The ISM band is quite crowded with
different systems from cordless phones to microwave ovens! There are other WLAN
technologies operating in this band including Bluetooth and HomeRF. HomeRF is an
industry driven standard specifying a 2.4 GHz FHSS system with data rate of 1.6
Mbps per channel and a range of about 150 feet. There are 15 channels and it is
possible to operate multiple WLANs in close proximity using different hopping
patterns. However this technology is mainly targeted, as the name implies, to
the the home voice/data network. Members of the consortium include Microsoft,
IBM, Motorola, Ericsson, Hewlett-Packard, Compaq and Intel.
Bluetooth is another technology operating
in the 2.4 GHz ISM band. It is also an industry driven standard by the Bluetooth
SIG specifying a low power, low cost frequency hopping scheme of 1600 hops per
second. Originally developed by Ericsson, it was really designed for a personal
area network (PAN) connecting cell phones, PDAs laptops and other peripherals.
Its members also include Motorola, Agere, 3Com, IBM, Intel, Nokia and Microsoft.
Table 1 gives some comparisons between the three technologies.
Table 1 Comparison of 802.11b, Bluetooth and HomeRF
| Parameter | 802.11b | Bluetooth | HomeRF |
| Data rate | 11 Mbps | 780 kbps | 1, 2, 10 Mbps |
| Modulation | DSSS | FHSS | FHSS |
| Range (ft) | 50-300 | 30 | 150 |
| Power amplifier | Highly linear - expensive | Nonlinear - easier to implement | Nonlinear - easier to implement |
| User devices | Laptops, PDAs | Laptop, PDAs, pager, car, cell phone and other mobile devices | Laptop, PDAs, internet appliance, modem, cordless phones |
| Environment | Office, campus LAN, hotel, airport, convention, home | Personal Area Network | Home |
| Interference | affected by other users of the spectrum. Particularly affected by Bluetooth PANs in the area | Less affected by interference due to frequency hopping | Less affected by interference due to frequency hopping |
| Encryption | Wireless Encryption Privacy (WEP) Optional 40 bit Rivest Code 4 (RC4) Pseudo Random Number Generator (PRNG) encryption algorithm |
128 bit (SAFER+) | 128 bit key encryption algorithm |
| Transmit power | 100 mW | < 10 mW (inactive mode) | |
| Voice | Voice over IP, therefore relies on TCP for error checking in segments. Delay may be unacceptable | Voice channels support links of 64 Kbps. | Bandwidth also reserved for voice. Up to 8 simultaneous toll quality calls |
Interference between Bluetooth and 802.11b
has been extensively studied [6]. It was shown that a DSSS receiver suffered
severe degradation when place 20m away from an AP with a Bluetooth piconet
located within 10m. For more on the topic please see [6]. For encryption, most
vendors have value-added features e.g. Cisco Aironet products provide 128 bit
encryption.