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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
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
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.
packet switching contd3
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.
packet switching contd4
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.
internet a network of networks
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
  • 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.
backbone network sites
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

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
ip internet protocol
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
ip internet protocol contd
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.
ip internet protocol contd11
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.
internal structure

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
ip datagrams network packets
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.
internet or ip address
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

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
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.
tcp contd
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.
tcp contd18
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.
names for computers
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.
names for computers contd
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.
names for computers contd21
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.
reasons for internet s success
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.