Redundancy Options - GE 350 Communications Manual

REDUNDANCY OPTIONS

Redundancy Options
3–2
Industrial real-time Ethernets typically demand much higher availability and uninterrupted
operation than office Ethernet solutions can provide. Even a short loss of connectivity can
result in loss of functionality, as for example in some automation, vehicular, power
generation, and power distribution systems.
To recover from a network failure, different standard redundancy schemes are applied
such as Parallel Redundancy Protocol (PRP), High-availability Seamless Redundancy (HSR)
and others.
The basic concept of both protocols, PRP and HSR, is to send practically identical frames
over different paths and discard one of the copies in reception, at best. If an error occurs or
one of the paths is down, the frame travelling through that path will not reach its
destination, but its copy will.
PRP operates on two independent networks. Each frame is replicated on the sending node
and transmitted over both networks. The receiving node processes the frame arriving first
and discards the subsequent copy. The PRP layer is responsible for this replicate/discard
function and hides the two networks from the upper layers. This scheme works without
explicit reconfiguration and switchover and therefore does not show a period of
unavailability.
Figure 3-1: Example of PRP with two LANs (LAN A and LAN B)
The two LANs, named LAN_A and LAN_B, are identical in protocol at the MAC level, but they
can differ in performance and topology. Transmission delays can also be different. The
LANs have no direct connection among them and they are assumed to be fail-
independent.
In some applications, only availability-critical nodes need a double attachment, while
others do not. In order to meet the specific requirements, PRP defines different kinds of
end nodes.
• The Dual Attached Node (DAN) is connected to both LANs.
• Uncritical nodes can be attached to only one LAN and are therefore called Single
Attached Nodes (SAN). SANs that need to communicate with each other are on the
same LAN.
• The Redundancy Box (RedBox) is used when a single interface node has to be
attached to both networks. Such a node can communicate with all other nodes. Since
a node behind a RedBox appears for other nodes like a DAN, it is called Virtual DANs
(VDAN). The RedBox itself is a DAN and acts as a proxy on behalf of its VDANs. The
RedBox has its own IP address for management purposes
Similarly to PRP, HSR is based in the duplication of every frame sent, but in a ring topology.
Each copy of the frame is injected in a different direction of the ring. If any of the links
between nodes is down, all nodes are still reachable. This topology forces every node in the
CHAPTER 3: ETHERNET INTERFACE
350 FEEDER PROTECTION SYSTEM – COMMUNICATIONS GUIDE