cse 447 lecture 1.ppt
TRANSCRIPT
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Introduction
Course Code: CSE 447
Course Name: Computer NetworksCourse Teacher: Shebuti Rayana
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What is a Computer Network?
It is basically a collection of autonomous computers
interconnected by a single technology.
Computers can be geographically located anywhere.
Two computers are said to be interconnected if they
are able to exchange information.
Connection via copper wire, fiber optics,
microwaves, infrared or communication satellite.
Internet is not a single network it is a network of
networks.
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Uses of Computer Network
Business Applications Resource sharing
Information Sharing
Communication medium
Client-server model E-commerce
Home Applications Access to remote information
Person-to-person communication
Interactive Entertainment
Electronic commerce
Peer-to-peer communication
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Uses of Computer Network (contd)
Mobile Users
Wireless network
Communication
Email Message broadcast
M-commerce
Social Issues
Security
Privacy
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Network Hardware
Two important dimensions:
Transmission Technology
Scale
Two types of transmission technology are in
widespread use:
Broadcast Link
Point-to-point Link
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Broadcast Link
A single communication channel shared by allmachines.
It allows the possibility of addressing a packet
to all destinations by a special code(Broadcasting).
It also supports transmission to a subset of
machines (multicasting). Reserve 1 bit to indicate multicasting
Remaining bits hold a group number
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Point-to-point Link
It consists of many connections between
individual pairs of machines.
On the way from source to destination the packet
may visit one or more intermediate machines.
When there are multiple paths, finding the best
path is the main challenge.
Point-to-point transmission with one sender andone receiver is sometimes called unicasting.
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Example of P2P
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Classifying network using Scale
Classifying the network using their physical
size.
Personal Area Network (PAN)
Local Area Network (LAN)
Metropolitan Area Network (MAN)
Wide Area Network (WAN)
The Internetwork (Internet)
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PAN
Networks that are meant for one person.
1 meter range
For example: A wireless network connecting acomputer with its mouse, keyboard and
printer.
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LAN
Privately owned networks within a singlebuilding or campus of up to a few kilometersin size.
Resource sharing Information exchange etc.
Three characteristics:
Size Transmission Technology
Topology
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LAN (contd)
Speed
10Mbps to 100Mbps for traditional LANs
10Gbps for new ones
Various Topology: Star
Bus etc.
IEEE 802.3 standard, popularly called Ethernetis a bus based broadcast network withdecentralized control(10Mbps-10Gbps).
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LAN (contd)
IEEE 802.5 is a ring based LAN (4 16 Mbps).
Depending on channel allocation broadcast
networks are of two types:
Static
Dynamic
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Static Allocation
Divide the time into discrete intervals and use
a round robin algorithm, allowing each
machine to broadcast only when its time slot
comes up.
It waste channel capacity.
So, most system attempt to allocate the
channel dynamically.
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Dynamic Allocation
Either centralized or decentralized.
Centralized Allocation
A single entity determines who goes next
It may do this by accepting request or by internal
algorithm.
Decentralized Allocation
Each machine decide by itself whether to
transmit.
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Ethernet bus Ethernet hub
transceivers
Figure 1.17Leon-Garcia & Widjaja: Communication
Networks
Copyright 2000 The McGraw Hill
Companies
Wired LANs
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Wireless LANs
Figure (a) Wireless networking with a base station. (b)Ad hoc networking.
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MAN
It covers a city.
Example: Cable
television
network, high-speed wireless
internet.
A metropolitan area network based on cable TV.
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WAN
Spans a large geographic area, a country or a
continent.
Hosts are connected by communication
subnet.
Customer owns the host
Telephone company or ISP owns the subnet
Subnet carries the message from host to host.
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WAN (contd)
In most WANs, subnet consists of twocomponents:
Transmission Lines
Move bits between machines
Made of copper wire, optical fiber or radio link
Switching Elements
Specialized devices connect three or more transmission
lines. When data arrives at incoming line it forward them by
outgoing line.
Example: Router
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WAN (contd)
Packet Switched Subnet(store-and-forward)
Sending host cuts the message into packets.
Each packet has a sequence number. Packets are injected into the network one at a
time.
At the receiver packets are reassembled. Routing decision of the packets are made
locally (Routing Algorithm needed).
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Packet-Switched Subnet
Figure: A stream of packets from sender to receiver.
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WAN (contd)
Satellite system
Each router has an antenna through which it can
send and receive.
Routers hear the output of the satellite and
upward transmission of fellow routers to the
satellite.
Sometimes routers are connected point-to-point subnet, with only some of them having
satellite antenna.
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Network Topology
The network topologydefines the way in whichcomputers, printers, andother devices are
connected.
It describes the layout ofthe wire and devices aswell as the paths used bydata transmissions.
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Bus Topology
Commonly referred to
as a linear bus
All the devices on a bus
topology are connectedby one single cable.
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Star and Tree Topology
The star topology is the most
commonly used architecture
in Ethernet LANs.
When installed, the star
topology resembles spokes
in a bicycle wheel.
Larger networks use the
extended star topology also
called tree topology.
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Star and Tree Topology (contd)
When used with network devices that filter
frames or packets, like bridges, switches, and
routers, this topology significantly reduces the
traffic on the wires by sending packets only tothe wires of the destination host.
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Ring Topology
A frame travels around the
ring, stopping at each node.
If a node wants to transmit
data, it adds the data as well
as the destination address tothe frame.
The frame then continues
around the ring until it
finds the destination node,which takes the data out of
the frame.
Single ringAll the devices
on the network are
organized in a single loop
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Ring Topology
A frame travels around the
ring, stopping at each node.
If a node wants to transmit
data, it adds the data as well
as the destination address tothe frame.
The frame then continues
around the ring until it
finds the destination node,which takes the data out of
the frame.
Dual Ring
The dual
ring topology allows
data to be sent in both
direction.
The primary disadvantage of ring topology is
the failure of one machine will cause the entire
network to fail.
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Mesh Topology
The mesh topology connectsall devices (nodes) to eachother for redundancy and faulttolerance.
It is used in WANs tointerconnect LANs and formission critical networks likethose used by banks andfinancial institutions.
Implementing the meshtopology is expensive anddifficult.
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Network Software
Protocol Hierarchies
Design Issues for the Layers
Connection-Oriented and ConnectionlessServices
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Protocol Hierarchies Networks are organized as a stack of layers or levels, each
one build upon the one below it.
The number of layers, the name of each layer, the contentsof each layer and the function of each layer varies from
network to network.
Each layer is a kind of virtual machine offering certainservice to the layer above it.
Layer n of one machine communicate with layer n of anothermachine using some protocol called layer n protocol(agreement between the two parties on howcommunication is to proceed).
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Layered Tasks
An example from the everyday life
Hierarchy?
Services
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Layers, Protocols, Interfaces
Layer 5
Layer 4
Layer 1
Layer 2
Layer 3
Layer 5
Layer 4
Layer 1
Layer 2
Layer 3
Physical Medium
Host 1 Host 2Layer 5 Protocol
Layer 4 Protocol
Layer 3 Protocol
Layer 2 Protocol
Layer 1 Protocol
Layer 4/5 interface
Layer 3/4 interface
Layer 2/3 interface
Layer 1/2 interface
Five Layer Network
Entities from the same Layer called- Peers
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Layers
In reality, no data are directly transferred from layer
n on one machine to layer n on another machine.
Each layer passes data and control information tothe layer immediately below it, until the lowest
layer is reached.
Below layer 1 is the physical medium throughwhich actual communication occurs.
Between each pair of adjacent layers is an interfacewhich offer service to the upper layer.
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Layers and Protocols
A set of layers and protocols is called network
architecture.
Networks are designed following the network
architecture.
A list of protocols used by a certain system,
one protocol per layer, is called a protocol
stack.
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Example: Protocol Stack
Peer protocol
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Design Issues for the Layers
Addressing To identify source and destination
Rules for data transfer Unidirectional or bidirectional
At least two logical channel for each connection Normal data
Urgent data
Error Control Error-detecting and error-correcting codes are used
The receiver tells the sender which messages arecorrectly received and which are not.
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Design Issues for the Layers
Flow Control
To synchronize the sender and receiver
Multiplexing and Demultiplexing
Routing
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Layer Services
Layers can offer two different types of
services:
Connection-Oriented Services
Connectionless Services
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Connection-Oriented Services
Three Step Service:
User first establish a connection
Use the connection
Release the connection In most cases the order of transmission is
preserved.
After connection is established a negotiation is
conducted between sender, receiver and subnet.
We can compare it with a Telephone service.
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Connectionless Services
It can be compared with a postal service.
Each packet carries a full destination address.
Routed through the network independently.
Ensure Quality of service. Message sequence
byte streams
Unreliable connectionless service is often calleddatagram service like telegram with noacknowledgement.
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Why unreliable connection is preferred
over reliable connection?
Reliable connection may not be available
Ethernet does not provide reliable connection
Packets can occasionally be damaged in transit.
It is up to higher protocol to deal with the
problem.
The delay in the reliable service is
unacceptable.
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Thank You