Specifications

December 2008 Page 5–9
Chapter 5. Alternate Path Mode (APM)
5
and “X2-1 to “X2-2.” Note that if you set the loop
definitions in this example to “X1-1 to X2-2” and
“X1-2 to X2-1” the loop might arm, but APM
would not operate correctly.
Figure 5–5. Defining Two Valid APM Loops
for One FOCUS Chassis.
5.1.4.8 Blinking LED with Two Loops
Defined
A blinking green ARMED LED (on the
Maintenance Module) indicates that two loops
have been defined, but only one loop is ARMed.
This LED’s blinking has no adverse effects. If you
prefer a solid LED, however, you can set the defi-
nition of the unARMed loop to “Not Defined.”
Normally, the second loop definition defaults to
this "Not Defined" state, except when FOCUS is
booted and two dual transceiver modules are
installed. In this case, loop definitions default to
"X1-1 to X1-2" for Loop 1 and "X2-1 to X2-2" for
Loop 2.
To set the APM loop definition, perform the
"APM Define" command from FCS.
5.1.4.9 Minimizing Sync Slips with Two
APM Loops ARMed
A FOCUS chassis with two dual transceiver
modules can be part of two APM loops, and you
can protect both loops with APM. In some situa-
tions, however, sync slips may occur when one or
both loops are in alternate path mode. To
minimize these sync slips, we recommend that
you set the FOCUS chassis with the two
loops defined as the master. The following
scenarios show why this is best.
Scenario 1
In this scenario, shown in Figure 5-6, chassis 386
is initially set as the master. When a break occurs
in either Loop 1 or Loop 2, all chassis can still
sync to one master (chassis 386).
Figure 5–6.
Two ARMed APM Loops – Scenario 1.
Scenario 2
In this scenario, shown in Figure 5-7, chassis 386
is initially set as the master. When a break occurs
in Loop 2 between chassis 206 and 439, chassis
386 switches its direction of synchronization to
X2-1 and becomes a slave. Chassis 206 becomes
a master.
It is important to note here that if a break occurs
on Loop 1 while Loop 2 is still broke, chassis 386
will not switch its sync back into Loop 1. Instead,
there will probably be two masters in the system,
and synchronization slips will most likely occur