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LECTPA

NETWORLAYERS IN THE OSI

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URE 3T 2

MODELSnd TCP/IP MODEL

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PHYSICAL LAYER

The physical layercoordinates the functions

required to transmit a bit

stream over a physical

medium. It also defines theprocedures and functions

that physical devices and

interfaces have to perform

for transmission occur.

The physical layer is responsible for trans

next.

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itting individual bits from one node to the

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Services provided by the Phys

The physical layer is concerned with t

Physical characteristics of interf

defines the characteristics of th

transmission media, including it

Representation of the bits: the

of bits without any interpretati

encoded into signals electrical

the type ofencoding.

Data rate: The physical layer def

number of bits sent each secon

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cal layer to the upper layers

e following:

aces and media: The physical layer

interface between devices and the

s type.

hysical layer data consist of a stream

n. To be transmitted, bits must be

or optical-. The physical layer defines

nes the transmission rate, the

.

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Services provided by the Phy

Line configuration: the physiconnection of devices to the

Physical topology; here it defi

make a network e.g. mesh h o

Transmission Mode; it also de

between two devices e.g. sim

Synchronization of bit; the se

use the same bit rate but also

level;

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ical layer to the upper layers

al layer is concerned with theedium.

es how devices are connected to

star topology

ines the direction of transmission

lex, duplex, half duplex

der and receiver not only must

ust be synchronized at the bit

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DATA LINK LAYER

The data link layer transforms

the physical layer, a raw

transmission facility, to a

reliable link and is

responsible for node-to-node

delivery. It makes the physical

layer appear error free to the

upper layer (network layer).

The data link layer is responsi

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le for transmitting frames

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Services provided by the Data

Framing; the data link layer divid

the network layer into manageabl

Physical addressing; if frames are

systems on the network, the data

to define the sender and or the re

Flow control; if the rate at which

is less than the rate at which data

link layer imposes a flow control

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Link layer to the upper layers

s the stream of bits received from

data units called frames

o be distributed to different

link layer adds a header to the frame

eiver of the frame

the data are absorbed by the receiver

re produced in the sender, the data

mechanism to prevent this.

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Services provided by the Data

Error control; the data link layer

by adding mechanisms to detect a

frames. It also uses a mechanism t

control is normally achieved thro

frame

Access control; when two or mor

link, data link layer protocols are

device has control over the link at

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Link layer to the upper layers

dds reliability to the physical layer

d retransmit damaged or lost

o recognise duplicate frames. Error

gh a trailer added to the end of the

e devices are connected to the same

ecessary to determine which

any given time

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hop-to-hop delivery

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NETWORK LAYER

The Network layer is responsible for t

of a packet possible across multiple net

If two systems are connected to the sa

for a network layer. However, if the tw

networks, there is often a need for the

source-to-destination delivery.

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he source-to-destination delivery

works.

me link, there is usually no need

systems are attached to different

network layer to accomplish

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Services provided by the Net

The network layer is responsible for the

source to the final destination.

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ork Layer to the Upper layers

elivery of packets from the original

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Logical addressing. The physical addr

layer handles the addressing problem

The network layer adds a header to t

among other things, includes the log

Routing. When independent networ

create an internetwork (a networ

connecting devices (called routers or

to their final destination.

Services provided by the Net

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essing implemented by the data link

locally.

e packet coming from the upper layer,

cal address of the sender and receiver.

s or links are connected together to

of networks) or a large network, the

gateways) route or switch the packets

ork Layer to the Upper layers

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Source-to-destination del

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ivery

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TRANSPORT LAYER

The transport layer is responsible

the entire message.

While the network layer oversees

individual packets, it does not rec

those packets.

The transport layer ensures that t

in order, overseeing both error c

process-to-process level.

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for process-to-process delivery of

host-to-destination delivery of

ognize any relationship between

e whole message arrives intact and

ntrol and flow control at the

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Transport layer

The transport layer is responsible

one process to another.

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or delivery of a message from

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Services provided by the Trans

Port addressing: computers ofteprograms) at the same time. Proc

delivery from a specific process o

on the other. The transport layercalled port address. The network

computer; the transport layer get

process on that computer.

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port Layer to the Upper layers

run several processes (runningss-to-process delivery means

one computer to a specific process

eader include a type of addressayer gets each packet to the correct

the entire message to the correct

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Services provided by the Tran

Segmentation and reassembly: a m

segments, each having a sequence

transport layer to reassemble the

destination.

Connection control: The transportconnection-oriented. A connectio

segment as an independent packet

at the destination machine. A conn

makes a connection with the trans

first before delivering the packets.

connection is terminated.

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port Layer to the Upper layers

essage is divided into transmittable

umber. These numbers enable the

essage correctly upon arrival at the

layer can be either connectionless orless transport layer treats each

and delivers it to the transport layer

ection-oriented transport layer

ort layer at the destination machine

After all the data are transferred, the

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Services provided by the Trans

Flow control: the transport lay

control

Error control: the transport lay

error control rather than acrosstransport layer makes sure that

receiving transport layer witho

duplication). Error control is usretransmission

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ort Layer to the Upper layers

r performs end to end flow

r performs process to process

a single link. The sendinghe entire message arrives at the

t error (damage, loss,

ually achieved through

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Th T t l td t d ti ti d li


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The Transport layer contd: so

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rce to destination delivery

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THE SESSION LAYER

The session layer is the network dialog c

maintain, and synchronize the interactio

Services provided to the upper layers:

Dialog control; it allows two systems to

communication between two processes t

duplex.

Synchronization; allows a process to add

a data stream of data e.g. if a system is se

checkpoints after every 100 pages to ens

acknowledged independently. Incase a cr

523, the only pages that need to be rese

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ntroller. It was designed to establish,

between communicating devices.

enter into a dialog. It allows the

o take place in either half duplex or full

checkpoints, or synchronization points, to

nding 2000 pages, it is advisable to insert

ure that each 200mpage unit is received and

ash happens during the transmission of page

t will be from page 501 to 523

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The session layer

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THE PRESENTATION LAYER

The presentation layer is concer

the information exchanged betw

for data translation, encryption, d

Services provided to the upper

Translation; the processes (run

usually exchanging information

numbers e.t.c. the information m

being transmitted. Because d

encoding systems the preseinteroperability between these di

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ed with the syntax and semantics of

en the two systems. It was designed

ecryption, and compression.

layers:

ing programs) in two systems are

in the form of character streams,

ust be changed to bit streams before

ifferent computers use different

tation layer is responsible forferent encoding methods.

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Services provided to the upper

Encryption; encryption means th

information into another form anover the network

Compression; data compression r

in the information. Data compres

in the transmission of multimedia

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layers

t the sender transforms the original

sends the resulting message out

duces the number of bits contained

ion becomes particularly important

like video and audio

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THE APPLICATION LAYER

The application layer enables theuser interfaces and support for serviaccess, WWW, and so on

It provides the following services

Network virtual terminal; is a softwallows a user to log on to a remote hsoftware emulation of a terminal at t

talks to the software terminal which File transfer, access and management

files on a remote host (to make chanremote computer to a local computeremote computer.

Mail services; the application providstorage.

Directory services; the application paccess for global information about v

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user to access the network. It provideses such electronic email, remote file

o the user

re version of a physical terminal, and itst; to do so the application creates ae remote host. The users computer

in turn talks to the host, and vice-vicer; the application allows a user to accesses or read data), or retrieve flies on a

r and to manage or control files on a

s the basis for mail forwarding and

ovides distributed database sources andarious objects and services

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Application layer


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Application layer

The application layer is responsible for

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roviding services to the user.

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Summary of the services pr

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vided by each layer

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Why OSI did not take over the Bad timing

Bad technology

Bad implementations

Bad politics

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A Critique of the OSI Mod

orld

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l and Protocols


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NETWORK MODEL 2-TCP

Most widely used interoperable n

Specified and extensively used be

OSI was slow to take place in

Funded by the US Defense Advan

(DARPA) for its packet switched

DoD automatically created an

Used by the Internet and WWW

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IP PROTOCOL SUITE

etwork protocol architecture

ore OSI

he market

ed Research Project Agency

etwork (ARPANET)

enormous market for TCP/IP


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TCP/IP Protocol Suite

TCP/IP does not have an official l

But protocols imply one Application layer

Transport (host to host) layer

Internet layer

Network access layer Physical layer

Actually TCP/IP reference model

That is why that reference mo and this is why it is not so imp

TCP/IP; understanding TCP, Iwould be enough

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yer structure

has been built on its protocols

el is only for TCP/IP protocol suitertant to assign roles to each layer inand the application protocols

OSI TCP/IP


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OSI vs. TCP/IP

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IP


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PHYSICAL LAYER (NETWOR

The network access layer is conce

packet requires to actually make It includes the LAN and WAN tec

contained in the OSI physical and

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ACCESS LAYER)

ned with all of the issues that an IP

physical link to the network media.nology details, and all the details

data link layers.

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INTERNET LAYER


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INTERNET LAYER

The purpose of the Internet layer

node and have them arrive at thepath taken.

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is to send packets from a network

estination node independent of the

TRANSPORT LAYER


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TRANSPORT LAYER

Five basic services:

Segmenting upper-layer applicati

Establishing end-to-end operation

Sending segments from one end h

Ensuring data reliability

Providing flow control

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n data

s

ost to another end host

Transport Layer


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Transport Layer

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APPLICATION LAYER


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APPLICATION LAYER

Handles high-level protocols, issu

dialog control. The TCP/IP protocol suite combi

one layer and ensures this data ison to the next layer.

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s of representation, encoding, and

nes all application related issues intoroperly packaged before passing it

Si il iti f th OSI dTC /IP M d l


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Similarities of the OSI and TC

Both have layers.

Both have application layers, thou

services.

Both have comparable transport a

Packet-switched, not circuit-swit

Networking professionals need to

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/IP Models

h they include very different

d network layers.

hed, technology is assumed.

know both models.

Diff f th OSI d TC

/IP M d l


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Differences of the OSI and TC

TCP/IP combines the presentatioapplication layer.

TCP/IP combines the OSI data li

TCP/IP appears simpler because

TCP/IP transport layer using UD

delivery of packets as the transpo

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/IP Models

n and session layer into its

k and physical layers into one layer.

t has fewer layers.

P does not always guarantee reliable

t layer in the OSI model does.

IP (INTERNET PROTOCOL) ( 1)


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IP (INTERNET PROTOCOL) (gr

The core of the TCP/IP protocol

Two versions co-exist v4 the widely used IP protocol

v6 has been standardized in 1996,

IP (v4) header minimum 20 octet

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up 1)

suite

but still not widely deployed

s (160 bits)

TCP


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TCP

Transmission Control Protocol

end to end protocol Reliable connection = provides

In TCP terms, a connection is a

temporary association between entities

TCP PDU Called TCP segment

Includes source and destination

Identify respective users (a

pair of ports (together withconnection; such an identifitrack segments between en

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flow and error control

in different systems

port

plications)

the IP addresses) uniquely identify acation is necessary in order TCP toities.

TCP Header


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TCP Header

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UDP


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UDP

User Datagram Protocol

Alternative to TCP

end-to-end protocol

Not guaranteed delivery

No preservation of sequence No protection against duplication

Minimum overhead

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PDUs inTCP/IP


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PDUs in TCP/IP

Dest. Port

Sequence number

Checksum.

Dest. Address

Source address

.

Dest. Network Address

Priority info

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S f th f ti f th f th TCP /IP d l


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Summary of the functions of the

Applicat

Application progra

Transport La

Management of end-to-e

error detection a

Network

Handling of datagrams

Data Li Management of cost effecti

access to phy

Physic

Physic

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ayers of the TCP /IP model

on Layer

s using the network

er (TCP/UDP)

d message transmission,

d error correction

ayer (IP)

: routing and congestion

k Layere and reliable data delivery,

ical networks

l Layer

l Media

A Critique of theTCP/IP R ference Model


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A Critique of the TCP/IP R

Problems:

Service, interface, and protocol n

Not a general model

Host-to-network layer not reall

No mention of physical and data l Minor protocols deeply entrench

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ference Model

t distinguished

a layer

nk layersd, hard to replace

Next lecture


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Next lecture

We shall look at the layer 3 prot

detail (TCP and UDP)

We shall look at addressing

We shall review chapter 2 and int

Thanks

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cols of the TCP/IP in a little more

oduce chapter 3

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