A Field Guide to Wireless LANs for Administrators and Power Users

Another class of wireless devices is based on the fixed (initially) and mobile (eventually[8] ) broadband wireless metropolitan area network (WirelessMAN™) standards being created by the IEEE 802.16 WG. The reason why the IEEE 802.16 standard is referred to as a metropolitan-area standard is that the technology supports inter-node distances of up to 31 miles (50 km), and some of its PHYs do not require line-of-sight in order to operate.

[8] The IEEE 802.16 WG's TG "e" is developing a standard for mobile broadband wireless access (MBWA). There is another MBWA effort under way in the IEEE 802.20 WG.

The initial application of IEEE 802.16 technology will be as a high-speed last-mile access technology, so that a networking company can deliver services over microwaves instead of over wires that it must lease from another company (or over wires that it owns, but had to pay to install). IEEE 802.16 infrastructures could be used to wirelessly interconnect IEEE 802.11 wireless "hotspots," or to provide "T-1"-class[9] access circuits to corporate customers, or as an alternative to using wires to deliver DSL services. In the latter case, it may be possible to provide hundreds of DSL-class access "circuits" from a single base station.

[9] A T-1 circuit operates symmetrically at 1.544 Mbps, with a bi-directional payload capacity of 1.536 Mbps, sufficient to carry 24 digitized voice channels, each running at 64 kbps. A T-1 circuit may be configured to support a "channelized" service, for use as a voice trunk between a company's private branch exchange (PBX) and a telephone company's central office, or as an "unchannelized" service, in which all of the payload capacity is used to carry data. This type of circuit is frequently also referred to as "DS-1" (digital signal level one).

IEEE 802.16 provides integrated support for privacy and QoS in its MAC sublayer protocol, because it is unlikely that a business would purchase a wireless T-1 circuit unless they could be certain that no eavesdroppers could intercept their traffic, and the provider could meet contractually mandated service level agreements. More information on IEEE 802.16 and related technology can be found on the Web at www.wirelessman.org.

There are currently two varieties of IEEE 802.16 standards, the original IEEE 802.16-2001, which operates in the 10 66 GHz RF spectrum (where licenses are required), and the newer IEEE 802.16a-2003 standard, which operates in parts of the 2.5 11 GHz RF spectrum (which supports operation over both licensed and unlicensed portions of this spectrum). The introduction to the IEEE 802.16-2001 specification describes the standard as follows:[10]

[10] Excerpted from IEEE Std. 802.16a-2003, copyright 2003. All rights reserved.

This standard specifies the air interface of a fixed (stationary) point-to-multipoint broadband wireless access system providing multiple services in a wireless metropolitan area network (MAN). The WirelessMAN™ medium access control layer defined here is capable of supporting multiple physical layer specifications optimized for the frequency bands of application. The standard includes a particular physical layer specification applicable to systems operating between 10 and 66 GHz. This 10 66 GHz air interface, based on single-carrier modulation, is known as the WirelessMAN-SC™ air interface. An amendment to this standard, to support 2 11 GHz using an enhanced version of the same basic medium access control layer along with new physical layer specifications, is in development in IEEE-SA Project 802.16a.

One important difference between IEEE 802.11 and IEEE 802.16 devices is that the latter standard operates in licensed spectrum. One of the enablers of the growth of the market for IEEE 802.11 devices has been that they are designed to operate in unlicensed spectrum, which means that the owners of such devices need not get permission from the FCC before turning them on. It is appropriate for a provider of IEEE 802.16-based fixed broadband wireless services to get a license to operate in a given geographical area, since they will want assurance that no other provider will be interfering with them. The IEEE 802.16 WG created a TGb to work on a PHY that could operate in the unlicensed 5 6 GHz band, known as the Wireless High-speed Unlicensed MAN (or, WirelessHUMAN™). Eventually, IEEE 802.16 TGb was merged with IEEE 802.16 TGa, so the finished IEEE 802.16a-2003 standard includes support for unlicensed operation in the 5 6 GHz band, as well as for licensed operation in the 2.5 11 GHz band.

The speed of networks based on IEEE 802.16 depends on several factors, notably channel width and physical distance separating the devices. The further two devices are apart, the more the transmitted power is attenuated (by the inverse square law), which means that more robust modulations must be used to overcome the poor signal-to-noise ratio. Such modulations cannot transmit as many bits per symbol, so a side effect of increasing distance is that the maximum achievable transmission speed decreases (this effect is also present in IEEE 802.11, although on a much shorter distance scale). In particular, devices that are designed to operate in wider RF channels can achieve higher speeds.

Devices based on the IEEE 802.16 standard can use any of three channel widths in the 10 66 GHz band, viz. 20, 25, or 28 MHz. Depending on the selected modulation scheme and the available RF channel size, speeds of 32 44.8 Mbps are achievable with QPSK, the "slowest" modulation scheme; speeds of 64 89.6 Mbps are achievable with the intermediate-speed modulation known as 16-QAM; and the fastest modulation, 64-QAM, can achieve speeds in the range of 96 134.4 Mbps. Table 8-1 summarizes the speeds attainable with the various combinations of modulation and channel width in the 10 66 GHz band. A typical deployment may deliver aggregate network bandwidth on the order of 70 Mbps.

Table 8-1. IEEE 802.16 Speeds and Modulations

Channel Width

Symbol Rate (Megasymbols per second)

QPSK (2 bits per symbol)

16-QAM (4 bits per symbol)

64-QAM (6 bits per symbol)

20 MHz

16

32 Mbps

64 Mbps

96 Mbps

25 MHz

20

40 Mbp

80 Mbps

120 Mbps

28 MHz

22.4

44.8 Mbps

89.6 Mbps

134.4 Mbps

Due to the increasing maturity of WirelessMAN standards and the expected new products that will be based on these standards, a group of vendors has formed an organization to promote interoperable implementations of the technology, known as the Worldwide Interoperability for Microwave Access (WiMAX) Forum, which is logically equivalent to the Wi-Fi Alliance, which provides a similar interoperability testing and promotion function for IEEE 802.11.

The WiMAX Forum (on the Web at www.wimaxforum.org) was announced on April 15, 2002, and will certify IEEE 802.16 products throughout the entire 2.5 66 GHz range (i.e., covering products based on either the IEEE 802.16-2001 or IEEE 802.16a-2003 standards). The full text of the press release announcing the WiMAX™ Forum is provided here (in the time since the four companies listed here formed it, the WiMAX™ Forum membership has increased to 26 members):

The WiMAX™ Forum is formed to promote interoperability standards for broadband wireless access. Executives from Nokia, Ensemble Communications, Harris Corporation, and Crosspan resolve to support interoperability standards.

ANTIBE, FRANCE --April 15, 2002-- Representing a significant step forward in catalyzing the growth and future viability of the global fixed broadband wireless market, leading equipment and component manufacturers have formed the Worldwide Interoperability for Microwave Access (WiMAX™) Forum. As a non-profit organization, the objective of WiMAX™ is to promote wide-scale deployments of point-to-multipoint networks operating between 2.5 and 66 GHz by leveraging new global consensus standards and certifying the interoperability of various products and technologies from multiple manufacturers.

As has been proven many times in the past, true market success of a particular technology can only be realized through a global standard and a concerted effort to ensure the interoperability of multi-vendor products which in turn result in economies of scale and, ultimately, a healthy competitive environment.

The recent completion and formal approval of IEEE® Standard 802.16™ ("Air Interface for Fixed Broadband Wireless Access Systems"), which defines the IEEE 802.16 WirelessMAN air interface, was achieved through successful collaboration of many individuals and companies in the wireless industry. Together, they produced the industry's first broadband wireless access standard to be published by an accredited standards body.

As the next step in widespread adoption and use of the WirelessMAN standard, WiMAX™ has been created to further define and commercialize this global specification.

Related

Категории