Frequency Allocation

The 802.11 offers two types of PHY layers, each with distinct RF usage
through either FHSS or DSSS. Both FHSS and DSSS options were created
to adhere to regulatory rules set by the FCC to operate in the 2.4-
GHz ISM. The unlicensed ISM band is allocated slightly differently
worldwide. Table 10.1 shows that the actual break in the spectrum of
usage varies by country.

TABLE 10.1
Spectrum of Usage
Varies by Country

Country Frequency
United States 2.4000–2.4835 GHz
Europe 2.4000–2.4835 GHz
Japan 2.471–2.497 GHz
France 2.4465–2.4835 GHz
Spain 2.445–2.475 GHz

Both FHSS and DSSS support 1 and 2 Mbps, but 11-Mbps radios utilize
DSSS. In fact, DSSS setups utilize the same technology as global
positioning system (GPS) and satellite cell phone equipment.
The specifications of this technology require that each information bit
is linked through an XOR function that has a long numerical value or a
pseudorandom numerical value (PN) that produces a high-speed digital
frequency modulated spectrum on a carrier frequency using differential
phase shift keying (DPSK).

When a DSSS signal is received, it is matched to a filter correlator
that removes the PN sequence and regains the original data stream.
The data rates of 11 Mbps and 5 Mbps are achieved only when DSSS
receivers use different banks of correlators and PN codes in order to
recover the transmission stream of network data.
The high-speed rate modulation mechanism is designated as a complementary
mechanism.

The PN sequences actually spread the data stream transmission
bandwidth of the signal, which defines its mechanism as spread spectrum.
The objective is to reduce power, and the total power used
remains the same. When the signal is received, it is correlated with the
same PN sequences so it can reject any narrowband interference and
reassemble the binary data in its original form.

The exact speed is not as important as the fact that the transmission
uses about 20 MHz for DSSS systems. This means that the ISM band
can support as many as three non-overlapping channels.

The fundamental methods that 802.11 uses involve the distributed
coordination function (DCF). It then uses carrier sense multiple access
with collision avoidance (CSMA/CA). This means that the wireless
workstation must listen for other users on the network. The station then
transmits once the channel is idle; however if it is busy, the wireless
workstation pauses until the transmission stops and executes a random
backoff until it can transmit safely on the radio spectrum.

The space of time between the packet transmission and the start of the
“ACK” frame is one short interspace (SIFS). The ACK frames have a higher
priority than other network traffic, requiring fast acknowledgment since
ACKs need to be supported by the MAC sublayer in the 802.11 standard.
Some transmissions wait for at least one DCF interframe space
(DIFS) prior to sending any data across the network. Should the transmitter
perceive that the network is very busy, it can then decide a specific
random backoff period by determining a value for the internal
timer for a specific number of slot times. When DIFS expires, the timer
starts to decrease. When the timer approaches zero, the station can then begin transmitting. Should the channel be in use by another wireless
workstation prior to the timer’s approaching zero, the timer setting is
kept the same at the decreased value for each future transmission
across the network. The mechanism behind this setup depends on the
physical carrier sense with the understanding that every wireless workstation
can listen to all other stations on the wireless network. However,
it should be noted that every wireless workstation may not necessarily
be able to hear all the other wireless workstations.

One solution to this problem is to define a second carrier sense
method. The virtual carrier sense permits a wireless workstation to
reserve the medium for a certain period of time using RTS/CTS frames.
For example, the first wireless workstation sends an RTS frame to
the access point. The second wireless workstation will not hear the RTS.
An RTS frame has a duration/ID field that designates the measure of
time for which the medium is reserved for the next wireless transmission.
This reservation information, used in the network allocation vendor
(NAV) of all stations, is used to detect the RTS frame.

The access point answers the CTS frame when an RTS is received
because it contains a duration/ID field that designates a measure of
time for which the medium was reserved. When the second wireless
workstation (stated above) does not detect a RTS, it will detect the CTS
and update the NAV. Thus, collision is avoided through using hidden
nodes from other wireless workstations.

RTS/CTS is utilized with respect to user-specified parameters such
that it can always or never be used with packets that exceed a designated
length. Note that DCF is the basic media access control method necessary
for all wireless workstations. In addition, there is an optional
extension to the DCF called the point coordination function (PCF) which
yields the functionality for time division duplexing (TDD). TDM is the
ability to deal with time-bounded and connection services.