802.11 Variants

In 1997, the IEEE adopted IEEE Standard 802.11-1997, the WLAN standard. This standard defines the Medium Access Control (MAC) and PHY layers for a LAN with wireless connectivity. It addresses local area networking where the connected devices communicate over the air to other devices that are within close proximity to each other. Figure 2-1 illustrates this.

The industry group Wireless Ethernet Compatibility Alliance (WECA) certifies its members' equipment as conforming to the 802.11b standard and enables compliant hardware to be certified as Wi-Fi compatible. This is an attempt to guarantee intercompatibility between hundreds of vendors and thousands of devices. Table 2-1 lists the variants of 802.11 and provides an overview of the relationship between 802.11b with other 802.11 variants.

Table 2-1: IEEE 802.11 Variants

802.11 Variant	Description
802.11a	Created a standard for WLAN operations in the 5 GHz band with data rates of up to 54 Mbps. Published in 1999.
802.11b	Created a standard (also known as Wi-Fi) for WLAN operations in the 2.4 GHz band with data rates of up to 11 Mbps. Published in 1999. Products based on 802.11b include public-space Internet kiosks, WLAN services such as Wayport, and wireless home networking products such as the Macintosh AirPort.
802.11c	Provided documentation of 802.11-specific MAC procedures to the International Organization for Standardization/International Electrotechnical Commission (ISO/IEC) 10038 (IEEE 802.1d) standard. Work has completed.
802.11d	Publishing definitions and requirements to enable the 802.11 standard to operate in countries that are not currently served by the standard.
802.11e	Attempting to enhance the 802.11 MAC to increase the quality of service (QoS) possible. Improvements in capabilities and efficiency are planned to allow applications such as voice, video, or audio transport over 802.11 wireless networks.
802.11f	Developing recommended practices for implementing the 802.11 concepts of APs and distributed systems (DSs). The purpose is to increase compatibility between AP devices from different vendors.
802.11g	Developing a higher-speed PHY extension to the 802.11b standard while maintaining backward compatibility with current 802.11b devices. The target data rate for the project is at least 20 Mbps.
802.11h	Enhancing the 802.11 MAC and 802.11a PHY to provide network management and control extensions for spectrum and transmit power management in the 5 GHz band. This will allow regulatory acceptance of the standard in some European countries.
802.11i	Enhancing the security and authentication mechanisms of the 802.11 standard.
802.1x	Also aimed at enhancing security of 802.11b.
Source: Wave Report, November 29, 2001.

FHSS (802.11a)

Spread spectrum radio techniques originated in the U.S. military in the 1940s. The unlikely co-patent holders on spread spectrum technology are the actress Hedy Lamar and musician George Antheil. Lamar had been married to a German arms dealer and fled Germany as the Nazis came to power. One of Antheil's techniques involved the use of player pianos. These two came together to create one of the twentieth century's most influential radio technologies.

The military had started to use radio as a remote control mechanism for torpedoes, but this technique suffered from a vulnerability to jamming. Aware of this, Lamar suggested to Antheil that the radio signal should be distributed randomly over time across a series of frequencies. The transmission on each frequency would be brief and make the aggregate less susceptible to interruption or jamming. The problem was synchronizing the transmitter and receiver to the frequency being used at any point in time. Antheil used his musical expertise to design a synchronization mechanism using perforated paper rolls like those found in player pianos.

Lamar and Antheil were awarded patent number 2,292,387 and gave the rights to the U.S. Navy in support of the war effort. Although the Navy did not deploy the technology, engineers at Sylvania Electronic Systems applied electronic synchronization techniques to the concept in the late 1950s. The U.S. military began using these systems for secure communications in the early 1960s. The spread spectrum technique spawned from the work of Hedy Lamar and George Antheil is what we now call FHSS.

Local authorities also regulate the hopping rate. In North America, the hopping rate is set at 2.5 hops per second with each transmission occupying a channel for less than 400 milliseconds. Channel occupancy is also called dwell time. In 2001, the FCC proposed to amend its Part 15 rules to allow adaptive hopping techniques to be used. This rulemaking is designed to reduce interference with other systems operating the 2.4 GHz frequencies. Studies have shown that up to 13 IEEE 802.11 FHSS systems can be co-located before frequency channel collisions become an issue.^[4]

DSSS

DHSS systems mix high-speed bit patterns with the information being sent to spread the RF carrier. Each bit of information has a redundant bit pattern associated with it, effectively spreading the signal over a wider bandwidth. These bit patterns vary in length and in the rate at which they are mixed into the RF carrier. They are called chips or chipping codes and range from 11 bits to extremely long sequences. The speed at which they are transmitted is called the chipping rate. To an observer, these sequences appear to be noise and are also called pseudorandom noise codes (Pncodes). Pncodes are usually introduced into the signal through the use of hardware-based shift registers, and the techniques used to introduce them are divided into several groups including Barker codes, Gold codes, M-sequences, and Kasami codes.

These spreading codes also allow the use of statistical recovery methods to repair damaged transmissions. Another side effect of spreading the signal is lower spectral density-that is, the same amount of signal power is distributed over more bandwidth. The effect of a less spectrally dense signal is that it is less likely to interfere with spectrally dense narrowband signals. Narrowband signals are also less likely to interfere with a DSSS signal because the narrowband signal is spread as part of the correlation function at the receiver.

The frequency channel in IEEE 802.11 DSSS is 22 MHz wide. This means that it supports three nonoverlapping channels for operation. This is why only three IEEE 802.11b DSSS systems can be co-located.

In addition to spreading the signal, modulation techniques are used to encode the data signal through predictable variations of the radio signal. IEEE 802.11 specifies two types of DPSK modulation for DSSS systems. The first is BPSK and the second is QPSK. Phase-shift keying (PSK), as the name implies, detects the phase of the radio signal. BPSK detects a 180-degree inversion of the signal, representing a binary 0 or 1. This method has an effective data rate of 1 Mbps. QPSK detects 90-degree phase shifts. This doubles the data rate to 2 Mbps. IEEE 802.11b adds CCK and packet binary convolutional coding (PBCC), which provide data rates up to 11 Mbps.^[5]

Orthogonal Frequency Division Multiplexing (OFDM) and IEEE 802.11a

IEEE 802.11a (5 GHz) uses OFDM as its frequency management technique and adds several versions of quadrature amplitude modulation (QAM) in support of data rates up to 54 Mbps. In 1970, Bell Labs patented OFDM, which is based on a mathematical process called Fast Fourier Transform (FFT). FFT enables 52 channels to overlap without losing their individuality or orthoganality. Overlapping channels is a more efficient use of the spectrum and enables them to be processed at the receiver more efficiently. IEEE 802.11a OFDM divides the carrier frequency into 52 low-speed subcarriers. Forty-eight of these carriers are used for data and four are used as pilot carriers. The pilot subcarriers allow frequency alignment at the receiver.

One of the biggest advantages of OFDM is its resistance to multipath interference and delay spread. Multipath is caused when radio waves reflect and pass through objects in the environment. Radio waves are attenuated or weakened in a wide range depending on the object's materials. Some materials (such as metal) are opaque to radio transmissions. As you can see, a cluttered environment would be very different from an open warehouse environment for radio wave transmission and reception. This environmental variability is why it is so hard to estimate the range and data rate of an IEEE 802.11 system. Because of reflections and attenuation, a single transmission can be at different signal strengths and from different directions depending on the types of materials it encounters. This is multipath. IEEE 802.11a supports data rates from 6 to 54 Mbps. It utilizes BPSK, QPSK, and QAM to achieve the various data rates.

Delay spread is associated with multipath. Because the signal is traveling over different paths to the receiver, the signal arrives at different times. This is delay spread. As the transmission rate increases, the likelihood of interference from previously transmitted signals increases. Multipath and delay spread are not much of an issue at data rates less than 3 or 4 Mbps, but some sort of mechanism is required as rates increase to mitigate the effect of multipath and delay spread. In IEEE 802.11b, it is CCK modulation. In 802.11a, it is OFDM. The IEEE 802.11g specification also uses OFDM as its frequency management mechanism.^[6]

The adoption and refinement of advanced semiconductor materials and radio transmission technologies for IEEE 802.11 provides a solid basis for the implementation of higher-level functions. The next step up the protocol ladder is the definition of access functionality. Without structured access, the physical medium would be unusable.^[7]

^[4]James and Ruth LaRocca, 802.11 Demystified (New York: McGraw-Hill, 2002), 124-126.

^[5]James and Ruth LaRocca, 802.11 Demystified (New York: McGraw-Hill, 2002), 126-128.

^[6]James and Ruth LaRocca, 802.11 Demystified (New York: McGraw-Hill, 2002), 131.

^[7]James and Ruth LaRocca, 802.11 Demystified (New York: McGraw-Hill, 2002), 99, 129-131.