802.11 Concepts, Bands, Channels, Data Rates and Regulations

What it is

Wireless networking uses radio waves instead of cable, and the 802.11 standards define how it works.

Each standard names a frequency band and a family of data rates. A radio also has a channel, which is a slice of the band, and a transmit power, which is how strongly it sends.

What you may use depends on the law of the country where the device runs. RouterOS therefore asks you to set the country before it allows certain frequencies and power levels.

How it works

The standard sets the band and the maximum rate, and the radio conditions set the rate you really get.

The 802.11 standards

Newer standards are normally backward compatible. A mixed network can still carry older clients, but those clients may slow the cell down.

Bands and channels

RouterOS names the band in the wireless settings, such as 2ghz-b/g/n or 5ghz-a/n/ac. The name tells the radio which standards to offer.

The 2.4 GHz band is crowded, and its channels are narrow slices that overlap one another. In practice only a few channels are free of overlap, and the usual set is channels 1, 6 and 11.

The 5 GHz band has more room and more channels that do not overlap. It is usually cleaner, though its signal passes through walls less well than 2.4 GHz. Some 5 GHz channels must also be vacated when radar is detected.

A channel width can be set. A wider channel gives a higher data rate, but it uses more spectrum and leaves fewer free channels. The common 20 MHz width is the safe choice in a crowded area.

/interface wireless set wlan1 band=2ghz-b/g/n channel-width=20mhz frequency=2437

The frequency is given in MHz. The value 2437 is the centre of channel 6.

Data rates and chains

A chain is one transmit and receive path, with its own antenna connection. More chains allow several data streams at once, which is the idea behind 802.11n and later. A radio with two chains can reach a higher rate than one with a single chain.

The data rate that a link really uses is not fixed. It depends on the standard, the channel width, the number of chains and the signal quality. When the signal is weak, the radio moves to a lower rate that it can still decode.

Transmit power and receive sensitivity

Transmit power is how strongly the radio sends, measured in dBm. Receive sensitivity is the weakest signal the radio can still decode at a given rate, also in dBm.

Two points follow from this:

• Higher data rates need a stronger received signal, so rx sensitivity gets worse as the rate rises.
• Signal values are negative dBm numbers, and a number closer to zero is a stronger signal.

More transmit power is not always better. A radio that shouts louder than its neighbour can hear back creates a link that works in one direction only.

Country regulations

Each country limits the frequencies, channels and power that wireless devices may use. RouterOS has a country setting that applies these limits.

/interface wireless set wlan1 country=latvia frequency-mode=regulatory-domain

With frequency-mode=regulatory-domain, the router allows only the channels and power permitted for the chosen country. Other modes exist that give the installer more control. Using them is the responsibility of the installer, who must stay within the law.

The country name must match the place where the equipment is used.