Understanding CSMA/CD

Understanding CSMA/CD
	Chapter 3. Network Architecture

I said Ethernet was a contention-based technology and in fact, it's called CSMA/CD or Carrier Sense Multiple Access Collision Detection.

To break this up - Carrier Sense means essentially - I put my ear say to the wire and if I don't sense that there 's anybody transmitting at the time, I jump on the wire, a bit like a railway line. If I don't hear a train coming I hop on the wire and off I go.

Multiple Accesses means there could be another train there, just around the corner that I haven't managed to hear or see. So multiple people access this railway line at the same time and, of course we're going to get a collision. As soon as there is a collision, there 's a ripple effect down the railway line.

Everybody that 's listening to the railway line hears this collision and we all back off for a random amount of time. When we sense the coast is clear, we hop back on the railway line and off we go.

So you can see that it 's not exactly the most efficient way of communicating but it does work and it 's simple.

Maximum transmission unit (MTU)

With this CSMA/CD we have what 's known as the MTU (Maximum Transmission Unit).

To continue our previous analogy with the train, the MTU is how long the train can be.

There 's a maximum size that our train can be when we stick it on the railway line. The maximum length a train could be in Ethernet is 1500 bytes and that 's the maximum transmission unit.

If you run the ifconfig command on the command line (logged in as root), you'll see there'll be an entry there called MTU in capital letters and that will be set to 1500 which is the standard size of a Ethernet train.

Process that can only talk MAC address to MAC address.

Now we've seen from the TCP/IP stack, that at the bottom there, we'll have a physical layer and this is what we are now referring to as Ethernet.

	Note
	Of course if you have Token Ring then the physical layer will be Token Ring.

Above that, we have Media Access Control layer, the MAC layer and in fact any 2 servers or workstations, can only talk to one another via the MAC address. They should be using the same physical connection. (Not strictly true but we'll cover that just now.)

In other words, 2 workstations or a server workstation or 2 clients want to talk to one another, they can only talk to one another using their MAC address.

You can use the "ifconfig" command to determine what the MAC address of your NICs are.

You cannot talk between two machines if they contain the same MAC address. It would be like the postman trying to deliver mail to No. 21 in the same street where 2 houses, 3 houses have the same No. 21. Clearly the postman's not going to know which one to deliver to.

Every Network Interface Card on the network needs a unique MAC address and these are supplied by the manufacturers of the NIC.

The MAC address is broken down into 2 by 6 byte numbers. The first set of 6 bytes is the manufacturer 's identifier and the remaining 6 bytes is the unique identifier given to this particular network interface card by that manufacturer. There are hundred of manufacturers of network interface devices around the world.

To find a out the MAC address on the workstation you are on you can type:

ifconfig eth0
eth0      Link encap:Ethernet  HWaddr 00:00:86:3C:A3:0A
              inet addr:192.168.2.233  Bcast:192.168.2.255  Mask:255.255.255.0
              inet6 addr: fe80::200:86ff:fe3c:a30a/64 Scope:Link
              UP BROADCAST NOTRAILERS RUNNING MULTICAST  MTU:1500  Metric:1
              RX packets:13655 errors:0 dropped:0 overruns:0 frame:0
              TX packets:5502 errors:0 dropped:0 overruns:0 carrier:0
              collisions:6 txqueuelen:1000
              RX bytes:7570021 (7.2 Mb)  TX bytes:632146 (617.3 Kb)
          Interrupt:3 Base address:0x300

The MAC address is the hexadecimal number after HWaddr. As in: HWaddr 00:00:86:3C:A3:0A


How do we connect the machines Physically		Broadcasts, Unicasts and Multicasts