WiFi amplitude effects throughput?

[themacman33]

Why does the amplitude drop caused by distance from a WiFi router decrease speed? Transmission frequency doesn't change, and if the AC waveform doesn't reach the trigger level, wouldn't there be no signal at all? If it is atmospheric distortion, how does the client ensure that it receives the proper information?
Sorry for all the questions! Thanks for responding, feel free to respectfully dispute my understanding of the theory.

-Mac

[themacman33]

For clarification, by throughput, I mean networking "bandwidth"
Networking bandwidth is the number of packets sent in an amount of time, whereas bandwidth is a section of the AC spectrum.

[amyk]

The error rate increases as the SNR decreases, and more retries are needed to ensure correct transmission. Read more about 802.11 for more information.

[Marco]

Noise both from outside sources and inside the receiver stays roughly the same, so the signal to noise ratio drops. The achievable bitrate over a channel is tied to SNR.

You don't really go from no packetloss to 100% packetloss, it happens smoothly (which in and of itself will lower througput) and the sender will lower it's rate to lower packetloss when it becomes too high (which will also lower throughput).

[themacman33]

Ok, that makes sense. By what mechanism can the reciever determine if a packet is an error? Is there some sort of algorithm that can be applied to each packet to determine the validity?

Thanks for your responses!

[Marco]

The packet has a CRC checksum, which the receiver checks before sending an ACK (or for a number of received packets at once).

[rs20]

Is that the only explanation, or is a slower data rate (for example, 16-QAM instead of 64-QAM or something) also negotiated if a particular SNR is detected?

[scatha]

Is that the only explanation, or is a slower data rate (for example, 16-QAM instead of 64-QAM or something) also negotiated if a particular SNR is detected?

It's not specified in the 802.11 standard how link quality is determined, nor how the transmission rate adapts to link quality. This behaviour is implementation dependant, with manufacturers unwilling to divulge what techniques they are using.

Generally though, if an ACK isn't received then the frame is retransmitted using lower rate/more robust modulation each time, e.g. for 802.11g the QAM levels are dropped on the subchannels, before dropping down to 802.11b modulation (CCK or barker spread QPSK/BPSK).

[aveekbh]

Typically, if a packet is not ACK'ed a few times (for example, 5 times), the transmitter drops the data rate to a more robust modulation type. This continues until the packet error rate drops below some manufacturer-defined threshold.