Pulse or digital communications – Pulse code modulation
Reexamination Certificate
1999-01-29
2002-12-31
Corrielus, Jean (Department: 2631)
Pulse or digital communications
Pulse code modulation
Reexamination Certificate
active
06501802
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method for generating and transmitting “silence” for use in a Pulse Code Modulation (PCM) data communication system. The communication system of particular interest herein uses the public digital telephone network (DTN) to transmit data directly from a digital source to a remote unit, where the remote unit is connected to the DTN either digitally or via an analog local loop. Within this PCM data communication system it is desirable to utilize a “zero” signal. The signaling method described herein a method of forming and transmitting a zero signal, and is compatible with both the &mgr;-Law and A-Law PCM coding, and minimizes the impact of coding differences between the two PCM coding systems.
Presently, typical modems used to communicate over the public telephone system represent binary data by an analog waveform that is modulated in response to the binary data. As an example, one such standard for modem communications is detailed in the International Telecommunication Union, Telecommunication Standardization Sector (“ITU-T”) Recommendation V.34 (1994). The waveform is in turn analyzed at a receiving modem to recover the binary data. For modem signals transmitted over the public telephone system, the analog waveforms are treated by central office facilities in the same manner as if the waveforms were analog voice signals. In other words, the waveforms are digitized into eight bit octets by an analog to digital converter (ADC) codec at the central office, and the octets are transmitted in digital format between central offices until they are converted back to an analog signal by a digital to analog converter (DAC) codec at the central office that is connected to the receiving subscriber loop. The public switched telephone network has operated in this manner for many years.
The data rate attainable by a modem operating in such an environment is limited by numerous factors including, in particular, the codec sample rate and the number and spacing of quantization levels of the codec converters at the central office switches. The effect on an analog signal associated with sampling the signal amplitude and representing the sample by one of a finite number of discrete (digital) values is generally referred to as quantization noise. Most telephone switches utilize voice codecs that perform nonlinear A/D and D/A conversions known as &mgr;-law or A-law conversion. In these conversion formats, the 8-bit codec codewords, also referred to as octets, represent analog voltages that are nonlinearly spaced. This type of conversion performs well for voice signals intended for a human listener (especially when transmitted over a noisy line), but have a negative impact on modulated analog waveforms associated with modems. Specifically, codecs that adhere to these standard nonlinear conversion formats use nonlinearly spaced quantization levels, and have the effect of increasing quantization noise which is detrimental to modulated analog waveforms.
Until recently, it was thought that the maximum attainable data rate for signals passing through the DTN was limited by the quantization noise associated with the codecs. However, it has been recognized that a data distribution system can overcome certain aspects of the aforesaid limitations by providing a digital data source connected directly to the DTN, without any intervening ADC or DAC. In such a system, the telephone network routes digital signals from the data source to the client's local subscriber loop without any intermediary analog facilities, such that the only analog portion of the link from server to client is the client's local loop (plus the associated analog electronics at both ends of the loop). The only DAC in the transmission path is the one at the Telephone Company's end of the client's subscriber loop. In such a system digital data can be converted into PCM codes, and fed to the DTN as 8-bit bytes (octets) at the network's clock rate of 8 kHz. At the distant end, the DTN's DAC converts each byte to one of 255 analog voltage levels in a system utilizing &mgr;-law encoding (or 256 levels in an A-law system), which is sent over the client's subscriber loop and received by a subscriber device (i.e., a modem) at the client's location.
FIG. 1
shows a block diagram of a PCM data distribution system. The system includes a data source
10
, or server, having a direct digital connection
30
to a digital telephone network (DTN)
20
. A client
40
is connected to the DTN
30
by a subscriber loop
50
that is typically a two-wire, or twisted-pair, cable. The DTN routes digital signals from the data source
10
to the client's local subscriber loop without any intermediary analog facilities such that the only analog portion of the link from the server to the client is the client's local loop
50
. The analog portion thus includes the channel characteristics of the local loop transmission line plus the associated analog electronics at both ends of the line. This typically includes a subscriber line interface card at the central office that includes a codec, as well as circuitry used to generate and interpret call progress signals (ring voltage, on-hook and off-hook detection, etc.). The only D/A converter in the transmission path from the server to the client is the one at the DTN end of the client's subscriber loop. It is understood that the client-side, or subscriber-side, equipment may incorporate an A/D and D/A for its internal signal processing, as is typical of present day modem devices. For the reverse channel, the only A/D converter in the path from the client to the server is also at the Telephone Company's end of the client's subscriber loop.
An alternative system is one where connection
50
, like connection
30
, is a digital connection to subscriber unit
40
. In such a system there are preferably no analog transmission links, that is, the digital PCM codewords are not converted to one of a plurality of an analog voltage levels, but transmitted directly to unit
40
in binary form. Of course, at the physical layer, the transmission of the binary signals or codewords is performed by transmitting voltage levels representing logic signals having values of “1” and “0”.
The conversion from octet to analog voltage is well known, and is based on a system called &mgr;-law coding in North America. In Europe, a format known as A-law coding is used. Theoretically, there are 256 points represented by the 256 possible octets, or &mgr;-law/A-law codewords.
FIGS. 2A-C
show the positive values of the &mgr;-law and A-law codewords. There are one hundred twenty eight values, and a total of two hundred fifty six values including the negatives. The codewords are given in hexadecimal format, and are ordered according to the corresponding analog voltage level. Note that the analog level is calibrated in integers from 0 to 32,124. These numbers correspond to a linear 16-bit A/D converter. As is known to those of ordinary skill in the art, the sixteenth bit is a sign bit which provides integers from 0 to −32124 which correspond to the first 128 octets (hexadecimal 0 to 7F), not shown in
FIG. 2
or
3
.
FIG. 3
plots the &mgr;-law codewords versus the analog voltage level.
FIG. 3
can be viewed as a conversion between the logarithmic binary data and the corresponding linear 16-bit binary data. It can also be seen in
FIG. 3
that the logarithmic function of the standard &mgr;-law conversion format is approximated by a series of 8 linear segments.
The format of the &mgr;-law codewords is shown in
FIG. 4
, where the most significant bit b
7
indicates the sign, the three bits b
6
-b
4
represent the linear segment, and the four bits, b
0
-b
3
indicate the step along the particular linear segment. These points are symmetric about zero; i.e., there are 128 positive and 128 negative levels, including two encodings of zero. Since there are 254 points not including zero, the maximum number of bits that can be sent per signal
3Com Corporation
Corrielus Jean
McDonnell & Boehnen Hulbert & Berghoff
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