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Now let�s look at a MAC address in more detail, its once again 6 bytes in length
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and if you remember a byte is 8 bits in length, so 6 x 6 gives you 48 bits
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3 bytes or 24 bits is the OUI portion of the address
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3 bytes or 24 bits is Network Interface Card specific
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and is the unique identifier of that Network Interface Card.
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Now in the OUI portion in the first octet or most significant octet,
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in other words the first byte in the OUI.
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The least significant bit, in other words the last bit of the first octet or first byte
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is either set to 0 which indicates unicast or it set to 1 which indicates multicast.
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Unicast traffic if you remember is a conversation between 2 devices where
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one devices sending the traffic and the other devices receiving the traffic.
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So device A is talking to device B.
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Multicast is where one device is sending traffic to multiple devices
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that have subscribe to the multicast.
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Now this make it very efficient for Ethernet switches
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to know whether they should flood the frame out of all ports.
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When multicast traffic is receive by a layer 2 switch that traffic is flooded
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out of all ports whereas unicast traffic is typically not flooded.
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So by reading the bit in the frame, the layer 2 switch knows how to process traffic.
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The second least significant bits in the first octet, so in the other words we're still
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looking at the first octet but the 2nd least significant bit, is either set to 0
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which means that it's a globally unique MAC address
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or it set to 1 which means that an administrator has change the MAC address.
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So that would be for the example that I did previously
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where I change the MAC address on my PC, the 0 means it's a unique MAC address
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designated by manufacturer where as a 1 means that a administrator locally change
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the MAC address of the interface.
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Now in Ethernet, when a bus topology is used, devices used
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what�s called Carrier Sense Multiple Access/Collision Detection or CSMA/CD.
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This operates as follows, when a device wants to send traffic
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it should first check to hear if any other devices speaking.
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So the device will not communicate unto the network if it hears another device
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that's called Carrier Sense, Carrier sense is essentially sensing the network
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to hear if another device is speaking.
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Multiple Access means that any device can communicate across that segment
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as long as no other device is communicating.
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Now this is different to the old main frame days where a central device
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would poll terminals to allow them to communicate.
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In Ethernet were using a distributed environment where each device can independently
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communicate across the network without permission from other devices.
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However a device should only change traffic if no other device is speaking
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and that�s because we want to avoid collisions in an Ethernet environment.
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As another analogy when traditional telephones are connected to PDX,
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the PDX is in charge of the communications.
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That�s not true in an Ethernet environment
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every device is independent of other devices
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However if collisions do take place
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there�s an option in Ethernet to detect collisions.
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When a devices detects that a collision has taken place it may send
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a back off or jamming signal to indicate that a collision has taken place.
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Once again in this environment, terminators are used at the end of the cable
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to ensure that signals don�t bounce back causing additional collisions.
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Now in a given scenario it may happen that 2 devices want to communicate
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at exactly the same time, but at that point on time, no devices are speaking.
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So let say that in this example A wants to communicate with C.
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so A wants to send traffic unto the network
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with the source address of A and destination address of C.
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But that exact point in time, D also wants to communicate.
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In this case D wants to communicate with B.
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So it wants to send the frame unto the network
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with the source address of D and destination address of B.
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Now in line with CSMA/CD both A and D firstly check to see if anyone is speaking.
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So they use Carrier Sense or CS to check the wire.
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At this point in time no devices communicating on the network.
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However, because of Multiple Access any device can access the cable
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without permission from any other device.
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So both A and D send traffic unto the network but because this is 10base2
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or in other words baseband, only 1 signal is allowed across the wire at any given time.
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So in this example a collision takes place.
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Now if A transmitting data station or PC detects another signal on the wire
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while transmitting its frame, it will stop transmitting that frame
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and then send a jamming signal as well as waiting a random period of time
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known as back off delay before trying to send the signal again.
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This will prevent machine or PCs
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from repeatedly attempting to transmit at the same time.
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However, the probability of collisions becomes greater as the cable length increases
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and as more devices are added unto the network.
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In other words, its more likely that collisions will takes place
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with longer cable lengths and more devices.
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So as you add more and more devices to this network
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and extend the cable length, the probability of collisions increases dramatically.
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