AX.25 is the most widely used amateur radio packet-switching protocol. The AX.25 protocol dates back to 1978. It was developed based on the standard X.25 packet-switching network protocol (Amateur X.25). The 2.0 version of AX.25 was released in 1984. This version became very popular. In 1997, version 2.2 of the AX.25 protocol was released. The AX.25 protocol is a data link layer protocol. In practice, AX.25 is used almost exclusively for 300 baud Bell and 103 tone HF and 1200 or 9600 baud Bell and 202 tone VHF, although NRZI encoding is different from phone modem standards, as it uses NRZS instead of NRZM.