Interesting article in the current AOPA Pilot magazine (part of their Future Flight series) that deals with this topic. I've cut and pasted the relevant extract below.
Frequency congestion and 8.33 kilohertz spacing
With the across-the-board increase in air traffic we've seen in recent years, our existing pool of allocated VHF communications frequencies is rapidly loading up. On some ATC frequencies it often seems that you can hardly get a word in edgewise. The growth in air traffic has already prompted two jumps in the communications frequency spectrum—from 180 to 360 channels, and from 360- to 720-channel frequency spacing—and now we can expect more splitting of the aviation communications band.
The VHF spectrum currently allocated to aviation communications runs from 118.000 to 135.975 megahertz. Notice that there are three numerals after the decimal point. A new initiative would give us four numerals after the decimal point, and 2,280 more badly needed channels. These additional channels would come about by dicing up the comm spectrum into 8.33-kilohertz slices. It's a proven technique that's already being used to reduce delays in Europe, where airline traffic has exploded in volume over the past 10 years.
Now the United States must deal with the same problem, using the same method. This means a vast overhaul of ATC and aircraft hardware in the next few years. Translation: Unless you already have a brand-new comm radio (the newest radios are already equipped with this feature, to make them competitive in the European market), you'll have to buy one with 8.33-kHz spacing in about six years or so. Of course, so will ATC. The inevitable delays in implementing a project of this magnitude suggest that the timetable for making the switch to the new spacing probably won't happen until sometime after 2005.
To ease the transition, it's very likely that the new frequencies will be phased in gradually, say, with usage initially limited to the high-altitude airspace structure. But there'll be nothing to ease the economic burden of gearing up your panel to play the expanding-decimal game of the future.
VDLs 3 and 4
There's an alternative to the 8.33-kHz expansion. Some advocate the use of digital communications, using VHF datalink (VDL) frequencies 3 and 4, saying the demand for additional communications and other uplinked functionality can't be served any other way. By 2009, proponents say, we may well be using VDL 3 and 4 for voice, uplinked weather information, and ADS-B (automatic dependent surveillance-broadcast) and other traffic advisories. These VDL frequencies would be squeezed into the one-kHz segment between 136.00 and 137.00 MHz.
VDL uses what experts call time differential multiple access (TDMA) technology. Under this scheme, the pilot with a VDL 3 radio makes a radio call—or gets a textual clearance change from ATC—from a ground-based transceiver. The radio traffic works both ways in a request-reply arrangement. VDL 4 adds ADS-B or TIS traffic information, weather graphics, and other information in its uplink capabilities; it was mentioned briefly in an earlier installment of "Future Flight" (see "Links to Tomorrow," February Pilot). The difference between VDL 3 and VDL 4 is that VDL 3 "listens" for one of four time slots to open within each of 20 channels spaced 20 kHz apart, then sends or receives its transmissions when an opportunity avails itself. It doesn't take long, however, with transmissions able to come and go at almost instantaneous speed. VDL 4 doesn't need to operate under this scheduling scheme—its messaging is self-organizing, depends on airborne equipment to process messages, and can operate independently of ground-based equipment.
Though its implementation could take more than a decade, those in the know say that a VDL-capable radio ought to cost the approximate equivalent of $5,000 in today's dollars. The good news about VDL 3 and 4? They should serve aviation's communications needs for a 30-year period, and their use will probably be limited to the high-altitude airspace structure, where it would primarily serve airliners.
What about VDL 2?
It's understandable if you've gone acronym-happy by now. The happiest of you are probably wondering: Hey, he mentioned VDL 3 and 4. What happened to VDL 1 and 2?
VDL 1 and VDL 2 are alive and well these days, serving as the vehicle for transmitting digital ACARS (airborne communications addressing and reporting system) messages. ACARS is used by the airlines and larger corporate jets to forward messages. The airlines, for example, use ACARS to learn of gate changes, and transmit passenger, maintenance, and other information between their dispatchers and aircraft. VDL 2 is also being used in an experimental program aimed at streamlining ATC workload. Known as CPDLC (controller-pilot datalink communications), the idea is to free controllers to use their time focusing on sequencing and separation of traffic.
One study found that 40 percent of a controller's workload consisted of such routine—yet time-consuming—tasks as establishing initial radio contact with air traffic or handing traffic off to another frequency. With CPDLC these contacts are uplinked to the cockpit in text format using VDL 2.
In spite of the glitz surrounding VDL, so much progress has been made in the 8.33-kHz area that it will likely become the method of choice. The 8.33-kHz option promises more frequencies, much sooner than VDL. And with the threat of imminent "frequency gridlock," 8.33-kHz spacing appears a simpler, more attractive choice.