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.