Afar NX-2E1, Operator'S Manual

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NetCrossing Gateway NX-2E1/T1 Operator’s Manual
2-6
The gateway keeps statistics on the network jitter and this is a great tool in establishing the optimum
jitter buffer size. When you don’t know the best jitter buffer value, set it to a high number to avoid
underrun errors and let the link run over the peak traffic period of the WAN. Use the
show command
to examine the worst case jitter encountered. Once you know the worst case jitter reduce the jitter
buffer size to be slightly above that value.
The total link latency can be approximated as:
Latency = 4.0 ms + jitter_ WAN delay.
where the value for the “WAN delay” is the best case delivery across the WAN.
2.3.4 Clock Sources
In a serial synchronous connection it is required to have a single source to clock the data stream in
both directions. In each TDM line the Rx clock is always recovered from the incoming Rx data on
that line. You can specify the Tx clock source for each gateway in one of four different modes, as
described below:
Internal : The Tx clock is generated by the gateway using a crystal controlled frequency synthesizer.
External : The gateway uses the recovered Rx clock as the source for the Tx clock. This is
sometimes called “loopback” mode.
Remote : The Tx clock is generated in the gateway frequency synthesizer. Once a link is established
this clock is locked to the recovered Rx clock of the remote gateway.
Off : The Tx clock is turned off. This is used to completely disable one of the TDM ports.
The three active configurations can be combined in only three compatible ways as shown in the
following figures.