High-flying data

Large amounts of information hurtling through the air gives broadband a whole new dimension. Luckily, testing can create the safety net wireless operators need

You can find broadband pipes almost anywhere - even in the air. Wireless access, the one remaining last-mile pathway to the Internet, is fast becoming a reality. "Unwired" no longer means "disconnected."

In some cases, a broadband radio connection may be the easiest and cheapest way for an enterprise to get high-speed Internet access. But as with any RF technology, service delivery entails a lot of work resolving propagation problems, spectrum interference, signal reflection and other issues inherent in the medium. Add the additional quality demands for data, and carriers such as Advanced Radio Telecom, Nextlink Communications and Teligent have their test and measurement work cut out for them.

Wireless DSL

Up to a point, fixed broadband wireless is directly analogous to DSL, only without copper and more robust. Using a 38 GHz digital microwave radio, broadband wireless can deliver up to 16 Mb/s on a single link - about 10 times as much as copper-based DSL (Figure 1). Harry Hirsch, chairman and CEO of ART, sees it as the ultimate IP delivery service - IP telephony, IP TV, IP streaming and general Internet access - from a single radio path.

For its part, Nextlink treats wireless as just another access platform for its customers when a copper DSL is unavailable or not economical. The company does not define its operational environment by its physical facilities, says Margaret Marino, vice president of engineering at Nextlink. For these wireless carriers, the trunk side of their networks looks almost the same as their wired competitors. They link via T-3 or OC-3 connections to a backbone network provider. They may share space in an incumbent local exchange carrier central office.

But for radio-based service providers, network points of presence (POPs) take the form of nodes - base stations set up at various sites in a city, similar to cellular and PCS systems. This time, however, one or more radio links have a direct line of sight at a specific building or group of buildings, not as an omnidirectional canvass over a wide area.

Only when it comes to individual network design do differences in architecture appear among carriers.

For example, Teligent integrates point-to-point and point-to-multipoint microwave radios. Base station nodes aggregate customer links and backhaul them to a switching center where ATM switches and routers hand them off to the public switched voice network, the Internet or to private data networks. Teligent has licenses in 74 markets, with commercial service launched in 15 of them in 1998 and 16 more added in 1999. It offers high-speed dedicated access to the Internet for business customers ranging from 64 kb/s to 45 Mb/s.

ART began rolling out service with the launch of its San Jose network in September, which initially served 11 buildings. Its unique approach stems from its "consecutive point" radio architecture. Unlike other fixed wireless service providers, ART's base station nodes are deployed in a ring configuration. This can help in providing quality radio transmission in dense urban areas, Marino says. ART's initial plan calls for rollout in 40 cities, but it holds licenses in 210 markets nationwide.

ART backhauls traffic via its own fiber optic rings but uses POPs from Qwest Communications. Qwest purchased a 19% share of ART in June, and the wireless carrier generally is seen as part of Qwest's plan to enter the local access market.

Nextlink turned up trial customers with a point-to-multipoint wireless hub site in Los Angeles. Like ART, the company is trunking traffic on a local fiber network. Following the completion of the field tests, Nextlink will offer service commercially to selected Los Angeles customers. The company also is on track to offer commercial service in the Dallas area by year-end.

Meanwhile, wireless point-to-multipoint and point-to-point equipment from many vendors is being tested in lab and field environments to assure efficient use of the local wireless spectrum and fiber facilities.

Advance planning

Of all the types of broadband access media, wireless, because it is over the air, requires the most advance planning. Buildings, bodies of water,rain and heavy foliage can affect radio propagation, as can the preponderance of other RF energy, especially from nearby frequency bands. Before and during deployment, all these factors must be tested and measured.

For Teligent, it begins with a database of potential customers and a detailed map of the service area. "In designing the RF environment, we try to lay out the most ideal places for nodes," says Keith Kaczmarek, senior vice president of network services.

"In the RF environment, you can design reliability," adds Hamid Akhaven, senior vice president and chief technology officer at Teligent. "You can make sure every customer will see the same quality of services."

The RF preparation phase comes down to three parts: spectrum and frequency planning, market readiness planning and cell design.

Frequency planning, Akhaven says, calls on "the art of radio planning," especially when the link is point-to-multipoint. The proper power levels have to be set so the signal is strong enough to go the necessary distance without fading. However, the carrier must be careful not to power up the signal so high that it causes interference with service on adjacent bands. This is easy when working with frequencies in one's own licensed spectrum allocation. But carriers must be just as mindful of "the other guy," especially when it comes to calibrating service on frequencies that border on spectrum blocs licensed to other carriers.

Market readiness planning sets the parameters for what customers will experience. Teligent sets its standards high. "We want to be equal or better than landline," Kaczmarek says. In addition, Teligent wants to maintain consistent quality levels across all markets.

Lastly, cell design applies the first two parts to the actual RF environment. Here, nodes are plotted carefully, and weather and terrain factors are calculated into the equation. The propagation characteristics of each base station are tailored individually "to get the maximum bang for the buck," Akhaven says.

In the planning phase, Teligent uses a wide array of test and measurement equipment. To identify, isolate and prove out potential node locations, the service provider uses equipment to gather precise geopositioning satellite data. In addition, for line-of-site determination precision, laser measurement gear comes into play.

"We have to see building-to-building," says Ron Olexa, executive vice president and chief technology officer for ART.

ART deploys base station nodes in a ring configuration, allowing for a degree of redundancy. Links also can be relatively short, Olexa says, which tempers problems with rain fade and interference in cities with densely packed skyscrapers such as Boston, Chicago and New York. "We know the analysis, we know the gain, we know what the attenuation characteristics are," he says, referring to the company's scope of RF know-how. "Rarely do we stretch it to the limits."

What if a new high-rise building springs up between a node and a customer location, blocking the line of sight? One solution would be to add a repeater and shoot the signal around the building. Olexa's preference, however, is to make the new building an ART customer, and then use the building as a new base station in its series of consecutive nodes.

Beyond the radio net

Nextlink has built switching networks and fiber rings in most of its markets, so it has more flexibility when it comes to hub design. Marino even refers to some sites as "partial hubs."

"We're designing our system for fiber-like capabilities," she explains. Spectrum analyzers and noise testers examine the way the local RF environment affects the new link. Finally, packet sniffers and other data communications test equipment are used to test how well service is running over the link.

Most of the equipment Teligent uses is off the shelf, although it has been enhancing the equipment with proprietary software, Akhaven says.

Once past the service's RF characteristics, broadband wireless operators deal with the same issues as other carriers rolling out high-speed data communications services. These extend into the operations and maintenance of the network as a whole and involve centralized management, QOS measurement and network alarm monitoring.

"Switches and routers have monitoring capabilities. Through hourly data dumps we monitor packet delay and packet loss," ART's Olexa says. "We also have packet sniffers to monitor traffic flow, and, if there's a problem, determine what it is."

If anything, the data aspect has made testing more crucial to successful operations. "These networks have evolved far beyond the ability of a single technician to test and monitor," Olexa says. Data networks today must be managed on the physical and logical level. "Test equipment is an absolute necessity to understand what's happening in the network when it's not an obvious hardware failure," he adds.

One of the unique aspects of the broadband wireless networks is that there are two sets of interfaces, says Teligent's Akhaven. The radio and base station interfaces are proprietary to the manufacturer. On the network side, interfaces tend to be standardized. Still, Teligent maintains a lab where it tests all the equipment before it's deployed in the network. "Nothing goes out before it meets lab standards," Kaczmarek says.

"We've had to put effort into integrating equipment, cost and features," Akhaven adds. Sometimes this leads to product ideas. For example, to accommodate the small amounts of space in co-location cages, Teligent worked with Ascend Communications, now part of Lucent Technologies, to build a DSL access multiplexer (DSLAM) "the size of a pizza box," he says.

On the data end, test equipment is used to ensure that the equipment works and that the customer is getting the level of service for which he paid.

>From its network operations center (NOC) in Herndon, Va., Teligent can >monitor its entire network. "Every component is monitored, every component >can send alarm information," Akhaven says. "We can remotely loop back >information to check the quality of any line. When there's a failure, we >know it immediately."

Simplicity and speed are the important aspects for Nextlink, Marino says. Nextlink has been testing broadband wireless equipment in its research lab in Plano, Texas, for the past year. The carrier does not seek specialized equipment but wants equipment to be simple to use and to handle the provisioning, testing and monitoring of lines efficiently as they come up.

Also in Plano is Nextlink's NOC, where it can monitor and test its entire network. Like several landline-based DSL providers, Nextlink uses DSLAM management systems from Turnstone Systems.

Repairs and troubleshooting, for cost reasons, have to be handled from centralized locations. "Windshield time must be avoided," Marino says.

ART's lab facilities in Bellevue, Wash., currently are being expanded to include all the elements that are in the field, Olexa says. All the San Jose equipment went through the labs prior to deployment. "Test equipment is all off the shelf, but it's customized to our needs," he adds, describing ART's data network as "elegant."

"Purely as a high-speed Ethernet - 100BaseT or 10BaseT - or IP packet over Sonet at OC-3, we're not doing anything that's not standard," Olexa says.

On the data communications level and the RF level, the test process always has been vitally important to ensure a correct buildout and QOS. In a competitive era, however, testing takes on that much more importance because of its usefulness in documenting problems and solutions.

An ordinary spectrum analyzer or RF signal generator can just as easily be a valuable tool in the quest for improved quality.

"A tool is only as good as its operator," Akhaven says. "As you become more experienced, you find you are using tools in a different way." For any carrier, the testing experience is in part troubleshooting, but it's also a collection of proprietary knowledge - RF secrets that can be used to gain a competitive advantage. "You can buy any component off the shelf," Akhaven says. "It's how you put it together. It's no different froma craftsman who creates a work of art."