Network Security Blog

CSMA/CD (Carrier Sense Multiple Access/Collision Detection)

The first Ethernet standards were 10Base5 and 10Base2.

Initially there were no hubs, switches.

the only things which is there is coaxial cable and the Computers NIC card.

all are connected in a shared bus topology.

++If one host send data--no problem.

++If two hosts sent data--Collision will Occur.

===>to over come these collisions we use CSMA/CD

CSMA/CD:-

it is Rules that all PC has to listen.

Rule1)Computer has to listen to Ethernet if any hosts are sending any frame, if no one then it will send

Rule2)After sending the frame also it will still continiously listen if there is any collisions.

Rule3)If collision heard, Both senders send JAM signals to all Hosts telling not to send any packets.

and a BACKOFF algorithm is calculated

backoff algorithm:- telling other computers not to transmit new data for a random amount of time..

When BACKOFF TIMER is expired then it willl again send the data.process is REPEATED...

REASON FOR MINIMUM ETHERNET HEADER:-

DMAC + SMAC + EtherType + Payload + CRC

6 + 6 + 2 + 46 + 4 = 64

++The reason for minimum length because of collision-detect mechanism.

Reason:-(collision-detect mechanism.)

++anything lesser than 64bytes then rex will treat it as a collision.

Example:-we don't allow bicycles on high-speed highways (national highways)

tx---sending signals ----to --->RX

basically at head of the packets it adds 0's and sends into the wire.

if the collision occurs then how TX knows?

all Rx'ers will send JAM signal to TX, thats how TX knows there is collision happened.

so TX will receive this JAM before it actually transmits the first 64bytes...

Frames below 64bytes==RUNT

POINTS:-

++switches with full-duplex links do not perform collision detection

++Wireshark doesn't show this, wireshark will capture before padding.

++Needed 64bytes only in full duplex, as in half duplex it uses CSMA/CD.