But there is also technical part of the whole story and regardless of the outcome it's worth having a look at the different systems which are involved electronically.
There are four main systems which provide communication between an airplane and the ground. The oldest one, which helped the british army to identify their german counterparts, is the
Primary RadarHigh frequent impulses are sent out by a (ground) station into the air and if they are deflected they echo back to the receiver at the (ground) station. The time the signal runs can be calculated and from that the distance and direction of the "object" is derived.
Primary radar signals have to be interpreted so it's sometimes not easy to judge if the "object" is a plane or a flock of birds. There are also limits due to the range of the radar waves and other factors like weather.
Some producer of flying (military) objects - stealth planes - even use their outer shape to avoid any de- or reflection of the radar waves.
To support the findings of the passive primary radar most of the planes, helicopters etc. send out an active transponder signal which is termed
Secondary RadarWhere the word "radar" stands for an active transmission of a signal coming from aboard a flying object. Well known as "transponders" the device sends an active signal which can be of different quality. The earliest systems gave an identifier or answer code (aka "Squawk code") so that the signal of the primary radar and the received signal of the transponder could be matched.
Today a system known as "ADS-B" - Automatic Dependent Surveillance - Broadcast - is used in many (commercial) airplanes. Once the systems on the plane are on, ADS-B is also automatically activated. Already on ground the system gets automatically interrogated by ground (radar) stations so that the signals can be matched. But those newer systems not only give an identifier, but also lots of other data like speed, altitude, heading and more. The Boeing 777-200 MH370 is equipped with those systems. The range also depends both on the (height/ distance) of the airplane and the quality of the receiving (ground) station.
The frequency of ADS-B is 1090MHz.
Voice Radio TransmissionsEven if many of the processes in aviation are automatically done there is still a lot of live voice communication between pilots an (ground) stations. This is done mostly on VHF (Very high frequency). The normal air radio frequencies are between 108 and 137 MHz. Military often uses other (higher) frequencies.
As with all high frequencies the transfer of messages underlies technical constrains like range, weather, power of the sender, height, and receiver sensitivity. So in some parts of the world there still have to be used less sophisticated radio signals to get contact to the ground.
ACARS - Aircraft Communications Addressing and Reporting SystemAnother active system is known as ACARS - Aircraft Communications Addressing and Reporting System. It does what the name implies - it actively, automatically, un-interrogated sends data live to the ground or up to a satellite. Data can be of any quality - from simple status messages to extensive data reports from systems on the plane. Allmost all commercial aircrafts send those data to their headquarters relayed by radio stations around the world.
As allegedly done and reported by the Wall Street Journal the MH370 sent (engine) data for hours into the flight. Depending this is true it is still the question of what quality the data was.
-UPDATE Malaysian officials have told there were no ACARS radio transmission after losing contact to MH370. Last ACARS transmissions were at 01:07AM local time -
The radio transmissions are done in some of the air band frequencies so they underlie the some constraints as mentioned above.
Then there are sensors and radio transmissions more or less independent on the before mentioned systems like the one of the jet engines, which are capable of sending sensor data without using the ACARS system (e.g. http://www.rolls-royce.com/about/our-technology/enabling-technologies/engine-health-management.aspx#sense). Those were of special interest in the MH370 case because there were transmissions long after disappearing of the plane and some attempts of triangulation to get a possible location of it.
In case of an emergency EPIRBs - Emergency position-indicating radiobeacon stations - are automatically activated. They are also constrained in respect to range and especially the time (about 30 days) they are able to send signals. There are a lot of systems for different use cases available (More info: https://en.wikipedia.org/wiki/Emergency_position-indicating_radiobeacon_station).
Finally there are other means of identifying flying objects like satellites which are capable of visual or radio tracking, flying radar/ radio stations like AWACS-planes (Airborne Warning And Control System) and other and more secret ways to identfy objects in the airspace.
But those are special ways and normally not involved with commercial aviation.