Backhaul has become a strategic concern for ISPs - especially mobile ISPs - in recent years, where historically it has often been more a tactical concern.

There are several reasons, which are related to the ways devices and apps are used by end users. Smartphones now represent hundreds of millions of active devices, all able to connect to Wi-Fi hotspots.

And, because of the higher cost of mobile Internet access services, compared to fixed network alternatives using Wi-Fi, users have incentives to do so.

Generally speaking, the retail cost of a gigabyte of usage is an order of magnitude higher on a mobile network, compared to a fixed network.

Because of the high cost of supplying mobile Internet access bandwidth, mobile service providers also have incentives to encourage such behavior.

One salient example is the AT&T introduction of the Apple iPhone. When the iPhone was launched in June 2007, almost no AT&T customers used a smartphone.

By the the end of the first quarter of 2012, 59 percent, or 41.2 million, of AT&T's postpaid subscribers had smartphones, lifting AT&T mobile data traffic 20,000 percent in five years. In fact, AT&T mobile data volume has doubled every year since 2007.

You might recall the widespread problems AT&T experienced when 3G network demand suddenly increased, leading to extensive consumer irritation about slow speeds.

“It’s been a challenging year for us,” said John Donovan, AT&T CTO, in 2009. “Overnight we’re seeing a radical shift in how people are using their phones,” Donovan said. “There’s just no parallel for the demand.”

In many cases, iPhone users were consuming an order of magnitude more data than users of other smartphones and 24 times more data than feature phone users.

That leads Professor Philip Weiser, Dean of the University of Colorado Law School, and Silicon Flatirons Center executive director, to quip that “Wi-Fi offload saved AT&T” when the Apple iPhone was launched.

Likewise, Joan Marsh, AT&T VP, has acknowledged that AT&T Mobility now “depends on offloading” to support its mobile business. “We increasingly look to unlicensed spectrum to augment our licensed spectrum,” Marsh said.

Up to this point, such offloading of demand largely has relied on at-home Wi-Fi, fortified by public Wi-Fi hotspots.

In the next evolution, new small cell and carrier Wi-Fi will be overlaid, rather densely, on the existing macrocell network. And that makes backhaul a newly-important challenge. Not only will such networks represent hugely more dense deployments of base stations, with new challenges in the area of authentication and hand-off between cells, but such small cell and public Wi-Fi cells will feature very-high bandwidth requirements at the same time affordability also is required.

Cell geometry is key, in that respect.

When a carrier shrinks cell radius by 50 percent, the number of required cells quadruples. That implies an exponential increase in the number of cells, and therefore backhaul connections.

For example, where a macrocell might have a radius of four miles, a small cell might have a radius of just hundreds of feet. So blanketing that one area, with small cells of a quarter mile coverage, would require 256 new cell sites. And that implies backhaul connections two orders of magnitude greater than before.

According to ABI Research, 2014 sales of small cell transmission gear will grow 33 percent to $1.8 billion.

This growth comes thanks to operators like AT&T, Verizon, Vodafone, Telefonica, Softbank, SK Telecom, and Sprint driving shipments of both outdoor and Metrocell deployments.

In 2014, 4G small cells are the fastest growing small cell type in the market driven by venue and dense urban deployments, ABI Research argues.

In fact, ABI Research forecasts the number of LTE small cells will grow by 200 percent in 2014 and by a similar factor each year through 2019, when the value of LTE small cells will reach more than $5 billion in equipment, while another $5 billion will be spent for 3G solutions.

The Asia-Pacific region will represent over 50 percent of the global small cell equipment market by 2019.