Asus USB-AC51, User Manual

11

COFDM (for 802.11a or 802.11g)

Signal power alone is not enough to maintain 802.11b-like distances in an 

802.11a/g environment. To compensate, a new physical-layer encoding 

technology was designed that departs from the traditional direct-sequence 

technology being deployed today. This technology is called COFDM (coded 

OFDM). COFDM was developed specifically for indoor wireless use and offers 

performance much superior to that of spread-spectrum solutions. COFDM 

works by breaking one high-speed data carrier into several lowerspeed 

subcarriers, which are then transmitted in parallel. Each high-speed carrier is 

20MHz wide and is broken up into 52 subchannels, each approximately 300KHz 

wide. COFDM uses 48 of these subchannels for data, while the remaining four 

are used for error correction. COFDM delivers higher data rates and a high 

degree of multipath reflection recovery, thanks to its encoding scheme and 

error correction.

Each subchannel in the COFDM implementation is about 300KHz wide. At 

the low end of the speed gradient, BPSK (binary phase shift keying) is used 

to encode 125Kbps of data per channel, resulting in a 6,000Kbps, or 6Mbps, 

data rate. Using quadrature phase shift keying, you can double the amount of 

data encoded to 250Kbps per channel, yielding a 12Mbps data rate. And by 

using 16-level quadrature amplitude modulation encoding 4bits per hertz, 

you can achieve a data rate of 24Mbps. The 802.11a/g standard specifies 

that all 802.11a/g-compliant products must support these basic data rates. 

The standard also lets the vendor extend the modulation scheme beyond 

24Mbps. Remember, the more bits per cycle (hertz) that are encoded, the more 

susceptible the signal will be to interference and fading, and ultimately, the 

shorter the range, unless power output is increased.

Default Key

This option allows you to select the default WEP key. This option allows you to 

use WEP keys without having to remember or write them down. The WEP keys 

generated using the Pass Phrase is compatible with other WLAN products. The 

Pass Phrase option is not as secure as manual assignment.

Device Name

Also known as DHCP client ID or network name. Sometimes provided by an ISP 

when using DHCP to assign addresses.

DHCP (Dynamic Host Configuration Protocol)

This protocol allows a computer (or many computers on your network) to be 

automatically assigned a single IP address from a DHCP server.