High Wireless Act

They don't call it the world wide wait for nothing. Just ask Richard Lynch, who's accustomed to throughput dipping to zilch when paying bills on-line via wireline.

"On a Sunday afternoon, I can wait 10 minutes and have to try three or four times before I get them paid," said Lynch, Bell Atlantic Mobile (BAM) executive vice president & CTO."It's as slow as some people think the data rates we're talking about for wireless are. The reality is that they're plenty fast for e-mail and other applications if it weren't for the fact that the Web drags it down."

That's the rub: Users typically blame delays and glitches on the wireless network when the real culprit might be a bogged-down server somewhere in the Internet. Granted, data doesn't fly through the air with the greatest of ease, but the air link doesn't have to be the weak link.

Perhaps the best way to maximize throughput is by improving coverage. BellSouth Wireless Data, for example, spent $6 million last year to improve link budgets by as much as 6dB, and for good reason: Throughput steadily decreases as the user moves toward the cell's fringe because as errors increase, more packets have to be retransmitted. As a rule of thumb, where voice coverage is good, data coverage also should be good, unless they use different infrastructure.

"In CDPD, because the capacity demand is not that great, some companies tend to use macrocells that cover several voice cell sites," said Norman Toms, Sierra Wireless CTO. "Voice sites are usually much smaller than they need to be for just coverage. They're reduced in size for capacity reasons. It's a little expensive to fire up every single one of those with CDPD if you don't yet have a capacity crunch."

Optimization also can vary by how the vendor handles CDPD. If the radio originally was designed to accommodate data, adding CDPD could involve just a software upgrade to switch the radio from voice to data. That approach eliminates the need for CDPD-overlay base stations and, in turn, can make optimization more straightforward. One example is BTS sensitivity.

"Part of that comes down to a 1:1 engineering of the data network," said Brad Fink, Nortel Networks senior manager, wireless-data services. "If, for example, you have a sectorized, coverage-limited voice network, and you put an omni data network on top of that, then you lose sensitivity. That's one of the reasons to apply it 1:1."

Optimizing CDPD also can be more straightforward when it uses dedicated channels.

"One of the things that sold CDPD initially was that it could hop from frequency to frequency to keep things going," said Ken Carolus, Hewlett-Packard wireless-application engineer, wireless-service-providers division. "They never have gotten the hop quite working good, so most of the sites I've seen have a dedicated channel."

WEATHERING THE TRIP The biggest difference between voice and data is the ability to handle delay and errors.

"Voice is a real-time service, so you can sustain more error but less delay," Fink said. "Data is the other way around: You can handle variable delay, and you can recover from errors."

That's because in data, error correction mitigates RF gremlins such as interference and weak signals by retransmitting lost or corrupted packets. But for carriers that still charge by the kilobyte, that approach can be an expensive solution. It's also inefficient because duplicate packets don't wring the maximum revenue from available capacity.

"You're talking (about) double-digit-percentage retransmission in a bad network," said BAM's Lynch. "So (it) can have a significant impact on the amount of capacity you've got available."

Next-generation data protocols promise to cut retransmission overhead by assessing signal strength and C/I to avoid sending at a higher rate than the channel can accommodate. GPRS, for example, chooses among four channel-coding schemes.

"Depending on the situation and channel conditions, it will use the most robust scheme," said James Shi, Ericsson professional-services senior engineer. "As you move toward the base station, channel quality will improve, and then it can choose a better scheme, one that has less redundancy and thus higher throughput."

In the meantime, middleware such as Nettech's SmartIP help applications cope with 2G wireless' latency and finite bandwidth. Although pre-determined parameters dictate maximum throughput, compression can give the illusion of better performance.

"The most commonly deployed is v.42bis compression, an optional feature in the CDPD spec that some carriers have deployed," said Andrew Harries, Sierra Wireless vice president, marketing. "In the future, I'd expect to see this type of compression built into wireless infrastructures as a matter of course."

Knowing which devices and applications subscribers use most often can help determine which performance tweaks are a good fit. For example, v.42bis runs on both the network and the device, and smaller devices, such as smart phones, might not have sufficient memory and processor speeds. It's also more effective on text than on graphics.

"If you're surfing the Web and downloading a picture, v.42bis isn't going to give you that much improvement," Harries said. "If you're doing e-mail, it could give you terrific performance improvement."

WHOSE FAULT IS IT? Not all data subscribers are as understanding as BAM's Lynch. Indeed, in many cases, the carrier acts as the lightning rod for any problems, even when the real culprit is the ISP or corporate firewall. That's why a clear troubleshooting methodology is key to resolving problems quickly. For many carriers, the process starts at a data help desk, although finding people who understand both worlds isn't easy. Rather than fielding all calls itself, BellSouth Wireless Data trains its major clients' help desks on what to look for.

"Only when they can't sort it out do they call us," said Roger Shultz, BellSouth Wireless Data senior vice president, network. "It's very difficult for (our) customer-management organization to have broad-based expertise. We probably have several hundred different types of applications. That's tough for someone to have in-depth knowledge. That's why the partnerships with the larger user groups work very nicely."

Network-monitoring systems help determine whether the problem is inside the wireless network.

"We look at that (user's) IP address and the activity that's been on it inside our routers as a means of trying to isolate where the problem might be," Lynch said. "If the customer didn't even hit our network, then we probably have either an application or a CPE (customer-premise-equipment) problem. But if the customer hit the network, then we can start looking out toward the direction that customer was headed."

That visibility will improve as data and IP play greater roles in wireless. When every network node, including radios, is IP-addressable, then troubleshooting is easier, faster and even cheaper, thanks to off-the-shelf tools, such as Simple Network Management Protocol.

"You could use standard IP tools to issue a trace," said Nortel's Fink. "It will respond with every IP-addressable item along the path and the associated delay."