Pulse or digital communications – Transceivers – Modems
Reexamination Certificate
1999-01-14
2003-02-18
Chin, Stephen (Department: 2734)
Pulse or digital communications
Transceivers
Modems
C379S406080
Reexamination Certificate
active
06522688
ABSTRACT:
The subject matter of this application is closely related to the following copending application which deals with certain aspects of the present invention as disclosed herein and is incorporated herein by reference: “High speed modem with uplink remote-echo canceller,” serial number unknown, by Eric M. Dowling and filed on the same day as the present application, Jan. 14, 1999.
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to data communications. More particularly, the invention relates to high-speed modems designed to provide transmission speeds of 56 kbps in both downlink and uplink directions.
2. Description of the Related Art
In the field of wireline telephone modems, one type of modem is known as a “V.90 modem.” “V.90” references an international standard (recommendation V.90) agreed upon by the International Telecommunications Union (ITU-T). A V.90 modem is a specific member of a class of modems known as “PCM modems.” In fact, before the V.90 standard was agreed upon, V.90 was known colloquially as “V.PCM.” “PCM” is an acronym which stands for “pulse code modulation.” PCM was originally developed to transmit digitized speech signals through a telephone network such as the public switched telephone network (PSTN). PCM as used in the PSTN is defined in the ITU-T G.711 standard. In the United States, PCM involves sampling a speech signal, encoding each sample to roughly 13 bits of linear resolution, and then logarithmically companding and re-encoding the sample to an 8-bit mu-law code. PCM coding involves a well-known and accepted form of nonlinear logarithmic compression whereby smaller signal amplitudes are represented at greater resolution than larger amplitudes. European countries use a different logarithmic compression law known as A-law. PCM encoding techniques provide a way to efficiently encode analog speech waveforms. This is because speech waveforms have a wide dynamic range and the human hearing system itself performs logarithmic signal processing.
The ITU-T's V.90 standard represents an accepted technology used to transfer information between an analog telephone subscriber and a network server such as an Internet service provider (ISP). V.90 compliant modems are asymmetric in that different signal protocols are used in the uplink and downlink communication directions. In connection with V.90 modems, the term “uplink” corresponds to the communication channel from the analog telephone subscriber to a digital network server. The “downlink” corresponds to the communication channel from the digital network server to the analog telephone subscriber. The V.90 recommendation makes the assumption that communication must pass through a standard network interface comprising a PCM codec. A PCM “codec” is a converter which converts an analog signal to a PCM signal (COder) and also converts a PCM signal to an analog signal (DECocer). A coder and a decoder are thus collectively called a “codec.” The codec may be viewed as a communication impairment, because at this analog interface, noise and distortion are added to an otherwise pure digital communication path capable of carrying 64 kbps in both the uplink and downlink directions. Certain types of digital impairments such as robbed-bit signaling may also reduce sustainable data rates in the digital network to 56 kbps or lower.
FIG. 1
illustrates a network configuration as is present in a V.90 oriented communication system. A digital network server
102
typically represents an ISP or a database server which is connected to a digital network
110
. The digital network
110
typically involves a network such as the PSTN and the digital network server
102
typically connects to the digital network
110
via a digital connection such as an ISDN line or a T
1
line. The digital network server
102
thereby couples to the digital network
110
via a digital modem
105
. No codec is needed to coupling the digital modem
105
to the digital network
110
. The digital modem
105
transmits and receives digital information across the digital network
110
. At the edge of the digital network
110
is a network interface
115
typically implemented using a line card. The network interface
115
includes a codec
117
and a line interface circuit
130
. The codec
117
accepts a digital stream of information from the digital network
110
into a digital-to-analog converter (DAC)
120
. The DAC
120
produces an analog output voltage based on the decompressed value of a mu-law code applied to its input. This data stream passing through the DAC
120
is said to travel in the “downlink” direction. The output of the DAC
120
feeds to a low pass filter (LPF)
125
which is also part of the codec
117
. The LPF
125
attenuates high frequency components in the downlink signal to perform reconstruction and line filtering as is known in the art. The LPF
125
may also attenuate a small portion of the low frequency band, for example from 0 Hz to 250 Hz. The output of the LPF
125
feeds a line interface circuit
130
which in this example is illustrated as a hybrid circuit. The hybrid circuit
130
provides a four-wire to two-wire conversion and is typically implemented using transformers and electronic amplifiers. The hybrid circuit
130
interfaces to an analog subscriber line
137
. The analog subscriber line
137
involves a telephone line and the electrical signaling applied thereto. The downlink signal as converted by the DAC
120
and filtered by the LPF
125
is coupled by the hybrid circuit
135
onto the subscriber line
137
. An analog “uplink signal” is also coupled via the hybrid circuit
135
from the subscriber line
137
. The analog uplink signal is passed via the hybrid circuit
130
to an LPF
135
which performs antialiasing. In addition to attenuating high frequency components, the LPF
135
may also attenuate a small portion of the low frequency band, for example from 0 Hz to 250 Hz. The output from the LPF
135
is passed to an analog-to-digital converter (ADC)
140
. Besides converting the filtered analog uplink signal to digital, the ADC typically logarithmically compresses the data according a compression curve such as the mu-law curve. Both the LPF
135
and the ADC
140
are also typically implemented as a part of the codec
117
which is itself implemented on a single integrated circuit die. Because the ADC
140
is found within the network interface
115
, the ADC
140
is also referred to herein as the “network-interface ADC.” Similarly, the DAC
120
is also referred to herein as the “network-interface DAC.”
Signals are coupled from the subscriber line
137
to an analog modem
145
via a line interface circuit
150
. The subscriber line
137
is therefore coupled via a network-side coupling to the network interface
115
and via a subscriber-side coupling to the subscriber modem
145
. A transmitter
160
generates a digital representation of an analog uplink to be sent over the subscriber line
137
back to the network interface
115
. In present day V.90 systems, the digital representation of the analog uplink signal is defined by the physical layer signal specification of the ITU-T recommendation V.34. The digital signal output from the transmitter
160
is coupled to a DAC
162
. Typically the DAC
162
is a high precision linear converter and is followed by a bandpass reconstruction filter with a passband in the range from approximately 250 Hz to 3500 Hz (not shown). This filtered analog uplink signal is coupled onto the subscriber line
137
via the line interface circuit
150
. The analog downlink signal sent by the network interface
115
is coupled from the subscriber line
137
via the line interface circuit
150
. Although not shown, the line interface circuit
150
also typically includes a receive-antialiasing filter. The received analog downlink signal is then digitized by an ADC
152
which typically samples its input at 16000 Hz and quantizes each sample to 16 bits of linear resolution. This received and digitized downlink signal is then combined with
Chin Stephen
Dowling Eric M.
Kim Kevin
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