The timing and shelf controller (TSC) card performs all system-timing functions for each ONS 15600. The TSC card monitors the recovered clocks from each traffic card and two BITS interfaces for frequency accuracy. The TSC card is provisionable, allowing timing from any optical interface source, a BITS input source, or the internal Stratum 3E as the system-timing reference. You can provision any of the clock inputs as primary or secondary timing sources, but the ONS 15600 does not support mixed timing references. If you specify external timing references, your options are BITS1, BITS2, and the internal Stratum 3E sources. If you select line timing, you can specify up to two line ports from which to derive timing, and the internal stratum 3E sources. You cannot specify BITSx as the primary reference and a line source as the secondary reference. A slow-reference tracking loop allows the TSC to synchronize with the recovered clock, and enables holdover if the reference is lost.The TSC card also provides shelf control related functions. The TSC card has a 100 Mbps Ethernet link to each card on the shelf and monitors the presence of these cards. The TSC provides bulk memory for non-volatile storage of system software and data and provides RS-232 and Ethernet customer interfaces. The TSC card processes and routes line and section DCC traffic as well as routing the K1, K2, and K3 overhead bytes between traffic (line) cards and CXC cards. The TSC card controls and monitors the shelf fans and all of the alarm interfaces.