Never Enough?

Hardly a week goes by without at least one newspaper article about how the number of wireless subscribers has outstripped capacity. The most recent -- but certainly not the last -- was in The Wall Street Journal, where AT&T Wireless CEO Dan Hesse acknowledged that Digital One Rate's roaring success was tempered by "certain markets and locales where we have had capacity problems."

To be fair, anticipating capacity needs is as much an art as it is a science. Carriers already study usage patterns almost religiously, but a growing number of blocked calls isn't the only indicator that capacity is growing thin.

"Other switch statistics that are important to monitor are number of total calls made on each sector of a site, number of call drops, number of access failures, number of total handoffs attempted, number of handoff failures and channel-element usage," said Naveed Akbar, Comsearch senior engineer.

Dropped calls aren't just a sign of poor signal coverage. They also can identify areas where capacity is growing dangerously thin.

"If you're dropping calls that are waiting for a handoff to occur, it's a sign that the cell site that was supposed to get the handoff might have been congested," said David Mangini, AT&T Wireless vice president of network realization for the eastern United States. "So dropped calls can give you some insight to diagnose your blocking, as well."

One caveat: Many networks use directed retry, where the handset will automatically try to connect via other sites if the preferred site is full. Although that feature hides blocking from subscribers, it also can create some misleading statistics.

"You have to be able to tell the difference between a call that couldn't originate and a call that got on successfully the next time around," said Bob Berner, Rogers Cantel CTO. "From a customer standpoint, it all seems like the same call."

A lack of vacant voice channels isn't the only reason that a call is blocked, so the hunt for the cause should involve more than just RF engineers.

"You have all sorts of different events that happen during a call, and each one has to have enough switch resources assigned to it to be able to make it work," Berner said. "That's not all that evident to a radio engineer. That's why it's good to have switching specialists involved, as well."

Although network statistics identify areas plagued by blocking, numbers alone don't tell the whole story. Drive testing helps give a real-world feel to raw data.

"I've always backed it up with some empirical drive testing," said Scott Fox, former BellSouth Cellular CTO and now president of Wireless Facilities' (WFI's) network-management group. "There's nothing like drive testing the system to see it from a customer's perspective."

THE UNUSUAL SUSPECTSBlocking's causes often go beyond just a shortage of BTS transceivers. Sure, they're the most common culprit, but insufficient switch resources and under-provisioned interswitch trunks also top the list. That's why it's important to beef up capacity network-wide.

"You want to increase capacity across the full network and not just at any one of the elements," said Bob Sellinger, Lucent wireless-networks group director of wireless architecture. "You can have blockage literally anywhere: at the air interface, at the backhaul between the BTS and the MSC, within the MSC and within the IN. Unless you can increase the capacity across all the networks and keep capacity balanced, you don't fully solve the problem."

Worse, adding just transceivers can dump more traffic into other elements that might be unable to handle the additional workload.

"You can't just open up more capacity at the cell sites by adding radios without following through the rest of your network to be sure that you have an open pipe all the way through," said AT&T Wireless's Mangini. "You've got the in taken care of. Now make sure the out matches the in."

HOW MUCH IS ENOUGH?One school of thought says that it's more prudent to add capacity only when and where it's needed, la just-in-time manufacturing. The other school argues that maintaining excess capacity is better in the long run even if it represents an unproductive capital expenditure in the short term.

"If you keep your network dimensioned so you have only the number of transceivers you need, and you move them around at midnight from one site to another, what you might save on capital you're going to burn up on operating expenses," said Clyde Smith, Triton PCS CTO. "I don't view that as a practical way to run a business."

Nor does it make for good PR if the carrier can't react quickly enough. Fortunately, for every embarrassing newspaper article about blocking, there's usually another about a carrier buying hundreds of millions of dollars worth of new switches and transceivers. Even so, adding capacity is easier and faster in some parts of the network than others. One factor is vendor lead time, although Lucent has cut that period to as little as 11 days for minicells. Not so with switches.

"You're not going to augment the capacity of a switch overnight, so you need to dimension that for a longer time frame," Smith said. "You're talking several months. On the radio side, you've got a little more flexibility."

For Rogers Cantel, the ultimate in flexibility was a dual 800MHz/1.9GHz system developed in conjunction with Ericsson. Deploying a 1.9GHz system alongside its existing 800MHz system was a good fit because Rogers Cantel already had the 1.9GHz licenses and, just as important, it had built sites capable of accommodating additional antennas and radios.

"Assuming that we put in all the channels that we're capable of putting in within the 10MHz frequency, we will have more than doubled our capacity on the same cell site," said Bob Berner, Rogers Cantel CTO. "That's a tremendous boon because one of the more difficult things to do these days is build sites in urban areas. We've totally avoided the need to do that."

A cross-band solution addressed other quality-of-service concerns such as spotty coverage.

"A lot of urban cell-site construction is to fix C/I problems or weak signal strength," Berner said. "By putting in an overlay, instead of having to do all those fixes on both frequencies, all you need to do is hand down off 1,900MHz in that area to 800MHz and back to 1,900MHz when you leave the area. That's all done through parameters that cause the handoff to occur at the right place at the right time."

For subscribers with dual-band, dual-mode phones, the chances of a call failing to complete are slim.

"That customer has an enormous probability of success," Berner said. "The phone prefers 1,900MHz. If there isn't a channel available, it tries again. Then it goes to 800MHz digital and does that all over again. If it still can't get on, it goes to (800MHz) analog. So the joint probability of success approaches 100% every time."

That's good news for carrier and caller alike.

"If you're not able to complete the call, you're not generating revenue," said Scott Fox, former BellSouth Cellular CTO and now president of Wireless Facilities' network-management group. "So nobody wins."