Specifications

Copyright © AMETEK
5
5.1 Description
The Alternate Path Mode (APM) feature gives
FOCUS the ability to detect a T1/E1 break on a
closed loop and reroute the affected channel data
around the opposite side of the loop, using the
same time slot (a time slot may be used only one
time within a loop).
The FOCUS network consists of network nodes
(FOCUS chassis) connected in one or more paths.
We define a path as the connection between any
one transceiver to any other transceiver in a
different FOCUS chassis.
When the nodes in a network are configured in a
loop, the Alternate Path Mode (APM) provides a
means for any channel module to link to its
complement should the primary path (i.e., the link
defined by the DS0 channel assignments) become
non-functional.
Channels may be rerouted if one node-to-node
link fails. The channels should be used only once
in the loop. When a link fails, the nodes adjacent
to the failure send a message in the opposite
direction of the loop connection that specifies
which channels are disrupted, and each node that
drops that channel reverses the direction for that
channel. This flip-flop reconnects the channels in
the reverse direction. When the link is re-estab-
lished, a complementary message is sent, causing
each node to switch back to the original direction.
All APM messages are sent using the overhead
bits known as Facilities Data Link (FDL). This
makes all 24 (30/E1) channels available to you for
channel assignment.
A node determines if a problem requires the APM
to be activated in the following ways:
1. Each node sends an Exchange ID (XID)
message out each active port on a regular
basis. When a node receives an XID, it
compares the XID’s source address to that of
its nearest neighbor. If it matches, the
neighbor is in normal mode. If it does not
match (i.e., if a bad XID event occurred), it
will be treated as a link failure. The mismatch
can occur if a node is inserted. The APM is
then started and remains in effect either until
the nodes detecting the failure recognize the
correct XID from their neighbor, or until the
node receives a message to change the
neighbor map to reflect the inserted node or
new Maintenance Module. Also, if no XID is
received for a specified period of time, an
XID time-out occurs and the APM is
activated.
2. Each node also checks the T1/E1 (and XCVR
indirectly through the T1/E1 Sync) status
before it checks for a valid XID from its
neighbor. If three consecutive T1/E1 errors
occur, APM will be started.
The APM feature operation is based on the
following assumptions:
1. A maximum of two loop connections are used,
and each transceiver is used only once.
The valid loop definitions you may select are
as follows:
• XCVR1-1—XCVR1-2 (E/W)
• XCVR1-1—XCVR2-1 (E/N)
• XCVR1-1—XCVR2-2 (E/S)
• XCVR1-2—XCVR2-1 (W/N)
• XCVR1-2—XCVR2-2 (W/S)
• XCVR2-1—XCVR2-2(N/S)
• NOT DEFINED
2. The channels that are used in the loop are only
used once.
3. All pass throughs made between both sides of
the loop-definition MUST be on the same
channel number. Cross-channel pass
throughs are not supported in APM.
4. The user configurable parts of APM are the
normal channel map setup, the entering of
5. Alternate Path Mode (APM)