Major functions of data link layer photocol
- Media Access Control (control that which computer can send data)
- Error Control (make sure that data send from node to another node correctly)
- Message Delineation (Identify the beginning and ending of message)
Media Access Control - make sure that 2 party not send at the same time
- Point to point half duplex links (can't send and receive at the same time)
- Multipoint configuration (more than 1 computer share link)
Main approaches
- controlled access
1 device that control point, to allow another device send data (Ex. token)
- contention based access
check medium that available or not before send data (collision : don't know that link free or not and send data. Have to wait the random of time if data get the small amount of number, can send data first )
Polling
- Roll call polling
use in mainframe computer, can send data when it is asked by server and send data to server. Don't have to call by order and spend a lot of time to polling.
- Hub polling
use token ring
Error Control
- Corrupted (data changed), the data reached the destination but incorrect
- Lost data, the data not reached the destination but loss data in somewhere
Major function
- Preventing errors (task of the human, have to check it by yourself)
- Detecting and Correcting errors (data link layer, use after human can't find error)
* In a single bit error, only one bit in the data unit has changed (noise just occur and disappear very fast)
* A burst error means that 2 or more bits in the data unit have changed (have to count error from starting point to ending point)
Error Detection
- Parity Checking (even and odd parity, parity not enough to check error) count number of 1 to check that it odd or even number
LRC - Longitudinal Redundancy Checking (98% error detection rate) the receiver may not know the error occur
1. find parity for each character
2. calculate LRC (except last line)
3. the last number of BCC calculate from all number in row BCC
Polynomial Checking
- checksum
- cyclic redundancy check (CRC)
if use n bit CRC, put n zero and divisor is n + 1 bits (more CRC bits, more detect the error)
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