Download The OSI Model - Institute of Technology Sligo

The OSI Model
Institute of Technology Sligo - Dept of Computing
Semester 1
Chapter 2
The OSI Model
Paul Flynn
Institute of Technology Sligo - Dept of Computing
The OSI Model
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 Systems 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.
Institute of Technology Sligo - Dept of Computing
The OSI Model
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.
Institute of Technology Sligo - Dept of Computing
The OSI Model
What are the 7 layers of the OSI Model?
Institute of Technology Sligo - Dept of Computing
The OSI Model
Why use a Layered Model?
Institute of Technology Sligo - Dept of Computing
The OSI Model
Remembering the OSI Model
• All
• People
• Seem
• To
• Need
• Domino’s
• Pizza
7 - Application
6 - Presentation
5 - Session
4 - Transport
3 - Network
2 - Data Link
1 - Physical
Institute of Technology Sligo - Dept of Computing
The OSI Model
Remembering the OSI Model
• Away
• Pizza
• Sausage
• Throw
• Not
• Do
• Please
7 - Application
6 - Presentation
5 - Session
4 - Transport
3 - Network
2 - Data Link
1 - Physical
Institute of Technology Sligo - Dept of Computing
The OSI Model
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…
Institute of Technology Sligo - Dept of Computing
Layer 1 – Physical Layer – Continued…
The OSI Model
• 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.
Institute of Technology Sligo - Dept of Computing
The OSI Model
Some Layer 1 Examples…
10 Base 2 – Co Ax
Fibre Optic Connectors
10 Base 5 – Thicknet
UTP
Institute of Technology Sligo - Dept of Computing
The OSI Model
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…
Institute of Technology Sligo - Dept of Computing
Layer 2 – Data Link Layer – Continued…
The OSI Model
• 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) sublayer
• Manages communication between devices over a single link of a network.
• Enables multiple higher-layer protocols to share a single physical data link.
• MAC sublayer
• Manages protocol access to the physical network medium.
• Determines hardware addresses.
Institute of Technology Sligo - Dept of Computing
The OSI Model
Some Layer 2 Examples…
Network Interface
Card (NIC)
Workgroup Switch
Institute of Technology Sligo - Dept of Computing
Layer 3 – Network Layer
• Defines logical addressing for nodes and networks/segments.
The OSI Model
• 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.
Institute of Technology Sligo - Dept of Computing
Layer 4 – Transport Layer
The OSI Model
• 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 connectionoriented, 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.
Institute of Technology Sligo - Dept of Computing
The OSI Model
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.
Institute of Technology Sligo - Dept of Computing
The OSI Model
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.
Institute of Technology Sligo - Dept of Computing
The OSI Model
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.
Institute of Technology Sligo - Dept of Computing
The OSI Model
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.
Institute of Technology Sligo - Dept of Computing
The OSI Model
Data Encapsulation Diagram
Institute of Technology Sligo - Dept of Computing
What are PDU’s?
The OSI Model
Each layer contains a Protocol Data Unit (PDU). PDU’s are
used for peer-to-peer conversations.
Institute of Technology Sligo - Dept of Computing
The OSI Model
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.
Institute of Technology Sligo - Dept of Computing
The OSI Model
Exploring the 4 Layers of the TCP/IP Model
Application
Includes all the
functions of the OSI’s
Application,
Presentation, & Session
layers including:
Transport
• Data representation
Internet
• Data encryption and
• Dialog control
Network Access
Institute of Technology Sligo - Dept of Computing
The OSI Model
Application Layer Protocols
Application
• FTP-File Transfer
Protocol
• HTTP-Hypertext
Transfer Protocol
Transport
• SMTP-Simple Mail
Transfer Protocol
Internet
• DNS-Domain Name
Service
Network Access
• TFTP-Trivial File
Transfer Protocol
Institute of Technology Sligo - Dept of Computing
The OSI Model
Exploring the 4 Layers of the TCP/IP Model
Application
Transport
Uses the TCP protocol
and is responsible for
quality of service issues
including:
• Reliability
• Flow Control and
Internet
Network Access
• Error Correction
Institute of Technology Sligo - Dept of Computing
The OSI Model
Transport Layer Protocols
Application
• TCP-Transmission
Control Protocol
• UDP-User Datagram
Protocol
Transport
Internet
Network Access
Institute of Technology Sligo - Dept of Computing
The OSI Model
Exploring the 4 Layers of the TCP/IP Model
Application
Uses the IP protocol and
is responsible for:
• Path determination and
• Packet switching.
Transport
Internet
Network Access
Institute of Technology Sligo - Dept of Computing
The OSI Model
Internet Layer Protocols
• IP – Internet Protocol
Application
Transport
Internet
Network Access
Institute of Technology Sligo - Dept of Computing
The OSI Model
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:
• Processes required by
IP to ensure a packet
reaches its destination.
• All the various LAN &
WAN Technologies such
as 100BaseTX & Frame
Relay.
Institute of Technology Sligo - Dept of Computing
The OSI Model
Network Access Layer Protocols
Application
Transport
Internet
Network Access
• LAN and WAN
Technologies
Institute of Technology Sligo - Dept of Computing
The OSI Model
Comparing the 2 Models
TCP/IP
OSI
Application
Application
Presentation
Session
Transport
Transport
Internet
Network
Network Access
Data Link
Physical
Institute of Technology Sligo - Dept of Computing
The OSI Model
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.
Institute of Technology Sligo - Dept of Computing
The OSI Model
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.
Institute of Technology Sligo - Dept of Computing
The OSI Model
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
Institute of Technology Sligo - Dept of Computing
The OSI Model
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
Institute of Technology Sligo - Dept of Computing
The OSI Model
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
Institute of Technology Sligo - Dept of Computing
The OSI Model
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