Hardware Reference
In-Depth Information
on the same networks, when mixing different standards on the same network, the network will
often slow down to the lowest common denominator speed. To prevent these slowdowns, you
can configure access points to disable “mixed mode” operation, but this will limit the types of
devices that can connect. For example, you can configure a 2.4GHz Wireless-N access point to
allow 802.11b/g/n connections (full mixed mode), or to only allow 802.11g/n (partial mixed
mode) connections, or to only allow 802.11n connections. The latter offers the highest
performance for Wireless-N devices. Similarly, you can configure Wireless-G access points to
allow 802.11b/g (mixed mode) operation, or to only allow 802.11g connections. Restricting or
disabling the mixed mode operation offers higher performance at the expense of restricting the
types of devices that can connect.
802.11n
The most common wireless standard in current use, 802.11n (also known as
Wireless-N
), was
published in October 2009. 802.11n hardware uses a technology called
multiple input, multiple
output (MIMO)
to increase throughput and range. MIMO uses multiple radios and antennas to
transmit multiple data streams (also known as
spatial streams
) between stations. Unlike earlier
802.11 implementations, in which reflected radio signals slowed down throughput, reflected radio
signals can improve throughput as well as increase useful range.
802.11n uses 64QAM signaling. Quadrature amplitude modulation (QAM) signaling uses two signals
that are 90 degrees out of phase with each other, are modulated during transmission, and are
recombined when received. QAM signaling modulates signals into multiple points, and the greater the
number of points, the higher the signal rates that can be achieved.
Note
To learn more about QAM and other modulation methods, see the tutorials at the Radio-
modulation/what-is-pm-tutorial.php
802.11n is the first wireless Ethernet standard to support two frequency ranges or bands:
•
2.4GHz
(same as 802.11b/g)—Required by the 802.11n specification
•
5GHz
(same as 802.11a)—An optional feature supported by the 802.11n specification
Thus, depending on the specific implementation of 802.11n in use, a dual-band 802.11n device may
be able to connect with 802.11b, 802.11g, and 802.11a devices, whereas a single-band 802.11n
device will be able to connect with 802.11b and 802.11g devices only.
Wireless-N devices can contain radios in a number of different configurations supported by the
standard. The radios are defined or categorized by the number of transmit antennas, receive antennas,
and data streams (also called
spatial streams
) they can support. A common notation has been devised
to describe these configurations, which is written as a x b:c, where a is the maximum number of
transmit antennas, b is the maximum number of receive antennas, and c is the maximum number of
simultaneous data streams that can be used.
The maximum performance configuration supported by the standard is 4 × 4:4, (4 transmit/receive
antennas and 4 data streams), which would support bandwidths of up to 600Mbps; a few high-end
devices currently support this performance configuration. However, more common configurations that
