LECTURE 3 PART 2

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LECTURE 3
PART 2

NETWORK MODELS
LAYERS IN THE OSI and TCP/IP MODEL
2/15/2011

LECTURE 3
PART 2

NETWORK MODELS
LAYERS IN THE OSI and TCP/IP MODEL
1
PHYSICAL LAYER


The physical layer
coordinates
the functions
required to transmit a bit
stream over a physical
medium. It also defines the
procedures and functions
that physical devices and
interfaces have to perform
for transmission occur.
The physical layer is responsible for transmitting individual bits from one node to the 
next.
 
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The physical layer is responsible for transmitting individual bits from one node to the
 
2
Services provided by the Physical layer to the upper layers

The physical layer is concerned with the following:


Physical characteristics of interfaces and media: The physical layer
defines the characteristics of the interface between devices and the
transmission media, including its type.


Representation of the bits: the physical layer data consist of a stream
of bits without any interpretation. To be transmitted, bits must be
encoded into signals

electrical or optical
the type of
encoding
.


Data rate: The physical layer defines the transmission rate, the
number of bits sent each second.
2/15/2011

Services provided by the Physical layer to the upper layers

The physical layer is concerned with the following:
Physical characteristics of interfaces and media: The physical layer
defines the characteristics of the interface between devices and the
transmission media, including its type.
Representation of the bits: the physical layer data consist of a stream
of bits without any interpretation. To be transmitted, bits must be
electrical or optical
-. The physical layer defines
Data rate: The physical layer defines the transmission rate, the
number of bits sent each second.
3
Services provided by the Physical layer to the upper layers


Line configuration: the physical layer is concerned with the
connection of devices to the medium.


Physical topology; here it defines how devices are connected to
make a network e.g. mesh h or star topology


Transmission Mode; it also defines the direction of transmission
between two devices e.g. simplex, duplex, half duplex


Synchronization of bit; the sender and receiver not only must
use the same bit rate but also must be synchronized at the bit
level;
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Services provided by the Physical layer to the upper layers

Line configuration: the physical layer is concerned with the
connection of devices to the medium.
Physical topology; here it defines how devices are connected to
make a network e.g. mesh h or star topology
Transmission Mode; it also defines the direction of transmission
between two devices e.g. simplex, duplex, half duplex
Synchronization of bit; the sender and receiver not only must
use the same bit rate but also must be synchronized at the bit
4
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 responsible for transmitting frames 
from one node to the next.
 
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The data link layer is responsible for transmitting frames
 
5
Services provided by the Data Link layer to the upper layers


Framing; the data link layer divides the stream of bits received from
the network layer into manageable data units called frames


Physical addressing; if frames are to be distributed to different
systems on the network, the data link layer adds a header to the frame
to define the sender and or the receiver of the frame


Flow control; if the rate at which the data are absorbed by the receiver
is less than the rate at which data are produced in the sender, the data
link layer imposes a flow control mechanism to prevent this.
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Services provided by the Data Link layer to the upper layers

Framing; the data link layer divides the stream of bits received from
the network layer into manageable data units called frames
Physical addressing; if frames are to be distributed to different
systems on the network, the data link layer adds a header to the frame
to define the sender and or the receiver of the frame
Flow control; if the rate at which the data are absorbed by the receiver
is less than the rate at which data are produced in the sender, the data
link layer imposes a flow control mechanism to prevent this.
6
Services provided by the Data Link layer to the upper layers


Error control; the data link layer adds reliability to the physical layer
by adding mechanisms to detect and retransmit damaged or lost
frames. It also uses a mechanism to recognise duplicate frames. Error
control is normally achieved through a trailer added to the end of the
frame


Access control; when two or more devices are connected to the same
link, data link layer protocols are necessary to determine which
device has control over the link at any given time
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Services provided by the Data Link layer to the upper layers

Error control; the data link layer adds reliability to the physical layer
by adding mechanisms to detect and retransmit damaged or lost
frames. It also uses a mechanism to recognise duplicate frames. Error
control is normally achieved through a trailer added to the end of the
Access control; when two or more devices are connected to the same
link, data link layer protocols are necessary to determine which
device has control over the link at any given time
7
hop-to-hop delivery
2/15/2011 
8
NETWORK LAYER


The Network layer is responsible for the source
of a packet possible across multiple networks.

If two systems are connected to the same link, there is usually no need
for a network layer. However, if the two systems are attached to different
networks, there is often a need for the network layer to accomplish
source-to-destination delivery.
2/15/2011

The Network layer is responsible for the source
-to-destination delivery
of a packet possible across multiple networks.
If two systems are connected to the same link, there is usually no need
for a network layer. However, if the two systems are attached to different
networks, there is often a need for the network layer to accomplish
9
Services provided by the Network Layer to the Upper layers
The network layer is responsible for the delivery of packets from the original 
source to the  final destination.
 
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Services provided by the Network Layer to the Upper layers
The network layer is responsible for the delivery of packets from the original
 
10


Logical addressing. The physical addressing implemented by the data link
layer handles the addressing problem locally.


The network layer adds a header to the packet coming from the upper layer,
among other things, includes the logical address of the sender and receiver.


Routing. When independent networks or links are connected together to
create an
internetwork
(a network of networks) or a large network, the
connecting devices (called routers or gateways) route or switch the packets
to their final destination.

Services provided by the Network Layer to the Upper layers
2/15/2011

Logical addressing. The physical addressing implemented by the data link
layer handles the addressing problem locally.
The network layer adds a header to the packet coming from the upper layer,
among other things, includes the logical address of the sender and receiver.
Routing. When independent networks or links are connected together to
(a network of networks) or a large network, the
connecting devices (called routers or gateways) route or switch the packets

Services provided by the Network Layer to the Upper layers
11
Source­to­
destination delivery
 
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destination delivery
 
12
TRANSPORT LAYER


The transport layer is responsible for process
the entire message.


While the network layer oversees host
individual packets, it does not recognize any relationship between
those packets.


The transport layer ensures that the whole message arrives intact and
in order, overseeing both error control and flow control at the
process-to-process level.
2/15/2011

The transport layer is responsible for process
-to-process delivery of
While the network layer oversees host
-to-destination delivery of
individual packets, it does not recognize any relationship between
The transport layer ensures that the whole message arrives intact and
in order, overseeing both error control and flow control at the
13
Transport layer
The transport layer is responsible for delivery of a message from 
one process to another.
 
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The transport layer is responsible for delivery of a message from
 
14
Services provided by the Transport Layer to the Upper layers


Port addressing: computers often run several processes (running
programs) at the same time. Process
delivery from a specific process on one computer to a specific process
on the other. The transport layer header include a type of address
called port address. The network layer gets each packet to the correct
computer; the transport layer gets the entire message to the correct
process on that computer.
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Services provided by the Transport Layer to the Upper layers

Port addressing: computers often run several processes (running
programs) at the same time. Process
-to-process delivery means
delivery from a specific process on one computer to a specific process
on the other. The transport layer header include a type of address
called port address. The network layer gets each packet to the correct
computer; the transport layer gets the entire message to the correct
15
Services provided by the Transport Layer to the Upper layers


Segmentation and reassembly: a message is divided into transmittable
segments, each having a sequence number. These numbers enable the
transport layer to reassemble the message correctly upon arrival at the
destination.


Connection control: The transport layer can be either connectionless or
connection-
oriented. A connectionless transport layer treats each
segment as an independent packet and delivers it to the transport layer
at the destination machine. A connection
makes a connection with the transport layer at the destination machine
first before delivering the packets. After all the data are transferred, the
connection is terminated.
2/15/2011

Services provided by the Transport Layer to the Upper layers

Segmentation and reassembly: a message is divided into transmittable
segments, each having a sequence number. These numbers enable the
transport layer to reassemble the message correctly upon arrival at the
Connection control: The transport layer can be either connectionless or
oriented. A connectionless transport layer treats each
segment as an independent packet and delivers it to the transport layer
at the destination machine. A connection
-oriented transport layer
makes a connection with the transport layer at the destination machine
first before delivering the packets. After all the data are transferred, the
16
Services provided by the Transport Layer to the Upper layers


Flow control: the transport layer performs end to end flow
control


Error control: the transport layer performs process to process
error control rather than across a single link. The sending
transport layer makes sure that the entire message arrives at the
receiving transport layer without error (damage, loss,
duplication). Error control is usually achieved through
retransmission
2/15/2011

Services provided by the Transport Layer to the Upper layers

Flow control: the transport layer performs end to end flow
Error control: the transport layer performs process to process
error control rather than across a single link. The sending
transport layer makes sure that the entire message arrives at the
receiving transport layer without error (damage, loss,
duplication). Error control is usually achieved through
17
The Transport layer cont’d: source to destination delivery
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The Transport layer cont’d: source to destination delivery
18
2/15/2011

The transport layer
–cont’d
19
THE SESSION LAYER


The session layer is the network
dialog controller
maintain
, and
synchronize
the interaction between communicating devices.

Services provided to the upper layers:

Dialog control; it allows two systems to enter into a dialog. It allows the
communication between two processes to take place in either half duplex or full
duplex.


Synchronization; allows a process to add checkpoints, or synchronization points, to
a data stream of data e.g. if a system is sending 2000 pages, it is advisable to insert
checkpoints after every 100 pages to ensure that each 200mpage unit is received and
acknowledged independently. Incase a crash happens during the transmission of page
523, the only pages that need to be resent will be from page 501 to 523
2/15/2011

dialog controller
. It was designed to
establish,
the interaction between communicating devices.

Services provided to the upper layers:

Dialog control; it allows two systems to enter into a dialog. It allows the
communication between two processes to take place in either half duplex or full
Synchronization; allows a process to add checkpoints, or synchronization points, to
a data stream of data e.g. if a system is sending 2000 pages, it is advisable to insert
checkpoints after every 100 pages to ensure that each 200mpage unit is received and
acknowledged independently. Incase a crash happens during the transmission of page
523, the only pages that need to be resent will be from page 501 to 523
20
The session layer
2/15/2011 
21
THE PRESENTATION LAYER


The
presentation
layer
is
concerned
the
information
exchanged
between
for
data
translation,
encryption,
decryption,

Services
provided
to
the
upper


Translation;
the
processes
(running
usually
exchanging
information
numbers
e
.t.c
.
the
information
must
being
transmitted.
Because
different
encoding
systems
the
presentation
interoperability
between
these
different
2/15/2011

concerned
with
the
syntax
and
semantics
of
between
the
two
systems
.
It
was
designed
decryption,
and
compression.

layers:

(running
programs)
in
two
systems
are
in
the
form
of
character
streams,
must
be
changed
to
bit
streams
before
different
computers
use
different
presentation
layer
is
responsible
for
different
encoding
methods
.
22
Services provided to the upper layers


Encryption; encryption means that the sender transforms the original
information into another form and sends the resulting message out
over the network


Compression; data compression reduces the number of bits contained
in the information. Data compression becomes particularly important
in the transmission of multimedia like video and audio
2/15/2011

Services provided to the upper layers

Encryption; encryption means that the sender transforms the original
information into another form and sends the resulting message out
Compression; data compression reduces the number of bits contained
in the information. Data compression becomes particularly important
in the transmission of multimedia like video and audio
23
THE APPLICATION LAYER


The
application layer
enables the user to access the network. It provides
user interfaces and support for services such electronic email, remote file
access, WWW, and so on

It provides the following services to the user


Network virtual terminal; is a software version of a physical terminal, and it
allows a user to log on to a remote host; to do so the application creates a
software emulation of a terminal at the remote host. The user’s computer
talks to the software terminal which in turn talks to the host, and vice


File transfer, access and management ; the application allows a user to access
files on a remote host (to make changes or read data), or retrieve flies on a
remote computer to a local computer and to manage or control files on a
remote computer.


Mail services; the application provides the basis for mail forwarding and
storage.


Directory services; the application provides distributed database sources and
access for global information about various objects and services
2/15/2011

enables the user to access the network. It provides
user interfaces and support for services such electronic email, remote file

It provides the following services to the user

Network virtual terminal; is a software version of a physical terminal, and it
allows a user to log on to a remote host; to do so the application creates a
software emulation of a terminal at the remote host. The user’s computer
talks to the software terminal which in turn talks to the host, and vice
-vicer
File transfer, access and management ; the application allows a user to access
files on a remote host (to make changes or read data), or retrieve flies on a
remote computer to a local computer and to manage or control files on a
Mail services; the application provides the basis for mail forwarding and
Directory services; the application provides distributed database sources and
access for global information about various objects and services
24
Application layer 
The application layer is responsible for providing services to the user.
 
2/15/2011
 
The application layer is responsible for providing services to the user.
 
25
Summary of the services provided by each layer
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Summary of the services provided by each layer
26
Why OSI did not take over the world

Bad timing

Bad technology

Bad implementations

Bad politics
2/15/2011

A Critique of the OSI Model and Protocols

Why OSI did not take over the world
27

A Critique of the OSI Model and Protocols
NETWORK MODEL 2-
TCP/IP PROTOCOL SUITE


Most widely used interoperable network protocol architecture


Specified and extensively used before OSI


OSI was slow to take place in the market


Funded by the US Defense Advanced Research Project Agency
(DARPA) for its packet switched network (ARPANET)


DoD automatically created an e


Used by the Internet
and WWW
28

TCP/IP PROTOCOL SUITE

Most widely used interoperable network protocol architecture
Specified and extensively used before OSI
OSI was slow to take place in the market
Funded by the US Defense Advanced Research Project Agency
(DARPA) for its packet switched network (ARPANET)
DoD automatically created an e
normous market for TCP/IP
TCP/IP Protocol Suite


TCP/IP does not have an official layer structure


But protocols imply one


Application layer


Transport (host to host) layer


Internet layer


Network access layer


Physical layer


Actually TCP/IP reference model has been built on its protocols


That is why that reference model is only for TCP/IP protocol suite


and this is why it is not so important to assign roles to each layer in
TCP/IP; understanding TCP, IP and the application protocols
would be enough
29

TCP/IP does not have an official layer structure
Actually TCP/IP reference model has been built on its protocols
That is why that reference model is only for TCP/IP protocol suite
and this is why it is not so important to assign roles to each layer in
TCP/IP; understanding TCP, IP and the application protocols
OSI vs. TCP/IP
30 
IP
PHYSICAL LAYER (NETWORK ACCESS LAYER)


The network access layer is concerned with all of the issues that an IP
packet requires to actually make a physical link to the network media.


It includes the LAN and WAN technology details, and all the details
contained in the OSI physical and data
2/15/2011

PHYSICAL LAYER (NETWORK ACCESS LAYER)

The network access layer is concerned with all of the issues that an IP
packet requires to actually make a physical link to the network media.
It includes the LAN and WAN technology details, and all the details
contained in the OSI physical and data
link layers.
31
INTERNET LAYER


The purpose of the Internet layer is to send packets from a network
node and have them arrive at the destination node independent of the
path taken.
32

The purpose of the Internet layer is to send packets from a network
node and have them arrive at the destination node independent of the
TRANSPORT LAYER

Five basic services:


Segmenting upper-
layer application data


Establishing end-to-
end operations


Sending segments from one end host to another end host


Ensuring data reliability


Providing flow control
33

layer application data
end operations
Sending segments from one end host to another end host
2/15/2011

Transport Layer
34
APPLICATION LAYER


Handles high-
level protocols, issues of representation, encoding, and
dialog control.


The TCP/IP protocol suite combines all application related issues into
one layer and
en
sures this data is properly packaged before passing it
on to the next layer.
35

level protocols, issues of representation, encoding, and
The TCP/IP protocol suite combines all application related issues into
sures this data is properly packaged before passing it
2/15/2011

Similarities of the OSI and TCP/IP Models


Both have layers
.


Both have application layers, though they include very different
services
.


Both have comparable transport and network layers


Packet-switched, not circuit-
switched, technology is assumed


Networking professionals need to know both models
36

Similarities of the OSI and TCP/IP Models

Both have application layers, though they include very different
Both have comparable transport and network layers
.
switched, technology is assumed
.
Networking professionals need to know both models
.
2/15/2011

Differences of the OSI and TCP/IP Models


TCP/IP combines the presentation and session layer into its
application layer
.


TCP/IP combines the OSI data link and physical layers into one layer


TCP/IP appears simpler because it has fewer layers


TCP/IP transport layer using UDP does not always guarantee reliable
delivery of packets as the transport layer in the OSI model does
37

Differences of the OSI and TCP/IP Models

TCP/IP combines the presentation and session layer into its
TCP/IP combines the OSI data link and physical layers into one layer
.
TCP/IP appears simpler because it has fewer layers
.
TCP/IP transport layer using UDP does not always guarantee reliable
delivery of packets as the transport layer in the OSI model does
.
IP (INTERNET PROTOCOL)
(group 1)


The core of the TCP/IP protocol suite


Two versions co-exist


v4

the widely used IP protocol


v6

has been standardized in 1996, but still not widely deployed


IP (v4) header minimum 20 octets (160 bits)
38

(group 1)

The core of the TCP/IP protocol suite
has been standardized in 1996, but still not widely deployed
IP (v4) header minimum 20 octets (160 bits)
TCP


Transmission Control Protocol


end to end protocol


Reliable connection
= provides flow and error control


In TCP terms, a
connection
is a

t
emporary association between entities in different systems


TCP PDU


Called “TCP segment”


Includes source and destination port


Identify respective users (applications)


pair of ports (together with the IP addresses) uniquely identify a
connection; such an identification is necessary in order TCP to
track segments between entities
39

= provides flow and error control

emporary association between entities in different systems

Includes source and destination port
Identify respective users (applications)
pair of ports (together with the IP addresses) uniquely identify a
connection; such an identification is necessary in order TCP to
track segments between entities
.
TCP Header
40
UDP


User Datagram Protocol


Alternative to TCP


end-to-end protocol


Not guaranteed delivery


No preservation of sequence


No protection against duplication


Minimum overhead
41
PDUs in TCP/IP
Dest. Port 
Sequence number 
Checksum 
…. 
Dest. Address 
Source address 
…. 
Dest. Network Address 
Priority info
 
42
Summary of the functions of the layers of the TCP /IP model
Application Layer
 
Application programs using the network
 
Transport Layer
 
Management of end­to­
end message transmission, 
error detection and error correction
 
Network
Layer
 
Handling of datagrams : routing and congestion
 
Data Link
 
Management of cost effective and reliable data delivery, 
access to physical networks
 
Physical Layer
 
Physical Media 
43

Summary of the functions of the layers of the TCP /IP model
Application Layer
 
Application programs using the network
 
Transport Layer
(TCP/UDP)
 
end message transmission, 
error detection and error correction
 
Layer
(IP)
 
Handling of datagrams : routing and congestion
 
Data Link
Layer
 
Management of cost effective and reliable data delivery, 
access to physical networks
 
Physical Layer
 
Physical Media
2/15/2011

A Critique of the TCP/IP Reference Model

Problems:

Service, interface, and protocol not distinguished

Not a general model

Host-to-
network “layer” not really a layer

No mention of physical and data link layers

Minor protocols deeply entrenched, hard to replace
44

A Critique of the TCP/IP Reference Model

Service, interface, and protocol not distinguished
network “layer” not really a layer
No mention of physical and data link layers
Minor protocols deeply entrenched, hard to replace
Next lecture 


We
shall
look
at
the
layer
3
protocols
detail
(TCP
and
UDP)


We
shall
look
at
addressing


We
shall
review
chapter
2
and
introduce
Thanks
2/15/2011

protocols
of
the
TCP/IP
in
a
little
more
introduce
chapter
3
45