The 802.11 Wireless LAN Standards
In 1997, the IEEE ratified the 802.11 Wireless LAN standards, establishing a global standard for implementing and deploying Wireless LANS. The throughput for 802.11 is 2Mbps, which was well below the IEEE 802.3 Ethernet counterpart. Late in 1999, the IEEE ratified the 802.11b standard extension, which raised the throughput to 11 Mbps, making this extension more comparable to the wired equivalent. The 802.11b also supports the 2 Mbps data rate and operates on the 2.4GHz band in radio frequency for high-speed data communications.
As with any of the other 802 networking standards (Ethernet, Token Ring, etc.), the 802.11 specification affects the lower layers of the OSI reference model, the Physical and Data Link layers.
The Physical Layer defines how data is transmitted over the physical medium. The IEEE assigned 802.11 two transmission methods for radio frequency (RF) and one for Infrared. The two RF methods are frequency hopping spread-spectrum (FHSS) and direct sequence spread-spectrum (DSSS). These transmission methods operate within the ISM (Industrial, Scientific, and Medical) 2.4 GHz band for unlicensed use. Other devices that operate on this band include remote phones, microwave ovens, and baby monitors.
FHSS and DSSS are different techniques to transmit data over radio waves. FHSS uses a simple frequency hopping technique to navigate the 2.4GHz band which is divided into 75 sub-channels 1MHz each. The sender and receiver negotiate a sequence pattern over the sub-channels.
DSSS, however, utilizes the same channel for the duration of the transmission by dividing the 2.4 GHz band into 14 channels at 22MHz each with 11 channels overlapping the adjacent ones and three non-overlapping channels. To compensate for noise and interference, DSSS uses a technique called “chipping”, where each data bit is converted into redundant patterns called “chips”.
The Data Link layer is made up of two sub-layers, the Media Access Control (MAC) layer and the Logical Link Control (LLC) layer. The Data Link layer determines how transmitted data is packaged, addressed and managed within the network. The LLC layer uses the identical 48-bit addressing found in other 802 LAN networks like Ethernet where the MAC layer uses a unique mechanism called carrier sense multiple access, collision avoidance (CSMA/CA). This mechanism is similar to the carrier sense multiple access collision detect (CSMA/CD) used in Ethernet, with a few major differences. Opposed to Ethernet, which sends out a signal until a collision is detected before a resend, CSMA/CA senses the airwaves for activity and sends out a signal when the airwaves are free. If the sender detects conflicting signals, it will wait for a random period before retrying. This technique is called “listening before talking” (LBT) and probably would be effective if applied to verbal communications also.
To minimize the risk of transmission collisions, the 802.11 committee decided a mechanism called Request-To-Send / Clear-To-Send (RTS/CTS). An example of this would be when an AP accepts data transmitted from a wireless station; the AP would send a RTS frame to the wireless station that requests a specific amount of time that the station has to deliver data to it. The wireless station would then send an CTS frame acknowledging that it will wait to send any communications until the AP completes sending data. All the other wireless stations will hear the transmission as well and wait before sending data. Due to the fragile nature of wireless transmission compared to wired transfers, the acknowledgement model (ACK) is employed on both ends to ensure that data does not get lost in the airwaves
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“Wireless Lan Security” Seminar Report
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- Wireless LAN Deployment
- Wireless LAN Security Overview
- Protecting Wireless LANs
- Wireless LAN Security Summary
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- WLAN Authentication
- WLAN Authentication Vulnerabilities
- WEP Encryption
- WEP Encryption Weaknesses
- Component of WLAN Security
- Future of WLAN Security
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