6:20 PM -- As Long Term Evolution (LTE) networks get deployed more quickly and more widely in the U.S., it's time to start looking beyond raw data speeds and ask about depth of coverage and the quality of the 4G networks.

Verizon Wireless said recently that it is ahead of schedule to cover its 3G footprint with LTE by mid-2013. The operator is launching another batch of smaller towns in Colorado, Wyoming and Missouri on 4G this Thursday.

AT&T Inc. (NYSE: T), meanwhile, is planning to have 300 million potential customers covered with LTE by the end of 2014. Sprint Corp. (NYSE: S) and T-Mobile US Inc. have similarly ambitious schedules, meaning the grunt work of deploying an LTE network for the first time should be done by 2014, assuming no carrier falls behind schedule.

Now the thing about coverage is that not all coverage is created equal. RootMetrics CEO Bill Moore told me recently that Verizon typically has the best coverage across separate towns and cities -- but even Verizon isn't at 100 percent yet. The strategy established by the carriers, especially Sprint, is to get some useful coverage up if they can't blanket the entire city with 4G.

So, we need a way -- beyond carrier-provided coverage maps -- to establish the density of coverage in a market as a useful metric. This would give us another angle to grade performance.

AT&T appears to be moving toward trying to address the density issue. Executives announcing its $14 billion wired and wireless upgrade program last week said part of the money will be spent on deploying 40,000 small cells through 2015.

Verizon is being more coy about its small cell situation. Verizon CFO Fran Shammo said last week, however, that the operator is already using the tiny radios where necessary. It's just not clear yet if the boxes are of a 3G or 4G LTE persuasion.

Of course, soon enough, we'll be talking about quality-of-service for voice-over-LTE and mobile video and the technical challenges that entails. It also seems to me that if the mobile operators want LTE to replace copper phone lines, then there should be some more service-level agreement work done with the new networks. (See 4G Kills the Copper Plant.)

No doubt all these topics will be covered at some length in 2013. Density of LTE coverage, however, seems like the best way to understand how the second major phase of operators' 4G deployments are shaping up. (See The Best 3G & 4G Cities in America.)

— Dan Jones, Site Editor, Light Reading Mobile