WiFi amplitude effects throughput?  - Page 1
Forum section: electronics / projects

author: themacman33
author_rank: Contributor
timestamp: November 30, 2014, 01:44:37 am
content: 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

author: themacman33
author_rank: Contributor
timestamp: November 30, 2014, 01:48:52 am
content: 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.

author: amyk
author_rank: Super Contributor
timestamp: November 30, 2014, 01:55:35 am
content: 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.

author: Marco
author_rank: Super Contributor
timestamp: November 30, 2014, 02:10:03 am
content: 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).

author: themacman33
author_rank: Contributor
timestamp: November 30, 2014, 02:21:54 am
content: 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!

author: Marco
author_rank: Super Contributor
timestamp: November 30, 2014, 03:05:41 am
content: The packet has a CRC checksum, which the receiver checks before sending an ACK (or for a number of received packets at once).

author: rs20
author_rank: Super Contributor
timestamp: November 30, 2014, 04:41:30 am
content: 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?

author: scatha
author_rank: Regular Contributor
timestamp: November 30, 2014, 05:59:26 am
content: Quote from: rs20 on November 30, 2014, 04:41:30 amIt'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).

author: aveekbh
author_rank: Regular Contributor
timestamp: November 30, 2014, 07:16:25 am
content: 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.