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  • Circuit vs. Packet Switching

    Taxonomy of Switched Wide Area Networks

    intelligent coredumb nodes

    Virtual Circuit(e.g. ATM)

    Datagram Networks(e.g. the Internet)

    connectionless connection-oriented




    (e.g. telephone networks)Packet-Switched


    connection-orientedservice (TCP)

    connectionlessservice (UDP)

    dumb coreintelligent nodes


  • Circuit vs. Packet Switching (cont.)Network Core mesh of routers/switches that interconnect end systems

    two fundamental approaches to building a WAN core:

    (1) circuit switching (example: telephone networks) a sequence of links (communication path) between

    two communicating nodes is determined ahead ofthe actual communication

    on each physical link, a channel is dedicated to the connection

    data is sent as a stream of bits through the network

    (2) packet switching (example: the Internet) data is sent through network in short blocks packets network links are dynamically shared by many packets;

    each packet uses full link bandwidth

    LAN 1

    LAN 2WAN Core Network


  • Circuit vs. Packet Switching (cont.)


    (e.g. telephone networks)


  • Circuit vs. Packet Switching (cont.)

    Packet Switching:Datagram Networks(e.g. the Internet)


  • Communication via involves three phases:Circuit Switching

    Circuit Switching

    (1) circuit establishment before any data is transmitted, an end-to-end circuit

    must be established, i.e. network resources on path/links between end-devices must be reserved

    (2) data transfer data transmission and signaling may each be digital

    or analog

    (3) circuit disconnect after some period of data transfer, the connection

    is terminated, by action of one of two stations, anddedicated resources are released




  • Circuit Switching (cont.)

    Multiplexing inCircuit-SwitchedNetworks

    each link can be shared among (up to) n circuits each circuit gets a fraction 1/n of the links bandwidth multiplexing = set of techniques that allows simultaneous

    transmission of multiple signals across a single data link

    frequency division multiplexing (FDM) = each circuitcontinuously gets a fraction of the links bandwidth

    time division multiplexing (TDM) = each circuit gets allof the bandwidth periodically during brief intervals of time








  • Circuit Switching (cont.)

    Advantages of Circuit Switching

    inefficient use of capacity channel capacity isdedicated for the duration of a connection, even ifno data is being transferred(example: silent periods in a phone call)

    circuit establishment delay circuit establishmentintroduces initial delay

    network complexity end-to-end circuit establishmentand end-to-end bandwidth allocation is complicatedand requires complex signaling software to coordinateoperation of switches

    Disadvantages of Circuit Switching

    guaranteed Quality of Service data are transmittedat fixed (guaranteed) rate; delay at nodes is negligible


  • Packet Switching

    Communication viaPacket Switching

    (1) message segmentation longer message is broken up into series of packets packets contain users data + control data control data (header) contains information that network

    requires to route the packet

    (2) data transfer intermediate nodes perform following operations:

    (a) receive entire packet(b) determine next node and link on route(c) queue packet to go out on that link

    when link is available, packet is transmitted to nextnode


  • Further Details ofPacket Switching

    each packet is treated independently withno reference to packets that have gonebefore!

    each packet contains the full (IP) addressits destination as well as its source

    each packet switch has a forwarding (routing)table that maps destination addresses to an output link

    when packet arrives at a packet switch, theswitch examines packets destination addressand chooses the next node on packets pathbased on current traffic, line failure, etc.

    packets with the same destination addressdo not necessarily follow the same route packets may arrive out of sequence atthe destination !

    if packets arrive out of order, resequencing must be performed at the destination

    Packet Switching (cont.) 9

  • Packet Switching (cont.)Main Principle ofPacket Switching

    statistical multiplexing () on-demand rather thanpre-allocated sharing of resources link capacity is sharedon packet-to-packet basis only among those users whohave packets that need to be transmitted over the link(1) router buffers packets and arranges them in a queue(2) as the transmission line becomes available, packets

    are transmitted one by one

    store-and-forward () switch must receive entire packetbefore it can begin to transmit the first bit of the packet ontothe outbound link


    B statistically multiplexed packets: packets are interleaved

    based on the statistics of the senders

    queue of packets waiting for output

    Bandwidth division into piecesDedicated allocationResource reservation


  • Packet Switching (cont.)

    Example [ circuit switching vs. packet switching ]

    N=35 users share a 1 Mbps link each user generates 100kbps when active each user is active 10% of time

    How many users can be supported with circuitand how many with packet switching?

    Circuit SwitchingWith circuit switching, 100kbps must be reserved for each user at all times. Hence, theoutput link can support 1Mbps/100kbps = 10 simultaneous users.

    Packet Switching 10 or fewer simultaneously active users aggregate rate 1 Mbps users

    packets flow through output link without delay, as in case of circuit switching

    more than 10 simultaneously active users aggregate rate exceeds output capacity

    With 35 users, probability of 10 or less simultaneously active users = 0.9996.Thus, packet switching can support all 35 users with virtually no delay!

    N users 1 Mbps link

    100 kbps

    100 kbps


  • Advantages of Packet Switching

    transmission delay each time a packet passes througha packet-switching node, it incurs a delay not present incircuit switching = the time it takes to absorb the packetinto an internal buffer

    variable delay each node introduces additional variabledelay due to processing and queueing

    overhead to route packets through a packet-switchingnetwork, overhead information including the address ofdestination and/or sequence information must be added toeach packet

    Disadvantages of Packet Switching

    greater line efficiency node-to-node link dynamicallyshared by many packets / connections

    data rate conversion two stations of different datarates can exchange packets, because each connects toits node at its proper data rate nodes act as buffers

    no blocked calls packets are accepted even underheavy traffic, but delivery delay increases

    Packet Switching (cont.) 12

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