PTP Synchronisation timing

The PTP600 and PTP500 both support synchronisation using PTP-SYNC. When using LinkPlanner to design a link, synchronisation can be selected for each link in question. I have 3 links of which the longest is 4.696km. Two PTP500s and one PTP600. My question is:

Why do the 100Mbps PTP500 links degrade to 86Mbps (upon selecting PTP-SYNC) when signal propagation only takes 3us per km?

LinkPlanner shows a TX/RX gap of greater than 189us. The PTP600 suffers a similar degradation. I would like to know where the extra time is going to (processing perhaps?).

Any answers are most welcome.

In non-synchronized links, the gaps between transmit and receive bursts are determined by (a) the minimum processing and switching time, and (b) the round trip time between the master and slave ODUs. In a synchronized network, the gaps in the TDD frame must be extended to guard against delayed interference from other links in the same network. The frame duration in a synchronized network depends on the length of the longest link, and on the maximum distance over which interference can occur. The link capacity at a given modulation mode is necessarily lower for a synchronized link than for the equivalent unsynchronized link. There is no additional processing or switching in a synchronized link, and it is the additional guard periods that account for the lower capacity rather than additional processing.

In PTP 500, the 1309 us burst duration and the minimum 3145 us frame duration provides approximately 90.7 Mbit/s capacity in the highest modulation mode and widest channel bandwidth, compared with 103.3 Mbit/s in unsynchronized operation.

The PTP 500 allows two settings for burst duration in TDD sync operation: 1309 us and 1451 us. It is important to understand that the PTP 500 ODU transmits a burst with actual duration of about 1309 us in either case. The 1451 us option configures the PTP 500 to transmit its native 1309 us burst in the centre of a 1451 us window, such that the 1451 us window can be aligned with the PTP 600 frame structure. The 1309 us option should always be used in a synchronized network consisting only of PTP 500 units.

The frame duration must be selected from the following permitted values: 3145, 3311, 3460, 3610, 3817, 4000, 4184 and 4367 us. LINKPlanner helps with the selection of suitable frame durations considering the length of the longest links and interference paths in the network. For some links, the 1451 us burst duration option triggers use of a longer frame duration, resulting in lower TDD frame rate and thus reduced capacity relative to the 1309 us burst duration setting.

At present LINKPlanner is not fully restricting the list of burst duration options for PTP 500 and we will address this in a future release.