Computer Network

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 Computer Networks
• A computer network is an interconnected
collection of autonomous computers.
– The goals of a computer network include:
• Sharing: programs (O.S., applications), data,
equipment (printers, disks).
• High reliability: users are more immune from
hardware/software failure.
• Less cost: It is easy to increase the capacity by
adding new machines.
• Communications medium: Users have access to
email and the Internet 2
Computer Networks

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 Computer Networks
Uses of Computer Networks
– Business Application
• Resource Sharing: programs (O.S., applications), equipment
(printers, disks).
• Information Sharing: data (Client-server model)
– Home Application
• Access to remote information (www, news paper, business,
cooking, health, history, etc..)
• Person-to-person communication (text messaging, twitter, etc…)
• Interactive entertainment (Facebook, WhatsApp, hike, etc…)
• Electronic commerce (pay bill, manage bank a/c, online
sale/purchase, etc…)
– Mobile Users
• Mobile computers/note book computers
• PDAs 4
 Network Hardware
• The user machines in a network are called
hosts.
• The hosts are connected by a subnet which
carries messages between hosts.
• The subnet is made up of transmission lines
(trunks, channels, circuits) and switching
elements (computers).
• Computer Networks can be classified by two
dimensions:
– Transmission Technology
– Scale 5
 Network Hardware
There are two types of transmission technology (subnet
design):
• Point-to-Point subnets: Point-to-point links connect
individual pairs of machines. (ex. Postal Service,
mobile).
– Unicasting – one sender and exactly one receiver
• Broadcast subnets: In this system a message is
broadcast over the network and all machines have the
possibility of receiving the message (ex. LAN, WAN).
– Broadcasting – received and processed by every
machine on the network
– Multicasting – received and processed by subset of
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machines
 Network Hardware
• In Point to point model, nodes either employ
circuit switching or packet switching.
• In circuit switching,
– a dedicated communication path is allocated
between A and B, via a set of intermediate
nodes.
– the data is sent along the path as a
continuous stream of bits.
• In packet switching,
– data is divided into packets which are sent
from A to B via intermediate nodes. 7
Network Hardware

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Network Topologies

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 Personal Area Networks (PAN)
• PANs let devices communicate over the
range of a person (ex. Computer and its
peripherals).
• PANs can also be built with other
technologies that communicate over short
ranges, such as RFID, Bluetooth, etc..
• These short range technologies use master-
slave
paradigm.

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Local Area Networks (LAN)

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 Local Area Networks (LAN)
• A LAN is a privately owned network that operates within
and nearby a single building like a home, office or factory.
• LANs are widely used to connect personal computers and
consumer electronics to let them share resources (ex.
printers) and exchange information.
• In another configuration it can be used as wireless LAN
consisting of a radio modem and an antenna (Access Point).
• Typically, wired LANs run at speeds of 100 Mbps to 1 Gbps,
have low delay (microseconds or nanoseconds), and make
very few errors.
• Newer LANs can operate at up to 10 Gbps.
• Various topologies are possible for LAN (bus-based, ring-
based N/W)
• Channel allocation can be Static or dynamic 12
 Metropolitan Area Networks (MAN)
• A MAN is a network with a size between a LAN
and a WAN.
• It normally covers the area inside a town or a city.
• It is designed for customers who need a high-
speed connectivity, normally to the Internet, and
have endpoints spread over a city or part of city
(ex. Cable tv, high speed internet access WiMax).
• It may be private or public.
• It support data & voice.
• It has one or more cable and does not contain
switching element.
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Metropolitan Area Networks (MAN)

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 Wide Area Networks (WAN)
• A WAN spans a large geographical area, often a
country or continent.
• It is available in two configurations namely
switched WAN and poit-to-point WAN.
• The switched WAN connects the end systems,
which usually comprise a router
(internetworking connecting device) that
connects to another LAN or WAN (ex. ATM).
• The point-to-point WAN is normally a line leased
from a telephone or cable TV provider that
connects a home computer or a small LAN to an
Internet service provider (lSP). 15
Wide Area Networks (WAN)

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 Internetworks
• A collection of interconnected networks is called
an internetwork or internet.
• Connecting a LAN and a WAN or connecting two
LANs is the usual way to form an internetwork.
• People on one n/w can communicate with people
on different n/w.
• It is widely used to connect universities,
government offices, companies and also private
individuals.
• Applications: Email, News, Remote login, file
transfer, etc…
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Network Software

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 Protocols and Standards
• A Protocol is a set of rules that govern data
communications
• A Protocol defines: what is communicated,
how it is communicated, & when it is
communicated
• There are three elements of a protocol:
– Syntax: The term syntax refers to the
structure or format of the data, meaning
the order in which they are presented.
– Semantics: The word semantics refers to
the meaning of each section of bits. How is 19
 Protocols and Standards
• Standards provide guidelines to manufacturers, vendors,
government agencies, and other service providers to
ensure the kind of interconnectivity necessary in today's
marketplace and in international communication.
• Standards are developed through the cooperation of
standards creation committees, forums, and government
regulatory agencies.
• The various standard creation committees are:
– International Organization for Standardization (ISO)
– International Telecommunication Union-Telecommunication
Standards Sector (ITU-T)
– American National Standards Institute (ANSI)
– Institute of Electrical and Electronics Engineers (IEEE)
– Electronic Industries Association (EIA) 20
The OSI Reference Model
The OSI Reference Model

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The OSI Reference Model
The OSI Reference Model

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Transmit Data from One Network to Another
 The Physical Layer

• This is lowermost layer of the OSI model This layer

consists of simply the wire or media by which the
network signals are conducted. Physical layer
includes hardware (wire, plugs and sockets etc.).
• The basic functions of this layer are handles
voltages, electrical pulses, connectors and
switches so that data can be transmitted from one
network device to another. 37
 The Data Link Layer
• The data link layer provides access to the networking
media and physical transmission across the media and
this enables the data to locate its intended destination
on a network.
• The data link layer provides reliable transit of data
across a physical link by using the Media Access Control
(MAC) addresses.
• The data link layer uses the MAC address to define a
hardware or data link address in order for multiple
stations to share the same medium and still uniquely
identify each other.
• Concerned with network topology, network access,
error notification, ordered delivery of frames, and flow
control. 38
 The Network Layer
• This layer establishes the route between the
sending and receiving stations.
• It handles the routing of data (sending in the
right direction to the right destination on
outgoing transmissions and receiving
incoming transmission at the packet). The
layer does routing & forwarding of data.
• The network layer also defines how to
fragment a packet into smaller packets to
accommodate different media.
• This layer uses the Internet protocol (IP). 39
 The Transport Layer
• The transport layer is responsible for constructing stream of
data packets, sending and checking for correct delivery.
• This layer manages the end to end control (for example
determining whether all packets have arrived) and error
checking.
• The transport layer ensures data is successfully sent and
received between two nodes.
• If data is sent incorrectly, this layer has the responsibility to
ask for retransmission of the data.
• Specially it provides a reliable network independent message
interchange service to the application group.
• This layer acts as an interface between the bottom and top
three layers.
• This layer uses of TCP (Transmission Control Protocol) & UDP
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(User Datagram Protocol).
 The Session Layer
• The session layer defines how to start, control
and end conversations (called sessions) between
applications.
• This includes the control and management of
multiple bi-directional messages using dialogue
control.
• It also synchronizes dialogue between two hosts'
presentation layers and manages their data
exchange.
• The session layer offers provisions for efficient
data transfer.
• This layer uses POP, TCP/IP protocols. 41
 The Presentation Layer
• The presentation layer ensures that the
information that the application layer of one
system sends out is readable by the
application layer of another system.
• If necessary, the presentation layer
translates between multiple data formats by
using a common format.
• Provides encryption and compression of
data.
• In this layer POP, SMTP, FTP protocol are
used. 42
 The Application Layer
• The application layer is the OSI layer that is closest to
the user.
• It provides network services to the user’s applications.
• It differs from the other layers in that it does not
provide services to any other OSI layer, but rather,
only to applications outside the OSI model.
• Examples of such applications are spreadsheet
programs, word processing programs, and bank
terminal programs.
• The application layer establishes the availability of
intended communication partners, synchronizes and
establishes agreement on procedures for error
recovery and control of data integrity. 43
 The TCP/IP Reference Model
• A set of protocols allowing communication across
diverse networks
• It is named from two of the most important
protocols in it:
– the Transmission Control Protocol (TCP) and
– the Internet Protocol (IP).
• The TCP/IP protocol suite is the engine for the
Internet and networks worldwide.
• The main design goal of TCP/IP was to build an
interconnection of networks, referred to as an
internetwork, or internet, that provided universal
communication services over heterogeneous
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physical networks.
The TCP/IP Reference Model

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 Network Interface Layer (Link)
• Responsible for sending and receiving TCP/IP
packets on the network medium
(physical/Data Link)
• Applicable LAN technologies
– Ethernet, Token Ring, FDDI (Fiber
Distributed Data Interface) etc.
• Applicable WAN technologies
– X.25 (old), Frame Relay, ATM etc.
• Note that some technologies such as ATM
and FDDI may be used at both the WAN and
the LAN levels 46
 Internet Layer
• Packaging
• Addressing
• Routing
• IP
– A connectionless unreliable protocol that is part of
the TCP/IP protocol suite
• ARP (Address Resolution Protocol)
– Resolves IP addresses to MAC addresses
• ICMP (Internet Control Message Protocol)
– Diagnostics and error reporting
• IGMP (Internet Group Management Protocol)
– Management of group multicast 47
 Internet Layer
• The internetwork layer, also called the internet
layer or the network layer, provides the “virtual
network” image of an internet (this layer
shields the higher levels from the physical
network architecture below it).
• Internet Protocol (IP) is the most important
protocol in this layer. It is a connectionless
protocol that does not assume reliability from
lower layers.
• IP does not provide reliability, flow control, or
error recovery. These functions must be
provided at a higher level. 48
 Transport Layer
• Sequencing and transmission of packets
• Acknowledgment of receipts
• Recovery of packets
• Flow control
• In essence, it engages in host-to-host transportation of
data packets and the delivery of them to the application
layer.
• TCP (Transmission Control Protocol): provides
connection-oriented reliable data delivery, duplicate data
suppression, congestion control, and flow control.
• UDP (User Datagram Protocol): provides connectionless,
unreliable, best-effort service. UDP is used by
applications that need a fast transport mechanism and
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can tolerate the loss of some data.
 Application Layer
• The application layer is provided by the program
that uses TCP/IP for communication.
• The interface between the application and transport
layers is defined by port numbers and sockets.
• This layer contains all the high level protocols: virtual
terminal (TELNET), file transfer (FTP) and electronic
mail (SMTP).
• The virtual terminal protocol allows a user on one
machine to log into a distant machine and work
there.
• The file transfer protocol provides a way to more
data efficiency from one machine to other.
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Comparison of OSI & TCP/IP Models
• The OSI & TCP/IP models are more or less similar.
The layer functionality is similar.
• The two models can be distinguished based on
concepts:
– Service; Interfaces; Protocols
• OSI: Each layer in OSI performs some service for the
layer above it. A layer’s interface tells the processes
above it how to access it. The peer protocol used in
the layer are the layer’s own business. It can use
ant protocol it want to.
• TCP/IP: The TCP/IP model did not clearly distinguish
between service, interface and protocol. 52
Comparison of OSI & TCP/IP Models
• The OSI model was devised before the
invention of protocols, hence they are not
biased towards one particular set of it.
• The OSI model have 7 layers while TCP/IP
have only 4 layers.
• The OSI model supports both connectionless
and connection-oriented communication in
the network layer, but only connection-
oriented communication in the transport
layer.
• The TCP/IP model has only one 53
 X.25 Network
• X.25 is a standard for WAN communications that
defines how connections between user devices and
network devices are established and maintained.
• It is typically used in the packet-switched networks
(PSNs) of common carriers, such as the telephone
companies.
• Subscribers are charged based on their use of the
network.
• The devices used in X.25 network fall into three
general categories:
– data terminal equipment (DTE),
– data circuit-terminating equipment (DCE),
– packet-switching exchange (PSE) 54
X.25 Network

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 X.25 Network
• Data Terminal Equipment (DTE) devices are end systems
that communicate across the X.25 network.
• They are usually terminals, personal computers, or
network hosts, and are located on the premises of
individual subscribers.
• Data communication Equipment (DCEs) are
communications devices, such as modems and packet
switches that provide the interface between DTE devices
and a PSE, and are generally located in the carrier's
facilities.
• Packet-switching Exchange (PSEs) are switches that
compose the bulk of the carrier's network. They transfer
data from one DTE device to another through the X.25
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PSN.
 X.25 Network
Packet Assembler/Disassembler
• The packet assembler/disassembler (PAD) is a
device commonly found in X.25 networks.
• The PAD is located between a DTE device and
a DCE device and it performs three primary
functions:
– buffering (storing data until a device is
ready to process it),
– packet assembly,
– and packet disassembly
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 X.25 Network
The X.25 protocol suite maps to the lowest three layers
of the OSI reference model. The layers are:
• Physical layer: Deals with the physical interface
between an attached station and the link that
attaches that station to the packet-switching node.
X.21 is the most commonly used physical layer
standard.

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 X.25 Network - Protocol Suite
• Frame (Link) layer: Facilitates reliable transfer of
data across the physical link by transmitting the
data as a sequence of frames. Uses Link Access
Protocol Balanced (LAPB), bit oriented protocol.
• Packet layer: Responsible for end-to-end
connection between two DTEs. Functions
performed are:
– Establishing connection
– Transferring data
– Terminating a connection
– Error and flow control
– With the help of X.25 packet layer, data are transmitted
in packets over external virtual circuits. 59
Asynchronous Transfer Mode (ATM)
• ATM is an international standard for cell relay
wherein information for multiple service types,
such as voice, video, or data, is conveyed in small,
fixed-size cells.
• ATM networks are connection-oriented.
• Technically, it can be viewed as an evolution of
packet switching.
• ATM integrates the multiplexing and switching
functions, is well suited for bursty traffic (in contrast
to circuit switching), and allows communications
between devices that operate at different speeds.
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ATM Protocol Architecture

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 ATM Protocol Architecture
• Physical layer of the ATM protocol architecture
involves the specification of a transmission
medium and a signal encoding scheme.
• The main functions of the ATM physical layer
are as follows:
– Cells are converted into a bit stream,
– The transmission and receipt of bits on the
physical medium are controlled,
– ATM cell boundaries are tracked,
– Cells are packaged into the appropriate
types of frames for the physical medium. 62
 ATM Protocol Architecture
• ATM Layer
– The ATM layer provides routing, traffic
management, switching and multiplexing
services.
– It processes outgoing traffic by accepting 48-byte
segment from the AAL sub-layers and
transforming them into 53-byte cell by addition
of a 5-byte header.
• ATM Adaptation Layer (AAL)
– Mapping the higher-layer information into ATM
cells to be transport over an ATM network.
– Collecting information from ATM cells for delivery
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to higher layers.
 ATM Protocol Architecture
• The ATM reference model consists of the following
planes, which span all layers:
• Control—This plane is responsible for generating and
managing signaling requests.
• User—This plane is responsible for managing the
transfer of data.
• Management—This plane contains two components:
– Layer management manages layer-specific
functions, such as the detection of failures and
protocol problems.
– Plane management manages and coordinates
functions related to the complete system.
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 ATM Network Interfaces
• An ATM network consists of a set of ATM
switches interconnected by point-to-point ATM
links or interfaces.
• ATM switches support two primary types of
interfaces: UNI and NNI.
• The UNI (User-Network Interface) connects ATM
end systems (such as hosts and routers) to an
ATM switch.
• The NNI (Network-Network Interface) connects
two ATM switches.
• UNI and NNI can be further subdivided into
public and private UNIs and NNIs. 65
 ATM Cell Format
• ATM transfers information in fixed-size units
called cells.
• Each cell consists of 53 octets, or bytes.
• The first 5 bytes contain cell-header
information, and the remaining 48 contain
the payload (user information).
• Small, fixed-length cells are well suited to
transfer voice and video traffic because such
traffic is intolerant to delays that result from
having to wait for a large data packet to
download, among other things. 66
 Frame Relay
• Frame Relay is a high-performance WAN protocol
that operates at the physical and data link layers
of the OSI reference model.
• Frame Relay originally was designed for use
across Integrated Services Digital Network (ISDN)
interfaces
• Frame Relay is based on packet-switched
technology.
• The following two techniques are used in packet-
switching technology:
– Variable-length packets
– Statistical multiplexing 67
Frame Relay

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 Frame Relay
• Devices attached to a Frame Relay WAN fall into
the following two general categories:
– Data terminal equipment (DTE)
– Data circuit-terminating equipment (DCE)
• Examples of DTE devices are terminals, personal
computers, routers, and bridges.
• DCEs are carrier-owned internetworking devices.
The purpose of DCE equipment is to provide
clocking and switching services in a network,
which are the devices that actually transmit data
through the WAN. In most cases, these are
packet switches. 69