Timestamp generation MAY be used in one of the following modes: 1. Absolute mode: the PSN-bound IWF sets timestamps using the clock recovered from the incoming TDM attachment circuit. As a consequence, the timestamps are closely correlated with the sequence numbers. All TDM implementations that support usage of the RTP header MUST support this mode. 2. Differential mode: Both IWFs have access to a common high-quality timing source, and this source is used for timestamp generation. Support of this mode is OPTIONAL.