RFC 3895
Definitions of Managed Objects for the DS1, E1, DS2, and E2 Interface Types
Network Working Group O. Nicklass, Ed. Request for Comments: 3895 RAD Data Communications, Ltd. Obsoletes: 2495 September 2004 Category: Standards Track Definitions of Managed Objects for the DS1, E1, DS2, and E2 Interface Types Status of this Memo This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited. Copyright Notice Copyright (C) The Internet Society (2004).Abstract
This memo defines a portion of the Management Information Base (MIB) for use with network management protocols in the Internet community. In particular, it describes objects used for managing DS1, E1, DS2 and E2 interfaces. This document is a companion to the documents that define Managed Objects for the DS0, DS3/E3 and Synchronous Optical Network/Synchronous Digital Hierarchy (SONET/SDH) Interface Types. This document obsoletes RFC 2495.
Table of Contents
1. The Internet-Standard Management Framework . . . . . . . . . . 2 1.1. Changes from RFC 2495. . . . . . . . . . . . . . . . . . 3 1.2. Changes from RFC 1406. . . . . . . . . . . . . . . . . . 3 1.3. Companion Documents. . . . . . . . . . . . . . . . . . . 4 2. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2.1. Use of ifTable for DS1 Layer . . . . . . . . . . . . . . 5 2.2. Usage Guidelines . . . . . . . . . . . . . . . . . . . . 6 2.2.1. Usage of ifStackTable for Routers and DSUs. . . . 6 2.2.2. Usage of ifStackTable for DS1/E1 on DS2/E2. . . . 8 2.2.3. Usage of Channelization for DS3, DS1, DS0 . . . . 9 2.2.4. Usage of Channelization for DS3, DS2, DS1 . . . . 10 2.2.5. Usage of Loopbacks. . . . . . . . . . . . . . . . 11 2.3. Objectives of this MIB Module. . . . . . . . . . . . . . 11 2.4. DS1 Terminology. . . . . . . . . . . . . . . . . . . . . 12 2.4.1. Error Events. . . . . . . . . . . . . . . . . . . 12 2.4.2. Performance Defects . . . . . . . . . . . . . . . 13 2.4.3. Performance Parameters. . . . . . . . . . . . . . 14 2.4.4. Failure States. . . . . . . . . . . . . . . . . . 18 2.4.5. Other Terms . . . . . . . . . . . . . . . . . . . 21 3. Object Definitions . . . . . . . . . . . . . . . . . . . . . . 21 4. Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . . 74 5. Security Considerations. . . . . . . . . . . . . . . . . . . . 74 6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 76 6.1. Normative References . . . . . . . . . . . . . . . . . . 76 6.2. Informative References . . . . . . . . . . . . . . . . . 78 Appendix A - Use of dsx1IfIndex and dsx1LineIndex. . . . . . . . . 79 Appendix B - The delay approach to Unavailable Seconds . . . . . . 81 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 83 Full Copyright Statement . . . . . . . . . . . . . . . . . . . . . 841. The Internet-Standard Management Framework
For a detailed overview of the documents that describe the current Internet-Standard Management Framework, please refer to section 7 of RFC 3410 [RFC3410]. Managed objects are accessed via a virtual information store, termed the Management Information Base or MIB. MIB objects are generally accessed through the Simple Network Management Protocol (SNMP). Objects in the MIB are defined using the mechanisms defined in the Structure of Management Information (SMI). This memo specifies a MIB module that is compliant to the SMIv2, which is described in STD 58, RFC 2578 [RFC2578], STD 58, RFC 2579 [RFC2579] and STD 58, RFC 2580 [RFC2580].
1.1. Changes from RFC 2495
The changes from [RFC2495] are the following: (1) The dsx1FracIfIndex SYNTAX matches the description range. (2) A value was added to dsx1TransmitClockSource. (3) Values were added to dsx1LineType. (4) Two objects were added, dsx1LineMode and dsx1LineBuildOut to better express transceiver mode and LineBuildOut for T1. (5) Reference was added to Circuit Identifier object. (6) Align the DESCRIPTION clauses of few statistic objects with the near end definition, the far end definition and with [RFC3593]. (7) Changes in Compliance Statements to include new objects. (8) A typographical error in dsx2E2 was fixed, new name is dsx1E2.1.2. Changes from RFC 1406
The changes from RFC 1406 [RFC1406] are the following: (1) The Fractional Table has been deprecated. (2) This document uses SMIv2. (3) Usage is given for ifTable and ifXTable. (4) Example usage of ifStackTable is included. (5) dsx1IfIndex has been deprecated. (6) Support for DS2 and E2 have been added. (7) Additional lineTypes for DS2, E2, and unframed E1 were added. (8) The definition of valid intervals has been clarified for the case where the agent proxied for other devices. In particular, the treatment of missing intervals has been clarified. (9) An inward loopback has been added.
(10) Additional lineStatus bits have been added for Near End in
Unavailable Signal State, Carrier Equipment Out of Service, DS2
Payload AIS, and DS2 Performance Threshold.
(11) A read-write line Length object has been added.
(12) Signal mode of other has been added.
(13) Added a lineStatus last change, trap and enabler.
(14) The e1(19) ifType has been obsoleted so this MIB does not list
it as a supported ifType.
(15) Textual Conventions for statistics objects have been used.
(16) A new object, dsx1LoopbackStatus has been introduced to reflect
the loopbacks established on a DS1 interface and the source to
the requests. dsx1LoopbackConfig continues to be the desired
loopback state while dsx1LoopbackStatus reflects the actual
state.
(17) A dual loopback has been added to allow the setting of an inward
loopback and a line loopback at the same time.
(18) An object indicating which channel to use within a parent object
(i.e., DS3) has been added.
(19) An object has been added to indicate whether or not this DS1/E1
is channelized.
(20) Line coding type of B6ZS has been added for DS2.
1.3. Companion Documents
This document is a companion to the documents that define Managed
Objects for the DS0 [RFC2494], DS3/E3 [RFC3896], and Synchronous
Optical Network/Synchronous Digital Hierarchy (SONET/SDH) [RFC3592]
Interface Types.
2. Overview
These objects are used when the particular media being used to
realize an interface is a DS1/E1/DS2/E2 interface. At present, this
applies to these values of the ifType variable in the Internet-
standard MIB:
ds1 (18)
The definitions contained herein are based on the AT&T T-1 Superframe (a.k.a., D4) [ANSI-T1.107] and Extended Superframe (ESF) formats [AT&T-UM-305] [AT&T-TR-54016], the latter of which conforms to ANSI specification [ANSI-T1.403], and the CCITT Recommendations [CCITT-G.703] [ITU-T-G.704], referred to as E1 for the rest of this memo. The various DS1 and E1 line disciplines are similar enough that separate MIBs are unwarranted, although there are some differences. For example, Loss of Frame is defined more rigorously in the ESF specification than in the D4 specification, but it is defined in both. Therefore, interface types e1(19) and g703at2mb(67) have been obsoleted. Where it is necessary to distinguish between the flavors of E1 with and without CRC, E1-CRC denotes the "with CRC" form (G.704 Table 4b) and E1-noCRC denotes the "without CRC" form (G.704 Table 4a).2.1. Use of ifTable for DS1 Layer
Only the ifGeneralInformationGroup needs to be supported. ifTable Object Use for DS1 Layer ====================================================================== ifIndex Interface index. ifDescr See interfaces MIB [RFC2863] ifType ds1(18) ifSpeed Speed of line rate DS1 - 1544000 E1 - 2048000 DS2 - 6312000 E2 - 8448000 ifPhysAddress The value of the Circuit Identifier. If no Circuit Identifier has been assigned this object should have an octet string with zero length. ifAdminStatus See interfaces MIB [RFC2863] ifOperStatus See interfaces MIB [RFC2863] ifLastChange See interfaces MIB [RFC2863] ifName See interfaces MIB [RFC2863].
ifLinkUpDownTrapEnable Set to enabled(1).
ifHighSpeed Speed of line in Mega-bits per second
(2, 6, or 8)
ifConnectorPresent Set to true(1) normally, except for
cases such as DS1/E1 over AAL1/ATM where
false(2) is appropriate
2.2. Usage Guidelines
2.2.1. Usage of ifStackTable for Routers and DSUs
The object dsx1IfIndex has been deprecated. This object previously
allowed a very special proxy situation to exist for Routers and CSUs.
This section now describes how to use ifStackTable to represent this
relationship.
The paragraphs discussing dsx1IfIndex and dsx1LineIndex have been
preserved in Appendix A for informational purposes.
The ifStackTable is used in the proxy case to represent the
association between pairs of interfaces, e.g., this T1 is attached to
that T1. This use is consistent with the use of the ifStackTable to
show the association between various sub-layers of an interface. In
both cases entire PDUs are exchanged between the interface pairs - in
the case of a T1, entire T1 frames are exchanged; in the case of PPP
and HDLC, entire HDLC frames are exchanged. This usage is not meant
to suggest the use of the ifStackTable to represent Time Division
Multiplexing (TDM) connections in general.
External&Internal interface scenario: the SNMP Agent resides on a
host external from the device supporting DS1 interfaces (e.g., a
router). The Agent represents both the host and the DS1 device.
Example:
A shelf full of CSUs connected to a Router. An SNMP Agent residing
on the router proxies for itself and the CSU. The router has also an
Ethernet interface:
+-----+
| | |
| | | +---------------------+
|E | | 1.544 MBPS | Line#A | DS1 Link
|t | R |---------------+ - - - - - - - - - +------>
|h | | | |
|e | O | 1.544 MBPS | Line#B | DS1 Link
|r | |---------------+ - - - - - - - - - - +------>
|n | U | | CSU Shelf |
|e | | 1.544 MBPS | Line#C | DS1 Link
|t | T |---------------+ - - - -- -- - - - - +------>
| | | | |
|-----| E | 1.544 MBPS | Line#D | DS1 Link
| | |---------------+ - - - - -- - - - - +------>
| | R | |_____________________|
| | |
| +-----+
The assignment of the index values could for example be:
ifIndex Description
1 Ethernet
2 Line#A Router
3 Line#B Router
4 Line#C Router
5 Line#D Router
6 Line#A CSU Router
7 Line#B CSU Router
8 Line#C CSU Router
9 Line#D CSU Router
10 Line#A CSU Network
11 Line#B CSU Network
12 Line#C CSU Network
13 Line#D CSU Network
The ifStackTable is then used to show the relationships between the
various DS1 interfaces.
ifStackTable Entries
HigherLayer LowerLayer
2 6
3 7
4 8
5 9
6 10
7 11
8 12
9 13
If the CSU shelf is managed by itself by a local SNMP Agent, the
situation would be identical, except the Ethernet and the 4 router
interfaces are deleted. Interfaces would also be numbered from 1 to
8.
ifIndex Description
1 Line#A CSU Router
2 Line#B CSU Router
3 Line#C CSU Router
4 Line#D CSU Router
5 Line#A CSU Network
6 Line#B CSU Network
7 Line#C CSU Network
8 Line#D CSU Network
ifStackTable Entries
HigherLayer LowerLayer
1 5
2 6
3 7
4 8
2.2.2. Usage of ifStackTable for DS1/E1 on DS2/E2
An example is given of how DS1/E1 interfaces are stacked on DS2/E2
interfaces. It is not necessary nor is it always desirable to
represent DS2 interfaces. If this is required, the following
stacking should be used. All ifTypes are ds1. The DS2 is determined
by examining ifSpeed or dsx1LineType.
ifIndex Description
1 DS1 #1
2 DS1 #2
3 DS1 #3
4 DS1 #4
5 DS2
ifStackTable Entries
HigherLayer LowerLayer
1 5
2 5
3 5
4 5
2.2.3. Usage of Channelization for DS3, DS1, DS0
An example is given here to explain the channelization objects in the
DS3, DS1, and DS0 MIBs to help the implementor use the objects
correctly. Treatment of E3 and E1 would be similar, with the number
of DS0s being different depending on the framing of the E1.
Assume that a DS3 (with ifIndex 1) is Channelized into DS1s (without
DS2s). The object dsx3Channelization is set to enabledDs1. There
will be 28 DS1s in the ifTable. Assume the entries in the ifTable
for the DS1s are created in channel order and the ifIndex values are
2 through 29. In the DS1 MIB, there will be an entry in the
dsx1ChanMappingTable for each ds1. The entries will be as follows:
dsx1ChanMappingTable Entries
ifIndex dsx1Ds1ChannelNumber dsx1ChanMappedIfIndex
1 1 2
1 2 3
......
1 28 29
In addition, the DS1s are channelized into DS0s. The object
dsx1Channelization is set to enabledDS0 for each DS1. When this
object is set to this value, 24 DS0s are created by the agent. There
will be 24 DS0s in the ifTable for each DS1. If the
dsx1Channelization is set to disabled, the 24 DS0s are destroyed.
Assume the entries in the ifTable are created in channel order and
the ifIndex values for the DS0s in the first DS1 are 30 through 53.
In the DS0 MIB, there will be an entry in the dsx0ChanMappingTable
for each DS0. The entries will be as follows:
dsx0ChanMappingTable Entries
ifIndex dsx0Ds0ChannelNumber dsx0ChanMappedIfIndex
2 1 30
2 2 31
......
2 24 53
2.2.4. Usage of Channelization for DS3, DS2, DS1
An example is given here to explain the channelization objects in the
DS3 and DS1 MIBs to help the implementor use the objects correctly.
Assume that a DS3 (with ifIndex 1) is Channelized into DS2s. The
object dsx3Channelization [RFC3896] is set to enabledDs2. There will
be 7 DS2s (ifType of DS1) in the ifTable. Assume the entries in the
ifTable for the DS2s are created in channel order and the ifIndex
values are 2 through 8. In the DS1 MIB, there will be an entry in
the dsx1ChanMappingTable for each DS2. The entries will be as
follows:
dsx1ChanMappingTable Entries
ifIndex dsx1Ds1ChannelNumber dsx1ChanMappedIfIndex
1 1 2
1 2 3
......
1 7 8
In addition, the DS2s are channelized into DS1s. The object
dsx1Channelization is set to enabledDS1 for each DS2. There will be
4 DS1s in the ifTable for each DS2. Assume the entries in the
ifTable are created in channel order and the ifIndex values for the
DS1s in the first DS2 are 9 through 12, then 13 through 16 for the
second DS2, and so on. In the DS1 MIB, there will be an entry in the
dsx1ChanMappingTable for each DS1. The entries will be as follows:
dsx1ChanMappingTable Entries
ifIndex dsx1Ds1ChannelNumber dsx1ChanMappedIfIndex
2 1 9
2 2 10
2 3 11
2 4 12
3 1 13
3 2 14
...
8 4 36
2.2.5. Usage of Loopbacks
This section discusses the behaviour of objects related to loopbacks. The object dsx1LoopbackConfig represents the desired state of loopbacks on this interface. Using this object a Manager can request: LineLoopback PayloadLoopback (if ESF framing) InwardLoopback DualLoopback (Line + Inward) NoLoopback The remote end can also request loopbacks either through the FDL channel if ESF or inband if D4. The loopbacks that can be requested this way are: LineLoopback PayloadLoopback (if ESF framing) NoLoopback To model the current state of loopbacks on a DS1 interface, the object dsx1LoopbackStatus defines which loopback is currently applied to an interface. This objects, which is a bitmap, will have bits turned on which reflect the currently active loopbacks on the interface as well as the source of those loopbacks. The following restrictions/rules apply to loopbacks: The far end cannot undo loopbacks set by a manager. A manager can undo loopbacks set by the far end. Both a line loopback and an inward loopback can be set at the same time. Only these two loopbacks can co-exist and either one may be set by the manager or the far end. A LineLoopback request from the far end is incremental to an existing Inward loopback established by a manager. When a NoLoopback is received from the far end in this case, the InwardLoopback remains in place.2.3. Objectives of this MIB Module
There are numerous things that could be included in a MIB for DS1 signals: the management of multiplexors, CSUs, DSUs, and the like. The intent of this document is to facilitate the common management of all devices with DS1, E1, DS2, or E3 interfaces. As such, a design
decision was made up front to very closely align the MIB with the set of objects that can generally be read from these types devices that are currently deployed. J2 interfaces are not supported by this MIB.2.4. DS1 Terminology
The terminology used in this document to describe error conditions on a DS1 interface as monitored by a DS1 device are based on the late but not final document of what became the ANSI T1.231 standard [ANSI-T1.231]. If the definition in this document does not match the definition in the ANSI T1.231 document, the implementer should follow the definition described in this document.2.4.1. Error Events
Bipolar Violation (BPV) Error Event A BPV error event for an AMI-coded signal is the occurrence of a pulse of the same polarity as the previous pulse (See T1.231 Section 6.1.1.1.1). 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 substitution code. Excessive Zeroes (EXZ) Error Event An Excessive Zeroes error event for an AMI-coded signal is the occurrence of more than fifteen contiguous zeroes (See T1.231 Section 6.1.1.1.2). For a B8ZS coded signal, the defect occurs when more than seven contiguous zeroes are detected. Line Coding Violation (LCV) Error Event A Line Coding Violation (LCV) is the occurrence of either a Bipolar Violation (BPV) or Excessive Zeroes (EXZ) Error Event. (Also known as CV-L; See T1.231 Section 6.5.1.1.) Path Coding Violation (PCV) Error Event A Path Coding Violation error event is a frame synchronization bit error in the D4 and E1-noCRC formats, or a CRC or frame synch. bit error in the ESF and E1-CRC formats. (Also known as CV-P; See T1.231 Section 6.5.2.1.) Controlled Slip (CS) Error Event A Controlled Slip is the replication or deletion of the payload bits of a DS1 frame (See T1.231 Section 6.1.1.2.3). 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 defect.
2.4.2. Performance Defects
Out Of Frame (OOF) Defect
An OOF defect is the occurrence of a particular density of
Framing Error events (See T1.231 Section 6.1.2.2.1).
For DS1 links, an Out of Frame 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 framing-bits.
For E1 links, an Out Of Frame defect is declared when three
consecutive frame alignment signals have been received with an
error (see G.706 Section 4.1 [CCITT-G.706]).
For DS2 links, an Out of Frame defect is declared when 7 or more
consecutive errored framing patterns (4 multiframe) are
received. The OOF is cleared when 3 or more consecutive correct
framing patterns are received.
Once an Out Of Frame Defect is declared, the framer starts
searching for a correct framing pattern. The Out of Frame
defect ends when the signal is in frame.
In-frame occurs when there are fewer than two frame bit errors
within 3 msec period for ESF signals and 0.75 msec for D4
signals.
For E1 links, in-frame occurs when a) in frame N the frame
alignment signal is correct and b) in frame N+1 the frame
alignment signal is absent (i.e., bit 2 in TS0 is a one) and c)
in frame N+2 the frame alignment signal is present and correct.
(See G.704 Section 4.1)
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% present for a time equal to or
greater than T, where 3 ms <= T <= 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 T
(See G.775, Section 5.4).
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 (see O.162 [CCITT-O.162] Section 3.3.2).
For DS2 links, the DS2 AIS shall be sent from the NT1 to the
user to indicate a loss of the 6,312 kbps frame capability on
the network side. The DS2 AIS is defined as a bit array of
6,312 kbps in which all binary bits are set to '1'.
The DS2 AIS detection and removal shall be implemented according
to ITU-T Draft Recommendation G.775 [ITU-T-G.775] Section 5.5:
- a DS2 AIS defect is detected when the incoming signal has
two (2) or less ZEROs in a sequence of 3156 bits (0.5 ms).
- a DS2 AIS defect is cleared when the incoming signal has
three (3) or more ZEROs in a sequence of 3156 bits (0.5 ms).
2.4.3. Performance Parameters
All performance parameters are accumulated in fifteen minute
intervals and up to 96 intervals (24 hours worth) 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 performance parameter.
Performance parameters continue to be collected when the interface is
down.
There is no requirement for an agent to ensure fixed relationship
between the start of a fifteen minute interval and any wall clock;
however some agents may align the fifteen minute intervals with
quarter hours.
Performance parameters are of types PerfCurrentCount,
PerfIntervalCount and PerfTotalCount. These textual conventions are
all Gauge32, and they are used because it is possible for these
objects to decrease. Objects may decrease when Unavailable Seconds
occurs across a fifteen minutes interval boundary. See Unavailable
Seconds discussion later in this section.
Line Errored Seconds (LES)
A Line Errored Second is a second in which one or more Line Code
Violation error events were detected. (Also known as ES-L; See
T1.231 Section 6.5.1.2.)
Controlled Slip Seconds (CSS)
A Controlled Slip Second is a one-second interval containing one
or more controlled slips (See T1.231 Section 6.5.2.8). This is
not incremented during an Unavailable Second.
Errored Seconds (ES)
For ESF and E1-CRC links an Errored Second is a second with one
or more Path Code Violation OR one or more Out of Frame defects
OR one or more Controlled Slip events OR a detected AIS defect.
(See T1.231 Section 6.5.2.2 and G.826 [ITU-T-G.826] Section
B.1).
For D4 and E1-noCRC links, the presence of Bipolar Violations
also triggers an Errored Second.
This is not incremented during an Unavailable Second.
Bursty Errored Seconds (BES)
A Bursty Errored Second (also known as Errored Second type B in
T1.231 Section 6.5.2.4) is a second with fewer than 320 and more
than 1 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. It
applies to ESF signals only.
Severely Errored Seconds (SES)
A Severely Errored Second for ESF signals is a second with 320
or more Path Code Violation Error Events OR one or more Out of
Frame defects OR a detected AIS defect (See T1.231 Section
6.5.2.5).
For E1-CRC signals, a Severely Errored Second is a second with
832 or more Path Code Violation error events OR one or more Out
of Frame defects.
For E1-noCRC signals, a Severely Errored Second is a 2048 LCVs
or more.
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 more.
Controlled slips are not included in this parameter.
This is not incremented during an Unavailable Second.
Severely Errored Framing Second (SEFS)
An Severely Errored Framing Second is a second with one or more
Out of Frame defects OR a detected AIS defect. (Also known as
SAS-P (SEF/AIS second); See T1.231 Section 6.5.2.6.)
Degraded Minutes
A Degraded Minute is one in which the estimated error rate
exceeds 1E-6 but does not exceed 1E-3 (see G.821 [CCITT-G.821]).
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 (a.k.a., 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 described below.
Unavailable Seconds (UAS)
Unavailable Seconds (UAS) 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 10 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 10 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 10 seconds. If the failure clearing time
is more than 10 seconds, the DS1 interface becomes available at
the onset of 10 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.
Note that this definition implies that the agent cannot
determine until after a ten second interval has passed whether a
given one-second interval belongs to available or unavailable
time. If the agent chooses to update the various performance
statistics in real time then it must be prepared to
retroactively reduce the ES, BES, SES, and SEFS counts by 10 and
increase the UAS count by 10 when it determines that available
time has been entered. It must also be prepared to adjust the
PCV count and the DM count as necessary since these parameters
are not accumulated during unavailable time. It must be
similarly prepared to retroactively decrease the UAS count by 10
and increase the ES, BES, and DM counts as necessary upon
entering available time. A special case exists when the 10
second period leading to available or unavailable time crosses a
900 second statistics window boundary, as the foregoing
description implies that the ES, BES, SES, SEFS, DM, and UAS
counts the PREVIOUS interval must be adjusted. In this case
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.
The agent may instead choose to delay updates to the various
statistics by 10 seconds in order to avoid retroactive
adjustments to the counters. A way to do this is sketched in
Appendix B.
In any case, a linkDown trap shall be sent only after the agent has
determined for certain that the unavailable state has been entered,
but the time on the trap will be that of the first UAS (i.e., 10
seconds earlier). A linkUp trap shall be handled similarly.
According to ANSI T1.231 unavailable time begins at the _onset_ of 10
contiguous severely errored seconds -- that is, unavailable time
starts with the _first_ of the 10 contiguous SESs. Also, while an
interface is deemed unavailable all counters for that interface are
frozen except for the UAS count. It follows that an implementation
which strictly complies with this standard must _not_ increment any
counters other than the UAS count -- even temporarily -- as a result
of anything that happens during those 10 seconds. Since changes in
the signal state lag the data to which they apply by 10 seconds, an
ANSI-compliant implementation must pass the one-second statistics
through a 10-second delay line prior to updating any counters. That
can be done by performing the following steps at the end of each one
second interval.
i) Read near/far end CV counter and alarm status flags from the
hardware.
ii) Accumulate the CV counts for the preceding second and compare
them to the ES and SES threshold for the layer in question.
Update the signal state and shift the one-second CV counts and
ES/SES flags into the 10-element delay line. Note that far-end
one-second statistics are to be flagged as "absent" during any
second in which there is an incoming defect at the layer in
question or at any lower layer.
iii) Update the current interval statistics using the signal state
from the _previous_ update cycle and the one-second CV counts
and ES/SES flags shifted out of the 10-element delay line.
This approach is further described in Appendix B.
2.4.4. Failure States
The following failure states are received, or detected failures, that are reported in the dsx1LineStatus object. When a DS1 interface would, if ever, produce the conditions leading to the failure state is described in the appropriate specification. Far End Alarm Failure The Far End Alarm failure is also known as "Yellow Alarm" in the DS1 case, "Distant Alarm" in the E1 case, and "Remote Alarm" in the DS2 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 5 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 zero. For DS2 links, if a loss of frame alignment (LOF or LOS) and/or DS2 AIS condition, is detected, the RAI signal shall be generated and transmitted to the remote side. The Remote Alarm Indication(RAI) signal is defined on m-bits as a repetition of the 16bit sequence consisting of eight binary '1s' and eight binary '0s' in m-bits(1111111100000000). When the RAI signal is not sent (in normal operation),the HDLC flag pattern (01111110) in the m-bit is sent. The RAI failure is detected when 16 or more consecutive RAI- patterns (1111111100000000) are received. The RAI failure is cleared when 4 or more consecutive incorrect-RAI-patterns are received. 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 cleared. (See
T1.231 Section 6.2.1.2.1)
An AIS defect at a 6312 kbit/s (G.704) interface is detected
when the incoming signal has two {2} or less ZEROs in a sequence
of 3156 bits (0.5ms). The AIS signal defect is cleared when the
incoming signal has three {3} or more ZEROs in a sequence of
3156 bits (0.5ms).
Loss Of Frame Failure
For DS1 links, the Loss Of Frame failure is declared when an OOF
or LOS defect has persisted for T seconds, where 2 <= T <= 10.
The Loss Of Frame failure is cleared when there have been no OOF
or LOS defects during a period T where 0 <= T <= 20. Many
systems will perform "hit integration" within the period T
before declaring or clearing the failure e.g., see TR 62411
[AT&T-TR-62411].
For E1 links, the Loss Of Frame Failure is declared when an OOF
defect is detected.
Loss Of Signal Failure
For DS1, 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% 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 10 consecutive zeroes are detected (see O.162
Section 3.4.4).
A LOS defect at 6312kbit/s interfaces is detected when the
incoming signal has "no transitions", i.e., when the signal
level is less than or equal to a signal level of 35dB below
nominal, for N consecutive pulse intervals, where 10 <=N<=255.
The LOS defect is cleared when the incoming signal has
"transitions", i.e., when the signal level is greater than or
equal to a signal level of 9dB below nominal, for N consecutive
pulse intervals, where 10<=N<=255.
A signal with "transitions" corresponds to a G.703 compliant
signal.
Loopback Pseudo-Failure
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).
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 (see G.732 Section 4.2.6).
This condition is never declared for DS1.
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 G.732 [CCITT-G.732] framing
(sometimes called "Channel Associated Signalling" mode).
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 (See G.732).
DS2 Payload AIS Failure
The DS2 Payload AIS is detected when the incoming signal of the
6,312 kbps frame payload (time-slots 1 through 96) has 2 or less
0's in a sequence of 3072 bits (0.5ms). The DS2 Payload AIS is
cleared when the incoming signal of the 6,312 kbps frame payload
has 3 or more 0's in a sequence of 3072 bits (0.5 ms).
DS2 Performance Threshold
DS2 Performance Threshold Failure monitors equipment performance
and is based on the CRC (Cyclic Redundancy Check) Procedure
defined in G.704.
The DS2 Performance Threshold Failure is detected when the bit
error ratio exceeds 10^-4 (Performance Threshold), and the DS2
Performance Threshold Failure shall be cleared when the bit
error ratio decreased to less than 10^-6."
2.4.5. Other Terms
Circuit Identifier This is a character string specified by the circuit vendor, and is useful when communicating with the vendor during the troubleshooting process (see M.1400 [ITU-T-M.1400] for additional information). Proxy In this document, the word proxy is meant to indicate an application which receives SNMP messages and replies to them on behalf of the devices which implement the actual DS1/E1 interfaces. The proxy may have already collected the information about the DS1/E1 interfaces into its local database and may not necessarily forward the requests to the actual DS1/E1 interface. It is expected in such an application that there are periods of time where the proxy is not communicating with the DS1/E1 interfaces. In these instances the proxy will not necessarily have up-to-date configuration information and will most likely have missed the collection of some statistics data. Missed statistics data collection will result in invalid data in the interval table.
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