Fixed Wireless

Fixed wireless can help you link sites separated by physical obstacles or get your network back online after a cable cut. We examine technologies and standards, including Wi-Fi, WiMAX and

September 9, 2005

13 Min Read
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The biggest buzzword in fixed wireless today is WiMAX. Despite some backers' heated promises to provide Internet access for your toothbrush at 400 miles, don't fall for the hype. When WiMAX begins to see greater deployment it may have an impact on the fixed wireless landscape. More on that later. For now, let's talk about some of the fixed wireless technologies that have been supplying network transport for some time.

Wi-Fi in the Backbone

Nearly 70 percent of network pros we polled currently running fixed wireless report that 802.11 products form the backbone of their networks. Two types of 802.11--a and g--provide a theoretical maximum of 54 Mbps and are available at a low cost from many distributors. An 802.11a/g bridge can serve as a link between the network main node and a remote location, and specialized antennas with high levels of gain, such as a yagi or parabolic dish, make the signal effectively stronger than if it had been transmitted from a standard patch or "rubber duck" antenna (the stubby rubber antenna you see on walkie-talkies and most Wi-Fi access points). As readily available, low-cost and highly adaptable as these systems are, why would anyone use anything else?

A few reasons. 802.11a/g products' limits in throughput and security may lead IT to consider other options for bridging and MAN (metropolitan-area networking) deployments. Although 45 percent of readers polled say they are most concerned about security, compared with 27 percent citing performance, the most significant limitation is that the protocols involved were created to support individual users in a point-to-multipoint topology, resulting in throughput levels that are, at best, only half the data rate because of network overhead, lack of signal strength, and various contention and interference issues. Wi-Fi links won't reach 54 Mbps and may miss that mark by a wide margin, especially if the endpoints are using noise-filled channels trying to avoid interference in the crowded 2.4-GHz band (for 802.11g) or the (as yet) less crowded 5-GHz band (802.11a). Because of less potential for interference and many more channels, 5 GHz is usually a better choice than 2.4 GHz for these links.In addition, the aforementioned wide availability means attackers can get their hands on a range of Wi-Fi gear, along with testing, surveying and security tools that will help them find and dissect your 802.11a/g signals. Transmissions can be made more secure through encryption, though we recommend going beyond the security methods contained within normal 802.11a/g implementations by including, for example, strategies such as 128-bit AES encryption. Although preshared keys are not acceptable for Wi-Fi because of key distribution issues, this method, implemented through third-party software systems, is fine for fixed wireless deployments consisting of a small number of nodes.

Still, if your demands are relatively low, say, 25 Mbps or less, and you don't need iron-clad quality of service or security, an 802.11a or g system may be the most cost-effective way to extend a wireless network. If your requirements are more rigorous, throughput in excess of 100 Mbps, interference reduction and consistent performance are available in systems developed for point-to-point purposes.

Beyond Wi-Fi

There are RF systems within the spectrum occupied by 802.11a/g that don't use the 802.11 protocols, modulations or channel allocations (see "RFing Without a License"), but crowded conditions mean enterprise IT groups often choose to acquire a licensed piece of spectrum. The FCC has pieces of RF spectrum available in a number of different bands, from 1.7 GHz up, including 10.5, 25, 26, 31, 38, and 39 GHz, for which they require users to be licensed to ensure that apps in a particular area don't interfere with one another.

Licensed spectrum offers another advantage in that the radiated power you may use can result in greater effective distances. Licenses do add to the cost of installation and maintenance, but prices are generally manageable when compared with the cost of obtaining right of way to run fiber across several miles or acquiring a DS-3, OC-3 or faster leased line. The throughput provided by these licensed services makes it possible to use RF as the carrier for a Sonet ring or ATM cloud. If an ATM layer is added over Sonet, quality of service comparable to wired metropolitan networks is possible.Moving further up the spectrum map, lasers, or "wireless fiber," can provide high-bandwidth connections across distances up to a couple of miles. Laser network links have some advantages over radio when it comes to security--it's almost impossible to intercept the laser's beam without disrupting the link--and resistance to interference. Where RF networking may take place in crowded bands and is subject to interference from man-made sources, such as the harmonics of signals in other bands or wide-band noise from motors, and natural causes, from sunspot activity to lightning, laser network points are typically problematic only for sites that have them receiving from the direction of the rising or setting sun.

Organizations looking to standardize wireless networking technologies across national boundaries can take advantage of an additional property of laser emissions: Although RF signals are regulated by every government, using rules that vary from country to country, light is not under the same restrictions. Lasers (and their emitted power) are regulated, but the frequency of the light isn't, so FSO acceptable in one country generally is acceptable in all. Oddly enough, lasers aren't certified by the FCC, but by the FDA. Virtually all lasers sold also have the EU seal of approval.

FSO handles interference better than RF, but laser is more sensitive to absorption caused by some weather events. Both RF and FSO can be affected by heavy rain, but laser networks can be shut down by heavy fog, smoke or snowfall. For infrared lasers, the rough rule of thumb is that a link can be maintained through two times the range of visibility in smoke or fog.

In addition, some people aren't comfortable around lasers, especially those they can't see. One common worry is the danger of looking into the beam. It almost goes without saying that lasers must be mounted in such a way as to make it impossible for someone to inadvertently intercept the beam with his or her eye. You should also be aware that lasers' tight beam patterns mean their aiming points can be affected by the gentle swaying of tall buildings. Some laser systems have autotracking mirrors to compensate for this, but mounting-point stability is still a concern. A new generation of systems combine FSO with RF backup, providing a wide-beam alternative if conditions make the FSO link unstable or unusable.

Fabric of our NetworkAll the technologies we've discussed have been in the context of single-point to single-point and, in laser's case, line-of-sight network links. For large campuses or metropolitan areas, however, single-point to multipoint, or multipoint to multipoint (mesh) may be more appropriate. We've seen interest in mesh topologies that provide a large area with coverage over wireless network points connected to a building infrastructure or 802.11a/g access points. More robust and higher-capacity mesh deployments use licensed-spectrum FHSS (frequency-hopping spread-spectrum) or proprietary transmission-scheme radios for the mesh.

Whatever the technology used, a mesh provides higher reliability because each point in the mesh has multiple connections to the rest of the network. Meshes are most often talked about in connection to a metropolitan deployment, and many assume that the mesh fabric provides connectivity to end-users, but mesh is a backbone technology: End users connect to a single access point or wall plate, though in most cases, a single node will handle both access and backhaul. (For more on implementing mesh technology,click here.)

Finally, WiMAX

At last we come to WiMAX, the industry consortium that is working to keep "wireless broadband" products adhering to the 802.16 standards interoperable, in much the same way that the Wi-Fi consortium has worked with products built on the 802.11 standard. There are several pieces of 802.16, and therefore of WiMAX, with each designed for a specific purpose.

According to the plans developed by the industry players readying WiMAX products, WiMAX and its flexible capabilities can be used to provide three broad types of access: Point-to-point fixed wireless backbone technology; a "last mile" delivery mechanism for Internet access to residential and business users; and a mobile wireless broadband service for roaming individual users. Proprietary technologies have long been available to provide these capabilities, but the allure of WiMAX is standardization, interoperability and, hopefully, lower cost.WiMAX will first be rolled out as a backbone and last-mile technology with maximum theoretical throughput of 75 Mbps, though real-world implementations will probably see a maximum in the 45-Mbps range. This is very good when compared with 802.11a/g, but it's considerably less than the throughput provided by dedicated point-to-point wireless systems available today. As a replacement for an OC-3 carrier, we expect WiMAX to be unacceptable. As the backup for an existing carrier or the link to a field office, it could be satisfactory. As the last mile to a residence or small office to replace ADSL or a cable modem, it should be a marked improvement over the status quo, with the caveat that because this is a multipoint technology, each user will not realize the maximum throughput.

The first pieces of WiMAX coming to market will be the fixed wireless, or wireless broadband, components. Like all the pieces of WiMAX, these will use Coded Orthogonal Frequency Division Multiplexing (COFDM) modulation, a variation on the modulation scheme used in 802.11. Designed to be a tool of carrier-class, highly reliable network services, WiMAX will make use of both unlicensed and licensed spectrum in the 2.5-, 3.5- and 5.8-GHz bands, and companies may find themselves implementing WiMAX network links for employee or remote-location access, buying access services based on WiMAX but provided by a third party, or both, depending on their location and circumstances. At least one unlicensed band and the licensed band should be available in most countries or regulatory jurisdictions.

WiMAX will break up its piece of the spectrum into channels, in much the same way that Wi-Fi does, but there are a couple of critical differences in the way the channels will be used. First, COFDM will allow the WiMAX frequency band to be broken into 256 signal carriers, with data distributed across multiple carriers simultaneously. Next, WiMAX will collect the carriers into channels, in much the same way that 802.11 is divided into channels. Unlike 802.11, though, channels will be able to grow or shrink in bandwidth and allocation (from uplink to downlink), allowing more bandwidth and, therefore, higher throughput in particular channels. The consortium is calling this capability SOFDMA (Scalable OFDM Multi-Access).

WiMAX could be an important standard and step forward in wireless networking deployment. Its main improvements, though, come when compared with 802.11 a/b/g technologies. If a network architecture calls for a true backbone with 100-MHz to 1-GHz throughput, existing RF and FSO systems will continue to be the best path for the foreseeable future.

We sent an electronic poll to a portion of our readership and received 1,005 responses. We analyzed your input in several key areas, beginning with whether you've deployed fixed wireless, and for those who have, the speeds, technologies and deployment strategies in use. For those who haven't yet launched fixed wireless projects, we asked about the factors keeping a system out of their organizations.Nearly two-thirds of respondents have deployed fixed wireless or have a system in the planning stage. Of those who've deployed fixed wireless, 91 percent say at least one of the links in place uses 802.11 a/b/g technology. This says a great deal about two things: the performance you feel is required across the fixed wireless link, and the budget you have to make that link happen. Clearly, neither is quite as high as many might like. Nineteen percent say they have some proprietary wireless fixed point system deployed, while 7 percent use FSO. These last two numbers are most intriguing when seen in light of the answers we received on performance, and your happiness with such.

More than 70 percent of our respondents say their fixed wireless deployments transmit at 54 Mbps or less. Add in those who say their fastest fixed wireless link is at 100 Mbps or less, and the percentage rises to 88. Regarding the adequacy of today's fixed wireless in terms of speed, just over half (54 percent) say current technology is adequate for most organizations. Answers to these two questions suggest many organizations are deploying lower-capacity systems because it's what they can afford or what they feel is available, rather than what they feel is acceptable.

As for significant barriers to fixed wireless adoption, security concerns top the list, with 45 percent. High cost (29 percent) and lack of performance (27 percent) follow. If WiMAX can keep its promise of lower cost through high adoption rates, a boom in fixed wireless could be on the horizon.

Fixed wireless can solve some seemingly unsolvable problems, whether you need to permanently link two sites but face physical obstacles or need a quick way to get your network back online after a cable cut. In "Retire the Wire" we look at the latest trends in fixed wireless, including Wi-Fi and WiMAX and new types of FSO laser and RF systems.

We tested seven fixed wireless systems--FSO devices from Canon, LaserBit Communications, LightPointe and Pav Data Systems, and RF systems from Adtran, BridgeWave Communications and Orthogon Systems--at our University of Florida partner labs during the summer, hoping for adverse weather conditions to stress the systems, and we weren't disappointed. At one point, we had to dismantle all the devices to keep them from being blown away by Hurricane Dennis: When the National Weather Service showed a 40 percent probability of tropical-storm-force winds there, the author and his wife lowered the systems off the roofs of the two test buildings with ropes. Once it was safe, they put them back. After all that, Orthogon's OS-Spectra took our Editor's Choice award thanks to its easy configuration. Canon's FSO Canobeam is a close second and a top choice for those who value performance. Finally, Pav's $8,995 PavLight 155 takes our Best Value award.

Note that unlicensed frequencies aren't limited to 802.11a/g allocations in the 2.4- and 5.4-GHz bands. Both of these are part of ISM (Industrial, Scientific and Medical) bands, which include other frequencies, including parts of 900 MHz. There is also the U-NII (Unlicensed National Information Infrastructure) band. which includes several pieces of the 5-GHz band not occupied by ISM, government and other uses. Beyond these, there are unlicensed possibilities in the 60-, 90- and 120-GHz bands. These higher frequencies allow wider bandwidths for each channel used to communicate between access points, so network transmission speeds can be correspondingly higher. The offsetting limitation is that higher frequencies for transmission are constrained to direct line-of-sight communications. They are also more limited in distance spanned because water vapor in the air can absorb the radio signals. These higher-frequency systems can be used for bridging the gap from one campus building to another but are not candidates for long-distance links.

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