Verilink 1557 (CG) Configuration/Installation Guide - Manual (Page 2)

Part Number 45-00036
Rev 2.0 T R A N S P O R T
®
1557 Transparent Automatic Protection Switch (TAPS)
This is true whether the selected mode is ‘transparent’ or
‘normal’.
Another feature of the TAPS product is a new switching
criterion. This parameter was added to enhance the TAPS
ability to efficiently detect incoming AIS and other severe
framing impairments. The new parameter is a measure of
Consecutive ESF Errored Frames (CEEF). A CEEF is
defined as any frame with a CRC error or an OOF defect.
This CEEF parameter has a fixed threshold value. Once
this threshold is exceeded, the line is considered ‘failed’
as with any other APS switching parameter. The Line
Availability Timer and all other aspects of APS switching
apply to this parameter. The threshold value is set to 31.
This means that 93 ms of AIS will typically cause a line
to be failed (because of hardware limitations, in some
cases 32 CEEFs are required to fail a circuit). This failed
condition is cleared when a non-errored frame is
received.
The standard TxPORT versions of the 1557 APS manual
and 1559 APSM manual still apply to the TAPS products.
Configuration Details
Switch S3-6, previously defined as a spare, now has a
particular function for TAPS units. Switch S3-6 is used to
configure the TAPS card mode as ‘normal’ or ‘transpar-
ent’. The drawings on the page below show how switch
S3 -6 should be set for the two different implementations
of the TAPS unit. The settings apply to both the 1557 APS
rack mount configuration and the 1558 APS stand -alone
configuration.
Since the TAPS card may now be used in a mode other
than CO-to-CPE, the meaning of switch S2 -5 has
changed somewhat from the original APS version. Before,
this switch denoted which APS unit was CO and which
was CPE. The CO unit was the master of the two. It con-
trolled status polling and management features. The CPE
Background
The 1557 Transparent Automatic Protection Switch
(TAPS) is an enhanced version of TxPORT’s previous
automatic protection switch (APS) product. The TAPS
product has an additional mode of operation that was
not previously available. This mode is what makes this
new automatic protection switch ‘transparent’.
Earlier APS products were designed to protect T1
access lines from a central office to the customer. This
was accomplished by providing automatic switching to
a standby T1 circuit when the normal circuit was
deemed failed.
The 1557 card was used in a rack mounted configura-
tion in the central office. The same 1557 card was a
component in a stand-alone unit for the customer end
of the circuit. Because of this dual use, the 1557 card
had to be configured for CO or CPE use. When config-
ured for CPE, the 1557 card provided many of the fea-
tures of an ESF CSU.
TAPS
The TAPS product is designed to provide the same type
of circuit protection but its transparent mode allows the
user to deploy it in configurations other than CO-to-
CPE. Particularly, TAPS units are designed to protect a
T1 circuit between two central offices.
When configured for transparent mode, the TAPS unit
will not provide CSU type functions. In the transparent
mode, the TAPS unit will not enforce ones density
requirements, respond to inband loop codes, or respond
to 54016 performance messages. These enhancements
allow the TAPS unit to be deployed in situations where
it is not required to serve as the network interface
equipment.
TAPS units configured for use as CPE do not generate
ANSI T1.403 performance report messages (PRM).
unit was therefore the slave. With TAPS, the CO/CPE
relationship is not always valid but the Master/Slave
concept remains. Therefore, switch S2-5 now denotes
whether a unit is a master or slave, regardless of where
the unit is physically located.
Note that in any pair of connected TAPS units, one unit
should be the master and one the slave. Both units
should be configured with the same mode selection,
either ‘Normal’ or ‘Transparent’.
Option switch S3-1, on the original APS units, was
used to enable or disable switching based on a single
out-of-frame (OOF) event. On TAPS units, switch S3-
1 is used to enable and disable the new CEEF parame-
ter described above. TAPS units no longer support the
option to switch on a single OOF. (OOF is referred to as
LOF in the APS Manager and its associated documenta-
tion.)
1559 APS Manager
The APS Manager may still show the LOF parameter.
Configuration screens may still allow the LOF parame-
ter to be enabled and disabled. In reality, this parameter
(shown as LOF) will be the new CEEF parameter. Set-
ting LOF [OFF] or LOF [ON] in the manager will be
the equivalent of setting CEEF [OFF] or CEEF [ON]
respectively. If this parameter is set to [ON], the CEEF
parameter is enabled as a protection switching criterion.
When the 1559 APSM is used to control a TAPS unit,
some operations will function differently depending on
the mode of operation of the TAPS units. For instance,
if a TAPS unit is configured for the Transparent mode,
the ‘CSU Loop’ function of the APS Manager will not
cause any action. This is because the slave TAPS will
not respond to the inband loop codes. And, since the
loop codes cannot be sent on the ‘active’ line, this func-
tion cannot be used to loop a far end CSU.