More options are available to developers of 802.11 wireless-local-area-networking (WLAN) products than ever before. To support a growing number of coding schemes and data rates, industry-standards bodies continue to establish new specifications. Each implementation requires a fairly well-established set of semiconductor devices including a baseband and medium-access-control (MAC) processor. Several other integrated circuits (ICs) also are required, mostly from third-party vendors.

As the industry explores new technology standards, the architecture is steadily evolving. This trend is especially evident at the MAC level. Here, the advent of multiplexing approaches and algorithms has the potential to improve both voice and multimedia performance.

The IEEE 802.11 specification for WLAN defines both MAC and physical-layer (PHY) protocols. Its base standard and extensions specify a number of options. Only a few of them are currently implemented in consumer and enterprise WLAN products. Most of the devices found in today's market operate under the IEEE 802.11a, 802.11b, and 802.11g physical layers. Each of these PHYs has its own proprietary variants. Support mainly falls to the distributed-coordination-function (DCF) MAC option.

In the 2.4-GHz ISM band, network interface cards (NICs) operate complementary code keying (CCK) up to 11 Mbps. They operate packet binary convolutional coding (PBCC) up to 33 Mbps and orthogonal frequency division multiplexing (OFDM) up to 54 Mbps. In the 5-GHz U-NII band, only Orthogonal Frequency Division Multiplexing (OFDM) is found with data rates up to 108 Mbps (and beyond).

As illustrated in Table 1, six WLAN NIC frequency allocations currently exist worldwide. This list includes two allocations each in the United States/Canada, Europe, and Japan. For each frequency allocation, government regulations stipulate the number of channels, non-overlapping channels, and power restrictions that must be supported. All of the aforementioned data rates are maximum bit rates as opposed to the actual average data throughput. The NIC will automatically drop to a lower rate when communications cannot be achieved at a specific rate. This problem arises because of impaired channel conditions.

Essentially, the standards-based 802.11b (2.4-GHz) CCK/PBCC physical layers are highly coded forms of binary-phase shift-keying (BPSK) and quadrature-phase shift-keying (QPSK) modulation. At the minimum, the BPSK scheme is used for the preamble of all packets. PBCC and CCK techniques are utilized to achieve a statistical advantage with this modulated energy in the presence of noise or "processing gain." To accomplish this goal, more symbols are sent than are required to represent the actual bit information.

IEEE 802.11b-compliant radios support four data rates: 1, 2, 5.5, and 11 Mbps. The three higher rates are coded into an 11-MegaSymbols-per-second (MSymbols/s) QPSK waveform. The 1-Mbps rate is coded into 11 MSymbols/s BPSK. A proprietary 22-Mbps PBCC mode also is included in some devices, although it isn't supported by the 802.11b specification.

The 5-GHz 802.11a specification uses OFDM, while the 2.4-GHz 802.11g specification offers it as one of the proposed options. In the latter case, however, OFDM is expected to be the main mode of operation. This standards-based OFDM is comprised of 52 independently modulated carriers. Four of them are BPSK "pilot," or synchronization, carriers. Depending on the data rate, the remaining 48 independent carriers are modulated as BPSK, QPSK, 16-quadrature amplitude modulation (16-QAM), or 64-QAM.

The supported data rates are 6, 9, 12, 18, 24, 36, 48, and 54 Mbps. Even though they're rarely omitted, the 36-, 48-, and 54-Mbps data rates are "optional." As with CCK and PBCC, the waveform is coded. Processing gain is thereby achieved over the gain of the actual information that's being conveyed. Additional proprietary modes of up to 108 Mbps (and beyond) also are available. But they require all stations to utilize similar hardware.

Table 2 provides a summary of the different modes. Today's PHY options span a broad range of data rates, modulation schemes, preamble/header formats, and additional proprietary modes. Maximum performance is available through 802.11a in the 5-GHz frequency band. Currently, 802.11a supports proprietary modes with data rates up to 108 Mbps.

NIC IMPLEMENTATIONS
802.11 WLAN NICs are built in several different form factors. The most common forms of client adapters include Peripheral Component Interconnect (PCI); PC card (Cardbus and/or Personal Computer Memory Card International Association or PCMCIA); Mini-PCI; Universal Serial Bus (USB)/USB dongle; and Compact Flash (CF+). Although some configurations do involve direct communications between NICs (ad-hoc mode), most users communicate via an access point or wireless router. A wireless router provides an access point and additional functionality.

Table 3 compares the various form factors and their associated power requirements. Depending upon the implementation, the access point and the wireless router can each vary in size and power requirements. PC cards (PCMCIA/Cardbus) have the widest power-efficiency margins. Compact Flash devices have some of the most stringent power and size restrictions. Yet today's solutions are capable of supporting both of them.

Regardless of form factor, WLAN NICs are generally built around chip sets. These chip sets may or may not be provided by a single vendor. In the current trend, an entire direct-conversion (or zero-IF) NIC consists of only a few integrated circuits (see figure). Among the main IC components that are found in most WLAN adapters are the baseband/MAC processor, radio chip, and power amplifier (PA). Additional ICs include RF switches, a serial EEPROM, a voltage regulator, and a voltage-controlled oscillator (VCO) that's external to the radio chip.