Multiplex communications – Diagnostic testing – Loopback
Reexamination Certificate
1996-09-12
2002-07-16
Vincent, David (Department: 2661)
Multiplex communications
Diagnostic testing
Loopback
C370S252000, C370S470000, C370S526000, C379S026020, C379S027010
Reexamination Certificate
active
06421323
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to telecommunications systems, and more particularly to a method and apparatus for detecting and determining the point of origin of events in a telecommunications system.
2. Description of Related Art
Public Switched Telephone Networks (PSTN) commonly utilize Time Division Multiplexing (TDM) transmission systems to communicate both voice and data signals over a digital communications link. For example, DS
1
(digital signal level 1) data paths are currently used to carry both voice and data signals over a single transmission facility. DS
1
paths carry DS
1
signals which are transmitted at a nominal rate of 1.544 Mb/s. DS
1
paths reduce the number of lines required to carry voice and data signals. Data paths, such as DS
1
paths, have a portion of the data transmission capability assigned to communicating customer information from one end of the data path to the other. This portion of the transmission capability is commonly referred to as the “payload”. In addition, another portion of the transmission capability is assigned to overhead functions, such as error detection and maintaining the data path. This portion of the transmission capability is commonly referred to as “overhead”.
DS
1
facilities are used in large part to carry signals switched by components of the PSTN. However, point-to-point DS1 data links are also used to interconnect equipment controlled by different data users. A typical DS1 signal path is shown in FIG.
1
. DS1 transmission systems, like the one shown in
FIG. 1
, include three general equipment types: (1) terminating equipment, (2) user interface equipment, and (3) transmission equipment. Terminating equipment
10
primarily serves to build the DS1 1.544 Mb/s TDM signal from the various sub-rate voice and data signals. Terminating equipment
10
typically performs Pulse Code Modulation (PCM) and TDM functions. The terminating equipment
10
also de-multiplexes the 1.544 Mb/s DS
1
signal to separate voice and data signals at their original sub-rates.
The user interface equipment typically comprises a Channel Service Unit (CSU)
20
which connects the terminating equipment
10
with the transmission equipment
30
, such as a DS
1
path and ensures that both ends of the DS
1
paths
30
send and receive a high quality DS
1
signal. The CSU
20
typically checks for conformance to certain standards which are set by the telecommunications industry. The CSU
20
corrects and detects errors in the DS
1
transmission path. For example, the CSU
20
corrects Bipolar Violations (BPV). In addition, the CSU
20
detects various errors and inserts alarm indications and zero substitution codes in the DS
1
transmission path, including Remote Alarm Indication (RAI), Alarm Indication Signal (AIS), and Bipolar with Eight-Zero Substitution (B8ZS) signals.
The DS
1
path
30
includes hardware used by the network providers to transmit DS
1
digital signals between equipment controlled by different data users. The DS
1
path
30
as shown in
FIG. 1
is implemented by a T1 line. However, other facilities, such as coaxial cables, fiber optic cables, and microwave links may be used by providing an appropriate transport interface between the Channel Service Unit (CSU)
20
and the facility.
DS
1
signals may be transmitted over a dedicated point-to-point network as simple as the one shown in
FIG. 1
utilizing twisted wire pairs and repeaters spaced at intermediate points. Alternatively, the network may be as complex as the one shown in
FIG. 2
which utilizes a combination of twisted wire pairs and repeaters, multiplexers, Digital Cross-connect Systems (DCS), Add Drop Multiplexers (ADM), Fiber Optic Terminals (FOT), Coaxial Cable, Microwave, Satellite, or any other transmission media capable of transporting a DS
1
signal. In some instances, DS
1
signals may be carried over a network similar to the point-to-point network, but having the added capability to switch the DS
1
signal (in a DCS or ADM) in a manner similar to the PSTN.
While DS
1
transmission systems such as the system shown in
FIG. 1
are well-known, customers more typically communicate using a public DS
1
network, as shown in FIG.
2
. In the DS
1
transmission system shown in
FIG. 2
, equipment is divided into categories based on the location of the equipment. Essentially, the equipment is broken into three categories: (1) the Customer Premises Equipment (CPE)
40
; (2) the Local Exchange Carrier (LEC) equipment, which comprises the local loop
42
and the central office equipment; and (3) the InterExchange Carrier (IEC)
52
. CPE
40
belongs to the network user (or customer). The customer that owns the CPE
40
is responsible for both its operation and maintenance. The customer must ensure that its equipment provides a healthy and standard DS
1
digital signal to the local exchange carrier equipment
42
. The equipment
40
on the customer premises typically consists of DS
1
multiplexers
46
, digital Private Branch Exchanges (PBXs), and any other DS
1
terminating equipment which connects to a CSU
20
at the CPE site. The local exchange carrier equipment
42
connects the CPE
40
with the central office
44
and the IEC
52
. LECs assume responsibility for maintaining equipment at the line of demarcation between the CPE
40
and the local loop
42
.
As shown in
FIG. 2
, a Network Interface Unit (NIU)
50
may be coupled between the CPE
40
and the LEC equipment
42
. The NIU
50
represents the point of demarcation between the CPE
40
and the LEC equipment
42
(which comprises local loop equipment
42
, and Central Office (CO) equipment
44
). Prior art NIUs may be relatively simple devices which allow network technicians to minimally test the operation and performance of both the CPE
40
and the DS
1
network
52
or they may be more sophisticated devices.
The CO equipment
44
may include equipment that can monitor for various DS
1
signal requirements. Independent of whether the DS
1
transmission system is simple (FIG.
1
), complex (FIG.
2
), or switched, all the circuits and network equipment required to transmit a DS
1
signal must be tested and maintained to operate at maximum efficiency. In order to perform such test and maintenance functions, equipment within the DS
1
path provides maintenance signals indicative of particular conditions on the incoming and outgoing signals. These signals are defined by standards established by the American National Standards Institute for operation of DS
1
communications links (ANSI T1.403 and ANSI T1.408, et. al). These indications include (1) RAI, which indicates that the signal that was received by the CPE equipment from the NIU
50
was lost (the detailed requirements for sending RAI are contained in ANSI T1.231); and (2) AIS, which is an unframed, all-ones signal which is transmitted to the network interface upon loss, or in response to the presence of a signal defect of the originating signal, or when any action is taken that would cause a service disruption (such as loopback). In addition, detection of signal defects in the digital hierarchy above DS
1
shall cause an AIS signal to be generated. The AIS signal is removed when the condition that triggered the AIS is terminated.
In addition to these indications, ANSI T1.403-1995 defines a performance report message (PRM) which is sent each second using a format which is detailed in the standard. PRMs contain performance information within a “message field” portion of the PRM for each of four previous one-second intervals. Counts of cyclic redundancy checking (CRC) errors are accumulated in each contiguous one-second interval by the reporting equipment. For example, a one bit (designated “G1” by the ANSI standard) within the variable portion of the PRM indicates that one CRC error had occurred within the last second of transmission. At the end of each one-second interval, a modulo-4 counter within the variable portion of the PRM is incremented, and the appropriate performance bits are set within the remainder of
Hartmann Paul R.
Nelson Derek J.
Tyburski Edward S.
Applied Digital Access, Inc.
Heller Ehrman White & McAuliffe
Phunkulh Bob A.
Vincent David
LandOfFree
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