The 802.11 Physical Layer

The three physical layers originally defined in 802.11 included two spread-spectrum radio
techniques and a diffuse infrared specification. The radio-based standards operate within the
2.4 GHz ISM band. These frequency bands are recognized by international regulatory agencies, such as the FCC (USA), ETSI (Europe), and the MKK (Japan) for unlicensed radio operations. As such, 802.11-based products do not require user licensing or special training. Spread-spectrum techniques, in addition to satisfying regulatory requirements, increase reliability, boost throughput, and allow many unrelated products to share the spectrum without explicit cooperation and with minimal interference.

Figure 2. Infrastructure Mode

The original 802.11 wireless standard defines data rates of 1 Mbps and 2 Mbps via radio waves using frequency hopping spread spectrum (FHSS) or direct sequence spread spectrum (DSSS). It is important to note that FHSS and DSSS are fundamentally different signaling mechanisms and will not interoperate with one another.

Using the frequency hopping technique, the 2.4 GHz band is divided into 75 1-MHz subchannels. The sender and receiver agree on a hopping pattern, and data is sent over a sequence of the subchannels. Each conversation within the 802.11 network occurs over a different hopping pattern, and the patterns are designed to minimize the chance of two senders using the same subchannel simultaneously.

Figure 3. Ad Hoc Mode

FHSS techniques allow for a relatively simple radio design, but are limited to speeds of no higher than 2 Mbps. This limitation is driven primarily by FCC regulations that restrict subchannel bandwidth to 1 MHz. These regulations force FHSS systems to spread their usage across the entire 2.4 GHz band, meaning they must hop often, which leads to a high amount of hopping overhead.

In contrast, the direct sequence signaling technique divides the 2.4 GHz band into 14 22-MHz channels. Adjacent channels overlap one another partially, with three of the 14 being completely non-overlapping. Data is sent across one of these 22 MHz channels without hopping to other channels. To compensate for noise on a given channel, a technique called “chipping” is used. Each bit of user data is converted into a series of redundant bit patterns called “chips.” The inherent
redundancy of each chip combined with spreading the signal across the 22 MHz channel provides for a form of error checking and correction; even if part of the signal is 5 Distribution system (DS) Extended Service Set (ESS)—multiple cells Basic Service Set (BSS)— single cell Access point(AP) Station Figure 2. Infrastructure Mode Independent Basic Service Set (IBSS) Figure 3. Ad Hoc Mode damaged, it can still be recovered in many cases, minimizing the need for retransmissions.

802.11b Enhancements to the PHY Layer
The key contribution of the 802.11b addition to the wireless LAN standard was to standardize
the physical layer support of two new speeds, 5.5 Mbps and 11 Mbps. To accomplish this, DSSS had to be selected as the sole physical layer technique for the standard since, as noted above, frequency hopping cannot support the higher speeds without violating current FCC regulations. The implication is that 802.11b systems will interoperate with 1 Mbps and 2 Mbps 802.11 DSSS systems, but will not work with 1 Mbps and 2 Mbps 802.11 FHSS systems.

The original 802.11 DSSS standard specifies an 11-bit chipping—called a Barker sequence—to encode all data sent over the air. Each 11-chip sequence represents a single data bit (1 or 0), and is converted to a waveform, called a symbol, that can be sent over the air. These symbols are transmitted at a 1 MSps (1 million symbols per second) symbol rate using a technique called Binary Phase Shift Keying (BPSK). In the case of 2 Mbps, a more sophisticated implementation called Quadrature Phase Shift Keying (QPSK) is used; it doubles the data rate available in BPSK, via improved efficiency in the use of the radio bandwidth.

To increase the data rate in the 802.11b standard, advanced coding techniques are employed. Rather than the two 11-bit Barker sequences, 802.11b specifies Complementary Code Keying (CCK), which consists of a set of 64 8-bit code words. As a set, these code words have unique mathematical properties that allow them to be correctly 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 carrier, while the 11 Mbps rate encodes 8 bits per carrier. Both speeds use
QPSK as the modulation technique and signal at 1.375 MSps. This is how the higher data
rates are obtained. Table 1 shows the differences.

To support very noisy environments as well as extended range, 802.11b WLANs use dynamic rate shifting, allowing data rates to be automatically adjusted to compensate for the changing nature of the radio channel. Ideally, users connect at the full 11 Mbps rate. However when devices move beyond the optimal range for 11 Mbps operation, or if substantial interference is present, 802.11b devices will transmit at lower speeds, falling back to 5.5, 2, and 1 Mbps. Likewise, if the device moves back within the range of a higher-speed transmission, the connection will automatically speed up again. Rate shifting is a physicallayer mechanism transparent to the user and the upper layers of the protocol stack.