The link budget assesses the quality of a satellite link. It compares the received signal power available via the uplink station, satellite and downlink station with the combination of noise and interference which arises in the link. A basic satellite link is illustrated which shows an uplink earth station, a satellite and a downlink station. In practice, especially in VSAT systems, there are a large number of small earth stations and usually a single large earth station, sometimes also known as a hub earth station. The need for a large earth station will become apparent from consideration of link budgets. The difference between the power of the wanted signal and the sum of interference and noise power in the same bandwidth for a satellite link is often small. A satellite communications link budget shows that the link is often severely power limited. Signalling techniques must be used that can provide an acceptable level of service in this situation.