Telephonic communications – Diagnostic testing – malfunction indication – or electrical... – Testing of subscriber loop or terminal
Reexamination Certificate
1999-09-10
2001-11-20
Tieu, Binh (Department: 2643)
Telephonic communications
Diagnostic testing, malfunction indication, or electrical...
Testing of subscriber loop or terminal
C379S026010, C379S029040, C379S032040, C379S399010
Reexamination Certificate
active
06320940
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a network termination device, and more particularly to a network termination device which determines whether the subscriber loop is in an idle state or busy state by detecting the polarity of a dc voltage supplied from local office equipment.
2. Description of the Related Art
The recent proliferation of the Internet has brought about increasingly widespread deployment of the Integrated Services Digital Network (ISDN) for home use. While conventional subscriber loops are provided originally for analog signal transmission, the ISDN makes it possible to utilize the existing metallic wires to transport high-speed digital signals.
The ISDN basic rate services are based on a reference model shown in
FIG. 18
, which is defined in the TTC standard JT-G961 (Digital Transmission System on Metallic Local Lines for ISDN Basic Rate Access). TTC stands for the Telecommunication Technology Committee (Japan), and JT-G961 is based on the ITU-T Recommendation G.961. Referring to
FIG. 18
, a public switched telephone network (PSTN)
1
is a telecommunications network that is accessible to the general public. An exchange termination device
2
and line termination devices
3
a
to
3
c
are deployed at a central office. The exchange termination device
2
permits a call from a subscriber to reach the desired destination by making an appropriate circuit connection. Being disposed at the exchange end of each metallic subscriber loop
4
, the line termination devices
3
a
to
3
c
serve as adapters that perform various operations for digital data communication services. The subscriber loop
4
is physically a twisted pair of copper wires, originally prepared for analog voice signal transmission.
In the system of
FIG. 18
, the subscriber-side equipment includes: a network termination device
5
, subscriber terminals
7
a
and
7
b,
a terminal adapter
8
, and an analog telephone set
9
. Located at the subscriber end of the metallic subscriber loop
4
, the network termination device
5
provides various functions as the peer system of the line termination device
3
a.
The subscriber terminals
7
a
and
7
b,
called terminal equipment (TE) in ISDN terminology, are digital telecommunications equipment such as digital telephone sets and G
4
standard-compatible fax machines. The terminal adapter
8
serves as a bridge to make the conventional analog telephone set
9
compatible with the ISDN protocol of S-bus
6
, converting conventional analog telephone signals to/from ISDN digital signals.
The metallic subscriber loop
4
, known as the U interface in ISDN terminology, carries basic-rate digital information between the network termination device
5
and the line termination device
3
a.
Since the U interface is not internationally standardized, various implementations are possible for the metallic subscriber loop
4
. In the United States, echo canceling techniques are used to provide simultaneous full-duplex transmission. Many other countries, on the other hand, use time-compression multiplexing techniques, which are also known as the ping-pong method.
FIG. 19
is a block diagram which presents the details of the line termination device
3
and network termination device
5
in the system of FIG.
18
. Here, the symbols “L
1
” and “L
2
” represent two individual wires constituting the metallic subscriber loop
4
.
FIG. 19
shows that the line termination device
3
comprises: a power supply
30
, a current detector
31
, a DC-AC splitter/combiner
32
, a polarity reversing switch
33
, a circuit termination unit
34
, and a switch controller
35
.
The power supply
30
provides the network termination device
5
with electric power.
FIG. 20
shows a voltage-current plot representing the output characteristics of the power supply
30
. Acting as a load of this power supply
30
, the network termination device
5
exhibits a high circuit impedance when in the on-hook (or idle) state, and a lower impedance when in the off-hook (or busy) state. In the former situation, the power supply
30
functions as a constant voltage source, while, in the latter situation, it serves as a constant current source, as can be seen from FIG.
20
. The current detector
31
monitors the load current of the power supply
30
and provides the switch controller
35
with a detection signal that indicates whether the current exceeds a predetermined threshold level.
Referring back to
FIG. 19
, the DC-AC splitter/combiner
32
allows a direct current from the power supply
30
to pass through to the polarity reversing switch
33
. Simultaneously, it permits the circuit termination unit
34
to send an outgoing data signal to the polarity reversing switch
33
, while preventing the signal from leaking to the power supply
30
. The DC-AC splitter/combiner
32
also receives an incoming data signal from the network termination device
5
via the polarity reversing switch
33
, and delivers it solely to the circuit termination unit
34
.
The polarity reversing switch
33
, composed of four switches S
1
to S
4
, manipulates the polarity of the supply voltage, when sending it out to the network termination device
5
. More specifically, the polarity reversing switch
33
changes over the supply voltage from normal polarity to reverse polarity, or vise versa, by alternating its internal connection paths according to control commands from the switch controller
35
. When the switches S
1
and S
4
are turned on, a straight connection path is made to place a positive voltage on the wire L
1
with respect to the other wire L
2
. This is referred to as the “normal polarity.” Oppositely, when the switches S
2
and S
3
are turned on, a crossed connection path is established so that a positive voltage will appear on the wire L
2
with respect to the other wire L
1
. This is referred to as the “reverse polarity.”
The circuit termination unit
34
performs, for example, bitrate conversion of data signals exchanged between the exchange termination device
2
and network termination device
5
. The switch controller
35
governs the polarity reversing switch
33
according to the aforementioned detection signal received from the current detector
31
. The switch controller
35
has a dead band which eliminates any possible instability in the current detection during a transitional period when the supply voltage changes from normal polarity to reverse polarity. It also contributes toward increasing noise immunity. Actually, this dead band is realized as a masking function with a predetermined time constant &tgr;.
FIG. 19
also shows that the network termination device
5
comprises: a DC-AC splitter/combiner
50
, a diode
51
, a switch
52
, a diode
53
, an internal power supply
54
, a circuit termination unit
55
, and a call request detector
56
. The structure and function of the DC-AC splitter/combiner
50
are similar to those of the aforementioned DC-AC splitter/combiner
32
in the line termination device
3
. In short, only a DC voltage appears at the left-hand port of the DC-AC splitter/combiner
50
, while data signals at the bottom port. The diode
51
applies the DC voltage to the switch
52
, only when it is with the normal polarity. The switch
52
comprises a semiconductor switch, which is activated by the call request detector
56
when it has detected a call originated from a subscriber terminal (not shown in FIG.
19
).
The diode
53
prevents the internal power supply
54
from receiving a DC voltage from the line termination device
3
that is working in normal polarity mode. The internal power supply
54
is typically a DC-DC converter. Operating only with a reverse-polarity DC voltage, it provides a predetermined voltage(s) to other portions of the network termination device
5
. As can be seen from the above, the internal power supply
54
will appear to the line termination device
3
as a high-impedance load when a normal-polarity voltage is applied, but as a low-impedance load when a reverse-polarity voltage is applied
Fujitsu Limited
Rosenman & Colin LLP
Tieu Binh
LandOfFree
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