IEEE 802.11 Tutorial

By Jim Zyren and Al Petrick

Go to the second section of the IEEE 802.11 Tutorial.

Approval of the IEEE 802.11 standard for wireless local area networking (WLAN) and rapid progress made toward higher data rates have put the promise of truly mobile computing within reach. While wired LANs have been a mainstream technology for at least fifteen years, WLANs are uncharted territory for most networking professionals. Some obvious questions come to mind when considering wireless networking:

• How can WLANs be integrated with wired network infrastructure?
• What is the underlying radio technology?
• How is multiple access handled?
• What about network security?

IEEE 802.11 is limited in scope to the Physical (PHY) layer and Medium Access Control (MAC) sublayer, with MAC origins to IEEE802.3 Ethernet standard. The following overview explains major differences between wired and wireless LANs and should answer some of the questions facing MIS professionals evaluating WLAN technology.

Network Topology

WLANs can be used either to replace wired LANs, or as an extension of the wired LAN infrastructure. The basic topology of an 802.11 network is shown in Figure 1. A Basic Service Set (BSS) consists of two or more wireless nodes, or stations (STAs), which have recognized each other and have established communications. In the most basic form, stations communicate directly with each other on a peer-to-peer level sharing a given cell coverage area. This type of network is often formed on a temporary basis, and is commonly referred to as an ad hoc network, or Independent Basic Service Set (IBSS).

In most instances, the BSS contains an Access Point (AP). The main function of an AP is to form a bridge between wireless and wired LANs. The AP is analogous to a basestation used in cellular phone networks. When an AP is present, stations do not communicate on a peer-to-peer basis. All communications between stations or between a station and a wired network client go through the AP. AP's are not mobile, and form part of the wired network infrastructure. A BSS in this configuration is said to be operating in the infrastructure mode.

The Extended Service Set (ESS) shown in figure 2 consists of a series of overlapping BSSs (each containing an AP) connected together by means of a Distribution System (DS). Although the DS could be any type of network, it is almost invariably an Ethernet LAN. Mobile nodes can roam between APs and seamless campus-wide coverage is possible.

Radio Technology

IEEE 802.11 provides for two variations of the PHY. These include two (2) RF technologies namely Direct Sequence Spread Spectrum (DSSS), and Frequency Hopped Spread Spectrum (FHSS). The DSSS and FHSS PHY options were designed specifically to conform to FCC regulations (FCC 15.247) for operation in the 2.4 GHz ISM band, which has worldwide allocation for unlicensed operation.

Both FHSS and DSSS PHYs currently support 1 and 2 Mbps. However, all 11 Mbps radios are DSSS. Operating principles of DSSS radios are described in the following paragraphs.

DSSS systems use technology similar to GPS satellites and some types of cell phones. Each information bit is combined via an XOR function with a longer Pseudo-random Numerical (PN) sequence as shown in Figure 3. The result is a high speed digital stream which is then modulated onto a carrier frequency using Differential Phase Shift Keying (DPSK).

When receiving the DSSS signal, a matched filter correlator is used as shown in Figure 4. The correlator removes the PN sequence and recovers the original data stream. Tat the higher data rates of 5.5 and 11 Mbps, DSSS receivers employ different PN codes and a bank of correlators to recover the transmitted data stream. The high rate modulation method is called Complimentary Code Keying (CCK). The effects of using PN codes to generate the spread spectrum signal are shown in Figure 5.

As shown in Figure 5a, the PN sequence spreads the transmitted bandwidth of the resulting signal (thus the term, "spread spectrum") and reduces peak power. Note however, that total power is unchanged. Upon reception, the signal is correlated with the same PN sequence to reject narrow band interference and recover the original binary data (Fig. 5b). Regardless of whether the data rate is 1, 2, 5.5, or 11 Mbps, the channel bandwidth is about 20 MHz for DSSS systems. Therefore, the ISM band will accommodate up to three non-overlapping channels

Go to IEEE 802.11 Tutorial Part 2.
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