Larscom ACST-1500B Series User manual

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Access-T 100, 200, 400, & 1500
Multi-port DSU/CSU System
Installation and Operation Manual
ACST-0351-005
June 1996

NOTICE
Specifications and performance characteristics described
in this document are subject to change without notice.
Access-T is a trademark of Larscom Incorporated.
❖
Larscom Incorporated
4600 Patrick Henry Drive
Santa Clara, CA 95054
Telephone: [phone]
Customer Service: [phone]
Fax: [phone]
World Wide Web: http://www.larscom.com
❖
Copyright 1996
PrintedinU.S.A.

Notice to U.S. Users
When using Access-T as an interface to a public T1 or Fractional T1 (FT1) service, ensure
compliance with FCC Rules, Part 68, as follows:
1. All direct connections to T1/FT1 lines must be made through standard plugs
and jacks furnished by the telephone company. No connection can be made
to party lines or coin lines.
2. Before connecting a unit, tell your local phone company that you have an FCC
registered device that you wish to connect to the company’s lines. Provide
them with the following information, as requested:
•The FCC registration number, GRPUSA-73975-DE-N (Series
100/200/400) or GRPUSA-75029-DE-N (Series 1500), listed on
the Access-T label.
•The service code: 6.0N.
•The facility code: 04DU9-B, 04DU9-C, 04DU9-S.
•The jack arrangement needed: RJ48C. After the telephone com-
pany has been notified and has installed any necessary jacks,
connect Orion to the Network Interface.
3. If Access-T appears to be malfunctioning, disconnect it from the telephone line
until it is determined whether the Access-T or the line is the source of the trouble.
If Access-T needs repair, do not reconnect it to the line until the repair has been
completed.
4. Access-T has been designed to prevent disruption of the T1/FT1 network. If
Orion is not meeting performance requirements, the telephone company can
temporarily disconnect service (giving advance notice, if possible).
5. Thetelephonecompanymustgiveadvancenoticebeforealteringequipmentor
operations in a way that will affect operation of Access-T or other customer
equipment.
6. Under FCC rules, the customer is not authorized to repair Access-T, regardless
of whether the unit is under warranty or not. Always return Access-T to the
factory if repair is required.
7. Do not remove equipment housing unless you are a qualified installer. There are
no user-servicable parts inside. The Larscom Access-T must be installed by a
qualified technician.
Access-T ACST-0351-005
Contents June 1996
xiv

Notice to Canadian Users
The Industry of Canada label identifies certified equipment. This certification means that the
equipment meets certain telecommunications network protective, operational, and safety
requirements. The Department does not guarantee the equipment will operate to the user’s
satisfaction.
Before installing this equipment, users should ensure that it is permissible to be connected
to the facilities of the local telecommunications company. The equipment must also be
installed using an acceptable method of connection. In some cases, the company’s inside
wiring associated with a single line individual service may be extended by means of a
certified connector assembly (telephone extension cord). The customer should be aware
that compliance with the above conditions may not prevent degradation of service in some
situations.
RepairstocertifiedequipmentshouldbemadebyanauthorizedCanadianmaintenancefacility
designated by the supplier. Any repairs or alterations made by the user to this equipment, or
equipment malfunctions, may give the telecommunications company cause to request the user
to disconnect the equipment.
Users should ensure for their own protection that the electrical ground connections of the
power utility, telephone lines, and internal metallic water pipe system, if present, are
connected together. This precaution may be particularly important in rural areas.
CAUTION: Users should not attempt to make such connections
themselves, but should contact the appropriate electric inspection
authority, or electrician, as appropriate.
This digital apparatus does not exceed the Class A limits for radio noise emissions from
digital apparatus set out in the Radio Interference Regulation of the Industry of Canada.
Le présent appareil numérique n’émet pas de bruits radioélectriques dépasant les limites applicables
aux appareils numériques de la class A prescrites dans le Réglement sur le brouillage radioélectique
édité par le ministére des Communication du Canada.
ACST-0351-005 Access-T
June 1996 Contents
xv

FCC Rules, Part 15
This equipment has been tested and found to comply with the
limits for a Class A digital device, pursuant to Part 15 of the
FCC Rules. These limits are designed to provide reasonable
protection against harmful interference when the equipment is
operated in a commercial environment. This equipment gener-
ates, uses, and can radiate radio frequency energy and, if not
installed in accordance with the instructions in this manual,
may cause harmful interference to radio communications. Op-
eration of this equipment in a residential area is likely to cause
harmful interference, in which case the user will be required to
correct the interference at his own expense.
Warning
Changes or modifications to this unit not expressly approved by
Larscom could void the user’s authority to operate the equipment.
Access-T ACST-0351-005
Contents June 1996
xvi

Introduction 1
Access-T refers to a DSU/CSU product family consisting of three groups:
•Access-T 01/02 DSU/CSUs. Older software version; no Aux port;
2 DTE ports.
•Access-T Series 100, 200, and 400 DSU/CSUs. Current software
version; Aux port option; integral SNMP; 1, 2, or 4 DTE ports.
•Access-T Series 1500 Multi-Port Network Access DSU/CSU. Same
software as Series 100/200/400; rack-mountable chassis holding up
to 15 Access-T modules; eachmoduleprovides 2DTE ports, Aux port,
and T1 CSU (and thus isequivalent to an Access-TSeries 200); integral
SNMP; one supervisory connection serves all modules.
This manual describes the functions and capabilities of the Larscom Access-T
Series 100, 200, 400, and 1500 products. Although identical in software, these
two versions of the product have significant physical differences, which are
noted throughout this manual as appropriate.
When the term “Access-T” is used in this manual, it refers to the Access-T 100, 200,
400, and 1500 products collectively. When the text refers to one product version in
particular, the full product name will be used.
Note: The Access-T Series 100, 200, and 400 are sometimes
referred to as the Access-T 1-Port, 2-Port, and 4-Port models.
ACST-0351-005 Chapter 1
June 1996 Introduction
1-1

Access-T installers and operators should be familiar with the information
presented in this manual, which is divided into the following 10 chapters:
Chapter 1. Introduction — This chapter provides general technical
information about T1 and FT1 networks and the Simple Network Man-
agement Protocol (SNMP).
Chapter 2. Access-T Functional Overview — This chapter provides
information about Access-T’s data and line interfaces, supervisory ports
and interfaces, performance monitoring, loopbacks, and specifications.
Chapter 3. Installation — This chapter provides Access-T installation
procedures, including all mounting, powering, and wiring instructions.
Chapter 4. Front-Panel Controls and Indicators — This chapter pro-
vides a detailed description of Access-T’s front-panel status indicators
and connectors.
Chapter 5. The Terminal Interface — This chapter provides a structural
overview of Access-T’s configuration and operation menus. It describes
each menu path and its associated options and parameters.
Chapter 6. Configuration Procedures — This chapter provides proce-
dures for configuring Access-T through the Terminal Interface.
Chapter 7. Alarms, Reports, and Tests — This chapter describes the
Access-T displays and commands used to monitor the performance and
status of the communication links and the procedures for performing all
diagnostics functions, including loopbacks, test pattern transmission,
and alarm monitoring.
Chapter 8. Using Access-T as a Master — This chapter describes how
to operate an Access-T which is configured as the master for a chain of
slave Access-Ts.
Chapter 9. Maintenance — This chapter provides a description of the
maintenance support available for Access-T.
There are also several appendices which provide the following supplementary materials:
Appendix A. Cable Drawings — This appendix contains drawings and
part numbers for each of the cables that can be used with Access-T.
Appendix B. Glossary — This appendix provides a list of T1, SNMP,
and Access-T terms and definitions.
Chapter 1 ACST-0351-005
Introduction June 1996
1-2

Appendix C. CP01 Chain Port Expander — This appendix describes the
Larscom CP01 Chain Port Expander, which can link the RS485 facilities
connecting multiple Access-Ts to the supervisory master via RS232 ports.
Appendix D. Access-T Operator Messages — This appendix lists all
Access-T status and error messages.
Appendix E. Access-T 1500 Accessory Kits — This appendix describes the
four accessory kits for use with the Access-T 1500 Line/Aux Port connectors.
Appendix F. Standard MIB-II Definitions —This appendix describes
the implementation of the SNMP MIB-II (RFC 1213).
Appendix G. Access-T DS1 MIB Support — This appendix lists theDS1
MIB commands supported by Access-T (RFC 1406).
Appendix H. SNMP Trap Support — This appendix describes the
implementation of the Larscom Private MIB, including trap support.
Appendix I. Point-to-Point Protocol — This appendix provides sup-
plementary information about PPP as is pertains to Access-T.
T1 Networks
The T1 digital transmission system, carrying DS1 signals, is the primary digital
communication system in North America. A T1 facility provides full-duplex
transmission at 1.544 megabits per second (Mbps). Bandwidth is divided into 8
kbps of overhead and 1.536 Mbps of user information. For digitized voice
applications, the information bandwidth typically consists of 24 multiplexed
64-kbps channels. For the transmission of data, a T1 facility may be channelized
as for voice, or it may carry from one to as many as several hundred multiplexed
signalsonanunchannelizedbasis.
Today’s T1 networks are typically multinodal, with multiple T1 line termina-
tions at each node. Nodes can be located at customer premises or at a carrier’s
Central Office, creating a hybrid public/private networking environment. The
high capacity of T1 facilities allows integration of previously separate voice,
data, facsimile, and image networks into high-speed backbone networks.
Currently, the T1 environment is in the midst of a major change in framing
format. The D4 or Superframe (SF) format is being supplanted by the Extended
Superframe (ESF) format. ESF allows performance monitoring of live traffic
with no decrease in information bandwidth.
ACST-0351-005 Chapter 1
June 1996 Introduction
1-3

Customer Premises Equipment
Various types of equipment are employed at the customer’s location. Digital
Terminating Equipment (DTE) provides the source for the transmitted signal
and the destination for the received signal. DTE includes such equipment as:
•Multiplexers.
•PBXs.
•T1 Channel Banks.
•Front-End Processors.
•Computers.
A Data Service Unit (DSU) converts one or more subrate signals (i.e., signals
transmitted at a subfrequency of the 1.544-Mbps T1 signal) to a T1 signal. Finally,
a Channel ServiceUnit (CSU) interfaces the customer’sT1 DTE or DSU to thepublic
T1 network. The CSU provides various termination and interface functions as
specified by AT&T Publication 62411 and FCC Rules, Part 68, including:
•Electrical interface.
•Surge and lightning protection.
•Signal regeneration and pulse density assurance.
•Keep-Alive and Yellow Signal.
•Loopback to the line controlled from the network.
T1 Signal Characteristics
The T1 signal is a bipolar pulse train. Data is encoded using pulse code modu-
lation (PCM) and time-division multiplexing (TDM). In other words, the signal
is divided into time slots of 648 nanoseconds each (1,544,000 time slots per
second), with data encoded by the presence or absence of a pulse in each time
slot. A pulse, if it exists, will have one-half the duration of the time slot and an
amplitude of three volts. When a pulse is present, the time slot data is a ONE;
when no pulse is present, the time slot contains a ZERO.
T1 signals employ Alternate Mark Inversion (AMI) line coding, in which con-
secutive pulses are expected to be of opposite polarity. Consecutive pulses of
the same polarity (called a “bipolar violation” or BPV) indicate a transmission
error. Figure 1-1 shows an example of AMI line coding.
Chapter 1 ACST-0351-005
Introduction June 1996
1-4

Transmission Facilities
T1 signals are transmitted primarily over standard twisted-pair copper wire.
Signal loss on the wire is approximately 5 or 6 dB per 1000 feet. Repeaters are
employed every 6000 feet along the transmission facility to compensate for the
signal losses and to ensure an adequate signal level at the Network Interface
(the termination of the T1 line at the customer premises). AT&T specifies the
maximum distance between the CSU and the last repeater to be 3000 feet;
Access-T CSUs accurately regenerate a -27 dB signal (equivalent to the attenu-
ation over 5000 feet of cable). Figure 1-2 shows one end of a typical T1 link.
T1 may also be transmitted via satellites, digital microwave radios, fiber optic
systems, and coaxial cable modems. In the carrier networks, T1 signals may be
multiplexed into even higher-speed signals (e.g., T3).
Figure 1-1. AMI Line Coding
Figure 1-2. One End of a Typical T1 Link
Bits
Signal
00011011
Customer Premises Equipment T1 Line
6000 ft. max.3000 ft. max.
Network
Interface (NI)
Repeater Repeater
DTE CSU
655 ft. max.
ACST-0351-005 Chapter 1
June 1996 Introduction
1-5

Pulse Density
To interpret and regenerate a T1 signal, repeaters and other T1 equipment must
be able to determine time slots based on the pulses in the received signal. Since
pulses occur only when ONES are transmitted, signals with too many consecu-
tive ZEROS cause timing problems.
The minimum frequency of ONES pulses (called pulse density) that is necessary
to ensure reliable timing varies with the equipment involved. However, a
standard pulse density requirement has been defined by AT&T:
N pulses in every 8(N+1) bits, where N = 1, 2, . . . 23
One of the corollaries of this formula is that no more than 15 ZEROS may be
transmitted consecutively. (If N = 1, there must be at least 1 pulse in every 16
bits.) This requirement was very important in the past, when repeaters would
tend to oscillate if they received more than 15 consecutive ZEROS, and an
oscillating repeater required manual resetting. However, most repeaters cur-
rently being deployed on T1 facilities can handle up to 80 consecutive ZEROS.
Pulse density requirements may be enforced by either the DTE or the DSU/CSU,
depending on the particular application. Pulse density may be guaranteed in
various ways:
•A particular bit position (generally, bit 7 or 8 of each data byte) may
be reserved for the transmission of a pulse.
•A pulse may be “stuffed” in the data stream (i.e., a ZERO may be
overwritten with a ONE) to ensure that pulse density is maintained.
•Data may be bundled or encoded in a way that guarantees an accept-
able pulse frequency.
Another pulse density issue involves the transmission of a keep-alive signal to
the T1 line whenever signal is lost from the DTE. The keep-alive signal is a fixed
bit pattern (usually unframed ALL-ONES) which provides required pulses to
the network while notifying the far end that a failure has occurred.
Note: Where dc power is available from the T1 span, a CSU
may be required to transmit a keep-alive signal to the network
in the event of a local power failure. Currently, however, the
implementation of dc-powered (“wet”) spans is decreasing.
Chapter 1 ACST-0351-005
Introduction June 1996
1-6

Clear Channel Capability
The primary drawback of pulse stuffing is that it corrupts the original data. This
is not a problem with voice transmissions, where the effect on the sound
reproduced at the receiving end is negligible. In data transmissions, however,
changing a single bit can have serious consequences. To eliminate the possibility
of data corruption, part of the bandwidth (for example, every eighth bit) must
be dedicated to pulses, or a method of data encoding must be employed that
ensures pulse density while allowing the entire bandwidth to be used for data.
Availability of the entire bandwidth for data is called “clear channel” capability.
The primary method for achieving clear channel capability in the T1 environ-
ment is a data-encoding scheme called B8ZS (Bipolar 8-Zero Substitution). A
modification of AMI line coding, B8ZS replaces any eight consecutive ZEROS
with a fixed code containing two bipolar violations (BPVs), as illustrated in
Figure 1-3.
There are two requirements for the use of B8ZS:
•The CSUs or DTE at each end of the T1 link must be able to encode
and decode B8ZS.
•Network equipment through which the signal passes must be trans-
parent to B8ZS—in other words, must not “correct” the BPVs in the
B8ZS code.
The latter requirement prevents the current implementation of B8ZS in many
T1 networks.
Framing Synchronization
T1 data is grouped into “frames” of 192 information bits plus 1 framing bit.
Framing bits occur in a fixed pattern of ONES and ZEROS. Network equipment
is able to synchronize on these patterns, and thus to identify frames properly.
An Out of Frame (OOF) event occurs whenever 5 consecutive framing bits
contain 2 or more errors.
As described in AT&T Publication 62411, a CSU declares Loss of Sync if an OOF
condition persists for at least 2.5 seconds. A CSU which loses sync on the signal
received from the T1 line declares a Red Alarm condition and transmits a
defined bit pattern called Yellow Signal to the T1 line.
ACST-0351-005 Chapter 1
June 1996 Introduction
1-7

Notes: Transmitted polarities are reversed if last ONE received was negative.
Bipolar violations always occur at 4th and 7th bit positions.
Figure 1-3. B8ZS Encoding
Framing Formats
Two framing formats are used in today’s T1 networks: D4 and ESF. Both are
based on the “frame” described above: 192 information bits preceded by a
framing/overhead bit. Thus, 8 kbps of the 1.544 Mbps is reserved for overhead,
leaving 1.536 Mbps for user information.
In the older D4 or Superframe (SF) format, a Superframe is defined as twelve
193-bit frames (2316 bits total). Figure 1-4 illustrates the D4 framing format. All
12 overhead (F) bits in each Superframe are used for frame synchronization.
As more T1 networks were deployed, a primary deficiency in D4—the inability
to monitor live data—became significant. To monitor performance on a D4
circuit, live traffic must be halted and test signals transmitted on the line. When
improvements in synchronizing circuits reduced the requirements for framing
synchronization to considerably less than 8 kbps, the Extended Superframe
(ESF) format was developed to allow performance monitoring of live data
without loss of information bandwidth.
Transmitted Bits
Received Bits
Transmitted Signal
1000+1
-1 0 -1 +1
100000000
8 consecutive ZEROS
Chapter 1 ACST-0351-005
Introduction June 1996
1-8

With ESF, the D4 Superframe is extended from 12 to 24 frames (4632 bits).
The 8 kbps of overhead is divided into three separate channels:
•A 2 kbps channel for framing.
•A 2 kbps channel for CRC-6, a six-bit Cyclic Redundancy Check code
for each ESF which is separately calculated by the transmitter and the
receiver and then compared to detect bit errors “on the fly”.
•A 4 kbps channel, called the Facility Data Link (FDL), for diagnostic
control and transmission of performance statistics.
Table 1-A shows bit-by-bit use of the 24 overhead bits and the “robbed”
signalingbitsineachESF.
Currently, there are two standards for ESF. The first was developed by AT&T
and is described in AT&T Publication 54016. More recently, the Exchange
Carriers Standards Association developed the ANSI T1.403 (T1E1) standard,
which differs from the AT&T standard primarily in the way the facility data link
is employed.
ACST-0351-005 Chapter 1
June 1996 Introduction
1-9

Figure 1-4. D4 Format and Channelization
F1 F2 F3 F4 F5 F6 F7 F8 F9 F10 F11 F12
100011011100
1
0
0
0
1
1
0
1
1
1
0
0
F
S
F
T
Combined F
Frame #
1 Superframe = 12 Frames
1.5 milliseconds, 2316 bits
125 microseconds, 193 bits
1 Frame
F12345678
F1234567812345678 12345678
Bit 3
Channel #
12 24
Frame
#
1
2
3
4
5
6
7
8
9
10
11
12
1
0
0
0
1
1
0
1
1
1
0
0
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
A
V
V
V
V
V
B
A Signaling Frame
B Signaling Frame
8-Bit Voice
8-Bit Voice
7-Bit Voice
7-Bit Voice
F = Framing (overhead) bit
V = Bit used for voice code
A = Primary signaling bit
B = Secondary signaling bit
Chapter 1 ACST-0351-005
Introduction June 1996
1-10

Table 1-A. ESF Format
Overhead Bits
(1 bit per frame) Bit Use, All 24 Channels (192 bits per frame) Signaling Bit Use Options (see Notes below)
Frame
Number Fe FDL CRC Traffic Signaling T 2 4 16
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
—
—
—
0
—
—
—
0
—
—
—
1
—
—
—
0
—
—
—
1
—
—
—
1
m
—
m
—
m
—
m
—
m
—
m
—
m
—
m
—
m
—
m
—
m
—
m
—
—
C1
—
—
—
C2
—
—
—
C3
—
—
—
C4
—
—
—
C5
—
—
—
C6
—
—
Bits 1-8
Bits 1-8
Bits 1-8
Bits 1-8
Bits 1-8
Bits 1-7
Bits 1-8
Bits 1-8
Bits 1-8
Bits 1-8
Bits 1-8
Bits 1-7
Bits 1-8
Bits 1-8
Bits 1-8
Bits 1-8
Bits 1-8
Bits 1-7
Bits 1-8
Bits 1-8
Bits 1-8
Bits 1-8
Bits 1-8
Bits 1-7
Bit 8
Bit 8
Bit 8
Bit 8
—
—
—
—
A
A
A
A
A
B
A
B
A
B
C
D
Notes: Fe = ESF Framing Bits (. . . 001011 . . .)
FDL = 4 kbps Facility Data Link (message bits m)
CRC = CRC-6 Block Check (check bits C1-C6)
Option T = Transparent (bit 8 for traffic)
Option 2 = 2-State Signaling (Channel A)
Option 4 = 4-State Signaling (Channels A, B)
Option 16 = 16-State Signaling (Channels A, B, C, D)
ACST-0351-005 Chapter 1
June 1996 Introduction
1-11

The AT&T Standard for ESF
AT&T Publication 54016 (revised 1989) defines the performance-monitoring and
data-link requirements for ESF CSUs. Per the latest revision of Publication 54016
(September 1989), the following performance parameters must be monitored:
•ESF Error Event—Any ESF in which an OOF event or CRC-6 error
occurs.
•Errored Second (ES)—A second with one or more CRC errors or
Out-of-Frame (OOF) conditions.
•Severely Errored Second (SES)—A second with 320 or more CRC
errors or one or more Out-of-Frame (OOF) conditions.
•Bursty Errored Second (BES)—A second with more than one but less
than 320 CRC errors.
•Unavailable Second (UAS)—A second during which Unavailable
Signal State is in effect. Unavailable Signal State is declared after 10
consecutive seconds of SES; it is cleared after 10 consecutive seconds
which are not SES.
•Loss of Frame Count (LOFC)—The number of times Loss of Frame
is declared. Loss of Frame is declared when a device loses sync on the
framing pattern for approximately 2.5 seconds. Loss of Frame is
cleared when a device maintains sync for about 15 milliseconds.
•Controlled Slips (CS)—Monitored and reported by devices which
perform controlled timing slips, optional.
ES, BES, SES, and UAS counts are to be kept in 24-hour registers divided into
96 intervals of 15 minutes each. In addition, a running total count of ESF errors
is to be kept. All ESF performance statistics can be requested, transmitted, and
cleared via the data link.
AT&T Publication 54016 defines two loopbacks which can be controlled from
the network. Line loopback (near the CSU’s line interface) is controlled by
in-band codes. Payload Loopback (near the CSU’s DTE interface) is controlled
by data link codes. These loopbacks correspond to Access-T’s LS Line and DS
Line loopbacks, respectively.
Chapter 1 ACST-0351-005
Introduction June 1996
1-12

The ANSI Standard for ESF
The ANSI T1-403 standard (informally called T1E1after the committee involved
in its formulation) defines performance-monitoring and data-link features
which are significantly different than those defined by AT&T in Publication
54016. The chief difference is that ANSI T1.403 replaces polled performance
registers with performance reports broadcast toward the network every second.
These one-second reports include various information about the received signal
(i.e., the signal from the network) over one second. Each second, the responsible
network element (DTE or CSU) broadcasts a report containing performance
data for each of the four pervious seconds.
As shown in Figure 1-5, one-second reports include the following information:
•CRC Errors.
•Framing Errors.
•Bipolar Violations (BPVs).
•Slip events.
•Loopback status.
ANSI T1.403 defines two loopbacks, Line Loopback and Payload Loopback,
which are the same as the loopbacks defined in AT&T Publication 54016.
However, under ANSI T1.403 both loopbacks are controlled via the data link.
Channelization and Robbed Bit Signaling
As shown in Figure 1-4, the 192 information bits in each frame may be divided
into twenty-four 8-bit channels for the transmission of digitized voice. This
channelization allows the multiplexing of twenty-four 64-kbps voice signals
onto a single T1 line. At each end of the T1 line, the DTE provides the multiplex-
ing and demultiplexing of the individual voice signals.
In addition to the overhead bits, bits are “robbed” from the information band-
width for special signaling in voice applications. Specifically, “robbed bit sig-
naling” employs the least significant bit in each channel in frames 6 and 12 (plus
frames 18 and 24 in ESF), as indicated in Figure 1-4 and Table 1-A. Signaling bits
are used to indicate such conditions as on-hook and off-hook.
ACST-0351-005 Chapter 1
June 1996 Introduction
1-13

Since for each channel only the least significant bit of every six frames is affected,
robbed bit signaling has a negligible effect on the quality of voice transmission.
Figure 1-5. Format of One-Second Reports per ANSI T1.403
Bits 87654321
Word 1 G3 LV G4 U1 U2 G5 SL G6
Word 2 FE SE LB G1 R G2 Nm N1
G1 = 1 1 CRC Error Event
G2 = 1 2 to 5 CRC Error Events
G3 = 1 6 to 10 CRC Error Events
G4 = 1 11 to 100 CRC Error Events
G5 = 1 101 to 319 CRC Error Events
G6 = 1 320 or more CRC Error Events
FE = 1 At least 1 Framing Bit Error; no SE
SE = 1 At least 1 Severely Errored Framing Event
LV = 1 At least 1 Line Code Violation (BPV)
SL = 1 At least 1slip event
LB = 1 Payload loopback activated
U1, U2 Usage under study
R Reserved
Nm, N1 Identifies second (previous, previous -1, etc.)
Chapter 1 ACST-0351-005
Introduction June 1996
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