2.4G FHSS and DSSS what is means?

[zennith]

Need to understand abt the reason and working principal , any one can help?
(Enggineers in radio & WLAN question), I am weak in this..
Found this :

DSSS
Direct Sequence Spread Spectrum. The data transmission scheme (sometimes referred to as a "modulation" scheme) used in 802.11b WLANs. DSSS uses a radio transmitter operating at a fixed centre frequency, but using a relatively broad range of frequencies, to spread data transmissions over a fixed range of the frequency band. 802.11a and 802.11g (when not operating in 802.11b mode) use Orthogonal Frequency Division Multiplexing (OFDM).

FHSS
Frequency-Hopping Spread-Spectrum (FHSS) is a spread-spectrum technique used by Bluetooth devices and some 802.11 1Mbps / 2Mbps WLANs. With FHSS, data is transmitted on a radio carrier which hops pseudo-randomly across several different frequencies at a pre-determined rate and hopping sequence. 802.11b devices use DSSS rather than FHSS. 802.11a and 802.11g devices use OFDM.

802.11a
Operates in the UNII (Unlicensed National Information Infrastructure) 5GHz band and supports transmission rates up to 54Mbps. 802.11a uses Orthogonal Frequency Division Multiplexing (OFDM), as opposed to the DSSS or FHSS schemes used in 802.11b and 802.11 networks. Due to its higher frequency of operation, the range of 802.11a equipment is less that of lower frequency systems such as 802.11b and 802.11g. This can increase the cost of an 802.11a network, because a greater number of access points may be required. Since it operates in a different RF band, 802.11a cannot work directly with 802.11b or 802.11g equipment. Using multi-mode 802.11a/b/g access points and/or client adapters will resolve this problem.

802.11b
Currently the most popular and widespread standard for wireless LANs, which operates in the 2.4GHz ISM band and supports data rates up to 11Mbps. Also known as 802.11 High-Rate (HR) and as Wi-Fi, 802.11b uses the DSSS spread spectrum transmission scheme, and operates at data rates of 1Mbps, 2Mbps, 5.5Mbps and 11Mbps.

802.11g
The latest in the 802.11 wireless LAN series, 802.11g operates in the 2.4GHz ISM band and uses Orthogonal Frequency Division Multiplexing (OFDM) to support data rates up to 54Mbps. 802.11g equipment, also known as 54g™, 802.11g can also fall back to 802.11b operation. Therefore an 802.11b device can work directly with an 802.11g device using 802.11b mode.

OFDM (Orthogonal Frequency Division Multiplexing)
OFDM employs multiple overlapping radio frequency carriers, each operating at a carefully chosen frequency that is Orthogonal to the others, to produce a transmission scheme that supports higher bit rates due to parallel channel operation. OFDM is an alternative transmission scheme to DSSS and FHSS.

My Question:
FHSS & DSSS using Operates in the UNII (Unlicensed National Information Infrastructure) 5GHz band and supports transmission rates up to 54Mbps. Is it a illegal band wave that we are operating?

Frequency-Hopping Spread-Spectrum (FHSS) is a spread-spectrum technique used by Bluetooth devices and some 802.11 1Mbps / 2Mbps WLANs.
Places where by heavy w bluetooth devices (e.g. trade fair and exhibition hall) 2.4G on rc cars, rc planes (rx/tx) will not be function properly am I correct?

Simple explaintion of OFDM function and operation anyone can explain? or refer link in internet for explaination

Both FHSS & DSSS why or which one can be on surface water? (Tested some 2.4g in bedok resivour on sail boat, some can use why some can't) is it got to do with the any of the design above?

Thanks

[zennith]

Found this 2.4G can get crowded thus ...

Descriptions
802.11g was the third modulation standard for Wireless LAN. It works in the 2.4 GHz band (like 802.11b) but operates at a maximum raw data rate of 54 Mbit/s, or about 19 Mbit/s net throughput (identical to 802.11a core, except for some additional legacy overhead for backward compatibility). 802.11g hardware is fully backwards compatible with 802.11b hardware. Details of making b and g work well together occupied much of the lingering technical process. In an 11g network, however, the presence of a legacy 802.11b participant will significantly reduce the speed of the overall 802.11g network.

The modulation scheme used in 802.11g is orthogonal frequency-division multiplexing (OFDM) copied from 802.11a with data rates of 6, 9, 12, 18, 24, 36, 48, and 54 Mbit/s, and reverts to CCK (like the 802.11b standard) for 5.5 and 11 Mbit/s and DBPSK/DQPSK+DSSS for 1 and 2 Mbit/s. Even though 802.11g operates in the same frequency band as 802.11b, it can achieve higher data rates because of its heritage to 802.11a.


Adoption
The then-proposed 802.11g standard was rapidly adopted by consumers starting in January 2003, well before ratification, due to the desire for higher speeds, and reductions in manufacturing costs. By summer 2003, most dual-band 802.11a/b products became dual-band/tri-mode, supporting a and b/g in a single mobile adapter card or access point.

Despite its major acceptance, 802.11g suffers from the same interference as 802.11b in the already crowded 2.4 GHz range. Devices operating in this range include: microwave ovens, Bluetooth devices, baby monitors and (in the USA) digital cordless telephones which can lead to interference issues. Additionally the success of the standard has caused usage/density problems related to crowding in urban areas. This crowding can cause a dissatisfied user experience as the number of non-overlapping usable channels is only 3 in FCC nations (ch 1, 6, 11) or 4 in European nations (ch 1, 5, 9, 13).

[zennith]

Am I correct?

802.11b and 802.11g use the 2.4 GHz ISM band, operating in the United States under Part 15 of the US Federal Communications Commission Rules and Regulations. Because of this choice of frequency band, 802.11b and g equipment may occasionally suffer interference from microwave ovens and cordless telephones. Bluetooth devices, while operating in the same band, in theory do not interfere with 802.11b/g because they use a frequency hopping spread spectrum signaling method (FHSS) while 802.11b/g uses a direct sequence spread spectrum signaling method (DSSS). 802.11a uses the 5 GHz U-NII band, which offers eight non-overlapping channels rather than the three offered in the 2.4 GHz ISM frequency band.

Am I right to say FHSS is least interference ?

The segment of the radio frequency spectrum used varies between countries. In the US, 802.11a and 802.11g devices may be operated without a license, as allowed in Part 15 of the FCC Rules and Regulations. Frequencies used by channels one through six (802.11b) fall within the 2.4 GHz amateur radio band. Licensed amateur radio operators may operate 802.11b/g devices under Part 97 of the FCC Rules and Regulations, allowing increased power output but not commercial content or encryption.[2]

Is our country have this rules ?
So 2.4G toys , esky 2.4G , home radio control 2.4G hand set, handphones, others product from china that is 2.4G, Security System uses 2.4G... will crash with our RC 2.4G (futaba, jr, spektrum) any time...? or they can jam us any time...?