Mode 2 is the main version of VDL being implemented for support of Controller Pilot Data Link Communications (CPDLC).  Mode 2 specifies 3 layers: the Subnetwork, Link and Physical Layer. The Subnetwork layer specifies and end to end data protocol for use with multiple ground stations and subnetworks including VDL. The Link layer consists of 2 sublayers (Data Link & media access control MAC) which provide aircraft with positive link connection to a ground station. The Physical Layer includes the use of 25 Khz VHF digital radios.