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