by April 12, 2002 0 comments

Whether it’s a wireless LAN in office that seamlessly keeps you connected when you take your notebook from one place to another; or a WPAN (Wireless Personal Area Network) that lets you receive best shopping deals on your Bluetooth-enabled cellphone, wireless networks are stepping out of the realm of sci-fi. 

IEEE’s 802.11 working group is fixing standards for WLANs, while its 802.15 working group is fixing them for WPANs such that both these networks should be able to work with each other, and with existing wired networks. The basic considerations in wireless networks are throughput, as future applications will need higher bandwidth, and cost. Also, since we’re working with mobile, battery-powered devices, the network shouldn’t be power-hungry. Security and privacy are other major considerations, because most wireless LAN equipment today uses radio frequency (RF), making it easy for others to tune into your frequency and steal information. Here, we look at the various standards and see how they handle these considerations.

IrDA: In the line of sight 
IrDA (Infrared Data Association) has been around much before WPAN. Notebooks and PDAs come with infrared ports that let them communicate with other infrared devices. So, you can fire a print from your PDA if your printer has an infrared port. Millions of devices today already have IrDA ports, something that works in this standard’s favor. Also, the cost of setting it up is the lowest vis-à-vis other standards.

IrDA also has several limitations. It assumes both communicating devices are fixed, and in line of sight of each other. It can’t penetrate walls or floors, and you can’t use it on the move. You can also have only one piconet (a short-range, small-scale network) supporting a maximum of 10 devices. It doesn’t support multiple overlaid networks. IrDA offers direct point-to-point, half-duplex connections, meaning the connection can either receive or transmit data at a time, and not both. However, specifications are coming up to handle full-duplex voice communication as well.

Throughput varies, depending on the application and range, as the IrDA protocol suite offers different services with different data rates and ranges. IrDA 1.1, for example, offers a gross throughput of 1.15—4 Mbps over a distance of 1m. IrDA 1.1-based transceivers are quite power-efficient too, as they supply only as much current as needed to maintain an IR link. In standby mode, the device will use about 1 mA of current, while it’ll use about 2.5 mA when it’s receiving data. Reliability and interference are non-issues, because it uses a very high frequency range, just below visible light. Since no other standard works in this frequency band, noise from other users is practically non-existent. Where security is concerned, it uses point-to-point, short-range, direct connections; so someone intercepting your information is highly unlikely. 

Bluetooth: More than just hype
The hype surrounding it was so all-pervasive that you couldn’t have missed it. This was the technology that would let control the lighting of your room, temperature of the air-conditioner, and the music your system plays, even before you unlocked the main door of your house. Though that vision is far, applications and devices using Bluetooth have begun debuting. Printers and notebooks from various vendors support Bluetooth, so does Windows XP. IEEE 802.15 working group has modeled the 802.15.1 standard for WPANs after the Bluetooth specification.

Bluetooth, promoted by the BSIG (Bluetooth Special Interest Group) is a short-range, full-duplex link that supports both voice and data. It transmits and receives at 1,600 hops/second (see box How wireless standards use RF), which is the fastest hopping rate vis-a-vis other standards. After IrDA, Bluetooth is the lowest-cost option for a WPAN. It can support up to 10 overlaid piconets, each with eight devices, making it a total of 80 devices that can communicate simultaneously. It’s also excellent for mobile usage, and works on both circuit-switched and packet-switched networks. Bluetooth works in a range of 10 m, which can be increased to 100 m using a separate amplifier on the network. It gives a transfer rate of 1 Mbps, and efforts are on to increase this to 2 Mbps. Bluetooth is also extremely power-conscious. In standby mode, it needs only 0.3 mA, which can go up to 30 mA when data is being transferred. Since its hopping rate is faster and packets are shorter, the interference in Bluetooth’s case is less than in other standards. 

It has robust built-in security, which is transparent to you–you don’t need to intervene when two Bluetooth-enabled devices talk to each other. The specification defines both encryption (128-bit) and authentication. Depending on the service a particular device will be used for, the device manufacturer can configure different modes of security. 

802.11b: For wireless LANs
This is the standard being used for wireless LANs. Also called Wi-Fi (Wireless Fidelity), it’s a follow up of the original 802.11 standard and offers a higher throughput. The original 802.11 standard was published in 1999, and provided data rates up to 2 Mbps in the 2.4 GHz ISM (Industrial, Scientific, and Medical) band. Later, IEEE set up many task groups to improve its data rate and quality of service (QoS), leading to the development of 802.11b. It provides transfer rates of up to 11 Mbps in the 2.4 GHz ISM band. This standard will eventually be replaced by better ones, and the two currently being worked on are 802.11a and 802.11g. This standard is also backward compatible with products that work on lower bandwidths of 1 or 2 Mbps. 

A major issue today is compatibility between WLANs and WPANs, specifically Wi-Fi and Bluetooth. Since Bluetooth works in the same frequency band as Wi-Fi, there could be interference between the two if both are deployed together. However, both Bluetooth and Wi-Fi have built-in measures to resend packets that get corrupted due to interference. This will slow down the data rate for both, but communication will not come to a stop. Bluetooth SIG and IEEE 802.15 working groups are working closely on defining mechanisms to let these two coexist better. 

OpenAir: Seamless roaming
OpenAir, promoted by the WLIF (Wireless LAN Interoperability Forum), a vendor-consortium founded by HP, IBM, Motorola, and Sharp, became a standard in 1996. Today, loads of products are in the market supporting this standard. It’s compatible with 802.11, but simpler. It is designed to support mobile data networks in small and medium-sized offices, and focuses on interoperability for seamless roaming from one access point to another. OpenAir can support up to 15 piconets, each with up to 16 devices. It transmits and receives at a relatively slow rate of 2.5 hops per second and gives a throughput of 1.6 Mbps. It works in a range of 150 m indoors and up to 300 m outdoors. Four power management modes are defined in its specification: transmit, receive, doze, and sleep. Transmit takes up 300 mA of power, receive uses 150 mA, sleep mode takes 2 mA, and 5 mA is used in doze mode. OpenAir has built-in security via encryption, a security ID function whereby every radio that has to communicate together is given a security ID, which is scrambled and stored. Optionally, authorization tables can be used to make sure that only authorized users are allowed into the network.

SWAP from HomeRF: Home networking
SWAP (Shared Wireless Application Protocol) from HomeRF supports both voice and data via a full-duplex connection, just like Bluetooth. It is a relatively new standard, and there aren’t too many products in the market that support it, though it’s in version 2.0 now. However, an interesting feature is that SWAP supports PSTN (Public Switched Telephone Networks). It is designed to support wireless networking at home, and supports 15 piconets with 127 devices each. SWAP transmits and receives at a relatively fast rate of 50 hops per second, and gives a gross data rate of 1 or 2 Mbps. It operates in a range of 50 m, has good power management, comparable to Bluetooth. Security is also robust, with a 24-bit network ID, and encryption for applications considered more sensitive. 

Applications of Standards
All the above standards have been designed for different applications. While Wi-Fi or OpenAir are for full-fledged WLANs, the wireless equivalent of today’s Ethernet networks, SWAP is useful for connecting PCs at home to share an Internet connection. Bluetooth is good for ‘cable replacement’ in the short range, like synchronizing e-mail between your notebook and PDA, something done by IrDA today. It also has wide applicability in futuristic uses, like the shopping example above, since two Bluetooth devices can automatically recognize and talk to each other. IrDA, on the other hand, can be used in environments like aeroplanes where radio transmission is banned. 

All wireless standards (except IrDA) operate in the 2.4 GHz ISM (Industrial, Scientific, Medical) band. This band, which has an 80 MHz-wide spectrum, has been allocated to unlicensed devices the world over. Since all wireless devices work in this band, they have to use spread spectrum techniques to minimize interference with each other (see article on wireless technologies). Today, very few of these standards can interoperate. So, we are looking at a market that’s segmented in terms of applications and the standards they use. It remains to be seen whether any consolidation or interoperability will take place.

Pragya Madan

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