28-05-2013, 03:27 PM
Wireless LANs
ABSTRACT
Wireless LANs provide high-speed data within a small region, e.g. a campus or small building, as users move from place to place. Wireless devices that access these LANs are typically stationary or moving at pedestrian speeds.
All wireless LAN standards in the U.S. operate in unlicensed frequency bands. The primary unlicensed bands are the ISM bands at 900 MHz, 2.4 GHz, and 5.8 GHz, and the Unlicensed National Information Infrastructure
(U-NII) band at 5 GHz. In the ISM bands unlicensed users are secondary users so must cope with interference from primary users when such users are active. There are no primary users in the U-
not required to operate in either the ISM or U-NII bands. However, this advantage is a double-edged sword, since
other unlicensed systems operate in these bands for the same reason, which can cause a great deal of interference nbetween systems. The interference problem is mitigated by setting a limit on the power per unit bandwidth for
unlicensed systems. Wireless LANs can have either a star architecture, with wireless access points or hubs placed throughout the coverage region, or a peer-to-peer architecture, where the wireless terminals self-configure into a
network. Dozens of wireless LAN companies and products appeared in the early 1990’s to capitalize on the “pentup demand” for high-speed wireless data. These first generation wireless LANs were based on proprietary and
incompatible protocols. Most operated within the 26 MHz spectrum of the 900 MHz ISM band using direct sequence spread spectrum, with data rates on the order of 1-2 Mbps. Both star and peer-to-peer architectures were used. The lack of standardization for these products led to high development costs, low-volume production, and
small markets for each individual product. Of these original products only a handful were even mildly successful. Only one of the first generation wireless LANs, Motorola’s Altair, operated outside the 900 MHz band.
system, operating in the licensed 18 GHz band, had data rates on the order of 6 Mbps. However, performance of Altair was hampered by the high cost of components and the increased path loss at 18 GHz, and Altair was discontinued within a few years of its release.
The second generation of wireless LANs in the U.S. operate with 80 MHz of spectrum in the 2.4 GHz ISM band. A wireless LAN standard for this frequency band, the IEEE 802.11b standard, was developed to avoid some of the problems with the proprietary first generation systems. The standard specifies direct sequence spread
spectrum with data rates of around 1.6 Mbps (raw data rates of 11 Mbps) and a range of approximately 150 m. The network architecture can be either star or peer-to-peer, although the peer-to-peer feature is rarely used.
Many companies developed products based on the 802.11b standard, and after slow initial growth the popularity of 802.11b wireless LANs has expanded considerably. Many laptops come with integrated 802.11b wireless LAN
cards. Companies and universities have installed 802.11b base stations throughout their locations,and many coffee houses, airports, and hotels offer wireless access, often for free, to increase their appeal. Two additional standards in the 802.11 family were developed to provide higher data rates than 802.11b. The IEEE 802.11a wireless LAN standard operates with 300 MHz of spectrum in the 5 GHz U-NII band. The 802.11a
standard is based on multicarrier modulation and provides 20-70 Mbps data rates. Since 802.11a has much more bandwidth and consequently many more channels than 802.11b, it can support more users at higher data rates. There was some initial concern that 802.11a systems would be significantly more expensive than 802.11b systems,
but in fact they quickly became quite competitive in price. The other standard, 802.11g, also uses multicarrier modulation and can be used in either the 2.4 GHz and 5 GHz bands with speeds of up to 54 Mbps. Many wireless
ABSTRACT
Wireless LANs provide high-speed data within a small region, e.g. a campus or small building, as users move from place to place. Wireless devices that access these LANs are typically stationary or moving at pedestrian speeds.
All wireless LAN standards in the U.S. operate in unlicensed frequency bands. The primary unlicensed bands are the ISM bands at 900 MHz, 2.4 GHz, and 5.8 GHz, and the Unlicensed National Information Infrastructure
(U-NII) band at 5 GHz. In the ISM bands unlicensed users are secondary users so must cope with interference from primary users when such users are active. There are no primary users in the U-
not required to operate in either the ISM or U-NII bands. However, this advantage is a double-edged sword, since
other unlicensed systems operate in these bands for the same reason, which can cause a great deal of interference nbetween systems. The interference problem is mitigated by setting a limit on the power per unit bandwidth for
unlicensed systems. Wireless LANs can have either a star architecture, with wireless access points or hubs placed throughout the coverage region, or a peer-to-peer architecture, where the wireless terminals self-configure into a
network. Dozens of wireless LAN companies and products appeared in the early 1990’s to capitalize on the “pentup demand” for high-speed wireless data. These first generation wireless LANs were based on proprietary and
incompatible protocols. Most operated within the 26 MHz spectrum of the 900 MHz ISM band using direct sequence spread spectrum, with data rates on the order of 1-2 Mbps. Both star and peer-to-peer architectures were used. The lack of standardization for these products led to high development costs, low-volume production, and
small markets for each individual product. Of these original products only a handful were even mildly successful. Only one of the first generation wireless LANs, Motorola’s Altair, operated outside the 900 MHz band.
system, operating in the licensed 18 GHz band, had data rates on the order of 6 Mbps. However, performance of Altair was hampered by the high cost of components and the increased path loss at 18 GHz, and Altair was discontinued within a few years of its release.
The second generation of wireless LANs in the U.S. operate with 80 MHz of spectrum in the 2.4 GHz ISM band. A wireless LAN standard for this frequency band, the IEEE 802.11b standard, was developed to avoid some of the problems with the proprietary first generation systems. The standard specifies direct sequence spread
spectrum with data rates of around 1.6 Mbps (raw data rates of 11 Mbps) and a range of approximately 150 m. The network architecture can be either star or peer-to-peer, although the peer-to-peer feature is rarely used.
Many companies developed products based on the 802.11b standard, and after slow initial growth the popularity of 802.11b wireless LANs has expanded considerably. Many laptops come with integrated 802.11b wireless LAN
cards. Companies and universities have installed 802.11b base stations throughout their locations,and many coffee houses, airports, and hotels offer wireless access, often for free, to increase their appeal. Two additional standards in the 802.11 family were developed to provide higher data rates than 802.11b. The IEEE 802.11a wireless LAN standard operates with 300 MHz of spectrum in the 5 GHz U-NII band. The 802.11a
standard is based on multicarrier modulation and provides 20-70 Mbps data rates. Since 802.11a has much more bandwidth and consequently many more channels than 802.11b, it can support more users at higher data rates. There was some initial concern that 802.11a systems would be significantly more expensive than 802.11b systems,
but in fact they quickly became quite competitive in price. The other standard, 802.11g, also uses multicarrier modulation and can be used in either the 2.4 GHz and 5 GHz bands with speeds of up to 54 Mbps. Many wireless