As has been previously mentioned, a CODEC is necessary to enable the sound and vision signals to be converted and then transmitted from the local site. Another CODEC is also required at the remote site to convert the digital signals back into sound and vision signals for the audience. The method of transmission for IP and ISDN are different and so require different CODECs. ISDN, once the mainstream medium for videoconferencing, has been overtaken by IP so that IP CODECs are now usually offered as standard with an ISDN option if required.