Telephonic communications – Subscriber line or transmission line interface – Network interface device
Reexamination Certificate
1997-11-17
2002-09-10
Isen, Forester W. (Department: 2644)
Telephonic communications
Subscriber line or transmission line interface
Network interface device
C379S399010, C379S397000
Reexamination Certificate
active
06449362
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to apparatus, systems, and methods for isolating Plain Old Telephone Service (POTS) signals from higher band communication signals and, more particularly, for apparatus, systems, and methods for isolating POTS signals from Asymmetric Digital Subscriber Line (ADSL) signals.
BACKGROUND OF THE INVENTION
In general, modems designed for use with conventional telephone lines accommodate relatively low data transmission rates. While current modems can process a high-end bit rate at about 33.6 Kbits/second, they are nonetheless significantly slower than a digital modem, such as one on an ISDN line which can operate at 64 Kbits/second. These rates, unfortunately, remain too low for many desired types of communication, such as full-motion video which requires a minimum of 1.5 Mbits/second for VHS quality using MPEG-1 (Motion Pictures Expert Group) compression and 3 to 6 Mbits/second for broadcast quality using MPEG-2 compression.
A recently pronounced standard in telecommunications defines an Asymmetrical Digital Subscriber Line (ADSL) system which executes a high speed transfer of data over a single twisted-wire pair, such as an existing telephone line. In addition to Plain Old Telephone Services (POTS), an ADSL system also permits full-duplex and simplex digital services with data rates from about 1.5 Mbits/second to 7 Mbits/second. An ADSL system uses a spectrum from about 26 kHz to 1.1 MHz for broadband data transmission and leaves the spectrum from about DC to 4 kHz for POTS. An ADSL system provides at least four downstream simplex channels having rates ranging from about 1.5 Mbits/second to 6 Mbits/second and four full duplex channels with rates ranging from about 64 Kbits/second to 640 Kbits/second. An ADSL system is therefore more than capable of providing video-on-demand capability, video conferencing, data file transfer capability and can provide all of this capability simultaneously with POTS. For additional information, reference may be made to American National Standards Institute Standard ANSI-T1.413-1995 which describes an ADSL system and an interface between a telecommunications network and a customer's installation and which is incorporated herein by this reference.
With reference to
FIG. 1
, a standard ADSL system
10
may comprise an ADSL transceiver unit
12
at a central office (ATU-C) which communicates with an ADSL transceiver unit
14
at a customer premises (ATU-R). The ADSL transceiver unit
12
at the central office receives data from a digital network
15
, performs various processing on the data, and transfers the processed data to a splitter
16
. The splitter
16
combines the signals from the transceiver unit
12
with signals from a public switched telephone network (PSTN)
18
and transfers the combined signals onto a line
20
. At the customer end, a splitter
22
supplies a lower-band set of signals to one or more POTS terminal devices
24
and a higher-band set of signals to the ADSL transceiver unit
14
. The ADSL transceiver unit
14
at the customer's end processes the received signals and supplies the processed signals to one or more service modules (SM)
26
. The processed data from the ADSL transceiver unit
14
may be supplied directly to the one or more service modules
26
or may be supplied through a customer installation distribution network
28
. The network
28
may be any type of network, such as a star or bus network. Reference may be had to ANSI T1.413-1995 for additional information on the ADSL transceiver units
12
and
14
and on other aspects of the ADSL system
10
, which is hereby incorporated by reference.
One difficulty with ADSL, however, is that the signals supplied to the ADSL transceiver
14
and the signals supplied to the POTS terminal device
24
must be isolated from each other. One reason requiring this isolation is that the POTS terminal device
24
, which may be a telephone or other non-linear device, produces inter-modulation harmonics from the ADSL system both in the frequency range of the ADSL signals and in the voice band. Likewise, the ADSL transceiver unit
14
can generate interference with the signals supplied to the POTS terminal devices
24
. Consequently, some type of filtering must occur between the ADSL transceiver
14
and the POTS terminal devices
24
.
The signals supplied to the POTS terminal devices
24
may be isolated from the signals supplied to the ADSL transceiver unit
14
in any one of a multitude of ways. One of these ways is to place a low-pass filter at each POTS terminal device
24
and to place a high-pass filter at either the ADSL transceiver unit
14
or at a network interface device (NID). For instance, the low pass filters may be placed in series between the POTS terminal devices
24
and their connection to a wall jack. These low pass filters would then filter out the higher band ADSL signals and prevent the ADSL signals from interfering with the POTS signals.
The placement of the low-pass filter at each POTS terminal device, however, adversely affects the overall performance of the ADSL system
10
. The lines connecting the POTS terminal devices
24
to the low pass filters look like bridge taps to the ADSL line and produce significant losses at the top end of the downstream ADSL band transfer function, such as losses from 5 dB to 15 dB between 400 kHz and 1.1 MHz. The reason for these losses and their effects on the ADSL system
10
are explained in more detail in Dennis J. Rauschmayer, “Effects of a Distributed POTS Splitter Topology on ADSL Line Transfer Functions,” American National Standards Institute T1E1.4 Technical Subcommittee Report T1E1.4/96-167, Jul. 22, 1996, which is incorporated herein by this reference. The placement of low-pass filters at each POTS terminal device
24
is therefore undesirable due to their effects on the ADSL signals.
In contrast to the placement of a low-pass filter at each POTS terminal device
24
, the use of a single low pass filter for all POTS terminal devices
24
produces more favorable results. For instance, a comparison between the placement of the low-pass filter at each phone drop versus the placement of the low-pass filter at a split is described in a report by Rick Roberts et al., “ADSL POTS LPF Placement,” American National Standards Institute Working Group Report T1E1.4/96-162, July, 1996, which is incorporated herein by this reference. This report suggests that a single low-pass filter at the split is preferred since a distributed low-pass filter at each phone causes several problems, such as a reduced bit rate and reduced reach of the ADSL system, an increase in line driver current, a hybrid/echo cancellation stress, and risk of improper installation or improper network modification. Thus, rather than placing a low-pass filter at each POTS terminal device
24
, the ADSL system
10
should preferably have a single low-pass filter installed at the split so that the signals supplied to all of the POTS terminal devices
24
are filtered by this single low pass filter.
A single low-pass filter, however, is not as easily installed at a split as are multiple low-pass filters at each POTS terminal device
24
. With multiple low-pass filters, a low-pass filter can be easily incorporated to the telephone network by simply adding a filter between each POTS terminal device
24
and its connection to the customer's telephony wiring, such between the POTS terminal device
24
and a wall jack. The single low-pass filter, on the other hand, must be located at a point along the customer's wiring which is shared by all POTS terminal devices
24
but not at a location which might affect ADSL signals traveling to and from the ADSL transceiver unit
14
.
When the customer's premises is an apartment, adding a low-pass filter to the telephony wiring becomes especially difficult. A typical wiring diagram
30
for an apartment is shown in FIG.
2
. The wiring
30
for an apartment includes an inside network interface (“INI”)
32
within which is contained a custome
Tennyson Gary
Wilmont D. A.
BellSouth Intellectual Property Corporation
Harold Jefferey
Isen Forester W.
Kilpatrick & Stockton LLP
Sutcliffe, Esq. Geoff L.
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