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FIGURE 1.2
Notional IEEE 802 wireless internet architecture.
technology proliferation is expected to increase substantially as network service providers adopt the technology. Sprint Corporation announced in 2006 that it plans to deploy a full WiMAX network across its entire U.S. cov- erage area to be operational in 2007. Other corporations across the world have also announced plans to increasingly deploy WiMAX technology, especially to underserved areas such as developing countries with limited infrastructure options. Furthermore, Intel’s announcement to support WiMAX as part of its wireless networking chipset in future laptop computers has further solidified WiMAX as a likely technology candidate for the next generation of wireless network-enabled devices.
1.2.1 IEEE 802.11
Of the wireless networking technologies specified by IEEE 802, IEEE 802.11 (Figure 1.3) has experienced the widest deployment to date with hundreds of thousands of IEEE 802.11 networks deployed all over the world. IEEE
802.11 supports data rates from 1 up to 54 Mbps using a variety of mod- ulation and coding methods. IEEE 802.11b operates using a direct-sequence spread spectrum (DSSS) waveform supporting data rates up to 11 Mbps, while IEEE 802.11g uses an orthogonal frequency division multiplexing (OFDM) waveform supporting data rates up to 54 Mbps. Both IEEE 802.11b and IEEE 802.11g operate in the 2.4 GHz industrial, scientific, and medical (ISM) band, while IEEE 802.11a operates in the 5 GHz Unlicensed National Information
FIGURE 1.3
IEEE 802.11 network architecture.
Infrastructure (UNII) band. IEEE 802.11a uses the OFDM waveform specified in IEEE 802.11g for data rates up to 54 Mbps, albeit at lower transmit powers (around a maximum of 20 mW for IEEE 802.11a compared to a maximum of 100 mW for IEEE 802.11g). More information on these standards can be found in Refs. 1–4.
The basic service set (BSS) is the foundation of an 802.11 network. The BSS is a group of stations that communicate with one another. These communica- tions take place in the basic service area (BSA). A station within the BSA can communicate with other members of the BSS. There are two types of BSS’: ad hoc (or independent) and infrastructural. An ad hoc BSS, also known as an independent basic service set (IBSS), is one in which stations communicate directly with one another. IBSS’ are typically short-lived in nature and are, thus, referred to as ad hoc. These are the least common types of 802.11 networks within the commercial domain. An infrastructural BSS is one in which all com- munications take place through the access point (AP) within that BSS. This is the most common type of 802.11 network within the commercial domain. Multiple BSS’ can be interconnected into an extended service set (ESS). An ESS is formed by chaining BSS’ together with a backbone network. The 802.11 does not specify the backbone network, but rather that this backbone network provides a certain set of services. From the perspective of the logical link control (LLC) sublayer that resides between the 802.11 MAC layer and the IP network layer, an ESS appears identical to a larger BSS (i.e., the concept of BSS versus ESS is transparent to the higher LLC sublayer). Figure 1.3 depicts the 802.11 network architecture from an infrastructural mode perspective.
An ESS or BSS is identified by its service set identity (SSID). The SSID is
a 0- to 32-byte identifier that is typically assigned a human-readable Ameri- can Standard Code for Information Interchange (ASCII) character string. As a result, it is alternatively known as the 802.11 network name. The first thing a mobile station (MS) wishing to join an 802.11 network must do is detect the presence of the network. There are two methods by which this can be accomplished: passive and active. In the passive case, the MS scans all fre- quency channels listening for the presence of network beacons, which are periodically transmitted by the stations of the network to announce their presence. These beacons contain essential information about that network, such as its SSID. The station can then begin the authentication and associa- tion procedures required to join the network. In the active case, the MS begins transmitting probes with the SSID of the network it wishes to join and then waits for a response from the probes. Upon receipt of a probe response, the MS can then begin joining the network. In fact, this active method is required if SSID broadcast is suppressed for security purposes.
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