Summary of IEEE 802.11 PHYs

The naming of the various PHYs defined in the context of IEEE 802.11 is not as descriptive as one might hope. The original RF-based PHYs were named Frequency Hopping Spread Spectrum (FHSS) and Direct Sequence Spread Spectrum (DSSS). In addition to these RF-based PHYs, the IEEE 802.11-1999 specification also specified a diffuse infrared PHY. All of those PHYs operate at either 1 or 2 Mbps. There was no specific "marketing" name for the 1 and 2 Mbps PHYs, other than "IEEE 802.11."

IEEE 802.11a

The IEEE 802.11a-1999 standard specifies a "High-Speed Physical Layer in the 5 GHz Band." To quote the standard, the following describes the actual attributes of IEEE 802.11a PHYs:

• The radio frequency LAN system is initially aimed for the 5.15 5.25, 5.25 5.35, and 5.725 5.825 GHz unlicensed national information structure [sic] (U-NII) bands, as regulated in the United States by the Code of Federal Regulations, Title 47, Section 15.407.

• The OFDM system provides a [W]LAN with data payload communication capabilities of 6, 9, 12, 18, 24, 36, 48, and 54 Mbps. A WLAN product claiming to support IEEE 802.11a must be capable of transmitting and receiving data at rates of 6, 12, and 24 Mbps.

• The OFDM system uses 52 subcarriers that are modulated using binary phase shift keying (BPSK), quadrature phase shift keying (QPSK), 16-quadrature amplitude modulation (16-QAM), or 64-QAM.

Figure 3-20 shows which modulations are used to achieve which speeds. The "coding rate" expresses the parameters governing forward error correction at a given speed.

Figure 3-20. OFDM modulations and their resulting speeds^[28]

^[28] Excerpted from IEEE Std. 802.11a-1999, copyright 1999. All rights reserved.

There are actually 12 non-overlapping channels that have been specified in IEEE 802.11a, of which eight contiguous channels occupy the "lower" U-NII band, and a disjoint four contiguous channels occupy the "upper" U-NII band. The increased number of non-overlapping channels is a significant deployment advantage for WLANs based on IEEE 802.11a, since it is possible to overlay many more APs in the same physical space.

IEEE 802.11b

The IEEE 802.11b-1999 standard is called "Higher Speed Physical Layer Extension in the 2.4 GHz Band." The IEEE 802.11b PHYs are commonly known as the "High Rate" (or simply "HR") PHYs, and include two different modulation choices in the 2.4 GHz band. The mandatory modulation scheme is known as Complementary Code Keying (CCK), and an alternate scheme is also defined, known as Packet Binary Convolutional Coding (PBCC). Both of these defined modulation schemes support operation at two speeds: 5.5 and 11 Mbps.

IEEE 802.11b is defined to operate between 2.401 GHz and 2.4835 GHz, which is a proper subset of the 2.4 GHz ISM band in the United States, and which is also similarly available for unlicensed use in many other parts of the world. The channel definition within the 2.4 GHz band, as specified in IEEE 802.11b-1999, allows for three non-overlapping channels: Channel 1, Channel 6, and Channel 11.

The complete name for the CCK PHYs is "High Rate Direct Sequence Spread Spectrum" (or HR/DSSS). They are "high rate" compared to plain DSSS that was defined by the original IEEE 802.11-1999, which operated at either 1 or 2 Mbps. There is an optional variant of HR/DSSS that uses the Short PLCP Preamble, which is known as HR/DSSS/short. In addition, there are two names for the PBCC-based PHYs, namely HR/DSSS/PBCC and HR/DSSS/PBCC/short, wherein the PLCP Header is preceded by the optional Short Preamble.

In all these cases, the PLCP Header is modulated such that any STA could understand it, to enable backward compatibility, so the PLCP header is modulated at either 1 Mbps or at a combination of 1 and 2 Mbps. The Short Preamble variants defined in IEEE 802.11b-1999 are able to be demodulated by DSSS STAs, but since there was no Short Preamble option defined at that time, these STAs would not be able to parse the PLCP Preamble of these frames.

IEEE 802.11d

IEEE 802.11d-2001 is not a PHY standard, but it is very deeply related to the IEEE 802.11b standard, so it is worth mentioning here. The IEEE 802.11d standard, entitled "Amendment 3: Specification for operation in additional regulatory domains," provides mechanisms that can enable products based on IEEE 802.11b-1999 to roam across differing regulatory domains.

Based on information conveyed by an IEEE 802.11d-capable AP, an IEEE 802.1d-capable STA will learn the regulatory domain in which it is operating, and thereby cease using channels that are not legal to use in that domain.

IEEE 802.11g

IEEE 802.11g-2003 adds the OFDM modulations based on IEEE 802.11a-1999 to the lower-cost radios of IEEE 802.11b-1999. It also extends the PBCC modulation, which optionally was defined to operate at 5.5 and 11 Mbps in IEEE 802.11b-1999, to the speeds of 22 and 33 Mbps. Additionally, a hybrid modulation is provided which uses the DSSS-style PLCP header, with OFDM-modulated data. The PHYs in the IEEE 802.11g standard are known as "Extended Rate PHYs" and have the following names:

• ERP-DSSS (5.5 and 11 Mbps using CCK with Short Preamble)

• ERP-OFDM (6, 9, 12, 24, 36, 48, and 54 Mbps)

• ERP-PBCC (22 and 33 Mbps)

• DSSS-OFDM (6, 9, 12, 24, 36, 48, and 54 Mbps)

The Short Preamble support, which was optional-to-implement in IEEE 802.11b, is mandatory-to-implement in IEEE 802.11g. The AP decides when it is safe to enable that mode of operation (e.g., when most, or all, of the associated STAs are ERP-STAs). IEEE 802.11g-2003 specifies rules to help the ERP-STAs safely interoperate with HR-STAs.

Winners and Losers

The proliferation of PHY choices in the IEEE 802.11 marketplace leads many to wonder which PHY will "win" over the others. As of late 2002, there was a selection of Wi-Fi-logoed products based on IEEE 802.11b-1999, plus the so-called "b+" products that offered 22 Mbps performance using PBCC at that speed (as well as at 5.5 and 11 Mbps). Products based on IEEE 802.11a-1999 were also on the market, and began to be issued with Wi-Fi logos as of late 2002. Several vendors also supported dual-band products; in other words, products that included support for both IEEE 802.11a-1999 and IEEE 802.11b-1999.

At the very end of 2002, and more so as 2003 got under way, products based on pre-standard IEEE 802.11g began to appear, to a warm welcome from the marketplace, despite their pre-standard status, and despite the fact that they lacked the Wi-Fi_™ "seal of approval." Products in the latter category are implicitly b/g combinations, and several vendors have announced chipsets that will enable future products to support all three PHYs (a, b, and g).

Based on past experience, it is likely that not all of the products may have equivalent mass-market success; however, niche applications may exist for the non-mass-market choice(s), perhaps even to the extent that there will be solid business justification for finding and serving those markets. Early indications are that IEEE 802.11g will be a winner, since it offers a logical upgrade path for users who have already deployed IEEE 802.11b equipment in the 2.4 GHz ISM band.

There was a time when the "conventional wisdom" was that IEEE 802.11a would displace IEEE 802.11b as soon as products supporting IEEE 802.11a were available. However, what actually happened was that IEEE 802.11b products became wildly popular, and their costs dropped rapidly, which further encouraged their adoption. There is no dispute that IEEE 802.11a has some considerable technical advantages over IEEE 802.11b, but the market would appear to be saying (at least for now) that IEEE 802.11b is good enough (at least for today's applications).

In the author's opinion, it is not likely that any of IEEE 802.11a, IEEE 802.11b, or IEEE 802.11g will disappear in a short time (due to obsolescence by a superior standard). It is more likely that they will all find some degree of market acceptance. The author is making no predictions as to which will still be in existence in five years, although by that time we should be beginning to see next-generation WLAN products based on the standards that will emerge from the new "High Throughput" Task Group that is being created within IEEE 802.11 in 2003 (we're already hearing talk about IEEE 802.11n, even though TGn doesn't even begin its formal existence until September, 2003…).