Multiplex communications – Communication techniques for information carried in plural... – Combining or distributing information via time channels
Reexamination Certificate
1999-08-16
2004-11-09
Chin, Wellington (Department: 2664)
Multiplex communications
Communication techniques for information carried in plural...
Combining or distributing information via time channels
C370S526000, C370S528000, C370S529000, C370S522000
Reexamination Certificate
active
06816511
ABSTRACT:
The present invention lies in the field of telecommunications and concerns transcoder apparatus. The function of such apparatus is to transform a receive signal that has been subjected to first encoding in a first network into a transmit signal satisfying second encoding used in a second network, it being understood that the signal in question is traveling from the first network to the second.
By way of example, the first network can be a GSM network for radio communication with mobile stations, and the second network can be the public switched telephone network (PSTN). In common practice, an analog speech signal is digitized in the form of 13-bit samples at a rate of 8 kHz, giving a data rate of 104 kilobits per second (kbps). At present, GSM provides for three types of encoding: full rate encoding, improved full rate encoding, and half-rate encoding which, starting from a signal at 104 kbps, produce signals that are respectively at 13 kbps, 12.2 kbps, and 5.6 kbps. The PSTN encodes the 104 kbps signal into a so-called “PCM” signal at 64 kbps using the A law or the &mgr; law, depending on country. Thus, a call coming from a GSM subscriber and going to a PSTN subscriber requires transcoding to transform the GSM encoded speech signal into a PCM encoded signal in the up link and for performing the opposite operation in the down link, where by convention the “up link” starts from the calling subscriber while the “down link” connects the called subscriber to the calling subscriber in the opposite direction.
It can also happen that a GSM subscriber makes a call to another GSM subscriber with the call transiting over the PSTN. Assuming that both subscribers are using the same GSM encoding, it can be seen that the GSM signals are subjected to two successive transcoding stages, one from GSM to PCM, and the other from PCM to GSM. These two transcoding stages are pointless since the overall purpose thereof is to reproduce a signal that is identical to the starting signal. In addition, such double transcoding significantly degrades the speech signal.
On this topic, International application CA 95/00704 proposes a system for avoiding two transcoding operations whenever possible. In that system, a transcoder device comprises a first transcoder which receives a signal originating from the mobile network and transcodes it to produce a transmitted signal. That device also has a generator to produce a digital signaling message indicating that the transmitted signal comes from a mobile subscriber and that, consequently, the signal received by said device has been subjected to coding specific to the mobile network. The signaling message is injected into the transmitted signal by stealing bits therefrom, i.e. some of the bits in the transmitted signal are replaced by bits of the signaling message. It is assumed that the transmitted signal travels over the PSTN. If this signal is destined for another mobile subscriber, it will be applied to a second transcoding device. A detector is thus provided in the second device to look for the signaling message coming from the first device. It will be understood that under such circumstances, both devices are identical and that each of them therefore possesses both a generator and a detector. Consequently, when the second device has detected that the first device is sending it a signal that comes from the mobile network, it uses its own generator to tell the first device that it has indeed received its signaling message and that it is likewise connected to a subscriber of the mobile network. It is then possible to eliminate the transcoding operations in the transcoders of each of the devices.
It should also be observed that subscribers to the PSTN gain access to that network over an analog local loop. A connector member connects said local loop, which has two wires, to the network proper by means of a hybrid circuit and a four-wire connection. Unfortunately, it often happens that the impedance matching of said connection member is of poor quality. This phenomenon gives rise to an echo known as “electrical” echo. This echo is not perceivable, providing transmission times are short, as is the case in the PSTN. However, in mobile networks, e.g. the GSM network, the go and return transmission time is subject to a delay of about 200 milliseconds (ms) and electrical echo can be very troublesome. Echo cancellers, where the term “echo cancellers” is used herein to cover both echo cancellers proper and echo suppressors, are therefore disposed in the mobile network on links coming from the PSTN. Echo cancellers should only be used for calls between a mobile subscriber and a PSTN subscriber. However, the mobile network control center does not always know how to tell whether a given subscriber is a mobile subscriber or a PSTN subscriber. As a result, echo cancellers are often used in calls between mobile subscribers.
Echo cancellers are digital signal processor members which serve, essentially, to modify the digital signal input thereto. This applies naturally to the signaling message received by a transcoding device from the PSTN. It follows that if the integrity of the message is affected, the device will not be able to identify the received message and will therefore not be able to tell whether the call in question involves two mobile subscribers. Under such circumstances, the procedure for eliminating double transcoding cannot be implemented.
It may be observed that such degradation of the signaling message could be due to a processor member other than an echo canceller. Such members are provided for processing audio signals and are therefore not appropriate for forwarding digital signaling. Filters can be mentioned by way of example.
An object of the present invention is thus to provide transcoding apparatus which takes the above constraints into account whether they are imposed by the mobile network or by the PSTN.
According to the invention, the transcoding apparatus designed to transcode a receive signal into a transmit signal in an up link, includes a signaling transmitter member for producing a signaling message and for introducing it into the transmit signal by stealing bits therefrom, and said signaling transmitter member also has a tone generator for adding a tone signal to said transmit signal prior to introducing said signaling message into the transmit signal.
The fundamental characteristic of the tone signal is a frequency and not a digital value as is the case of the signaling message. The tone signal will therefore pass through the or all of the processor members without losing its fundamental characteristics, even if it is subjected to attenuation or possibly amplification. Furthermore, inserting the signaling message into the transmitted signal after the tone signal has been added is practically of no consequence. The signaling message can be considered to be white noise of very low amplitude.
In addition, the apparatus is also designed in a down link to transcode a receive signal into a transmit signal, and it can have a detector for seeking a tone signal and a signaling message in the signal received by said down link.
This dual signaling by means of the tone signal and by means of the signaling message makes it possible to obtain detection advantages that are described in greater detail below in the description.
Naturally, the apparatus has means for not transcoding the signals received by the up and down links when the signaling message detected in the signal received by the down link so requires.
According to an advantageous characteristic of the invention, the apparatus includes means responsive to the tone signal detected in the signal received over the down link to produce a deactivation signal for application to any transmission-degrading processor member placed in the up and down links.
In particular, said processor member consists in an echo canceller.
Preferably, when the transmit and receive signals correspond to a speech signal restricted to a predetermined passband, the transcoder apparatus is adapted so that the tone signal is situa
Fauconnier Denis
Le Strat Evelyne
Navarro William
Chin Wellington
Ho Chuong
Nortel Matra Cellular
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