The Point-to-Point of Wireless Bridging

We tested six enterprise-class fixed-access systems running at 5 GHz, where bandwidth is plentiful. Proxim's QuickBridge 60 eked out our Editor's Choice.

November 18, 2002

22 Min Read
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Point-to-Point Performanceclick to enlarge

We cast a wide net for our tests, inviting a dozen vendors of wireless point-to-point systems to send their wares to our Syracuse (N.Y.) University Real-World Labs®. Some do not have 5-GHz units at all, while some did not have devices ready for test, but we got products from six big names: Airaya Corp., BitRage, Proxim, RadioLAN, Wi-LAN and Young Design.

Several of the products we tested had just been released. And, as is typical of many emerging technologies, the performance, range and overall capabilities varied considerably, making head-to-head comparisons challenging. If product A offers twice the performance of product B but at half the range and double the price, does that make product A better? The answer, of course, depends on your needs.

The Basics

the products we tested work as wireless Layer 2 bridges. In an age where Layer 3 switches and routers dominate the market, you may wonder why these products are still running at Layer 2. The positive spin is that with a point-to-point system, you don't really need the sophisticated traffic-management capabilities of Layer 3 devices. In addition, many point-to-point systems will be linked to backbone routers anyway. Nonetheless, we would like more Layer 3 control--say, to filter certain protocols to optimize performance.

FCC Regulations for Select Unlicensed Radio Bandsclick to enlarge

Although all the systems we tested are unlicensed and therefore must adhere to rules defined by the FCC and other international regulatory bodies, the underlying radios and modulation systems varied considerably from product to product in our tests. In most cases, it's a trade-off between price and performance. Packing more bits into each clock cycle requires more sophisticated radio technology--and you'll pay for that luxury. Because regulations vary by sub-band at 5 GHz, you'll get longer range from products that operate in the 5.8-GHz UNII-3 sub-band. The chart, "FCC Regulations for Select Unlicensed Radio Bands", shows some of the key regulations in the most commonly used unlicensed bands. We expect to see increased availability of low-cost chipsets that operate in the UNII-3 band, a development that will push the price-performance ratio lower.An interesting trend visible in some of these products involves integrating the antenna and modem into a single weatherproof enclosure. This makes installation easier and increases effective system range compared with older designs by eliminating RF cabling, which is a source of significant signal loss. An alternative yielding similar benefits is to collocate the modem and antenna on a single antenna mast; this strategy allows flexibility of antenna selection--and even more range--but is more complex.So just how much geography can you cover with these systems? The vendors whose products we tested claim ranges from 1 to 15 miles, and while more expensive offerings are available from some vendors that will extend range even farther, the products we tested hit the sweet spot for most organizations. For more information on range, see "How Far Can You Go?".

What about speed? The short answer is you're likely covered for most mainstream business applications. The products we tested had throughputs ranging from 7 Mbps to 78 Mbps. Even the slowest was faster than most inexpensive 2.4-GHz bridges. And if this isn't fast enough, Proxim offers a rather pricey unlicensed bridge that has a data rate of 480 Mbps.

Overall, we were disappointed in the devices' management and monitoring capabilities, which we consider important for mission-critical deployments. Some products include remote-status monitoring tools, system logs and firmware upgrade utilities. Others include none of the above. Again, you can enhance monitoring if you tie these systems directly to a backbone router, but we'd like more management intelligence at the units.

5-GHZ Ehternet Bridge Featuresclick to enlarge

Security is always a top concern for wireless implementations, but we don't consider it nearly as big an issue with fixed wireless as with wireless LANs. First, most of these products use proprietary radio-signaling schemes, and in this case, proprietary is good. Second, because these are point-to-point systems employing highly directional antennas, the most viable attack is literally a man-in-the-middle attack, though the man would need to be suspended in mid-air to intercept the signals. And if that's not enough, most vendors support some kind of encryption system.

With these caveats in mind, Proxim's QuickBridge 60 narrowly edged out Wi-LAN's AWE 120-58 as our Editor's Choice. Proxim's solution has just the right mix of performance, range, ease of installation and management functionality. We like the overall functionality of Wi-LAN's product, and if you can live with sub-10 Mbps performance, it's an excellent choice. RadioLAN offers nice features and the lowest price of any product tested. We were also intrigued by Airaya's innovative AI108, the first point-to-point bridge to be built around a low-cost commodity 5-GHz WLAN chipset.Proxim Corp. Tsunami QuickBridge 60 | Wi-LAN AWE 120-58 Ultima3 RD | RadioLAN Campus BridgeLink-II-P25 | BitRage CR45-A-53 DS3 Radio and IU45-E AC with Ethernet Bridge | Airaya corp. AI108-1-050 | Young Design EX-1 Wireless Bridge

Proxim Corp. Tsunami QuickBridge 60

Proxim's integrated design is both functional and stylish. The included mounting brackets and hardware make the installation easy enough for the most inexperienced network engineers to handle. The bridges are installed in a master-slave configuration. Both offer the same configuration control via the product's nicely designed Java-based management utility. We used the Java application to configure the bridges' communication channels, data rate, IP address and security key. The utility also includes an antenna-alignment tool, which offers audible signals that beep faster as the signal level increases. That's a nice touch that only Proxim and RadioLAN offer.Like many of the other products we tested, the QuickBridge is designed specifically for ease of use, so it doesn't have many advanced features or options. It supports Power-over-Ethernet, and has a special weatherproof connector that protects against moisture infiltration at the connection point. Proxim also includes a 16-character shared-key security system that encrypts all communications between bridges without degrading performance.

We used the Java management utility to view the status of the bridges. Once the link was operational, we could connect to the local or remote bridge using the same application. The utility provides monitoring information, including channel, data rate, receive signal level, packet loss rate and distance. But the product lacks logging and SNMP support.

The QuickBridge's bidirectional throughput of 47.4 Mbps was second only to BitRage's 78.7 Mbps, but its $5,500 price is almost 80 percent less than BitRage. Overall, QuickBridge clearly provides the best price-performance of any product we tested.

Proxim claims a maximum range of 2.5 miles when running at 60 Mbps, and we verified this based on our attenuation measurements. You can increase range to 6 miles by reducing the performance level to 20 Mbps.

Tsunami QuickBridge 60, $5,499. Proxim Corp., (800) 229-1630, (408) 731-2700. www.proxim.comWi-LAN AWE 120-58 Ultima3 RD

Wi-LAN is probably best known for its work in the development of OFDM (orthogonal frequency division multiplexing) technologies for the deployment of non-line-of-sight wireless access systems. The AWE 120-58 wireless bridge we tested uses more traditional DSSS (Direct Sequence Spread Spectrum) technology, and we were impressed with the overall system design. Performance is only in the 8-to-9-Mbps range, but if that's enough to get the job done in your environment, this product should be on your shortlist.

Vendors at a Glanceclick to enlarge

The AWE's monitoring and management capabilities were the best of any system we tested. Wi-LAN offers a range of configuration techniques, including a serial console, telnet and an embedded Web server. The AWE was more difficult to install than most of its rivals, but that's largely because of the added functionality it provides. One bridge is configured as a base station, while the other is a remote station. Both the console and Web interfaces provide antenna-alignment capabilities as well as traffic monitors and error counters.

The AWE 120-58 was the only product we tested that includes automatic power adjustment, which lets you optimize distance and minimize interference by automatically adapting to the least amount of power required for operation at a given distance. This feature would be helpful if you were deploying multiple systems in a multipoint star topology. Wi-LAN is also the only vendor to provide transparent VLAN and IP filtering support. The bridges can look at the data from a VLAN trunk link and pick which VLANs should transmit data across the wireless link. Some organizations will find the IP address-filtering capabilities useful as a mechanism for enhancing access to specific hosts and/or subnets from the other end of a wireless link. Not only were we able to filter on specific IP addresses, the product made it easy to filter on ranges of IP addresses. MAC (Media Access Control) address filtering is also supported.Wi-LAN's product provides strong security capabilities, including proprietary modulation, data scrambling and data-formatting algorithms. An optional 20-byte hard-coded password/shared-key can be configured to encode and decode data streams. System status for both the local and remote systems can be viewed via the console or Web interfaces, and SNMP support is included.

The AWE 120-58's performance was in the bottom half of the pack, with peak unidirectional throughput of 8.4 Mbps and average bidirectional throughput of 9.6 Mbps. Its range was the highest of any product tested.

AWE 120-58 Ultima3 RD, $3,297. Wi-LAN, (800) 258 6876, (403) 273-9133.

RadioLAN Campus BridgeLink-II-P25

RadioLan's BridgeLink finished at the bottom of the pack in performance, but it was the most feature-rich product we tested. Configuration options are both flexible and efficient. We started by connecting to the serial console port and setting the unit's IP address. DHCP support is included, as is an IPAssign utility that lets you bypass console configuration by assigning an IP address from a computer attached to the bridge.

The configuration Web page offers an antenna-alignment tool with speech capabilities. While you are pointing the antennas at one another, the alignment Web page will tell you the value in percent of signal strength.

Security in the bridge is accomplished by using a unique four-character subnet ID. This ID identifies a pair of bridges. For systems to exchange data, they must both be configured with the same ID. Together with a proprietary modulation, this provides a reasonable level of system security, but you can also purchase a 128-bit encryption option for less than $100 that can be installed on the bridges for an additional layer. We'd like this included in the base configuration. As with Wi-LAN, RadioLAN provides superior filtering. Traffic can be filtered by MAC address and by Layer 3 protocol type (IP/IPX/NetBEUI/NetBIOS). IP address filtering is not supported.

The BridgeLink includes excellent monitoring capabilities. Telnet and Web interfaces let you view bridge statistics, including packets received and packets dropped. The system also maintains its own log file, which can be sent to a central syslog server or is viewable from the bridge Web. Full SNMP support is built in.

Peak unidirectional throughput was 7.4 Mbps, while average bidirectional throughput was 6.6 Mbps. That means the BridgeLink doesn't stack up very well against other products we tested, but it is faster than most commodity 2.4-GHz bridges, and the overall feature set is quite strong. The range of 3.5 miles is about average as well.Campus BridgeLink-II-P25, $4,798. RadioLAN, (866) 272-3465, (408) 365-6200.

BitRage CR45-A-53 DS3 Radio and IU45-E AC with Ethernet Bridge

The BitRage CR45-A is a real screamer when it comes to performance, but you do give up a little in terms of features and elegant design. In essence, the product is designed as a DS-3 radio, but a companion box provides protocol translation between DS-3 and Ethernet. Its full-duplex orientation was obvious when we looked at the two ports on the radio, labeled "data in" and "data out."

BitRage offered the most convenient antenna-alignment tool in the form of LEDs on the modem that clearly display the receive-signal strength. One unit transmits at 5.3 GHz and the other at 5.7 GHz. There are no channels to configure, encryption keys to enter or IDs to input. Once you figure out how to plug it in and align the antennas, you are ready to start bridging traffic.BitRage stresses raw performance, and the product didn't let us down. We pushed almost 78 Mbps through the unit. The CR45-A's maximum range is advertised as 11 miles with appropriate antennas, and we verified in our attenuation tests that the units perform at full speed up to their minimum receive threshold. Latency is less than 1 millisecond, making the product an excellent choice for voice, video and other delay-sensitive apps.

Management and monitoring capabilities are quite limited. You can connect to the console and list the version of the OS that the bridge is running, but that's about it. The bridge does offer telnet, but this only reports the same status information as the console. You can't even upgrade the system via software: BitRage had to send us new chips so we could upgrade the software. The unit does support SNMP, including standard MIBs, so you can connect to the bridge and see stats such as uptime or system name.

At a cost of almost $24,000, the BitRage CR45-A is almost three times as expensive as the next highest-priced unit. For this extra cost, you get a high-performance, low-latency radio system.

CR45-A-53 DS3 Radio and IU45-E AC with Ethernet Bridge, $23,900 ($11,950 per bridge/protocol translator), BitRage, (888) 808-8169, (904) 808-0656.

Airaya corp. AI108-1-050

The Airaya AI108 was the only product we tested that is based on the same Atheros chipset that is widely implemented in 802.11a WLAN products. This lets Airaya take advantage of low component costs while delivering excellent performance. The big trade-off is range. Because the AI108 operates in the 5.25-to-5.35-GHz UNII-2 band, it is limited by the FCC to a maximum of 1 watt of radiated power at the antenna. At deadline, Airaya's Web site indicated that the product had not yet received FCC certification, though the company assured us certification was imminent. Airaya says it plans to add support for the UNII 5.8-GHz band, which allows for higher radio output levels, early next year.

Like Wi-LAN's and Proxim's offerings, the AI108 integrates the radio and antenna into a single enclosure. Power-over-Ethernet is supported, so a single cable supplies both power and network connectivity. During performance testing on the early production unit supplied to us we experienced unit failures that were quickly traced by Airaya back to a faulty power supply. Airaya sent us a new device that performed without problems, along with assurances that its quality-assurance program has been improved to ensure that customers don't experience this problem.

The bridge comes with a factory default IP address. This let us access the integrated Web configuration utility, which is protected by a username and password. The Web page included an option to choose a channel (only a single channel was available in the unit we received) and data rate. The bridge lets you select data rates of 108, 54, 48, 36, 24, and 12 Mbps, but if you choose "best," the rate will be set to the highest possible rate automatically. Those of you familiar with 802.11a will recognize these data rates. In essence, Airaya is binding two 11a channels to achieve a raw data rate of 108 Mbps. The Web configuration utility includes an antenna-alignment tool, but it didn't work in the version we tested. After consulting with Airaya, we were told the feature is available in the current firmware release, which we did not have time to test before deadline.

Some users may be concerned that using an 802.11a chipset for a point-to-point bridge could expose the system to hacking by 11a users who are able to intercept the radio signals. Airaya addresses this problem by using a nonstandard, center-channel frequency. It also implements a proprietary 152-bit encryption algorithm.The AI108's Web interface does not provide any status information once the unit is operational. In fact, once the units are configured and the bridges start passing data, the Web interface is disabled. Airaya says this enhances security because the bridge cannot be reconfigured during operation. We don't accept this proposition and are happy to hear that a new version of firmware will offer customers a choice. In the short term, you don't give up any functionality since the product does not include logging or SNMP support. Interface access, however, will let you upgrade firmware through the Web.

As you might expect from a product based on the Atheros 802.11a chipset, performance of the AI108 was very good. However, there were peaks and valleys in performance, likely a result of the half-duplex radio design, which needs to stop transmission to handle TCP acknowledgements. In our unidirectional tests, peak performance was 29.2 Mbps, while average performance was only 19.6 Mbps. In our full-duplex test, the difference was even more noticeable, with average throughput of 14.2 Mbps and peak throughput of 29.1 Mbps. Nonetheless, even the average throughput is quite good in light of the product's cost, and depending on your application environment, you may never notice the degradation we saw when we stressed the units in the lab.

AI108-1-050 Wireless Bridge, $3,599. Airaya, (866) 224-7292, (408) 776-9583.

Young Design EX-1 Wireless Bridge

EX-1 provides a raw data rate of 16 Mbps with a range of up to 7 miles when using a 2-foot dish antenna. The system is also FCC-certified for operation with 6-inch, 12-inch and 24-inch flat panels as well as with 1- and 2-foot solid dish antennas. The EX-1 was easy to set up because there is little to configure. The device offers no Web interface, no telnet support, not even a console port. Instead, the system is configured via small DIP switches, which let you choose one of eight channels on which the bridges will communicate, their transmit power, and half- or full-duplex Ethernet operation. The eight channels let you install multiple point-to-point systems in close proximity to one another. As for ease of use, the bridges worked right out the box.

The modem is designed to install outdoors and comes with two weatherized connectors that affix to the bridge. The two connectors are part of a special-purpose cable that comes with the unit, combining both power and Ethernet into a single cable shroud. This works fine, but a true Power-over-Ethernet design would provide for additional cabling flexibility.

Unlike the other products we tested, aligning the EX-1 antennas requires that a separate voltmeter be attached to a special port on the unit. Not only is this inconvenient, but it makes it difficult to troubleshoot physical-layer problems after the unit is operational. Young Design suggests you take preventative maintenance readings each month to catch weak-signal problems before an outage occurs.

We received much higher throughput when we switched the Ethernet port into full-duplex mode, so we configured the unit for full-duplex operation for all our performance tests. Not only did our full-duplex test work better, but this change also improved the device's unidirectional performance because our test used TCP and full-duplex facilitated ACKs.

Average performance in our unidirectional test was 7.7 Mbps, while bidirectional performance came in at around 14.5 Mbps. That placed it around the middle of the pack. At a cost of $8,500 with 2-foot antennas, the Young Design offering was the second most expensive of any of the products tested. In our range measurements, the EX-1 conformed to manufacturer's specifications.Although this unit doesn't stack up all that well against the competition, we're not sure we really played to Young Design's strength. The company offers a much broader array of products that operate in the 2.4-GHz band as well as a number of helpful RF utilities and some of the best technical information available on any vendor Web site.

Model EX-1, starts at $8,000 ($8,500 as tested), Young Design, (888) 297-9090, (800) 664-7060, (703) 205-0600.

Dave Molta is a Network Computing senior technology editor. He is also an assistant professor in the School of Information Studies and director of the Center for Emerging Network Technologies at Syracuse University in New York. Write to him at [email protected]. Cornell W. Robinson III is a research associate at the Center for Emerging Network Technologies and a freelance reviewer. His experience includes four years in IT and a position as network manager of Point Park College in Pittsburgh. Write to him at [email protected].

We tested the fixed wireless point-to-point systems in our Syracuse University labs using calibrated radio-frequency attenuators to simulate the free space path loss that would occur in a typical outdoor installation. Although our test bed let us simulate longer links, our objective was to simulate a 1-mile connection. At 5.3 GHz, the path loss for a 1-mile link is approximately 111 dB. Because some of the products include integrated antennas and others work with a variety of external antennas, we controlled for the effects of antenna gain by assuming that each product used an antenna on each end with a gain of 23 dBi, a number typical of several commercially available directional path antennas. We also corrected for path loss in our cabling system. For the products that included integrated antennas, we requested vendors provide an RF connector that bypassed the internal antenna.

Because an antenna on each end would provide 46 dBi of total signal gain and our expected 1-mile path loss is 111 dB, we expected each product to operate properly with 65 dB of attenuation (111 minus 46). All the products met that standard. We also measured the level of attenuation at which each product stopped operating and compared that result to what we would expect based on manufacturers' claims of radio output power and receive sensitivity. We found that all the products operated within 5 percent of these specifications. Thus, we're confident in stating that, when properly installed with appropriate antennas, all these products will operate at the distances stated by the manufacturers.

For performance testing, we used NetIQ Chariot's filercvl TCP test using 100 iterations of 1-MB records to evaluate the maximum throughput under optimal conditions. The test bed consisted of four 1.2-GHz Toshiba Satellite notebook computers with 256 MB of RAM running Microsoft Windows XP. All computers and wireless bridges were connected to a Cisco 3500 switch with VLANs defined for each bridge and its computer pair (see diagram). We measured peak and average performance in unidirectional half-duplex mode and average aggregate performance in bidirectional full-duplex mode. We also ran the same tests with the notebook computers connected directly to the switches to establish a baseline Ethernet measurement and ensure that the notebook computers themselves were not a bottleneck. The range of microwave systems is limited by a number of technical radio characteristics as well as by the curvature of the earth. Most high-performance systems require direct line of sight between antennas. Because designers must take into account the RF effects of a system's Fresnel zone (the pattern of electromagnetic radiation that is created by a transmitting station from its antenna to receiving antennas), bridges should be elevated sufficiently off the ground to ensure reliable operation.The major technical variables that affect range are radio output power, radio receiver sensitivity, antenna gain and path loss. Path loss is the attenuation that occurs when radio signals pass through air, and while generally easy to calculate, it varies with frequency (the higher the frequency, the higher the path loss per meter). While somewhat of an over-simplification, at a given frequency, high radio output power combined with high antenna gain and good receiver sensitivity translates into improved range. Most professional installers also will add a fade margin to their overall range calculations. This margin will vary depending on the geographic and climatic conditions of different geographic areas, which may introduce atmospheric and multipath-related fading that must be added to the free-space path loss simulated in our tests. It's not unusual to see engineered fade margins of 20 dBm for high-reliability applications. Your vendor or reseller should be able to help you with this, and several of the vendors whose products we review here include technical information and range calculation utilities on their Web sites.

Of course, in the real world, we also have to deal with government regulations, which vary somewhat by jurisdiction. In the United States, the FCC imposes a number of restrictions on unlicensed radio operations, and vendors must gain FCC certification of compliance in order to sell their products. Key regulations involve acceptable wave forms, radio output power and EIRP (effective isotropic radiated power), which is a combination of radio output power and antenna gain. Thus, all else being equal, receiver sensitivity--a radio's ability to separate a very weak signal from lots of noise--is the most important technical design element. This is where RF engineers earn their salaries.


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