We have a propose 750Mbps PTP820 link, and have been using LPlanner to make the path profile design. Distance is at 5km, notice LPlanner introduces the antenna configuration option. What does the following means:
1. Asymetric Antenna Coupling &
2. Symmetric Antenna Coupling
Will PTP820c be able to provide a working link for this case with required availability of 99.9999%.
Hi Gerry - great question!
When we refer to Asymmetric or Symmetric coupling we are talking about how the insertion/feeder losses are distributed across Primary and Secondary paths. This is generally to do with 1+1 HSB links.
Asymmetric couplers will generally have less loss in the Primary path and greater losses in the Secondary paths. I If the link is operating in Primary-Primary, then there would be significantly less loss than if a Secondary path was involved!
Symmetric couplers have EQUAL losses across both the Primary AND the Secondary path. This would mean that the losses are equal regardless of whether the link is operating in Primary-Primary, Secondary-Primary/Primary-Secondary or Secondary-Secondary.
So, this affects the overall transmission design as the link budget, fade margins and availabilities will all be affected depending on operating mode during a failover event. This also includes antennas size implications to maintain minimum availability and performance requirements.
When do you use Asymmetric or Symmetric? There may be many reasons - one important reason would be based on Mean Time To Repair (MTTR) committments - this represents the average time required to repair a failed component/device. A rudimentary example would be if it takes an hour to get to site and an hour to replace/repair fault then an asymmetric configuration would probably best suit since the system (link) would only be affected for a short while - the degraded fade margins and availability figures may not be felt by the customer during this period.
If, however, it takes days to get to site (permits, helicopter access only, weather, etc) then MTTR would be significantly greater. In this case it would make more sense to design the link with equal (symmetric) losses for all events.
Here is an example that shows the losses associated with an Asymmetric Coupling configuration. You can see that there are much greater imposing losses on the Secondary paths!
Here is an example that shows the losses associated with an Asymmetric Coupling configuration. Losses here are equal (symmetric) across all paths.