Csma/cd carrier monitoring multipoint access / collision detection protocol

“ Multipoint access ” It means that many computers are connected to a bus by means of multi-point access .

“ carrier sense ” It means that each station should check whether there are other computers sending data on the bus before sending data , If there is , Do not send data for the time being , In case of collision . “ carrier sense ” It is to use electronic technology to detect whether there are data signals sent by other computers on the bus .

“ collision detection ” That is, when the computer sends data, it detects the signal voltage on the channel . When several stations send data on the bus at the same time , The signal voltage swing on the bus will increase （ Superimpose on each other ）. When the signal voltage swing value detected by a station exceeds a certain threshold value , It is considered that at least two stations on the bus are sending data at the same time , Indicates a collision . So-called “ Collision ” It's a conflict . therefore “ collision detection ” Also known as “ Collision detection ”.

After collision detected ： In the event of a collision , The signal transmitted on the bus has serious distortion , There is no way to recover useful information . Every station that is sending data , Once a collision is found on the bus , Stop sending immediately , So as not to waste network resources , Then wait for a random period of time and send again .

characteristic ：

1、 Use CSMA/CD Protocol Ethernet can not carry out full duplex communication but only two-way alternate communication （ Half duplex communication ）.

2、 Each station is in a short period of time after sending data , There is a possibility of collision . This uncertainty of transmission makes the average traffic of the whole Ethernet far less than the maximum data rate of Ethernet .

Contention period ：

The first station to send data frames , At most time after sending data frame 2a （ Twice the end-to-end round trip delay ） We can know whether the transmitted data frame has suffered collision . After the contention period, no collision has been detected , In order to be sure that this transmission will not collide .

Ethernet end-to-end round-trip delay 2a It's called contention period , Or collision window . Usually , take 51.2 us For the length of the contention period . about 10 Mb/s Ethernet , It can be sent during the contention period 512 bit, namely 64 byte . When Ethernet sends data , If before 64 No bytes conflict , Then the subsequent data will not conflict .

If there is a conflict , It must have been before it was sent 64 In bytes . As soon as a conflict is detected, the transmission is stopped , At this time, the data already sent must be less than 64 byte . Ethernet specifies Shortest valid frame Long for 64 byte , Where the length is less than 64 Byte frames are invalid frames aborted due to conflicts .

“ Wait for a random period of time ” The explanation of —— Binary exponential type backoff algorithm ：

The collision station stops sending data , To postpone （ to retreat ） A random time to send data .

Determine the basic retreat time , Generally, it is taken as the period of contention 2a.

Defining parameters k ,
                      k = Min[ Number of retransmissions , 10]

From the set of integers [0,1,…, (2^k - 1)] Randomly take a number , Write it down as r. The delay required for retransmission is r Times the basic escape time .

When re communicating 16 Discard the frame if it still fails , And report to the top .