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Computer NetworksModule 1: Introduction
Computer Networks, Andrew Tanenbaum 4e
Dr. Vikram Shete
St. Francis Institute of Technology
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Administrative Trivia
• Marks Overview– Theory Exam (Univ.) = 80– Internal Exam = 20– Internal Termwork = 25
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Type Based on Comments Total
Labs Experiments + Mini group project (ns2)
Average out of 10 + 5 for project
15
Assignments Two Internal 20 marks test*
Mapped to 5 marks each
5
Attendance Lectures + Labs 75% minimum 5
Grand total 25
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Administrative Trivia
• Copying– From each other – From the web– From text book
• Paraphrase in your own words and cite the original sources– Unless you have developed significant theory
• Just because you are in India doesn’t mean you have to be corrupt
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Administrative Trivia
• Penalty if found copying/cheating on assignments, labs and exams– 0 marks on that deliverable– Take great care of what you submit
• Meet me if you need help
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Administrative Trivia
• Books– Computer Networks
• Andrew Tanenbaum & David Wetherall
– Computer Networks: A top down approach• James Kurose & Keith Ross
– Data Communications & Networking• Behrouz Forouzan
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Why Computer Networks?
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Why Study Computer Networks?
• Age of information
• Ever evolving
• Created jobs (both, Steve and paid!)
• Society has been affected by it
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What is this course about?
• Fundamental concepts of networking
• Types of networks
• Features of networks
• Design issues involved in networking
• Will develop concepts which are at core of networking
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What is this course NOT about?
• Not about a specific device or company
• May not be sufficient to get a job in itself
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Historical Perspective
• December 23, 1947– William Shockley– Walter Brattain– John Bardeen
Source: http://www.computerhistory.org/
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Historical Perspective
Source: Wikimedia Commons: http://en.wikipedia.org/wiki/File:Wafer_2_Zoll_bis_8_Zoll_2.jpg
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Historical PerspectiveNetwork of 1861
Source: Wikimedia Commons
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Historical Perspective
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Where does it stands today?
Produced with permission from Chris Harrison- Carnegie Mellon University
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Network Applications
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Introduction
• Network Applications: – Business– Home– Mobile– Social
Payroll System UpdatesEmailSystem InformationCompany wide broadcastsTrainingRecruitmentTaxesData centersE-CommerceCollaborative work
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Business Applications of Networks (Tanenbaum)
A network with two clients and one server.
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Network Applications (Tanenbaum)
• Some forms of e-commerce.
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Introduction
• Network Applications: – Business– Home– Mobile– Social
Access to remote information
Person-to-person communication
Interactive entertainment
Electronic commerce
“There is no reason for any individual to have a computer in his home” ~ Ken Olsen – President, Digital Equipment Corporation (# 2 computer vendor after IBM)
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Home Network Applications
• In peer-to-peer system there are no fixed clients and servers. (eg: Napster)
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Social Issues
• Ideas move beyond country boundaries• Countries have different cultural and legal
structures• Technical issues are not problematic• Issues escalate when people discuss
– Politics– Sex– Religion
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Social Issues
• Censorship– Internet operators similar to phone operators– Cannot control what users say or do online– Can censor but defies freedom of expression
• Government censorship– Patriot Act of USA– PIPA and SOPA?
• Internet provides true freedom but– Brings forth many other unsolved issues
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Network Hardware
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Network Hardware
• Technology• Scale
• Broadcast links• Point-to-Point links
• Different from P2P networks
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Network Hardware
• Technology• Scale
• Broadcast links• Point-to-Point links
• Different from P2P networks
PAN
LAN
MAN
WAN
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Point-to-Point Line Configuration
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Point-to-Point Line Configuration
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Multipoint Line Configuration
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Mesh Topology
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Star Topology
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Tree Topology
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Bus Topology
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Ring Topology
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Hybrid Topology
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Simplex
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Half-Duplex
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Full-Duplex
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Network Hardware
• Technology• Scale
• Broadcast links• Point-to-Point links
• Different from P2P networks
PAN
LAN
MAN
WAN
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Local Area Network
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Local Area Network
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Network Hardware (LAN)
• Local Area Networks (LAN)– Widely used for private network within
• Building, factory, campus etc.• Share common resources (printers, database etc)
– Small in size • Worst case transmission times known apriori• Tailor made designs are possible
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Network Hardware (LAN)
– Wired LANs• Use cables, high speeds and low delays with no
errors• Traditional LANs operate at 10 Mbps to 100 Mbps• Newer ones can go upto 1Gbps
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Network Hardware (LAN)
– LAN topologies• Bus
– At most one computer can transmit at a time– Rest must remain quiet– An arbitration method is required to resolve conflicts– IEEE 802.3 a.k.a Ethernet is a bus based broadcast
network. Operates at 10Mbps to 10Gbps
• Ring– IEEE 802.5, the IBM Token Ring protocol. Operates at 4
and 16 Mbps– Fiber Distributed Data Interface (FDDI) ring network
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Local Area Networks
• Two broadcast networks• (a) Bus• (b) Ring
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Metropolitan Area Network
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Metropolitan Area Networks
• A metropolitan area network based on cable TV.
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Figure 2-18
WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998
Wide Area Network
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Wide Area Networks
• Relation between hosts on LANs and the subnet.
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Internetwork (Internet)
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Network Hardware (Summary)
• Technology• Scale
• Broadcast links• Point-to-Point links
• Different from P2P networks
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Network Software
• The start was hardware• Networks evolved • Network software is more structured now
than ever before• Layered architecture of network
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Network Software
• Layered architecture– Reduces complexity
– Organized as a stack of layers
– Each layer has responsibilities and tasks
– Layers interact with the ones above and below
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Network Software
• Layered Architecture– Each layer provides a service to the layer
above– Layer to layer communication in different
devices– The rules of communication are called
protocols
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Network Software
• Data never moves directly between layers• Flows through the entire stack• Actual transmission at the lower most
layer– This is the physical medium
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Network Software
• Set of layers and protocols together is called the network architecture
• Each layer uses a protocol• A set of protocols used by layers is called
a protocol suite/stack
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Network Software
Protocol Hierarchies
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Network Software
• Example information flow supporting virtual communication in layer 5.
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Design Issues for Layers
• Addressing– Layers need to identify sources & destinations– Computers can have multiple processes
• Data transfer– Unidirectional flow– Bidirectional flow– Prioritized bidirectional flow
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Design Issues for Layers• Error Control
– Physical connections are noisy– Common standard between receiver and
transmitter– Means to communicate errors have occurred
• Order of Data Received – Order may not preserved– Mechanism to detect out of order pieces – Mechanism to put those in order
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Design Issues for Layers• Flow Control
– A sender may swamp the network with data– Feedback to reduce data transmission
• Length of Messages – Too long
• Disassemble, transmit and reassemble
– Too short• Assemble, transmit and disassemble
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Design Issues for Layers• Multiplexing-Demultiplexing
– Inefficient to setup channels for each processes
– Layers will multiplex data streams from different processes
• Routing– Choosing a right path based on various
parameters• Privacy laws, costs involved, infrastructure etc.
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Design Issues for LayersARE DATA FOrM
• Addressing
• Routing
• Error Control
• Data Transfer Mode
• Flow Control
• Order of Data Received
• Mux-Demux
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Types of Services Offered by Layers to Layers Above Them
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Connection-Oriented and Connectionless Services
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Reference Models for Layers
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Open Systems Interconnection
• OSI Reference Model– First introduced in 1970 by International
Standards Organization (ISO-OSI reference model)
– Aimed at enabling communication between two different systems
– No need to changed underlying hardware and technology
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Open Systems Interconnection
• OSI Reference Model– An open system is a set of protocols– OSI model is not a protocol – Model allows designing a network architecture
which is: • Flexible• Robust• Interoperable
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Open Systems Interconnection
• OSI Reference Model– Architecture is very general – Earlier protocols are rarely used today– Consists of 7 separate but related layers– Each layer defines a part of process in
moving data forward– Each layer is a group of closely related
functions– Each group is distinct from the other
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OSI Layers (Forouzan)
Interface 7/6
Interface 6/5
Interface 5/4
Interface 4/3
Interface 3/2
Interface 2/1
Interface 7/6
Interface 6/5
Interface 5/4
Interface 4/3
Interface 3/2
Interface 2/1
Peer-to-Peer Protocol 7th Layer7
6
5
4
3
2
1
7
6
5
4
3
2
1
Application
Presentation
Session
Transport
Network
Data Link
Physical
Application
Presentation
Session
Transport
Network
Data Link
Physical
Network
Da.Li
Physical
Network
Da.Li
Physical
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OSI Layers
• Layers belong to 3 subgroups– Network Support Layers (Layers 1,2,3)
• Electrical properties, physical connections, physical and logical addressing, transport timing and reliability
– User Support Layers (Layers 5,6,7)• Allow interoperability between unrelated software systems
– Link (Layer 4)• Ensures seamless communication between above 2 groups
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OSI Layers
• Upper layers are always implemented in software
• Lower layers are a combination of hardware and software
• The physical layers is almost always hardware
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An Exchange Using the OSI Model
Header
Hea
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Dat
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om P
revi
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laye
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Encapsulation
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Layers in Details
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Physical Layer
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Physical Layer
• Physical characteristics of interfaces and medium
• Representation of bits– electrical/optical encoding
• Data rate– Define duration of bit
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Physical Layer
• Bit synchronization– Clock synchronization between sender and
receiver
• Line configuration– Point to point or multipoint
• Physical topology– Mesh, star etc
• Transmission mode– Simplex, half duplex or full duplex
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Data Link Layer
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Data Link Layer
• Transforms a raw transmission facility into a reliable service
• Physical layer appears error free to the network layer
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Data Link Layer
• Broadly responsible for moving frames from one node to another
• Other functions include – Framing– Physical addressing– Flow control– Error control– Access control
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Data Link Layer
• Framing – Divide stream of bits from network layer into
manageable units called frames
• Physical addressing– Sender’s and receiver’s address of the frame
within the network– If outside the network then to default gateway
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Data Link Layer
• Flow control– Imposes flow control mechanism at the sender’s
end to avoid overwhelming the receiver
• Error control– Detect and retransmit damaged or lost frames– Recognize duplicate frames
• Access control– When 2 or more devices connect to same link,
data link layer protocols select the controlling device
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Data Link Layer
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Network Layer
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Network Layer
• Logical addressing– Enables communication across networks
• Routing– Enable moving of packets over the network
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Transport Layer
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Transport Layer
• Responsible for Process to Process delivery
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Transport Layer
• Other functions– Service-point addressing– Segmentation and reassembly– Connection control– Flow control– Error control
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Transport Layer (Other Functions)
• Service-point addressing– Transport layer header contains the port
address– Network layer gets each packet to the correct
computer– Transport layer delivers it to the correct
process
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Transport Layer (Other Functions)
• Segmentation and reassembly– Message is divided into segments each with a
sequence number– Sequence numbers allow reassembly
• Connection control– In connectionless each segment is treated
independently and delivered– In connection oriented service, a connection
is first setup and then segment is delivered
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Transport Layer (Other Functions)
• Flow Control– Flow control is end to end and not link to link
like in data link layer
• Error Control– Control is performed process to process and
not on a link to link basis– Correction is achieved through retransmission
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Transport Layer
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Self Study
• Session • Presentation • Application
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Summary of Layers
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TCP/IP Protocol Suite
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TCP/IP Protocol Suite
• TCP/IP was designed to have 4 layers• Compared to OSI it can be said to have 5
layers• Some of the functions in OSI are bundled
in TCP/IP
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TCP/IP Protocol Suite
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TCP/IP Protocol Suite
• 4 levels of addressing are used in internet using TCP/IP
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TCP/IP Protocol Suite
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TCP/IP v/s OSI
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Comparison of OSI & TCP/IP
• Concepts central to OSI model– Services: Tells what a layer does– Interfaces: How to access services?– Protocols: Layers internal business to get the
job done
• Analogous to objects in OOP• TCP/IP does not clearly differentiate
between the above three
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Comparison of OSI & TCP/IP
• Consequently protocols are well hidden in OSI compared to TCP/IP– Protocols can be changed as and when
technology changes– This is primary reason for a layered structure
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Comparison of OSI & TCP/IP
• OSI model was before protocols– Hence is not protocol biased– Designers were inexperienced and did not
know “what functionality belonged to which layer”
• TCP/IP came after protocols– Model fits the existing protocols very well– Unsuitable for non-TCP/IP networks
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Comparison of OSI & TCP/IP
• OSI model has 7 layers– Uneven distribution of functionalities– Very little in upper layers and transport and
network layers are overcrowded
• TCP/IP has 4 layers
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Comparison of OSI & TCP/IP
• OSI supports connection oriented and connectionless in the network layer – Only connection oriented in the transport layer– Transport layers services visible to user
• TCP/IP supports connectionless in network layer– But both in transport layer– Users get to choose between either