"Timing" indication after some modulation modes

What are the criteria LinkPlanner uses to decide that a specific combination of bandwidth and modulation will not be usable to transport a specific amount of T1s?  I am referring to the « Timing » indication after some modulation modes.

Thanks!

From the PTP 650 Series User Guide:

"In narrow channel bandwidths and lower modulation modes, the link may have insufficient capacity to relay the E1/T1 payload; in this case, the wireless link continues to carry timing information in order to maintain accurate clock synchronization.

The relay of TDM data resumes automatically when the link reaches a suitable modulation mode.

Links that are able to operate consistently in a high modulation mode can take advantage of lower link latency.

This option is configured by setting the "Lowest TDM Modulation Mode" during installation.

Appropriate settings for this control may be determined by using the LINKPlanner tool.

The reduction in latency is achieved by disabling the relay of TDM data in lower modulation modes, and this necessarily results in somewhat lower availability for the TDM circuit.

The loss of availability can be estimated using the Link Planner.

The unit will override the user setting of Lowest TDM Modulation Mode if the selected mode has insufficient capacity to carry the TDM data, or if the mode demands very high latency and requires more buffering than the link can provide."

Does this answer your question?

Dave

Thanks Dave for your answer!  It helps me to better understand the mechanism.

So, LinkPlanner will indicate us which lower modulation mode is required to relay the payload of the required T1s. But now is it better to choose that lower modulation mode and block it to that specific modulation or letting the radios manage themselves the right modulation mode between that lower one and the higner one possible?

If for example, the lower modulation mode is QPSK 0.63 (because "Timing" is indicated next to BPSK 0.63), do we gain something by forcing the radios to always use QPSK instead of letting the radios choose the right modulation between QPSK and 256 QAM?  In my understanding, I would get better availability if I force the radios to use QPSK instaed of letting them choose one between QPSK and 256 QAM. But maybe I am not right!

Thanks again!

There isn't much benefit in forcing the radio to use a low modulation mode, unless there is some intermittent radio interference that creates errors in the higher modulation mode. Choosing a low modulation mode does not generally improve availability, but it doesdeny you the upside of increased capacity in higher modes.

The attribute Lowest TDM Modulation Mode does a slightly different job. It provides a tradeoff between latency and availability. Consider that the buffer depth for TDM must be constant (as the external TDM equipment expects to receive a consistent stream of data without wander), whilst the underlying Ethernet latency of the wireless is degraded in low modulation modes. The buffer depth must be set to something greater than the worst case Ethernet latency.

This attribute switches the TDM transmission to timing only in the modulation modes lower than the configured threshold. For example, if the availability of 16QAM and above is good, you could set Lowest TDM Modulation Mode to 16QAM. The benefit of Lowest TDM Modulation Mode is that it sets the buffer depth for TDM data in the radio based on the threshold (16 QAM in this example), so that the end-to-end latency is reduced compared to the default setting. This action does necessarily affect availability of TDM somewhat, but that effect could be minimal with a good wireless link. 

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Thanks Mark for your explanations and suggestions!  I greatly appreciate.