The difference between A and B

Confused about the whole 802.11a/802.11b divide? Check out our guide to the differences between the two standards.

The purpose of this paper is to provide the reader with a basic understanding of how the IEEE 802.11b and 802.11a standards compare to one another.

This paper does not advocate the use of 802.11b over 802.11a or vice versa, nor does it provide a market analysis or economic speculation on the growth of the wireless LAN industry. However, this paper does factually inform the reader as to how the 802.11a and 802.11b standards compare to one another and additionally to provides detailed information on 802.11a.

Introduction
The IEEE 802.11 standard was established in 1997 to define both the MAC (Media Access Control) and PHY (physical) layer requirements for Wireless LAN. Since that time, 802.11b has proven to be enormously successful. Numerous products based upon 802.11b have been introduced to the marketplace and an organization devoted to interoperability of those products has been established. This organization is called WECA (Wireless Ethernet Compatibility Alliance). Part of WECA's charter is to bestow a logo called Wi-Fi (Wireless Fidelity) upon products that comply with the interoperability requirements. Although 802.11b and Wi-Fi are not strictly synonymous, all WECA tests are grounded firmly on the 802.11 standard. Products based upon 802.11a began to emerge in late 2001. WECA will initiate its WiFi5 logo program once there are at least two semiconductor companies producing 802.11a chips, and equipment manufacturers producing at least three types of 802.11a equipment. This is expected to occur in mid-2002.

Basic Comparison of 802.11b and 802.11a

Operating Frequency Band

802.11b operates in the 2.4-GHz ISM (Industrial, Scientific, and Medical) band. 802.11a operates in 5-GHz UNII (Unlicensed National Information Infrastructure) band.

Operating Bandwidth
802.11b occupies 83.5 MHz (for North America) from 2.4000 GHz to 2.4835 GHz, while 802.11a occupies 300 MHz in three different bandwidths of 100 MHz each.

Number of Channels
802.11b provides 11 channels (for North America), each channel being 22 MHz in width, and each channel centered at 5 MHz intervals (beginning at 2.412 GHz and ending at 2.462 GHz).

802.11a provides 12 channels, each channel being 20 MHz wide, and each centered at 20 MHz intervals (beginning at 5.180 GHz and ending at 5.320 GHz for the upper and middle U-NII bands, beginning at 5.745 GHz and ending at 5.805 GHz for the upper U-NII band).

Transmit Power Output
802.11b allows a maximum output of 1000 mW (1 Watt) of transmit power. However, most 802.11b products are designed to output no more than 30 mW for reasons of thermal dissipation and electrical power consumption (including battery life conservation for mobile applications).

Data Rates
802.11b supports data rates of 1, 2, 5.5, and 11 Mbps. 802.11a supports mandatory data rates of 6, 12, and 24 Mbps and optional data rates of 9, 18, 36, and 54 Mbps.

Single vs. Multicarrier Techniques
802.11b uses DSSS (Direct Sequence Spread Spectrum) with a single carrier per channel. 802.11a uses OFDM (Orthogonal Frequency Division Multiplexing) with multiple carriers (aka "subcarriers") per channel.

Performance Similarities to 802.11b

Range vs. Rate
As with 802.11b, connection speed decreases (falls back) as the distance between an 802.11a client and its access point increases, even though the connection is maintained. However, preliminary results indicate that for a given range, 802.11a is always faster than 802.11b, e.g., a range that can support a link speed of 11 Mbps for 802.11b should be able to support a link speed of 36 Mbps (or greater) for 802.11a.

Common MAC
802.11a's use of the same MAC as 802.11b means one less component to design. For adopters, this means that upgrading from 802.11b to 802.11a technology will not have significant impact on network operations. 802.11b's MAC uses CSMA/CA technology and implements a number of options to improve throughput, especially in congested areas.

The only drawback to using the 802.11b MAC is that 802.11a inherits the same inefficiencies hampering 802.11b wireless solutions. The 802.11b MAC is only about 70 percent efficient, so even at 54 Mbps, 802.11a maximum throughput is closer to 38 Mbps. Factor in driver inefficiencies and some additional overhead at the physical layer, and you can expect actual throughput to be about 30 Mbps. This estimate is based on the average throughput of 802.11b networks, which is now about 6 Mbps of a possible 11 Mbps for optimal implementations and environments. But unlike 802.11b, 802.11a does NOT have to transmit its headers at 1 Mbps, so 802.11a will gain some theoretical efficiency over 802.11b (but it's unlikely that throughput will exceed 38 Mbps).

Other Differences

Power Consumption
Initial 802.11a devices are estimated to consume 20 to 40 percent more power (when running at 5 4 Mbps) than today's 802.11b PC cards (when running at 11 Mbps).

Globalization
At the moment, 802.11a can only be used in North America and Japan. Its use has NOT been approved in Europe. The IEEE 802.11 Task Group H is working on spectrum management techniques called Dynamic Frequency Selection (DFS) and Transmit Power Control (TPC) to meet the challenges of European deployment. As of March 2002, this standard is still in process of completion.

Co-existence with Other Wireless Devices
Since 802.11a does not share the 5 GHz frequency band with other wireless devices (e.g., cordless phones, microwave ovens, Bluetooth, HomeRF, etc.), it will rarely be victimized by anything other than the actual physical environment.

Allen Huotari is Director of Research & Development at The Linksys Group, Inc.