Receiving Premium European satellite television is very similar to receiving free to air channels. However very often, premium satellite television comes encrypted and requires decryption in order to watch. Encryption is used to allow only permitted people to watch the premium content. Decryption is achieved with a decryption card and an appropriate CAM (Conditional Access Module). This can either be built into a purpose-built satellite receiver or an external unit which connects into the satellite receiver, the satellite receiver will need to have a common interface slot in order to connect a external CAM.