Transcript Document

CIT 513: Introduction
to Computer
Communications
Mohammed A. Saleh
http://ifm.ac.tz/staff/msaleh/CIT513.html
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The OSI Reference Models
What is the OSI reference model?
 In the 1970 the International Standards Organization
(ISO) developed the Open Systems Interconnection
(OSI) reference model to define the basic standards for
network communication
 It sets up the rules that all computers have to use no
matter what protocol they are using
 The OSI model is made up seven (7) layers.
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Cont …
•Application
•Presentation
•Session
•Transport
•Network
•Data Link
•Physical
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•All
•Away
•People
•Pizza
•Seem
•Sausage
•To
•Throw
•Need
•Not
•Data
•Do
•Processing
•Please
Easy way to remember the 7 layers of the OSI model
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Cheat Sheet
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Cont …
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This is what I would like to call the cheat sheet of the
OSI model.
The sheet gives out what the OSI model is comprised of,
not everything but covers what you need to know at this
level
The sheet clearly shows that there are two OSI models,
one on the sending computer and the other on the
receiving computer
From the sending computer the arrows are pointing
downwards whereas on the receiving side the arrows are
pointing upwards
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Cont …
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The reason for the upward and downward direction is to
show how data travels through the OSI model
This is how data travels through logically.
Even though data travels down travels in the upward and
downward directions, the dashed lines represents the
virtual communication that takes place.
Virtual communication means once data travels across
the network, the receiving physical layer only
acknowledges what the sending physical layer did to the
data and then passes it up to the data link layer.
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Cont …
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As data travels down the layers, each layers adds its
own piece of information to the data
Each layer adds a slice to one side of the data, except
data link layer that also adds information to the opposite
on the other side.
Whatever is added to the data by the layers can also be
said that the layers add header information to the data
(a)
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(b)
(a) Original data from the application layer moving down
the other layers
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(b) Data after having added header information
Cont …
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the additional data added by the data link layer is known
as trailer information
The reason is to make sure the data sent is not
damaged by the air
Once the data arrives on the receiving side, it is stripped
off as it moves up the layers, leaving only the original
sent data
Video on the OSI Layers
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About the OSI Model
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The OSI model divides network architecture into seven
layers.
The term layer refer to a collection of similar functions
that provide service to the layer above it and requests
service from the layer below it.
Each layer is responsible for a very specific task.
The function of one layer is not known by other layers,
which makes the OSI model a suitable model to be used
by network communications.
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Functions of the OSI Layers
Physical Layer
 It identifies the physical characteristics of the network,
including the following specifications:
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Hardware: The type of media used on the network such as type
of cable, type of connector, and pin out format for cables.
Topology: The physical layer identifies the topology to be used in
the network. Common topologies include ring, mesh, star, and
bus.
Data-Link Layer
 Is responsible for getting data to the physical layer so
that it can be transmitted over the network
 Also responsible for error detection, error correction, and
hardware addressing
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Cont …
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The term frame is used to describe the logical grouping
of data at the data-link layer.
The data-link layer has two distinct sub-layers the Media
Access Control (MAC) sub-layer and the Logical Link
Control (LLC) sub-layer.
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MAC layer: The MAC address is defined at this layer. The MAC
address is the physical or hardware address burned into each
NIC.
LLC layer The LLC layer is responsible for the error and flowcontrol mechanisms of the data-link layer
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Cont …
Network Layer
 The primary responsibility of the network layer is routing
providing mechanisms by which data can be passed
from one network system to another
 It does not specify how the data is passed, but rather
provides the mechanisms to do so.
 Functionality at the network layer is provided through
protocols
 Protocols at the network layer are also responsible for
route selection, which refers to determining the best path
for the data to take throughout the network
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Cont …
In contrast to the data-link layer, which uses MAC
addresses to communicate on the LAN, network
protocols use software configured addresses and special
routing protocols to communicate on the network.
 The main protocol operating at this layer is the Internet
Protocol (IP)
 The term packet is used to describe the logical grouping
of data at the network layer.
Transport Layer
 It provides mechanisms to transport data between
network devices
 Primarily it does this in three ways:
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Cont …
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Error checking Protocols at the transport layer ensure that data
is sent or received correctly.
Service addressing Protocols such as TCP support many
network services. The transport layer makes sure that data is
passed to the right service at the upper layers of the OSI model.
Segmentation: To traverse the network, blocks of data need to
be broken down into packets that are of a manageable size for
the lower layers to handle
Session Layer
 Is responsible for managing and controlling the
synchronization of data between applications on two
devices
 It does this by establishing, maintaining, and breaking
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sessions.
Cont …
Presentation Layer
 Its basic function is to convert the data intended for or
received from the application layer into another format
 Transforms data into the form that only the application
layer can accept
 Some common data formats handled by the presentation
layer include the following:
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Graphics files JPEG, TIFF, GIF.
Text and data: translate data into different formats such as American
Standard Code for Information Interchange (ASCII) and the
Extended Binary Coded Decimal Interchange Code (EBCDIC).
Sound/video MPEGs, QuickTime video, and MIDI files
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Cont …
A very important function of the presentation layer is
encryption, which is the scrambling of data so that it can't
be read by anyone other than the intended recipient.
Application Layer
 The function of the application layer is to take requests
and data from the users and pass them to the lower
layers of the OSI model
 Incoming information is passed to the application layer,
which then displays the information to the users
 The most common misconception about the application
layer is that it represents applications that are used on a
system such as a Web browser, word processor, or a
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spreadsheet
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Cont …
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Instead, the application layer defines the processes that
enable applications to use network services, like web,
email, file transfer, terminal emulation programs
Each of the services has is represented by a
corresponding protocol;
Service
Protocol
Web
Hyper Text Transfer Protocol (HTTP)
Email
Simple Mail Transfer Protocol
(SMTP)
File Transfer
File Transfer Protocol (FTP)
T E programs
telnet
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Questions