WO1994006237A1

WO1994006237A1 - Integrated services digital network complementary modem

Info

Publication number: WO1994006237A1
Application number: PCT/AU1993/000443
Authority: WO
Inventors: Chris Reilly
Original assignee: Chris Reilly
Priority date: 1992-09-01
Filing date: 1993-08-30
Publication date: 1994-03-17

Abstract

Apparatus (16, 26) for coupling a primary information processing device (30) and a plurality of secondary information processing devices (12) to a telecommunications network (20) such that information may be passed between the devices substantially uncorrupted by encoding and decoding apparatus (22) within the telecommunications network. The apparatus assists digital communications devices to operate at higher speed over lines designed for analogue communications. The apparatus substantially eliminates/offsets the effect of the encoding and decoding apparatus provided in the network by processing means at each end.

Description

INTEGRATED SERVICES DIGITAL NETWORK COMPLEMENTARY MODEM Field of the Invention

The present invention relates to an interfacing apparatus for integrated services digital networks, and more particularly to an apparatus for providing complementary digital interfacing of a primary information processing device, and analogue interfacing of a plurality of secondary information processing devices, to an integrated services digital network. Background

A modulator-demodulator (MODEM) is usually used to convert a digital signal into an analogue signal for transmitting data over an analogue telephone network, and also for reconverting the analogue signal received over the analogue telephone network back into a digital signal. Similarly, a coder-decoder (CODEC) is usually used for converting an analogue signal, such as speech, into a digital signal for transmitting over a digital network, and also for reconverting the digital signal received over the digital network back into an analogue signal.

CODECS also usually encode signals for minimum distortion using methods such as A-law or mu-law companding. More specifically, encoding seeks to minimise distortion of perceptually-important features of an analogue signal by assigning a proportionally larger number of bits to those perceptually-important features than the number of bits it assigns to less perceptually-important features. To achieve this low distortion, such encoding at a transmission end must be accompanied by decoding at a reception end.

Both MODEMS and CODECs can be present in a transmission path which includes a typical trunk or inter-exchange link. For example, in a typical system, a data terminal (or secondary information processing device) which transmits and receives digital information might be connected via a MODEM to an analogue transmission line. The transmission line could enter a local exchange and be connected via a CODEC to a digital trunk line. The digital trunk line could, in turn, be connected to a CODEC in a distant telephone exchange, and that CODEC might be connected via an analogue transmission line to a distant MODEM. Finally, the distant MODEM may be connected to a host computer (or primary information processing device) . In this typical example, the transmission of digital information involves digital to analogue conversion, analogue transmission, analogue to digital conversion, encoding, digital transmission, decoding, digital to analogue conversion, analogue transmission, and analogue to digital conversion. This is clearly an inefficient utilisation of transmission equipment because it involves multiple forward and reverse conversions between the analogue and digital domains. Moreover, a problem introduced by such multiple conversions is that analogue transmission limits the transmission rate to that which can be accommodated by the analogue transmission or telephone line.

Apart from this and other problems caused by multiple forward and reverse conversions, another problem arises when a host computer is connected directly and digitally (that is, without a CODEC) to a high-speed service of an integrated services digital network, and the host computer is to be made available to a large number of terminals which only have access to analogue transmission lines. In this case, the signal from the terminals is converted to analogue form by MODEMS and transmitted to local exchanges. At the exchanges, the received analogue information is converted into digital form and encoded by the CODECS. It is then transmitted digitally to the host computer. The problem in this case arises because the digital information is not decoded by a second CODEC before being sent to the host computer. This encoding without subsequent decoding can lead to erroneous data being received by the host computer. Similarly, digital signals from the host computer is transmitted directly without encoding, yet it is nevertheless subsequently decoded before being transmitted to the terminals. This can lead to erroneous data being received by the terminals.

Thus, there is a need for apparatus which will allow for more efficient transmission of digital information without requiring multiple conversion from digital form to analogue form, and vice versa. In addition, there is a need for apparatus which will allow a host computer which is connected digitally to a high-speed service of an integrated services digital network to be made available to terminals which only have access to analogue transmission lines, such availability not being subject to erroneous transmission due to encoding without subsequent decoding, or decoding without prior encoding. Summary of the Invention

The present invention consists in an apparatus for coupling a primary information processing device and a plurality of secondary information processing devices to a telecommunications network, the telecommunications network comprising a primary service and a plurality of secondary services, each one of the plurality of secondary services having an associated encoding and decoding apparatus, the apparatus comprising: a primary interface circuit for interfacing the primary information processing device to the primary service of the telecommunications network; a secondary interface circuit for interfacing one of the plurality of secondary information processing devices to one of the plurality of secondary services of the telecommunications network; and processing means for substantially eliminating the effect of the encoding and decoding apparatus; such that information may be passed between the primary information device and the plurality of secondary information processing devices, the information being substantially uncorrupted by the encoding and decoding apparatus.

Preferably, the primary interface circuit is a digital circuit for coupling information in digital form from the primary information processing device to the primary service of the telecommunications network.

Preferably, the secondary interface circuit performs a conversion between information in analogue form and information in digital form, the information in analogue form being transmitted to, and received from, each one of the plurality of secondary services of the telecommunications network, and information in digital form being transmitted to, and received from, the primary service of the telecommunications network. In an alternative embodiment, the present invention consists in an apparatus for coupling a primary information processing device to a telecommunications network, the telecommunications network comprising a primary service and a plurality of secondary services, each one of the plurality of secondary services having an associated encoding and decoding apparatus, the apparatus comprising: a primary interface circuit for interfacing the primary information processing device to the primary service of the telecommunications network; and processing means for substantially eliminating the effect of the encoding and decoding apparatus; such that information may be passed between the primary information device and a plurality of secondary information processing devices coupled to the plurality of secondary services, the information being substantially uncorrupted by the encoding and decoding apparatus.

In a further alternative embodiment, the present invention consists in an apparatus for coupling a plurality of secondary information processing devices to a telecommunications network, the telecommunications network comprising a primary service and a plurality of secondary services, each one of the plurality of secondary services having an associated encoding and decoding apparatus, the apparatus comprising: a secondary interface circuit for interfacing a corresponding one of the secondary information processing devices to a corresponding one of the plurality of secondary services of the telecommunications network; and processing means for substantially eliminating the effect of the encoding and decoding apparatus; such that information may be passed between a primary information processing device coupled to the primary service of the telecommunications network and the plurality of secondary information processing devices, the information being substantially uncorrupted by the encoding and decoding apparatus. Brief Description of the Drawings

The invention will be described with reference to the accompanying figure in which:

Figure 1 is a block diagram of an interfacing apparatus according to the present invention. Detailed Description

Figure 1 shows telecommunications system 10 in which the present invention is used to allow information to flow substantially uncorrupted between a plurality of terminals and a host computer. Briefly, the elements shown in Figure 1 represent the following. Terminal 12 represents one of a plurality of terminals, each terminal 12 being coupled via separate digital terminal lines 14 to corresponding modulator-demodulator (MODEM) 16. The digital information produced by terminal 12 is converted into an analogue signal by MODEM 16 and passed over telephone line 18 into telecommunications network 20. This analogue signal is converted into a digital signal and encoded by coder-decoder (CODEC) 22. The digital signal is then transmitted over digital transmission line 24 to reach digital termination unit 26. Digital computer line 28 then couples the digital information from digital termination unit 26 to host computer 30.

Information flow in the reverse direction, that is from host computer 30 to terminal 12, takes place in an analogous manner. In the following, the operation of the present invention is described in more detail. Although not shown explicitly in Figure 1, there may be a large number of terminals 12 all communicating substantially simultaneously with host computer 30. Ideally, a completely digital line could couple all terminals 12 to host computer 30. This would allow for high-speed, low-error rate communication. However, in existing telephone networks, there still remain a large number of analogue transmission lines, especially telephone lines, over which digital signals must be carried. Thus, to utilize these existing analogue lines, modulator-demodulators (MODEMS) are used to convert digital information into analogue form.

MODEM 16 has a digital port and an analogue port. It also contains a digital-to-analogue converter and an analogue-to-digital converter. Digital terminal line 14 couples the digital port of MODEM 16 to terminal 12.

MODEM 16 converts a digital signal received at its digital port from terminal 12 into an analogue signal which it sends to its analogue port. Telephone line 18 couples the analogue signal from MODEM 16 into telecommunications network 20. MODEM 16 also converts a signal received at its analogue port from telephone line 18 into a digital form which it transmits via its digital port to terminal 12.

A wide range of methods can be used by MODEM 16 for converting analogue information into digital information, and vice versa. For example, MODEM 16 may employ frequency- or phase-shift keying.

Telephone line 18 eventually terminates in CODEC 22 which is used to convert between the analogue signal representation present on telephone line 18 and the digital signal representation present on digital transmission line 24. Apart from including an analogue-to-digital converter and a digital-to-analogue converter to perform these tasks, CODEC 22 may also include a compander. The service provided by the telecommunication network over telephone line 18 may be called a secondary service.

The digital output from CODEC 22 is transmitted over digital transmission line 24 to termination unit 26. Finally, host computer, or primary information processing device, 30 is connected to termination unit 26 by digital computer line 28. The service provided by the telecommunications network to host computer 30 may be called a primary service. Such a primary service provides a direct digital connection into the network and may operate at a significantly higher data rate than the secondary service. This allows multiple terminals 12 to communicate substantially simultaneously with host computer 30. In converting an analogue signal into a digital signal, CODEC 22 may also encode the signal. This encoding is usually performed, in the case of speech for example, to achieve low distortion by assigning proportionally more bits to features in the speech which are perceptually important than to features that are not perceptually important. Possible encoding schemes include A-law encoding and mu-law encoding.

To achieve low distortion, encoding at a transmission end must be accompanied by decoding at a reception end. Thus, a signal is encoded on the assumption that it will subsequently be decoded. This is usually true in symmetrical communications channels, that is, channels in which a CODEC is present at both the transmission end and at the reception end. However, in the unsy metrical network shown in Figure 1, this assumption is not true. In a first preferred embodiment, the present invention includes a processing means in MODEM 16 to overcome the problem of data corruption due to encoding without subsequent decoding, or decoding without prior encoding.

A first function performed by the processing means in the first preferred embodiment is the pre-processing of information transmitted from terminal 12 to yield a pre-processed analogue signal for transmission over telephone line 18 to CODEC 22. This pre-processing substantially counteracts the subsequent encoding by CODEC 22 so that the digital signal transmitted on digital transmission line 24 to host computer 30 is a substantially accurate representation of the digital signal transmitted by terminal 12.

A second function performed by the processing means in the first preferred embodiment is the post-processing of information transmitted from CODEC 22 to yield a post-processed digital signal for transmission over digital terminal line 14 to terminal 12. This post-processing substantially counteracts prior decoding by CODEC 22 so that the digital information transmitted over digital terminal line 14 to terminal 12 is a substantially accurate representation of the digital signal transmitted by host computer 30. In a second preferred embodiment, the present invention includes a processing means in termination unit 26 to overcome the problem of data corruption due to encoding without subsequent decoding, or decoding without prior encoding.

A first function performed by the processing means in the second preferred embodiment is the pre-processing of information transmitted from host computer 30 to yield a pre-processed digital signal for transmission over digital transmission line 24 to CODEC 22. This pre-processing substantially counteracts the subsequent decoding by CODEC 22 so that the digital signal transmitted on digital terminal line 14 to terminal 12 is a substantially accurate representation of the digital signal transmitted by host computer 30.

A second function performed by the processing means in the second preferred embodiment is the post-processing of information transmitted from CODEC 22 to yield a post-processed digital signal for transmission over digital computer line 28 to host computer 30. This post-processing substantially counteracts prior encoding by CODEC 22 so that the digital information transmitted over digital computer line 28 to host computer 30 is a substantially accurate representation of the digital signal transmitted by terminal 12.

In alternative embodiments, various combinations of the first and second preferred embodiments can be used to overcome the problem of data corruption due to encoding without subsequent decoding, or decoding without prior encoding.

Other functions performed by the processing means, in any of the above embodiments, include pre-processing to avoid signal corruption due to random timing errors, "jammed bit" insertion, and "binary eight zero" substitution used in pulse code modulation systems. In addition, encryption and security functions such as message authentication codes and proofs of origin can be included in the processing means.

One of the advantages of the present invention is that, by eliminating conventional MODEMS, it can achieve much higher data transmission rates over telephone line 18 than are conventionally possible. For example, over a telephone line with a four kilohertz bandwidth, conventional MODEMS can achieve maximum data transmission rates of approximately ten thousand bits per second. However, with the present invention, data transmission rates of approximately thirty thousand bits per second can be achieved. This speed improvement can be achieved without physical intervention in the existing telecommunications network.

Another advantage is that the time and premium charges applicable to data transmission can be avoided in countries where timed charges are applied to local data transmission, but are not applied to voice transmissions. Still another advantage of the present invention is that a gradual transition to an entirely digital network can be achieved, without replacing physical components at the network end points.

Yet another advantage of the present invention is that the host computer can communicate substantially simultaneously with a large number of remote terminals over a simple four-wire connection. In conventional systems, the host computer would need to have a separate MODEM to communicate with each one of the remote terminals. Thus, an apparatus for an integrated services digital network has been described which allows a plurality of data terminals to be connected via existing analogue lines to a host computer which is connected to the network via a digital line. The apparatus substantially eliminates errors caused by the omission of a second CODEC in the transmission paths between the terminals and the host computer. It also allows for increased data transmission rates, as well as providing means for encrypting and verifying data transmissions. It will be appreciated by persons skilled in the art that numerous variations or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.

Claims

CLAIMS:

1. Apparatus for coupling a primary information processing device and a plurality of secondary information processing devices to a telecommunications network, the telecommunications network comprising a primary service and a plurality of secondary services, each one of the plurality of secondary services having an associated encoding and decoding apparatus, the apparatus comprising: a primary interface circuit for interfacing the primary information processing device to the primary service of the telecommunications network; a secondary interface circuit for interfacing one of the plurality of secondary information processing devices to one of the plurality of secondary services of the telecommunications network; and processing means for substantially eliminating the effect of the encoding and decoding apparatus; such that information may be passed between the primary information device and the plurality of secondary information processing devices, the information being substantially uncorrupted by the encoding and decoding apparatus.

2. The apparatus of claim 1 wherein the primary interface circuit is a digital circuit for coupling information in digital form from the primary information processing device to the primary service of the telecommunications network.

3. The apparatus of claim 1 or 2 wherein the secondary interface circuit performs a conversion between information in analogue form and information in digital form, the information in analogue form being transmitted to, and received from, each one of the plurality of secondary services of the telecommunications network, and information in digital form being transmitted to, and received from, the primary service of the telecommunications network.

4. Apparatus for coupling a primary information processing device to a telecommunications network, the telecommunications network comprising a primary service and a plurality of secondary services, each one of the plurality of secondary services having an associated encoding and decoding apparatus, the apparatus comprising: a primary interface circuit for interfacing the primary information processing device to the primary service of the telecommunications network; and processing means for substantially eliminating the effect of the encoding and decoding apparatus; such that information may be passed between the primary information device and a plurality of secondary information processing devices coupled to the plurality of secondary services, the information being substantially uncorrupted by the encoding and decoding apparatus.

5. Apparatus for coupling a plurality of secondary information processing devices to a telecommunications network, the telecommunications network comprising a primary service and a plurality of secondary services, each one of the plurality of secondary services having an associated encoding and decoding apparatus, the apparatus comprising: a secondary interface circuit for interfacing a corresponding one of the secondary information processing devices to a corresponding one of the plurality of secondary services of the telecommunications network; and processing means for substantially eliminating the effect of the encoding and decoding apparatus; such that information may be passed between a primary information processing device coupled to the primary service of the telecommunications network and the plurality of secondary information processing devices, the information being substantially uncorrupted by the encoding and decoding apparatus.