If you've owned a smartphone on AT&T for longer than a couple years, you likely remember this well: the network was pushed to the brink of disaster by the overwhelming popularity of the iPhone 3G, 3GS, and 4. Prior to the iPhone, it had been a commonly-accepted principle that the bulk of the wireless market would gradually move from feature phones to smartphones, but uptake of Apple's game-changing entry outpaced even the most generous estimates — carriers, particularly AT&T, were simply caught off-guard and ran into an unprecedented capacity crunch. The UMTS network, still relatively young at that time, couldn't handle it. Calls failed, web pages didn't load, text messages didn't send.

Since then, AT&T and its competitors have made big moves to hedge against a data-rich nation whose citizens are loading Facebook and uploading photos on a never-ending cycle: spectrum refarming, build-outs, licensing, increased backhaul capacity, and attempted acquisitions. But the ripple effects of the iPhone's explosive success persist to this day. In some markets, the data pipe still grinds to a halt during peak hours. Less populated networks — namely Sprint and T-Mobile — try to entice consumers with the promise of all-you-can-eat buckets, their towers largely unburdened by the overcrowding from which AT&T has never fully recovered.

LTE has to prove that it can take on the iPhone

That's where LTE comes in. Until now, the next-gen wireless technology has served as a relatively quiet, peaceful oasis, almost fully protected from the claustrophobic mayhem of the legacy 3G networks that it is designed to eventually replace: even in heavily populated areas, it hasn't been uncommon to see downlink speeds in excess of 20Mbps with latency of well under 100ms. In other words, LTE is fast — fast enough for a phone, fast enough for a mobile hotspot. That's partly because LTE is inherently a faster, better, more efficient technology, but it's also because of the obvious: it hasn't been graced with a worldwide phenomenon like the iPhone.

That's not to say LTE's arrival has been trouble-free. Verizon, which has scaled up its nationwide 4G footprint the fastest among American carriers, had stumbled through a series of growing pains through 2011 and early 2012 that only now seem to be ironed out. And here in Chicago, I've noticed that LTE speeds on AT&T's network have fallen into the single digits during peak hours since the launch of the wildly popular Samsung Galaxy S III — still plenty fast, but that'll only be impacted further by the launch of an iPhone 5 that's practically guaranteed to sell by the tens of millions over the coming months.

"We were able to take key learnings from our experience leading the smartphone revolution and build that into our 4G LTE network," says AT&T spokesman Seth Bloom, noting that the cellular topology is in many ways more advanced than it was even a few years ago: carriers are now investing in small cells, self-optimizing networks, and short-range solutions for particularly dense environments like sports venues. That's good news, but obviously, the proof will be in the pudding; only after several months of the iPhone 5's deployment will carriers reasonably be able to declare success in the fight against crushing data demand. Any outage or glacial web page load time will be perceived as a failure, and a very public one at that. Apple waited a generation with the iPhone until LTE was sufficiently mature to integrate it, and Cupertino is undoubtedly hoping that holding off has given the networks a chance to settle — another nationwide Verizon data outage will affect not just Verizon customers, but Apple customers as well.

Any outage or glacial web page load time will be perceived as a failure

In the meantime, there's no way of knowing whether AT&T, Verizon, or — in the handful of markets where it's live, Sprint — is best prepared to handle this launch. Ironically, AT&T's HSPA+ network will almost certainly begin to speed up as its customers upgrade to the iPhone 5 and offload usage to the LTE network. In the long term, carriers will siphon off 3G capacity and use it to augment the newer technology — just as they're beginning to do with 2G today — but for now, it's back to the age-old equation of supply and demand. Or in Apple's case, crushing, unprecedented demand.

Verizon did not respond to a request for comment.