The interface is often a DS1, with sync-status messages supplied by the clock and placed into the DS1 overhead. Line-derived timing A network element can choose (or be configured) to derive its timing from the line-level, by monitoring the S1 sync-status bytes to ensure quality. Holdover As a last resort, in the absence of higher quality timing, a network element can go into a holdover mode until higher-quality external timing becomes available again.