The OSI Model - Institute of Technology Sligo

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The OSI Model

Institute of Technology

Sligo

-

Dept of Computing

Semester 1

Chapter 2

The OSI Model

Paul Flynn

The OSI Model

Institute of Technology

Sligo

-

Dept of Computing

The OSI Reference Model



The OSI layer was introduced by the International Organization for
Standardization (ISO) in 1984 in order to provide a reference model to
make sure products of different vendors would interoperate in networks.



OSI is short for Open System
s

Interconnection.



The OSI layer shows WHAT needs to be done to send data from an
application on one computer, trough a network, to an application on another
computer, not HOW it should be done.



A layer in the OSI model communicates with three other layers: the layer
above it, the layer below it, and the same layer at its communication
partner.



Data transmitted between software programs passes all 7 OSI layers.



The Application, Presentation and Session layers are also known as the
Upper Layers.



The Data Link and Physical layers are often implemented together to
define LAN and WAN specifications.

The OSI Model

Institute of Technology

Sligo

-

Dept of Computing

What is a Protocol?



A Protocol is a set of rules that make communication on a network
more efficient.



E.G.
-

When answering the telephone, someone says, "Hello," then
the person calling says, "Hello. This is.... "; and so it goes back and
forth



One technical definition of a data communications protocol is: a set
of rules, or an agreement, that determines the format and transmission
of data. Layer n on one computer communicates with Layer n on
another computer.

The OSI Model

Institute of Technology

Sligo

-

Dept of Computing

What are the 7 layers of the OSI Model?

The OSI Model

Institute of Technology

Sligo

-

Dept of Computing

Why use a Layered Model?

The OSI Model

Institute of Technology

Sligo

-

Dept of Computing

Remembering the OSI Model



All


7
-

Application



People

6
-

Presentation



Seem


5
-

Session



To


4
-

Transport



Need


3
-

Network



Domino’s

2
-

Data Link



Pizza


1
-

Physical


The OSI Model

Institute of Technology

Sligo

-

Dept of Computing

Remembering the OSI Model



Away


7
-

Application



Pizza


6
-

Presentation



Sausage

5
-

Session



Throw


4
-

Transport



Not


3
-

Network



Do


2
-

Data Link



Please

1
-

Physical


The OSI Model

Institute of Technology

Sligo

-

Dept of Computing

Layer 1


Physical Layer



The physical layer defines the electrical, mechanical, procedural, and functional
specifications for activating, maintaining, and deactivating the physical link between
communicating network systems.



Physical layer specifications define characteristics such as:



voltage levels



timing of voltage changes



physical data rates



maximum transmission distances



physical connectors



Physical layer implementations can be categorized as either LAN or WAN
specifications.



more on next slide…

The OSI Model

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Dept of Computing

Layer 1


Physical Layer


Continued…



Examples of LAN specifications are:



Ethernet



FastEthernet



Token Ring



FDDI



Examples of WAN specifications are:



HSSI



V.24



V.35



BRI



RS
-
232



Transmits bits. (bitstream)



Repeaters operate at this layer.

The OSI Model

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Dept of Computing

Some Layer 1 Examples…

10 Base 2


Co Ax

Fibre Optic Connectors

10 Base 5


Thicknet

UTP

The OSI Model

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Dept of Computing

Layer 2


Data Link Layer



Defines psychical addressing, network topology, and is also concerned with error
notification, sequencing of frames and flow control.



Examples of network topologies are:



Star



Bus



Ring



Physical addresses are also known as hardware and BIA's (Burned In Addressess)
but most commonly as MAC addresses.



Examples of Data Link LAN specifications are:



Ethernet



FastEthernet



Token Ring



FDDI



More on next slide…

The OSI Model

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Sligo

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Dept of Computing

Layer 2


Data Link Layer


Continued…



Examples of Data Link WAN specifications are:



Frame Relay (operates also on the Physical layer)



PPP (operates also on the Physical layer)



X.25 (operates also on the Physical and Network layer)



Transmits Frames.



Bridges and Switches operate at this layer.



The Data Link layer consists of
two sublayers
:



LCC (Logical Link Control)
subl
ayer



Manages communication between devices over a single link of a network.



Enables multiple higher
-
layer protocols to share a single physical data link.



MAC
subl
ayer




Manages protocol access to the physical network medium.



Determines hardware addresses.

The OSI Model

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Dept of Computing

Some Layer 2 Examples…

Network Interface
Card (NIC)

Workgroup Switch

The OSI Model

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Dept of Computing

Layer 3


Network Layer



Defines logical addressing for nodes and networks/segments.



Enables internetworking, passing data from one network to another.



Defines the logical network layout so routers can determine how to forward packets
trough an internetwork.



Routing occurs at this layer, hence Routed and Routing protocols reside on this layer.



Routed protocols are used to encapsulate data into packets. The header added by the
Network layer contains a network address so it can be routed trough an internetwork.



Examples of Network layer Routed protocols are:



IP
,
IPX
,
AppleTalk



Routing protocols are used to create routing tables; routing tables are used to
determine the best path / route. Routing protocols provide periodic communication
between routers in an internetwork to maintain information on network links in a routing
table.



Examples of Network layer Routing protocols are:



OSPF
,
IGRP/EIGRP
,
RIP
,
BGP
.




Transmits Packets.



Routers operate at this layer.

The OSI Model

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Dept of Computing

Layer 4


Transport Layer



The main purpose of this layers is making sure that the data is delivered error
-
free
and in the correct sequence.



Establishes, maintains and terminates virtual circuits.



Provides error detection and recovery.



Is concerned with reliable and unreliable transport. When using a connection
-
oriented, reliable transport protocol, such as TCP, acknowledgments are send back
to the sender to confirm that the data has been received.



Provides Flow Control and Windowing.



Provides multiplexing; the support of different flows of data to different applications
on the same host.



Examples of Transport layer protocols are:



TCP (connection
-
oriented, reliable, provides guaranteed delivery.)



UDP (connectionless, unreliable, less overhead, reliability can be provided
by the Application layer)



SPX


Transmits Segments.

The OSI Model

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Dept of Computing

Layer 5


Session Layer



The session layer establishes, manages, maintains and terminates
communication channels between software programs on network
nodes.



Provides error reporting for the Application and Presentation layer.



Examples of Session layer protocols are:



NFS



SQL



RPC


Transmits Data.

The OSI Model

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Dept of Computing

Layer 6


Presentation Layer



Defines coding and conversion functions.



Ensures that information sent from the application layer of one system
is readable by the application layer of another system.



Includes common data representation formats, conversion of
character representation formats, common data compression schemes,
and common data encryption schemes, common examples of these
formats and schemes are:



MPEG, QuickTime



ASCII, EBCDIC



GIF, TIFF, JPEG



Transmits Data.

The OSI Model

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Dept of Computing

Layer 7


Application Layer



Provides network services directly to applications. Software programs itself are
not part of the OSI model.



Determines the identity and availability of communication partners, and
determines if sufficient resources are available to start program
-
to
-
program
communication.



This layer is closest to the user.



Examples of Application layer protocols are:



Telnet



SMTP



FTP



SNMP



Transmits Data.

The OSI Model

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Dept of Computing

Data Encapsulation



Data Encapsulation is the process of adding a header to wrap the data that flows
down the OSI model.



Each OSI layer may add it's own header to the data received from above. (from the
layer above or from the software program 'above' the Application layer.)



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 Network layer converts the SEGMENTS to PACKETS (or datagrams)

4. The Data Link layer converts the PACKETS to FRAMES

5. The Physical layer converts the FRAMES to BITS.



At the sending computer the information goes from top to bottom while each layers
divides the information received from upper layers in to smaller pieces and adds a
header. At the receiving computer the information flows up the model discarding the
corresponding header at each layer and putting the pieces back together.

The OSI Model

Institute of Technology

Sligo

-

Dept of Computing

Data Encapsulation Diagram

The OSI Model

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Dept of Computing

What are PDU’s?

Each

layer

contains

a

Protocol

Data

Unit

(PDU)
.

PDU

s

are

used

for

peer
-
to
-
peer

conversations
.


The OSI Model

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Sligo

-

Dept of Computing

The TCP/IP Model



The TCP/IP Model was develop by the Department of
Defense in the late 60s’ to ensure data communications
would not be interrupted even under the worse
circumstances.



Since then, TCP/IP has become the de facto method
we use for data communications on the Internet.

The OSI Model

Institute of Technology

Sligo

-

Dept of Computing

Exploring the 4 Layers of the TCP/IP Model

Application

Transport

Internet

Network Access

Includes all the
functions of the OSI’s
Application,
Presentation, & Session
layers including:



Data representation



Data encryption and



Dialog control

The OSI Model

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Dept of Computing

Application Layer Protocols

Application

Transport

Internet

Network Access



FTP
-
File Transfer
Protocol



HTTP
-
Hypertext
Transfer Protocol



SMTP
-
Simple Mail
Transfer Protocol



DNS
-
Domain Name
Service



TFTP
-
Trivial File
Transfer Protocol

The OSI Model

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Sligo

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Dept of Computing

Exploring the 4 Layers of the TCP/IP Model

Application

Transport

Internet

Network Access

Uses the TCP protocol
and is responsible for
quality of service issues
including:



Reliability



Flow Control and



Error Correction

The OSI Model

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Dept of Computing

Transport Layer Protocols

Application

Transport

Internet

Network Access



TCP
-
Transmission
Control Protocol



UDP
-
User Datagram
Protocol

The OSI Model

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Dept of Computing

Exploring the 4 Layers of the TCP/IP Model

Application

Transport

Internet

Network Access

Uses the IP protocol and
is responsible for:



Path determination and



Packet switching.

The OSI Model

Institute of Technology

Sligo

-

Dept of Computing

Internet Layer Protocols

Application

Transport

Internet

Network Access



IP


Internet Protocol

The OSI Model

Institute of Technology

Sligo

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Dept of Computing

Exploring the 4 Layers of the TCP/IP Model

Application

Transport

Internet

Network Access

Includes all the
functions of the OSI’s
Data Link & Physical
layers including:



P
rocesses required by
IP to ensure a packet
reaches its destination.



A
ll the various LAN &
WAN Technologies such
as 100BaseTX & Frame
Relay.

The OSI Model

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Dept of Computing

Network Access Layer Protocols

Application

Transport

Internet

Network Access



LAN and WAN
Technologies

The OSI Model

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Dept of Computing

Comparing the 2 Models

Application

Transport

Internet

Application

Presentation

Session

Transport

Network

Data Link

Physical

Network Access

TCP/IP

OSI

The OSI Model

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Dept of Computing

Why are there 2 models?



TCP/IP is the most popular “protocol specific” model
used on the Internet.



However, TCP/IP does not cover all the protocols and
standards we will be exploring.



The OSI Model is “protocol independent.” Therefore, all
topics covered in the curriculum can fit into its framework.




The OSI Model

Institute of Technology

Sligo

-

Dept of Computing

What should I know for the test?



The OSI Model’s layers & their key characteristics and
functions.



The process of data encapsulation and the various
PDUs.



The TCP/IP layers and its corresponding protocols.



Comparison of the two model’s layers.




The OSI Model

Institute of Technology

Sligo

-

Dept of Computing

Sample CCNA Question #1

In the TCP/IP model which layer would deal with
reliability, flow control, and error correction?

A


Transport Layer

B


Internet Layer

C


Network Layer

D


Application Layer




The OSI Model

Institute of Technology

Sligo

-

Dept of Computing

Sample CCNA Answer #1

In the TCP/IP model which layer would deal with
reliability, flow control, and error correction?

A


Transport Layer

B


Internet Layer

C


Network Layer

D


Application Layer




The OSI Model

Institute of Technology

Sligo

-

Dept of Computing

Sample CCNA Question #2

What is the control information called that is placed before
data when encapsulating that data for network
transmission?

A


Frame

B


Header

C


Capsule

D


Routing Information




The OSI Model

Institute of Technology

Sligo

-

Dept of Computing

Sample CCNA Answer #2

What is the control information called that is placed before
data when encapsulating that data for network
transmission?

A


Frame

B


Header

C


Capsule

D


Routing Information




The OSI Model

Institute of Technology

Sligo

-

Dept of Computing