It's no secret that the iPhone has taxed AT&T's network in densely populated areas, especially New York and San Francisco. Reports of problems using iPhones at major tech conferences, like SXSWi, Macworld Expo, CES, and NAMM are not unusual. The iPhone's ease of use and focus on mobile media generally lead to higher data usage on average, but despite claims by AT&T Mobility CEO Ralph de la Vega, the amount of data being consumed is rarely the problem. The issue has to do with how modern smartphones—beginning with the iPhone—save power by disconnecting from the network whenever possible.

Even though AT&T has made improvements to its network over the last couple of years—including moving towers to an 850MHz spectrum that can more easily penetrate building walls, as well as upgrading to faster 7.2Mbps HSPA+ protocols—those improvements have done little to stem the tide of complaints from consumers in larger urban areas. Those users experience frequent dropped calls and an inability to make data connections, and in general they feel that service is spotty.

To make matters worse, AT&T has announced initiatives to add an even greater number of 3G data devices to its network, including Android smartphones, e-book readers, mobile data modems, and now the iPad. Even if consumers aren't yet concerned about the effect of the increasing number of devices on the network, the FCC sure is.

"With the iPad pointing to even greater demand for mobile broadband on the horizon," wrote FCC director of scenario planning for Omnibus Broadband Initiative Phil Bellaria, "we must ensure that network congestion doesn't choke off a service that consumers clearly find so appealing or frustrate mobile broadband's ability to keep us competitive in the global broadband economy."

The fact that the US lags behind many other countries in both broadband capacity in general and wireless networks specifically is nothing new. But the fact that almost all of the complaints from iPhone users come from the US suggests that AT&T's network is at least partially to blame. In fact, users in other countries have told Ars that they don't experience the kinds of problems that US users often report. Well, other countries except one.

Several users in the UK, almost all in London, reported issues that were very similar to what we've heard from users in the US (and experienced ourselves): frequent dropped calls, lack of voice mail notifications, inability to make or receive calls even when the signal looks strong, and inability to make data connections. These problems were happening on the O2 network, which for several years was the UK's only iPhone carrier.

The carrier apologized to its customers late last year for the spotty service as it trumpeted network improvements meant to address the issues. An O2 employee contacted Ars to explain what caused the problem, and explained how newer smartphones are changing the assumptions that carriers use when configuring their network.

Good for the battery, but not so good for the network

The first problem that O2 encountered was that the iPhone uses more power saving features than previous smartphone designs. Most devices that use data do so in short bursts—a couple e-mails here, a tweet there, downloading a voicemail message, etc. Normally, devices that access the data network use an idling state that maintains the open data channel between the device and the network. However, to squeeze even more battery life from the iPhone, Apple configured the radio to simply drop the data connection as soon as any requested data is received. When the iPhone needs more data, it has to set up a new data connection.

The result is more efficient use of the battery, but it can cause problems with the signaling channels used to set up connections between a device and a cell node. Cell nodes use signaling channels to set up the data connection, as well as signaling phone calls, SMS messages, voicemails, and more. When enough iPhones are in a particular area, these signaling channels can become overloaded—there simply aren't enough to handle all the data requests along with all the calls and messages.

It's important to note, however, that this technique is not limited to the iPhone. Android and webOS devices also use a similar technique to increase battery life. While the iPhone was the first and currently most prolific device of this type, such smartphones are quickly becoming common, and represent the majority of growth in mobile phone sales in the past year.

Our source at O2 told us that network equipment that is configured to handle signaling traffic dynamically—shifting more spectrum to signaling channels when needed—can mitigate this problem. But even with more signaling capacity, network nodes may not be able to set up a data session, or may have problems getting a valid network address from an overloaded DHCP server. He said that data capacity is rarely the problem—nodes themselves can usually handle much more data than is flowing through them. However, the networks need to be configured to handle a growing number of devices connecting and disconnecting at a much higher rate than they've been accustomed to.

We spoke to another expert who works in the telecom field to find out why most European networks were not experiencing the problems that AT&T and O2 did. He told Ars that Europe embraced heavy text messaging and data use far earlier than users in the US. SMS and MMS messages rely heavily on signaling channels to operate, and so networks were generally configured to dynamically manage changes in signaling traffic.

O2 worked with its network equipment vendors to identify the problems and adjust the configuration to adapt to the changing needs of its smartphone users. For its part, AT&T has announced publicly that it is increasing backhaul capacity by running fiber to its cell sites, and plans an additional investment in network infrastructure—including adding up to 2,000 additional towers this year. The company would not comment on its efforts to address the kinds of issues described by our source at O2, though we know that O2 shared what it learned with AT&T and other carriers.

Apple has also stated that it is confident that AT&T can handle the additional network demands that the iPad would add to the growing tide of smartphone traffic. "As you know, AT&T has acknowledged that they are having some issues in a few cities and they have very detailed plans to address these," Apple COO Tim Cook said during the most recent quarterly earnings call. "We have personally reviewed these plans and we have very high confidence that they will make significant progress towards fixing them."