How does CPDLC work?

From the Canadian Aviation Safety Letter (Canadian Aviation Safety Letter 2014)

CPDLC employs a series of standardized text messages for most routine communications. These include over 200 “uplink messages” (from ATC to the cockpit) and more than 100 “downlink messages” (from the flight crew to controllers). Pilots and controllers also have the option of sending
free-text messages. For the most part, CPDLC works with the click of a mouse. Controllers have drop-down menus on their screens with the standard messages. Menus are divided into different categories to make the appropriate message easier to find. Each ACC can modify their drop-down menus and choose which messages are contained in each message group.

For instance, the Maintain (Alt), Climb to and Maintain (Alt), Descend to and Maintain (Alt), and At (POS) Climb to and Maintain (Alt) would likely go under the Altitude drop-down menu. Other common drop-down menus are Radio, Route, Speed, and Free text.

There are also quick-response buttons for Unable, Roger, Negative, Standby and Deferred.
For downlink messages that require a response, the controller just has to click on that message and the appropriate drop-down menu, and the response is highlighted in green (as opposed to white), making the messages easier to find.

Usage and Equipage
The number of monthly CPDLC contacts in domestic airspace has almost tripled in the 14-month period from November 2012 to December 2013: from just under 18 000 to nearly 53 000. Those numbers are expected to continue to rise as the CPDLC equipage rate increases. The percentage of CPDLC-equipped flights in Canada’s domestic high level airspace varies according to geographical location, from 27 percent in the central Canadian FIRs, to 79 percent near the country’s east coast.

In addition to the many safety enhancements, CPDLC adds an important efficiency benefit. As the need for voice communications decreases, the problem of radio frequency congestion becomes less of an issue. CPDLC also has a multiplier effect on alleviating frequency congestion when you calculate the number of flights using CPDLC. If one data link message can eliminate even 30 seconds of airtime, that can translate to 15 or more hours per day of voice communications
taken off the airwaves.

And finally, for those of you who are wondering about the safety of pilots “texting while flying”, it is always the pilot monitoring (PM) who sends and receives the messages.