Communication method

1. communication methods in LAN

Most method in LAN communication is Uni-Cast, Which sends data to specific computer by including destination MAC address.

 

Broad-Cast spreads data to all computers which on its group.

 

Multi-Cast is the combination of Broad-Cast and Uni-Cast.

That can sends to specific group, and it excepts other group. It means there's no effect to any other groups.

 

2. CRC example

When data is arrived to destination. They could not include origin data. May be influenced by external factors.

 

Then, how to fix these problems? Just take another data? No, There's always an answer.

If you send data like 10101101_(2) to David, it includes error fix code.

First, do Left-Shift 4 times. Result will be like 101011010000_(2)

 

Don't think about carry numbers (overflow, borrow), just divide code by Generator Polynormial bits. ( 11101_(2) )

This process goes on XOR(mod-2) calculation.

After the process, 1011_(2) will left.

then add it to origin code, so the conclusion is 10101101"1011"_(2)

When destination get this code, it divides again.

when left bits are 0, there's no error in communication.

Otherwise, bits are not 0, then error occurs.

 

3.Local network

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

CS -> detects the line is used by any others.

MA -> If its in idle, computer understands that bus allows to using itself.

CD -> after sending message, computers checks collision. this is called by "collision detection"

Collision detection is about works to preventing collision when more than 2 computers try to send packet in same time.
If Collision occurs, waiting for several times and allows re-send, but time will deploy randomly.

Though, some computer can have longer time to process.

 

*CSMA/CA ( same but different with Collision Avoidance )

A and B can not communicate each other, only AP can communicate all.

Belong the technology was improved, Wireless Communication appears to LAN. But this method has its own problems, so technicians developed CA.

If A hooked to AP first, then B's requires will be avoided.

4.TCP & UDP

TCP repeats method of read() and write(), so it seems like manually.

UDP has different in server process. Server doesn't need to repeat the data reading sequence, just keep tracking input and re-send data to request of computers.

Benefits at server process amount and two way communication, otherwise it can't detect connects when all cleared.