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Packet Switching

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  1. 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.

  2. 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.

  3. 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.

  4. 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.

  5. 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

  6. Routers • 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.

  7. 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.

  8. net at 2 net at 3 LAN at site 1 Router 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

  9. 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

  10. 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.

  11. 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.

  12. Router connecting networks a and b Network a a b c f e d Computer attached to network d Source:Comer, D.E. (1994), The Internet Book, Second Edition, Prentice-Hall, Inc., New Jersey. Internal Structure

  13. 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.

  14. 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.

  15. Computer C2 Router 1 C2 Computer C3 a b c C1 Router5 Router 2 C3 d e f Computer C1 Router 3 Router4 Source:Comer, D.E. (1994), The Internet Book, Second Edition, Prentice-Hall, Inc., New Jersey. Routing Example

  16. 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.

  17. 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.

  18. 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.

  19. 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.

  20. 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.

  21. 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.

  22. 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.