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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
from one node to the next.2/15/2011
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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2/15/2011
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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