This document describes various terms associated with T1 and E1 lines.
Use this document in conjunction with the following T1 and E1 troubleshooting
Readers of this document should have knowledge of the following
This document is not restricted to specific software and hardware
The information in this document was created from the devices in a
specific lab environment. All of the devices used in this document started with
a cleared (default) configuration. If your network is live, make sure that you
understand the potential impact of any command.
For more information on document conventions, refer to the
Technical Tips Conventions.
Many of the terms listed below are directly visible in the
show controllers t1 or show controllers
e1 command output. For more information, refer to the document
the show controllers e1 Command.
Bipolar Violation (BPV) Error Event
A BPV error event for an alternate mark inversion (AMI)-coded signal is
the occurrence of a pulse of the same polarity as the previous pulse. A BPV
error event for a B8ZS- or HDB3- coded signal is the occurrence of a pulse of
the same polarity as the previous pulse without being a part of the zero
Controlled Slip (CS) Error Event
A Controlled Slip is the replication or deletion of the payload bits of
a digital signal level 1 (DS1) frame. A Controlled Slip may be performed when
there is a difference between the timing of a synchronous receiving terminal
and the received signal. A Controlled Slip does not cause an Out of Frame
Excessive Zeroes (EXZ) Error Event
An EXZ error event for an AMI-coded signal is the occurrence of more
than fifteen contiguous zeroes. For a binary 8-zero substitution (B8ZS) coded
signal, the defect occurs when more than seven contiguous zeroes are
Line Coding Violation (LCV) Error Event
An LCV is the occurrence of either a Bipolar Violation or Excessive
Zeroes error event.
Path Coding Violation (PCV) Error Event
A PCV error event is a frame synchronization bit error in the D4 and
E1-no cyclic redundancy check (CRC) formats, or a CRC error in the Extended
Super Frame (ESF) and E1-CRC formats.
Alarm Indication Signal (AIS) Defect
For D4 and ESF links, the 'all ones' condition is detected at a DS1
line interface upon observing an unframed signal with a one's density of at
least 99.9 percent present for a time equal to or greater than T, where 3 ms is
less than or equal to T, which is less than or equal to 75 ms. The AIS is
terminated upon observing a signal not meeting the one's density or the
unframed signal criteria for a period equal to or greater than than T.
For E1 links, the 'all-ones' condition is detected at the line
interface as a string of 512 bits containing fewer than three zero bits.
Out Of Frame (OOF) Defect
An OOF defect is the occurrence of a particular density of Framing
For T1 links, an OOF defect is declared when the receiver detects two
or more framing errors within a 3 msec period for ESF signals and 0.75 msec for
D4 signals, or two or more errors out of five, or fewer consecutive
For E1 links, an OOF defect is declared when three consecutive frame
alignment signals have been received with an error.
When an OOF defect is declared, the framer starts searching for a
correct framing pattern. The OOF defect ends when the signal is
In-frame occurs when there are fewer than two frame bit errors within a
3 msec period for ESF signals and 0.75 msec for D4 signals.
For E1 links, in-frame occurs when:
in frame N, the frame alignment signal is correct
in frame N+1, the frame alignment signal is absent (that is, bit 2 in
TS0 is set to one)
in frame N+2, the frame alignment signal is present and
All performance parameters are accumulated in fifteen minute intervals
and up to 96 intervals (covering a 24 hour period ) are kept by an agent. Fewer
than 96 intervals of data will be available if the agent has been restarted
within the last 24 hours. In addition, there is a rolling 24-hour total of each
There is no requirement for an agent to ensure a fixed relationship
between the start of a fifteen minute interval and clock time; however some
agents may align the fifteen minute intervals with quarter hours.
Bursty Errored Seconds (BES)
A Bursty Errored Second (also known as Errored Second type B) is a
second with fewer than 320 and more than one Path Coding Violation error
events, no Severely Errored Frame defects and no detected incoming AIS defects.
Controlled slips are not included in this parameter.
This is not incremented during an Unavailable Second.
Controlled Slip Seconds (CSS)
A Controlled Slip Second is a one-second interval containing one or
more controlled slips.
A Degraded Minute is one in which the estimated error rate exceeds 1E-6
but does not exceed 1E-3.
Degraded Minutes are determined by collecting all of the Available
Seconds, removing any Severely Errored Seconds grouping the result in 60-second
long groups and counting a 60-second long group (minute) as degraded if the
cumulative errors during the seconds present in the group exceed 1E-6.
Available seconds are merely those seconds which are not unavailable as
Errored Seconds (ES)
For ESF and E1-CRC links an Errored Second is a second with one of the
one or more Path Code Violations
one or more Out of Frame defects
one or more Controlled Slip events
a detected AIS defect
For D4 and E1-noCRC links, the presence of Bipolar Violations also
triggers an Errored Second. This is not incremented during an Unavailable
Line Errored Seconds (LES)
A Line Errored Second, according to T1M1.3, is a second in which one or
more Line Code Violation error events were detected.
While many implementations are currently unable to detect the zero
strings, it is expected that interface manufacturers will add this capability
in deference to ANSI; therefore, it will become available in time.
In the T1M1.3 specification, near end Line Code Violations and far end
Line Errored Seconds are counted. For consistency, we count Line Errored
Seconds at both ends.
Severely Errored Framing Second (SEFS)
An Severely Errored Framing Second is a second with either one or more
OOF defects or a detected AIS defect.
Severely Errored Seconds (SES)
A Severely Errored Second for ESF signals is a second with one of the
For E1-CRC signals, a Severely Errored Second is a second with either
832 or more Path Code Violation error events or one or more OOF defects.
For E1-noCRC signals, a Severely Errored Second is a 2048 LCVs or
For D4 signals, a Severely Errored Second is a count of one-second
intervals with Framing Error events, or an OOF defect, or 1544 LCVs or
Controlled slips are not included in this parameter.
This is not incremented during an Unavailable Second.
Unavailable Seconds (UAS)
Unavailable Seconds are calculated by counting the number of seconds
that the interface is unavailable. The DS1 interface is said to be unavailable
from the onset of ten contiguous SESs, or the onset of the condition leading to
a failure (see Failure States). If the condition leading to the failure was
immediately preceded by one or more contiguous SESs, then the DS1 interface
unavailability starts from the onset of these SESs. Once unavailable, and if no
failure is present, the DS1 interface becomes available at the onset of ten
contiguous seconds with no SESs. Once unavailable, and if a failure is present,
the DS1 interface becomes available at the onset of 10 contiguous seconds with
no SESs, if the failure clearing time is less than or equal to ten seconds. If
the failure clearing time is more than ten seconds, the DS1 interface becomes
available at the onset of ten contiguous seconds with no SESs, or the onset
period leading to the successful clearing condition, whichever occurs later.
With respect to the DS1 error counts, all counters are incremented while the
DS1 interface is deemed available. While the interface is deemed unavailable,
the only count that is incremented is UASs.
A special case exists when the ten or more second period crosses the
900 second statistics window boundary, as the foregoing description implies
that the Severely Errored Second and Unavailable Second counters must be
adjusted when the Unavailable Signal State is entered. Successive "gets" of the
affected dsx1IntervalSESs and dsx1IntervalUASs objects will return differing
values if the first get occurs during the first few seconds of the window. This
is viewed as an unavoidable side-effect of selecting the presently-defined
The following failure states are received or detected failures that are
reported. The conditions under which a DS1 interface would, if ever, produce
the conditions leading to the failure state are described in the appropriate
Alarm Indication Signal (AIS) Failure
The Alarm Indication Signal failure is declared when an AIS defect is
detected at the input and the AIS defect still exists after the Loss Of Frame
failure (which is caused by the unframed nature of the 'all-ones' signal) is
declared. The AIS failure is cleared when the Loss Of Frame failure is
Far End Alarm Failure(Yellow Alarm)
The Far End Alarm failure is also known as a Yellow Alarm in the T1
case and a Distant Alarm in the E1 case.
For D4 links, the Far End Alarm failure is declared when bit 6 of all
channels has been zero for at least 335 ms and is cleared when bit 6 of at
least one channel is non-zero for a period T, where T is usually less than one
second and always less than five seconds. The Far End Alarm failure is not
declared for D4 links when a Loss of Signal is detected.
For ESF links, the Far End Alarm failure is declared if the Yellow
Alarm signal pattern occurs in at least seven out of ten contiguous 16-bit
pattern intervals and is cleared if the Yellow Alarm signal pattern does not
occur in ten contiguous 16-bit signal pattern intervals.
For E1 links, the Far End Alarm failure is declared when bit 3 of
time-slot zero is received set to one on two consecutive occasions. The Far End
Alarm failure is cleared when bit 3 of time-slot zero is received set to
Far End Loss Of Multiframe Failure
The Far End Loss Of Multiframe failure is declared when bit 2 of TS16
of frame 0 is received set to one on two consecutive occasions. The Far End
Loss Of Multiframe failure is cleared when bit 2 of TS16 of frame 0 is received
set to zero. The Far End Loss Of Multiframe failure can only be declared for E1
links operating in Channel Associated Signalling mode.
The Loopback Pseudo-Failure is declared when the near end equipment has
placed a loopback (of any kind) on the DS1. This allows a management entity to
determine from one object whether the DS1 can be considered to be in service or
not (from the point of view of the near end equipment).
Loss Of Frame(LOF) Failure
For T1 links, the Loss Of Frame failure is declared when an OOF or LOS
defect has persisted for T seconds, where T is more than or equal to two, but
less than or equal to ten. The Loss Of Frame failure is cleared when there have
been no OOF or LOS defects during a period T is more than or equal to zero, but
less than or equal to twenty. Many systems will perform "hit integration"
within the period T before declaring or clearing the failure.
For E1 links, the Loss Of Frame Failure is declared when an OOF defect
Loss Of MultiFrame Failure
The Loss Of MultiFrame failure is declared when two consecutive
multiframe alignment signals (bits 4 through 7 of TS16 of frame 0) have been
received with an error. The Loss Of Multiframe failure is cleared when the
first correct multiframe alignment signal is received. The Loss Of Multiframe
failure can only be declared for E1 links operating with framing (sometimes
called Channel Associated Signalling mode).
Loss Of Signal (LOS) Failure
For T1, the Loss Of Signal failure is declared upon observing 175 +/-
75 contiguous pulse positions with no pulses of either positive or negative
polarity. The LOS failure is cleared upon observing an average pulse density of
at least 12.5 percent over a period of 175 +/- 75 contiguous pulse positions
starting with the receipt of a pulse.
For E1 links, the Loss Of Signal failure is declared when greater than
ten consecutive zeroes are detected.
TS16 Alarm Indication Signal Failure
For E1 links, the TS16 Alarm Indication Signal failure is declared when
time-slot 16 is received as all ones for all frames of two consecutive
multiframes. This condition is never declared for T1.
This is a character string specified by the circuit vendor, and is
useful when communicating with the vendor during the troubleshooting