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8/7/2019 1-Introduction to Computer Networks
http://slidepdf.com/reader/full/1-introduction-to-computer-networks 1/39
Introduction to Computer
NetworksS.S. Satapathy
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Introduction- Computer Network
- Collection of Computer Systems interconnected bycommunication channels
HH
H
H
H
H
HH H
H Subnet
H - Host
- Switch
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Network Types• Based on the scope and size the networks
are classified as:– WAN – Satellite, Long distance optical
fiber, leased line based networks spreadacross cities, states or countries.
– MAN – Spread over an area with a radiusof up to several tens of km.• Wi Max, Cable based networks.
– LAN – Confined within a hall, building or a campus.• Ethernet, Wi Fi.
– PAN/ BAN – Confined within a room.•
Bluetooth
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Topology
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Network Topology• A network's topology is the layout of the
cables and devices that connect the nodes.• The four most common network topologies
are:
– Bus. Each node is connected in series along asingle conduit.
– Star. All nodes are connected to a central hub.
– Ring. Nodes are connected in a circular chain,
with the conduit beginning and ending at thesame computer.
– Mesh. Each node has a separate connection toevery other node.
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Switching Techniques
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Switching Techniques
• Circuit Switching
• Packet Switching
– Virtual Circuit– Datagram
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Simple Switched Network
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Circuit Switching• Dedicated communication path between two stations
• Three phases– Establish
– Transfer
– Disconnect
• Must have switching capacity and channel capacity to establish
connection• Must have intelligence to work out routing
• Inefficient– Channel capacity dedicated for duration of connection
– If no data, capacity wasted
• Set up (connection) takes time• Once connected, transfer is transparent
• Developed for voice traffic (phone)
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Packet Switching
• Data transmitted in small packets
containing
– user data &
– some control info
• Routing (addressing) info
• Packets are received, stored briefly
(buffered) and past on to the next node– Store and forward
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Use of Packets
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Advantages• Line efficiency
– Single node to node link can be shared by many
packets over time
– Packets queued and transmitted as fast as possible
• Data rate conversion– Each station connects to the local node at its own
speed
– Nodes buffer data if required to equalize rates
• Packets are accepted even when network is busy– Delivery may slow down
• Priorities can be used
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Datagram• Each packet treated
independently• Packets can take any
practical route
• Packets may arrive
out of order • Packets may go
missing
• Up to receiver to re-order packets andrecover from missingpackets
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VirtualCircuit• Preplanned route
established before any
packets sent• Call request and call
accept packets establishconnection (handshake)
• Each packet contains avirtual circuit identifier
instead of destinationaddress
• No routing decisionsrequired for each packet
• Clear request to dropcircuit
• Not a dedicated path
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Virtual Circuits v Datagram• Virtual circuits
– Network can provide sequencing and error control
– Packets are forwarded more quickly• No routing decisions to make
– Less reliable• Loss of a node looses all circuits through that node
• Datagram
– No call setup phase• Better if few packets
– More flexible• Routing can be used to avoid congested parts of the
network
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Packet Size
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Multiple Access Methods
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Multiple Access Methods
Three major types:
– Frequency Division Multiple Access (FDMA)
– Time Division Multiple Access (TDMA)
– Code Division Multiple Access (CDMA)
• Frequency hopping (FH-CDMA)
• Direct sequence (DS-CDMA)
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Frequency-Time Plane
Time
Frequency
Partition of signalspace into time slots and frequency bands
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FDMA
Time
Frequency
Different userstransmit at differentfrequency bands simultaneously.
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Frequency Division Multiple Access
(FDMA)
• The spectrum of each link (forward or reverse) is
further divided into frequency bands
• Each station assigned fixed frequency band
f r e q u e n c
y b a n d s
idle
idle
idle
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TDMA
Time
Frequency
Different userstransmit at differenttime slots.
Each user occupy thewhole freq. spectrum.
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Time Division Multiple Access
(TDMA)
• The mobile users access the channel in round-robin fashion.
• Each station gets one slot in each round.
Slots 2, 5 and 6 are idle
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FDMA/TDMA, example GSM
1 2 3 7 8
f
t
124
1
124
1
20 MHz
200 kHz
890.2 MHz
935.2 MHz
915 MHz
960 MHz
Each freq. carrier is divided into 8 time slots.
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CDMA
Received
signal
Use B’s
Chip
User A’s
Chip
• Spread spectrum modulation– originally developed for the
military
– resists jamming and many kindsof interference
• All users share same (large)block of spectrum
• Almost all accepted 3G radiostandards are based on
CDMA– CDMA2000, W-CDMA and TD-
SCDMA
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Frequency Hopping CDMA
Frequency
Time
At each successive time
slot, the frequencyband assignments arereordered.
Each user employs acode that dictates the
frequency hoppingpattern.
Benefits include improved privacy,decreased narrowband
interference
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Direct Sequence CDMA
Time
Frequency
All users occupy the wholebandwidth all the time.
Signals of different usersoverlap with one other.
How can it be done?
N t k A hit t
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Network Architecture– Has to deal with very wide range of issues
– Cable, connector, signal form, voltage, current . . .
– Procedural steps in sending signals.– Reliably exchanging data between adjacent switches.– Dealing with interference, errors, loss of data.– Specifying the destination host.– Finding the path to the destination.
– Security issues.– Issues specific to kind of use- Application– . . .
– Designed in layered modules- each module/
layer taking care of a set of issues– Each layer has a set of rules & mechanisms (protocol)
for the exchange of information.
– The set of layers, also called the protocol stack,define the architecture of the network.
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ISO/OSI Reference Model
• To address the growing tangle of incompatibleproprietary network protocols, in 1984 the ISO formed a
committee to devise a unified protocol standard.
• The result of this effort is the ISO Open Systems
Interconnection Reference Model (ISO/OSI RM).
• The ISO’s work is called a reference model because
virtually no commercial system uses all of the features
precisely as specified in the model.
• The ISO/OSI model does, however, lend itself to
understanding the concept of a unified communications
architecture.
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ISO/OSI Reference Model• The OSI RM contains seven protocol
layers, starting with physical media
interconnections at Layer 1, throughapplications at Layer 7.• OSI model defines only the functions of
each of the seven layers and theinterfaces between them.
• Implementation details are not part of themodel.
• Interface: It defines which primitiveoperations and services the lower layer offers to the upper layer.
• Peer: The similar layer on a differentmachine.
• Protocol: It is a set of rules andconventions used by the layer to
communicate with similar peer layer inanother (remote) system.
• The peer processes communicate with eachother using a protocol .
• A set of layers and protocols is called anetwork architecture.
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• The Physical layer receives a stream of bits from the Data Link layer above it,
encodes them and places them on thecommunications medium.• The Physical layer conveys transmission
frames, called Physical Protocol DataUnits, or Physical PDUs. Each physical
PDU carries an address and hasdelimiter signal patterns that surroundthe payload , or contents, of the PDU.
• It concerns with transmitting raw bitsover a communication channel.
• It deals with:– Voltage Levels for 0 and 1.– Connectors: Number of bins and purpose of
each bin.– Transmission Media.
– Attenuation and Distortion.
Physical layer
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• The Data Link layer negotiates framesizes and the speed at which they aresent with the Data Link layer at theother end.
– The timing of frame transmission iscalled flow control .
• Data Link layers at both endsacknowledge packets as they areexchanged. The sender retransmitsthe packet if no acknowledgement isreceived within a given time interval.
Data Link layer
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• At the originating computers, the
Network layer adds addressinginformation to the Transport layer PDUs.
• The Network layer establishes the routeand ensures that the PDU size iscompatible with all of the equipment
between the source and the destination.• Its most important job is in moving PDUs
across intermediate nodes.• It deals with:
– Routing: It determines how packets are
routed from source to destination.– Congestion Control: Many packets in the
subnet trying to use the same route.– Internetworking: It allows heterogeneous
networks to be interconnected.– Accounting Function.
Network layer
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• the OSI Transport layer provides end-to-end acknowledgement and error correctionthrough its handshaking with the Transportlayer at the other end of the conversation.
• Transport layer assures the Session layer that there are no network-induced errors inthe PDU.
• Disassembling and Reassembling: Itaccepts data from a session layer, split it upto smaller units if needed, pass these to thenetwork layer, and ensure that the pieces allarrive correctly at the other end.
• End-to-end error control.• End-to-end flow control.• It defines Quality of Service (QOS).• It is an end-to-end layer. Lower layers
communicate with intermediate nodes.
Transport layer
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• It allows users on different machines to establishsessions between them.
• Interaction Management: The Session layer arbitrates the dialogue between twocommunicating nodes, opening and closing thatdialogue as necessary. The data exchangeassociated with a dialog may be:– Duplex: Two-way simultaneous.
– Half-Duplex: Two-way alternate.– Simplex: One-way.
• Synchronization: For lengthy transaction, theuser may choose to establish synchronizationpoints associated with the transfer. If a faultdevelops during a transaction, the dialog may be
restarted at an agreed synchronization point• It also supplies recovery checkpoints during file
transfers.• Checkpoints are issued each time a block of data
is acknowledged as being received in goodcondition.
Session layer
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• The Presentation layer provideshigh-level data interpretationservices for the Application layer above it, such as EBCDIC-to-ASCII translation.
• Presentation layer services arealso called into play if we useencryption or certain types of data compression.
Presentation layer
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• The Application layer suppliesmeaningful information andservices to users at one end of the communication andinterfaces with system resources(programs and data files) at theother end of the communication.
• All that applications need to do isto send messages to the
Presentation layer, and the lower layers take care of the hard part.
Application layer
D t t i i i OSI R f
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Data transmission in OSI Reference
Model
Data
DataPH
DataAH
DataSH
DataTH
DataNH
DataDH DT
Application
Data Link
Presentation
Network
Physical
Session
Transport
Receiving
Process
Application
Data Link
Presentation
Network
Physical
Session
Transport
Sending
Process
Bits
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