Adopted last November after a long and often-heated debate, the IEEE 802.11g draft standard extends data rates for 2.4-GHz wireless-LAN (WLAN) systems to 54 Mbits/second and provides backward compatibility with existing 802.11b (Wi-Fi) equipment. Both mandatory and optional aspects are included. Basically, the draft standard mandates use of orthogonal frequency-division multiplexing (OFDM) for higher data rates (greater than 20 Mbits/s) and requires support for complementary code keying (CCK) to ensure backward compatibility with existing 802.11b radios. The draft also includes two optional elements, called CCK/OFDM and packet binary convolutional coding (PBCC.) Developers may elect to include either optional element or omit both options entirely.
IEEE 802.11a and 802.11g now share a common high-rate waveform (OFDM) and offer complementary advantages to consumers. IEEE 802.11a systems enjoy more spectrum at 5 GHz, thus allowing for more channels and, by extension, more users. On the other hand,802.11g systems provide backward compatibility with existing Wi-Fi devices and offer a range advantage relative to systems operating at 5 GHz.
The use of OFDM in the 2.4 GHz band will also facilitate development of dual-band radios. The reason is quite simple: Developers of dual-band radios will need to include OFDM capability for 5-GHz operations and CCK capability to support Wi-Fi at 2.4 GHz. By using OFDM at 2.4 GHz, implementing 802.11g in a dual-band device will require no additional complex hardware.
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Every packet of transmitted data can be thought of as consisting of two main parts: a preamble/header and a payload. The preamble/header alerts all radios sharing a common channel that data transmission is beginning. The preamble is a known sequence of 1's and 0's and allows radios to get ready to receive data (think of it as a wake-up call). The header immediately follows the preamble and conveys several important pieces of information, including the length (in mi-
croseconds) of the payload. Other radios will not begin trans-
mission during this period, thus preventing a network collision. The preamble/header and the payload are normally sent using the same modulation format (CCK for example).
However, there are exceptions to this rule in the optional elements of the 802.11g draft standard.
Complementary code keying (CCK) is the modulation format for current Wi-Fi (IEEE 802.11b) systems. The preamble/header and the payload can both be transmitted using CCK modulation. CCK is a single- carrier waveform, whereby data is transmitted by modulating a single radio frequency or carrier.
OFDM is just now beginning to reach the market in IEEE 802.11a devices operating at 5 GHz. A multicarrier access scheme, OFDM splits up the data among several closely spaced subcarriers and modulates each carrier using the same binary phase-shift keying as used in 802.11b. The multicarrier feature helps OFDM provide very reliable operation even in the presence of severe signal distortion due to multipath. In addition, OFDM systems can support higher data rates than single-carrier systems without incurring a huge penalty in terms of complexity. For data rates up to 11 Mbits/s, CCK is a good option. However, as data rates go higher, OFDM becomes the clear choice.
Until very recently the FCC prohibited the use of OFDM in the 2.4-GHz band. Now, a common modulation format can now be used in both bands.
OFDM employs a much shorter preamble length than CCK — it is just 16 microseconds in length as compared to 72 microseconds for CCK. This shorter preamble reduces network overhead. Note that OFDM modulation is used for the preamble/header and the payload.
CCK/OFDM is an optional part of the draft proposal. As the name implies, it is a hybrid scheme whereby CCK modulation is used for the header/preamble and OFDM is used for the payload. The CCK header alerts all legacy Wi-Fi devices that a transmission isbeginning and informs those devices of the duration (in microseconds) of that transmission. The payload can then be transmitted at a much higher rate using OFDM. Even though existing Wi-Fi devices will not receive the payload, collisions are prevented because the preamble/header is transmitted using CCK.
The mandatory OFDM waveform can also coexist and operate with existing Wi-Fi devices. However, a different method referred to as RTS/CTS is required. This method is described in greater detail below.
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Like CCK, packet binary convolutional coding is a single-carrier system, but that's where the similarity ends. It is a more complex signal constellation (8-PSK for PBCC vs. QPSK for CCK) and it employs a different code structure. PBCC can also be thought of as a hybrid waveform because it uses a CCK preamble/header with a PBCC payload. The maximum data rate for the PBCC option is 33 Mbits/s.
It is very likely that many IEEE 802.11g radios will implement only the mandatory modes. This section describes how radios
using OFDM modulation (OFDM preamble/header and OFDM payload) can interoperate with existing Wi-Fi radios (CCK preamble/header and CCK payload).
Normally, all of the radios on a given channel share access to the airwaves by means of a "listen-before-talk" mechanism referred to as carrier-sense, multiple-access/collision avoidance (CSMA/CA). In simple terms, the radios listen to determine if another device is transmitting. Each radio on a channel waits until there is no other transmission in progress before beginning to transmit. There are additional provisions to reduce the probability that more than one radio will attempt to transmit at the same moment.
IEEE 802.11g radios will be able to receive either CCK or OFDM transmissions. However, existing Wi-Fi devices can only receive CCK transmissions. This presents a problem: If existing radios cannot receive OFDM transmissions, how will they avoid colliding with those same transmissions? The CSMA/CA mechanism will not be suitable when CCK radios and OFDM radios operate on the same channel. Fortunately, another mechanism already exists in the 802.11 protocol that addresses this problem very efficiently.
Normally, all radios sharing a given channel (including the access point) can "hear" one another. However, this is not always the case. There are instances when all radios can hear and be heard by the access point (AP), but they cannot hear each other. Under these conditions, the listen-before-talk mechanism would break down because radios might detect a clear channel and begin transmitting to the AP while the AP is already in the process of receiving another transmission from a "hidden" radio. This is commonly referred to as the hidden node problem.
For this reason, another mechanism called request-to-send/clear-to-send (RTS/CTS) was included in the existing 802.11 standard. Under the RTS/CTS mechanism, each node must send an RTS message to the AP and receive a CTS reply before transmission can begin. The situation of CCK and OFDM radios operating on the same channel is very analogous to the hidden node problem because the CCK radios cannot detect the OFDM transmissions. However, via the use of the RTS/CTS mechanism, OFDM radios will be able to operate on the same channel as existing Wi-Fi radios without collision.
While the RTS/CTS mechanism results in additional network overhead, the penalty is fairly modest. The benefit is a migration path to higher data rates for radios operating in the 2.4 GHz band. In the future, networks may make exclusive use of OFDM in the 2.4 GHz band, thus removing the need to use RTS/CTS at some point.
The emergence of IEEE 802.11g is extremely beneficial for the WLAN market. OFDM is the mandatory high-rate waveform in the 2.4 GHz band. Data rates of up to 54 Mbits/s are now available in the 2.4 GHz band. In addition, backward compatibility with Wi-Fi devices is assured.
Longer term, the IEEE 802.11g draft standard represents an important step toward the realization of dual-band (2.4 GHz and 5 GHz) radios. Because OFDM is already required for operation in the 5 GHz band, implementing 802.11g in a dual-band device adds no extra hardware complexity to the resulting product. For dual-band devices, "G is free!"
