Russia's ability to eavesdrop on terrorists' and rival forces' communications in the Syrian battlefield may have suffered a monumental setback following the accidental shooting down of the Ilyushin Il-20M Electronic Intelligence (ELINT) aircraft.
The ELINT aircraft was supposedly only one of its kind deployed in Syria and possibly one of 10 in possession of the Russian Aerospace Forces. The Cold-War era planes were apparently not equipped with any counter-measures which could have alerted the aircraft the moment the 'enemy' missile had achieved a 'lock-on' to the aircraft.
The Il-20 platform is equipped with infrared and optical sensors, a Side-Looking Airborne Radar (SLAR) mounted ventrally and dorsal and fuselage-side and satellite communication equipment for real-time data sharing and a wide array of antennas. These aircraft are used to perform long-range Intelligence gathering, Surveillance and Reconnaissance (ISR) missions to serve accompanying aircraft.
At the time of the incident, the Il-20 was likely monitoring NATO warships operating in the Eastern Mediterranean.
The other factor can be the incorrect identification of targets by the Syrian forces despite the provisions of Identification Friend or Foe (IFF) systems.