The following are areas where advances in wireless LAN (WLAN) is taking place:
1. Higher WLAN speeds to support an adequate number of users in high-density environments and also voice over IP (VoIP) users. The transition to an IEEE 802.11g and/or 802.11n environment is a basic necessity in a high-density and/or high-bandwidth context.
2. Support of QoS over the wireless (and also core intranet) infrastructure. The deployment of IEEE 802.11e QoS-supporting technology is another basic necessity.
3. Secure communications is highly desirable. The deployment of IEEE 802.11i security capabilities is yet another requirement.
4. Roaming between access points, floors, and subnets is needed, as is a handoff to a cellular service when corporate WLAN service is no longer available or generally, for WN mobility situations. The deployment of IEEE 802.11r roaming capabilities addresses this requirement (capabilities not expected to be available and/or implemented until sometime in the future). Roaming also brings up the question of whether a traditional IP solution is adequate or if
one needs to utilize Mobile IP (MIP) (IETF RFC 3344) [4.9]; this is a fairly complex issue.
The IEEE 802.11b and 802.11g specifications postulate a partitioning of the spectrum into 14 overlapping staggered channels whose center frequencies are 5 MHz apart; within this partitioning of the ISM spectrum, channels 1, 6, and 11 (and if available in the regulatory domain, channel 14) do not overlap. These channels (or other sets with similar gaps) can be used so that multiple networks can
operate in close proximity without interfering with each other
wareless LAN (WAN) and Bluetooth typical topology
The spectral mask for 802.11b requires that the signal be at least 30 dB down from its peak energy at +-11 MHz from the center frequency and at least 50 dB down from its peak energy at +-22 MHz from the center frequency. Note that if the transmitter is sufficiently powerful, the signal can be relatively strong even beyond the +-22-MHz point (e.g., a powerful transmitter on channel 6 can easily overwhelm a weaker transmitter on channel 11); in most situations, however, the signal in a given channel is sufficiently attenuated to interfere only minimally with a transmitter on any other channel. The channels that are available for use in a particular country differ according to the regulations of that country. In the United States, for example, FCC regulations allow only channels 1 to 11 to be used. Channels 10 and 11 are the only channels that work in all parts of the world, because Spain has not licensed channels 1 to 9 for 802.11b operation.
The UNII band used in the IEEE 802.11a context is in the range 5.15 to
5.85 GHz. The 802.11a standard uses 300 MHz of bandwidth; the spectrum is divided into three domains, each having restrictions imposed on the maximum output power allowed. The first 100 MHz in the lower-frequency portion is restricted to a maximum power output of 50mW; the second 100 MHz has a higher maximum, 250mW; and the third, 100 MHz, intended primarily for outdoor applications, has a maximum power output of 1.0W. It is generally recognized that the higherfrequency UNII band is limited intrinsically to shorter ranges than the ISM band, due to higher path loss, limiting the utility of 802.11a relative to that of 802.11b/g in the WSN context, except for within-building applications. In particular, there is an increase of excess path loss with frequency. Table 4.8 provides a comparison between IEEE 802.11b/g and IEEE 802.11a. The IEEE 802.11a protocol uses a complex digital modulation method: specifically, orthogonal frequency-division multiplexing (OFDM); this digital modulation method requires more linearity in amplifiers because of the higher peak-to-average power ratio of the OFDM signal transmitted. In addition, better phase noise performance is required because of the closely spaced overlapping carriers. These issues tend to add to the implementation cost of 802.11a products. Although IEEE 802.11a was approved in the late 1990s, new product development has proceeded much more slowly than with 802.11b/g, due to the cost and complexity of implementation. Frequency-division multiplexing (FDM) is a multiplexing technology that transmits multiple signals from or for different users simultaneously over a single transmission path, such as a cable or wireless system (commercial FM radio is an example). Each signal occupies its own unique frequency range (carrier), which is modulated by the data (text, voice, video, etc.). The OFDM spread-spectrum technique distributes the data over a large number of carriers that are spaced apart at precise frequencies. This spacing provides the orthogonality, which prevents the demodulators from seeing frequencies other than their own. The benefits of OFDM are high spectral efficiency, resiliency to RF interference, and lower multipath distortion. This is useful because in a typical terrestrial broadcasting scenario there are multipath channels (i.e., the signal transmitted arrives at the receiver using various paths of different length). Since multiple versions of the signal interfere with each other [intersymbol interference (ISI)] it becomes difficult to extract the original information. OFDM is the modulation technique used for digital television in Europe, Japan, and Australia [4.10].
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