CKK modulation

CCK allows for multi-channel operation in the 2.4 GHz band by virtue of using the existing 802.11 1 and 2 Mbps DSSS channelization scheme. The spreading employs the same chipping rate and spectrum shape as the 802.11 Barker Sequence, allowing for three non-interfering channels in the 2.4 to 2.483 GHz band. Thus CCK modulation provides for a spectrum similar to that of the original 802.11 systems, at least at the low bandwidths. This allows interoperability with the original 802.11's DSSS modulation technique and it also allows for 802.11b multi-channel operation in the 2.4 GHz band using the existing 802.11 DSSS channel structure scheme.
CCK modulation consists of a set of 64 eight-bit code words. As a set, these code words have unique mathematical properties that allow them to be accurately distinguished from one another by a receiver even in the presence of substantial noise and multipath interference (e.g., interference caused by receiving multiple radio reflections within a building). The 5.5 Mbps rate uses CCK to encode 4 bits per symbol, while the 11 Mbps rate encodes 8 bits per symbol. Actually, to attain 11 Mbps CCK modulation, 6 bits of the 8 are used to select one of 64 symbols of 8 chip length for the symbol and the other 2 bits are used by QPSK to modulate the entire symbol. This results in modulating 8 bits onto each symbol. The chipping rate is maintained at 11 million chip bits per second for all modes. Both speeds use QPSK as the modulation technique and signal at 1.375 million symbols per second.

Figure 5.4: This graphic shows how the CCK modulation is formed. Graphic courtesy of Intersil.

The FCC regulations for the ISM band require at least 10 decibel (dB) of processing gain (11 dB for 802.11), which is normally achieved with spread spectrum techniques. CCK can achieve this gain too without having to be a conventional spread spectrum signal. Rather than using one or two 11-bit Barker sequences, CCK uses a series of codes called "complementary sequences." Because there are 64 unique code word sets that can be used to encode the signal, up to 6 bits can be represented by any one particular code word (instead of the single bit represented by a Barker symbol).
The wireless radio transmitter device generates a 2.4 GHz carrier wave (2.4 to 2.483 GHz) and modulates that wave using a various techniques, depending on the circumstances. For a 1 Mbps transmission, BPSK is used (one phase shift for each bit). To accomplish 2 Mbps or greater transmission, more sophisticated QPSK is used. QPSK can encode two bits of information in the same space as BPSK encodes one. The tradeoff is the need for increased power or else one must decrease the range to maintain signal quality.
Unfortunately, the FCC regulates the output power of portable radios to just one Watt; therefore, as the 802.11 transceiver moves away from the radio, the radio must adapt to the situation by using a less complex (and slower) encoding mechanism to send data. Ironically, the CCK code word is modulated with the same QPSK technology that was used in 2 Mbps wireless direct spread radios. This enables an additional 2 bits of information to be encoded in each symbol. Eight binary "chip" numbers are sent for each 6 bits, but each symbol encodes 8 bits thanks to the QPSK modulation. So, for a 1 Mbps transmission, 11 million chip bits per second times 2 MHz equals 22 MHz of spectrum. Likewise, for a 2 Mbps transmission, 2 bits per symbol are modulated with QPSK, 11 million chips per second, and thus you need 22 MHz of spectrum. In short, to transmit at a bit rate of 11 Mbps, you need 22 MHz of frequency spectrum.

802.11's Modulation Techniques

The original 802.11 standard specified two different spread spectrum transmission techniques: DSSS and FHSS. All radio equipment use the 2.4 GHz ISM band, and systems based on the original 802.11 standard provide data rates up to 2 Mbps. This is possible because DSSS utilizes an 11-bit chipping code called the Barker Sequence for signal spreading with modulation being achieved using either binary phase shift keying (BPSK) or quadrature phase shift keying (QPSK) techniques. (For FHSS, a modulation technique called Gaussian frequency shift keying or GFSK is employed.) Furthermore, in the U.S. DSSS deployments provide 11 independent channels by using different predefined chipping codes. (FHSS based implementations provide for 78 different logical channels through different hopping patterns, although in reality fewer channels would be actually usable due to frequency separation requirements.)

FHSS was dropped from the 802.11b specification because it was felt that "direct spread" could handle the tradeoff between wireless devices coexisting with other users, while extracting the greatest capacity from systems that are both power and band limited. Later this aspect of 802.11b underwent modification after the FCC indicated in a Notice of Proposed Rule Making from the FCC published in the year 2001: ET 99-231; FNPRM & ORDER 05/11/01 (adopted 05/10/01); FCC 01-158 Amendment of Part 15 of the Commission's Rules Regarding Spread Spectrum Devices, Wi-LAN, Inc. et al. that it would consider relaxing the spread spectrum requirement on the ISM band in order to abandon the peaceful "coexistence of equipment" requirement (interference rejection) in favor of support for greater wireless network capacity (higher bit-rate transmissions). Therefore, for high bit rates above 2 Mbps (5.5 Mbps to 11 Mbps and higher) 802.11b's purely spread spectrum techniques have been supplanted by CCK modulation so as to provide 4 or 8 bits per transmission symbol. The combination of QPSK and CCK is what enables 802.11b's maximum data rate of 11 Mbps. Lower data rates are accommodated through a dynamic rate shifting scheme. Also, the reader should note that 802.11g supports CCK modulation so as to provide backwards compatibility with 802.11b. (As an option for faster link rates, 802.11g also allows packet binary convolutional coding (PBCC) modulation.)

Wi-Fi Versus Bluetooth

Another wireless technology is on the market today, which is similar enough to Wi-Fi at first glance to add to the confusion. The Danish telephone equipment company Ericsson originally developed Bluetooth (BT)wireless. (Ericsson named their radio technology after a tenth-centuryViking king who had a sweet tooth for blueberries.) BT uses the same radio band as Wi-Fi and a similar frequency-hopping scheme to dodge interference. Some application overlap also exists, since some devices using the Bluetooth standard can interface a PC to a printer or to a wired network , just like Wi-Fi. Some equipment is being designed today, which , one for BT and another for Wi-Fi. But other than that special circumstance, Bluetooth and Wi-Fi are not compatible.When used in close proximity, Bluetooth devices are likely to interfere with Wi-Fi devices. They both employ a frequency-hopping scheme that minimizes fixed-frequency interference, but they both hop among the same range of frequencies, and sometimes hop on top of one another.
Moving the BT device can often solve the conflict, since its operating power and range are less than Wi-Fi. Newer BT models, using technical tweaks approved bythe FCC, will most likely peacefully coexist with Wi-Fi.An argument exists in the wireless industry about which of these two technologies will dominate in years to come. But even now the two standards are becoming more differentiated and applied to different customer needs.Wi-Fi is a method for connecting PCs and printers to a LAN over adistance of 75 to 300 meters. Bluetooth, with its shorter 10-meter range, deals with a personal area network (PAN). Its unique application is the elimination of wires to hand held or wearable devices, such as PDAs. A wireless headset for a cell phone would be another good example. Unlike a structured LAN, these personal devices link up on an ad hoc basis. Is This Trip Necessary?
Your Best Bet If all these standards are puzzling, your safest bet remains the 802.11b standard. So many people are using it that it won’t anytime soon. A manufacturer who wants to prove that his product will work with other manufacturer’s Wi-Fi equipment must submitit to the Wi-Fi Alliance for testing. Before you buy a piece of Wi-Fi hardware,you should first examine the package for a prominent label that indicates conformance. Very likely you will find another label indicating which radio band the equipment uses.

IEEE Wireless Standards

The first attempt at a wireless standard was the HomeRF protocol, which did not catch on because of its slow (1.6 Mbps) speed. It was replaced by the 802.11 standard, which ran at 1 or 2 Mbps. Because of its limited speed, it is also history. In 1999, the IEEE added the “a” and “b” refinements.
Products conforming to the 802.11a standard operate at speeds of up to 54 Mbps on a very short-wave frequency of 5 billion cycles per second, or 5 gigahertz, abbreviated GHz. Its speed advantage is offset by its shorter range, which is typically 50 to 200 meters. Unlike the more popular b standard, it uses a modulation scheme with the hefty name of Orthogonal Frequency Division Multiplexing (OFDM) that makes possible data speeds as high as 54 Mbps and cuts down on cross-channel and reflected-signal interference.
More commonly, communication takes place at 6, 12, or 24 Mbps. Today the most widely followed standard by far is known as IEEE 802.11b. It moves data at a top speed of 11 Mb ps in the 2.4 GHz frequency band. It is more prone to interference than 802.11a, but the lower frequency gives it a longer range, estimated at between 75 to 300 meters.
The 802.11g standard is now under development and discussion by members of the IEEE, but it won’t become official until released in 2003.
The g standard will be backward compatible with the b standard and will operate on the same 2.4 GHz frequencies. But it will be faster at 54 Mpbs.
Is This Trip Necessary?
and less vulnerable to radio noise. Its greater capacity makes it a promising media for wireless streaming video.
As often happens with a burgeoning technology, as soon as the Committee handed down the standard, it began amending it. Other additions are in the IEEE pipe:

802.11d and h will accommodate European regulations governing radio devices.

802.11e is due in January 2003. It adds quality of service (QoS) features.

802.11f will add protocols that enable data sharing between disparate systems in 2003.

802.11i addresses security holes in the present standards.

The Wireless Next Generation (WNG) specification seeks to combine all the above into one universal standard.

Ultrawideband (UWB) was granted a limited license in February 2002 for use in the 3.1 GHz and 10.6 GHz bands, but only indoors or in handheld peer-to-peer applications for now.

A Brief History of Wireless Computer Networks

Hawaii presented a unique challenge and opportunity for those engineers and graduate students. At the time, computers were capable of being linked, but only across a computer room floor using a cable the diameter of your thumb. The only alternative was known as Sneaker net, in which data was put onto a floppy disk or tape and carried to a distant computer, presumably by someone wearing comfortable sneakers. That was inadequate for the islands, which were separated by distance and natural barriers.
Moving a spreadsheet by courier might entail an expensive boat or helicopterride. The tapes or printouts could get lost or damaged, and in any case the information they contained would be aging while it was in transit.
Multiple copies would accumulate, and corrections or updates required more boat rides. Those limitations are actually familiar to today’s commuters, who often carry their CD-ROMs or laptops from their offices to their homes and back again day after day. It would be so much easier if they could just move the information and leave the battery packs, wires, and floppies behind.
But Hawaii’s isolation in the middle of the Pacific also handed the AlohaNet engineers a solution. Plenty of radio frequencies were available for use as transmission media. The islands consisted of relatively few people and therefore relatively little man-made interference. The flat ocean provided unobscured line-of-sight paths for the signals to travel. Soona blizzard of data was flying from Lanai to Oahu at the speed of light. The concept had been proven, but could it be exported to the mainland? The answer was: not easily.
Coaxial Cables California abounded in mountains, which do not pass radio waves. Wherever you tuned on the radio dial in metropolitan areas, someone was already there. When a new frequency band became available through regulation or a technology advance, lots of entrepreneurs waitedto fill the vacuum. Wireless networking had to be put on hold as engineers were forced to return to coaxial cable. This wire used a central elements urrounded by a layer of insulation and a cylindrical metal or woven shield. It was expensive because it had to be manufactured to precise tolerances. But data signals did not escape from it to pollute the airwaves, nor did interference penetrate. The scheme was given the name broadband,because as the amount of digital data increased, it could be apportioned onto additional frequency channels. Often those channels were shared with traditional video signals on the same type of cable that brings television signals into your home.Multiconductor Cables When cheaper technologies were developed a decade ago, the coaxial cable was deemed obsolete and replaced with the local area network (LAN) drops we know today. These are unshielded multiconductor cables, about the diameter of a chopstick, adapted from use by the telephone industry. Their immunity from signal pollution comes from the arrangement of the wires inside. To this day, universal twisted pair wiring remains the most common way of making computers talk to one another.
It is rare when a technology rises from the dead, but propelled by an unquenchable popular demand, broadband TV cable is once again being used to deliver data to millions of homes. Likewise, recent advances and refinements have revived wireless as a means of easily interconnecting computers inside homes, businesses, and home-based businesses. That real has been given the acronym SOHO, for Small or Home Office. Over a hundred manufacturers are sending SOHO-targeted products to market.The Internet Arrives As the means of interconnection has evolved, sohas “the end.” The Internet began with the National Science Foundation Actof 1950, as an A-bomb-proof means for academics and military researchers to send text to one another. Until 1992, using it for profit was actually illegal.
Since then it has become a commercial, all-encompassing medium unto itself, augmenting and even displacing traditional communication channels, such as radio, television, telephones, newspapers, and even the postal service. Some new businesses, such as Amazon.com or the Google search engine, could not exist without it.At one time, home telephones were considered to be either a luxury ora business imposition. But they quickly turned into a necessity, such that most homes now have several, with many on second or even third home telephone lines. In turn, these wired phones are being augmented or displaced by cell phones. The same process is happening with home computers, as they migrate from the business desktop to the kitchen tabletop. Home owners know that their PC power is multiplied when they share information.
Is This Trip Necessary?
DSL Arrives The stage was set for Wi-Fi home networks with the arrival of digital subscriber line (DSL). This development enables your connection to the telephone or cable TV systems to double as high-speed data channels to the Internet. To get the 1.5 Mbps speeds that DSL now offers, customers previously would have to purchase a dedicated T1 line that would cost between $700 and $1,100 per month. A DSL connection, which can carry regular phone calls as well as data, costs about a tenth of that. Like broadband cable, telephone infrastructure has the considerable advantage of already being in place under streets and hanging from telephone poles.You will not need to dig a trench across your front lawn to get connected.
All these technologies can now be applied in combination so that the Internet or the World Wide Web can finally cross the last mile into apartments, shops, and split-levels from Cleveland to Fairbanks. Practically anyone can afford it. As time marches on, few can afford to ignore it.