Ring trip detection in a communication system

Details Ring trip detection in a communication system Ring trip detection in a communication system

1997-06-06

2001-04-17

Isen, Forester W. (Department: 2747)

Telephonic communications

Supervisory or control line signaling

Using line or loop condition detection

C379S382000, C379S399010, C379S413000

Reexamination Certificate

active

06219417

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to communication systems and particularly to ring trip detection with short duration short circuit discrimination.
2. Description of the Related Art
Communication systems utilizing transmission lines such as subscriber loops are commonplace throughout much of the world. Subscriber loops are terminated on one end by terminal equipment and provide a path for the terminal equipment to communicate with other terminal equipment via, for example, a vast network of central offices, private branch exchanges, satellite relay systems, transmission lines, repeaters, and wireless systems.
FIG. 1
illustrates a portion of a conventional subscriber loop communication system
100
. The subscriber loop
102
is modeled as a balanced two-wire transmission line
103
, with loop resistances R and inductances L and leakage impedance modeled by capacitor
104
and resistor
106
. The subscriber loop
102
is terminated on respective ends by terminal equipment
110
and a central office
122
line card
120
. The subscriber loop
102
provides a communication path for information transmission such as voice signals and signaling information between a subscriber's terminal equipment and the central office
108
.
Terminal equipment
110
is illustratively modeled as a telephone with off-hook resistance
112
, nominally 200 ohms, and ringer impedance Z, which may be modeled, for example, as a series RC or series RLC circuit. Terminal equipment
110
includes a switch hook
118
which loads the subscriber loop
102
on the subscriber end with resistance
112
when the terminal equipment
110
is off-hook (as shown) and loads subscriber loop
102
with ringer impedance Z when the terminal equipment
110
is on-hook. Terminal equipment
110
may be any of a variety of devices besides the familiar, ubiquitous telephone such as facsimile machines, private branch exchanges, voice mail systems key telephone systems, computers, modems, telephone answering machines, alarm systems, and radio control systems, as well as many other devices.
The other end of the subscriber loop
102
, opposite terminal equipment
110
, converges on line card
120
of central office
122
. The line card
120
terminates subscriber loop
102
at conductors A (Tip) and B (Ring) with a feed impedance of 900 ohm or other standard feed impedance. The line card
120
provides a gateway to the public switched telephone network (PSTN) through switching network
124
.
Referring to
FIG. 2
, the subscriber line interface circuit (SLIC)
202
of line card
120
provides a two-wire interface
204
to the generally analog signal carrying subscriber loop
102
. The SLIC
202
performs a variety of interface functions that allow terminal equipment
110
to communicate with other terminal equipment (not shown). The SLIC
202
and the subscriber loop audio-processing circuit (SLAC)
206
carry out the well-known BORSCHT (Battery feed, Overvoltage protection, Ringing, Supervision, Coding, Hybrid, and Test) functions. The SLIC
202
monitors direct current (DC) levels on the subscriber loop
102
with ground key detector circuitry
208
and off-hook detector circuitry
210
. Input decoder and control circuitry
214
provides a mechanism for other circuitry (not shown) in the central office
122
and for SLAC
206
to control such SLIC
202
functions as subscriber loop
102
activation, ringing, and polarity reversal. Analog two-wire interface
204
and signal transmission circuitry
212
cooperate in sensing subscriber loop
102
metallic voltage (voltage at conductor A minus voltage at conductor B or Vab) while generally having a high rejection of longitudinal voltages (Vab_long). Alternating current (AC) signals, such as voice signals, are transmitted over subscriber loop
102
to terminal equipment
110
by two-wire interface
204
and signal transmission circuitry
212
in response to voice information input signals received from central office
122
through SLAC
206
.
The power feed controller
222
includes a battery feed circuit and a polarity reversal circuit. The battery feed functions supply direct current from a central office battery (not shown) to the subscriber loop
102
through balanced feed resistances at conductors A and B. Loop current is generally limited to no more than 45 to 75 milliamperes (mA) in a low-resistance subscriber loop. Higher subscriber loop resistances generally result in lower subscriber loop current. The on-hook subscriber loop powering voltage is typically the battery voltages minus 48 Volts DC (Vdc) less any overhead voltage, typically about 4 Vdc, necessary to prevent SLIC
202
saturation. Battery feed specifications are regionally provided and conform with specifications provided by, for example, BELLCORE, the Electronic Industries Association (EIA), British Telecom, and the International Telegraph and Telephone Consultative Committee (CCITT).
The SLAC
206
generally filters and converts analog output signals received from SLIC
202
into digital signals (A/D), processes the signals in accordance with control and timing information, and compresses the digital signals. The pulse code modulation (PCM) interface
220
provides PCM signals to the central office
122
. SLAC
206
also receives digital input signals from the central office
122
via PCM interface
220
, expands the digital input signals, process the signal in accordance with control and timing information, and converts the digital signals into analog signals (D/A) for input to SLIC
202
. Additional information on SLICs and SLACs is found in the 1995 Advanced Micro Devices of California data book entitled “Linecard Products for the Public Infrastructure Market.”
Referring to
FIGS. 1 and 2
, when a call is directed to terminal equipment
110
, the ring relay driver
216
activates a relay
126
which connects a central office
122
ringing AC voltage generator
128
and DC bias source
130
to subscriber loop
102
. The AC voltage generator
128
supplied ringing voltage generally varies from a nominal 105 Vac at 16 Hz to 140 Vac at 66 ⅔ Hz. The DC bias source
130
biases the ringing voltage with a DC voltage of between 48 Vdc and 105 Vdc. Additionally, line circuit
120
may provide an integral (internally generated) ringing voltage signal to subscriber loop
102
. While terminal equipment
110
is on-hook, switch hook
118
connects subscriber loop
102
across the ringer impedance Z (FIG.
1
). The ring trip detector
218
detects an off-hook condition of terminal equipment
110
while the ringing voltage is being applied by detecting the DC current change generated by the closing of subscriber loop
102
across resistance
112
and the DC bias source
130
. The ring trip detector
218
must initiate cessation of the ringing voltage signal application to subscriber loop
102
within a predetermined amount of time after an off-hook event, which is generally between 150 and 200 ms. Furthermore, some geographical regions require discriminating between short duration short circuits which may also cause a current while the ringing signal is applied to subscriber loop
102
.
A low pass filter (not shown) is generally used to detect the off-hook current change while discriminating against the AC ringing voltage. However, discriminating against the short duration short circuit is a nontrivial task for the low pass filter. A sudden current change due to a short duration short circuit will cause the output waveform of the low pass filter to slowly rise. After cessation of the short circuit, the low pass filter output waveform will slowly decay. The decay time may exceed the maximum time allotted to detect an off-hook condition, and, thus, the short duration short circuit may be detected by ring trip detector
218
in error as an off-hook condition. The low pass filter may be tuned to successfully meet off-hook detection and short duration short circuit discrimination requirements for some subscriber loops. However, generically tuning a low pass filter to accommo

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