Chapter One - Introduction to Computer
Networks And Data Communications
Network Architectures
• A reference model that describes the layers of
hardware and software necessary to transmit data
between two points or for multiple devices /
applications to interoperate
• Reference models are necessary to increase likelihood
that different components from different
manufacturers will converse
• Two models to learn: OSI model and TCP/IP protocol
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suite
Chapter One - Introduction to Computer
Networks And Data Communications
Network Architectures
• The OSI model’s seven layers:
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Chapter One - Introduction to Computer
Networks And Data Communications
Network Architectures - OSI
• Application layer – where the application using the
network resides. Common network applications
include web browsing, e-mail, file transfers, and
remote logins
• Presentation layer – performs a series of
miscellaneous functions necessary for presenting the
data package properly to the sender or receiver
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Chapter One - Introduction to Computer
Networks And Data Communications
Network Architectures - OSI
• Session layer – responsible for establishing sessions
between users
• Transport layer – provides an end-to-end error-free
network connection. Makes sure the data arrives at
the destination exactly as it left the source.
• Network layer – responsible for creating, maintaining
and ending network connections. Transfers a data
packet from node to node within the network.
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Chapter One - Introduction to Computer
Networks And Data Communications
Network Architectures - OSI
• Data link layer – responsible for taking the data and
transforming it into a frame with header, control and
address information, and error detection code
• Physical layer – handles the transmission of bits over
a communications channel. Includes voltage levels,
connectors, media choice, modulation techniques
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THE SEVEN OSI REFERENCE MODEL LAYERS
Each layer defines a family of function distinct from those of
the other layers.
An Exchange Using the OSI Model
Physical Layer
• Coordinates the function required to
transmit a bit stream over a physical
medium.
• Defines the procedures and function that
physical devices and interfaces have to
perform for transmission to occur.
Physical Layer
Physical layer concerns
• Physical characteristics of interface and media.
The physical layer defines the characteristics of
the interface between devices and the
transmission medium.
• Representation of bit.
The physical layer defines the type of encoding
(how 0s and 1s are changed to signals)
• Data rate: The number of bits send per second.
• Synchronization of bits: The sender and the
receiver clocks must be synchronized.
Physical layer concerns
• Line configuration.
The physical layer is concerned with the
connection of devices to the medium.
• Physical topology.
The physical topology defines how devices
are connected to make a network.
• Transmission mode.
The physical layer also defines the direction
of transmission between two devices.
Data Link Layer
Transforms the physical layer, a raw
transmission facility, to a reliable link
and it responsible for node-to-node
delivery.
Data Link Layer
Data Link layer responsibilities
• Framing: The data link layer divides the stream
of bits received from the network layer into
manageable data units called frames.
• Physical addressing: The data link layer adds a
header to the frame to define the physical address of the
sender (source address) and/or receiver (destination
address) of the frame.
• Flow control: If the rate at which the data are absorbed
by the receiver is less than the rate produced in the
sender, the data link layer impose a flow control
mechanism to prevent overwhelming the receiver.
Data Link layer responsibilities
• Error control: The data link layer adds reliability
to the physical layer by adding mechanism to
detect and retransmit damage or lost frames and
prevent duplication of frames.
• 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.
Data Link layer
Network layer
• Responsible for the source-to-destination
delivery of a packet possibly across
multiple networks (links).
• The network layer ensures that each
packet gets from its point of origin to its
final destination.
Network Layer
Network layer responsibilities
• Logical addressing:
– The network layer adds a header to the packet
coming from the upper layer, includes the logical
addresses of the sender and receiver.
• Routing:
– When independent networks or link are connected
together to create an internetwork ( a network of
networks) the connecting devices (router or
gateways) route the packet to their final
destination.
Network Layer Example
Network Layer Example
Transport layer
• Responsible for source-to-destination
(end-to-end) delivery of the entire
message.
• Ensure that the whole message arrives
intact and in order.
• Transport layer, may create a
connection between the two end port.
Transport layer responsibilities
• Service-point addressing: gets the entire message to
the correct process on that computer, not only from
one computer to the next but also from specific
process on the other.
• Segmentation and reassembly: divides the message
into segments, each segment containing a sequence
number, these numbers enable the T.L to reassemble
the message when it arrived correctly.
• Connection control:
– Connectionless : each segment is an
independent packet
– Connection-oriented: make a connection with
the transport layer on the receiving machine
before delivering the packets
Transport layer responsibilities
• Flow control: end-to-end flow control
• Error control: the sending transport layer
makes sure that the entire message
arrives at the receiving transport layer
without error (damage, loss,…)
Transport Layer
Transport Layer Example
Session layer
Is the next dialog controller, it establishes,
maintain, and synchronizes the interaction
between communicating systems.
Session layer responsibilities
• Dialog control:
The Session layer allows two systems to enter into
dialog. Allows the communication between two
processes to take place either in half-duplex (one
way at a time) or full-duplex ( two ways at a time)
• Synchronization:
Session layer allows a process to add checkpoints
(synchronization points) into a stream of data.
Session Layer
Presentation layer
Is concerned with the syntax and
semantics of the information exchanged
between two systems.
Presentation layer responsibilities
• Translation:
Presentation layer is responsible for interoperability
between these different encoding methods.
• Encryption:
– A system must be able to assure privacy.
– Encryption means, that the sender transforms the
original information to another form and sends
the resulting message out over the network.
• Compression:
Data compression reduce the number of bits to be
transmitted.
Presentation Layer
Application layer
The application layer enables the user,
wither human or software, to access the
network. It provides user interfaces and
support for services, such as electronic
mail, remote file access and transfer….
Application layer services
• Network Virtual Terminal.
– Network virtual terminal is a software version of a physical
terminal and allows a user to log on a remote host.
• File transfer, Access, and management (FTAM)
– This application allows a user to access files in a remote
computer, to retrieve file files from a remote computers.
• Mail Services.
– This application provides the basis for e-mail forwarding and
storage.
• Directory services
– This application provides distributed database sources and
access for global information about various objects and
services.
Application Layer
Summary of Layer Functions
Chapter One - Introduction to Computer
Networks And Data Communications
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Chapter One - Introduction to Computer
Networks And Data Communications
Network Architectures
• The TCP/IP protocol suite (DoD protocol suite, Internet
model):
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Chapter One - Introduction to Computer
Networks And Data Communications
Network Architectures – TCP/IP
• Application layer – equivalent to OSI’s application
and presentation layers
• Transport layer – equivalent to OSI’s transport layer
• Network (Internet or internetwork) layer – equivalent
to OSI’s network layer
• Network access (data link/physical) layer –
equivalent to OSI’s data link and physical layers
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Chapter One - Introduction to Computer
Networks And Data Communications
Network Architectures
• Logical and physical connections – A logical
connection is one that exists only in the software,
while a physical connection is one that exists in the
hardware
• Note that in a network architecture, only the lowest
layer contains the physical connection, while are
higher layers contain logical connections
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Chapter One - Introduction to Computer
Networks And Data Communications
The TCP/IP protocol suite in action
• Note the flow of data from user to web browser and
back
• At each layer, information is either added or
removed, depending on whether the data is leaving or
arriving at a workstation
• The adding of information over pre-existing
information is termed encapsulation
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TCP/IP model in summary
Protocol Layering:
The Internet is build this way
• Internet Protocol (IP) provides a way to
deliver packets to a destination
SSH, FTP, HTTP, SMTP
DNS, VoIP
TCP
UDP
Internet Protocol
Internet (IP) addresses
mmhh@dmu.ac.uk (email)
http://www.apoptygma.eu.org (www)
ftp://ftp.uk.debian.org (file transfer)
telnet://towel.blinkenlights.nl (telnet)
144.32.100.24
These are the IP addresses
148.122.211.110 of the above sites. IP addresses
195.224.53.39 are 32 bits grouped into 4 octets.
(Octet = 8 bits – a number from
62.250.7.101
0-255)
Three different kinds of addresses
– Host names (e.g., www.cnn.com)
– IP addresses (e.g., 64.236.16.20)
– MAC addresses (e.g., 00-15-C5-49-04-A9)
MAC Address vs. IP Address
• MAC addresses
– Hard-coded in read-only memory when adaptor is
built
– Like a social security number
– Flat name space of 48 bits (e.g., 00-0E-9B-6E-49-76)
– Portable, and can stay the same as the host moves
– Used to get packet between interfaces on same
network
• IP addresses
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–
–
–
–
Configured, or learned dynamically
Like a postal mailing address
Hierarchical name space of 32 bits (e.g., 12.178.66.9)
Not portable, and depends on where the host is
attached
Protocol Layering:
The Internet is build this way
• TCP—Transmission Control Protocol, reliable
connect-oriented transfer of a byte stream.
• TCP uses packets to maintain connections”
across a network, and thus is layered above IP.
SSH, FTP, HTTP, SMTP
DNS, VoIP
TCP
UDP
Internet Protocol
Transmission Control Protocol
• TCP is connection-oriented.
• HTTP is an application layer protocol
which uses TCP as its transport.
• Each host has a very formal way of
ensuring the accuracy of the message it
receives in a connection-oriented transport.
• Being connection-oriented introduces a
guarantee of reliability in the connection.
• Reliability – every byte of data is
guaranteed to be received at the other end.
Protocol Layering:
The Internet is build this way
• UDP—User Datagram Protocol, best-effort
connectionless transfer of individual messages.
• UDP just sends or receives raw packets with a
best-effort approach, also layered above IP
SSH, FTP, HTTP, SMTP
DNS, VoIP
TCP
UDP
Internet Protocol
Connection-Oriented and Connectionless Services
• Six different types of service.
Protocol Layering:
The Internet is build this way
• SSH, FTP, HTTP, SMTP and many more
applications use TCP connections to
communicate data back and forth
SSH, FTP, HTTP, SMTP
DNS, VoIP
TCP
UDP
Internet Protocol
Protocol Layering:
The Internet is build this way
• DNS, VoIP, and many more applications
use UDP packets to communicate data
SSH, FTP, HTTP, SMTP
DNS, VoIP
TCP
UDP
Internet Protocol
Layers in the Example
HTTP
HTTP protocol
HTTP
TCP
TCP protocol
TCP
IP
Ethernet
Ethernet
argon.tcpiplab.edu
128.143.137.144
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IP
IP protocol
Ethernet
IP protocol
Ethernet
Ethernet
router71.tcpip- router137.tcpiplab.edu
lab.edu
128.143.137.1
128.143.71.1
00:e0:f9:23:a8:20
IP
Ethernet
neon.tcpip-lab.edu
128.143.71.21
Layers in the Example
HTTP
TCP
IP
Frame is an IP
datagram
Ethernet
Send HTTP Request
to neon
Establish a connection to 128.143.71.21 at
port 80Open TCP connection to
128.143.71.21 port 80
IP datagram is a TCP
segment for port 80
Send
IP data-gram
to
Send a datagram (which
contains
a connection
Send IP datagram
to
IP
128.143.71.21
request) to 128.143.71.21
128.143.71.21
Frame is an IP
datagram
Send the datagram to 128.143.137.1
Ethernet
Ethernet
HTTP
TCP
IP
Send the datagram
Ethernet
to 128.143.7.21
argon.tcpipneon.tcpip-lab.edu
router71.tcpip- router137.tcpipSend Ethernet frame
Send Ethernet frame
lab.edu
128.143.71.21
lab.edu
to 00:20:af:03:98:28
to 00:e0:f9:23:a8:20 lab.edu
128.143.137.144
128.143.137.1
128.143.71.1
00:e0:f9:23:a8:20
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Layers and Services
• Service provided by TCP to HTTP:
– reliable transmission of data over a logical
connection
• Service provided by IP to TCP:
– unreliable transmission of IP datagrams across
an IP network
• Service provided by Ethernet to IP:
– transmission of a frame across an Ethernet
segment
• Other services:
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– DNS: translation between domain names and IP addresses
– ARP: Translation between IP addresses and MAC addresses
Sending a packet from Argon to Neon
argon.tcpip-lab.edu
"Argon"
128.143.137.144
neon.tcpip-lab.edu
"Neon"
128.143.71.21
router137.tcpip-lab.edu
"Router137"
128.143.137.1
router71.tcpip-lab.edu
"Router71"
128.143.71.1
Router
Ethernet Network
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Ethernet Network
is notArgon
on my local
Sending128.143.71.21
a packet from
to network.
Neon
Therefore, I need to send the packet to my
128.143.71.21
on my local
network.
default
gateway withisaddress
128.143.137.1
DNS:
DNS:
The is
IPisthe
address
address
of
Therefore, I can send the packet directly.
ARP:What
What
theIPMAC
of“neon.tcpip-lab.edu”
“neon.tcpip-lab.edu”?
is of
address
128.143.137.1?
ARP:
TheofMAC
address
128.143.71.21
128.143.137.1 is 00:e0:f9:23:a8:20
argon.tcpip-lab.edu
"Argon"
128.143.137.144
ARP: What is the MAC
ARP:
TheofMAC
address of
address
128.143.71.21?
128.143.137.1 is neon.tcpip-lab.edu
00:20:af:03:98:28
"Neon"
128.143.71.21
router137.tcpip-lab.edu
"Router137"
128.143.137.1
router71.tcpip-lab.edu
"Router71"
128.143.71.1
Router
frame
Ethernet Network
58
frame
Ethernet Network
• DNS – Domain Name System
• DNS is the application that turns our web
site addresses into Internet Protocol
addresses.
• Like an operator, given a name it will
return a phone number.
Chapter One - Introduction to Computer
Networks And Data Communications
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