Support of Ring Topology

We have a project with 3 sites namely A, B and C where we will have PTP670 to connect between site A and B, Site B and C and Site A and C to form a ring topology. Site A and C has no connection to internet except Site B where Site B is the central site.

If the wireless connection between Site A and B is lost (may be due to breakdown of the ODU), data from Site A will go to Site C and then to Site B. The same will apply to Site C where data at site C will go via Site A to Site B.

Does PTP670 supports Ring topology ?

If not, how can we achieve the support of Ring Topology by using the PTP670 ? 

That's a good question. PTP 670 doesn't support ring protection in the embedded switch, but it does have some features that make it easier to create effective ring protection using external Ethernet switches.

The simplest option is to use basic Ethernet switches at A, B and C with Rapid Spanning Tree Protocol (RSTP) enabled. RSTP will block one port in the ring, transforming the ring into a simply-connected tree. RSTP has a much quicker reconvergence than STP when the connectivity changes, but it might still take a few seconds.

The other option is to use Ethernet switches with a dedicated ring protoction protocol like G.8032. This approach should provide protection switching time of about 50 ms.

In PTP 670, ensure that Data Port Wireless Down Alert is set to Enabled. The ODU will briefly disconnect the Ethernet port as soon as the wireless link drops to trigger the external switch to take action. This provides a quicker response than waiting for the switch to detect missing control messages. Consider setting Lowest Data Modulation Mode > BPSK 0.63, but lower than the mode that you expect to achieve in normal operation. This ensuires that the ring switches when the link is degraded, rather than waiting for it to drop completely.

Dear Mark,

Thanks for your explanation and solution. 

Hi, Mark,

If I also have the PMP450M at Location A and C together with the PTP670, do I need to have GPS sync at location A, B and C together (the traffic from PMP450M will go to Site B form site A and Site C via the PTP670 link) ?

Which GPS sync unit should I use, cnPulse or PTP650 Sync unit ?

Can we assume that the PMP 450M and PTP 670 links are on different channels?

If so, you might need to synchronize PTP 670 and PMP 450M but this depends on the spacing and relative positions of the radios on the mast or building, on the transmit power used, and on the modulation mode you need to achieve.

Thinking about just the ring of three PTP 670 links, synchronization is not a simple choice because you have an odd number of links. Suppose we locate two Masters at A, and two Slaves at B; at C we now need one Master and one Slave, and synchronization won't help us.

Dear Mark,

We can assume the PMP450m and PTP670 are working on the different channels but on the same pole. We understand this is difficult. With the cental site (Site B) having both PTP670 ODU set to Master, the other two sites will have one Master PTP670 ODU and one Slave PTP670 ODU and the PMP450m. Or, as you mentioned, we locate two Masters at A, and two Slaves at B; at C we now need one Master and one Slave. This is the reason why I want to have a solution.