IEEE 802.11 Family

Within the IEEE 802.11 family, a few wireless LAN technologies represent the evolution and refinement of wireless LANs. Table 1.6 provides a comparison of these technologies. Note that, to the general public, Wi-Fi is probably the term that links to 802.11 wireless LANs. The Wi-Fi Alliance is a nonprofit industry association formed in 1999 to certify the interoperability of wireless LAN products based on IEEE 802.11 specifications. It has over 200 member companies. The goal of the Wi-Fi Alliance is to enhance the user experience through product interoperability and, understandably, promote the wireless technology and products for business interest.

The initial 802.11:1997 standard contained three incompatible options — infrared, FHSS, and DSSS — to support data rates of 1 to 2 Mbps. For FHSS, 79 channels are allocated in the 2.4-GHz ISM band in the United States and Europe. For DSSS, an 11-chip Barker sequence is used. A Barker sequence is a special binary sequence of _ 1 and _ 1 possessing mathematical characteristics that can be utilized to improve a coding scheme’s robustness and error-correction capability. Only a few Barker sequences are known. The one that is 11 in length is used in the initial 802.11 DSSS, which only supports data rates of 1 and 2 Mbps. The updated 802.11b:1999 standard discontinued further specification of infrared and FHSS, focusing instead on only enhancements to DSSS WLANs. 802.11b added new 5.5- and 11-Mbps data rates based on CCK modulation, a new chip sequence using 8-chip complementary code keying. Wi-Fi-certified products implement DSSS as defined by 802.11b:1999, supporting both 1 and 2 Mbps with the Barker code and 5.5 _ 11 Mbps with CCK. 802.11b defines a total number of 14 channels separated by a 5-MHz gap, from 2414 to 2484 MHz, but only 11 are usable due to FCC regulations in the United States. Furthermore, for DSSS to operate, the bandwidth of these channels should be 22 MHz apart in the frequency domain. As a result, only channel 1 (2412 MHz), channel 6 (2437 MHz), and channel 11 (2462 MHz) can be used at the same time. In Europe, these channels are channel 1 (2412 MHz), channel 7 (2442 MHz), and channel 13 (2472 MHz).

In 802.11a, OFDM is used instead of DSSS. 802.11a operates on the UNNI 5-GHz band with a total number of 12 non-overlapping channels. Channel spacing is 20 MHz. Recall that OFDM leverages multiple carriers (52 in the case of 802.11a) of different frequencies to transmit the same bitstream. Each channel of 802.11a leverages 52 subcarriers that are evenly separated by a distance of 312.5 KHz, plus some virtual subcarriers that are not used. The data rates of 802.11a are 6, 9, 12, 18, 24, 36, 49, and 54 Mbps, each of which is realized by a combination of a specific PSK or QAM digital modulation scheme and OFDM symbol setting.

802 .11g wireless LANs operate at the 2.4-GHz band but can offer much higher data rates,
up to 54 Mbps. To be backward compatible, 802.11g incorporates 802.11b’s CCK to achieve bit transfer rates of 5.5 and 11 Mbps in the 2.4-GHz band. To obtain higher data rates at the 2.4-GHz band, it adopts 802.11a’s OFDM scheme. Use of the 2.4-GHz ISM band permits 802.11g to have almost the same signal coverage as 802.11b.

802 .11n is the latest wireless LAN standard and promises to offer data rates up to 108 _ 320 Mbps at the 2.4-GHz ISM band. As of this writing, no offi cial release has been made by the IEEE 802.11n working group. Two proposals are being considered, and it is unclear which one will finally win. One group, TG n Synch, advocates using a 40-MHz bandwidth for each channel. The competing World Wide Spectrum Efficiency (WWiSE) group wants to retain the 20-MHz bandwidth (as in 802.11b, a, and g) and utilize 2 _ 2 MIMO (two transmitters and two receivers in each device) and OFDM. Recall that MIMO is in essence a spatial-division multiplexing technology that leverages multipath propagation to generate quasi-independent paths in space in order to boost the capacity of the system.

In addition to new wireless LANs, some other 802.11 working groups are focusing on specific issues of general wireless LANs. For example, 802.11c and 802.11d work on wireless switching that enables extension of wireless LANs, while 802.11e emphasizes providing QoS support at the MAC layer for audio and video services. Probably the most notable one is 802.11i, the new security mechanism to replace WEP and intermediate WPA. HIPERLAN is a wireless standard developed by ETSI. HIPERLAN version 1 offers up to 10 Mbps of data rate within a range of 50 m, targeting the wireless home networking market.

HIPERLAN version 2 was actually codeveloped with 802.11a. As a result, HIPERLAN/2 uses the 5-GHz UNNI band and provides data rates up to 54 Mbps. An interesting component of the HIPERLAN/2 is the so-called convergence layer defined in its protocol stack. The convergence layer unifies the data-link layer (data-link control layer in HIPERLAN terminology) functionality of various wireless access technologies and provides a unified interface and services to the network layer. This enables a HIPERLAN/2 node to interconnect with heterogeneous networks such as UMTS and the Internet. The standard specifies a cellbased convergence layer for ATM networks and a packet-based convergence layer for general packet-switching networks.

Source of Information : Elsevier Wireless Networking Complete 2010


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