Fixed wireless links mobile nets

While ad hoc and mesh networks are focused on the interconnection of independent nodes, at some stage these networks will need to connect to the wired Internet. But what is the most efficient connection strategy for high-speed wireless LANs that are scattered throughout a local site? For this problem, fixed wireless systems have shown promise.

The WLAN market is experiencing tremendous growth in the indoor LAN segment and is also expected to continue its growth outdoors. The FCC allocations in the United States have focused on indoor and outdoor applications in order to help foster this growth. The incredibly attractive pricing of 802.11 end-user equipment, and its inclusion in current and next-generation business and entertainment devices, will fuel the mass deployment of this technology.

Construction of LANs using this technology is highly attractive since intra-LAN roaming is possible and the costs and problems associated with moves, additions and changes are reduced or nearly eliminated. In many networks, roamable computing is an essential application. Examples of these are hospitals, warehouses, universities, large enterprises and distributed government/municipal networks.

Newer 802.11 standards, namely 802.11g and 802.11a, create the basis for very high-speed wireless Internet Protocol (IP) data networks, which are highly attractive for LAN applications. This technology is readily able to support approximately 50-Mbit-per-sector, multisectored, multibasestation networks, the cumulative result of which can be many hundreds of megabits of traffic. Routine applications such as file sharing/serving, e-mail, videoconferencing and gaming are supported by the speed and flexibility of these networks.

Ease of development

High-speed fixed wireless connectivity is an increasingly popular way to link WLAN segments thanks to the low cost, ease of deployment and avoidance of transport service contracts. When considering the use of fixed wireless, a number of system design elements need to be addressed in order to define the best overall approach. Several candidate licensed and unlicensed RF technologies can be employed to create a campus environment in which numerous WLAN-enabled buildings are interconnected to form a larger, extended LAN structure that can cost-effectively support a wide range of LAN applications, including roaming.

In many instances, multiple buildings form a functional entity in which the people in the various sites need to function as if they are under one roof. Modern, IP-based LANs have gone a long way toward collapsing the distance between communicating locations. Wireless LANs have taken this a step further in that roaming users can be connected anywhere in the coverage area, indoors or out.

Examples of campus infrastructure include multibuilding corporate complexes; university and college campuses; schools within a school board district; metropolitan (municipal) governments operating out of multiple buildings; hospitals; utilities; and metro disaster-recovery networks.

Although most LANs are constructed with wired infrastructure today, there is growing use of wireless LANs in order to "unwire" buildings and to support roaming of end-user terminals. Additionally, a number of the campus types may also include outdoor coverage, which is normally implemented using wireless technology.

Construction of a multibuilding virtual-LAN environment requires the interconnection of LAN segments between the various building sites. Since this virtual LAN must support numerous peer-to-peer applications within the LAN, bandwidth demands tend to be relatively high (as compared with Internet access links from a LAN).

Use of unlicensed wireless technology in the lowest access layer is highly attractive since the cost of the end-user equipment can be very low. Examples of this technology can be readily seen in the form of IEEE 802.11-based technologies now entering widespread use. Unlicensed wireless has the drawback of being prone to interference, particularly as user density grows. Interference in this layer of the network is generally reconciled through self-frequency planning and movement of the user's end-terminal.

Wireless technology in the fixed high-speed intrabuilding links cannot stand the same degree of interference since these links cannot "move" in order to rectify any link quality problems. Additionally, if these links fail, then the mission of the campus virtual LAN fails, since the various building sites are no longer connected to one another. Generally, debugging interference and implementing fixes is a challenging and time-consuming process.

A number of wireless-technology choices are available for interconnecting the buildings within a campus LAN. These tend to boil down to the following basic categories:

• Free-air optics

• 60-GHz, 5.8-GHz and 24-GHz unlicensed

• Licensed 18 to 38 GHz

Free-air optics (FAO) technology is highly attractive for short-haul applications (< 500 meters). Being unlicensed and optical is a reasonably good combination of attributes when considering interference immunity. The short range of operation is generally driven by FAO's ability to operate reliably in various weather conditions. Bandwidths of 2.5 Gbits/second are available from various manufacturers. For most LAN applications, relying primarily on 10/100-Mbit/s Ethernet technologies, this data rate performances is higher than what's usually required.

The 60-GHz unlicensed technology has recently surfaced in the United States, where a large piece of spectrum has been allocated for this purpose. In this frequency range, severe atmospheric and rain absorption attributes limit the range of this technology to the same order as FAO devices. Bandwidths of up to 1 Gbit/s are available. As with FAO technology, widespread use of 10/100-Mbit/s LAN technology tends to push LAN interconnects toward the 100-Mbit/s range.

Unlicensed wireless technology at 5.8 GHz can be based on IEEE 802.11a technology. Since this technology is also applied to wireless LANs, low-cost ASIC technology is available. In some cases, this has been exploited to realize low-cost fixed wireless point-to-point link products. The 802.11a technology provides maximum speeds of 54 Mbits/s using a half-duplex TDD over-air protocol. This works out to approximately 20 Mbits/s (or less) full-duplex.

Perils of interference

The central drawback of 5.8-GHz technology is the potential presence of interference. In addition to other point-to-point, multipoint and WLAN sources of interference, this band is available for a wide variety of high-power, unlicensed applications. Randomly deployed commercial electronics, such as high-power home "handi-phones," are one such example. These types of potential interferers can equally affect links in larger cities and rural areas.

As a result, 5.8 GHz for fixed applications must be deployed with the full recognition of the perils of interference. When such a link becomes interfered with, LAN IT personnel are often ill-equipped to debug and rectify the problems due to the specialized skills and equipment needed to do so. Also, once an interference source has been detected and isolated, the matter of resolution still remains. Aside from "good neighbor" motivation, there may be very little incentive for the source of interference to cease or modify operation.

Wireless systems at 5.8 GHz employing 802.11 technology may also use dynamic modulation. When link conditions allow-short range, no multipath, no interference-they operate on the highest modulation that supports the highest transmission bandwidths. When interference is present, the link can cut the speed to improve link integrity through reduced operating signal-to-noise ratio. One drawback of "downspeeding" is that certain types of applications can deteriorate in performance. Time-sensitive and bandwidth-sensitive applications, such as voice-over-IP and videoconferencing, may be adversely affected, for example.

Interference immunity

A new band available for unlicensed use in the United States is the 24-GHz ISM allocation. Aside from narrow-beam point-to-point links, this band is not available for broad-beam, high-power uses. These limitations provide a significant interference immunity for users, thus making it a highly attractive solution. Full-duplex bandwidths in the range of 50 to 250 Mbits/s are available from products in this allocation.

The traditional 18- to 38-GHz range provides numerous licensed bands in which wireless links can be operated without any interference. These bands therefore provide the ultimate reliability and insurance of availability of the link. Different frequency bands are available in different countries, generally carrying licensing fees of a few hundred dollars a year by the national regulator.

In the 18- to 38-GHz range, numerous wireless products are available from a variety of suppliers. Data rate ranges of a few megabits to 311 Mbits/s are available in TDM, ATM, Sonet and Ethernet formats.

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