Ethernet collisions

							           Ethernet collisions
                      CCNA1 module 6




1-Feb-12         S Ward Abingdon and Witney College   1
Media Access Control
    Many networks have a shared medium
     environment where several hosts have
     access to the same cable or airspace.
    If hosts transmit at the same time, their
     signals will become confused.
    A Media Access Control protocol is needed to
     regulate the access of hosts to their shared
     media

    1-Feb-12       S Ward Abingdon and Witney College   2
Shared cable
    The original versions of Ethernet were 10Base5 and
     10Base2
    They used a physical bus topology with all hosts on
     a shared coaxial cable.




    1-Feb-12          S Ward Abingdon and Witney College   3
Collisions
    10BASE5 and 10BASE2 provide only one
     wire for all the hosts to put the signals on
    If two hosts transmit at the same time, there
     will be a collision.
    The signals will interfere with each other and
     become unreadable
                                  Adding gives:


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Dealing with collisions
    You could try to prevent collisions
    Or you could let collisions happen, let the
     collided signals be destroyed, then send the
     signals again
    Ethernet lets the collisions happen and then
     deals with them
    Too many collisions will affect the network’s
     performance

    1-Feb-12        S Ward Abingdon and Witney College   5
CSMA/CD
    Carrier Sense: ‘Listen’ to see if there are
     signals on the cable
    Multiple Access: Hosts share the same
     cable and all have access to it
    Collision Detection: Detect any collisions of
     signals when they occur
    This is the ‘first come, first served’ method of
     letting hosts put signals on the medium

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Listen for signals

                 Are there signals on
                 the cable?
                 Yes.




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Wait if there are signals

                  Wait until there are
                  no more signals




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Listen for signals

                 Are there signals on
                 the cable now?
                 No.




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Put signals on cable
                   Put my signals on the
                   cable.




1-Feb-12    S Ward Abingdon and Witney College   10
Listen for collisions: no
                    No collision.
                    All is well.
                    My message was
                    sent.




 1-Feb-12    S Ward Abingdon and Witney College   11
Listen for collisions: yes
                    There is a collision.
                    Stop sending signals.
                    Send jamming signal.
                    My message is lost.




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Listen again

                    No signals now.
                    Wait for a random
                    length of time.
                    Send message again.




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Collision domain
    The shared medium where collisions can occur
    Repeaters and hubs pass signals on through all
     ports except the input port so devices connected by
     repeaters or hubs are in the same collision domain




    1-Feb-12          S Ward Abingdon and Witney College   14
Splitting collision domains
    Bridges and switches inspect messages and send
     them only where they need to go
    They split networks into separate collision domains
     and improve performance




    1-Feb-12          S Ward Abingdon and Witney College   15
10BASE-T Ethernet
    Twisted pair cable
    Separate pairs of wires to transmit and
     receive
    Physical topology, star
    Hosts linked by hubs
     or switches




    1-Feb-12         S Ward Abingdon and Witney College   16
Collisions with 10BASE-T?
    We still get collisions when hosts are
     connected through a hub because all the hosts
     share the medium
    But collisions do not show
     up as double height
     signals because hosts
     have separate transmit
     and receive wires
    Hosts must check both
     wires

    1-Feb-12        S Ward Abingdon and Witney College   17
Listen for signals

                 Are there signals on
                 the receive wire?
                 Yes. Wait.




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Listen for signals

                 Are there signals on
                 the receive wire?
                 No. All clear.




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Put signals on cable

                 Put my signal on
                 transmit wire.




1-Feb-12    S Ward Abingdon and Witney College   20
Listen for collisions – no.

                  No signal on receive
                  wire. My signal was
                  sent successfully.




 1-Feb-12    S Ward Abingdon and Witney College   21
Listen for collisions –yes.

                  Signals on receive
                  wire and transmit
                  wire. Collision. Send
                  jam signal.




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Wait for random time

                Wait for random time.
                All clear. Send signal
                again.




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Segmenting with a switch
    A switch breaks up collision domains




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Fully switched
    A fully switched
     network with full duplex
     transmission will have
     no collisions
    Hosts do not share the
     medium
    They have exclusive
     use of their section
    This is
     microsegmentation
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                                          The End




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