To Overlay or Not to Overlay

Data will account for almost 25% of all wireless traffic within the next five years, according to Pascal Debon, Nortel GSM Networks president. And that number will continue to increase. Because most networks are not designed to handle such a high volume of data traffic effectively, you probably will need to modify or upgrade your network.

If you have legacy equipment, chances are you installed overlays when you converted analog systems to digital and now may be considering implementing them for increased data. But there are a variety of technology and infrastructure issues that determine whether or not you should take an overlay approach.

Brad Fink, Nortel Networks senior manager of data services, said overlays are not always ideal because they require hardware upgrades, which usually means adding equipment to hundreds or even thousands of cell sites.

Mixing and matching base-station infrastructure with network elements creates operational challenges such as billing and network management. Fink said network management is a key technical challenge with overlays because you're adding different equipment, and it presumably has different network management than the legacy equipment. It becomes even more challenging because you're merging two different worlds -- data and voice.

"The ideal situation is that you have an investment in existing infrastructure and evolve that infrastructure to support future services, including voice capacity, with minimal incremental investment -- ideally via software," Fink said.

The preferred solution is adding software at the switching center, purchasing the functionality from the supplier and turning it on via software. Although digital carriers can support many of the new features with simple software upgrades, others don't have that option.

If you go with a base-station overlay approach, make sure you can interface and reuse antenna systems or as much of the existing equipment as possible. If you have to use an overlay, Nortel's Fink said it should be future-proof.

"The base station as well as the network components need to be software-defined as much as possible to protect the investment," he said. "You don't want to be doing this again two years from now."

Because all access technologies (CDMA, GSM, TDMA) have different evolution options defined by specific customer requirements, each carrier must look at its options carefully. If an overlay is necessary, there are potential ways of recouping your investment: You can add new services via overlay with a path to lower-cost voice.

"Data's the sizzle and low-cost voice is the steak," Fink said. "Lower-cost voice pays for some of these data investments."

AN OVERLAY ROUTE Before making your decision, compare the cost of an overlay approach vs. integrated. Weigh the functionality of the technology against the cost savings. After these considerations, run a business model. By following these steps, BellSouth Cellular found a strategy that works for it.

"The question of overlay all depends on where your starting point is," said Keith Radousky, BellSouth Cellular director of engineering. "In our cellular properties, we will do an overlay for 3G. In our GSM networks, we don't have to do an overlay for 3G. We can leverage some of the existing platform."

Radousky said BellSouth Cellular couldn't justify clearing out spectrum for data only, so it amended its path. BellSouth Cellular's current path for high-speed data is called EDGE Compact, which is a variation of EDGE Classic, the European-born evolution for GSM technology. EDGE Classic required carriers to deploy 2.4MHz, a significant piece of spectrum. It was difficult for BellSouth to clear that much spectrum in its domestic 800MHz properties and continue to maintain the capacity it needed as well as good quality of service.

"(So) we put our PhDs to work on the problem of trying to change EDGE so that it requires less spectrum and the ability to do an overlay technology," Radousky said.

BellSouth's EDGE Compact technology allows it to overlay its 800MHz network and offers more choices because it can leverage vendor "quotes" for 3G equipment rather than being obligated to use the incumbent infrastructure provider.

Another advantage of the EDGE overlay concept is that if BellSouth has 250 cell sites in a market, it can deploy EDGE in only 100 with a little higher power and get the same footprint it would with 250 sites. To enable a true overlay, the carrier also changed the standard so that high-speed data customers use an EDGE control channel and not the TDMA control channel.

Overlays haven't been Bell-South's strategy always. According to Radousky, when it migrated from analog to TDMA, it was an integrated transition in which it leveraged cell-site equipment to go digital. The integration approach made sense because the same 30KHz-channel bandwidth was required for analog and TDMA.

Because high-speed data requires a different bandwidth channel, 200KHz, it calls for overlay technology. From a technical standpoint, the viability of an overlay has to do with the technology's RF bandwidth. The other issue is how much of the existing equipment in the cell site and the switch you can leverage for the new service. It's analogous to adding onto your house or buying a new house, Radousky said.

"If you can add on to your existing house and meet your requirements for how big of a house you need, fine. That's an integrated approach," he explained. "If you need a bigger house, and it can't fit on the land you've got, then the bandwidth is too small, and you have to build a new house. That's an overlay."

OTHER OPTIONS Typically, carriers implement overlays to dedicate digital spectrum to data-only use.

"The theory is to avoid disrupting your voice traffic so that if data uses start to explode, they don't push everyone else off the channel," said Jonathan Marshall, AirTouch Communications spokesperson.

But not all carriers will use an overlay approach to next-generation networks. Marshall said although AirTouch is open to the possibility of using overlays, it doesn't plan to use them because 3G technology will inherently permit much faster data rates and double the spectrum efficiency for voice.

"Because that's going to greatly increase our voice capacity as well, there's no special reason to move to an overlay," he said.

Marshall said dedicating a piece of spectrum to data is just not an attractive option today. He cited low carrier interest in Qualcomm's high-data-rate technology. Although it is more efficient for data than 3G, it requires using its own CDMA channel for implementation.

"You may be wasting most of that spectrum until the data (market) gets to be big enough," he said.

If you can increase both your voice and data capacity by moving straight to 3G, Marshall said, you'll get most of what you need.

GSM carriers probably will choose integrated solutions instead of overlays.

"I remember when I was doing analog-to-digital conversions where we literally had to change everything we did, and the only way to do that was a complete overlay," said William A. Diaz, Omnipoint director of network engineering. "(But GSM has) inherent things already that allow us to upgrade to these new technologies in a manner that it's not really an overlay. It is an integrated part of the overall network, and it is an integrated service."

Chris Resavy, senior director of network engineering and operations, said Omnipoint's data services are an integrated part of its voice service.

"We anticipate that anywhere you have voice, you'll have GPRS-based data as well," he said.

According to Diaz, GSM's inherent data capability puts GSM carriers in a good position for offering 3G services because they won't have to make as many infrastructure changes.

"If I were an independent true CDMA player, I'd be worried because those are probably the people that are going to have to spend the most money to upgrade and make their networks 3G-ready," he said.

According to Diaz, although a digital carrier will be OK on the base-station side, it will probably have to make major network upgrades. Because the GSM standard is based on wideband CDMA, GSM networks will require only minimal upgrades. Such carriers won't have to change the message structure drastically, or the basic translation coding in the switches. They can continue to build on what they have. Diaz said Omnipoint will add a few more network elements per vendor, and add SS7 and IP connections to reach an integrated voice/data service.

Resavy said GPRS will require two additional nodes in MSC locations. For each of the switching locations, Omnipoint will have a serving GPRS service node that will help move all of the packet-data traffic around. Traffic will send data to the gateway GPRS service node, which is the gateway handler for all of the network's packet data.

"Each of the nodes that are being added have defined interfaces for fault, performance and configuration management," Resavy said. "We have to modify it and add the new rules to the existing package, but since we've gotten good at it, it's not a major issue for us."

Omnipoint's plans include a GPRS beta test later this year and commercial service the first half of next year.

Resavy predicts almost every GSM carrier will integrate GPRS in its existing footprint in the near future.

"They may try to introduce it in smaller areas first before they go to a wide area of deployment, but if we're going to have a true ubiquitous service equal to voice, then that'll require you put it out there everywhere you have voice," he said.

But according to Diaz, soon it won't matter whether you have deployed an overlayed TDMA network or an integrated GSM network because 3G will bring them all together.

"Once we get to 3G, you're going to see a lot of the network infrastructures start to look alike," he said. "At the network level, we're all going to start to look more similar, and at the RF level, we'll still be as different as we are today."