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1. Introduction (8%)
Computer Networks
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Introduction
• Interconnection of number of computers
• Interconnection through• copper wire
• fiber optics
• microwaves
• infrared
• communication satellite
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Uses of Networks
• Distributed Systems– Software system built on top of computer networks
– Primary objective of Computer Networks:
• Transfer data from machine A to machine B• Facilitate access to remote information• Facilitate sharing of data• Facilitates person to person communication• Facilitate Interactive Entertainment
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Social Issues
• The topics for bulletin boards, news groups etc should be kept under watch for sensitive topics
• Hacking
• Virus spreading
• Anonymous messages can be sent
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Transmission Technology
• Transmission Technology– Broadcast Technology
– Point-to-Point Technology
• Broadcast Technology– Single Communication Channel
– Data sent by sender will be received & processed by all
– Multicasting: Transmission to a subset of machines
– Used for small networks
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Transmission Technology
• Point to Point Technology– Many connections between individual pairs of
machines – Used for big networks
• Unicasting: Point to point transmission with one sender and one receiver.
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Local Area Networks
• 3 Characteristics:1. Size
• Restricted to limited size
• Worst case transmission time is bounded and known in advance
2. Transmission Technology• Connected through cable
• LANs run at 10 to 100 Mbps or now Gbps
3. Topology– Bus & Ring
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Two Examples of Broadcast NWs
BusRing
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The Broadcasting in LAN• At any instance only one machine is allowed to send • Arbitration mechanism is required (Common Bus)
– Static• divide time into discrete intervals and use a round-robin algorithm • Waste of time
– Dynamic• Decentralized
– Sender check the channel before sending
• Centralized– Bus arbitration unit decides who goes next
• Ethernet (also known by no 802.3)– bus-based broadcast network with decentralized control
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Metropolitan Area Network• It is a bigger version of LAN and may
encompass the whole city– Ex Cable Television
• In MANs we use DQDB and also known by no 802.6– DQDB= Dual Queue Dual Bus– DQDB contains two unidirectional busses to
which all the computers are connected
• Upper bus for right, lower bus for left
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Architecture of DQDB MAN
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Wide Area Networks
• It spans a large geographical area, usually a country or continent
• The hosts are connected by a communication subnet, or just subnet for short
• Subnet is operated by Telephony Company or ISP• The job of the subnet is to carry messages from
host to host
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Relation Between Hosts and SubnetSubnet = Transmission lines + Switching Element
Transmission Line – copper wires,optical fiber, radio links
Switching Element - Router
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Internetwork
• A collection of interconnected networks is called an internetwork or internet.
• Many networks exist in the world, often with different hardware and software.
• To fulfill this Gateway is used– Also known as Protocol converters
• Internet is a collection of LANs connected by WAN– If system contains only routers => subnet– If system contains routers & hosts => WAN
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Wireless Networks• Categories
1. System Interconnection– Short-range radio signals– bluetooth– Keyboard, mouse connection
2. Wireless LANs– Require radio modem and antenna– Less range
3. Wireless MANs– Radio networks– Wide range– Used in cellular telephones– 1G – analog and voice– 2G – digital and voice– 3G – digital voice and data
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Figure 2-19Internetwork
(Internet)
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Need For Protocol Architecture
• Reduce Complexity• Data exchange can involve complex procedures like
file transfer• Better if task broken into subtasks• Implemented separately in layers in stack
– each layer provides functions needed to perform communication for layers above
• Peer layers communicate with a protocol
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Design Issues in layers• Identification of Host• Rules for data transfer
– In one direction or both• Error Control
– Error detecting and correcting codes• Sequencing of data• Flow Control • Inability of accepting long messages
– Disassembling, transmitting and reassembling messages• Avoid separate connection because of inconveniency and expensive,
– Multiplexing and Demultiplexing• Multiple paths between source and destination, a route must be chosen
– Routing
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Connection Oriented & Connectionless Services
• CO – Establish connection, use the connection and
releases the connection– Eg: Telephone system
• CL – Eg: Postal service
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Addressing Requirements
• Two levels of addressing required
• Each host on a subnet needs a unique global network address– its IP address
• Each application on a (multi-tasking) host needs a unique address within the host– known as a port
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Reference Models
There are two very famous models– OSI Reference model
• OSI = Open Systems Interconnection
• Developed by ISO (International Standard Organization)
• Protocols are not used but the model is valid
– TCP/IP Reference model• TCP/IP = Transport Control Protocol/Internet
Protocol
• Here protocols are widely used
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The TCP/IP Reference
Model
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The OSI Reference Model
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Communication between End-Systems
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The Physical Layer• It is concerned with transmitting raw bits
• 1 should be received as 1 and not zero!
• It deals with mechanical, electrical, timing interfaces & physical transmission
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Figure 3-4
Physical Layer
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The Data Link Layer
• It’s job is to provide error-free transmission between two adjacent computers
• It breaks the sender data in data frames• Assign sequence no. to each frame • If the service is reliable, the receiver confirms
correct receipt of each frame by sending back an acknowledgement frame.
• Checks for the buffer space at the receiver end
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The Data Link Layer
• Flow control– Sender and Receiver may not match in speed
• MAC (Medium access control)– For Broadcast Networks Only– Decides “ Who will access the Medium “
• Medium = Ethernet (802.11)
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Figure 3-5
Data Link Layer
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Figure 3-6
Data Link Layer Example
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The Network Layer• Controls the operation of subnet• To route the packets from ultimate source to
ultimate destination• Congestion control
– When many no. of computers want to send data destined to another network simultaneously
– Each data has to pass from same router
• Problems with heterogeneous NWs like addressing, packet size etc are important
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Routing
• Directing packets over number of networks
Computer A Computer B
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Figure 3-8
Network Layer Example
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Figure 3-8-continued
Network Layer Example
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The Transport Layer• It accepts data from the upper layer, split it
into smaller units if needed, pass to NWL, and ensure it arrives intact at the other end
• Every process send data to network with the help of port
• So transport layer adds port nos to the data received from upper layer (i.e. Session Layer)
• Hence it’s first true end-to-end layer
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Transport Protocols
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Session Layer• To establish a session betn 2 processes• It establishes, manages and terminates the
connections between the local and remote application.
• The Session layer is typically completely unused
• Dialogue control – To decide whose turn to speak (send data)
• Synchronization– To add checkpoints to the stream of data for
roll backing
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Presentation Layer
• Presentation of the data to be sent
• Translation– Translate data to be sent in bit streams
• Encryption– Transform the original info to another form
• Compression– Reduce the no of bits to be transmitted
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The Application Layer
• Enable us to access network
• User friendly software application
• Examples– Network virtual terminal (eg: telnet)– Mail services (eg: Eudora)– Directory services
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Figure 3-14
Summary of Layer Functions
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OSI Model
Data unit Layer Function
Hostlayers
Data
7. Application Network process to application
6. Presentation Data representation and encryption
5. Session Interhost communication
Segment 4. Transport End-to-end connections and reliability (TCP)
Medialayers
Packet/Datagram 3. Network Path determination and logical addressing (IP)
Frame 2. Data link Physical addressing (MAC & LLC)
Bit 1. Physical Signal and binary transmission
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The TCP/IP Reference Model
• Used in the Internet
• History– First used by ARPANET (Advanced Research
Projects Agency Network)– ARPANET was a research network
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Protocols and Networks in TCP/IP Model
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Internet Layer
• Packet switching network based on connectionless internetwork layer.
• Packets travel independently in the network
• Order can be different– It’s the job of higher levels to rearrange this
packet.
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A Comparison of OSI and TCP/IP• TCP/IP model is widely used compared to
OSI model
• OSI model explicitly defines services, interfaces and protocols (Well organized)
• OSI Model designed first and then protocols
• TCP/IP Protocols designed first and then model
• No of layers are 4 compared to 7