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Introducing 802.11

IEEE 802.11 is the Working Group within the IEEE responsible for wireless LAN standards. IEEE 802.11 became a standard in July 1997, and defined two RF technologies operating in 2.4-GHz band:

Direct-sequence spread spectrum (DSSS) 1 Mbps and 2 Mbps

Frequency-hopping spread spectrum (FHSS) 1 Mbps and 2 Mbps

Each of these two different technologies relies on a similar MAC layer protocol, but the physical layer differs drastically. Although they use the same frequency band, their approach to utilizing the actual RF is miles apart.

Because 802.11b and 802.11a operate in different frequency bands, the two standards have different propagation characteristics. A thorough understanding of how performance is affected when moving from one band to another or from one standard to another is essential to a successful design. Details such as capacity, data rates, throughputs, performance, and ranges and their varying effects when moving between 802.11b and 802.11a technologies need to be understood. Do not assume that you can just replace 802.11b products with 802.11a devices without other system implications. RF engineering is a very complex field of study, and this book outlines the necessary information for a solid understanding of the required RF basics.

Direct-Sequence Spread Spectrum

The DSSS approach involves encoding redundant information into the RF signal. Every data bit is expanded to a string of chips called a chipping sequence or Barker sequence. The chipping rate as mandated by the IEEE 802.11 is 11 chips (Bipolar Phase Shift Keying [BPSK]/Quadrature Phase Shift Keying [QPSK]) at the 1- and 2-Mbps rates. At these rates, 11 bits are transmitted for every 1 bit of data. The chipping sequence is transmitted in parallel across the spread-spectrum frequency range. The rationale here is that because the energy is so spread across the band, the signal looks more like noise to standard RF receivers, and with the information spread across a wide spectrum, it tends to be more immune to interference than a signal with a narrow spectrum. For this reason, DS is considered to have good interference immunity. Chapter 2, "Understanding RF Fundamentals," covers DS in more detail.

The DS protocol transmits out multiple "chips," or bits, for every data bit from the information. In bit was a 1 or a 0, even if some of the chips are missing.

IEEE 802.11b Direct-Sequence Channels

Fourteen channels are now defined in the IEEE 802.11 direct-sequence (DS) channel set. Each DS channel as transmitted is 22 MHz wide; however, the channel separation is only 5 MHz. This leads to channel overlap such that signals from neighboring channels can interfere with each other. In a 13-channel DS system (11 usable in the U.S.), only three nonoverlapping (and hence, noninterfering) channels 25 MHz apart are possible (for example, Channels 1, 6, and 11). For the 14-channel systems (Japan only), there are 4 possible nonoverlapping channels (1, 6, 11, and 14).

This channel spacing governs the use and allocation of channels in a multi-access point (AP) environment such as an office or campus. APs are usually deployed in "cellular" fashion within an enterprise where adjacent APs are allocated nonoverlapping channels. Alternatively, APs can be collocated (placed in the same physical area) using Channels 1, 6, and 11 to deliver 33-Mbps bandwidth to a single area (but only 11 Mbps to a single client). Figure 1-2 shows the channel-allocation scheme.

There are up to 14 22-MHz-wide channels, with only 3 nonoverlapping channels (1, 6, and 11 in the U.S. and 1, 7, 13 in Europe). This allows three APs to occupy the same space for a total of 33-Mbps aggregate throughput. (Each channel supports an 11-Mbps data rate.)

Frequency Hopping

The frequency-hopping (FH) approach to data transmission is basically what the name implies. The transmitting signal "hops" or moves around the band on a predetermined sequence. The receiver must also have the same sequence in order to "follow" the transmitter. The 802.11 standard specifies 78 different sequences or hopping patterns. In the face of interference, the radio just transmits the data packets, and if they do not reach the intended destination (as verified by an acknowledgment [ACK] sent back after successful reception of the packet), the transmitter just retransmits the packets on the next frequency, which is theoretically free of interference. For this reason, FH is said to have good interference avoidance.

Figure 1-3 is an example of FH in action. Packet 1 is sent out at a frequency near the bottom edge of the band, followed in time by packet 2. Notice that this second packet is located at a higher frequency in the band. Each successive packet in time is transmitted on a different frequency. In the actual FH implementation, the transmitter may send out multiple packets on the same frequency before moving to another frequency, but the amount of time that it is permitted to reside on any one frequency is limited to 400 milliseconds (ms).

802.11 Working Groups

Within the 802.11 Working Group, a number of different Task Groups are responsible for various elements of the 802.11 WLAN standard. MIB deficiencies in 802.11b

802.11a, b, and g represent the different radio technologies of the 802.11 specification. A good understanding of the advantages and disadvantages of these technologies will assist you in making the proper product-selection decision for your WLAN. The following sections focus on the important facets of these three technologies.

802.11a

In 1999, the IEEE released two new specifications for higher-bandwidth WLANs. One of these was the 802.11a specification, which identified a protocol that permitted data rates up to 54 Mbps, using the 5-GHz frequency band. At the time, very little in the way of development was being done in that RF band, which delayed the introduction of this technology into the market place for several years. In 2001, the first products based on the 802.11a standard started to appear on the market, and by 2002, most WLAN vendors were shipping some type of 802.11a products.

802.11b

802.11b standard was released at the same time as the 802.11a standard, but because of development that was already in place in the 2.4-GHz band, this technology hit the market much sooner. In fact, one vendor, Aironet Wireless Communication (now part of Cisco Systems), released their first 802.11b-designed product almost nine months before the standard was completed.

This new 802.11b standard used the same band as previous 802.11 standardsthat is, 2.4 GHzbut increased the data rate to 11 Mbps. This provided the much-needed bandwidth to utilize standard office application over the wireless link, and many users started to consider implementation in the standard networks.

When the 11-Mbps standard was completed, all of the main WLAN vendors jumped into development; within a year, the WLAN market had shifted to an 802.11b market.

802.11g

With the push for higher and higher bandwidth and for backward compatibility to the industry "standard" of 802.11b, a new standard was needed. The goal of more than 20 Mbps was initially set by the Task Group, but because there was already much development around 802.11a and 54 Mbps, it was finally decided (after many months of debate in the Task Group) to move this data rate to 54 Mbps, similar to 802.11a. This permitted many vendors to utilize some of the same components for the new 802.11g technology that they had used in their 802.11a devices.

One requirement for 802.11g was that the devices must be backward compatible to 802.11b. This means any devices that meet the new standard would be able to communicate with an existing 802.11b device, at the 802.11b data rates. This enabled users to migrate from 802.11b to 802.11g without a complete replacement of all devices at once.

802.11g was completed in June 2003. Some prestandard products were already shipping at that point, but many companies were waiting for the completion of the standard before making final design changes and beginning production.