`Yabusaki et al.
`
`[54] RADIO COMMUNICATION SYSTEM WITH
`CONTROLLED VOICE CODE SWITCHING
`
`[75]
`
`Inventors: Masami Yabusaki; Kouji Yamamoto,
`both of Saitama; Shinji Uebayashi,
`Kanagawa. all of Japan
`
`[73] Assignees: Nippon Telegraph and Telephone
`Corp; NTT Mobile Communications
`Network Inc., both of Tokyo, Japan
`
`121) Appl. No.: 249,077
`[22] Filed:
`
`May 25, 1994
`
`Related U.S. Application Data
`
`[63] Continuation of Ser. No. 849,197, Mar. 11, 1992, aban(cid:173)
`doned.
`
`[30]
`
`Foreign Application Priority Data
`
`[JP}
`
`Japan .. : ................................. 3-246430
`
`Sep. 2, 1991
`Int. Cl.6
`...•........... .....•..••........ H04B 1/38; H04L 5/16
`[51)
`[52] U.S. Cl. .......................... 375/219; 455/53.1; 379/258
`[58] Field of Search .................................. 379/58, 59, 90,
`379/258, 230; 455/53.1, 54.1, 38.1, 14;
`375/377, 242, 243, 219
`
`11111111m111111111m 111111111 1111 911 m1111111 H11111111111111111111
`US005513211A
`5,513,211
`[11) Patent Number:
`[451 Date of Patent:
`Apr. 30, 1996
`
`[56]
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`4,330,886 5/1982 Fukuda et al ............................. 370/62
`4,723.T72
`211988 Maat ....................................... 379/230
`4,924,480 5/1990 Gay et al. ................................... 375/8
`4,996,708
`211991 Kakizawa ................................ 379/216
`5,148,27 1 9/1992 Kato ct al .•............................... 375125
`
`Primary Examiner-Stephen Chin
`Assistant Examiner-Hai H. Phan
`A11omey, Agent, or Finn-Martin Novack
`
`[57]
`
`ABSTRACT
`
`A radio communication system operates between two radio
`terminals, and/or between a radio terminal and a fixed
`terminal through a network. In the present disclosure, the
`switching of codec-release mode and codec mode in the
`network is controlled by using a specific control line which
`is installed in addition to a speech line. Thus, when a radio
`terminal communicates with another radio terminal through
`a network, no codec is used in the network, and so, speech
`distortion and signal delay are not deteriorated due to signal
`conversion by using a codec.
`
`3 Claims, 6 Drawing Sheets
`
`RADIO
`SPEECH
`PATH 131
`&
`IV
`RADIO
`CONTROL
`PATH 111
`
`BASE
`STATION 31
`SPEECH
`FATH 51
`
`STATIQ\I 41
`
`EXCHANGE
`SWITCH 71
`
`CODEC
`
`RADIO
`TERrvllNAL
`21
`
`RADJO
`SPEECH
`PATH 13t.
`'V
`IV
`RADIO
`is -u PATH 1 lz
`r1, CONTF\OL
`
`RADIO
`TERMINAL
`22
`
`CONTROL
`LINE 121
`
`81
`'--~~~~~~-i 61
`
`CONTROL
`CIRCUIT
`
`EXCHANGE SWITCH 7z
`42
`
`BASE
`STATION 32
`
`SPEECH
`PATH 5z
`
`SPEECH LINE 53
`
`(ol
`
`CONTROL
`LINE 122
`
`'---1--- --
`
`CQ\ITROL
`--1. CIRCUIT
`62,,...__,,,__ _ _ _ _
`
`CONTRCL LINE 123
`
`RING BACK TONE
`GENERATOR 10
`
`IPR2016-01710
`UNIFIED EX1006
`
`
`
`~
`~
`"' N
`~
`ii(cid:173)
`"' 01
`01
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`
`NAL
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`
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`
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`
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`
`XED TERMINAL I
`
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`RED LINE
`
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`
`CODEC 82
`
`._
`
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`bo
`
`92
`SELECTOR
`
`>TATION 32
`3ASE
`
`COOEC 81
`
`'--
`
`ac ._
`
`b r. 1/-
`
`SWITCH 7
`~~LECTOR EXCHANGE
`
`EXCHANGE STATION 4
`
`>TATION 31
`~ASE
`
`Fig. 1
`
`-cJ VOICE CODE
`
`SIGNAN
`
`SPECIFIC
`RADIO
`
`TERMINAL 22
`RADIO
`
`zs-
`
`VOICE COOE
`SPECIFIC
`
`RADIO TERMINAL 21
`n-L_J N
`r1. SIGNAL
`
`. RADIO
`
`
`
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`-.
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`
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`
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`
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`d
`
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`
`~@s6lstjs4js3js2ls1 I
`
`(a)
`
`SPEECH LINE 53
`
`RING BACK TONE
`
`H---J GENERATOR 10
`
`I
`
`CONTROL LINE 123
`
`~-.-------1 CIRCUIT
`CONTROL
`
`62,-.A' -.
`
`LINE 122
`CONTROL
`
`82
`
`CODEC
`
`1SfittECTOR 92
`
`PATH 5~l
`SPEECH
`
`/
`
`SWITCH 71
`EXCHANGE
`
`EXCHANGE STATICN 41
`
`IS~CTOR 91
`
`\
`
`STATION 31
`BASE
`
`EXCHANGE SWITCH 72
`
`(
`
`EXCHANGE STATION 42
`
`CIRCUIT
`CONTROL
`
`81
`
`CODEC
`
`STATION 32
`BASE
`
`Lll\E 121
`CONTROL
`
`PATH 5t
`SPEECH
`
`Fig. 2
`
`~s2]8!]
`( b)
`
`.
`
`PATH 13?.
`SPEECH
`RADIO
`
`PATH 112
`CONTROL
`RADIO
`/V
`'V
`
`22
`TERMINAL
`RADIO
`
`~ -cJ
`
`2f
`TERTvlNAL
`RADIO
`
`PATH 111
`CONTROL
`RADIO
`IV
`ill
`
`PATH 131
`SPEECH
`RADIO
`
`n{J
`
`
`
`1-4
`~
`'6 N
`(.N
`~
`01
`'6
`(./)
`
`y
`
`~
`
`CONN ACK
`
`ACKNOWLEDGEMENT
`CODEC RELEASE
`
`------------___ ...,
`0 CODEC 81 RELEASE
`~--------------
`c uDEC RELEASE REQUEST
`l
`(
`
`-----·--·-. --·--·--· RBT ·--·
`.
`
`ANM
`
`CPG
`
`-
`
`-
`
`CONN
`
`ALERT
`
`Cl" a
`
`Cl.l
`
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`$,
`w
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`:"" p
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`
`~-
`
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`
`,
`
`GENERATOR 10
`CONNECT RING BACK TONE
`
`CONN
`
`ALERT
`
`CALL PROC
`
`SETUP
`
`-
`
`RADIO CHANNEL SET
`
`0
`
`AUTHENTICATION RESPONSE
`AUTHENTICATION REQUEST
`
`_TERMINATION RESPONSE
`TERMINATION REQUEST
`
`(DESTINATION SIOEl
`RADIO TERMINAL 22 ~
`
`~
`
`d • 00
`
`•
`
`MODE
`CODEC 82 RELEASE
`
`·~
`
`ACM
`IAM
`
`~
`
`YO CODEC MOOE <CODEC 81)
`
`~010 CHMJNEL SEL
`AUTHENTICATION RESPONSE_
`
`-
`
`STATION 42 -
`EXCHANGE
`
`AUTHENTICATION REQUEST
`
`-
`
`CALL PROC
`
`REQUEST I SETUP
`ORIGINATING CONDITION
`
`STATION 41
`EXCHANGE
`
`3
`
`· g.
`
`F1'
`
`CORIGINA~rNGMINAL 21
`
`RADIO T
`
`SIDE)
`
`
`
`~
`~
`N
`....
`~ w
`tit
`....
`tit
`
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`
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`
`--= ~
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`"""'
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`
`= """'
`~
`
`~
`•
`00
`0 •
`
`RADIO SPECIFIC VOICE CODE SIGNAL
`
`\
`
`STATION 32
`
`RADIO TERMINAL 22
`
`~-0
`
`N
`SIGNAL
`VOICE CODE
`RADIO SPECIFIC
`
`FIXED TERMINAL l
`
`~
`
`I
`
`-
`
`~
`I SIGNAL
`
`/
`
`SWITCH 7
`VOICE CODE
`EXCHANGE WIRED LINE
`
`EXCHANGE STATION 4
`
`STATION 31
`BASE
`
`Fig. 4
`
`", 't--1coDEC s
`
`'1----J
`
`I/
`
`I
`
`'
`
`I
`/ \
`lJ../ "" \
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`I
`I
`·"'-
`
`L--+----+----r----,
`I [
`I I
`
`I
`
`RADIO TERMINAL 21
`
`SIGNAL
`VOICE CODE
`RADIO SPECIFIC
`
`/l,/
`
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`
`
`
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`
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`0 ....,
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`
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`
`5
`
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`
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`
`ODE SIGNAL
`
`STATION 4
`EXCHANGE
`
`CODEC 82
`
`{
`
`,__
`
`0
`
`-
`
`~
`
`SELECTOR 92
`
`BASE STATION 32
`
`RADIO TERMINAL 22
`
`~-0
`N
`SIGNAL
`VOICE CODE
`RADIO SPECIFIC
`
`SIGNAL
`VOICE CODE
`SPECIFIC
`RADIO
`
`CODEC 81
`
`~
`
`I
`
`~
`
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`
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`
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`
`SELECTOR 91
`BASE STATION 3t
`
`Fig. 5
`
`RADIO TERMINAL 2
`
`~-0
`
`N
`SIGNAL
`CODE
`VOICE
`SPECIFIC
`RADIO
`
`
`
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`......,
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`
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`
`SPEECH SIGNAL
`VOICE CODE
`
`Fl~ED
`
`VOICE CODE
`WIRED LINE
`
`I ltr WIRED LINE
`I . TERMINAL
`I 11 lPEECH Sl~L ~
`
`SWITCH 7-~
`EXCHANGE
`
`EXCHANGE STATION ft
`
`F i g. 6 PRIOR ART
`
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`
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`
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`
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`
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`
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`SPEECH
`VOICE CODE BASE
`SPECIFIC
`RADIO
`
`n -0
`
`TERMINAL 21
`RADIO
`
`
`
`5,513,211
`
`1
`RADIO COMMUNICATION SYSTEM WITH
`CONTROLLED VOICE CODE SWITCHING
`
`This is a continuation of U.S. application Ser. No.
`07/849,197, filed Mar. 11, 1992, abandoned.
`
`5
`
`BACKGROUND OF THE INVENTION
`
`2
`B 1 . That is to say, one bit of 8 bits of speech encoding is used
`merely for switching of a codec beween codec-free mode
`and codec mode, and a speech is encoded by using only 7
`bits. As the number of bits for speech encoding is decreased
`for the switching of a codec, that prior art has the disad(cid:173)
`vantage that the speech quality is deteriorated.
`
`SUMMARY OF THE INVENTION
`
`10
`
`The present invention relates to a radio communication
`system between a radio terminal and a fixed terminal, and/or
`between radio terminals. In particular, the present invention
`relates to such a system which provides excellent speech
`quality avoiding multiple encoding/decoding operations
`between a radio link and a network.
`In order to save bandwidth, and thus reduce transmission
`costs, in a digital radio communication system, a speech
`signal is encoded with a low bit rate, for instance 8 kilobits/
`second (kbps), by exploiting redundancies in the speech
`signals and compressing the signals. On the other hand, a
`fixed network uses a coding system with typically 64 kbps. 2o
`Therefore, when a radio terminal communicates with a
`fixed terminal, a coding system must be converted between
`a low bit rate signal in a radio link and a high bit rate signal
`in a fixed link.
`FIG. 6 shows a prior radio communication system with a
`network. In the figure, the numeral 1 is a fixed terminal
`coupled with a network, 2 1 and 22 are radio terminals which
`communicate not only with a fixed terminal, but also with a
`radio terminal, 31 and 33 are base stations for coupling radio
`terminals with the network through radio link, 4 is an
`exchange station for exchange and control of the terminals
`1, 21 and 22 , and 7 is an exchange switch installed in the
`exchange station 4 for the exchange of the terminals. The
`numerals 81 and 82 are codecs for converting a voice code
`between a radio-specific voice code which is used in a radio
`terminal, and a wired-line voice code which is used in a
`network.
`When a fixed terminal 1 communicates with a radio
`terminal 21, a codec 81 converts a radio-specific voice code
`of the radio terminal to a wired-line voice code, and vice 40
`versa.
`Similarly, when the radio terminal 2 1 communicates with
`another radio terminal 22 , the codec 81 converts the radio(cid:173)
`specific voice code from the radio terminal 21 to the wired- 45
`line voice code which is used in the network, and then,
`another codec 82 converts said wired-line voice code from
`the codec 8 1 to the radio-specific voice code which is
`forwarded to the radio terminal 22 through the base station
`32 . A voice signal from the radio terminal 22
`is also
`converted to the radio specific voice code through the
`wired-line voice code by two codecs 81 and 82 .
`Accordingly, the conventional radio communication sys(cid:173)
`tem has the disadvantage that when a radio terminal com(cid:173)
`municates with another radio terminal, a voice code is first 55
`converted to a wired-line voice code, and then, converted
`back to a radio-specific voice code. The process of voice
`signal twice in codecs deteriorates speech quality in speech
`distortion and/or delay time.
`The U.S. Pat. No. 4,924,480 describes codecs with sup- 60
`pression of multiple encoding/decodings across a connec(cid:173)
`tion. In that prior art, a fixed network operates with high bit
`rate of 64 kbps, and each speech signal is encoded by using
`8 bits (B 1 through B8). A codec communicates its presence
`to another codec on the high bit rate side is based on the 65
`transmission of predetermined synchronization patterns
`inserted in the signals, for instance the least significant bit
`
`15
`
`It is an object of the present invention to provide a new
`and improved radio communication system through fixed
`network by overcoming the disadvantages and limitations of
`such a system in a prior art.
`It is also an object of the present invention to provide a
`radio communication system through a fixed network, with
`excellent speech quality by avoiding speech degradation
`because of multiple encoding/decoding.
`The above and other objects are attained by a radio
`communication system comprising a plurality of radio ter(cid:173)
`minals which operate with a first voice coding system; a
`network coupled with said radio terminals through radio
`link, operating with a second voice coding system, said
`network having a plurality of base stations for coupling the
`25 network with said radio terminals through radio link, a
`plurality of exchange stations each having an exchange
`switch for exchanging calls, and a plurality of codecs for
`converting voice coding systems between said first coding
`system and said second coding system; wherein said net-
`30 work has a selector for switching each of said codecs
`between a codec release mode and a codec mode; and
`wherein when a radio terminal is coupled with another radio
`terminal through said network, said selector selects a codec
`release mode so that multiple coding/decoding is avoided,
`35 and when a radio terminal is coupled with a fixed terminal,
`said selector selects a codec mode for conversion between
`said first voice coding system and said second voice coding
`system.
`Preferably, a specific control line for controlling said
`selector is provided among said exchange stations.
`
`50
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`The foregoing and other objects, features, and attendant
`advantages of the present invention will be appreciated as
`the same become better understood by means of the follow(cid:173)
`ing description and accompanying drawings wherein;
`FIG. 1 shows a radio communication system according to
`the present invention,
`FIG. 2 shows the detailed view of the radio communica-
`tion system according to the present invention,
`FIG. 3 shows the operation sequence of the system shown
`in FIG. 2,
`FIG. 4 shows another radio communication system
`according to the present invention, ,
`FIG. 5 shows still another radio communication system
`according to the present invention, and
`FIG. 6 shows the conventional radio communication
`system.
`
`DESCRIPTION OF THE PREFERRED
`EMBODIMENTS
`
`FIG. 1 shows a block diagram of a radio communication
`system through network according to the present invention.
`
`
`
`5,513,211
`
`5
`
`15
`
`3
`In the figure, the numeral 1 is a fixed terminal coupled
`with a fixed network, 2 1 and 22 are radio terminals which
`communicate with a fixed terminal 1, or a radio terminal 22
`or 21, 3 1 and 32 are base stations which are coupled with
`radio terminals 21 and 22 , respectively, through radio link.
`The numeral 4 is an exchange station which effects exchange
`service and communication control for the related terminals
`1, 21 and 22 • The numeral 7 is an exchange switch for
`selectively connecting a base station with a fixed terminal.
`The exchange switch 7 is installed in the exchange station 4.
`The numerals 8 1 and 82 are codecs installed in the exchange
`station 4 for the conversion between a first voice coding
`system which is used in a radio link, and a second voice
`coding system which is used in network. The numerals 9 1
`and 92 are selectors for controlling said codecs between
`codec release mode and codec mode.
`When a fixed terminal 1 communicates with a radio
`terminal 2 1, a radio link is established between the radio
`terminal 21 and the base station 31, and the exchange switch
`7 in the exchange station 4 connects the base station 3 1 and
`the fixed terminal 1. The codec 8 1 effects the conversion of
`the radio-specific voice code from the radio terminal 21 to
`the wired-line voice code which is sent to the fixed terminal
`1, and the conversion of the wired-line voice code from the
`fixed terminal to the radio-specific voice code which is sent
`to the radio terminal 2 1•
`When the radio terminal 2 1 communicates with the other
`radio terminal 22, the radio links are established between the
`radio terminal 21 and the base station 3 1, and between the
`radio terminal 22 and the base station 32 , respectively, and
`the exchange switch establishes the wired line between the
`base stations 3 1 and 32 . The selector 91 and 92 are switched
`so that the codecs 81 and 82 operate in codec-release mode
`by being connected to the contact (b). Therefore, the radio(cid:173)
`specific voice code of the radio terminal 21 is forwarded to
`the radio terminal 22 through the base station 31, the
`exchange station 4 and the base station 32 , with no code
`conversion.
`As described above, when a fixed terminal communicates
`with a radio terminal which uses different voice code system 40
`from that of the fixed terminal, the codec 8 1 in the exchange
`station 4 effects the conversion of the voice codes, and when
`a radio terminal 21 communicates with another radio termi-
`nal 22 which uses the same voice code system as that of the
`radio terminal 21, the codecs 8 1 and 82 operate in codec- 45
`release mode so that no code conversion is effected. There-
`fore, the voice communication between different voice code
`systems is of course possible, and no code conversion is
`effected for the communication between terminals which use
`the same voice code system as each other so that the speech 50
`quality is not deteriorated because of the distortion by the
`conversion and the delay time.
`FIG. 2 shows a modification of the present radio com(cid:173)
`munication with network. In the figure, the same numerals
`as those in FIG. 1 show the same members. The numerals 6 1 55
`and 62 are control circuits installed in the exchange stations
`41 and 42 , respectively. Those control circuits effect the call
`origination and termination control for the radio terminals 21
`and 22 through the radio control line 111 and 112 , and for the
`base stations 3 1 and 32 through the control lines 121 and 122, 60
`and the connection control for the exchange stations 41 and
`42 through the control line 123 • The control circuits 61 and
`62 effect also the control of the selectors 91 and 92 which
`switch the codecs between codec-release mode and codec
`mode, and the connection control of the exchange switches 65
`71 and 7 2 for the speech path through the control line 123 •
`The numeral 10 is a ring back tone generator, which gen-
`
`4
`erates a ringing signal indicating that a radio terminal is
`alerting with the wired-line voice code.
`In the figure, a coding system in a radio link between a
`radio terminal and a base station is a first voice coding
`system, which is for instance 8 kbps which outputs one bit
`in every 125 µS, or 160 bits in every 20 mS, and a coding
`system in a network is a second voice coding system which
`is for instance 64 kbps which outputs 8 bits in every 125 µS.
`As a fundamental period in operation of a codec is 20 mS,
`10 a synchronization bit Sl is attached to an information bit B2
`so that an end of a burst of 20 mS is indicated.
`A codec 81' or 82 effects the conversion between a first
`coding system (B2, Sl) as shown by (b) and a second coding
`system (BS-Bl) as shown by (a).
`When a selector 91 or 92 is connected to the contact (a),
`a codec is in codec mode, and said conversion is carried out,
`and a codec outputs 64 kbps signal having 8 bits in every 125
`µS. On the other hand, when the selector 91 or 92 is
`connected to the contact (b), the codec operates in codec-
`20 release mode, in which code conversion operation is not
`carried out.
`In order to synchronize two related exchange stations so
`that both stations operate in codec mode, or codec-release
`mode, a control line 123 is provided among the exchange
`stations in addition to a speech line 53 •
`The selectors 91 and 92 select codec-release mode when
`two radio terminals communicate with each other, and select
`codec mode when a radio terminal communicates with a
`30 fixed terminal in a network.
`FIG. 3 shows the time sequence of call origination and
`termination control, when the radio terminal 2 1 originates a
`call, which is terminated by the radio terminal 22 .
`First, the radio terminal sends a SETUP signal which is an
`originating condition request to the control circuit 61
`through the radio control link 11 1 and the control link 121 •
`Then, the CALLPROC signal which is an authentication
`request and an authentication response is sent and received
`between the radio terminal 2 1 and the exchange station 4 1.
`Next, the control circuit 61 makes the selector 91 switch
`to the contact (a) so that the codec 8 1 operates in codec
`mode. This initial codec mode is necessary for forwarding a
`ring back signal which is generated by the ring back tone
`generator 10 to the originating radio terminal 21• The ring
`back signal is obtained in the form of wired-line voice code
`signal, and it is converted to radio specific voice code signal,
`and is forwarded to the radio terminal 2 1 .
`Next, a radio channel set signal is sent to a radio terminal
`21 so that a radio speech path between the radio terminal 2 1
`and the base station 31 and the speech path between the base
`station 3 1 and the exchange station 41 are established.
`The control circuit 61 also anylyzes the SETUP (origi(cid:173)
`nating condition request), which includes a terminated
`address, and recognizes that the terminated address is a radio
`terminal, and that a codec 8 1 must be in codec release mode
`during speech.
`Next, the control circuit 61 forwards IAM (initial address
`message) signal to the terminated exchange station 42
`through the control line 123 , establishes the speech path 53
`between the exchange stations 41 and 42 , and makes the
`exchange switch 71 connect the speech paths 5 1 and 53 .
`The control circuit 62 in the terminated exchange station
`42 sends back the ACM (address complete message) signal
`to the originating exchange station 41 upon receipt of said
`IAM signal. The control circuit 62 also recognizes the
`terminated address which is included in the IAM signal, and
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`25
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`5,513,211
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`10
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`6
`radio terminal with no conversion of said voice code to
`wired line voice code. And, an excellent speech quality with
`no code conversion is obtained.
`FIG. 4 shows the modification of the present radio com-
`5 munication system, in which a selector 9 is removed, and
`instead, both input and output of a codec 8 are connected to
`an exchange switch 7.
`When a fixed terminal 1 communicates with a radio
`terminal 21, the fixed terminal 1 is connected to the codec 8
`through the exchange switch 7, which also connects the
`codec 8 to the base station 31 which is coupled with the radio
`station 21, so that the codec 8 converts the radio specific
`voice code from the radio terminal 21 to the wired line voice
`code which is forwarded to the fixed terminal 1, and also
`15 converts the wired line voice code from the fixed terminal 1
`to the radio specific voice code which is forwarded to the
`radio terminal 21.
`When the radio terminal 21 communicates with the radio
`terminal 22 , the exchange switch 7 connects the base station
`20 31 which is coupled with the radio station 21 to the base
`station 32 which is coupled with the radio terminal 22 , and
`the codec 8 is released. Thus, the radio specific voice code
`from each radio station is forwarded to another radio ter(cid:173)
`minal with no code conversion.
`A control circuit is not shown in FIG. 4 for the sake of the
`simplicity of the figure, but of course a control circuit
`corresponding to that of FIG. 2 is provided in the apparatus
`of FIG. 4, and also a control line for forwarding a codec
`control signal or a selector control signal is provided in FIG.
`4.
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`sends the destination radio terminal 22 the termination
`request signal through the control line 122 .
`Next, the termination response signal, the authentication
`request signal, and the authentication response signal are
`exchanged between the exchange station 42 and the radio
`terminal 22 . Then, the control circuit 62 recognizes that an
`originating terminal is a radio terminal by analyzing the
`originating address included in the IAM signal, and then,
`makes the selector 92 switch to the contact (b) to codec
`release mode. Then, the control circuit 62 forwards the radio
`terminal 22 the radio channel set signal, and establishes the
`radio circuit 132 between the radio terminal 22 and the base
`station 32, and the speech path 52 between the base station
`32 and the exchange station 42 .
`Next, the control circuit 62 makes the exchange station 42
`send and receive SETUP signal, and CALLPROC (call
`processing) signal to and from the radio terminal 22 , through
`the radio control line 112 , and the wired control line 122 .
`When the exchange station 42 receives the ALERT signal
`from the radio terminal 22 , the exchange station 42 sends the
`exchange station 4 1 the CPG (call progress message) signal
`through the control line 123 . Then, the exchange station 4 1
`sends the ALERT signal to the originating radio terminal 21
`through the wired control line 12 1 and the radio control line
`111.
`Then, the control circuit 62 in the exchange station 42
`makes the exchange switch 7 2 connect the ring back tone
`generator 10 to the speech path 53 so that the ring back tone
`(RBG) is sent back to the originating radio terminal 21 to
`inform the same that the destination radio terminal is ring- 30
`ing. Thus, the ring back tone which is in wired-line voice
`code generated by the ring back tone generator 10, is
`forwarded to the originating radio terminal 21, through the
`exchange switch 72 , the speech path 53 , the exchange switch
`71, the codec 81 which converts the wired-line voice code to 35
`the radio specific voice code, the selector 91, the speech path
`51, the base station 3 1, and the radio path 131.
`When the destination radio terminal 22 hangs up a handset
`to make a response to ringing, the destination radio terminal
`22 sends the CONN (connect) signal to the exchange station 40
`42 through the radio control path 112 and the wired control
`path 122 . Then, the control circuit 62 in the exchange station
`42 forwards ANM (answer message) signal to the originating
`exchange station 41 through the control line 123 . Then, the
`originating exchange station 41, upon receipt of the ANM 45
`signal, forwards the CONN signal to the originating radio
`terminal 21 through the wired control line 121 and the radio
`control line 11 1.
`At the same time, the destination control circuit 62 in the
`destination exchange station 42 makes the exchange switch
`7 2 release the ring back tone generator 10, and connect the
`speech paths 52 and 53 , and the control circuit 62 forwards
`the originating exchange station 41 the codec release request
`~al.
`Upon receipt of said codec release request signal in the
`originating exchange station 4 1, the control circuit 6 1 in the
`exchange station 4 1 makes the selector 91 connect to the
`contact (b) so that the codec 81 is released, and forwards the
`destination exchange station 42 the codec release acknowl- 60
`edgement signal.
`Upon receipt of the codec release acknowledgement sig(cid:173)
`nal, the destination exchange station 42 forwards the radio
`terminal 22 the CONNACK (connect acknowledge) signal.
`Thus, the speech path between the radio terminals 2 1 and 65
`22 is established, so that the radio specific voice code
`generated in each radio terminal is forwarded to another
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`50
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`FIG. 5 shows another modification of the present radio
`communication system. The structure of FIG. 5 is essentially
`the same as that of FIG. 2, except that codecs 81 and 82 are
`installed in a base station together with a related selector,
`while the codec and the related selector are installed in an
`exchange station in FIG. 2. A control line for controlling a
`selector is provided although it is not shown in the drawing.
`From the foregoing it will now be apparent that a new and
`improved radio communication system has been discovered.
`It should be understood of course that the embodiments
`disclosed are merely illustrative and are not intended to limit
`the scope of the invention. Reference should be made to the
`appended claims, therefore, rather than the specification as
`indicating the scope of the invention.
`What is claimed is:
`1. A radio communication system comprising;
`a plurality of radio terminals which operate with a first
`voice coding system,
`a network coupled with said plurality of radio terminals
`through a plurality of radio links, said network having
`a plurality of base stations for coupling the network
`with said plurality of radio terminals through said
`plurality of radio links, a plurality of exchange stations,
`each having an exchange switch coupled with a corre-
`sponding base station of said plurality of base stations
`and also with fixed terminals of a fixed network oper-
`ating with a second voice coding system, for exchang(cid:173)
`ing calls between terminals, and a plurality of codecs
`coupled between said plurality of base stations and said
`fixed network for converting voice coding systems
`between said first voice coding system and said second
`voice coding system, each of said voice coding systems
`having voice carrying lines that carry voice signals
`represented by a plurality of voice-representative digi(cid:173)
`tal bits,
`said network including a plurality of selectors for switch(cid:173)
`ing respective codecs between a codec release mode
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`5,513,211
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`and a codec mode, a plurality of control circuits for
`controlling the switching by said selectors, and a con(cid:173)
`trol line, separate from voice carrying lines of said
`voice coding systems, coupled between said control
`circuits in different exchange stations for control of said 5
`control circuits during operation on said voice carrying
`lines, such that said selectors are operative to select a
`codec release mode when a radio terminal communi(cid:173)
`cates with another radio terminal through said network,
`and to select a codec mode for conversion between said
`first voice coding system and said second voice coding
`system when a radio terminal communicates with a
`fixed terminal/such that none of the digital bits on the
`voice carrying lines is used for switching between
`codec release mode and codec mode, and the digital bit
`resolution on said voice carrying lines is retained
`without deterioration by the switching.
`
`8
`2. A radio communication system according to claim 1,
`wherein said selectors are implemented by said exchange
`switches.
`3. A radio communication system according to claim 1,
`wherein said control circuits are operative to control said
`codecs such that when two radio terminals communicate
`with each other, a codec in an originating side is first in
`10 codec mode, and switched to codec release mode after a
`response for ringing by a destination terminal is acknowl(cid:173)
`edged, and a codec in a destination side is in codec release
`mode when a destination terminal is recognized by said
`15 control circuits as a radio terminal.
`
`* * * * *