The original PON specifications set by FSAN and ratified by the ITU-T as G.983.1 (1998). APON specifies asynchronous transfer mode (ATM) as the Data Link Layer protocol. APON runs in asymmetric mode at a signaling rate of 622 Mbps downstream and 155 Mbps upstream, or in symmetric mode at 155 Mbps.The more contemporary broadband passive optical network (BPON) is an APON variant. See also asymmetric, ATM, BPON, Data Link Layer, downstream, G Series, PON, protocol, signaling rate, and upstream.