Pulse or digital communications – Equalizers – Automatic
Reexamination Certificate
1998-05-27
2002-04-16
Pham, Chi (Department: 2631)
Pulse or digital communications
Equalizers
Automatic
Reexamination Certificate
active
06373889
ABSTRACT:
BACKGROUND OF THE INVENTION
A. Field of the Invention
The present invention relates to a method and apparatus for exception processing in a pulse code modulation (“PCM”) communication environment. More particularly, the present invention relates to an in-band signal for exception processing in a PCM modern. A detection method and apparatus is also provided.
B. Description of the Related Art
For many years the public digital telephone network (“DTN”) has been used for data transmission between moderns. Typically, a modulated carrier is sent over a local loop to a service provider (e.g., a Regional Bell Operating Company), whereupon the service provider quantizes the signal for transmission through the DTN. A service provider that is located near the receiving location converts the digital signal back to an analog signal for transmidssion over a local loop to the receiving modem. This system is limited in the maximum achievable data rate at least in part by the sampling rate of the quantizers, which is typically 8 kHz (which rate is also the corresponding channel transmission rate, or clock rate, of the DTN).
Furthermore, the analog-to-digital (A/D) and digital-to-analog (D/A) conversions are typically performed in accordance with a non-linear quantizing rule. In North America, this conversion rule is known as &mgr;-law. A similar nonlinear sampling technique known as A-law is used in other areas of the world such as Europe. The non-linear A/D and D/A conversion is generally performed by a codec (coder/decoder) device located at the interfaces between the DTN and local loops. Alternatively, these devices are referred to herein as a DAC (digital-to-analog converter) and an ADC (analog-to-digital converter).
It has been recognized that a data distribution system using the public telephone network can overcome certain aspects of t he aforesaid limitations by providing a digital data source connected directly to the DTN, without an intervening codec. In such a system, the telephone network routes digital signals from the data source to a client's local subscriber loop without any intermediary analog facilities, such that the only analog portion of the link from the data source to the client is the client's local loop (plus the associated analog electronics at both ends of the local loop). The only codec in the transmission path is the one at the DTN end of the client's subscriber loop.
FIG. 1
shows a block diagram of a 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
20
by an analog subscriber loop
50
that is typically a two-wire, or twisted-pair, cable. The DTN
20
routes digital signals from the data source
10
to the client's local subscriber loop
50
without any intermediary analog facilities such that the only analog portion of the link from the server
10
to the client
40
is the subscriber loop
50
. The analog portion of the system thus includes the channel characteristics of the subscriber loop
50
plus the associated analog electronics at both ends of the subscriber loop
50
.
The analog electronics are well known to those skilled in the art and typically include 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.). In the system of
FIG. 1
, the only codec in the transmission path from the server
10
to the client
40
is a DAC located at the DTN
20
end of the subscriber loop
50
. It is understood that the client-side, or subscriber-side, equipment may incorporate an ADC and DAC for its internal signal processing, as is typical of present day modem devices. For the reverse channel, the only ADC converter in the path from the client
40
to the server
10
is also at the DTN
20
end of the subscriber loop
50
.
In the system of
FIG. 1
, the server
10
, having direct digital access to the DTN
20
may be a single computer, or may include a communications hub that provides digital access to a number of computers or processing units. Such a hub/server is disclosed in U.S. Pat. Nos. 5,528,595 and 5,577,105, the contents of which are incorporated herein by reference.
In the system shown in
FIG. 1
, digital data can be input to the DTN
20
as 8-bit bytes (octets) at the 8 kHz clock rate of the DTN. This is commonly referred to as a DS-0 signal format At the interface between the DTN
20
and the subscriber loop
50
, the DTN
20
codec converts each byte to one of 255 analog voltage levels (two different octets each represent 0 volts, i.e plus 0 volts and minus 0 volts) that are sent over the subscriber loop
50
and received by a decoder at the client's location. The last leg of this system, i.e., the local loop
50
from the network codec to the client
40
, may be viewed as a type of baseband data transmission system because no carrier is being modulated in the transmission of the data. The baseband signal set contains the positive and negative voltage pulses output by the codec in response to the binary octets sent over the DTN. The client
40
, as shown in
FIG. 1
, may be referred to herein as a PCM modem.
FIG. 2
is a graph showing a &mgr;-law to linear conversion for one-half of the &mgr;-law codeword set used by the DTN
20
codec. As shown in
FIG. 2
, the analog voltages (shown as decimal equivalents of linear codewords having 16 bits) corresponding to the quantization levels are non-uniformly spaced and follow a generally logarithmic curve. In other words, the increment in the analog voltage level produced from one codeword to the next is not linear, but depends on the mapping as shown in FIG.
2
. Note that the vertical scale of
FIG. 2
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 octets from 0 to 127, not shown in FIG.
2
. Thus
FIG. 2
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. 2
that the logarithmic function of the standard conversion format is approximated by a series of 8 linear segments.
The conversion from octet to analog voltage (or a digital representation of the analog voltage, as discussed above) is well known, and as stated above, is based on a system called &mgr;-law coding in North America and A-law coding in Europe. Theoretically, there are 256 points represented by the 256 possible octets, or &mgr;-law codewords. The format of a &mgr;-law codeword is shown in
FIG. 3
, where the most significant bit b
7
indicates the sign of the codeword, and the remaining seven bits, b
6
-b
0
, represent the magnitude of the codeword. Referring to
FIG. 2
, it may be observed that the three bits b
6
-b
4
represent the linear segment in the conversion graph, and the four bits, b
0
-b
3
indicate the step along the particular, linear segment in the conversion graph. 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 non-zero points, the maximum number of bits that can be sent per signaling interval (symbol) is just under 8 bits.
A &mgr;-law or A-law codeword utilized by the DTN
20
may be referred to herein as a PCM codeword. It is actually the PCM codeword that results in the DTN
20
codec to
5
output a particular analog voltage. The codeword and the corresponding voltage may be referred to herein as “points.” The client
40
, shown in
FIG. 1
, may be referred to herein as a PCM modem.
Other factors, such as robbed-bit signaling, digital attenuation (pads), channel distortion and noise introduced by the subscriber loop, and the crowding of points at the smaller voltage amplitudes and the associated difficulty in distingu
Alelyunas Carl H.
Norrell Andrew
Parizhsky Vladimir
3Com Corporation
Burd Kevin M
McDonnell & Boehnen Hulbert & Berghoff
Pham Chi
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