CCSDS AOS provides two levels of multiplexing. The virtual channel access sublayer provides virtual channels that allow several users to use the same physical channel simulateously. The vitual channel link control sublayer multiplexes packetized data allowing several users to utilize the same virtual circuit. The virtual channel access sublayer provides virtual channel service data units (VC_SDUs). These units are transmitted over the channel using VCDU (virtual channel data units). When error correction is used the transmitted units are called CVCDU (coded virtual channel data units).