Intelsat has managed to regain communications with the Galaxy 15 spacecraft which failed in orbit earlier this year:
Data from various subsystems ( fuel level values, battery voltages, online/standy state of amplifiers and receivers etc ) is converted to digital data where appropriate and multiplexed together. This data is then used to PSK modulate ( as in many Wi-Fi systems ) a carrier prior to being transmitted down to earth where it is received via a TTC+M ( Tracking, Telemetry, Command and Monitoring ) station for analysis.
The carrier which is used to transport the data from the spacecraft is known as the satellite beacon ( which is where the Wi-Fi world got the term from ). These may be at several different frequencies. For example, 3947.5 MHz ( 3.9475 GHz ) and 3952.5 MHz ( 3.9525 GHz ).
Normally communication signals are sent via directional antennas towards the earth ( usually parabolic antennas ). Beacon signals can be sent via an omni-directional antenna however due to the fact that we may not know the orientation of the spacecraft relative to the earth. It could have its directional antennas facing away from earth or even be spinning around.
Command signals are uplinked on a special frequency from a TTC+M facilty. Security precautions are put in place to help prevent "anyone" from sending commands to the satellite to do something undesirable.