What is CSMA / CD?
CSMA / CD multiple access, carrier sensing, collision detection abbreviation of three words.
Multiple Access: multiple sites connected to the main line, the competition to use the bus.
Carrier sensing: the first test at each site before the retransmission if there are other stations on the bus in the transmission frame (1) upon detecting 96 bit bus idle period, the transmission of this frame. (2) if the bus is busy, it continues to monitor the bus and wait for the 96 bit idle time, and then continues to send this frame.
Collision detection: each station is transmitting side transmits a collision detection side, the transmission is stopped if the monitored after a collision, and the random wait some time to continue transmission. At the same time increasing the impact enhanced Ethernet operation, once the collision found that in addition to the corresponding station stops transmitting, it will also artificially increase the interference signal, the purpose is to allow all hosts know that a collision has occurred.
The following re-introduce the concept of collision window:
As shown above, the main monitor site A first idle bit 96, then starts transmitting a frame, but the frame during transportation of another A, D sites also monitored main line is idle for 96 bits, then it starts to transmit the frame, which will the two sides leading to a collision, only when the frame data to and from back to square one again collision may determine not happen, so we called the end of the round-trip time of the collision window or expropriation. Obviously, the long-term requisition, the more the number of sites, the greater the probability of collision, not even so much shared Ethernet host bus can not be too long.
Minimum frame length:
As shown, if the frame length is too short, then cause the host before a collision is detected completes the transmission of the frame, so that the frame has been not know collision, even if the destination station of the frame dropped, not the original site re-issued. Therefore, we require a minimum frame size is 64 bytes, if the data is not long enough to fill a number of bytes so that it can guarantee that a collision can be detected before the transmission is completed, as long as the collision is detected to stop transmission, it is possible to know all less than 64 byte length of the frame is a frame after collision!
The maximum frame size:
Once the frame length is too long, cause bus has been occupied, the receiver would buffer overflow, it is necessary to specify a maximum frame length, the maximum frame size of 1518 bytes, 18 bytes by subtracting the length of the head and tail after the data load of 46 to 1500 bytes.
What is CSMA / CA?
上面的CSMA/CD是用在以太网上的，而CSMA/CA使用在无线网络上的，它们相差在一个叫CA另一个叫CD上，为什么无限网络不能进行碰撞检测CD呢？原因在于无线传播的特殊性，信号强度的动态范围很大，导致如果要进行碰撞检测那么对硬件的要求过高，所以就改成碰撞避免了。