11.5 802.11 Family Physical Layer

This section examines the various PHY layer mechanisms that have been standardized in the IEEE 802.11 family. The architecture described in the previous section is common to all the physical layer technologies. This section begins with the IEEE 802.11 standard and then discusses the IEEE 802.11b standard and the IEEE 802.11a standard. The other evolving standards, such as IEEE 802.11g and IEEE 802.11h, are not discussed as they are still in their formative stages.

The protocol model of the IEEE 802.11 standard shows a new PHY layer that was introduced for wireless transmission. The PHY layer portion of the IEEE 802.11 standard has been partitioned into multiple layers for separate functions. The physical layer is responsible for transmission and reception over the air. However, many physical layer mechanisms are available, such as RF and IR mechanisms. The physical layer also needs to work with the MAC layer to transfer and receive MAC layer protocol data units (MPDUs).

11.5.1 IEEE 802.11 Physical Layer

There are three different types of PHY layer transmission technologies that have been standardized in the initial IEEE 802.11 standard. Two of these methods are based on RF technologies, and the third is an infrared mechanism. The two RF mechanisms employ the spread spectrum technique to achieve the PHY layer transmission.

SPREAD SPECTRUM TECHNIQUES

The wireless spectrum is always a scarce resource, with tight regulations in terms of usage and power radiated along with licenses required to operate . This is true for most of the wireless systems in place. However, the ISM band is one of those bands that does not require a license to operate.

One of the key issues in an unlicensed band is the interference due to various technologies operating in the unlicensed band. The spread spectrum technique is inherently less sensitive to interference. The two types of spread spectrum techniques are frequency hopping spread spectrum (FHSS) and direct sequence spread spectrum (DSSS).

In the case of the IEEE 802.11 physical layer that employs FHSS, every country has designated a fixed number of 1-MHz channels for use, along with details such as the power to be used and the dwell time. In the United States, there are 79 of these 1-MHz channels available for use in the IEEE 802.11 standard. Each country also defines hopping sequences. For example, in North America, there are three sets of hopping sequences, and the hopping sequence to be used is selected when a FH device is configured for wireless LAN operation.

Frequency Hopping Spread Spectrum

Frequency hopping is one of the spread spectrum techniques used in the IEEE 802.11 standard.

The 2.4-GHz frequency spectrum that is available for use is divided into several 1-MHz frequency bands. The transmitter and receiver "hop" from one 1-MHz frequency band to another, in a near-random sequence. The transmitter will send data in each of the 1-MHz frequency bands, and if the receiver is locked onto the appropriate 1-MHz band, it should be able to receive the information from the transmitter.

The amount of time spent by the transmitter or the receiver in a 1-MHz band is referred to as the dwell time. Typically, any narrowband interference is limited to the dwell time in each band.

The wideband used in the IEEE 802.11 DSSS mechanism is 22 MHz. The allocation of DSSS frequencies is dependent on the country of operation. For example, in most of North America and Europe, the available 2.4-GHz spectrum in the ISM band is divided into 14 channels of 22 MHz each that overlap one another (Figure 11-4). Any of these 14 channels may be used, based on the configuration of the WLAN.

Figure 11-4. Direct sequence spread spectrum channel allocation in North America.

If multiple WLAN systems are needed for increased capacity, there are three non-overlapping or distinct 22-MHz channels available. This allows three 22-MHz DSSS systems to coexist without causing the WLAN systems to interfere with one another.

Direct Sequence Spread Spectrum

DSSS is another mechanism used in IEEE 802.11 to provide data rates of 1 and 2 Mbps.

In the IEEE 802.11 DSSS system, an 11-bit code called a Barker sequence is used to transform the original data bits. The resulting transformed bits are modulated to send over a carrier frequency using one of two modulation techniques: differential binary phase shift keying (DBPSK) or differential quadrature phase shift keying (DQPSK).

INFRARED TECHNIQUE

Infrared is the third mechanism for physical layer transmissions available for use in the IEEE 802.11 standard. This technique uses infrared light in the 850 to 950 nm range. This mechanism typically operates with a range of up to 10 m and does not require a direct line of sight. Operation is possible in a diffused mode, where reflection off walls is allowed. This mechanism is typically used in indoor environments such as classrooms or conference rooms, since infrared light cannot penetrate obstructions and is attenuated easily. This technology is not as popular as FHSS or DSSS since it is not suitable for mobile users.

11.5.2 IEEE 802.11b Physical Layer

The key need for the IEEE 802.11b standard was to provide data rates of 5.5 Mbps and 11 Mbps in addition to the basic 1Mbps and 2 Mbps data rates introduced by the IEEE 802.11 standard. In contrast to the IEEE 802.11 standard, which provides multiple physical layer options for wireless mediums, the IEEE 802.11b standard only allows the DSSS mechanism to achieve the higher data rates of 5.5 Mbps and 11 Mbps. This is because use of the FHSS mechanism at these higher data rates may violate government regulations for operating in the unlicensed band.

The following table lists the various techniques used in the physical layer of the IEEE 802.11b standard:

Data Rates	Code	Modulation	Symbol Rate	Bits/Symbol
1 Mbps	11 bit Barker	DBPSK	1 Msps	1
2 Mbps	11 bit Barker	DBPSK	1 Msps	2
5.5 Mbps	8 bit CCK or PBCC	DQPSK	1.375 Msps	4
11 Mbps	8 bit CCK or PBCC	DQPSK	1.375 Msps	8
CCK: complementary code keying PBCC: packet binary convolutional coding

The IEEE 802.11b physical layer uses the same formats for 1 and 2 Mbps. However, if higher throughput and no interworking with legacy is required, the newer format allows 5.5 and 11 Mbps. The IEEE 802.11b standard also allows dynamic rate switching depending on radio conditions. If good transmission conditions exist, then higher data rates of 5.5 and 11 Mbps may be used. If radio conditions are poor, the data rates may drop down to 1 and 2 Mbps.

11.5.3 IEEE 802.11a Physical Layer

The IEEE 802.11a standard uses the 5-GHz range, which is not as interference-prone (compared to the 2.4-GHz band) due to the relatively minimal technologies operating in this domain. According to the IEEE, the role of the IEEE 802.11a standard, which was approved in September 1999, is to "develop a higher speed PHY for use in fixed, moving or portable wireless local area networks."

Several PHY layer technologies were investigated, and the IEEE finally decided on orthogonal frequency division multiplexing (OFDM) as the technology for transmission. This is a new encoding scheme that offers benefits over spread spectrum. In OFDM, each user transmits using 20-MHz, which is in turn divided into 52 subcarriers of 300 KHz each.

The subcarriers are transmitted in parallel, with each subcarrier transmitting at a much lower rate than the total combined data rate. The receiving device processes and combines these individual signals, each one representing a fraction of the total data, to make up the actual signal. The mandatory data rates are 6, 12, and 24 Mbps, and other data rates up to 54 Mbps are optional.

Orthogonal Frequency Division Multiplexing

To achieve high data rates, a divide and conquer approach is chosen . One high-speed channel is split into several low-speed channels when transmitting information. At the receiver, all of these different low-speed channels or carriers are combined to re-create the high-speed channel.

The advantage of these many subcarriers is that high data rates can be achieved easily. Another advantage is that as the subcarriers tend to carry data at lower rates, multipath or reflective signals do not cause undue concern. The IEEE 802.11a standard also adds additional features in the PHY layer, such as forward error correction (FEC) codes, to recover from any errors, especially given the high data rates.

As the technology is complex, the market has not been flooded with new IEEE 802.11a standard devices. Higher-frequency radio technologies have historically required exotic, expensive semiconductor processes, and the technical challenge before the industry is to implement 802.11a functionality economically. This would allow for mass production, bringing about the desired side effect of reduced prices.

11.5.4 Compatibility of 802.11, 802.11b, and 802.11a Technologies

A wireless LAN user may take several paths depending on the throughput needed and the applications employed. One such evolution may be to max out the capacity of the 802.11 system in the 2.4-GHz range by going from 802.11 to 802.11b and 802.11g (in the future).

If bandwidth is an immediate need, then operators may deploy 802.11 or 802.11b for their current users and then jump to another new standard, 802.11a. The issue with this is that these technologies are not backward compatible, and significant costs may be involved in the changeover.

However, 802.11a can be overlaid over an 802.11 or an 802.11b system as they operate in different bands. So the network administrator may deploy 802.11a in hot spots and provide extended coverage using 802.11b or 802.11 systems.

Between 802.11 and 802.11b systems, the network administrators should go for 802.11 DSSS systems if backward compatibility with 802.11b is desired. Otherwise, the option that best suits current requirements and meets future needs should be selected.