At the link layer, AX.25 uses HDLC frame syntax and procedures. (ISO 3309) frames are transmitted with NRZI encoding. HDLC specifies the syntax, but not the semantics, of the variable-length address field of the frame. AX.25 specifies that this field is subdivided into multiple addresses: a source address, zero or more repeater addresses, and a destination address, with embedded control fields for use by the repeaters. To simplify compliance with amateur radio rules, these addresses derive from the station call signs of the source, destination and repeater stations.