Alstom MiCOM P54 Series - Page 191

Operation P54x/EN OP/La4
MiCOM P543, P544, P545 & P546 (OP) 5-17
OP
If the link fails in one direction, say between the transmitter at node B and the receiver at
node C, the self-healing ring can continue to transfer signals from node B to node C via the
standby route through nodes B, A, F, E, D and then C (obviously a longer path). In this case
the communication propagation delay times between nodes B and C differ in the two
directions, and if the difference is greater than 1ms the traditional time alignment technique
described in section 1.1.1.1 is no longer adequate.
t
p2

t
p2
t
p1
t
p1

Node
C
Relay
End 2
Relay
End 1
Node
B
Node
F
Node
E
Node
A
Node
D
Standby
path
Healthy
Path
P1003ENa
P1003ENa
Figure 4 Example of switched synchronous digital hierarchy
P54x make use of the timing information available from the GPS system to overcome the
limitation of the traditional technique, and therefore allow application to communications that
can provide a permanent or semi-permanent split path routing.
A 1 pulse per second output from a GPS receiver is used to ensure that the re-sampling of
the currents at each relay occurs at the same instant in time. The technique is therefore not
dependant on equal transmit and receive propagation delay times; changes in one or both of
the propagation delay times also do not cause problems. These factors make it suitable for
use with switched SDH networks.
The GPS technique is taken further, however, to overcome concerns about the reliability of
the GPS system. Consider a similar two ended system to that of Figure 3 where the
re-sam
pling instants (tAn, tBn) are synchronized using the GPS timing information. Here the
re-sampling instants at the two ends will be coincidental as shown in Figure 5.
Not
e: Figure 5 demonstrates a case where the communications path
prop
agation delay times are not the same.