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 cre- ated 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
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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 GHzBoth FHSS and DSSS support 1 and 2 Mbps, but 11-Mbps radios uti- lize 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 com- plementary mechanism.
The PN sequences actually spread the data stream transmission bandwidth of the signal, which defines its mechanism as spread spec- trum. 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 trans- mitter perceive that the network is very busy, it can then decide a spe- cific 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 sensewith the understanding that every wireless work-
station 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 transmis- sion. This reservation information, used in the network allocation ven- dor (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 designat- ed length. Note that DCF is the basic media access control method nec- essary 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.