In particular, five functions are specified to ensure interoperability: connectivity, timing,
signaling, MEN performance criteria, and MEN services OAM. MEF 8 focuses
especially on timing and signaling issues. The specification also augments the performance
characteristics defined in MEF 3.
Timing/Synchronization Synchronization is an important consideration in any circuit
emulation scheme and the clock of the incoming signal (into the IWF) and outgoing
signal (to the IWF) should be synchronized (i.e., the frequency should be the same).
There are four options for this clock:
?– TDM line timing Use the clock from the incoming TDM line.
?– External timing Use an external reference clock source.
?– Free run timing Use a free-running oscillator.
?– Ethernet line timing Recover the clock from the Ethernet interface.
82 Chapter 2
The last option, Ethernet line timing, covers all methods where information is extracted
from the Ethernet, including adaptive timing, where the clock is recovered from data in
the CESoE frames and the arrival time of the frames, and differential timing, where the
clock is recovered from a combination of data contained in the CESoE frames and knowledge
of a reference clock common to both the SEN-bound and TDM-bound IWFs.
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