High-speed WLANs: 80211a and 80211g in VS .NET

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1443 High-speed WLANs: 80211a and 80211g
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There is a continuous demand for increased bandwidth in WLANs To tackle this issue, IEEE 802 Working Groups de ned two standards: 80211a and 80211g, both providing a maximum data rate of 54 Mbps
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80211a provides data rates up to 54 Mbps by operating in the 5 GHz band, which is called the universal networking infrastructure (UNI) band The UNI band is further divided into three subbands Devices operating in each subband are subjected to different transmission constraints For implementing WLANs, only the UNI-1 (515 525 GHz) and UNI-2 (525 535 GHz) can be used2 Speci cally, UNI-1 is reserved for indoor use, while UNI-2 is both indoor and outdoor Compared to the 80211b technology, the 80211a technology guarantees a more ef cient frequency reuse, since four nonoverlapping channels exist within each subband, while only (up to) three nonoverlapping channels are available in 80211b Therefore, 80211abased WLANs can use eight different channels This larger number of available channels greatly encourages WLANs deployment The increased spectral ef ciency of the 80211a systems is due to the use of the orthogonal frequency division multiplexing (OFDM) modulation technique [19] Speci cally, in traditional frequency division multiplexed (FDM) systems, nonoverlapping channels are obtained if the spacing between channels is greater than the symbol rate Instead, OFDM systems adopt a xed spacing among the channels because the orthogonality among carriers is achieved by distributing the data over multiple carriers spaced at planned frequencies Hence, guard bands are eliminated so as to increase the spectral ef ciency of the overall system An 80211a-compliant device can transmit using one of the available transmission rates In particular, a multirate station can operate at the following throughput steps: 6, 9, 12, 18, 24, 36, 48 and 54, where 9 and 18 are optional However, the throughput increase is obtained at the cost of using modulation techniques that are less robust to interference, thus reducing the allowed transmission ranges Furthermore, by using the 5 GHz spectrum, the reduction in the transmission ranges is more marked than the corresponding reductions in the 24 GHz spectrum [19] To overcome these limitations, the 80211g standard has been proposed Speci cally, the
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Table 141 The relationship between 80211b, 80211g and 80211a Single/Multicarrier 1 2 55 6 9 11 12 18 24 36 48 54 Single carrier Single carrier Single carrier Multicarrier Multicarrier Single carrier Multicarrier Multicarrier Multicarrier Multicarrier Multicarrier Multicarrier 80211 b Barker Barker CCK 80211 g Barker Barker CCK OFDM OFDM CCK OFDM OFDM OFDM OFDM OFDM OFDM 80211 a
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80211g is interoperable with the 80211b systems because it operates in the 24 GHz band, while offering the higher data rates of the 80211a standard In detail, the 80211g system uses the same modulation technique as the a and b systems, providing a maximum transmission rate of 54 Mbps for WLANs that operate at 24 GHz The 80211g standard uses OFDM modulation but, for backward compatibility with 80211b, it also supports complementary code keying (CCK) modulation (see Table 141 for a synoptic view of these standards)
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1444 Quality of Service in 4G WLANs: the 80211e standard
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The basic 80211 MAC protocol is the distributed coordination function (DCF) that works as a listen-before-talk scheme, based on carrier sense multiple access (CSMA) Furthermore, the 80211 de nes a collision avoidance (CA) mechanism to reduce the probability of collisions due to two or more stations transmitting concurrently Since the CSMA/CA protocol is a random access MAC protocol, it does not provide support either to differentiate ows or to guarantee QoS requirements such as delay, jitter, etc To support time-bounded services, the IEEE 80211 standard de nes the point coordination function (PCF) to let stations have priority access to the wireless medium, coordinated by a station called the point coordinator (PC) The PCF operates similarly to a polling system: the PC provides (through a polling mechanism) the transmission rights at a single station at a time There are problems with the PCF that can induce a poor QoS performance, these are leading to the design of enhancements to the MAC protocol discussed in this section Among many others, it is worth pointing out the unpredictable delays before the beginning of the contention free periods,3 and the unknown transmission durations of the polled stations To support applications with QoS requirements, the IEEE 80211e WG is developing a supplement to the MAC layer to provide QoS support for LAN applications It will apply to 80211 physical standards a, b and g and will add to them the ability to support time-sensitive