Analysis: Fixed Wireless

If you'e ever had to link multiple local buildings, you know the budgetary pain interconnection fees can inflict--you may pay tens of thousands of dollars per month just to span a few miles. There's got to be a better way, right? If you're lucky, you can take advantage of new options like metro Ethernet, which we discussed in "The Qwest for Last-Mile Connectivity". If you're really lucky, the city or county will provide right-of-way access to run fiber. But let's face it: Most of us have to make our own luck. Enter today's fixed-wireless systems. This low-profile technology is growing at a decent clip: The point-to-point wireless market is expected to reach $7 billion by 2009, up from $4 billion in 2004, according to analyst group Visant Strategies.

We've always seen P2P (point-to-point) wireless as having attractive ROI, and products have evolved considerably since the first time we tested them--back in 2002, most were little more than 802.11a radios with high gain, directional antennas. Now, licensed and unlicensed microwave and FSO (Free Space Optical) systems from a raft of vendors compete to fill local interconnection needs. QoS (quality of service) means you can route voice over IP (VoIP) over P2P links for even more savings.

Organizations with critical systems will find fixed wireless a cost-effective backup to a wired infrastructure, especially since security capabilities have improved. Most vendors offer the ability to encrypt data, some using AES (Advanced Encryption Standard).

No line of sight? Alternate point-to-multipoint and mesh topologies can bring fixed wireless to more locations, but beware: Running afoul of the FCC can cause more than your wardrobe to malfunction.

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Neither Snow, Nor Wind ... Well, Maybe Wind

Last time we evaluated fixed wireless systems, in summer 2005, we set up an outdoor test bed at our University of Florida Real-World Labs®--just in time for Hurricane Dennis (see "Bridging the No Cable Divide"). After scrambling to haul the gear off roofs to avoid the tropical-storm-force winds predicted by the National Weather Service, we made an appointment to have our collective head examined.

This time out, in the depths of winter, we decided to connect fixed wireless products from Alvarion, Motorola and Proxim Wireless in our Syracuse University Real-World Labs® using a combination of variable and fixed attenuators to simulate distance; see our testing scenario, on page 60, for more details. Yes, removing obstructions, including trees and precipitation, means we couldn't see how the great outdoors influenced performance. On the flip side, eliminating extraneous environmental factors simplified troubleshooting--when we had problems with a radio, the vendor couldn't blame Mother Nature.

We were impressed by what we saw. Actual throughput generally equaled 70 percent to 80 percent of the products' advertised data rate; this compares favorably to 802.11's throughput, which is only 40 percent to 50 percent of the advertised data rate, mostly due to overhead in the protocol. Performance ran the gamut, from 23.5 Mbps (Proxim Tsunami MP.11) on the low end to 277.5 Mbps (Motorola PTP 600) on the high end, not surprising considering each caters to a different market segment.

Latency was variable, from 0.6 milliseconds to 14 milliseconds. We were pleasantly surprised at the sub-millisecond latency times offered by the higher-end Tsumani.GX 90, Tsunami.GX 200 and Motorola PTP 600 systems, which use more complex modulation schemes and radios with higher receive sensitivity to achieve better performance. Because the products support 802.1Q VLAN trunking, enterprises could use them to extend a LAN to another building to tie a branch office to a main office or to assign a remote location to its own subnet.DIAL 1-800-P2P4SAVINGS

We've always believed that fixed wireless could deliver fast ROI when implemented in the right setting. Costs vary depending on requirements; a simple link of two wireless access points set in bridging mode and connected to directional antennas might cost $1,500 to build and provide 15 Mbps to 20 Mbps of throughput. The same link with high-gain antennas and purpose-built radios can provide several hundred megabits of throughput for $20,000. Distances theoretically can reach 30 miles, though going beyond a few miles raises performance challenges.

Of course, pricing for connectivity on the metro ring is highly variable, based on region and building location (see "E-LAN Cost Comparison" in the gallery )Keep in mind that you'll lower operational costs in favor of a one-time capital expenditure and be able to forego telecom service provider support. While operational costs for fixed wireless installations aren't zero, they are much lower than competing leased-lines. IT will be able to handle day-to-day maintenance, while a support contract from a system integrator or the wireless vendor covers larger issues.

But How's The Quality?

Need more justification for fixed wireless? We decided to see whether routing VoIP calls across our fixed wireless links exacted a cost in quality.We set up a VoIP testing gambit that involved sending 12 simultaneous voice calls across the P2P links--when the links were already at full capacity. All three systems effectively prioritized the voice traffic over the background traffic, confirming that you do benefit if you select a system with the appropriate QoS capabilities.

Different radios supported different QoS types, ToS (type of service) or DSCP (DiffServ)--which define QoS at Layer 3 in an IP packet's header--or 802.1p, which defines QoS at Layer 2 in a three-bit user priority field added to an 802.1Q header. ToS and DSCP packets were tagged at the application layer using Chariot, while 802.1p QoS tagging was defined in our Cisco switches on a per-port basis. We tagged all traffic before it entered the radios.

Alvarion supports ToS or DSCP, while Motorola is limited to 802.1p tagging; its devices can also prioritize TDM traffic over IP traffic. Proxim's MP.11 radio offered the widest array of QoS options, including ToS, DSCP and 802.1p. Oddly enough, though, its high-end Tsunami.GX radios offer no QoS. Proxim says the GX-series devices are meant to act as transparent bridges; traffic would be relayed across the link in a FIFO (first in, first out) manner, with QoS handled by the switch or router connected to the radio. We were surprised at this lack of native prioritization--after all, whenever the input into a device is greater than its ability to transport traffic, QoS is needed, and the GX offers both Ethernet and T1/E1 interfaces. See more on our VoIP results.

Unlicensed And Licensed And Fso, Oh My

Saying "fixed wireless" is a lot like saying "motor vehicle." It's a broad term that encompasses a variety of more clearly defined segments.Licensed microwave systems, available from companies like Alcatel, BridgeWave, Ceragon Networks, DragonWave, Gigabeam and Harris Stratex Networks, operate in frequencies that range from 11 GHz to 95 GHz with licenses from the FCC. Fortunately, because of the low power and limited propagation of a typical fixed wireless installation, the licensing process is relatively painless, albeit not speedy. Depending on frequency, a license could be approved within 48 hours of submission; however, it takes 30 days before it can be used for your organization. During this waiting period the FCC issues a "Prior Coordination Notice" that gives other license holders in your area a chance to contest issuance based on overlap. Once approved, licenses are renewable and good for 10 years.

Licensed systems, though expensive, offer some of the best fixed wireless data performance, with throughput ranging from 50 Mbps to 10 Gbps. And, because your organization "owns" the spectrum for that installation, you're immune from interference posed by other installations. On the downside, if you need a link up within a week, the lengthy licensing process will be a roadblock.

FSO (Free Space Optical) systems are available from Canon USA, LaserBit Communications, LightPointe Communications and other vendors. Whereas fixed microwave installations operate in various parts of the traditional RF spectrum, FSO uses infrared or visible light for communications. FSO can offer high performance (up to 1.25 Gbps) over short distances (generally under one mile), and these links are extremely hard to intercept and thus very secure. However, FSO can be adversely affected by heavy fog or other atmospheric conditions.

Finally, license-free microwave systems run the gamut in range, price and quality. In the United States, the FCC has set aside numerous frequency bands for fixed wireless, including the 2.4-GHz and 5-GHz ISM bands, most often associated with Wi-Fi, as well as the 24-GHz and 60-GHz bands. These higher bands are relatively immune to interference compared with the 2.4-GHz and 5-GHz bands, which are becoming increasingly cluttered.

A 24-GHz system, like those available from DragonWave, offer up to 500 Mbps of throughput, while 60 GHz, or millimeter microwave, can provide speeds of 1 Gbps to 1.5 Gbps and are available from vendors like Terabeam and Ceragon. These 60-GHz links are unique in that radio signals are actually absorbed by oxygen in the atmosphere, which attenuates the signal. This offers advantages and disadvantages: On one hand, 60-GHz links are better isolated from interference because absorption by oxygen keeps the signal from straying. However, this also limits the distance a 60-GHz link can cover. Moreover, 60-GHz systems can be negatively impacted by rain. Depending on your geographic location, expect maximum distances of .5 to 1.5 miles.The 5-GHz market is probably the broadest in terms of vendor participation and is where we focused our testing. The lower frequency lets systems span longer distances than any other license-free choice--in theory, over 30 miles, though this depends on the environmental aspects of the installation, the particular radio used, power output and antennas. Throughput can reach as high as 300 Mbps. The 5-GHz band especially is becoming a victim of its own popularity: Because it's used for so many apps, these links can be very vulnerable to interference.

Alternative Topologies

While fixed wireless has enjoyed increasing success in recent years, with great ROI reward comes risk: If you need absolute reliability and you're not deploying on your own campus, a wired link may still be the way to go, with P2P wireless as backup. Long links--say, those spanning more than a couple of miles--will vary in performance depending on conditions; weather, trees, new construction--all can hinder quality. Say a new building goes up midway between two of your sites, causing performance issues. Tough luck! It's time to re-examine your installation and, possibly, rework it.

Still, fixed-wireless engineers we spoke with said that with good site planning and knowledge of environmental conditions, installations can be created with uptime rivaling wired connections--four or five nines reliability. In networks where interference is a concern, providers may set up a ring of radios, reminiscent of a Sonet ring.

That brings us to architectures.If you're trying to link multiple sites on a campus wirelessly, point-to-point can be complex. Each building must be tied together with its own, independent radio link. While this is certainly an option, there are alternatives.

A point-to-multipoint system, available from a variety of vendors including Alvarion and Proxim, is one way of serving multiple sites wirelessly in a star topology. Generally, a central building with a radio and an omindirectional antenna radiates a radio signal out equally in all directions, as opposed to a directional antenna that focuses a signal in one direction. Remote sites connect to the central location using directional antennas. Point-to-multipoint installations can be cost-effective, as you buy only one central radio to serve multiple buildings.

However, while a point-to-multipoint architecture can simplify deployment, it does so at the detriment of distance. For radio installations, the FCC places limitations on EIRP (effective isotropic radiated power). EIRP defines the amount of gain from both an antenna and an amplifier, emanating from the point of an antenna before the signal is attenuated as it interacts with the environment.

The FCC's restrictions on EIRP for point-to-multipoint links are much more restrictive than equivalent P2P links: A point-to-point link operating in the UNII-3 band (5.725 GHz to 5.825 GHz) can be up to 62 times more powerful than a point-to-multipoint link in the same band, simply due to EIRP restrictions. This limits the distance between sites in a point-to-multipoint topology compared with a similar P2P installation.

Another alternative is a mesh, where every radio can transmit to every other radio. This fault-tolerant topology has proved popular in the metro Wi-Fi market, where it provides wireless backhaul for access points, and in other situations where extremely reliable, fault-tolerant links are required. Not surprisingly, the cost-to-performance ratio can be high compared with equivalent P2P. And, mesh networks like those from BelAir Networks, Cisco, Motorola and Tropos Networks generally offer lower performance than competing P2P setups.Given advances in design, security and QoS, fixed wireless is a viable option that more enterprises should consider instead of typical leased lines when connecting buildings in close proximity. Of course, no P2P system can replace a 10-Gbps Ethernet link, or other high-bandwidth fiber connections. But the cost savings just might make that a worthwhile trade-off. NWC Reports: Point-to-point Wireless

To qualify for our point-to-point wireless review, radios had to operate in the 2.4-GHz, 5.3-GHz or 5.8-GHz range and support network traffic management, including honoring QoS (802.1p) and VLAN tags (802.1Q); operate at full data rates at a distance of at least half a mile, with minimum throughput of 10 Mbps; and provide an Ethernet interface that supports a minimum of full-duplex, 100-Mbps connectivity. We also wanted SNMP management/trapping and Layer 2 encryption using 3DES or AES. Radios also had to include support for external antenna connectors.

PARTICIPATING VENDORS

Alvarion, Motorola, Proxim Wireless

TESTING SCENARIOTo evaluate P2P wireless radios we ran the following tests:

RESULTS

Alvarion sent its BreezeNet B-100 radio. Motorola sent its PTP 600 radios, formerly known as the Orthogon Spectra 300, which won our Editor's Choice in the 2005 review. Not to be outdone, Proxim sent us three products: its entry-level Tsunami MP.11, along with two models from its higher-end Tsunami.GX series, the Tsunami.GX 90 and the Tsunami.GX 200.

Each system worked as advertised, with throughput results between 70 percent and 80 percent of maximum data performance. With the exception of a few glitches with Motorola's PTP 600 and Proxim's MP-11, all radios passed our tests with ease, and all make our shortlist.

Alvarion's BreezeNet B-100 radios solidly occupied the midtier of our lineup. They're ideal for companies needing near-100-Mbps Ethernet connections at an affordable price. Although we were a little disappointed that Alvarion offered no Web GUI, its menu-driven CLI was easy enough to use for configuration and management.

Motorola's PTP 600 radios were the best performers, suitable for backing up or supplanting optical fiber connections. The PTP 600 is also well-suited for installations that don't have perfect line of sight; because they use two antennas--one horizontally polarized, one vertically polarized--the radios can overcome environmental factors better than single-antenna solutions. PTP 600s don't come cheap, however.Proxim's Tsunami.GX radios were the simplest to set up and are well-suited for enterprises looking for easy-to-deploy backup links. We simply plugged them in and, without any configuration, were able to start sending data. However, we question Proxim's decision not to include QoS support--while you can install a switch or router that will handle QoS at either end of the wireless link, we wonder how well the Tsunami.GXs will perform for organizations that handle a lot of time-sensitive traffic, like VoIP or video. The MP.11 radios also performed well and may be a valid option for businesses looking for a system that's more robust than deploying Wi-Fi APs configured as bridges.

Sean Ginevan is a technology analyst with the Center for Emerging Network Technologies at Syracuse University. Write to him at [email protected].