Chapter 18 - Communications
Figure 198: Redundant Ethernet ring architecture with IED, bay computer and Ethernet switches after failure
DUAL HOMING PROTOCOL
The Dual Homing Protocol (DHP) implemented in the REB is a proprietary protocol. It is designed, primarily to be
used on PACiS systems that employ the C264-SWD212 and/or H36x multimode switches.
DHP addresses the constraints of critical time applications such as the GOOSE messaging of IEC 61850.
DHP is applied to double-star network topologies. If a connection between two devices is broken, the network
continues to operate correctly.
The Dual Homing Manager (DHM) handles topologies where a device is connected to two independent networks,
one being the "main" path, the other being the "backup" path. Both are active at the same time.
Internet frames from a sending device are sent by the DHM to both networks. Receiving devices apply a "duplicate
discard" principle. This means that when both networks are operational, the REB receives two copies of the same
Ethernet frame. If both links are healthy, frames are received on both, and the DHM uses the first frame received.
The second frame is discarded. If one link fails, frames received on the healthy link are used.
DHP delivers a typical recovery time of less than 1 ms. The mechanism is outlined in the figures below.