Thursday, June 21, 2007

Comparison between 802.11a and 11b

IEEE ratified the 802.11a and 802.11b wireless networking communications standards in 1999, to create a standard

• Ultrahigh spectrum efficiency: More data can travel over a smaller amount of bandwidth than competing technologies
• High resistance to multi-path: Reflected multi-path signals are less likely to cancel the main signal, making it much more suitable for indoor wireless networking
• Relative immunity to interference: If interference happens to block one data pathway, the other carrier waves remain unaffected

The disadvantages of OFDM include:

• Expense: Components are typically more expensive to produce due to their added complexity
• Higher Power Consumption: OFDM-based systems draw more power than 802.11b-based systems. This is a problem for notebook users.

OFDM was developed specifically for indoor wireless use and offers performance much superior to that of spread spectrum solutions. OFDM works by breaking one high speed data carrier into several lower speed sub-carriers, which are then transmitted in parallel in a DMT way, the same that is used by the ADSL modems. Each high-speed carrier is 20MHz wide and is broken up into 52 sub-channels, each approximately 300 KHz wide. OFDM uses 48 of these sub channels for data.

The advantages of IEEE 802.11a are:
• Operating speeds up to 54 Mbps.
• This difference is primarily a result of 802.11a's modulation scheme.
• The larger bandwidth allocation in the 5 GHz range can be exploited for greater data rates.
• Less interference in the 5 GHz frequency range. The crowded 2.4 GHz band is shared by cordless phones, microwave ovens, Bluetooth, and WLANs.
• Greater potential to handle more users, as a result of more radio frequency channels and increased operating bandwidth.

802.11a Implementation Barriers
• The total cost of ownership (TCO) for 802.11a must be close to that of 802.11b before wide-scale iimplementation takes place. Since the range of 802.11a (approximately 50 meters) is roughly half that of802.11b, this will be difficult.
• Unlike 802.11b, 802.11a is not accepted worldwide. For example, Japan only permits the use of a smaller band containing half the channels. And Europe is still holding onto the promise of High Performance Radio Local Area Network Type 2 (HiperLAN2). In fact, it's illegal to use 802.11a in Europe, as the standard doesn't
comply with various EU requirements.
• Furthermore, vendors are uncertain whether to deploy at 5.2 GHz or 5.8 GHz. Certain military and government installations use portions of the 5 GHz band for ground tracking stations and satellite communications, creating additional barriers to worldwide 802.11a deployment.
• OFDM is inherently less power-efficient than DSSS. This means a 54 Mbps OFDM transceiver operating at a given range will consume much more power than an 11 Mbps DSSS transceiver with the same range. This presents an extra burden on the battery life of notebook PCs.
• Currently, there is no interoperability certification available for 802.11a products. Wi-Fi certification (performed by WECA) ensures multi-vendor interoperability of 802.11b products.
• 802.11a is not compatible or inter-operable with the 802.11b protocol
• The 802.11a standard does not address growing concerns over wireless networking security
• Although there is less interference in the 5 GHz frequency range, signals at 5 GHz have a higher absorption rate, and are therefore blocked more easily by walls and other building structures.

No comments: