Overall difference between each specification shown on below table.
IEEE 802.11
the original wireless LAN standard that specifies the slowest data transfer rates in both RF and light-based transmission technologies.
IEEE 802.11b
Although Wi-Fi consumer market continues to grow at a tremendous rate, 802.11b compatible WLAN equipment gave the industry the first needed huge shot in the arm. In 1999, the IEEE Task Group b (TGb) published the IEEE Std. 802.11b-1999, and it was later amended and corrected as IEEE Std. 802.11b-1999/Cor1-2001. The Physical layer medium that is defined by 802.11b is strictly direct sequence spread spectrum (DSSS). The frequency space in which 802.11b radio cards can operate is the unlicensed 2.4 to 2.4835 GHz ISM band.
The TGb Task Group’s main goal was to achieve higher data rates within the 2.4 GHz ISM band. 802.11b radio cards accomplish this feat by using a different spreading/coding technique called Complementary Code Keying (CCK) and modulation methods using the phase properties of the RF signal. 802.11 cards used a spreading technique called the Barker Code
. The end result is that 802.11b radio cards support data rates of 1, 2, 5.5 and 11 Mbps. 802.11b systems are backward compatible with the 802.11 DSSS data rates of 1 Mbps and 2 Mbps. The transmission data rates of 5.5 and 11 Mbps are known as High-Rate DSSS (HR-DSSS). Once again, understand that the supported data rates refer to available bandwidth and not aggregate throughput.
[ IEEE 802.11b transmit spectrum mask ]
[ Sideband lobe interference ]
IEEE 802.11a
During the same year the 802.11b amendment was approved, another very important amendment was also ratified and published as IEEE Std. 802.11a-1999. The engineers in the TGa Task Group set out to define how 802.11 technologies would operate in the newly allocated Unlicensed National Information Infrastructure (UNII) frequency bands. 802.11a radio cards can transmit in three different 100 MHz unlicensed frequency bands in the 5 GHz range, as shown in below table.
The 2.4 GHz ISM band is a much more crowded frequency space than the 5 GHz UNII bands. Microwave ovens, Bluetooth, cordless phones, and numerous other devices all operate in the 2.4 GHz ISM band and are potential sources of interference. In addition, the sheer number of 2.4 GHz WLAN deployments has often been a problem in environments such as multi-tenant office buildings.
One big advantage of using 802.11a WLAN equipment is that it operates in the less crowded 5 GHz UNII bands. Eventually the three UNII bands will also become crowded. Regulatory bodies such as the FCC are opening up more frequency space in the 5 GHz range.
[ 802.11a spectrum mask ]
IEEE 802.11g
Another amendment that generated a lot of excitement in the Wi-Fi marketplace was published as IEEE Std. 802.11g-2003. The IEEE defines 802.11g cards as clause 19 devices, which transmit in the 2.4 to 2.4835 GHz ISM frequency band.
The main goal of the TGg Task Group was to enhance the 802.11b Physical layer to achieve greater bandwidth yet remain compatible with the 802.11 MAC. To achieve the higher data rates, Extended Rate Physical OFDM (ERP-OFDM) technology is used exactly as defined in the 802.11a amendment. Therefore, data rates of 6, 9, 12, 18, 24, 36, 48, and 54 Mbps are possible using OFDM technology, although once again the IEEE only requires the data rates of 6, 12, and 24 Mbps. To maintain backward compatibility, the DSSS data rates of 1, 2, 5.5, and 11 are supported as well.
The 802.11g amendment defines the use of several PHYs but requires support for both DSSS and ERP-OFDM. The good news is that an 802.11g AP can communicate with 802.11g client stations as well as 802.11b stations. The ratification of the 802.11g amendment triggered monumental sales of Wi-Fi gear in both the small office, home office (SOHO) and enterprise markets because of both the higher data rates and the backward compatibility with older equipment. As mentioned earlier in this chapter, spread spectrum technologies cannot communicate with each other, yet the 802.11g amendment mandates support for both DSSS and ERP-OFDM. In other words, ERP-OFDM and DSSS technologies can coexist, yet they cannot speak to each other. Therefore, the 802.11g amendment calls out for a protection mechanism that allows the two technologies to coexist. The goal of the 802.11g “protection mechanism” is to prevent ERP-OFDM radio cards from transmitting at the same time as DSSS radio cards.
The 802.11g amendment also specifies other optional technologies, including Packet Binary Convolutional Coding (PBCC. This technology is optional and is rarely used.
[ 802.11 Channels and OFDM subcarriers ]
How Does 802.11b Affect 802.11g Throughput?
When an 802.11b station causes an 802.11g BSS to enable protection mechanism, a large amount of overhead is added to every 802.11g data transmission. This overhead will reduce the 802.11g aggregate data throughput to below 13 Mbps, and possibly as low as 9 Mbps.
802.11b mode can be disabled on 2.4GHZ interface to enhance data throughput on 2.4Ghz radio interface.
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