Network timing is derived from the PSTN and distributed through the network to gateways capable of deriving timing from the incoming UDP
stream. The derived timing has the correct frequency for voice
telephony without using external timing sources or extraneous hardware components. For example, a
digital signal processor (DSP) can derive the timing from a timed
TDM bus and distribute messages, such as IP messages, to other gateways or port networks. The other gateways and port networks use the incoming
stream to extract the timing which is then used to time their
TDM bus. The port networks and gateways can also distribute other streams to other gateways in a fan-out type of arrangement. This internally generated timing can be used, for example, for Circuit Emulated Services (CES).