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Wireless Local Area Network (WLAN) 413

8 bit block

6 bits are used to generate
DQPSK an 8 bit code word
phase change
between the Phase change during the
symbols transmission of
the symbol
Symbol

Figure 6.14 Complementary code keying for 11 Mbit/s transmissions.

correctly do not cause collisions because of their inability to detect an ongoing transmis-
sion of a high‐datarate frame. Therefore, it is necessary to ensure that at least the begin-
ning of the frame is received correctly by all devices. Thus, the PLCP header of a frame
is always sent at a speed of 1 Mbit/s regardless of the speed and coding scheme of the rest
of the frame. To inform other stations about the duration of the transmission, the PLCP
header also contains information about the total duration of the transmission.
If the actual speed of an 11 Mbit/s WLAN network is compared to that of a 10 Mbit/s
fixed Ethernet, it becomes apparent that is not quite as fast as its fixed‐line counterpart.
A 10 Mbit/s fixed‐line Ethernet can reach a maximum speed of about 700–800 kB/s
under ideal conditions. In an 11 Mbit/s WLAN, the maximum speed is about 300 kB/s
between two wireless devices. This is due to the following properties, which have
already been described in this chapter:
The PLCP header of a WLAN frame is always transmitted at 1 Mbit/s.
●
Each frame has to be acknowledged with an ACK frame.
●
In a wired network, a frame is directly sent from the source to the destination device.
●
In a WLAN BSS, a frame is sent from the source to the AP, which then forwards the
frame to the destination. The frame, therefore, traverses the air interface twice. In
practice, this cuts the maximum transmission speed in half.

6.6.2 IEEE 802.11g with up to 54 Mbit/s
A first step to higher data transmission speeds was the 802.11g standard, which introduced a
new modulation scheme called Orthogonal Frequency Division Multiplexing (OFDM).
This modulation scheme enables speeds up to 54 Mbit/s while using almost the same
bandwidth as the 802.11b standard. Although 802.11g is not widely used anymore
today, the OFDM modulation scheme introduced here was reused with only a few
modifications in the PHYs that followed with 802.11n and 11 ac.
The OFDM modulation scheme is fundamentally different from the modulation
schemes used in the 802.11b standard. As shown in Figure 6.15, OFDM divides the
bandwidth of a single 20 MHz channel into 52 subchannels.
The subchannels are ‘orthogonal’, as the amplitudes of the neighboring subchannels
are exactly zero at the middle frequency of a subchannel. Therefore, they do not