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Detection of LOS on T1/E1 Interfaces
Guidelines for Configuring LOS Detection
Copyright © 2018, Juniper Networks, Inc.
state, the external clock is verified every second to ensure that its frequency is still present
and within range. If not, the bundle transitions from RUNNING to TtFAIL.
In the TtFAIL state, the bundle periodically transitions back to the EVAL state, where the
external clock is checked again. If the clock fails or a bad frequency occurs, the bundle
returns to the TtFAIL state. If the clock is properly functional, then the bundle transitions
to the various states that eventually end in the RUNNING state. Such a method of change
of states enable a graceful (if not instantaneous) recovery of a circuit where a cable is
disconnected, but subsequently reconnected. Because removal of the cable on a serial
port that is using an external clock can cause the bundle to exit the RUNNING state, that
bundle switches offline, if currently online in a Y-cable redundancy setup.
The clock and data signals are embedded together on a T1/E1 interface in a single AMI
(alternate mark inversion) electrical signal. The hardware line interface unit (LIU) that
recovers the composite AMI signal into its component clock and data signals recovers a
clock from the incoming AMI signal, even when none is present because it is based on a
free running phase-locked loop (PLL) that generates a clock, even when it is not locked
to an incoming signal. As a result, the CTP port interface receives an incoming external
clock from the LIU, whether a valid T1/E1 signal is connected to the CTP or not. The LIU,
however, cannot determine when it has a valid incoming T1/E1 signal, and in such a
condition, the LIU indicates as a LOS status bit. This indication serves as the basis for
detecting a cable disconnect in a Y-cable redundant configuration.
To use LOS as a way to take down a RUNNING bundle, the effective method implemented
is to treat a T1/E1 LOS condition exactly the same as a serial port with a bad or missing
external clock. When the CTP device performs its "check external clock" function, instead
of returning an automatic success on T1/E1 ports, the LOS status bit is analyzed to
determine whether it is a T1/E1 port. If the LIU LOS status indicates that there is no
incoming signal, then the function returns a failure, which causes the bundle to move to
the TtFAIL state. This state is the same as a missing external clock processing for a serial
port. In this manner, the T1/E1 ports behave exactly the same way as serial ports.
Keep the following points in mind when you configure the capability to detect LOS
conditions on T1E1 interfaces:
A cable disconnection of a serial port cannot be detected when no external clock is
being used by the port. The following clock configurations use an external clock:
DCE/DTE: Cfg Rate – Ext Clk
DCE/DTE: All Clock – Ext Clk
DCE/DTE: Adap Rate – Ext Clk
DCE/DTE: Auto Rate – Ext Clk
DTE: All clocked by Ext Clk (ST/RT)
Chapter 2: Configuring CTP Bundles
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