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 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 .