An ATM-based PON specification described in ITU-T G.983.3 (2001) and based on the original APON (ATM-base PON) specifications ratified by the ITU-T in 1998. BPON runs in asymmetric mode at 622 Mbps downstream and 155 Mbps upstream, or in symmetric mode at 155 Mbps over a distance of as much as 20 kilometers (12 miles). BPON supports as many as 32 splits, that is, splitters can divide the signal to serve as many as 32 premises from a single optical fiber. BPON employs wavelength division multiplexing (WDM) for downstream transmission, with as many as 16 wavelengths with 200 GHz spacing and 32 wavelengths with 100 GHz spacing between channels. BPON provides for enhanced security through a technique known as churning in which the encryption key is changed at least once a second between the Optical Line Terminal (OLT) at the headend and the Optical Network Terminal (ONT) at the customer premises. PON variants also include Ethernet-based PON (EPON), and gigabit PON (GPON). See also APON, asymmetric, ATM, churning, downstream, EPON, GPON, ITU-T, PON, splitters, symmetric, upstream, and WDM.