Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
2001-02-09
2004-11-30
Appiah, Charles (Department: 2686)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S433000, C455S560000, C370S523000, C375S219000
Reexamination Certificate
active
06826404
ABSTRACT:
This application claims priority under 35 U.S.C. §§119 and/or 365 to 00102223.5 filed in Europe on Feb. 11, 2000; the entire content of which is hereby incorporated by reference.
TECHNICAL FIELD OF THE INVENTION
The present invention relates to method, system and apparatus for tone signalling with TFO (Tandem Free Operation) communication links.
TECHNICAL BACKGROUND AND RELATED ART
In public switched telephone networks (PSTNs) tone signalling, in particular DTMF (Dual Tone Multi-Frequency) signalling, is employed as an in-band signalling method to offer the customer various services, for example control of a voice mailbox, phone banking/booking systems or other authentication procedures. By pressing one of the push buttons on a phone, a DTMF generator compiles a signal which consists of a pair out of eight different pre-defined tones. It is actually the combination of the two tones which codes a digit or letter on the pressed button. The characteristics of the transmitted tones follow a specification as described for example in “CEPT T/CS 46-02 “Multifrequency signalling to be used for push-button telephones” (Ref. 1).
On older analog transmission lines the characteristics of the signal could be changed for example by modulation/demodulation, level loss, added noise, or the like. As a consequence, the specification of a DTMF receiver defines the allowed deviations of a DTMF signal from its mean characteristic (see Ref. 1). In a DTMF receiver a signal is recognized as a DTMF signal when its characteristic lies within the defined limits in terms of time, level and signal-to-noise ratio (SNR).
Tone signalling, in particular DTMF signalling, can also be used in mobile communications systems, such as GSM systems. However, in the development of GSM systems it was considered that a problem would arise insofar as speech coding or speech compression is applied at a mobile telephone or mobile station MS. In such GSM systems, compressed speech is sent from the MS at 16 kbps or 8 kbps (Full Rate/Enhanced Full Rate or Half Rate). Conventionally, the speech is then transcoded and carried in the infrastructure of the system at a higher bit rate of 64 kbps (PCM). It was considered that, if generated in the MS, tone signals such as DTMF signals would, as a result of being subject to the speech coding or compression in the MS, lie outside the defined limits after transcoding to 64 kbps PCM and so would likely not be recognised by a DTMF receiver. For this reason, in GSM systems, tone signals such as DTMF signals are not generated in the MS itself but are generated on the infrastructure side, in the local mobile switching center MSC via which the MS is communicating, so that the DTMF signals are not subject to the speech coding or compression. When keys in the MS are pressed, separate out-of-band signalling messages are sent by the MS, upon reception of which a generator in the local MSC generates the DTMF signals. The DTMF signals are then sent from the local MSC in a clear, uncompressed form, at 64 kbps PCM.
This handling of DTMF is specified in the uplink (MS→network/infrastructure) direction in GSM systems. See “Support of Dual Tone Multi-Frequency (DTMF) via the GSM System”, GSM 03.14, ETSI TS 100 532 V7.01, 1999-07 (Ref. 2). In the downlink direction (network→telephone), however, handling of DTMF is not specified.
Generally, it was considered that specification of the handling of DTMF was not needed in the downlink direction because in most cases the DTMF signals sent from an MSC as explained above, in response to keypresses on a mobile station MS, would be intended for a destination other than a mobile station (MS), for example a voice mail service at an MSC or other services at public switched telephone network (PSTN) destinations. Therefore it was considered that the DTMF signals sent from the MSC would pass clear and uncompressed over a 64 kbps link and should not again be subjected to speech coding or compression such that they would be made unrecognisable for a DTMF receiver at the destination. Although, there is no reason in principle why DTMF signals sent from an MSC, or originating from another source such as a PSTN telephone, should not be destined for a mobile station MS, it was accepted that the DTMF signals would be subjected to speech compression or coding in the downlink to the MS and so would probably not be recognisable by a DTMF receiver at the mobile station.
In mobile communication systems, such as GSM systems, speech coding or speech compression applied as mentioned above degrades the quality of signals by introducing non-linear distortions. Generally, in Mobile-to-Mobile calls (MS-MS calls) speech coding/decoding happens at least two times, i.e. in a tandem manner. First, in the uplink direction encoding to lower bitrates (e.g. 16 kbps or 8 kbps) takes place in the first MS, with decoding/transcoding to a higher rate (64 kbps in GSM systems) in its counterpart on the infrastructure side, a first transcoder (TRA or TRAU—Transcoder and Rate Adaptation Unit). The second coding process happens in downlink direction, with encoding/transcoding in a second TRAU and decoding in the second MS. More coding processes could occur on long distance lines between the two TRAUs. Unfortunately, each instance of coding/decoding (codec) in such a tandem configuration adds new noise resulting in more and more degraded speech.
In order to avoid the “tandem” codec situation, ETSI (European Telecommunications Standards Institute) has recently specified a “Tandem Free Operation” (TFO) protocol for Mobile-to-Mobile calls in GSM systems—see “Inband Tandem Free Operation of Speech Codecs”, GSM 08.62 (Ref. 3) and “Tandem Free Operation (TFO)”, GSM 03.53 version 7, Release 1998; ETSI TS 101 732 V7.01 (1999-07) (Ref. 4)—which allows speech coding devices to be bypassed by applying inband signalling.
Using the TFO protocol, in a Mobile-to-Mobile call (MS-MS calls), speech is coded or compressed in the first MS but passes through the first and second TRAUs, in the uplink and downlink, without transcoding, to be decoded in the second MS.
The TFO protocol, in its basic operation, affects only the TRAUs and is fully compatible with existing GSM equipment. The TFO protocol is standardized for example for GSM speech traffic channels, for Enhanced Full Rate, Full Rate (16 kbps) and Half Rate (8 kbps). Using the protocol in GSM systems, the speech quality is lifted up to the high level of a single-codec configuration, once the in-band signalling protocol has verified that the prerequisites for establishing a TFO connection between two TRAUs are fulfilled.
As explained above speech signals are compressed in the MS→Network/Infrastructure uplink and in the Network/Infrastructure→MS downlink. However, proposals have been made to compress speech signals also in the infrastructure of a mobile communications network such as a GSM network, to enable a number of speech channels to be carried over a 64 kbps MSC-MSC link in the infrastructure. The equipment used for this purpose is known as Digital Circuit Multiplication Equipment or DCME. Two DCME “heads” are placed between the pair of MSCs between which the MSC link is established, which heads provide complementary speech compression and decompression so that a number of channels, for example four 16 kbps channels, can be combined into one 64 kbps link between MSCs. This enables a saving of transmission costs in the inter-MSC network/infrastructure, but introduces the disadvantage that a further instance of speech compression/decompression (codec) is involved, with consequent degradation of speech quality. The speech compression used may for example be standard compression, i.e. 1:4 via EFR (Enhanced Full Rate).
The TFO protocol now offers the possibility of a cost efficient transmission within the fixed or infrastructure part of the network, but without the disadvantage of a further instance of speech compression/decompression (codec). Since a TFO connection actually requires only a reduced bitrate, for example 8/16 kbps (GSM Half Rate/Full Ra
Delfs Eckhard
Ertel Emilian
Hellwig Karl
Hoffmann Rudolf
Appiah Charles
Telefonaktiebolaget LM Ericsson
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