Traditional PSTN networks (Plain Service Telephone Network) include 2 wire to 4 wire hybrids. These hybrids introduce what is known in the art as “echo” in telephony, e.g. a signal reflected from a far end circuit, and returned to a transmitting (near end) circuit. This phenomenon of a returned echo is well recognized both while transmitting voice signals and VBD type of signals.
FIG. 1 illustrates an example of a traditional PSTN network 1, in which facsimile device 5 receives signals originated by facsimile 3 at a significantly higher power level than that of the echo signals reflected from hybrid 7 (which path is indicated by 11). The echo signals are formed as a result of hybrid 7 reflecting back signals received from facsimile 5. The attenuation of the returned signal (echo) introduced by the hybrid is called Echo Return Loss (and will be referred to hereinafter as “ERL”). Typical echo control devices require a minimum ERL of 6 db (in accordance with ITU-T Recommendation G.122). Facsimile machines are designed to operate under such PSTN/ISDN conditions, i.e. attenuated echo signals.
When dealing with the transmission of voiceband data (“VBD”) type of signals, e.g. when there is a need to relay a facsimile, a modem or a DTMF transmission so that it can be transmitted over a media other than PSTN, and/or in order to reduce bandwidth requirements and even improve call completion rate, a VBD relay is often used. The term “VBD relay” as used hereinafter, is used to denote a device adapted to operate by processing analog signals (or digitally sampled analog signals) transmitted from a suitable originating machine and by generating a new (corresponding) VBD signal that carries information of the original signals transmitted towards a matching destination device over the applicable network, e.g. over an IP-based network. In the case that the VBD signals are of the facsimile type or of the modem type, the analog signals (or their digitally sampled presentation) are demodulated in the VBD relay, transmitted over to the corresponding relay at the other side of the transmission path and remodulated thereat. Principles of such VBD relay method are illustrated in FIG. 2. When the VBD signal is of the DTMF type, the analog signals (or their digitally sampled presentation) are encoded (i.e. identified and the information retrieved is represented in a coded form), transmitted over to the corresponding relay at the other side of the transmission path, decoded and regenerated thereat. As will be appreciated by those skilled in the art the term “VBD relay mode” should be understood to encompass a mode by which a VBD relay may operate, i.e. encoding a VBD signal by retrieving data included in the VBD signal (e.g. demodulation of a facsimile or of a modem signal) and representing it in a coded form, decoding an encoded VBD signal and regenerating a second VBD signal that carries information based on the information comprised in the original VBD signal (e.g. re-modulating the demodulated facsimile or modem). As would be appreciated by those skilled in the art, the regenerated second VBD signal may be, but not necessarily, identical to the original VBD signal.
A typical VBD relay as known in the art, comprises a Tx unit (module) and an Rx unit (module). In the example shown in FIG. 2, the Tx unit of the near end VBD relay 31 receives VBD type of transmission from the near end VBD signals' originating machine 25 (e.g. fax machine), detects and classifies the VBD signals, retrieves the information comprised therein, formats the information into a coded form and transmit their coded representation towards the Rx unit of the far end VBD relay 33. The Rx unit decodes the coded representation, regenerates a corresponding second VBD signal and transmits the regenerated signal to the far end VBD terminal 23 via hybrid device 27. In such a scenario, echo signals are reflected back from hybrid device 27, but contrary to the situation described above for PSTN networks, in the present case there is no guaranteed signal transparency between hybrid 27 and hybrid 29 due to the existence of VBD relays 31 and 33 along the transmission path. Therefore, appropriate measures must be taken in order to let the VBD transmission be successfully completed under these conditions.
As opposed to fax machines and dial-up modems, which are typically equipped with their own mechanisms to manage the progress of the communication session, a VBD relay is an entity that does not initiate nor is provided with information allowing independent handling of the communication session thereby. In other words, such a VBD relay typically operates as a “slave” of the VBD communication device that is linked thereto. This fact raises a problem that since a VBD relaying device is not provided with effective means to reduce the impact of the return echo, the encoded return echo signal received by the VBD relay is decoded and regenerated, or in other words, is treated as a legitimate VBD signal that should be processed when in fact such a signal should be discarded.
Another problem associated with the methods used in the art to handle fax relay devices, is that the fax relay might amplify the echo signal to a level which the fax machines themselves will not be capable of handling by using the conventional means which they are provided with.
Fax Relay methods have been described in a number of international standards. ITU-T Recommendation G.766 describes a Fax Relay method for TDM networks. ITU-T Recommendation I.366.2 describes such a method for ATM networks, ITU-T Standard Recommendation T.38 describes a Fax Relay method for IP networks, whereas Frame Relay Forum “Voice over frame relay implementation agreement” FRF.11 describes a Fax Relay method for Frame Relay networks.
However, neither of these publications suggests a method to control the echo signals during facsimile transmissions. When implementing fax relay over a packet network, one possible implementation of the T.38 standard is to re-modulate all signals at a predefined nominal level (e.g. −13 dbm0). In this case both the original source A signal as well as the echo signal of source B will arrive at facsimile terminal B at the same nominal level. Under these conditions the correct completion of the fax transaction cannot be guaranteed.
One method of addressing the return echo problem fax relay is described in ITU-T Standard recommendation G.766. The method provided in this Recommendation is to control the power level of the signals. According to this method, the demodulator side of the fax relay, relays also the power level of the received signal to the far end remodulator side. Consequently, this method guarantees similar echo levels like when operating without fax relay. However, this method does not provide a solution to ensure that the echo returned signals are discarded already at the transmitting fax relay, avoiding the transmission of the echo return signals over the whole transmission path.
The disclosure of the references mentioned throughout the present specification are hereby incorporated by reference.