ISM frequency band is reserved for the Industrial/ Scientific / Medical use, which is specialized by FCC. Nowadays, most countries all over the world open this frequency band by following this standard. But the ISM frequency is not unified in these countries. We cannot be allowed to use these frequency bands, i.e., 900MHz, 2.4GHz, 5.7GHz; but the transmitted power must be lower the specialized power (generally lower 1W) and the utilized frequency cannot interfere with other frequencies. At present, most of the wireless products are working at 2.4GHz. And the 802.11 protocol families are shown in Table 1-1.
| 802.11b | 802.11a | 802.11g | 802.11n | |
|---|---|---|---|---|
| Release time | Sept. 1999 | Sept. 1999 | June. 2003 | June. 2003 |
| Frequency | 2.4Ghz | 5Ghz | 2.4Ghz | 2.4Ghz or/and 5Ghz |
| Modulation | CCK/DSSS | OFDM | CCK/OFDM | MIMO OFDM |
| Range | About 100m | About 50m | <100m | Several Km |
| Max rate | 11Mbps | 54Mbps | 54Mbps | 600Mbps |
| Compatible | 11g | Not with 11b/g | 11b | 11b/a/g |