Most of the talk about future fourth-generation (4G) mobile broadband networks appears to focus on speed. These networks will be far, far faster than current 3G networks. But speed isn't actually the main reason why every carrier in the developed world is trying to extend the life of 3G networks by speeding them up, and build 4G networks as fast as equipment and financing is available. It's all about congestion and capacity, not speed. (In the developing world, 2G networks are being boosted to 3G speeds, too.)

"They're not really for speed, they're not really for voice, they're for capacity," said Allen Nogee, a principal analyst at the In-Stat research firm. Overwhelmed network operators need to handle more devices, each of them owned by high-revenue customers.

Regardless, this year we have already seen significant 3G network upgrades by AT&T and T-Mobile, the mass-market expansion of Clearwire's 4G WiMax network, and the commercial launch of Verizon's 4G LTE (Long Term Evolution) network.

What's the road to 4G look like? So far, it's remarkably free of missteps; delay has been the watchword instead of disaster.

The current landscape

If you've used an iPhone in San Francisco or New York, I don't have to explain the problem of congestion to you.

The iPhone is the flag bearer for mobile networks that were underbuilt and overutilized. AT&T hemmed and hawed, but the company has now quite openly admitted that it wasn't spending enough on its mobile network, particularly on backhaul, and has stepped up tremendously.

There are two major parts of a cellular network that handle data relevant to our purposes: the local link, or the wireless connection from your phone, laptop, MiFi router, or other device equipped with mobile broadband, to the nearest cellular base station on a roof or tower; and the backhaul, which connects all the radio gear from a carrier to its core network.

(There are, of course, a whole lot of other components, too, such as the core network, the base station software, the network's interchanges with other networks, and so on, but these pieces have less obvious and direct impact on what an end user experiences.)

Let's address backhaul first, because it's much simpler. Most cell base stations had relatively paltry backhaul until the last two years, even in dense urban markets. Before 3G started to be heavily used by mobile devices, and for previous 2G networks, a 1.5 Mbps T-1 line or an even slower link (whether wireless or wired) could handle all the voice and data capacity for a carrier. Hundreds of voice calls and multiple data devices wouldn't flood the line.

Generation Gap

There are four defined generations of cellular technology. 1G was all analog. 2G is digital, with a small provision for data networking at speeds slower than dial-up modems. 2.5G was a stopgap of 100 to 200 Kbps data networking while 3G was finalized and deployed. 3G networks needed to have a raw downstream rate of 2 Mbps or higher, but voice and data traffic are managed separately. 4G networks are ostensibly all Internet protocol with low latency to allow voice and video to work well without extra effort.

But a cellular node can have multiple pieces of gear and handle multiple cellular channels at once. In the 3G-and-beyond world, that could mean devices capable of pulling down 3 to 4 Mbps per channel. A T-1 doesn't cut it, and the move from T-1 to 10 Mbps isn't a smooth one unless the carrier is the incumbent local exchange carrier (ILEC) and it's in an urban area with plenty of wire or fiber.

Carriers have been struggling and investing many billions of dollars to build out fiber or high-speed, high-frequency, point-to-point wireless links to handle the additional backhaul needed. In areas where one carrier has to deal with another—for Sprint and T-Mobile, that's everywhere—it can be even harder.

In a recent interview with T-Mobile, in which the firm announced that a faster 3G flavor was in place (see below), the company said it had paired its new installations with the fastest backhaul it could arrange.

"That's the most complicated part of the upgrade and something we've been at for some time," said Jeremy Korst, a T-Mobile director in charge of broadband products and services. T-Mobile spokesperson David Henderson noted that the new service was a "pretty basic upgrade from a radio side, and then the backhaul is really the biggest gating factor." Fiber and wireless are both being used.

AT&T seems to agree. While announcing an identical network service upgrade in January, the company said it wouldn't be switching on the faster flavor on the back-end (only on the local link) until it had finished a project to "dramatically increase the number of high-speed backhaul connections to cell sites, primarily with fiber-optic connections."

It's odd—possibly even ironic—how wireless networks are so heavily constrained by wires and cables, but that's how the business works.

For the local link, there are currently four dominant network standards for 3G and 4G that are deployed or in the process of being deployed: the GSM flavors of 3G HSPA and 4G LTE, CDMA's 3G EVDO Rev. A, and WiMax, which is considered a 4G standard.

GSM network standards, which dominate cellular markets worldwide, have been broken into many incremental improvements, which allowed a lot of stairstepping and retrofitting of 2G and 3G networks. For instance, the so-called 2.5G (a spot between 2G digital and 3G broadband mobile networks) EDGE standard was developed to provide speeds several times faster than 2G data (GPRS) both before 3G could be deployed and in markets where it was cost-prohibitive to build 3G. EDGE was a relatively minor software and hardware upgrade on the base station side, and less expensive to build into handsets and other devices. Apple's very first iPhone could work only as fast as EDGE.

From EDGE (usually 100 to 200 Kbps), networks moved to UMTS (384 Kbps), and then various flavors of paired HSDPA (High Speed Download Packet Access) and HSUPA (the U for Uplink). HSDPA rates commonly deployed are 3.6, 7.2, 14, and 21 Mbps depending on the network; HSUPA rates run from 1.5 Mbps to 6 Mbps on production networks. AT&T has paired HSPDA on nearly all of its phones with UMTS for upstream.

On the CDMA side, the US market halts at EVDO Rev. A, even though Qualcomm, CDMA's prime mover, had a Rev. B update and a 4G standard in development. Rev. A tops out at 3.1 Mbps downstream, and 1.8 Mbps upstream. (Rev. B will appear in non-US markets, while Qualcomm's 4G Ultra Mobile Broadband was abandoned.)

The two CDMA carriers in the US, Verizon Wireless and Sprint Nextel, took different 4G courses. Verizon opted for LTE, which it knew would be years away and which it didn't possess the right spectrum for at the time it made its technology decision. Sprint chose WiMax, which was a near-term solution for which it had enough spectrum to deploy right away. Interesting choices in both cases.

I'll get to 4G in a moment.

Bumping up 3G

We all know that 4 follows 3, but just as the sequence in cell generations went 2G, 2.5G (EDGE), 3G in order of deployment, we'll see a lot more tweaks to 3G networks before 4G networks have anything like full national coverage.

T-Mobile already has plans to leapfrog over AT&T, however, with HSPA+, a 21 Mbps variant that it can generally add as a software update.

In January and February, AT&T and T-Mobile respectively announced that their networks had moved from the 3.6 Mbps to 7.2 Mbps (often called HSPA 7.2) flavor of HSDPA. Both networks have also moved to 2.0 Mbps HSUPA. HSPA 7.2 doubles the local link, and it may have an advantage even without increasing backhaul. With a faster local link, you have less congestion among devices contending to have any communication with a cell base station. That means each device can be more fairly dealt with, even while receiving slender slivers of Internet access.

The rub for both firms is backhaul and coverage. As noted above, T-Mobile and AT&T are both focused on improving backhaul. T-Mobile has a little advantage because the company is building its 3G network fresh with spectrum acquired a few years ago. T-Mobile had no 3G network until May 2008, and now covers over 200 million people in nearly 300 cities with service.

AT&T has been building 3G service for years, but hasn't built as fast as one might expect. That telco claims it passes—has the ability to offer service to—just 233 million people with its 3G flavor.

For those who can get access, AT&T and T-Mobile will likely shortly be slugging it out for the fastest 3G network. In a series of PCWorld tests conducted by Novarum in December 2009 and January 2010, AT&T had substantially higher speeds than the other three major 3G network providers, including T-Mobile. That's partly due to AT&T's pre-announcement HSPA 7.2 testing. While HSPA 3.6 typically performs in a downstream rate of 700 Kbps to 1.7 Mbps, AT&T claims, HSPA 7.2 can average 1 to 2 Mbps in typical usage based on results from existing deployments.

Ken Biba, the founder and chief technology officer of Novarum, said that his firm found spikes of near 4 Mbps in nearly all the 13 cities tested. Average raw TCP downloads—which score better than, say, a Web download—averaged over 3 Mbps for 1 minute in 1 percent of tested cities, and 2.5 Mbps in 10 percent of those cities. (Biba said Novarum has no past or current contracts with carriers to test 3G data performance.)

The iPhone 3GS and nine other current AT&T devices use HSPA 7.2, although the company doesn't appear to provide a list. T-Mobile has several HSPA 7.2 compatible devices and smartphones, including its USB webConnect adapter.

T-Mobile already has plans to leapfrog over AT&T, however, with HSPA+, a 21 Mbps variant that it can generally add as a software update. The GSM Association lists 17 networks worldwide already using this flavor, including three in Canada. The company plans to start with coastal US cities in the next few weeks, following commercial availability in Philadelphia, its pilot city, and then fill in most of the rest of its territory during 2010. So far, only one HSPA+ device has been announced—another USB adapter—but there's a variety of HSPA+ gear already for sale in Europe and Canada that can be adapted. (T-Mobile has a nearly unique slice of 3G spectrum that isn't harmonized with any other North American or European carrier.)

These efforts add some life into 3G, and allow AT&T and T-Mobile to both ease congestion and have greater performance than Sprint and Verizon. Sprint and Verizon won't be able to speed up their existing 3G networks, and both are relying on future deployments to let them jump up a notch to 4G, as discussed next.

T-Mobile, as the trailing fourth-largest carrier, and a relatively small amount of mobile broadband spectrum may be able to take the technical advantage for two to three years, as AT&T has no present plans to adopt HSPA+. Monica Paolini, the principal at analysis firm Senza Fili Consulting, said, "An operator with less spectrum, but also fewer subscribers may be better off than a larger operator with more spectrum," but that "HSPA+ is at best a short term advantage" as competitors ramp up for true 4G networks.

Biba of Novarum agreed: "I came away with the clear impression that HSPA 7.2 and HSPA+ have legs over WiMax—much less LTE," he said.

But how long can those flavors compete? It all depends on how fast LTE and WiMax blanket the country.