Lee H. Badman

Network Computing Blogger


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The Evolution of Wireless Bridging

Wireless bridges are a great option for providing network connectivity between physical locations. On the college campus I help support, there are buildings that need connectivity but can't be reached with fiber. Leased lines or site-to-site VPNs are options, but they can be expensive and complex. Wireless bridges are my solution of choice where clear line-of-sight exists, and I'm seeing a change for the better in my bridging options.

Before we talk products, be aware that in a bridging project, RF knowledge is the key to successful installations. You must understand principles such as free-space path loss, wind-loading, and how distance and modulation affect data. Most failed bridge projects tend to trace back to poor installation or the wrong bridge for the use case, so make sure you don't underestimate the challenge.

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To understand the significance of the bridge options that are available today, it's worth a quick personal history lesson. Back in the days of Cisco's autonomous 802.11b wireless access points (the mid-90s), a bridge version of the venerable model 350 and a high-directional antenna gave networkers a then-powerful weapon for getting a few megabits per second of backbone connectivity to a far-flung building.

Then came Cisco's 1300 (11g) and 1400 (11a) 54-Mbps bridges, and a new mode in its1200-series (and newer) APs that let an access point be turned into a bridge. I've installed all, and still have a 1400 in service that generally works flawlessly to around 14 Mbps at a mile distance. That's double the bandwidth--at no cost beyond initial install--that we would otherwise pay monthly if we opted for a business-class Road Runner connection or similar wired option.

Unfortunately, the 1300s weren't as reliable as Cisco's 1200 APs in bridge mode, and eventually they fell victim to the crowded 2.4-GHz space as Wi-Fi signals spread across our campus and the neighborhoods that we shoot our bridges across.

In 2010, Cisco stopped developing its own bridges and partnered with Exalt to sell and support the Exalt r5005. The r5005 has been a dream to use, and provides quick ROI versus paying for local ISP connectivity and then finding a logical solution to get back to the home network.

The r5005 works in unlicensed 5-GHz spectrum, and provides 80 Mbps in each direction (aggregate 160 Mbps) to 3 miles when aligned right. This has become my bridge of choice for many point-to-point needs, as most installers find it to be a breeze to install and align. But we're still talking networking here, and sometimes you need even more bandwidth.

With roots in an overseas project I was involved in, my team ended up with a licensed-spectrum link, a carrier-grade Bridgewave FE80XU. Providing 100-Mbps full-duplex in the 80-GHz spectrum, this was my first foray into dealing with the licensing process for bridge links.

Should you opt for a licensed solution, expect both delay and additional cost for the FCC-required paperwork. Also be advised; aligning a link with a tight beamwidth between endpoints can kick your butt. For perspective, the Exalt r5005 has a 10-degree beamwidth, where the Bridgewave FE80XU is .4 degrees (yes, four-tenths of one degree) and is therefore almost a laser beam at a distance of one mile. This is a challenge to line up if you've never done it before.

Invariably, backbone links that approach or exceed Gigabit are needed in larger remote sites, even when wireless bridging is the only path to the core. My first Gig bridge was a short-range, unlicensed Bridgewave AR60 which struggles beyond around a few thousand feet. Working in the 60-GHz spectrum is a curious study on the lofty topic of oxygen absorption. This bridge proved to be the most difficult to align that my team has ever installed, but it delivered the promised Gig connectivity nicely.

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You learn as you go in this space, and our next high-capacity bridge link was also licensed, in the 18-GHz space, with the ExtremeAir from Exalt. With the same slick interface as the r5005, this Exalt bridge was also great to work with, and currently provides a remote building full of happy users with a reliable full-duplex 700 Mbps of wireless uplink to the campus network.

To keep installations and performance consistent, it's good to standardize as much as possible. At the same time, you have to stay on top of what's out there as available bandwidth increases at more attractive price points.

We're getting ready to evaluate the Ubiquity AirFiber, which promises the best of all worlds: low-cost, license-free (in 24 GHz), long distance, and Gigabit+ aggregate speeds. We also are looking at LigoWave 2x2 MiMO 5-GHz bridges as a contender to the Exalt r5005, as it comes highly recommended for similar performance at a fraction of the cost.

My own point-to-point story is just the tip of the iceberg for product options. Ruckus and Aruba customers have native bridging options, but sometimes these products can't be centrally managed alongside APs with the vendors' native management tools (check before you purchase).

There are open-source-based bridges (Linux/Free BSD, DD-WRT and OpenWRT) to be had for well under $100 if you feel adventurous, and there are also high-capacity, six-figure, free-space optics (laser) options for hyper-critical links.

Don't let the lack of fiber to a remote building get you down, and don't get sucked into costly ISP connections if wireless bridging is an option.


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