As we know the speed of wireless connection wifi depends on SNR The speed (bitrate) maybe 20,10 or 5 Mbps The question is: how is wifi can reduce or increase its bitrate? It use the same carrier (2.4 ghz) ! Is the bitrate changed by manipulate bit duration?
This is done via Dynamic Rate Shifting (also known as Adaptive Modulation and Coding and other names).
Basically, this allows the WiFi device to adjust its Modulation and Coding Scheme (MCS) according to current RSSI and SNR.
Modulation refers to how many bits are transmitted per subcarrier. Trying to put it simple, subcarriers are then transmitted over the air as symbols (waveforms). The more complex the modulation scheme, the more bits are transmitted per subcarrier, but also the more sensitive to interference and noise, therefore requires better channel. In 802.11g modulations are:
- BPSK (1 bit per subcarrier)
- QPSK (2 bits per subcarrier)
- 16-QAM (4 bits per subcarrier)
- 64-QAM (6 bits per subcarrier)
Coding refers to redundant bits added for protecting the information bits. This is expressed as fraction of useful information bits. In 802.11g they are:
1/2 (half of transmitted bits are useful, the other half are redundant)
The last one provides more useful bits within transmission (higher data rate) but also less protection and therefore requires better channel.
To give an idea, in 802.11g using OFDM as transmission mode (the main one), the symbol duration is 4 microseconds, during which are transmitted 48 subcarriers carrying data.
The best channel condition allows for usage of 64-QAM (highest modulation) and 3/4 coding rate (less protection). This gives: 6 bits per subcarrier (64-QAM) x 48 subcarriers = 288 bits per symbol, 3/4 of which are useful data = 216 information bits per symbol. These are transmitted in 4 microseconds which gives 54 Mbps (highest nominal data rate of 802.11g).
The worst link condition allows for usage of BPSK (strongest modulation) and 1/2 coding rate (more protection). This gives: 1 bit per subcarrier (BPSK) x 48 subcarriers = 48 bits per symbol, 1/2 of which are useful data = 24 information bits per symbol. These are transmitted in 4 microseconds which gives 6 Mbps (lowest nominal data rate of 802.11g using OFDM)
In the middle of these extreme channel conditions, there are several combinations of modulation and coding rate which give different data rates (e.g. BPSK 3/4, ..., QPSK 1/2, ..., 16-QAM 3/4, ...)