Multiplex communications – Wide area network – Packet switching
Patent
1992-04-06
1994-04-05
Safourek, Benedict V.
Multiplex communications
Wide area network
Packet switching
370 68, 370 84, H04J 322
Patent
active
053011902
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a communication system which compresses signals, such as speech signals and multiplexes these signals with each other to send these signals to a high-bit-rate digital line in the form of a multiplexed signal. More specifically, the present invention is concerned with a communication device having a function of terminating a received call at a terminal, or relaying it to the high-bit-rate digital line.
BACKGROUND ART
Recently, there has been in practical use a communication system which sends to a high-bit-rate digital line a signal obtained by compressing and multiplexing speech signals. Conventionally, speech signals are multiplexed at a bit rate of 64 Kbps, and a multiplexed signal thus obtained is sent to a high-bit-rate digital line. Further, recently there has also been provided a system in which speech signals have been multiplexed at a bit rate of 32 Kbps.
FIG. 1 shows a conventional communication system having a function of compressing and multiplexing speech signals in order to send these signals to a high-bit-rate digital line. The communication system shown in FIG. 1 functions as a WAN (Wide Area Network), and comprises exchanges 10A, 10B and 10C, such as PBXs (Private Branch Exchanges), transmission devices 20A, 20B and 20C, and transmission paths L1 and L2 which are high-bit-rate digital lines. The transmission path L1 connects the transmission devices 20A and 20B to each other, and the transmission line L2 connects the transmission devices 20B and 20C to each other.
Each of the exchanges 10A, 10B and 10C accommodates terminals T1, T2 and T3, such as telephone sets, and terminals other than telephone sets, such as data terminals and facsimile machines A circuit of a communication path system of each of the exchanges 10A, 10B and 10C switches 64 Kbps signals. Hence, each of the exchanges 10A, 10B and 10C has a PCM (Pulse Code Modlation) converter which converts analog signals from the terminals T1, T2 and T3 into a PCM speech signal having a bit rate of 64 Kbps and executes the reverse operation. It may be possible to provide the above PCM converter for each analog terminal.
The transmission device 20A connected to the exchange 10A comprises a compressor 21, an expander 22 and a multiplexing circuit 23. The compressor 21 converts the 64 Kbps PCM speech signal into an ADPCM (Adaptive Differential Pulse Code Modulation) signal having a bit rate of 32 Kbps. As shown, the compressor 21 is comprised of an adaptive quantizer 21a, a predictor 21b and a difference device 21c. Generally, a plurality of compressors 21 are provided. The multiplexing circuit 23 multiplexes the 32 Kbps ADPCM signals with other digital signals (image signals and data signals), and sends a multiplexed signal to the transmission path L1. Further, the multiplexing circuit 23 separates a plurality of digital signals from a multiplexed signal received from the transmission line L1. The expander 22 demodulates the received 32 Kbps ADPCM signal to generate a 64 Kbps PCM speech signal. The demodulated PCM speech signal is sent to the exchange 10A. The compressor 21 and the expander 22 form a speech codec. The transmission device 20C and the exchange 10C connected thereto are respectively configured in the same manner as the above transmission device 20A and the exchange 10A.
The transmission device 20B comprises a multiplexing circuit 24 connected to the transmission line L1, and a speech codec 25 connected thereto. Further, the transmission device 20B comprises a multiplexing circuit 27 connected to the transmission line L2, and a speech codec 26 connected thereto. Each of the speech codecs 25 and 26 has the functions of both the compressor 21 and the expander 22.
It will now be assumed that the terminal T1 accommodated in the exchange 10A generates a call requesting to communicate with the terminal accommodated in the exchange 10B, a speech signal from the terminal T1 being converted into a 64 Kbps PCM speech signal in the exchange 10A, which is then converted into a 32 Kbps ADP
REFERENCES:
patent: 4641303 (1987-02-01), Vogl
patent: 4672662 (1987-06-01), Nishino et al.
patent: 4805165 (1989-02-01), Kawamura et al.
patent: 4825436 (1989-04-01), Kobayashi et al.
Patent Abstracts of Japan, vol. 10, (E-1038) & JP-A-2 298 132 (Mitsubishi).
Patent Abstracts of Japan, vol. 11, (E-821) & JP-A-1 152 894 (NEC).
Electronic Switching, "Studies in Telecommunications"; vol. 2, Elsevier Science Publ., Amsterdam, NL; vol. 2, 1984; pp. 161-164.
Iribe Yoshiya
Tsukuda Hiroyuki
Fujitsu Limited
Patel Ajit
Safourek Benedict V.
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