msc

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Basics of COMPUTER NETWORKS
Dr. Nagesh Yagnam
Ph.D.
Professor
Dept of IT, Debre tabor University

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Basics of
Computer Networks and Internet
What is a network?
 Set of devices communicating with each other
 Could be a CPU, monitor and other peripheral
devices connected (and exchanging data) to each
other
 Could be a group of people …. A network of friends
 Or, could be a set of computers communicating with
each other

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Computer Network
 Technology and architecture of the
communications networks used to interconnect
devices
 An interconnected collection of autonomous
computers is called computer network
 Examples: LAN,MAN,WAN, Internet etc.

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Why Networks?
 Point-to-point communication is not usually
practical because
 Devices are too far apart
 Requires large number of connections
between all devices
 Too expensive
 Solution is a communications network

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Uses of Computer Networks
 Availability of Resource / Resource sharing
- Resources become available regardless of the
user’s physical location
 Load Sharing
- Jobs processed on the least crowded machine
 High Reliability
- File and processor redundancy
 Human-to-Human Communication
- Telephone
- Long distance education and collaboration

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Applications of Computer Networks
 Business Applications
 Home Applications
 Mobile Users
 Social Issues

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Business Applications of Networks
 The client-server model involves requests and
replies
Fig: A network with two clients and one server

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Home Network Applications
 Access to remote information
- World Wide Web
 Person-to-person communication
- Electronic mail, Videoconference
 Interactive entertainment
- Video-on-Demand, Games
 Electronic commerce

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Home Network Applications
 Some forms of e-commerce.

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Mobile Network Users
 Combinations of wireless networks and mobile
computing
-Cellular Phones, Personal Digital Assistant (PDA)
and Smart hones (PDA+ Handset+ GPS+…)

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Network Hardware
Network design dimensions:
 Transmission technology( broadcast networks, Point-
to-point networks)
 Scale( LAN, WAN, Internet etc..)

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Network Classifications
 Based on transmission technology networks are
divided into Broadcast and Point to Point networks
 Broadcast Networks use one communication
channel that is shared by all the machines.
Packets are sent to the shared channel and are
“listened to” by all machines.(for smaller,
geographically localized networks)

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Network Classifications (cont’d)
 Point - to - Point network : A network in which
a physical communication path exists between 2
end-systems with no other devices involved

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Network Classifications (cont’d)
Classification based on scale
 Local Area Networks( LAN):
Room, Building /Campus
 Metropolitan Area Networks
( MAN): City
 Wide Area Networks
(WAN): Country, Continent
 Wireless Networks
 Home networks
 Internetworks: Global

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Local Area Network
 Network in small geographical Area (room
,building or a campus) is called LAN
 A LAN is a data communication system allowing
a number of independent devices to
communicate directly with each other, within a
moderately sized geographic area over a
physical communication channel of moderate
data rates.

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Local Area Network
(Cont’d)
 Short geographical distance (a few kilometers)
 High speed (Larger than 1 Mbps up to 100
Mbps )
 Multiple access (Many can use it at the same
time)
 Sharing (hardware, software etc.)
 IEEE standard 802.3
 The most reliable network

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LAN Topologies

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Metropolitan Area Networks
 Network in a City is call MAN (Metropolitan Area
Network)
 MAN is distinguished by its adopted standard
DQDB (Distributed Queue Dual Bus (IEEE
standard 802.6) and it contains two unidirectional
buses which all the computers are connected
A metropolitan area network based on cable TV

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Definitions
 Network: Interconnected collection of
autonomous computers
 Host: machine running user application
 Subnet: Communication subnet carries
messages between hosts
 Channel: Logical Line of communication (circuit)
 Router: Network router is a device or a piece of
software in a computer that forwards and routes
data packets along networks

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Wide Area Networks
 Network spread geographically (country or
across Globe) is called WAN.
 WAN contains hosts that are connected by
a communication subnet
 The job of the subnet is to carry messages
from host to host like telephone system
carries words from speaker to listener
 Store-and-forward network

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Wide Area Networks (Cont’d)
Fig: Relation between hosts on LANs and the subnet

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Wide Area Networks (Cont’d)
Figure: A stream of packets from sender to receiver

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Networking Devices
HUB, Switches, Routers,
Wireless Access Points,
Modems etc.

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Wireless Networks
Categories of wireless networks:
 (used for) System interconnection
 Wireless LANs
 Wireless WANs

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Wireless Networks (Cont’d)
(a) Bluetooth configuration
(b) Wireless LAN

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Wireless Networks (Cont’d)
(a) Individual mobile computers
(b) A flying LAN

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Internetworks
 internet: a collection of interconnected networks
 Internet: specific worldwide internet
 Subnets: (WAN) collection of routers and
communication lines (without hosts). Ex:
telephone subnet
 Networks: combination of subnet and its hosts.
Ex: telephone network = telephone subnets+
telephones
 internetwork: formed when distinct networks
are connected together. Ex: connecting LAN and
WAN

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Home Network Categories
 Computers (desktop PC, PDA, shared peripherals
 Entertainment (TV, DVD, VCR, camera, stereo,
MP3)
 Telecomm (telephone, cell phone, intercom, fax)
 Appliances (microwave, fridge, clock, furnace etc.)
 Telemetry (utility meter, burglar alarm etc.)

29. What’s the Internet: “nuts and bolts” view
millions of connected
computing devices:
hosts = end systems
 running network
apps
Home network
Institutional network
Mobile network
Global ISP
Regional ISP
router
PC
server
wireless
laptop
cellular
handheld
wired
links
access
points
communication links
 fiber, copper,
radio, satellite
 transmission
rate = bandwidth
routers: forward
packets (chunks of
data)
Introduction 1-29

30. What’s the Internet: “nuts and bolts” view
 protocols control
sending, receiving of
msgs
 e.g., TCP, IP, HTTP,
Ethernet
 Internet: “network of
networks”
 loosely hierarchical
 public Internet versus
private intranet
 Internet standards
 RFC: Request for
Home network
Institutional network
Mobile network
Global ISP
Regional ISP
Introduction 1-30

31. What’s the Internet: a service view
 communication infrastructure
enables distributed
applications:
 Web, VoIP, email, games,
e-commerce, file sharing
 communication services
provided to apps:
 reliable data delivery from
source to destination
 “best effort” (unreliable)
data delivery
Introduction 1-31

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Network Software
 Protocol Hierarchies
 Design Issues for the Layers
 Connection-Oriented and Connectionless Services
 Service Primitives
 The Relationship of Services to Protocols

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 A protocol is an agreement that standardizes the
way communication will be handled
 It is a set of rules and conventions that governs
exchange of data between two systems
 A set of layers and protocols is called a network
architecture
 A list of protocols used by a certain system, one
protocol per layer, is called a protocol stack
Protocol

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Protocol Hierarchies
 Networks are organized as a stack of levels or
layers
 We build each layer on the one below it
 Layers differ in number and function from one
network to another
 Each layer hides underlying details from the one
above it – sort of like a virtual machine
 Each layer talks to the ones above & below it

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Network Software
Protocol Hierarchies
Fig: Layers, protocols, and interfaces

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Data Transfer
 No data is transferred directly from one machine
to another on that layer – the layers can only talk
to the ones above or below them on their host.
 A message from layer 5 will have to travel to
layer 1, move across the physical medium, and
then back up to layer 5 on the different machine.
 Layer 1 is the only layer able to move data from
one machine to another, through the physical
medium.

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Connection-Oriented and
Connectionless Services
 Six different types of service

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Service Primitives
 Five service primitives for implementing a
simple connection-oriented service

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The Relationship of Services to
Protocols
 A service defines what operations the layer is
prepared to perform on behalf of its users, but it
says nothing at all about how these operations
are implemented.
 A protocol, in contrast, is a set of rules governing
the format and meaning of the frames, packets,
or messages that are exchanged by the peer
entities within a layer. Entities use protocols in
order to implement their service definitions.

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Reference Models
The OSI Reference Model
In the late 1970s, to promote the compatibility of
network designs, the International Organization
for Standardization (ISO) proposed an architecture
model called the open systems interconnection
references model (OSI model)
layer N
layer N-1
layer N
layer N-1
PDUs (protocol data unit)
actual data flow on the lowest level
services

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The OSI Reference Model

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The OSI Reference Model
application
application
presentation
session
transport
network
data link
physical
7
6
5
4
3
2
1 transmission of bits
transmission of packets on one given link
end-to-end transmission of packets
end-to-end delivery of messages
setup and management of end-to-end conversation
formatting, encryption, and compression
network services (email, file transfer)

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OSI Layers
Application
Presentation
Session
Transport
Network
Data link
Physical
Application
Presentation
Session
Transport
Network
Data link
Physical
Application
Presentation
Session
Transport
Network
Data link
Physical
App Y
App X
Data
Data
Data
Data
Data
Data
Data
H
H
H
H
H
H
Outgoing Packets Incoming Packets
Physical Path

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The Physical Layer
 Transmission of a raw bit stream
- forms the physical interface between devices
 - Makes sure that when one side sends a 1 bit ,the
other side receives it as a 1.
 - Covers all aspects for communication
 Mechanical interface defines the connectors used,
number of pins, their placement, size, material
used.
 Electrical interface defines the voltage/current levels
of signals.

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The Data Link Layer
 Data link layer attempts to make physical link reliable
 May divide upper layer packet into multiple frames
 Ensure that peer entity will recognize frame
boundaries
 May introduce sequence numbers and
acknowledgments
 Provides reliable transfer of information between two
adjacent nodes
 Creates frames, or packets from bits and vice versa
 Provides frame-level error control
 Provides flow control

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The Network Layer
 Network layer is designed to facilitate communication
between systems across a communication network
 Implements network routing and message delivery
through networks to the correct destination
 Congestion control can also be done at network layer
 Internetworking
 Internet protocol or IP is one example
 Responsible for routing decisions

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The Transport Layer
 Provides reliable end-to-end communication
 May detect erroneous packets
 Reorders packets which arrive out-of-sequence.
 Ensures that there is no loss or duplication of
packets
 May provide connection-less or connection-oriented
type of service
 Provides for the connection management.
 Multiplexing and demultiplexing
 Packetization, flow control, etc.
 Examples: TCP and UDP
 Establishes and deletes connections across the
network

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The Session Layer
 Allows users on different computers to establish a
session
 This layer requests for a logical connection to be
established based on an end-user's request
 This layer provides services like dialogue discipline
which can be full duplex or half duplex
 Session layer can also provide check pointing (or
synchronization) mechanism
 If a failure occurs between checkpoints, all data can
be retransmitted from the last checkpoint
 May perform synchronization and token management
between several communicating applications

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The Presentation Layer
 Performs specific functions that are requested
regularly by applications
 Presentation layer defines the data format to be
exchanged between the programs.
 Manages abstract data structures, and converts
them host-representation to network- representation
and vice versa.
 Encryption

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The Application Layer
 Application layer protocols are application dependent
 Implements communication between two applications
of the same type
- FTP
- SMTP (email) etc.

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What is TCP/IP ?
 The name TCP/IP refers to a suite of data
communication protocols
 Its name comes from two of the more important
protocols in the suite: the Transmission Control
Protocol (TCP) and the Internet Protocol (IP)
 The most common of all network protocol
suites, is the ‘standard’ in modern networks,
used for communication on the Internet
 Although designed for the Internet it is used to
build LANs, WANs and MANs
 Most widely used protocol suite, used within
Unix, Windows and Macintosh platforms

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The TCP/IP Reference model

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The TCP/IP Reference model
Protocols and networks in the TCP/IP model initially

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Network Access
(Host-to-network)
Ethernet, Token Ring, FDDI
Frame Relay, ATM, ISDN
Access commn network
LAN and WAN
technologies
Internet
Transport
Application
Path determination –
routing IP, ARP, ICMP, DHCP
TCP, UDP
DNS
FTP
HTTP
Telnet
SMTP
SNMP
TFTP
HTTP
POP3
Provides access to
network services for the
user and application
programs
Provides end-to-end
transport
The TCP/IP Protocol Suite
Four-Layered Model Protocols

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Host-to-Network Layer
 The TCP/IP model does not really say much about
what happens here, except to point out that the host
has to connect to the network so it can send IP
packets to it
 Access the communication network

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Internet Layer
 Routes data between hosts
 Connectionless
 Every packet routed independently
 Does not guarantee reliable or in-sequence delivery

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Internet Layer (cont’d)
 The internet layer defines an official packet format for
and protocol called Internet Protocol (IP)
 It is the responsibility of the internet layer to get the IP
packets to their destination
 The TCP/IP internet layer is similar in functionality to
the OSI network layer

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Transport Layer
 The transport layer is intended to allow two machines
to carry on a conversation, just like the OSI transport
layer
 There are two protocols defined for this:
 TCP (Transmission Control Protocol)
reliable, connection-oriented
 UDP (User Datagram Protocol)
unreliable, connectionless

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Application Layer
 This layer contains all of the higher-level protocols
 telnet
 FTP
 SMTP
 DNS
 HTTP

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The differences between the TCP/IP
and OSI models
 OSI model clearly defined the distinction between
services, interfaces and protocols, where the
TCP/IP model does not
 This fits in nicely with OO programming concepts as
the layer hide information
 Protocols can easily be replaced in the OSI model, but
not in the TCP/IP model

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The Differences (cont’d)
 The OSI model has no biases for protocols, but the
OSI model was overly general and did not help with
the creation of new protocols
 OSI supports connectionless and connection-oriented
in the network layer, but only connection-oriented in
the transport layer, providing no choice to the user
 TCP/IP supports only connectionless in the network
layer, but both in the transport layer, giving user a
choice

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Example Networks
 The Internet(ARPA NET)
 Connection-Oriented Networks:
X.25, Frame Relay, and ATM
 Ethernet
 Novell Netware
 Wireless LANs: 802:11

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Internet Usage
 Traditional applications (1970 – 1990)
 E-mail
 News
 Remote login
 File transfer