Switch hook status determination in a communication system

Details Switch hook status determination in a communication system Switch hook status determination in a communication system

1997-06-06

2001-01-23

Chan, Wing F. (Department: 2743)

Telephonic communications

Supervisory or control line signaling

Using line or loop condition detection

C379S377000, C379S399010

Reexamination Certificate

active

06178241

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a communication system and more particularly relates to determining the switch hook status of terminal equipment of the communication system.
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 leakage resistor
106
is generally at least tens of thousands of ohms. 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 ring relay driver
216
activates a relay(s) which connects a ringing signal from a central office
122
ringing AC voltage generator (not shown) and DC voltage bias source, to terminal equipment
110
when a third party is calling. The ring trip detector circuitry
218
detects an off-hook condition of terminal equipment
110
and initiates cessation of the ringing signal application to subscriber loop
102
.
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 generally receives digital audio input signals from the central office
122
via PCM interface
220
, expands the digital input signals, processes 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.”
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).
Call establishment performance is of considerable importance to telephone companies. Generally, terminal equipment
110
initiates a call by seizing subscriber loop
102
by loop start signaling. Another method, referred to as ground start signaling, is generally used by private branch exchanges (PBX). Loop start signaling occurs after the terminal equipment
110
goes off-hook and switch hook
118
closes the subscriber loop
102
across resistance
112
as shown in FIG.
1
. The resulting DC subscriber loop current is detected by line circuit
120
, which then connects equipment capable of receiving dialed address information.
Address information may be transmitted by
110
using dual tone multifrequency (DTMF) signal generators (not shown) or with dial pulses from a dial pulse instrument. The pulse rate is typically in the range of 8-12 pulses per second (pps) and may be as high as 20 pps. Pulses are generated by making and breaking (off-hook and on-hook, respectively) the subscriber loop
102
switch hook
118
connection. Generally, the pulse duration is about 100 milliseconds (ms) with the break interval varying from about 55-65 percent of the total pulse duration. The minimum time between dial pulses is typically about 200 ms.
Regional communication authorities and central office
108
service providers generally require a line card to detect transitions between on-hook and off-hook events within a predetermined time, such as 2 ms, after the actual initiation of the transition event. However, meeting this criteria is a non-trivial task due to widely varying subscriber loop
102
characteristic such as impedance.
Subscriber loop
102
off-hook impedance varies widely from as little as about 200 ohms to over 2 kohms. When the terminal equipment
110
goes off-hook

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