Sunday, June 17, 2007

Introduction to 802.11b standards

802.11b extends the original 802.11 standard, which included specifications for 1- and 2-Mbps wireless Ethernet transmissions using spread spectrum RF signals in the 2.4-GHz Industrial, Scientific, and Medical (ISM) band. The transmissions use 100 milliwatts (mw) of transmitter power in North America (and less in other parts of the world). In the original standard, two different spread spectrum transmission methods for the physical layer (PHY) were defined:Direct Sequence Spread Spectrum (DSSS) and Frequency Hopping Spread Spectrum (FHSS). The 802.11b standard extends the original DSSS PHY standard to provide a theoretical maximum data rate of 11 Mbps. Spread spectrum technology is a modulation technique that spreads data transmissions across the entire available frequency band in a prearranged scheme. This type of modulation makes the signal less vulnerable to noise, interference, and snooping. Spread spectrum technology also permits many users to share a frequency band with minimal interference from otherusers and from devices such as microwave ovens.

With FHSS, a transmitting and receiving station are synchronized to hop from channel to channel in a predetermined pseudorandom sequence. The prearranged hop sequence is known only to the transmitting and receiving station. In the U.S. and Europe, IEEE 802.11 specifies 79 channels and 78 different hop sequences. If one channel is jammed or noisy, the data is simply retransmitted when the transceiver hops to a clear channel. 802.11 networks using FHSS are limited to1- and 2-Mbps data rates.

Under DSSS, each bit to be transmitted is encoded with a redundant pattern called a chip, and the encoded bits are spread across the entire available frequency band. The chipping code used in a transmission is known only to the sending and receiving stations, making it difficult for an intruder to intercept and decipher wireless data encoded in this manner. The redundant pattern also makes it possible to recover data without retransmitting it if one or more bits are damaged or lost during transmission. DSSS is used in 802.11b networks.

Wi-Fi Certification Program
In a heterogeneous wireless network environment, it is important to select 802.11b standards-based wireless products that are interoperable. The main measure of 802.11b equipment interoperability is the Wireless Fidelity (Wi-Fi)certification program. (802.11b networks are sometimes referred to as Wi-Fi networks.) Administered by the industrygroup, Wireless Ethernet Compatibility Alliance (WECA), the Wi-Fi logo on a product certifies its interoperability with other products containing the logo. An independent lab, the Agilent/Silicon Valley Networking Labperforms the actual testing. The Wi-Fi interoperability program tests for association and roaming capabilities, throughput, and required features such as 64-bit encryption. WECA tracks standards developments and enhances the interoperability testing to reflect these advancements.

802.11b Options and Proprietary Extensions
Some vendors differentiate their 802.11 products with additional features. Some are options in the 802.11 standard such as 128-bit encryption, and some are proprietary features such as security/authentication schemes, roaming capabilities, key management, and "Power Over Ethernet." Using or enabling proprietary extensions usually requires that the wireless equipment, including APs and network cards, be supplied by a single vendor. Proprietary extensions are not suitable for heterogeneous environments with a mix of hardware. Although the extensions provide specific benefits, they limit future flexibility. Before choosing to implement these features, it is important to assess all the environments that must be supported in addition to the corporate wireless LAN, including home office and public (airports and hotels)wireless LANs.

Infrastructure Mode vs. Ad hoc Mode
802.11b networks can be implemented in "infrastructure" mode or "ad hoc" mode. In infrastructure mode-referred to inthe IEEE specification as the basic service set-each wireless client computer "associates" with an access point (AP) via a radio link. The AP connects to the 10/100-megabits per second (Mbps) Ethernet enterprise network using a standardEthernet cable, and provides the wireless client computer with access to the wired Ethernet network. Ad hoc mode isthe peer-to-peer network mode, which is suitable for very small installations. Ad hoc mode is referred to in the 802.11b specification as the independent basic service set.

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