explain about downlink and uplink????

These are two types of frequencies used in communication satellites . Uplink is the frequency which is received by the receiving antenna of satellite. Hence the signal received by satellite from ground station may be called uplink. Similarly , after receiving the signal, the satellite removes noise, increases the signal strength and then transmits to the ground station. This is called downlink.

The satellite uses two different frequencies for down link and up link to avoid the interference between strong transmitting station and weak receiving station.

Uplink frequency is always greater then the down link frequency for effective utilization of power in the satellite.

The above figure shows the uplink and downlink frequencies for two separate communication bands.