Telephonic communications – Telephone line or system combined with diverse electrical... – Having transmission of a digital message signal over a...
Reexamination Certificate
1999-05-12
2004-11-02
Harvey, Minsun Oh (Department: 2644)
Telephonic communications
Telephone line or system combined with diverse electrical...
Having transmission of a digital message signal over a...
C379S377000, C379S387010
Reexamination Certificate
active
06813343
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to data and voice communications over digital subscriber lines and, more particularly, to a method and apparatus for filtering signals in a splitterless asymmetric digital subscriber line (ADSL) system.
BACKGROUND OF THE INVENTION
Asymmetric Digital Subscriber Line (ADSL) refers to a new modem technology that allows existing twisted pair telephone lines to be converted into a high-performance access path for multimedia and high-speed data communications. An ADSL circuit connects an ADSL modem on each end of the twisted pair telephone line, creating three information channels—a high speed downstream (central office to end user) channel, a medium speed upstream (end user to central office) channel, and a POTS (“Plain Old Telephone Service”) channel. The POTS channel is separated from the ADSL modem by filters, thus guaranteeing uninterrupted POTS, even if the ADSL circuit fails.
While description is provided in terms of the POTS channel, telephone voice communications signals, telephone instruments, and the like for the benefit of familiarity, it should be understood that telephone equipment and signals need not be limited to voice communications, but may also include other technologies, for example equipment and signals compatible with regular telephone lines, such as facsimiles machines, voiceband modems (for example, V.90 modems), answering machines, and the like.
Two variants of ADSL systems are available today —full-rate ADSL in accordance with the T1E1.413 or ITU G.992.1 standards and “splitterless” ADSL defined by the ITU G.992.2 standard. Full-rate ADSL uses POTS splitters to separate the POTS channel from the ADSL data signals. A POTS splitter is installed at each end of the line and includes a lowpass filter for separating out POTS telephone voice communication signals and a highpass filter for separating out data communication signals.
The POTS splitter divides the subscriber line into two separate twisted pairs—one for data communication (ADSL) and one for telephone voice communication signals (POTS). As a result, the existing two-wire internal house telephone wiring is not usable for ADSL. New wiring must be installed from the splitter to the modem, resulting in increased installation cost.
Splitterless ADSL can be installed without the need for additional home wiring. In this case, the ADSL modem includes a high-pass filter that rejects the POTS telephone voice communication signal, while every telephone instrument in the house is connected to the telephone line through a low-pass filter that rejects the ADSL data signals.
When only one telephone instrument is used, the well-known design of LC filters is adequate to implement the required low pass filter. However, in real life, several telephone instruments are usually connected to each telephone line, each of them in an on-hook or off-hook state. In “splitterless” ADSL, the result of such a configuration is that several lowpass filters are connected in parallel on the same telephone line. Certain electrical properties of a telephone instrument, for example its input impedance, depends on the operational state or hookswitch condition (e.g., whether the telephone instrument hookswitch is on-hook or off-hook). Hookswitch condition can refer to the on-hook or off-hook states of the hookswitch or the transitions of the hookswitch between these states. As a result, certain filter characteristics, for example the frequency response, of a low pass filter connected to a telephone instrument will change when the telephone changes its state.
In reality, the behavior is much more complicated. A low pass filter connected to an on-hook telephone has zero impedance at 4 kHz, which will produce distortion in another telephone instrument, should it happen to be off-hook. Several on-hook telephones connected in parallel will create several resonance frequencies (Universal ADSL Technical Group Contribution, Document # [TG/98-121]; “Preliminary Report of the POTS Filter and Power Reduction Ad-hoc;” Bob Beeman; Redmond, Wash.; Apr. 14, 1998; pp. 1-9).
FIG. 1
is a block diagram illustrating a splitterless ADSL system of the prior art. Customer premises equipment (CPE)
101
is coupled to central office (CO)
102
by digital subscriber line (DSL)
103
. CPE
101
includes a highpass filter
104
, ADSL modem
107
, computer
108
, lowpass filters
105
and
106
, and telephone instruments
109
and
110
. Computer
108
is coupled to ADSL modem
107
, which is coupled to highpass filter
104
, which is coupled to DSL
103
. Telephone instrument
109
is coupled to lowpass filter
105
, which is coupled to DSL
103
. Telephone instrument
110
is coupled to lowpass filter
106
, which is coupled to DSL
103
.
CO
102
includes a POTS (“plain old telephone service”) splitter
111
, ADSL modem
112
, data switch
113
, voice switch
114
, data network
115
, and voice network
116
. DSL
103
is coupled to POTS splitter
111
, which is coupled to voice switch
114
and ADSL modem
112
. Voice switch
114
is coupled to voice network
116
. ADSL modem
112
is coupled to data switch
113
, which is coupled to data network
115
.
Voice communications passing through voice switch
114
are passed through POTS splitter
111
and applied to DSL
103
as baseband signals. Data communications passing through data switch
113
are modulated at a frequency range higher than that of the baseband POTS signals and passed through POTS splitter
111
and applied to DSL
103
. Since the data communications are transmitted at a different frequency range than the voice communications, frequency-division-multiplexing (FDM) allows simultaneous transmission of both voice communications (POTS) and data communications over a single DSL
103
.
Since data communications are suitably processed by ADSL modem
107
and computer
108
, while voice communications are intended for telephone instruments
109
and
110
, highpass filter
104
and lowpass filters
105
and
106
provide selective filtering of the voice and data communications. Highpass filter
104
passes the higher frequency data communications to ADSL modem
107
and computer
108
, while blocking the lower frequency baseband voice communications. Lowpass filters
105
and
106
pass the lower frequency baseband voice communications to telephone instruments
109
and
110
, respectively, while blocking the higher frequency data communications.
Unfortunately, lowpass filters
105
and
106
exhibit a deficiency that can adversely affect the performance of the ADSL system. The frequency response of lowpass filters
105
and
106
changes based on the status of telephone instruments
109
and
110
, respectively. For example, while lowpass filter
105
might properly differentiate between voice communications and data communications when telephone instrument
109
is off-hook (e.g, when telephone instrument
109
is in use), the electrical characteristics of lowpass filter
105
are altered when telephone instrument
109
is returned to its on-hook state (e.g., when the user hangs up). This change in the electrical characteristics of lowpass filter
105
can cause interference with the data communications between ADSL modem
107
of CPE
101
and ADSL modem
112
of CO
102
. Thus, a circuit is needed that will allow telephone instruments, such as telephone instruments
109
and
110
, to change between their off-hook and on-hook states without adversely affecting ongoing data communications over DSL
103
.
FIG. 2
is a block diagram illustrating a lowpass filter and telephone instrument of the prior art. Lowpass filter
201
is coupled to telephone instrument
202
. Telephone instrument
202
includes a load
203
, which exhibits a load impedance. Load
203
is coupled in series with hookswitch
204
. When telephone instrument
202
is off-hook, hookswitch
204
is closed, coupling load
203
to lowpass filter
201
. However, when telephone instrument
202
is on-hook, hookswitch
204
is open, disconnecting load
203
from lowp
Darby & Darby
Harvey Minsun Oh
Orckitt Communications Ltd.
Swerdlow Daniel
LandOfFree
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