Information Technology Reference
In-Depth Information
a wireless backbone, and does not require any physical network cable for that AP
node. For example, you might want to interconnect two buildings using IEEE
802.11 hardware, in addition to supporting wireless workstations in either building.
A dual W-NIC access point could be placed in each remote building, with an omni-
directional range extender antenna for the local workstations, and a directional Yagi
outdoor antenna for the remote building link. In the remote building, either wire-
less stations or interconnection to a wired Ethernet could be accommodated.
Fall-Back Operation.
The most sterling feature of the IEEE 802.11 implementa-
tions is that they are backwards compatible. That is, the very latest high-speed 54
and 108 Mbps W-NICs can interoperate with the earlier 1, 2, and 11 Mbps cards.
As a matter of fact, this feature is exploited to extend the range of the WLAN at all
speeds by allowing the cards to downshift as necessary to maintain transmission
integrity.
Control packets, including beacons, are normally sent at 1 Mbps operation,
and the subsequent data rate is negotiated up, according to the capabilities of the
access point and the station. In addition to the built-in speed range of each WiFi sta-
tion—some are “b” and some are “g” or better—signal strength determines the data
rate the air link will support. In other words, the higher the data rate, the shorter
the range. So, IEEE 802.11 allows the data rate to fall back to a lower speed if the
packet error rate is too high.
IEEE 802.11 WLANs can operate at any channel within either the 2.4 or the 5 GHz
bands. (See Table 13.2.) The 2.4 GHz band ranges from 2.400 to 2.4835 GHz (to
2497 in Japan), a total bandwidth of 83.5 MHz (97 MHz in Japan), divided into 13
channels with center frequencies that are 5 MHz apart, and one additional channel
at 2484 in Japan. In the United States and Canada, the FCC and IC plans allow 11
channels for DSSS or OFDM (Channels 1 to 11), while the ETSI allows channels 1
to 13 in most of Europe (only channels 10 and 11 in Spain, and 10 to 13 in France),
while Japan allows all 14.
IEEE 802.11a has a total of only 12 “operating” channels in the United States
(Table 13.3), but they have 20 MHz spacings. Two of the “a” bands are contiguous,
running from 5150 to 5350 MHz. The third band goes from 5725 to 5825. That is
a total allocation of 300 MHz. Channel numbering for 5 GHz is rather strange. In
the IEEE scheme, basic channel numbers start at an arbitrary channel 1 at 5005
MHz (5.005 GHz) and are sequentially numbered in 5 MHz steps. The first IEEE
802.11a operating channel within the lower bands is numbered “Channel 36,”
