2-2008UP-Copyrights reserved1 ITGD4103 Data Communications and Networks OSI Reference Model...

2-2008 UP-Copyrights reserved 1

ITGD4103 Data Communications and Networks

Lecture-5: OSI Reference ModelOSI Reference Model

week 5- q-2/ 2008

Dr. Anwar Mousa

University of Palestine InternationalFaculty of Information Technology

Contents:

1.1.OSI Reference ModelOSI Reference Model

2.TCP/IP

Layers and Stacks

Many different protocol stacks:

OSI Open Systems Interconnection.

TCP/IP Internet Protocols.

SS7 Signaling System #7

The number of layers, and the name, contents, and

function of each layer differ for each protocol stack.

OSI Reference ModelOSI Reference Model

Layering

The separation of the communication functions is

called layering. layering is the communication into separate steps that

are performed sequentially, defined by specific

interfaces for passing the result of each step to the next

layer until the overall function, such as the sending or

receiving of some amount of information, is completed.

Each layer communicates with the layers above and

below through a standard interface.

Reasons for layering

Divides interrelated aspects of network operation into

less complex elements.

Simplifies the network model.

Enables programmers to specialize in a particular level

or layer of the networking model.

Provides design modularity. Prevent changes in one area from impacting other

areas, so each area can evolve more quickly.

Layering Responsibility

End- to- end delivery of packets to the network.

Selecting routes for the packets to take.

Implies knowledge of the network topology.

Managing utilization of the links.

Provide flow control (across multiple links).

Spread load among different routes.

Layering Principle

Each layer uses its own protocol to communicate with

its peer layer in the other system (receiver).

Layer N software on the destination computer must

receive exactly the message sent by layer N software

on the sending computer.

A layer N+1 entity sees the lower layers N only as a

service provider.

Each layer depends on the service function of the layer

below.

An entity of a particular layer can only communicate with:

Peer layer entity using a common protocol (Peer Protocol)

Adjacent layers to provide services and to receive services.

OSI Reference ModelOSI Reference Model

Open Systems Interconnection Reference Model (OSI).

A seven-layer structure designed to describe computer

network architectures and the way that data passes

through them.

It is a layered model used to describe network protocols

and services.

Seven Layer Reference Model

Layered model consists of seven layers:

7. Application

6. Presentation

5. Session

4. Transport

3. Network

2. Data Link

1. Physical

1. The Physical Layer

Responsibility: Transmission of raw bits over a communication

channel (how to transfer bits correctly).

Issues: Physical characteristics of interfaces and

media. Representation of bits. Data rate and transmission mode.

The physical layer coordinates all the functions required

to transmit a bit stream over the physical medium.

2. The Data Link Layer

Responsibility: Transfer data frames correctly and successfully. Provide an error-free communication link. Error control and flow control.

Issues: Framing (dividing data into chunks). Addressing.

10110110101 01100010011 10110000001

3. The Network Layer

Responsibilities:

Move packet inside the network and send it to the

destination.

Path selection between end-systems (routing).

Fragmentation & reassembly.

Translation between different network types.

Issues:

Forwarding, routing, addressing, switching, and

congestion control.

4. The Transport Layer

Responsibilities:

Delivery of the data between two peer processes.

Connection establishment, management,

termination, error control and flow control.

Issues: Headers. Error detection. Reliable communication.

5. The Session Layer

Responsibilities:

Enables two applications on the network to have an

ongoing conversation or dialog.

Allows users to establish, manages, and terminate

session.

Issues: Control for data exchange

Data synchronization.

Failure recovery.

6. The Presentation Layer

Responsibilities:

Data translation.

Data formatting.

Data encryption.

Data compression.

Data syntax restructuring.

7. The Application Layer

Responsibilities:

Providing services to applications.

Enable the user to access the network.

Variety of protocols that are commonly used.

Anything not provided by any of the other layers.

Issues:

File Transfer.

Appropriate selection of “type of service”.

Data transmission using the OSI model:

Layering & Headers

Each layer needs to add some control information to

the data in order to do its job.

This information is added to the data before being

given to the lower layer.

Once the lower layers deliver the data and control

information, the peer layer uses the control

information.

Important Notes

Data-Link Layer :

Communication between machines on the same network.

Network Layer :

Communication between machines on possibly different networks.

Transport Layer :

Communication between processes (running on machines on possibly different networks).

Conclusion

TCP/IP

TCPTransmission Control Protocol

TCP is one of the main protocols in TCP/IP networks.

Whereas the IP protocol deals only with packets.

TCP enables two hosts to establish a connection and

exchange data.

TCP - is responsible for verifying the correct delivery

of data from client to server.

TCP guarantees delivery of data and also

guarantees that packets will be delivered in the

same order in which they were sent.

TCP provides:

Connection-oriented

Reliable

Full-duplex

Byte-Stream

Connection-Oriented

Connection oriented means that a virtual connection

is established before any data is transferred.

If the connection cannot be established, the user

program is notified.

If the connection is ever interrupted, the user

program is notified.

Reliable means that every transmission of data is

acknowledged by the receiver.

If the sender does not receive acknowledgement

within a specified amount of time, the sender

retransmits the data.

Full Duplex:

TCP provides transfer in both directions.

Buffering: TCP is responsible for buffering data and

determining when it is time to send a datagram.

Stream means that the connection is treated as a

stream of bytes.

The user application does not need to package data

in individual datagrams.

TCP Segments

The chunk of data that TCP asks IP to deliver is called

a TCP segment.

Each segment contains:

Data bytes from the byte stream.

Control information that identifies the data bytes.

TCP/IP

The TCP/IP is a protocol suite.

It was initially successful because it delivered a few basic

services that everyone needs (file transfer, electronic mail,

remote logon) across a very large number of client and

server systems.

All TCP/IP protocol suite specifications are in the

public domain. No license fees.

Used to create open system networking environments.

TCP/IP

TCP/IP is a way to communicate reliably between hosts.

The advantages of TCP/IP:

TCP/IP is bound to no manufacturer.

TCP/IP is usable in LANs and WANs.

TCP/IP makes application independent of the transmission system.

TCP/IP Model

TCP/IP Model consists of only five layers.

Physical layer

Network access layer (Data link layer)

Internet layer (Network layer)

Transport layer

Application layer

Session layer and Presentation layer are not actually

defined by TCP/IP.

Protocol layering and data

Each layer takes data from above. Adds header information to create new data unit. Passes new data unit to layer below.

ApplicationTransportNetwork

LinkPhysical

ApplicationTransportNetwork

LinkPhysical

Source Destination

HtHnHl

message

segment

datagram

5. Application Layer : Concerned with how data at both ends is handled.

4. Transport Layer : Manages flow of data.

3. Internet Layer : Consists of several protocols, primary protocol is IP

(providing addressing scheme).

2. Network Access Layer : Manages transmission of data within the network.

1. Physical Layer : TCP/IP leaves the physical connection to manage itself.

1. Physical Layer

Covers the physical interface between a data

transmission device and a transmission medium or

network.

Physical layer specifies:

Characteristics of the transmission medium.

The nature of the signals.

The data rate.

2. Network Access Layer

Equivalent to the OSI’s physical and data link layers.

Concerned with the exchange of data between an end

system and the network to which it's attached.

Software used depends on type of network

Circuit switching

Packet switching

3. IP Internet Layer

Uses Internet Protocol (IP)

Provides routing functions to allow data to traverse

multiple interconnected networks.

Its function is:

Permit hosts to inject packets into any network and

have them travel independently to the destination.

They may even arrive in a different order than they

were sent, in which case it is the job of higher layers

to rearrange them, if in-order delivery is desired.

4. Transport Layer

Defines two standard transport protocols: TCP and UDP

TCP implements a reliable data-stream protocol.

(connection oriented)

TCP is responsible for data recovery.

Provides reliability during data exchange.

UDP implements an unreliable data-stream.

(connectionless)

TCP How it works

The source establishes a connection and sends a limited

number of segments and waits for an ACK

(acknowledgment) from the destination for some of the

segments before sending more.

If no ACK is received within a "timeout" period, then

source assumes unacknowledged segments were lost and

resends those segments (flow control).

The destination sends a NACK (negative acknowledgement)

for any segments with errors and source resends it.

Protocol Data Unit (PDU)

A Protocol Data Unit comprises of data and control

information which is passed between layers in a protocol

stack.

Unreliable, meaning no guarantees that data coming from

the upper layer :

Will arrive at their destination.

Will arrive in the order in which the were sent.

Will arrive intact.

The User Datagram Protocol (UDP) is composed at the

Transport Layer.

Done by taking the data received from the application

program at the Application Layer and adding a small

header to it.

This datagram is then sent down to the Internet layer to

be encapsulated into an IP datagram and sent out in a

connectionless mode.

UDP can send its datagrams to multiple destinations.

UDP Features

Simple protocol:

No connection establishment delays.

No connection state at sender and receiver.

No congestion control.

Small segment header.

UDP Applications

Useful for Applications such as voice and video,

where:

Retransmission should be avoided.

The loss of a few packets does not greatly affect

performance.

Rate sensitive.

5. Application Layer

This layer provides services suitable for the

different types of application that might wish to

use the network .

Common TCP/IP Applications:

Simple mail transfer protocol (SMTP)

File Transfer Protocol (FTP)

TCP/IP Protocol Graph

The TCP/IP Communication

The flow of data from user to web browser and back.

At each layer, information is either added or removed

Depends on whether data is leaving or arriving at a workstation

Encapsulation - adding information over pre-existing information

TCP/ IP Network Communication

OSI versus TCP/IP Model TCP/IP does not map its layers precisely to OSI

OSI = 7 layers

TCP/IP = 5 layers

The OSI and TCP/IP reference models have much in

common. Both are based on the concept of a stack of independent

protocols.

The functionality of the layers is roughly similar.

OSI versus TCP/IP Model

In TCP/IP,

Session Layer’s

characteristics(OSI) are

provided by the

Transport Layer.

The function of

presentation layer(OSI)

is provided by the

Application Layer.

OSI vs TCP/IP

Session

Physical

Transport

Network

Data Link

Presentation

ApplicationApplication

Transport

Data Link

Internet

Conclusion TCP enables two hosts to exchange data.

TCP provides: Connection-oriented, Reliable,

Full-duplex, and Byte-Stream.

TCP/IP Layers: five layers

Physical layer, Network access layer, Internet layer,Transport layer, and Application layer.

TCP and UDP

OSI versus TCP/IP

Conclusion

Summary

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