Far echo canceller for PCM modems

Details Far echo canceller for PCM modems Far echo canceller for PCM modems

1999-09-09

2004-04-13

Olms, Douglas (Department: 2661)

Multiplex communications

Duplex

Transmit/receive interaction control

C370S523000, C375S148000

Reexamination Certificate

active

06721287

ABSTRACT:

FIELD OF INVENTION
The invention pertains to modems. More particularly, the invention pertains to modems designed to operate in accordance with the CCITT V.90 and V.90PLUS standardized protocols.
BACKGROUND OF THE INVENTION
Modems are transceiver devices that allow digital data to be transmitted between pieces of digital equipment, e.g., computers, via the telephone lines. The transmitting modem receives serial digital data from a computer (typically passed from the computer to the modem through a UART (Universal Asynchronous Receiver Transmitter) in order to convert it from parallel to serial format. The modem converts the data to a signal form that can be transmitted effectively via the public telephone system. The receiving modem receives that data and converts it back to serial digital format and passes it to the receiving computer (typically through another UART, which converts the data back to parallel).
Over the past few decades, several protocol standards for modems have been developed. One of the more recent standards has been promulgated by the ITU (International Telecommunications Union) formerly known as the CCITT and is known as ITU-T recommendation V.90, incorporated herein by reference. Earlier generation standards developed by the ITU/CCITT include V.22, V.22bis, V.32, V.34, V.42 and V.42bis.
In the relevant industries, communication in the transmit direction from a network node, such as a telephone or a modem, in the direction of the telephone company central office is termed the upstream direction. Receive direction communications from the network towards a node is termed the downstream direction. In accordance with the V.90 protocol, the data format is different in the downstream direction than it is in the upstream direction. In the V.90 standard, modem transmission in the upstream direction is an analog signal in accordance with the older V.34 standard. However, downstream communication is a PCM (pulse code modulated) signal.
There also is a proposed V.90PLUS recommendation, also incorporated herein by reference, which presently is not in commercial use. In the V.90PLUS standard, PCM is used in both the upstream and downstream directions.
FIG. 1
is a block diagram generally illustrating modem to modem communications through a public telephone network. The system will be described in connection with a public telephone network customer exchanging data with his Internet service provider (ISP) through the public telephone network. For purposes of fully illustrating the various factors contributing to noise in this type of communication, let us assume the customer and his ISP are coupled to different central offices of the public telephone network.
The customer at computer
12
inputs and sends data to the ISP at
28
. The computer
12
includes a built-in UART and, therefore, sends out a serial digital signal to the modem
14
. The modem converts the serial digital signal to comply with the V.90 standard (which, in the upstream direction, is the analog V.34 standard) and puts it out on the public telephone network
20
.
Under the V.34 standard, data rates as great as 31.2 kilobits per second (Kbps) can be achieved.
Within the telephone network, telephony communications between central offices are digital, rather than analog. Accordingly, the analog signal is encoded by a codec
22
into a 64 Kbps signal. In particular, the received analog signal is sampled at a rate of 8 KHz and digitized at an 8 bit resolution to produce a 64 kbps digital PCM signal. The 64 kbps standard is known in the United States as the &mgr;-law standard and in Europe as the A-law standard. The information is digitally transmitted between central office
24
and central office
26
.
For voice and data communications between two normal customers of the public telephone network, the digital signals received at central office
26
from other central offices on the network, e.g., central office
24
, would be passed through another codec (not shown) to be decoded back to analog form. The decoded analog signals would then be forwarded to the receiving customer. However, a high volume customer of the public telephone network, such as an ISP
28
, would likely have a high bandwidth digital connection to the central office
26
, such as a T-1 line
30
. Accordingly, ISP
28
would not use a codec in central office
26
, but instead would receive the data in digital form over a digital link, such as a T1 line
30
.
In the opposite direction, ISP
28
outputs digital data to central office
26
via T1 line
30
. This data is transmitted in digital form to central office
24
. Codec
22
in central office
24
decodes the digital data to a PCM analog version of the digital signal in accordance with the V.90 protocol and transmits it to the customer. The customer's modem
14
receives the data and converts it to a serial digital data format detectable by computer
12
. Finally, the UART in computer
12
converts the data from serial to parallel. In the downstream direction, data can be received at rates as great as 56 Kbps.
As can be seen, under the V.90 standard, upstream communications are at a different data rate, i.e., 31.2 Kbps, than downstream communications, i.e., 56 kbps. Further, the communications in the upstream direction, is in an analog format, i.e., V.34, and, in the downstream direction, are in PCM format.
FIG. 2
is a more detailed block diagram of the interface between a customer's modem
14
and the local central office
24
. As shown, the modem
14
accepts transmit data from the computer's UART
201
on a transmit data path
202
and sends data to the computer's UART
201
on a receive data path
204
. In the transmit (upstream) direction, V.90 transmitter
203
in modem module
206
of modem
14
converts data between the serial digital format generated by the UART
201
to the analog V.34 format. Codec
209
converts the data from digital to analog for transmission over the telephone lines. In the receive direction (i.e., downstream), V.90 receiver
205
in modem module
206
converts data from the V.90 PCM format to the serial digital format used by UART
201
. Codec
209
converts data from analog to digital in the receive direction.
The customer's equipment (to the left of hybrid circuitry
208
in
FIG. 2
) is a four wire system. That is, there are two wires for the transmit direction (i.e., each of lines
202
and
204
comprises two wires) and two wires for the receive direction (i.e., each lines
204
and
207
comprises two wires). The public telephone network, however, is a two wire system in which the transmit data and the receive data are transmitted over the same wire pair. Accordingly, a hybrid circuit
208
interfaces between the codec
209
and the public telephone network
210
. In the transmit direction, it takes the transmit data from the codec and places it on the two wires
211
(tip and ring) of the telephone network. In the receive direction, it selects and isolates the receive data from wires
211
and forwards it to the modem module
206
on the receive wire pair
207
. There is almost always an impedance mismatch between the customer's telephone equipment and the public telephone network. This impedance mismatch has the unfortunate effect of causing an echo at the hybrid circuit. The echo occurs in both directions. For instance, data transmitted from the computer
12
through the modem module
206
to the hybrid
208
is reflected back on the receive wire pair
207
to the modem module
206
and computer
12
. Likewise, data received from the public telephone network over the tip and ring wire pair
211
also is reflected back onto the public telephone network.
At the central office, there is another hybrid circuit
224
and codec circuit
226
serving essentially the same functions. Hybrid circuit
224
also creates echos in both directions. The echo from hybrid circuit
224
passes back through hybrid circuit
208
and reach the receive data path
204
,
207
. Such echoes are not particularly bothersome for voice commun

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