Hardware Reference
In-Depth Information
are used in Wireless-N devices include 1 × 1:1, 1 × 2:1, and 2 × 2:1, which include radios with 1 or
2 antennas supporting only a single data stream for up to 150Mbps in bandwidth. Other common
configurations include 2 × 2:2, 2 × 3:2, and 3 × 3:2, which include radios with 2 or 3 antennas
supporting up to two data streams for up to 300Mbps in bandwidth. Those using more antennas than
data streams allow for increased signal diversity and range. The highest performance Wireless-N
devices widely available on the market today use a 4 × 4:3 or 3 × 3:3 radio configuration, which
supports three data streams for up to 450Mbps in bandwidth.
802.11n is significantly faster than 802.11g, but by how much? That depends mainly on how many
data streams are supported, as well as whether a couple of other optional features are enabled or not.
The base configuration uses 20MHz wide channels with an 800ns guard interval between transmitted
signals. By using channel bonding to increase the channel width to 40MHz, more than double the
bandwidth can be achieved in theory. I say “in theory” because using the wider channels works well
under very strong signal conditions but can degrade rapidly under normal circumstances. In addition,
the wider channel takes up more of the band, causing more interference with other wireless networks
in range. In the real world I've seen throughput decrease dramatically with 40MHz channels, such that
the use of 40MHz channels is disabled by default on most devices.
Another optional feature is using a shorter guard interval (GI), which is the amount of time (in
nanoseconds) the system waits between transmitting OFDM (orthogonal frequency division
multiplexing) symbols in a data stream. By decreasing the guard interval from the standard 800ns to
an optional 400ns, the maximum bandwidth increases by about 10%. Just as with channel bonding
(40MHz channel width), this can cause problems if there is excessive interference or low signal
strength, resulting in decreased overall throughput due to signal errors and retries. However, in the
real world the shorter guard interval doesn't normally cause problems, so it is enabled by the default
configuration in most devices.
Combining the use of three data streams using standard 20MHz channels and the standard 800ns guard
interval, the maximum throughput of a Wireless-N connection would be 195Mbps. Using the shorter
400ns guard interval would increase this to up to 216.7Mbps. As with other members of the 802.11
family of standards, 802.11n supports fallback rates when a connection cannot be made at the
maximum data rate.
802.11ac
The latest wireless network standard, 802.11ac (also known as
Wireless-AC
), has not yet been
certified by the Wi-Fi Alliance in final form, though it is expected to be in late 2013. However,
hardware meeting 802.11ac draft specification 3.0 or 4.0 has been available since late 2012.
802.11ac is essentially an improved version of 802.11n, with the following major differences:
• Channel bonding to provide 80MHz wide channels (double the maximum of 40MHz available
in 802.11n) up to 160MHz wide channels
• 256QAM modulation (802.11n uses 64QAM); the higher the QAM modulation, the higher the
data rate
• Up to eight MIMO data streams (four was the maximum in 802.11n)
• Support for 5GHz frequencies only
Typical 802.11ac hardware also supports channel-bonded 802.11n 2.4GHz frequencies for
backwards compatibility with existing networks.
