Wifi technologies transmit data across a range of frequencies referred to as the frequency bandwidth. Each specific frequency within a channel is known as a subcarrier, which is used to transmit data and manage communication.
As an example, a 20 MHz channel on 5 GHz consists of 64 subcarriers, of which 52 are used for data transmission and the rest (pilot and null subcarriers) are used for communication management. The transmission technology is called OFDM (orthogonal frequency-division multiplexing).
Among the various factors that impact wifi data rates, which include the number of spatial streams and modulation, the one with the most direct impact is probably channel width. Doubling the channel width also doubles the data rate, or nearly so.
Wi-Fi 4 (802.11n) introduced the concept of channel bonding to be able to increase the bandwidth from the original 20 MHz in OFDM.
Channel bonding means combining multiple 20 MHz channels to increase the number of available subcarriers for data transmission, and thereby the data rates. This is particularly desirable when wifi networks are expected to deliver Gigabit internet access.
Since a larger channel width provides more subcarriers for data transmission, increasing the channel width increases the overall network throughput in an ideal condition.
For example: A 2×2 Wi-Fi 6 (802.11ax) client using a 160 MHz channel width can achieve a maximum throughput of 2.4 Gbps in ideal conditions.
However, channel bonding not only provides more subcarriers, it also increases the noise floor by 3 dB per doubling on 2.4 and 5 GHz frequency bands.
Increasing the noise floor means a lower signal-to-noise ratio (SNR). The lower the SNR, the more impact to:
Increasing the channel width also increases the probability of co-channel interference from neighboring networks due to contention overhead.
Because there are only three non-overlapping 20 MHz channels available at 2.4 GHz, channels larger than 20 MHz do not scale.
The 2.4 GHz frequency band is already congested, and we therefore recommend sticking with the 20 MHz channel width and not using channel bonding on 2.4 GHz.
Using a 160 MHz channel requires higher (greater than 38 dB) SNR, low co-channel interference and no radar nearby operating on DFS channels.
This means that it is only advisable for homes where:
As the majority of wifi frames are small, a 40 MHz channel satisfies the requirements of most applications.
However, if providing gigabit WAN, we recommend a 80 MHz channel, as a 2×2 client operating on a 40 MHz channel can achieve a maximum of 570 Mbps.
Generally, wifi access points are configured to use automatic channel width selection, the behavior of which varies from vendor to vendor.
We tested the auto channel width selection feature on Zyxel Wi-Fi 6 (802.11ax) capable CPEs in our lab while generating different types of interference on the medium.
The access point:
However, the channel width capabilities of wifi clients also varies, and the AP adjusts its communication based on client capabilities, meaning that it transmits on 20 MHz channel width towards a 20 MHz-only client and on 80 MHz channel width towards a 80 MHz-capable client.
320 MHz is a channel width only available on 6 GHz, and with the frequencies available in Europe, only a single 320 MHz channel is possible.
Theoretically, SNR decrease should be less of a worry on 6 GHz, but co-channel interference for other networks would still be a major concern.
We advise that 320 MHz channel bandwidth only be used where there are no or very few neighboring networks.
In summary, we:
In communication between Wi-Fi 4 and more recent clients, primary and secondary channels are used for data transmission, while management and control frames are transmitted only on the primary channel.
Therefore, when doing packet capture for wireless analysis of bonded channels, make sure to capture both primary and secondary channels if you need the data packets. Otherwise, only management and control frames will be captured when the channel size is not specified.
Article by Dr. Maghsoud Morshedi Chinibolagh and Jorunn Danielsen