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WIRELESS STANDARDS

Now that you know all about RF signals, hardware, and modulations, you can examine how they are used in various wireless standards. Since most of the heavy lifting has already been done, this section may seem somewhat light. Additional coverage of the various wireless standards is covered in Chapter 3.

Wireless LAN (802.11 a/b/g/n)

Wireless LAN standards are referred to by their corresponding IEEE standard number. All the Wireless LAN specifications are contained within these various IEEE 802.11 standards. Most of the 802.11 standards are, in fact, ratifications or additions to the original 802.11 standard. The suffix letter attached to the 802.11 designation is used to refer to each specific standard. There are numerous 802.11 standards in existence today, and the most prevalent of these are the 802.11a, 802.11b, 802.11g, and 802.11n ratifications. Each of these standards has particular characteristics that make it unique. Table 2-3 provides a summary of these four standards.

Table 2-3: The Common 802.11X Standards

Protocol

Frequency Band

Typical Data Rate

Maximum Data Rate

Legacy

2.4 GHz

1 Mbit/s

2 Mbit/s

802.11a

5 GHz

25 Mbit/s

54 Mbit/s

802.11b

2.4 GHz

6.5 Mbit/s

11 Mbit/s

802.11g

2.4 GHz

11 Mbit/s

54 Mbit/s

802.11n

2.4 and 5 GHz

200 Mbit/s

540 Mbit/s

In each of the 802.11 standard ratifications, a great deal of consideration is made toward ensuring interoperability of the various standards. For example, the 802.11g standard took a long time to become an official standard, and most of the time was spent ensuring that it would be backward compatible with the earlier 802.11b standard.

802.11a

The 802.11a ratification made two important additions to the wireless LAN standards. The first of these is the addition of the 5 GHz frequency band, which opened up numerous badly needed frequency channels. Second, a new type of modulation was implemented in 802.11a that provided an increased maximum theoretical data throughput of 54 Mbit/ second.

The new 5 GHz channels utilized in the 802.11a standard do not suffer from the same interference problems as those in the 2.4 GHz ISM band. Table 2-4 lists the channels and frequencies available in the 802.11a standard. Once again, the "no free lunch" rule applies here since 5 GHz has a shorter range than the 2.4 GHz standards. The reduced range is due to two factors. First, the path loss at 5 GHz is greater than it is for 2.4 GHz. The second attribute of the 5 GHz reduced range comes from the increased line-of-sight behavior as compared to that of 2.4 GHz. Table 2-4 provides a list of all the 802.11a channels and their corresponding frequencies.

Table 2-4: Channel Numbers and Frequencies for 802.11a

Channel

Frequency (MHz)

[A]

[E]

[J]

[W]

34

5170

  

Y

 

36

5180

Y

Y

 

Y

38

5190

  

Y

 

40

5200

Y

Y

 

Y

42

5210

  

Y

 

44

5220

Y

Y

 

Y

46

5230

  

Y

 

48

5240

Y

Y

 

Y

52

5260

Y

Y

 

Y

56

5280

Y

Y

 

Y

60

5300

Y

Y

 

Y

64

5320

Y

Y

 

Y

100

5500

 

Y

 

Y

104

5520

 

Y

 

Y

108

5540

 

Y

 

Y

112

5560

 

Y

 

Y

116

5580

 

Y

 

Y

120

5600

 

Y

 

Y

124

5620

 

Y

 

Y

128

5640

 

Y

 

Y

132

5660

 

Y

 

Y

136

5680

 

Y

 

Y

140

5700

 

Y

 

Y

149

5745

Y

  

Y

153

5765

Y

  

Y

157

5785

Y

  

Y

161

5805

Y

  

Y

[A]Americas

[E]EMEA (Europe, Middle East, Africa)

[J]Japan

[W]Rest of the world

The modulation utilized in the 802.11a standard is a combination of Orthogonal Frequency Division Multiplexing (OFDM) coupled with advanced modulation techniques such as 16-QAM and 64-QAM. An 802.11a signal consists of 52 individually modulated subcarriers that are combined to construct the complete signal. The combination of these two techniques solved the inherent problems with the high data rate spread-spectrum signals encountered with the prior standards. As a result of this improved modulation technique, 802.11a is capable of achieving a theoretical maximum data rate of 54 Mbit/ second. Table 2-5 shows a list of all the available data rates in 802.11a along with the specific modulations used for each.

Table 2-5: 802.11a Data Rates and Modulations

Data Rate (Mbit/s)

Modulation

54

64-QAM (3/4)

48

64-QAM (2/3)

36

16-QAM (3/4)

24

16-QAM (1/2)

18

QPSK (3/4)

12

QPSK (1/2)

9

BPSK (3/4)

6

BPSK (1/2)

802.11b

The 802.11b standard operates in the 2.4 GHz ISM band and supports increased data rates as compared to the original 802.11 standard. The addition of the higher data rates in 802.11b was accomplished through the use of complementary code keying (CCK) modulation. A great deal of time was spent by the standards committee to find a modulation technique that allowed higher data rates while still maintaining interoperability with the lower data rate. Table 2-6 provides a list of the 802.11b channels and their corresponding frequencies. Table 2-7 shows the available data rates in 802.11b along with the associated modulation technique for each.

Table 2-6: Channel Numbers and Frequencies for 802.11b

Channel

Frequency (MHz)

[A]

[E]

[J]

[W]

1

2412

Y

Y

Y

Y

2

2417

Y

Y

Y

Y

3

2422

Y

Y

Y

Y

4

2427

Y

Y

Y

Y

5

2432

Y

Y

Y

Y

6

2437

Y

Y

Y

Y

7

2442

Y

Y

Y

Y

8

2447

Y

Y

Y

Y

9

2452

Y

Y

Y

Y

10

2457

Y

Y

Y

Y

11

2462

Y

Y

Y

Y

12

2467

 

Y

Y

Y

13

2472

 

Y

Y

Y

14

2484

  

Y

 

[A]Americas

[E]EMEA (Europe, Middle East, Africa)

[J]Japan

[W]Rest of the world
Table 2-7: 802.11b Data Rates and Modulations

Data Rate (Mbit/s)

Modulation

11 Mb/s

CCK

5.5 Mb/s

CCK

2 Mb/s

DBPSK / DQPSK+DSSS

1 Mb/s

DBPSK / DQPSK+DSSS

802.11g

The 802.11g standard is somewhat of a hybrid combination of both the 802.11a and the 802.11b standards. It operates in the 2.4 GHz ISM band and shares the same channels as 802.11b, but it implements the same type of modulation technique as 802.11a. This allows the addition of the 802.11a data rates to the 2.4 GHz band. 802.11g is fully interoperable with 802.11b networks. In fact, much of the time spent on the 802.11g ratification was spent on ensuring interoperability with the 802.11b standard. Although the standard committee went to great lengths to achieve this, operating 802.11b and 802.11g networks in close proximity negatively impacts the performance of the 802.11g network. The frequency channels used by 802.11g are identical to 802.11b and the modulations used in 802.11g are listed in Table 2-8.

Table 2-8: Data Rates and Modulations Used by 802.11g

Data Rate (Mbit/s)

Modulation

54

64-QAM (3/4)

48

64-QAM (2/3)

36

16-QAM (3/4)

24

16-QAM (1/2)

18

QPSK (3/4)

12

QPSK (1/2)

11

CCK

9

BPSK (3/4)

6

BPSK (1/2)

5.5

CCK

2

DBPSK / DQPSK+DSSS

1

DBPSK / DQPSK+DSSS

802.11n

802.11n, at the time of this writing, is the latest and greatest addition to the suite of wireless LAN standards. When the 802.11n standard was commissioned by the IEEE, the primary goal was to develop a wireless standard that could achieve a raw data rate at the MAC level of over 100 Mbit/sec. Presently the 802.11n standard is still undergoing ratification, but it is scheduled to be completed in mid-2007. This standard will make use of both the 2.4 and 5 GHz frequency bands.

An interesting aspect of 802.11n is that it exploits the phenomena of multipath interference, which typically causes a severe reduction in data throughput, to actually increase data throughput with a technology called Multiple Input Multiple Output (MIMO). MIMO devices have multiple antennas that are capable of transmitting and receiving different copies of the same signal. By analyzing the signals transmitted and received from each of the antennas using digital signal processing, the effects of multipath propagation are used to increase the amount of data that can be communicated by the signal.


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