GE B30 Communications Manual page 246

OVERVIEW CHAPTER 3: IEC 61850 COMMUNICATION
To facilitate rapid processing in both the publisher and subscriber, the MMS/TCP/IP stacks are bypassed; GOOSE messages
interface directly to the Ethernet layer. To avoid the publisher having to compose and send sequentially individual
messages to each subscriber, multicast addressing is used whereby the network copies each transmitted GOOSE
messages to all subscribers on the same VLAN. Finally, to avoid the long delays that handshaking type protocols can
introduce when a packet is lost, GOOSE instead employs a scheme whereby messages encoding a change of state are
repeated several times; if the first event message is lost, the closely following repetition gets through.
The figure shows the timing of GOOSE transmissions.
Figure 3-4: TxGOOSE timing
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When none of the published member values are changing, the GOOSE message is sent periodically to allow subscribers to
monitor the connection. These messages, called heartbeat messages, also allow a subscriber recovering from an
interruption or outage to obtain the values of the members in the absence of event messages. A timeAllowedtoLive field
including in each GOOSE message informs the subscriber of the maximum amount of time to wait for another message
before declaring the connection lost. In UR devices, the heartbeat period is controlled by the TxGOOSE UPDATE TIME
setting, and the timeAllowedtoLive value is controlled by the TIME TO LIVE setting.
When any published member value changes state, the GOOSE message is immediately sent with the new values. Such a
message is called an event message. A short time after the initial event message is sent, it is resent several times. In UR
devices there are normally three retransmissions, with intervals controlled by the TxGOOSE RETRANS TIME settings.
However, if the TxGOOSE RETRANS TIME setting is set to zero, there are no retransmissions.
UR devices implement the publisher side of each GOOSE service under the name TxGOOSE. See the applicable UR
instruction manual for the number of TxGOOSE elements available, and the performance of each.
3-8 UR FAMILY – COMMUNICATIONS GUIDE