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 3 bytes or 24 bits is the UI portion of the Address 3 bytes or
24 bits.
Is a network interface card specific and is the unique identifier of that network interface card.
Now in the UI portion in the first octet will most significant octet.
In other words the first bite in the UI the least is significant.
But in other words the last bit of the first octet or first byte is either set to 0 which indicates
he unicast or it's set to 1 which indicates multicast unicast traffic if you remember is a conversation
between two devices.
One device is sending the traffic and the other devices receiving the traffic.
So device say is talking to device B multicast is a way one device is sending traffic to multiple devices
that have subscribed to the multicast.
Now this makes it very efficient for Ethan it switches to know whether they should flood the frame out
of all ports when multicast traffic is received by layer to switch that traffic is flooded out of all
ports whereas unicast traffic is typically not flooded.
So by reading the but in the frame the layer to 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 the least significant but he's 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 to 0
means it's a unique MAC address designated by the manufacturer where as a one means that a administrator
locally changed the MAC address of the interface.
Now in Ethernet when a bus topology is used to Vice's use what's called carious sains multiple access
slash collision detection for CSM a slash 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 onto the network if it hears another device that's called Carry a
sense carious 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 devices
are communicating.
Now this is different to the old mainframe days where a central device would pull terminals to allow
them to communicate in Ethernet wi using a distributed environment where each device can independently
communicate across the network without permission from other devices.
However a device should only same traffic if no other devices speaking.
And that's because we want to avoid collisions in an Ethernet environment.
As another analogy with 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 the net to detect collisions when a device
detects that a collision has taken place.
It may send a backoff or jamming signal to indicate that a collision has taken place.
Once again in this environment terminate is 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 one 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 he wants to send traffic onto the
network with a source address Ave and destination address of C..
But at that exact point time D also wants to communicate.
In this case D wants to communicate with B said wants to send a frame onto the network with a source
address of D and destination address of B.
Now in line with CSM a slash CD both A&E fusee check to see if anyone is speaking.
So they use carious saints or C-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 10 days too.
Or in other words baseband 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 backoff 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 a 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 the 10 base 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 wouldn't
be received by another host the host on one side of the cable might send a signal.
But because of segregation 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.
Homestay cannot communicate with other devices in the network.
Those devices cannot communicate with state however because of those cable break there's no terminator
on the cable the cable is also damaged.
So what happens is signals get reflected deam might send a signal to see 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 10 base 2 implies 10 megabits per second Ethernet.
However this is not 10 megabits per second for each device.
It's 10 megabits per second shared between all devices on that segment.
In addition because of collisions you can only use between 30 and 40 percent.
See only getting 30 to 40 percent utilization collisions increased dramatically above that utilization.
So a conservative figure would be 30 percent utilization.
That means 10 megabits per second would be shared between all devices on that segment.
So in this case we have four devices.
So that means that 10 megabits per second divided by four devices times 30 percent only gives you zero
point seven five megabits per second and not 10 megabits per second per device.
This is not ideal because the bandwidth available to your PC is very low.
He's spacially in a large network so it is 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.