Now let's look at a mac address in more detail.
It's once again six bytes in length.
And if you remember, a byte is eight bits in length.
So six times eight gives you 48 bits.
Three bytes or 24 bits is the UI portion of the address.
Three bytes or 24 bits.
Is network interface card specific and is the unique identifier of that network interface card.
Now in the UI portion in the first octet or most significant octet.
In other words, the first byte in the UI, the least significant bit.
In other words, the last bit of the first octet or first byte is either set to zero, which indicates
unicast or it's set to one which indicates multicast unicast.
Traffic, if you remember, is a conversation between two devices where one device is sending the traffic
and the other device is receiving the traffic.
So device A is talking to device B.
Multicast is where one device is sending traffic to multiple devices that have subscribed to the multicast.
Now, this makes it very efficient for Ethernet switches to know whether they should flood the frame
out of all ports.
When multicast traffic is received by a layer two 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 two switch knows how to process traffic.
The second least significant bit in the first octet.
So in other words, we still looking at the first octet, but second least significant bit is either
set to zero, which means that it's a globally unique Mac address or it's set to one, which means that
an administrator has changed the MAC address.
So that would be for the example that I did previously where I changed the Mac address on my PC.
The Zero means it's a unique Mac address designated by a manufacturer, whereas a one means that a administrator
locally changed the Mac address of the interface.
Now in Ethernet, when a bus topology is used, devices use what's called carrier sense, multiple access
slash collision detection or CSA slash CD.
This operates as follows When a device wants to send traffic, it should first check to hear if any
other device is speaking so the device will not communicate onto 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 mainframe days where a central device would pull terminals to allow
them to communicate.
In Ethernet, we're using a distributed environment where each device can independently communicate
across the network without permission from other devices.
However, a device should only send 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 a PBX, the PBX 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 device 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, terminates 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 two devices want to communicate at exactly the same time,
but at that point in time, no devices are speaking.
So let's say that in this example A wants to communicate with C, so A wants to send traffic onto the
network with a source address of a and destination address of C.
But at that exact point in time, D also wants to communicate.
In this case, D wants to communicate with B, so it wants to send a frame onto the network with a source
address of D and destination address of B now in line with C, SMA Slash CD, both A and D.
Firstly, check to see if anyone is speaking so they use carrier sense or S to check the wire.
At this point in time, no device is 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 onto the network.
But because this is ten base two or in other words, base band, only one 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 a back off delay before trying to send the signal again.
This will prevent machines 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 onto the network.
In other words, it's more likely that collisions will take 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.
Now there were other issues with ten BS too.
The first issue is cable length.
The longer the cable, the greater the signal degradation was.
In other words, as your cable increased in length, the more likely it was that one host's signal would
not be received by another host.
The host on one side of the cable might send a signal, but because of degradation, a host at the other
end of the cable may not be able to receive or interpret the signal.
Another problem is cable breaks.
A cable break at any point would cause the entire network to fail.
So if someone accidentally broke the cable at this point, the whole network would fail.
Host A cannot communicate with other devices in the network.
Those devices cannot communicate with host A.
However, because of this cable break, there is no Terminator on the cable.
The cable is also damaged.
So what happens is signals get reflected.
D might send a signal to C, but it's going to continue across the cable and then it's going to be reflected
back causing collisions in the network.
So this network wasn't very robust in that cable breaks could bring down the entire network.
Now it gets worse.
Ten base two implies ten megabits per second Ethernet.
However, this is not ten megabits per second for each device.
It's ten megabits per second shared between all devices on that segment.
In addition, because of collisions, you can only use between 30 and 40%, so you only getting 30 to
40% utilization.
Collisions increase dramatically above that utilization.
So a conservative figure would be 30% utilization.
That means ten megabits per second would be shared between all devices on that segment.
So in this case, we have four devices.
So that means that ten megabits per second divided by four devices, times 30%, only gives you 0.75
megabits per second and not ten megabits per second per device.
This is not ideal because the bandwidth available to your PC is very low, especially in a large network.
So as more devices are added to the network, the bandwidth available to each device is decreased.
This is also known as a single collision domain.
In other words, if a collision takes place at any point in the network, all devices in this network
are affected by that collision and would need to back off.