Parallel Redundancy Protocol; Figure 171: Ied Attached To Separate Lans - GE P4A Technical Manual

Chapter 17 - Communications P54A/B/C/E
5.2

PARALLEL REDUNDANCY PROTOCOL

PRP (Parallel Reundancy Protocol) is defined in IEC 62439-3. PRP provides bumpless redundancy and meets the
most demanding needs of substation automation. The PRP implementation of the REB is compatible with any
standard PRP device.
PRP uses two independent Ethernet networks operating in parallel. PRP systems are designed so that there should
be no common point of failure between the two networks, so the networks have independent power sources and
are not connected together directly.
Devices designed for PRP applications have two ports attached to two separate networks and are called Doubly
Attached Nodes (DAN). A DAN has two ports, one MAC address and one IP address.
The sending node replicates each frame and transmits them over both networks. The receiving node processes the
frame that arrives first and discards the duplicate. Therefore there is no distinction between the working and
backup path. The receiving node checks that all frames arrive in sequence and that frames are correctly received
on both ports.
Devices such as printers that have a single Ethernet port can be connected to either of the networks but will not
directly benefit from the PRP principles. Such devices are called Singly Attached Nodes (SAN). For devices with a
single Ethernet port that need to connect to both LANs, this can be achieved by employing Ethernet Redundancy
Boxes (sometimes abbreviated to RedBox). Devices with a single Ethernet port that connect to both LANs by
means of a RedBox are known as Virtual DAN (VDAN).
The figure below summarises DAN, SAN, VDAN, LAN, and RedBox connectivity.
DAN
DAN
DAN
SAN
LAN B
LAN A
REDUNDANCY
BOX
VDAN
SAN SAN
VDAN
VDAN
E01028

Figure 171: IED attached to separate LANs

In a DAN, both ports share the same MAC address so it does not affect the way devices talk to each other in an
Ethernet network (Address Resolution Protocol at layer 2). Every data frame is seen by both ports.
When a DAN sends a frame of data, the frame is duplicated on both ports and therefore on both LAN segments.
This provides a redundant path for the data frame if one of the segments fails. Under normal conditions, both LAN
segments are working and each port receives identical frames.
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