WO2004062253A1

WO2004062253A1 - Enabling robbed bit identification

Info

Publication number: WO2004062253A1
Application number: PCT/IL2003/000002
Authority: WO
Inventors: Assaf Kasher; Udi Dahan; Abraham Fisher
Original assignee: Surf Communication Solutions, Ltd.
Priority date: 2003-01-01
Filing date: 2003-01-01
Publication date: 2004-07-22
Also published as: AU2002356416A1

Abstract

A method of transmitting signals on a modem connection. The method includes providing, by a modem, signals to be transmitted on a modem connection, actively adding noise to one or more, but not all, of the provided signals of the modem connection, and transmitting the provided signals with the added random noise, toward a remote unit.

Description

ENABLING ROBBED BIT IDENTIFICATION FIELD OF THE INVENTION

The present invention relates generally to modem communications and particularly to identification of robbed bits by a modem. BACKGROUND OF THE INVENTION

Telephone networks generally include analog lines which connect end users to the network and digital trunks which form the backbone of the network. In addition to their use for telephone connections, telephone networks are used for transferring data over modem connections. While digital trunks generally transfer exactly the bit content that they receive, analog lines do not perfectly transfer the transmitted signals but instead operate as a complex transfer system which provides for each sample of the transmitted signals, a weighted sum of the current sample and one or more previous samples, as well as added noise. The receiving end- modem reconstructs the transmitted signals from the received signals. While the affects of the analog line do not substantially affect voice connections, the end-modem must learn the analog line transfer function in order to reconstruct the transmitted signals. The learning of the characteristics of the analog line is performed in a training session at the beginning of the modem connection.

The digital trunks are connected to each other through switches. The switches along a path of a single connection may communicate with each other in-band, for signaling, using a lowest significant bit of the connection. Generally, the digital backbone carries 8-bit words at a rate of 8000 words per second. The in-band signaling generally uses a least significant bit of the 8-bit words once every predetermined number of words, for example once in every 6 words or once in every 12 words. The bits used for in-band switch communication are referred to as robbed bits.

The robbed bits generally do not affect the quality of telephone voice sessions as they only affect a small number of least significant bits. Modem connections of some protocols, however, may deteriorate due to loss of data in the robbed bits if they do not adapt their transmission to the pattern of the robbed bits. Some modem protocols ignore the robbed bit problem, at the cost of achieving a lower transmission rate. Other modem protocols suggest performing, during the training session of the modem connection, a robbed bit identification procedure in which the pattern of the robbed bits is determined. According to the determination, the modems are instructed when they can use the entire 8-bit word length for transmission of data and when to use only 7 of the bits, leaving unused by the end modems the bit to be used for in-band inter-switch signaling.

Exemplary methods for identifying robbed bit patterns are described in US patents 6,084,883 to Norrell et al, 6,088,334 to Davenport et al., and 6,104,730 to Marks, the disclosures of all of which are incorporated herein by reference.

Generally, the robbed bit pattern identification is performed by identifying regular patterns in the least significant bits of signals transmitted during a training session of the modem connection. The identification of the robbed bit pattern requires that the bit values inserted by the network into the robbed bits be in accordance with a predictable pattern and that the values of the least significant bits of the modem signals at the entrance to the network are not equal to the robbed bit values inserted by the network. The robbed bit identification generally requires reviewing about 144 samples for about 18 milliseconds.

Therefore, some robbed bit identification methods involve changing the training session to include a signal suitable for robbed bit identification. These methods, however, require preplanning the modem protocol for robbed bit pattern identification. Other robbed-bit identification methods utilize signals that were not specifically generated for robbed bit identification but that have many variations. For example, when the signals pass through an analog line they generally change sufficiently for jobbed bit identification.

When the modem connection is entirely digital, while the modem training was planned basically for at least one analog connection (for example, the N.92 protocol), there may be a problem in identifying the robbed bit pattern, as the transmitted training signals may all have a least significant bit equal to the robbed bit value.

SUMMARY OF THE INVENTION

An aspect of some embodiments of the invention relates to a modem which purposely adds noise to, or otherwise degrades, at least some of the signals it transmits, in order to allow robbed bit identification. By adding the noise, the regularity of the transmitted signals is countered, allowing easier identification of robbed bit patterns. The addition of the noise is optionally used when a modem is connected digitally to the telephone network and therefore, an analog line which ordinarily enriches the variability of the transmitted signals, is not used. In some embodiments of the invention, the noise is added during transmission of one or more modem training signals, used for robbed-bit pattern identification. Alternatively or additionally, for example when the signals are degraded in a manner which can at least partially be compensated for by the receiving modem, the signals are degraded during the entire modem connection. In this alternative, the degradation may be performed by a unit external to the fransmitting modem, not necessarily synchronized with the modem. hi some embodiments of the invention, the noise is added by adding a random signal having a peak to peak level of the least significant bit of the telephone network, to the transmitted signal. The added random numbers may be generated using any type of a random generator (e.g., a scrambler) known in the art. Alternatively or additionally, the random numbers include a pre-generated sequence. Further alternatively or additionally, the noise is added by filtering the output signals of the modem, optionally by a linear filter. The filtering is optionally performed in a manner similar to the effect of analog lines on transmitted signals or otherwise in a manner which allows at least partial compensation for the filtering by the receiving modem.

There is therefore provided in accordance with an exemplary embodiment of the invention, a method of transmitting signals on a modem connection, comprising providing, by a modem, signals to be transmitted on a modem connection, actively adding noise to one or more, but not all, of the provided signals of the modem connection, and transmitting the provided signals with the added random noise, toward a remote unit.

Optionally, actively adding noise to one or more of the provided signals comprises adding noise to a predetermined framing signal required by a protocol governing the modem connection. Optionally, adding noise to a predetermined training signal comprises adding noise to a predetermined training signal of the N.92 protocol. Optionally, adding the noise comprises adding to the provided one or more signals a pre-stored random noise sequence, used for a plurality of different modem connections.

Optionally, adding the noise comprises adding noise generated randomly, separately for each connection. Optionally, adding the noise comprises combining a noise sequence to the one or more provided signals, using a 'xor' or 'addition' operation. Optionally, adding the noise comprises applying a filter to the one or more provided signals. Optionally, adding the noise comprises adding noise to signals transmitted on the modem connection, during a predetermined time from setup or after a retrain of the connection. Optionally, adding the random noise comprises examining the provided signals and adding noise to signals deteraiined to require addition of noise. Optionally, adding the noise comprises examining the provided signals and adding noise to signals having fewer than a predetermined number of signal levels.

Optionally, adding the random noise comprises adding noise which affects the level of at least one hit of at least some samples of the signals. Optionally, adding the random noise comprises adding noise having a level which does not affect more than one bit of samples of the transmitted signal. Optionally, fransmitting the provided signals comprises fransmitting the provided signals with the added noise directly onto a digital line. Optionally, adding the random noise comprises adding a digital noise sequence. Optionally, the method includes identifying, by the remote unit, a robbed bit pattern of the network using the fransmitted signals with the added noise.

There is further provided in accordance with an exemplary embodiment of the invention, a method of identifying a robbed bit pattern, comprising determining a signal expected by a remote modem, generating a signal similar to the determined signal, but not identical, fransmitting the generated signal, and identifying a robbed bit pattern, by the remote modem, based on the fransmitted signal as received by the remote modem.

Optionally, determining the signal expected by the remote modem comprises preprogramming the modem with the expected signal according to a protocol, for example the N.92 protocol. Optionally, determining the signal expected by the remote modem comprises receiving an indication of the desired signal from the remote modem. Optionally, generating the similar signal comprises adding noise to the determined signal. Optionally, adding the noise comprises adding noise which at most affects a single least significant bit of each sample of the signal. Optionally, generating the similar signal comprises retrieving a pre-stored similar signal.

There is further provided in accordance with an exemplary embodiment of the invention, a modem, comprising a signal generator adapted to generate signals for fransmission, a noise generator adapted to add noise to at least one of the signals, but not all the signals, generated by the signal generator, for at least one modem connection; and a network interface adapted for transmitting generated signals to which noise was added onto a network.

Optionally, the signal generator is adapted to generate training signals according to a protocol. Alternatively or additionally, the signal generator is adapted to generate one or more signals according to instructions from a remote modem. Optionally, the modem includes a switch adapted to enable or disable the noise generator for specific connections. Optionally, the modem includes a connection identifier adapted to control the switch according to whether the modem is connected to an analog line or to a digital line. There is further provided in accordance with an exemplary embodiment of the invention, a method of fransmitting signals on a modem connection, comprising providing, by a modem, signals to be fransmitted on a modem connection, applying a filter to the provided signals of the modem connection and transmitting the filtered signals, onto a digital line, toward a remote unit.

Optionally, applying the filter comprises applying the filter to substantially all the signals fransmitted by the modem on the modem connection. Alternatively, applying the filter comprises applying the filter to fewer than all the signals fransmitted by the modem on the modem connection. Optionally, applying the filter comprises filtering by calculating a weighted sum of samples of the provided signals.

Optionally, applying the filter comprises applying a filter included within a single casing as the modem. Alternatively, applying the filter comprises applying a filter located in a casing separate from the modem. Optionally, applying the filter comprises applying a filter which does not receive input from the modem except for the provided signals.

BRIEF DESCRIPTION OF THE FIGURE Particular non-limiting embodiments of the invention will be described with reference to the following description of embodiments in conjunction with the figure. Fig. 1 is a schematic illustration of a modem connection, in accordance with an exemplary embodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

Fig. 1 is a schematic illustration of a modem connection 100, in accordance with an exemplary embodiment of the invention. A client modem 102 connects digitally to a telephone network 104, for example through an ISDN connection, a cellular interface and/or directly to a network trunk. Direct digital connection of client modem 102, is commonly used, for example, when client modem 102 is a gateway in a modem over IP (MoIP) connections. On another end of telephone network 104, a remote access server (RAS) 106 that services client modem 102 is digitally connected to network 104. Client modem 102 includes a signal generator 110, which generates the training and data signals fransmitted by the modem. A noise generator 112, belonging to client modem 102 or external to the client modem, adds noise to some or all of the signals transmitted by modem 102, in order to allow robbed bit pattern identification by

RAS 106. The noise added by noise generator 112 is optionally of a sufficient magnitude to affect the value of the lowest significant bit of the fransmitted signals, but of a low enough level so as not to affect any other bits of the fransmitted signals.

In some embodiments of the invention, noise generator 112 is implemented in software, optionally on a same processor as signal generator 110, which may also be implemented in software. Alternatively, noise generator 112 is implemented in hardware. Optionally, the noise is generated by noise generator 112 separately from the signal and is then combined to the modem signal from signal generator 110, for example using a 'xor' or addition operation. The noise optionally includes a series of single bits which affect the least significant bits of the modem signal. The noise is optionally generated by a pseudo random number generator. In an exemplary embodiment of the invention, the noise is generated by applying a scrambler, such as n(k) = 1 xor n(k-5) xor n(k-23), to an initial stream bits, e.g., all '1' bits. The term 'xor' refers herein to the 'exclusive or' logical operation, which provides a value of 1' if the operands have different values and '0' if the operands have the same value. Alternatively or additionally, the added noise is a pre-stored sequence of random bits or one of a plurality of pre-stored sequences of random bits. In an exemplary embodiment of the invention, the same noise sequence is added in every modem connection of client modem 102 to which noise is added.

In some embodiments of the invention, instead of generating a separate noise sequence and combining the noise series to the modem signal, the noise is generated directly on the modem signal, for example by applying a filter (finite or infinite) to the modem signal. Optionally, the applied filter comprises a low pass filter (LPF) applied over time to the modem signal. For example, the filter may output the sum or difference of the current and previous samples of the modem sample stream forming the fransmitted signal. In some embodiments of the invention, the filter comprises a linear filter, for example performing a weighted sum or average of the samples of the fransmitted signals. Optionally, the filter comprises a digital filter.

Optionally, noise generator 112 adds noise only during fransmission of one or more specific training signals, which are used by RAS 106 in identifying robbed bit patterns. In an exemplary embodiment of the invention, for the N.92 protocol, noise is added during fransmission of the tmι_u and/or the trn2_u signals. Alternatively, noise generator 112 adds noise to the fransmitted signals during a duration covering part or all of the training session of the connection. In some embodiments of the invention, noise generator 112 adds noise for a predetermined time after the beginning of the connection and/or after a retrain signal is transmitted on the connection. Optionally, when noise generator 112 is not adding noise, the added noise is set to a constant stream of zeros or noise generator 112 is bypassed.

In some embodiments of the invention, client modem 102 is adapted only for digital connection. Alternatively, client modem 102 may be connected to network 104 either through an analog line or through a digital line, and is adapted for both analog and digital connection. Optionally, in this alternative, modem 102 senses the type of connection to the network (analog or digital) and accordingly enables (for a digital connection) or disables (for an analog connection) noise generator 112. The sensing is optionally performed by monitoring the signals received from the remote modem and determining whether they include the richness caused by analog lines. Alternatively or additionally, a user of modem 102 sets a switch which enables or disables noise generator 112. Further alternatively, noise generator 112 adds noise to the transmitted signals regardless of the type of connection of modem 102 to the network. Noise added during the training session by noise generator 112 while modem 102 is connected through an analog line will generally have little or no affect on the modem connection, as training signals are generally less susceptible to levels of noise that only affect the least significant bit.

In some embodiments of the invention, noise generator 112 operates only during some connections, for example high priority connections.

The modem signals generated by signal generator 110 may be generated in accordance with a predetermined protocol and/or may be signals requested by a remote modem. In some embodiments of the invention, instead of determining whether to add noise according to predetermined factors, noise generator 112 determines whether to add noise responsive to an analysis of the transmitted signal. Optionally, noise generator 112 examines the signals from signal generator 110 (e.g., during a training session) and adds noise to signals which have a limited number of signal levels, e.g., 2, 4, 8, or to signals that always have a same least significant bit value. hi some embodiments of the invention, for each fransmitted signal, noise generator 112 examines a predetermined number of leading samples of the signal without adding noise, and responsive to the examination determines whether to add noise to the signal. This procedure is optionally performed only during a predetermined time after connection and/or retrain, while thereafter no noise is added regardless of the fransmitted signal. Alternatively, noise in a form which maybe compensated for (at least partially) by RAS 106, for example in accordance with a filtering function as described above, is added throughout the entire duration of the modem connection according to the structure of the signal. In some embodiments of the invention, noise generator 112 is included within client modem 102, for example within a same package as signal generator 110. Alternatively or additionally, noise generator 112 may be included in an add-on unit connected externally to client modem 102. Optionally, noise generator 112 is instructed by signal generator 110 when to add noise. Alternatively, client modem 102 is not adjusted in any way to the operation with noise generator 112. In some embodiments of the invention, noise generator 112 may be at a distance from client modem 102, for example at a connection point with an external telephone network and/or at a central office of the telephone network, closest to modem 102. It will be appreciated that the above described methods may be varied in many ways, including, changing the order of steps, and the exact implementation used. It should also be appreciated that the above described description of methods and apparatus are to be interpreted as including apparatus for carrying out the methods and methods of using the apparatus.

The present invention has been described using non-limiting detailed descriptions of embodiments thereof that are provided by way of example and are not intended to limit the scope of the invention. It should be understood that features and/or steps described with respect to one embodiment may be used with other embodiments and that not all embodiments of the invention have all of the features and/or steps shown in a particular figure or described with respect to one of the embodiments. Variations of embodiments described will occur to persons of the art.

It is noted that some of the above described embodiments may describe the best mode contemplated by the inventors and therefore may include structure, acts or details of structures and acts that may not be essential to the invention and which are described as examples. Structure and acts described herein are replaceable by equivalents which perform the same function, even if the structure or acts are different, as known in the art. Therefore, the scope of the invention is limited only by the elements and limitations as used in the claims. When used in the following claims, the terms "comprise", "include", "have" and their conjugates mean "including but not limited to".

Claims

1. A method of fransmitting signals on a modem connection, comprising: providing, by a modem, signals to be fransmitted on a modem connection; actively adding noise to one or more, but not all, of the provided signals of the modem connection; and fransmitting the provided signals with the added random noise, toward a remote unit.

2. A method according to claim 1, wherein actively adding noise to one or more of the provided signals comprises adding noise to a predetermined training signal required by a protocol governing the modem connection.

3. A method according to claim 2, wherein adding noise to a predetermined fraining signal comprises adding noise to a predeteπnined framing signal of the N.92 protocol.

4. A method according to any of claims 1-3, wherein adding the noise comprises adding to the provided one or more signals a pre-stored random noise sequence, used for a plurality of different modem connections.

5. A method according to any of claims 1-3, wherein adding the noise comprises adding noise generated randomly, separately for each connection.

6. A method according to any of claims 1-5, wherein adding the noise comprises combining a noise sequence to the one or more provided signals, using a 'xor' or 'addition' operation.

7. A method according to any of claims 1-3, wherein adding the noise comprises applying a filter to the one or more provided signals.

8. A method according to any of the preceding claims, wherein adding the noise comprises adding noise to signals fransmitted on the modem connection, during a predetermined time from setup or after a refrain of the connection.

9. A method according to any of the preceding claims, wherein adding the random noise comprises examining the provided signals and adding noise to signals determined to require addition of noise.

10. A method according to claim 9, wherein adding the noise comprises examining the provided signals and adding noise to signals having fewer than a predetermined number of signal levels.

11. A method according to any of claims 1-10, wherein adding the random noise comprises adding noise which affects the level of at least one bit of at least some samples of the signals.

12. A method according to any of claims 1-11, wherein adding the random noise comprises adding noise having a level which does not affect more than one bit of samples of the fransmitted signal.

13. A method according to any of the preceding claims, wherein fransmitting the provided signals comprises fransmitting the provided signals with the added noise directly onto a digital line.

14. A method according to any of the preceding claims, wherein adding the random noise comprises adding a digital noise sequence.

15. A method according to any of the preceding claims, comprising identifying, by the remote unit, a robbed bit pattern of the network using the fransmitted signals with the added noise.

16. A method of identifying a robbed bit pattern, comprising: determining a signal expected by a remote modem; generating a signal similar to the determined signal, but not identical; fransmitting the generated signal; and identifying a robbed bit pattern, by the remote modem, based on the fransmitted signal as received by the remote modem.

17. A method according to claim 16, wherein determining the signal expected by the remote modem comprises pre-programming the modem with the expected signal according to a protocol.

18. A method according to claim 17, wherein determining the signal expected by the remote modem comprises pre-programming the modem with the expected signal according to the N.92 protocol.

19. A method according to claim 16, wherein determining the signal expected by the remote modem comprises receiving an indication of the desired signal from the remote modem.

20. A method according to claim 16, wherein generating the similar signal comprises adding noise to the determined signal.

21. A method according to claim 20, wherein adding the noise comprises adding noise which at most affects a single least significant bit of each sample of the signal.

22. A method according to claim 16, wherein generating the similar signal comprises retrieving a pre-stored similar signal.

23. A modem, comprising: a signal generator adapted to generate signals for transmission; a noise generator adapted to add noise to at least one of the signals, but not all the signals, generated by the signal generator, for at least one modem connection; and a network interface adapted for transmitting generated signals to which noise was added onto a network.

24. A modem according to claim 23, wherein the signal generator is adapted to generate fraining signals according to a protocol.

25. A modem according to claim 23, wherein the signal generator is adapted to generate one or more signals according to instructions from a remote modem.

26. A modem according to claim 23, comprising a switch adapted to enable or disable the noise generator for specific connections.

27. A modem according to claim 26, comprising a connection identifier adapted to control the switch according to whether the modem is connected to an analog line or to a digital line.

28. A method of fransmitting signals on a modem connection, comprising: providing, by a modem, signals to be fransmitted on a modem connection; applying a digital filter to the provided signals of the modem connection; and fransmitting the filtered signals, onto a digital line, toward a remote unit.

29. A method according to claim 28, wherein applying the filter comprises applying the filter to substantially all the signals fransmitted by the modem on the modem connection.

30. A method according to claim 28, wherein applying the filter comprises applying the filter to fewer than all the signals fransmitted by the modem on the modem connection.

31. A method according to any of claims 28-30, wherein applying the filter comprises filtering by calculating a weighted sum of samples of the provided signals.

32. A method according to any of claims 28-31, wherein applying the filter comprises applying a filter included within a single casing as the modem.

33. A method according to any of claims 28-31, wherein applying the filter comprises applying a filter located in a casing separate from the modem.

34. A method according to claim 33, wherein applying the filter comprises applying a filter which does not receive input from the modem except for the provided signals.