Multiplex communications – Fault recovery
Reexamination Certificate
1999-04-08
2003-10-14
Cangialosi, Salvatore (Department: 2732)
Multiplex communications
Fault recovery
C370S242000
Reexamination Certificate
active
06633536
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a signalling protocol and an apparatus enabling a transmitter in a speech-transmitting digital telecommunications system to transmit predetermined messages to a receiver. In many digital telecommunications systems, it is necessary to transmit not only encoded speech and/or other information but also messages that may for example relate to the control of that particular connection or that may transfer data fully independent of the information to be transmitted. Such messages are often called signalling. To provide an illustrative description within the scope of this application, the term “speech” is used even though the information to be transmitted in the system may comprise other types of sound, music, a video signal, multimedia, etc. instead of or in addition to speech. In terms of a practical embodiment, the invention is disclosed in the context of a mobile communications system, particularly a speech channel in the GSM system. It is to be borne in mind, however, that the technology in accordance with the invention is suitable for use in many other environments as well.
FIG. 1
shows the parts of a cellular mobile communications system essential for understanding the invention. Mobile stations MS communicate with Base Transceiver Stations BTS over the air interface Um. The base stations are controlled by Base Station Controllers BSC associated with Mobile Services Switching Centres MSC. A subsystem administered by a base station controller BSC—including the base stations BTS controlled by it—is commonly called a Base Station Subsystem BSS. The interface between an exchange MSC and a base station subsystem BSS is called the A interface. The part of the A interface on the MSC side is called a Network Subsystem NSS. The interface between a base station controller BSC and a base station BTS is called the Abis interface. The mobile services switching centre MSC switches incoming and outgoing calls. It performs tasks similar to those of the exchange of a public switched telephone network PSTN. Additionally, it performs tasks characteristic of mobile telecommunications only, such as subscriber location management, in co-operation with network subscriber registers (not separately shown in FIG.
1
). A Transcoder and Rate Adaptation Unit TRAU is an element of the base station subsystem BSS and may be located in association with the base station controller BSC, as shown in this figure, or also in association with the mobile services switching centre, for example. The transcoders convert speech from digital format into another format, for instance convert the 64 kbit/s A-law PCM from the exchange over the A interface into encoded speech of 13 kbit/s to be sent to the base station line and vice versa. Rate adaptation for data is carried out between the rate 64 kbit/s and the rates 3.6, 6, or 12 kbit/s.
In digital telecommunications systems transmitting speech, a speech signal is usually subjected to two coding operations: speech coding and channel coding. Speech coding comprises speech encoding performed in the transmitter by a speech encoder, and speech decoding performed in the receiver by a speech decoder.
FIG. 2
illustrates various operations to be performed on the speech. The most significant steps in view of the present invention include speech encoding and decoding and channel encoding and decoding. In the GSM system, for example, channel encoding in the network is performed at the base station, whereas speech encoding is performed in a discrete transcoder unit that may be located remote from the base station and even when located at the base station is a fully separate logic unit. References Tx and Rx will be explained in connection with FIG.
4
.
FIG. 2
further illustrates an exemplary frame F, comprising a header H, a payload portion P, and a check portion C. The frame F also often contains bit patterns for synchronization. The header H typically comprises the identifiers of the sender and receiver of the frame, a consecutive number for the frame, or the like. The actual information is carried in the payload portion P. Parts essential to the present invention include the payload portion P and the check portion C. The check portion C is usually implemented in the form of a cyclic redundancy check (CRC) value, but it may also be a parity having one or more bits, or equivalent. Essential to the invention is mainly the fact that the system in some way defines a “good” and a “bad” frame, which may be distinguished from one another by means of an implicit or explicit information element in the frame, permitting the system to conclude whether the frame has been transferred correctly. In the present context, “implicit” means that, as is well-known, the cyclic redundancy check (CRC) value does not directly indicate whether the frame is good or bad, but the receiver calculates the CRC value from the frame and compares it with the check sum sent with the frame. If the check sums are identical, the frame is good. An “explicit” indicator of a bad frame is for instance the Bad Frame Indicator BFI used in the fixed parts of a telephone network.
FIG. 3
illustrates the type of message transmission most widely known in the art.
FIG. 1
shows both a transmitter
100
and a receiver
102
. In this arrangement, messages and speech are transmitted on completely different channels. In the transmitter
100
, a digital speech signal
104
is supplied to a speech encoder
106
, which, from this signal, generates compressed speech coding bits, which are sent to the receiver on a speech channel
108
. In the transmitter, a message
114
to be sent to the receiver is supplied to a message encoder
116
, which generates message bits, which are then sent to the receiver on a separate message channel
118
. The receiver
102
receives the speech coding bits from a speech channel
108
and supplies them to a speech decoder
110
, which synthesizes the speech signal
112
to be heard. The receiver
102
receives the message bits from a separate message channel
118
and supplies them to a message decoder
120
, which interprets the transmitted message
122
.
A speech encoder
106
located in a transmitter
100
compresses a speech signal so that the number of bits used to represent it per unit of time is reduced. The speech encoder
106
typically processes speech as speech frames containing a certain amount of speech samples. On the basis of sampled speech, the speech encoder
106
calculates speech parameters, each of which is encoded as a separate binary code word. The speech parameters produced by the RPE-LTP speech encoder used in the full-rate channel of the pan-European GSM mobile telephone system are described in ETSI GSM Recommendation 06.10. These parameters are also disclosed in Table 1 of Appendix 1. The RPE-LTP (Regular Pulse Excitation—Long Term Prediction) produces 76 speech parameters from one speech frame of 20 ms (corresponding to 160 speech samples at a sampling frequency of 8 kHz). Recommendation GSM 06.10 also discloses the length of the binary code word assigned for each parameter.
Very often speech encoders also group speech parameters together, in which case each group—instead of a single speech parameter—is encoded into a separate code word. Encoding parameters in groups is called vector quantization. Modem speech encoders usually encode some speech parameters separately and some in groups (the RPE-LTP speech encoder of the example does not employ vector quantization). The RPE-LTP speech encoder of the invention produces 260 speech coding bits per each speech frame of 20 ms.
The speech decoder
110
of a receiver
102
performs a reverse operation and synthesizes a speech signal
112
from the bits produced by the speech encoder. The decoder
110
receives binary code words and generates corresponding speech parameters on the basis of them. The synthesization is performed by the use of the decoded speech parameters. The speech synthesized in the receiver is, however, not identical with the original speech compressed by
Cangialosi Salvatore
Nokia Telecommunications Oy
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