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Packet Switching Packet is a unit of data that can be transferred at one time. Packet includes a header and the data. Packets are of variable sizes with a maximum size limit. All data is transferred across the Internet in packets. Packet Switching (contd.)

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Packet switching l.jpg
Packet Switching

  • Packet is a unit of data that can be transferred at one time.

  • Packet includes a header and the data.

  • Packets are of variable sizes with a maximum size limit.

  • All data is transferred across the Internet in packets.

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Packet Switching (contd.)

  • Packet switching is the process of routing communications traffic in the form of data packets from a sender to the (correct) receiver.

  • LANs and WANs use packet switching.

  • Packet switching avoids delays; short messages need not wait for long transfers to complete.

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Packet Switching (contd.)

  • Packet switching permits multiple pairs of computers to communicate simultaneously in a shared network.

  • Computers take turns sending packets.

  • Interface hardware handles sharing automatically.

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Packet Switching (contd.)

  • Each device connected to a network contains a small computer that handles the communication.

  • Network software on the sender side divides data into packets.

  • Network software on the receiver side reassembles the packets.

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Internet - A Network of Networks

  • Interconnection of multiple packet switched networks.

  • Multiple packet switching technologies -

    • different speed, distances, & cost characteristics

  • Internet provides

    • a mechanism to interconnect arbitrary networks

    • software to transfer data across the connections

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  • Computers with special purpose software that interconnects networks.

  • Forward packets from one computer to another.

  • Routing is the process of selecting a network over which to send a packet.

  • Router can interconnect diverse technologies, for example, a LAN to a WAN.

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Backbone Network & Sites

  • A major WAN to which other networks attach is called a backbone network.

    • Example: vBNS - very high-speed Backbone Network System (by MCI in 1995).

  • Sites reached by the backbone are called backbone sites.

  • At each backbone site, a router connects a LAN to the backbone.

Wide area backbone l.jpg

net at 2

net at 3

LAN at site 1


Wide Area Backbone

LAN at site 2

LAN at site 3

LAN at site 7

LAN at site 6

LAN at site 5

LAN at site 4

net at 7

net at 6

Source:Comer, D.E. (1994), The Internet Book, Second Edition, Prentice-Hall, Inc., New Jersey.

Wide Area Backbone

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IP - Internet Protocol

  • A Communication Protocol is an agreement that specifies a common language two computers use to exchange messages.

  • A protocol specifies

    • exact format & meaning of each message

    • conditions under which a message can be sent

    • how a computer should respond to a message

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IP - Internet Protocol (contd.)

  • IP specifies how a packet must be formed, and how a router must forward it to its destination.

  • IP software is needed on a computer connected to the Internet.

  • IP software is memory resident.

  • Internet packets are called IP datagrams.

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IP - Internet Protocol (contd.)

  • IP transforms a collection of networks and routers into a seamless communication system.

  • IP makes the complex internal physical structure of the Internet transparent to the end user.

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Router connecting networks a and b

Network a







Computer attached

to network d

Source:Comer, D.E. (1994), The Internet Book, Second Edition, Prentice-Hall, Inc., New Jersey.

Internal Structure

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IP Datagrams & Network Packets

  • IP datagram defines a standard format for all Internet packets.

  • IP datagram travels inside a network packet.

  • IP datagram is data within a network.

  • Router creates a new network packet for transmission across another network.

  • Datagram is processed by software on the destination computer.

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Internet or IP Address

  • IP address is a unique number assigned to a computer.

  • Computer stores an IP address in 4 bytes.

  • IP address is displayed as 4 decimal numbers separated by periods,


  • IP addresses are not random; prefix is the same for all computers on the same network.

  • Routing example l.jpg

    Computer C2

    Router 1


    Computer C3






    Router 2





    Computer C1

    Router 3


    Source:Comer, D.E. (1994), The Internet Book, Second Edition, Prentice-Hall, Inc., New Jersey.

    Routing Example

    Tcp transmission control protocol l.jpg
    TCP - Transmission Control Protocol

    • In addition to IP software, most of the computers that connect to the Internet also run TCP software.

    • TCP/IP - an entire set of Internet communication protocols.

    • TCP enables two computer programs to communicate across the Internet - connect, exchange data, and disconnect.

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    TCP (contd.)

    • TCP makes the Internet reliable.

      • Checks for lost datagrams that were discarded by routers.

      • Puts incoming datagrams in the order they were sent.

      • Checks for duplicate datagrams.

    • TCP handles the problem of lost datagrams by using timers and acknowledgements.

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    TCP (contd.)

    • TCP software on the receiver side sends an acknowledgement back to the sender.

    • TCP on the sender side starts a timer when it sends data. If an ack. arrives before the timer expires then the TCP cancels the timer, else it retransmits another copy.

    • TCP’s timer mechanism adapts to “distance” of destination and Internet traffic delays.

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    Names for Computers

    • Users prefer alphabetic names to IP addresses.

    • Each computer on the Internet must have a unique name.

    • Names with many parts

      • Full Name = local name + organization suffix

        • E.g. = www +

      • Full Name = computer name + dept. name + organization suffix

        • E.g.

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    Names for Computers (contd.)

    • Domain Name System (DNS) is a software that translates a domain name to an IP address.

    • DNS uses the client/server approach; application program contacts the domain name server to get the IP address.

    • Domain name servers store names of computers at only one company or enterprise.

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    Names for Computers (contd.)

    • Computer name lookup is automatic.

      • Application asks a local domain name server and if needed this server then contacts a remote domain name server.

    • There is no correspondence between the parts of a domain name and the parts of its IP address.

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    Reasons for Internet’s success

    • IP provides flexibility

      • Accommodates many types of hardware - WAN or LAN, high-speed or slow, wired or wireless, etc.

    • TCP/IP standards specify how to send IP datagrams on each type of network.

    • TCP provides reliability

      • Automatically adapts to Internet conditions

      • Makes reliable communication possible even during periods of congestion.