Specifications

December 2008 Page 5–11
Chapter 5. Alternate Path Mode (APM)
5
When APM is activated, each chassis around the
loop will skip the same channels you selected
here.
When should you skip a channel? When you have
a channel assignment that is not supported by
APM. This is true if:
1. A channel is used for more than one
connection around the loop.
2. A PLD connection is made, and the next
time slot is not cleared around the entire
loop.
3. A pass-through channel assignment is
made between both transceivers of the
APM loop definition, and the pass-through
is not on the same time slot.
Also, once you have armed the loop, the skipped
channels are displayed in the APM status dialog
box. Before changing the status of a channel in an
armed loop from disabled (skipped) to enabled or
vice versa, you must first disarm the loop. You
then change the status when you re-arm the loop.
5.1.5 How to Analyze and Respond
to an APM Event
This section tells you how to determine the cause
of an Alternate Path Mode (APM) event and how
to make sure the alternate path maps remain in
effect while the originating failure is being
repaired.
5.1.5.1 APM Basic Terminology and
Definitions
As a first step, let’s identify the meanings of a few
APM related terms we will be using throughout
this discussion (and the rest of this manual, as
well):
Inactive State An APM software state which
indicates that the APM loop definition has not
been defined and/or the neighbors addresses are
not defined. In this state, an APM loop cannot
be armed.
Ready For Arm State An APM software state
which indicates that the APM loop definition
and neighbors addresses have been defined, but
the chassis will not detect communication
failures and initiate the rerouting of DS0
channels.
Armed State An APM software state indicating
that there are no current failures on the loop . In
this state, the FOCUS chassis is ready to initiate
APM if a problem should occur.
Down State An APM software state which
indicates this FOCUS Chassis has initiated an
APM event due to a failure adjacent to this
chassis. An adjacent failure could indicate a
fiber break, a local node failure, or an adjacent
node failure. The FOCUS chassis responds to
the failure by flipping all mapped channels in
the direction of the failure and sending a
message to other FOCUS chassis which causes
them to also flip the appropriate channels.
Flipped State An APM software state the
FOCUS chassis is in when a failure has been
detected on the loop and the chassis has
responded by flipping the appropriate mapped
channels, based on the location of the failure.
This state indicates that this FOCUS chassis did
NOT initiate APM (detect the failure), but there
is a chance the failure may still be adjacent to
this FOCUS chassis.
Primary Map The original DS0 Channel
Assignment map which indicates the communi-
cations path for the channel modules and
pass-throughs.
Alternate Path Map A temporary, dynamic
DS0 Channel Assignment map which reroutes
channels affected by a communications failure
in a loop of FOCUS chassis.
XID Messages An internal message sent to the
FOCUS neighboring chassis to determine if the
communications path is working. The message
must be received from the FOCUS chassis
You can view the current APM state via the
FCS “APM Status” command or by observing
the system target, “APM Loop 1 Going Down”.
NOTE