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The easiest way of synchronising between streams at different sites is
based on providing a globally synchronised clock. There are two ways this
might be done:
- Have the network provide a clock. This is used in H.261/ISDN based
systems. A single clock is propagated around a set of CODECs and MCUs.
- Have a clock synchronisation protocol, such as NTP (the network Time
Protocol) or DTS (Digital Time Service). This operates between all the
computers in a data network, and continually exchanges messages between
the computers to monitor:
Alternatively, the media streams between sites could carry clock offset
information, and the media timestamps with arrival times could be used to
measure network delays, and clocks adjusted accordingly, and then used to
insert a baseline delay into the adaptive playout algorithms so that the
different streams are all synchronised.