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nel in a way that maintains compatibility to IEEE 802.11a/g devices. 802.11n
transmits data over two contiguous channels. One of these assumes the task
of a control channel that, among other things, handles the administration of
data transmission. Concentrating these basic tasks into the control channel
means that devices supporting a transmission at 20 MHz only can also be
connected. The second channel is an extension that only comes comes into
effect if the remote client also supports data transmission at 40 MHz. The use
of the second channel remains optional throughout, with transmitter and
receiver deciding dynamically whether one or two channels should be emplo-
yed.
Control channel
20 MHz
As the implementation of 40 MHz with separate control and extension chan-
nels is more efficient in the 802.11n draft than in the conventional turbo
mode, more than double the amount of carrier signals can be obtained (108
in total). The maximum data throughput when using improved OFDM modu-
lation and two parallel data streams thus rises to 270 Mbps.
Short guard interval
The final improvement of the 802.11n draft is the improvement in the chro-
nological sequence of data transmission. A signal that is to be transmitted in
a WLAN system is not broadcast at a distinct point in time but is "held up" for
a certain, constant transmission period. In order to prevent interference at the
receiving end, a short break is made following the transmission period before
the transmission of the next signal commences. The entire duration of trans-
mission period and break are referred to in WLAN terminology as "symbol
length" and the break itself is known as the "guard interval".
IEEE 802.11a/g uses a symbol length of 4 μs: the information transmitted on
the carrier signal changes following transmission of 3.2 μs and a break of
0.8 μs. 802.11n reduces the break between transmissions to the so-called
"short guard interval" of only 0.4 μs.
LANCOM L-300 Access Point Serie
Chapter 1: Introduction
Extension channel
20 MHz
17
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