits for an acknowledgement
The Cyclical Redundancy Check (CRC) represents errorcorrection and verification
Data Link Layer is sub-divided into two sub-layers, i.e. Logical Link Control (LLC),
Media Access Control (MAC).
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Data Link Layer
LOGICAL LINK CONTROL
Responsible for establishing and maintaining links (i.e.either logical or physical) between the communicatingdevices.
MEDIA ACCESS CONTROL
controls the way in which multiple devices share the samemedia channel.
It communicated directly with the NICs. All NICs has a unique 12-digit hexadecimal MAC
addresses that are used to establish logical linksbetween computers on the same LAN.
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Data Link Layer
Major duties
Responsible for Node-To-Node Delivery
Framing
Physical addressing
Flow control
Error control
Access control
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Data Link Layer
Hop-to-hop (node-to-node) delivery
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Network Layer
Responsible for the delivery of individual packets from thesource host to the destination host.
Ensures that each packet gets from its point of origin to its final
destination responsible for
Logical addressing
and translating it to physical addresses.
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Network Layer
break large packets into smaller chunks.
Route determination.
Also manages traffic problems on the network
packets switching Routing
Controlling the congestion of data.
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Network Layer
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Network Layer Logical addressing
Routing
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Transport Layer
The transport layer is responsible for the deliveryof a message from one process to another.
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Transport Layer
This layer ensures that packets are delivered error free, in
sequence, and with no losses or duplications.
This layer repackages messages, dividing long messages into
several packets to be transmitted efficiently over the network. At the receiving end, the transport layer unpacks the
messages, resembles the original messages and typically sends
an acknowledgement of receipt.
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Transport Layer
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Transport Layer
Service port addressing
Segmentation and reassembly
Connection control
Flow control
Error control
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Session Layer The session layer is responsible for dialog control and
synchronization.
Session Creation, Maintenance and termination.
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Session Layer
allows two applications on different computers to
establish, use and end a connection called a session.
performs name recognition and the function, such as
security, needed to allow two applications tocommunicate over the network.
provides synchronization between user tasks by placing
checkpoints in the data stream.
Also implements dialog control between communicating
processes, regulating which side transmits, when, for
how long, and so on.
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Presentation Layer
The presentation layer is responsible for translation,
compression, and encryption
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Presentation Layer
determines the format used to exchange data amongnetworked computers.
It can be called the networks translator.
At the sending computer this layer translates data from aformat sent down from the Application layer into acommonly recognized, intermediary format.
At the receiving computer, this layer translates theintermediary format into a format useful to that computers
Application layer.
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Presentation Layer
Responsible for
protocol conversion
translating the data
encryption
Changing on converting the character set
and expanding graphics commands
data compression to reduce the number of bits that needs to betransmitted.
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Application Layer
The application layer is responsible for providing services
to the user.
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Application Layer
serves as the window for the application processes toaccess network service.
The major duties of the application Network virtual terminal
File transfer, access, and management Mail services
Directory services
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Summary
The OSI Model
Function of the Layers
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Suggested Reading
Section
2.1,
2.2, 2.3,
Data Communications and Networking 4th Edition by
Behrouz A. Forouzan