CA2041030C - Cordless telephone system capable of quickly establishing connection during call setup phase - Google Patents
Cordless telephone system capable of quickly establishing connection during call setup phaseInfo
- Publication number
- CA2041030C CA2041030C CA002041030A CA2041030A CA2041030C CA 2041030 C CA2041030 C CA 2041030C CA 002041030 A CA002041030 A CA 002041030A CA 2041030 A CA2041030 A CA 2041030A CA 2041030 C CA2041030 C CA 2041030C
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- Prior art keywords
- channel
- cordless
- station
- channels
- candidate
- Prior art date
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Classifications
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04M—TELEPHONIC COMMUNICATION
- H04M1/00—Substation equipment, e.g. for use by subscribers
- H04M1/72—Mobile telephones; Cordless telephones, i.e. devices for establishing wireless links to base stations without route selection
- H04M1/725—Cordless telephones
- H04M1/72502—Cordless telephones with one base station connected to a single line
- H04M1/72505—Radio link set-up procedures
- H04M1/72508—Radio link set-up procedures using a control channel
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04M—TELEPHONIC COMMUNICATION
- H04M1/00—Substation equipment, e.g. for use by subscribers
- H04M1/72—Mobile telephones; Cordless telephones, i.e. devices for establishing wireless links to base stations without route selection
- H04M1/725—Cordless telephones
- H04M1/72502—Cordless telephones with one base station connected to a single line
- H04M1/72505—Radio link set-up procedures
- H04M1/72511—Searching for available channels
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04M—TELEPHONIC COMMUNICATION
- H04M1/00—Substation equipment, e.g. for use by subscribers
- H04M1/72—Mobile telephones; Cordless telephones, i.e. devices for establishing wireless links to base stations without route selection
- H04M1/725—Cordless telephones
- H04M1/733—Cordless telephones with a plurality of base stations connected to a plurality of lines
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- Engineering & Computer Science (AREA)
- Computer Networks & Wireless Communication (AREA)
- Signal Processing (AREA)
- Mobile Radio Communication Systems (AREA)
Abstract
In a cordless telephone system, several speech channels are periodically scanned by a base station during a standby mode to detect idle channels from the scanned channels. In response to receipt of a call request from a cordless station or an incoming call from a switched telephone network, the base station transmits a channel indicating signal to the cordless station over a control channel indicating that the detected idle channels can be used as possible candidate channels, switches to a first candidate channel and waits for an end-of-switching signal from the cordless station until a prescribed period of time expires. On receiving the channel indicating signal, the cordless station switches to the first candidate channel and makes a channel check on it and transmits an end-of-switching signal if it determines that the first candidate channel is idle. If the end-of-switching signal is not received from the cordless station before the prescribed period expires, the base station switches to a second candidate channel. If the cordless station determines that the first candidate channel is busy, it switches to the second candidate channel and makes a channel check on it. If the cordless station determines that the second candidate channel is idle, it transmits the end-of-switching signal over the second candidate channel.
Description
-1- 20~10~0 TITLE OF THE INVENTION
2"Cordless Telephone System Capable Of Quickly Establishing 3Connection During Call Setup Phase"
sThe present invention relates to a cordless telephone system.
6Cordless telephone systems have currently been in widespread use 7due to their excellent mobility. The system includes a base station 8interfacing the public or private switched telephone network and a gcordless station for exchanging radio control signals with the base station 10 to establish a connection. During a standby mode, the base station 11 constantly scans several speech channels to detect idle channels. In 12 response to a call request from the cordless station, the base station 13 specifies one of the idle speech channels as a possible candidate and 14 informs the cordless station of the identifier of the specified channel over 1 5 the control~ channel and wa;ts for a response from the cordless station 16 until a prescribed period expires. On receiving this information through 17 the control channel, the cordless station switches to the specified channel 18 and checks for the availability of the specified channel. Because of 19 different field intensities between the sites of the base station and cordless station, the latter may determine that the specified channel is not actually 21 available. Under such circumstances, the cordless station switches to the 22 control channel and repeats the transmission of a call request to the base 2 3 station. Failing to receive a response from the cordless station before the 24 prescribed period expires, the base station recognizes that the specified 2 5 channel is not available and switches to the control channel to wait for the26 retransmission of a call request. In response to the retransmission, the 27 base station specifies another idle channel and informs this channel 2 8 identifier to the cordless station to allow it to perform a channel check on29 the newly specified channel. The process will be repeated until an ~-available speech channel is detected. Similar situations occur when an incoming call is received from the network.
Therefore, the time taken to establish a connection i5 substantial if busy conditions are repeatedly encountered at the site of the cordless station.
SUHHARY OF THE INVENTION
It is therefore an object of the present invention to provide a cordless telephone system capable of establishing a connection in a short period of time following the reception of an incoming call from the network or an originating call from the cordless station.
According to one aspect of the present invention, there is provided in a cordless telephone system having a control channel and a plurality of speech channels, a method comprising the steps ofs A) scanning said speech channels during a standby mode and detecting idle channels from the scanned channels; B) receiving, at a base station, a call request from a cordless station or an incoming call from a switched telephone network; C) transmitting a channel identification signal from said base station to the cordless ætation over said control channel identifying the detected idle channels as possible candidate channels, and receiving said channel identification signal over said control channel at said cordless station; D) switching the base station to a first one of the candidate channels and waiting for receipt of an end-of-switching signal from said cordlesæ
station and switching the cordless station to said first candidate channel and testing said first candidate channel for availability '- 204 1 030 at the site of the cordless station and transmitting an end-of-switching signal if said first candidate channel is determined as being idle; and E) switching the base station to a second one of the candidate channels if the end-of-switching signal is not received from the cordless ætation within a prescribed period of time following the selection of the first candidate channel and switching the cordless station to the second candidate channel and testing said second candidate channel for availability at the site of the cordless station if the first candidate channel is determined as being busy and transmitting said end-of-switching signal over the second candidate channel if same is determined as being idle.
According to another aspect, the present invention provides a cordless telephone system having a control channel and a plurality of speech channels, comprising: a base station coupled to a switched telephone network, and a cordless station, said base station including means for scanning said speech channels during a standby mode and detecting idle channels from the scanned channels, means responsive to a call request from the cordless station or an incoming call from the network for transmitting a channel identification signal to the cordless station over said control channel identifying the detected idle channels as possible candidate channels, and means for selecting a first one of the candidate channels and waiting for receipt of an end-of-switching signal from the cordless station, and for selecting a second one of the candidate channels if the end-of-switching signal is not received within a prescribed period of time following the selection of the first candidate channel; and said cordless station including means for receiving said channel identification signal over said control channel, and means for selecting and testing said first candidate channel identified by the received channel identification signal for availability at the site of the cordless station and transmitting said end-of-switching signal over the selected first candidate channel if same is determined as being idle, and for selecting and testing said second candidate channel for availability at said site if the selected first candidate channel is determined as being busy and for transmitting said end-of-switching signal over the selected second candidate channel if same is determ$ned as being idle.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will be described in further detail with reference to the accompanying drawings, in which:
Fig. 1 is a block diagram of a cordless telephone system of the present invention;
Figs. 2A, 2B and 2C are flowchartæ describing programmed instructions which are performed by the base station;
Figs. 3A and 3B are flowcharts describing programmed instructions which are performed by the cordless station; and Figs. 4A and 4B are illustrations of control signals exchanged between the base station and cordless station during a call originating mode and a call terminating mode, respectively.
DETAILED DESCRIPTION
In Fig. 1, two cordless telephone systems are illustrated as adjacent systems which share the same frequency ~.
spectrum. Each cordless telephone system comprises a base station 2 connected to a public or private switched telephone network 1 through a subscriber loop 3, and a cordless station 4. Base station 21 includes a hybrid 10 having a two-wire circuit coupled through subscriber loop 31 to the network 1, the transmit portion of the four-wire circuit being coupled to the input of a radio transmitter 11 and the receive portion of the four-wire circuit being coupled to the output of a radio receiver 13. Transmitter 11 and receiver 13 are provided with a respective frequency synthesizer which is controlled by a control circuit 12. The output of transmitter 11 is connected to a transmit antenna 16 and the input of receiver 13 iæ connected to a receive antenna 17. A
channel status memory 14 is connected to control circuit 12 to store busy/idle ætatus of all speech channels. As will be described, this is done by sweeping the frequency synthesizer of receiver 13 across the allocated spectrum so that all speech channels are scanned at periodic intervals and detecting their field intensities. For this purpose, a carrier detector 15 is connected to receiver 13 to supply the control circuit 12 with a busy or idle status signal indicating that the detected carrier intensity of a switched channel is higher or lower than a prescribed threshold, respectively. The busy/idle conditions of all speech channels are stored into channel status memory 14 and periodically updated during a standby mode. During a call processing mode, the memory 14 is accessed by control circuit 12 to select idle speech channels to generate a channel identification signal indicating that the detected idle channels 4a 2041 030 7l024-l60 can be used as possible candidate channels.
In cordless station 41' radio signals from base station 21 are received by a receive antenna 28 and supplied to a radio receiver 20. Radio signals from base station 22 of adjacent system are also received by antenna 28 and fed into receiver 20.
The audio-frequency output of receiver 20 is applied to a speech circuit 24 whose output is, in turn, coupled to the input of a radio transmitter 21. Speech circuit 24, which is normally turned off, is activated by control circuit 22 in response to a turn-on signal sent from base station 21. The output of transmitter 21 is coupled to a transmit antenna 29 for tranæmission of signals to receive antenna 17 of the base station. Receiver 20 and transmitter 21 are provided with a respective frequency synthesizer which is controlled by a control circuit 22. A
carrier detector 23 is coupled to receiver 20 to detect the intensity of transmitted carrier to supply control circuit 22 with information on the busy/idle status of a channel to which the station is being switched. A keypad 25 supplies dialing information as well as off-hook signal to control circuit 22. An idle channel memory 27 is connected to control circuit 22 to store idle channel data supplied from 4b 20410~0 base station 21 to enable control circuit 22 to sequentially switch the 2 station from one idle speech channel to the next until it detects an actually 3 available speech channel. Receipt of a ringing signal sent from base 4 station 21 causes control circuit 22 to activate a tone ringer 26 and the off-hook condition of the station or receipt of a turn-on signal for speech 6 circuit causes control circuit 22 to deactivate it.
7 The system has a single control channel and several speech 8 channels. These control and speech channels are commonly used by 9 adjacent systems, so radio signals from antenna 16 of base station 21 are 10 also received by cordless station 42 as shown at Al and signals from base 11 station 22 are also received by antenna 2B of station 41 as shown at A2.
12 Likewise, signals from antenna 29 of station 41 are also received by base 13 station 22 as shown at Bl and signals from station 42 are also received by14 antenna 17 of base station 21 as shown at B2.
16 The control circuits 12 and 22 are a microprocessor-based controller 17 which is programmed to perform a sequence of instructions as will be 18 described hereinbelow.
19 ln Fig. 2A, during standby modes, base-station control circuit 12 is shown to perform a standby routine in which it constantly scan all speech 21 channels and examines their busy/idle status and updates the channel 22 status memory 14. The program execution starts with step 31 in which 2 3 variable i is set equal to 1. Exit then is to step 32 to cause base-station 24 transmitter 11 and receiver 13 to switch to speech channel S; and 2 5 proceeds to step 33 to check the carrier intensity of channel S; detected 26 by carrier detector 15 against the prescribed threshold to determine if 27 channel Sj is idle or currently used by any cordless telephone system and 28 store the busy/idle status of the channel into channel status memory 14.
29 Exit then is to step 34 to increment variable i by one and moves ahead to 204 1 030~
decision step 3S to check to see if all speech channels are examined. If 2 the answer is negative, control returns to step 32 to repeat the process on 3 the next speech channel S;+1 until all speech channels are tested.
4 If the answer is affirmative in step 35, control proceeds to step 36 to s control the transmitter 11 and receiver 13 so that the base station is 6 switched to the control channel to receive control signals (decision steps 7 37 and 38). If a call request is made from cordless station 41, the decision 8 in step 37 is affirmative and control exits to step 41 (Fig. 2B), and if there is g an incoming call from the network, the decision in step 38 is affirmative 10 and control exik to step 51 (Fig. 2C). If negative decision is made in both 11 steps 37 and 38, control returns to step 31 to repeat the standby routine.
12 In Fig. 2B, if there is an originating call from cordless station 41, base 13 station 2 returns an acknowledgment (ACK) signal over the control 14 channel by containing in it a list of idle channels S; stored in memory 14 5 (step 41). Exit then is to step 42 to set variable i to 1 and switch the base 16 station to an idle speech channel 5; (step 43). A timeout period of T
17 seconds is then set in a timer (step 44), and the receipt of an end-of-18 switching signal from the cordless station is checked (step 45). If the end-19 of-switching signal is not received, control proceeds to step 46 to check 20 to see if timeout period T1 has expired. If the answer is negative in step 21 46, control repeats steps 45 and 46 until timeout period T1 expires. If an 22 end-of-switching signal is received within the timeout period T1, control 23 advances to step 49 to send a turn-on signal to the cordless station and 24 enters a talking mode. Otherwise, it exits to step 47 to check to see if allchannels contained in the idle channel list are tested. If the answer is 26 negative, variable i is incremented by 1 (step 48) and control returns to 27 step 43 to repeat the process so that the next idle speech channel S;+1 is 28 examined. If no end-of-switching signal is received on each of the 29 successively switched idle speech channels, the decision is affirmative in step 47 and control abandons the attempt to establish a connection and 2 returns to the standby routine.
3 In Fig. 2C, if there is an incoming call from the network, base station 2 4 sends a channel assignment signal indicating a list of idle channels S;
s stored in memory 14 and the timer is set to period Tl (step 51). Exit is to 6 step 52 to check for receipt of an ACK signal from the cordless station. If 7 the answer is negative in step 52, step 53 is executed to check for the 8 expiration of period T1. If no ACK signal is returned within the period T1, 9 control returns to step 51 to retransmit the channel assignment signal. If1 0 an ACK signal is received, control moves ahead to step 54 to set variable 11 i to 1 and goes to a looped sequence of steps 55 through 60 which are 12 respectively identical to steps 43 through 48 of Fig. 2B.
13 If an end-of-switching signal is received so the answer is affirmative in 14 step 57, control proceeds to step 61 to transmit a ringing signal over the swikhed speech channel to receive an off-hook signal (step 62) and 16 transmit a turn-on signal (step 63) to enter a talking mode.
17 In Fig. 3A, during standby modes, the cordless station is switched to 18 the control channel. A call originating routine begins with step 71 which 19 checks to see if an off-hook condition has occurred. If the answer is affirmative, control exits to step 72 to switch the station to the control 21 channel and checks the output of carrier detector 23 against the 22 prescribed threshold to determine if the control channel is in use by 23 another cordless telephone. If the answer is negative, control proceeds 24 to step 73 to send a call request signal to base station 2. Exit then is to step 74 to check for the presence of a channel assignment signal. If there 26 is none, control returns to step 73 to retransmit the call request signal, and 27 if there is one, control proceeds to step 75 to store into idle channel 28 memory 27 the list of idle speech channels contained in the transmitted 2 9 channel assignment signal.
Variable i is then set equal to 1 (step 76). The cordless station is then 2 switched to an idle speech channel Sj (step 77) and a timer is set to a 3 timeout period T1 (step 78). Exit is to step 79 to check the output of 4 carrier detector 23 against the threshold to determine if that channel is busy. If it is, control exits to step 80 to check for the expiration of the 6 tirneout period and goes to step 81 to check to see if all channels of the 7 idle list are tested. If the answer is negative, variable i is incremented by 1 8 (step 82) and control returns to step 77 to repeat the process on the next 9 idle speech channel Si+1. If the answer is affirmative in step 81, control returns to the starting point of the call originating routine.
11 If the switched idle channel is not busy, exit then is to step 83 to send 12 an end~f-switching signal, followed by the setting of the timer to a 13 timeout period T2 seconds in step 84. Exit then is to step 85 to check for 14 the presence of a turn-on signal. If there is one, control proceeds to step 87 to tum on the speech circuit 24 to enter a talking mode. If the turn-on 16 signal is not received, a timeout check is made (step 86) to repeat step 85 17 until period T2 expires, whereupon control returns to step 71.
18 In Fig. 3B, the cordless station starts a call terminating routine with 19 step 91 which checks for the presence of a channel assignment signal. If there is one, control proceeds to step 92 to store the list of idle speech 21 channels contained in the received signal into memory 27 and goes to 2 2 step 93 to return an ACK signal. Steps 94 through 100 are executed in a 23 manner identical to steps 76 through 82 of Fig. 3A, so that idle channels 24 will be successively tested if each test encounters a busy channel.
If an idle speech channel is available, control exits from step 97 to 26 step 101 to send an end-of-switching signal, causing the base station to 27 send a ringing signal (step 61, Fig. 2C). If the ringing signal is received by 28 the cordless station (step 102), the tone ringer 26 is activated (step 103) 29 to urge the user to go off hook. If an off-hook condition is detected (step -- ~0~10~0 g 104), control proceeds to step 105 to deactivate the tone ringer and goes 2 to step 106 to send an off-hook signal, causing the base station to send a 3 turn-on signal (step 63). The timer is set to period T2 (step 107) to wait 4 for the reception of the turn-on signal (step 108) until period T2 expires s (step 109). If the answer is affirmative in step 108, control exits to step 6 110 to turn on the speech circuit 24.
8 The operation of the system will be described with reference to Figs.
9 4A and 4B together with the flowcharts described above. Assume that, 10 in a simplified example, speech channels S1 and S2 are detected as being idle by the base station during a standby mode.
12 In Fig. 4A, if a call is originated from cordless station 41, a call request 13 signal is sent (step 73, Fig. 3A) to base station 21. On receiving it (step 37, 14 Fig. 2A), base station 21 returns an ACK signal containing a list of idlespeech channels S1 and S2 (step 41, Fig. 2B). Data indicating the idle 16 speech channels S1 and S2 are stored into idle channel memory 27 (step 17 75) and the cordless station is switched from the control channel to 18 speech channel S1 (steps 76, 77) and the busy/;dle status of channel S1 is 19 checked during timeout period T1 (steps 78, 79, 80). If channel S1 is used by cordless station 42, the signal from base unit 22 will be received by 21 antenna 29 and carrier detector Z3 produces an output. Therefore, the 22 answer in step 79 is negative. After period T1 expires, control exits from 23 step 80 to step 81, and repeats the process so that cordless station 21 is 24 then switched to speech channel S2.
Following transmission of the ACK signal, base station 21 is switched 26 to idle channel S1 (steps 42, 43, Fig. 2B) expecting the receipt of an end-27 of-switching on that channel from cordless station 41. Since this signal is 28 not received within the timeout period T1 (step 46), base station 2 29 switches to channel S2. If this channel is detected by cordless station 4 20410~0 .
~o -as being idle, an end-of-switching signal is sent on channel 52 (step 83, 2 Fig. 3A). In response to this signal (step 45), base station 21 transmits a3 turn-on signal (step 49), which is received by the cordless station (step 4 85) to tum on the speech circuit 24.
In Fig. 4B, if an incoming call is received from the network (step 38, 6 Fig. 2A), base-station controller 12 sends a channel assignment signal 7 (step 51, Fig. 2C) containing in it the list of idle channels S1 and S2.
8 Cordless station 41 responds to this signal by returning an ACK signal 9 (step 92, Fig. 3B) and switching it to channel S1 to check for its busy/idle 10 status. If this channel is detected by cordless station 41 as being busy, 11 period T1 lapses and the cordless station switches to channel 52 and 12 transmits an end-of-switching signal on channel S2 (step 101).
sThe present invention relates to a cordless telephone system.
6Cordless telephone systems have currently been in widespread use 7due to their excellent mobility. The system includes a base station 8interfacing the public or private switched telephone network and a gcordless station for exchanging radio control signals with the base station 10 to establish a connection. During a standby mode, the base station 11 constantly scans several speech channels to detect idle channels. In 12 response to a call request from the cordless station, the base station 13 specifies one of the idle speech channels as a possible candidate and 14 informs the cordless station of the identifier of the specified channel over 1 5 the control~ channel and wa;ts for a response from the cordless station 16 until a prescribed period expires. On receiving this information through 17 the control channel, the cordless station switches to the specified channel 18 and checks for the availability of the specified channel. Because of 19 different field intensities between the sites of the base station and cordless station, the latter may determine that the specified channel is not actually 21 available. Under such circumstances, the cordless station switches to the 22 control channel and repeats the transmission of a call request to the base 2 3 station. Failing to receive a response from the cordless station before the 24 prescribed period expires, the base station recognizes that the specified 2 5 channel is not available and switches to the control channel to wait for the26 retransmission of a call request. In response to the retransmission, the 27 base station specifies another idle channel and informs this channel 2 8 identifier to the cordless station to allow it to perform a channel check on29 the newly specified channel. The process will be repeated until an ~-available speech channel is detected. Similar situations occur when an incoming call is received from the network.
Therefore, the time taken to establish a connection i5 substantial if busy conditions are repeatedly encountered at the site of the cordless station.
SUHHARY OF THE INVENTION
It is therefore an object of the present invention to provide a cordless telephone system capable of establishing a connection in a short period of time following the reception of an incoming call from the network or an originating call from the cordless station.
According to one aspect of the present invention, there is provided in a cordless telephone system having a control channel and a plurality of speech channels, a method comprising the steps ofs A) scanning said speech channels during a standby mode and detecting idle channels from the scanned channels; B) receiving, at a base station, a call request from a cordless station or an incoming call from a switched telephone network; C) transmitting a channel identification signal from said base station to the cordless ætation over said control channel identifying the detected idle channels as possible candidate channels, and receiving said channel identification signal over said control channel at said cordless station; D) switching the base station to a first one of the candidate channels and waiting for receipt of an end-of-switching signal from said cordlesæ
station and switching the cordless station to said first candidate channel and testing said first candidate channel for availability '- 204 1 030 at the site of the cordless station and transmitting an end-of-switching signal if said first candidate channel is determined as being idle; and E) switching the base station to a second one of the candidate channels if the end-of-switching signal is not received from the cordless ætation within a prescribed period of time following the selection of the first candidate channel and switching the cordless station to the second candidate channel and testing said second candidate channel for availability at the site of the cordless station if the first candidate channel is determined as being busy and transmitting said end-of-switching signal over the second candidate channel if same is determined as being idle.
According to another aspect, the present invention provides a cordless telephone system having a control channel and a plurality of speech channels, comprising: a base station coupled to a switched telephone network, and a cordless station, said base station including means for scanning said speech channels during a standby mode and detecting idle channels from the scanned channels, means responsive to a call request from the cordless station or an incoming call from the network for transmitting a channel identification signal to the cordless station over said control channel identifying the detected idle channels as possible candidate channels, and means for selecting a first one of the candidate channels and waiting for receipt of an end-of-switching signal from the cordless station, and for selecting a second one of the candidate channels if the end-of-switching signal is not received within a prescribed period of time following the selection of the first candidate channel; and said cordless station including means for receiving said channel identification signal over said control channel, and means for selecting and testing said first candidate channel identified by the received channel identification signal for availability at the site of the cordless station and transmitting said end-of-switching signal over the selected first candidate channel if same is determined as being idle, and for selecting and testing said second candidate channel for availability at said site if the selected first candidate channel is determined as being busy and for transmitting said end-of-switching signal over the selected second candidate channel if same is determ$ned as being idle.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will be described in further detail with reference to the accompanying drawings, in which:
Fig. 1 is a block diagram of a cordless telephone system of the present invention;
Figs. 2A, 2B and 2C are flowchartæ describing programmed instructions which are performed by the base station;
Figs. 3A and 3B are flowcharts describing programmed instructions which are performed by the cordless station; and Figs. 4A and 4B are illustrations of control signals exchanged between the base station and cordless station during a call originating mode and a call terminating mode, respectively.
DETAILED DESCRIPTION
In Fig. 1, two cordless telephone systems are illustrated as adjacent systems which share the same frequency ~.
spectrum. Each cordless telephone system comprises a base station 2 connected to a public or private switched telephone network 1 through a subscriber loop 3, and a cordless station 4. Base station 21 includes a hybrid 10 having a two-wire circuit coupled through subscriber loop 31 to the network 1, the transmit portion of the four-wire circuit being coupled to the input of a radio transmitter 11 and the receive portion of the four-wire circuit being coupled to the output of a radio receiver 13. Transmitter 11 and receiver 13 are provided with a respective frequency synthesizer which is controlled by a control circuit 12. The output of transmitter 11 is connected to a transmit antenna 16 and the input of receiver 13 iæ connected to a receive antenna 17. A
channel status memory 14 is connected to control circuit 12 to store busy/idle ætatus of all speech channels. As will be described, this is done by sweeping the frequency synthesizer of receiver 13 across the allocated spectrum so that all speech channels are scanned at periodic intervals and detecting their field intensities. For this purpose, a carrier detector 15 is connected to receiver 13 to supply the control circuit 12 with a busy or idle status signal indicating that the detected carrier intensity of a switched channel is higher or lower than a prescribed threshold, respectively. The busy/idle conditions of all speech channels are stored into channel status memory 14 and periodically updated during a standby mode. During a call processing mode, the memory 14 is accessed by control circuit 12 to select idle speech channels to generate a channel identification signal indicating that the detected idle channels 4a 2041 030 7l024-l60 can be used as possible candidate channels.
In cordless station 41' radio signals from base station 21 are received by a receive antenna 28 and supplied to a radio receiver 20. Radio signals from base station 22 of adjacent system are also received by antenna 28 and fed into receiver 20.
The audio-frequency output of receiver 20 is applied to a speech circuit 24 whose output is, in turn, coupled to the input of a radio transmitter 21. Speech circuit 24, which is normally turned off, is activated by control circuit 22 in response to a turn-on signal sent from base station 21. The output of transmitter 21 is coupled to a transmit antenna 29 for tranæmission of signals to receive antenna 17 of the base station. Receiver 20 and transmitter 21 are provided with a respective frequency synthesizer which is controlled by a control circuit 22. A
carrier detector 23 is coupled to receiver 20 to detect the intensity of transmitted carrier to supply control circuit 22 with information on the busy/idle status of a channel to which the station is being switched. A keypad 25 supplies dialing information as well as off-hook signal to control circuit 22. An idle channel memory 27 is connected to control circuit 22 to store idle channel data supplied from 4b 20410~0 base station 21 to enable control circuit 22 to sequentially switch the 2 station from one idle speech channel to the next until it detects an actually 3 available speech channel. Receipt of a ringing signal sent from base 4 station 21 causes control circuit 22 to activate a tone ringer 26 and the off-hook condition of the station or receipt of a turn-on signal for speech 6 circuit causes control circuit 22 to deactivate it.
7 The system has a single control channel and several speech 8 channels. These control and speech channels are commonly used by 9 adjacent systems, so radio signals from antenna 16 of base station 21 are 10 also received by cordless station 42 as shown at Al and signals from base 11 station 22 are also received by antenna 2B of station 41 as shown at A2.
12 Likewise, signals from antenna 29 of station 41 are also received by base 13 station 22 as shown at Bl and signals from station 42 are also received by14 antenna 17 of base station 21 as shown at B2.
16 The control circuits 12 and 22 are a microprocessor-based controller 17 which is programmed to perform a sequence of instructions as will be 18 described hereinbelow.
19 ln Fig. 2A, during standby modes, base-station control circuit 12 is shown to perform a standby routine in which it constantly scan all speech 21 channels and examines their busy/idle status and updates the channel 22 status memory 14. The program execution starts with step 31 in which 2 3 variable i is set equal to 1. Exit then is to step 32 to cause base-station 24 transmitter 11 and receiver 13 to switch to speech channel S; and 2 5 proceeds to step 33 to check the carrier intensity of channel S; detected 26 by carrier detector 15 against the prescribed threshold to determine if 27 channel Sj is idle or currently used by any cordless telephone system and 28 store the busy/idle status of the channel into channel status memory 14.
29 Exit then is to step 34 to increment variable i by one and moves ahead to 204 1 030~
decision step 3S to check to see if all speech channels are examined. If 2 the answer is negative, control returns to step 32 to repeat the process on 3 the next speech channel S;+1 until all speech channels are tested.
4 If the answer is affirmative in step 35, control proceeds to step 36 to s control the transmitter 11 and receiver 13 so that the base station is 6 switched to the control channel to receive control signals (decision steps 7 37 and 38). If a call request is made from cordless station 41, the decision 8 in step 37 is affirmative and control exits to step 41 (Fig. 2B), and if there is g an incoming call from the network, the decision in step 38 is affirmative 10 and control exik to step 51 (Fig. 2C). If negative decision is made in both 11 steps 37 and 38, control returns to step 31 to repeat the standby routine.
12 In Fig. 2B, if there is an originating call from cordless station 41, base 13 station 2 returns an acknowledgment (ACK) signal over the control 14 channel by containing in it a list of idle channels S; stored in memory 14 5 (step 41). Exit then is to step 42 to set variable i to 1 and switch the base 16 station to an idle speech channel 5; (step 43). A timeout period of T
17 seconds is then set in a timer (step 44), and the receipt of an end-of-18 switching signal from the cordless station is checked (step 45). If the end-19 of-switching signal is not received, control proceeds to step 46 to check 20 to see if timeout period T1 has expired. If the answer is negative in step 21 46, control repeats steps 45 and 46 until timeout period T1 expires. If an 22 end-of-switching signal is received within the timeout period T1, control 23 advances to step 49 to send a turn-on signal to the cordless station and 24 enters a talking mode. Otherwise, it exits to step 47 to check to see if allchannels contained in the idle channel list are tested. If the answer is 26 negative, variable i is incremented by 1 (step 48) and control returns to 27 step 43 to repeat the process so that the next idle speech channel S;+1 is 28 examined. If no end-of-switching signal is received on each of the 29 successively switched idle speech channels, the decision is affirmative in step 47 and control abandons the attempt to establish a connection and 2 returns to the standby routine.
3 In Fig. 2C, if there is an incoming call from the network, base station 2 4 sends a channel assignment signal indicating a list of idle channels S;
s stored in memory 14 and the timer is set to period Tl (step 51). Exit is to 6 step 52 to check for receipt of an ACK signal from the cordless station. If 7 the answer is negative in step 52, step 53 is executed to check for the 8 expiration of period T1. If no ACK signal is returned within the period T1, 9 control returns to step 51 to retransmit the channel assignment signal. If1 0 an ACK signal is received, control moves ahead to step 54 to set variable 11 i to 1 and goes to a looped sequence of steps 55 through 60 which are 12 respectively identical to steps 43 through 48 of Fig. 2B.
13 If an end-of-switching signal is received so the answer is affirmative in 14 step 57, control proceeds to step 61 to transmit a ringing signal over the swikhed speech channel to receive an off-hook signal (step 62) and 16 transmit a turn-on signal (step 63) to enter a talking mode.
17 In Fig. 3A, during standby modes, the cordless station is switched to 18 the control channel. A call originating routine begins with step 71 which 19 checks to see if an off-hook condition has occurred. If the answer is affirmative, control exits to step 72 to switch the station to the control 21 channel and checks the output of carrier detector 23 against the 22 prescribed threshold to determine if the control channel is in use by 23 another cordless telephone. If the answer is negative, control proceeds 24 to step 73 to send a call request signal to base station 2. Exit then is to step 74 to check for the presence of a channel assignment signal. If there 26 is none, control returns to step 73 to retransmit the call request signal, and 27 if there is one, control proceeds to step 75 to store into idle channel 28 memory 27 the list of idle speech channels contained in the transmitted 2 9 channel assignment signal.
Variable i is then set equal to 1 (step 76). The cordless station is then 2 switched to an idle speech channel Sj (step 77) and a timer is set to a 3 timeout period T1 (step 78). Exit is to step 79 to check the output of 4 carrier detector 23 against the threshold to determine if that channel is busy. If it is, control exits to step 80 to check for the expiration of the 6 tirneout period and goes to step 81 to check to see if all channels of the 7 idle list are tested. If the answer is negative, variable i is incremented by 1 8 (step 82) and control returns to step 77 to repeat the process on the next 9 idle speech channel Si+1. If the answer is affirmative in step 81, control returns to the starting point of the call originating routine.
11 If the switched idle channel is not busy, exit then is to step 83 to send 12 an end~f-switching signal, followed by the setting of the timer to a 13 timeout period T2 seconds in step 84. Exit then is to step 85 to check for 14 the presence of a turn-on signal. If there is one, control proceeds to step 87 to tum on the speech circuit 24 to enter a talking mode. If the turn-on 16 signal is not received, a timeout check is made (step 86) to repeat step 85 17 until period T2 expires, whereupon control returns to step 71.
18 In Fig. 3B, the cordless station starts a call terminating routine with 19 step 91 which checks for the presence of a channel assignment signal. If there is one, control proceeds to step 92 to store the list of idle speech 21 channels contained in the received signal into memory 27 and goes to 2 2 step 93 to return an ACK signal. Steps 94 through 100 are executed in a 23 manner identical to steps 76 through 82 of Fig. 3A, so that idle channels 24 will be successively tested if each test encounters a busy channel.
If an idle speech channel is available, control exits from step 97 to 26 step 101 to send an end-of-switching signal, causing the base station to 27 send a ringing signal (step 61, Fig. 2C). If the ringing signal is received by 28 the cordless station (step 102), the tone ringer 26 is activated (step 103) 29 to urge the user to go off hook. If an off-hook condition is detected (step -- ~0~10~0 g 104), control proceeds to step 105 to deactivate the tone ringer and goes 2 to step 106 to send an off-hook signal, causing the base station to send a 3 turn-on signal (step 63). The timer is set to period T2 (step 107) to wait 4 for the reception of the turn-on signal (step 108) until period T2 expires s (step 109). If the answer is affirmative in step 108, control exits to step 6 110 to turn on the speech circuit 24.
8 The operation of the system will be described with reference to Figs.
9 4A and 4B together with the flowcharts described above. Assume that, 10 in a simplified example, speech channels S1 and S2 are detected as being idle by the base station during a standby mode.
12 In Fig. 4A, if a call is originated from cordless station 41, a call request 13 signal is sent (step 73, Fig. 3A) to base station 21. On receiving it (step 37, 14 Fig. 2A), base station 21 returns an ACK signal containing a list of idlespeech channels S1 and S2 (step 41, Fig. 2B). Data indicating the idle 16 speech channels S1 and S2 are stored into idle channel memory 27 (step 17 75) and the cordless station is switched from the control channel to 18 speech channel S1 (steps 76, 77) and the busy/;dle status of channel S1 is 19 checked during timeout period T1 (steps 78, 79, 80). If channel S1 is used by cordless station 42, the signal from base unit 22 will be received by 21 antenna 29 and carrier detector Z3 produces an output. Therefore, the 22 answer in step 79 is negative. After period T1 expires, control exits from 23 step 80 to step 81, and repeats the process so that cordless station 21 is 24 then switched to speech channel S2.
Following transmission of the ACK signal, base station 21 is switched 26 to idle channel S1 (steps 42, 43, Fig. 2B) expecting the receipt of an end-27 of-switching on that channel from cordless station 41. Since this signal is 28 not received within the timeout period T1 (step 46), base station 2 29 switches to channel S2. If this channel is detected by cordless station 4 20410~0 .
~o -as being idle, an end-of-switching signal is sent on channel 52 (step 83, 2 Fig. 3A). In response to this signal (step 45), base station 21 transmits a3 turn-on signal (step 49), which is received by the cordless station (step 4 85) to tum on the speech circuit 24.
In Fig. 4B, if an incoming call is received from the network (step 38, 6 Fig. 2A), base-station controller 12 sends a channel assignment signal 7 (step 51, Fig. 2C) containing in it the list of idle channels S1 and S2.
8 Cordless station 41 responds to this signal by returning an ACK signal 9 (step 92, Fig. 3B) and switching it to channel S1 to check for its busy/idle 10 status. If this channel is detected by cordless station 41 as being busy, 11 period T1 lapses and the cordless station switches to channel 52 and 12 transmits an end-of-switching signal on channel S2 (step 101).
13 Base station 21 responds to the ACK signal from cordless station 4 14 by switching to channel S1 (steps 54, 55). Since this channel is found busy on th`e cordless station side of the system, period T1 expires also in 16 the base station (steps 58, S9) and it switches to channel S2 (steps 60, SS) 17 to receive the end-of-switching signal (step 57). A ringing signal is then 18 sent to the cordless station (step 61).
19 VVhen cordless station 41 activates its tone ringer (steps 102, 103) in response to the ringing signal, goes off hook (step 104) and deactivates 21 the tone ringer (step 105). An off-hook signal is then sent (step 106), 22 which is received by the base station (step 62) and a turn-on signal is sent23 to the cordless station (step 63) to allow it turn on its speech circuit.
24 It is seen that, while the cordless station is switched successively to an idle channel if each channel test reveals a busy condition, the base station 26 iS switched successively to the same channel in response to the end-of-27 switching signal from the cordless station signaling that the switched 28 speech channel is also idle on the cordless station side of the system.
29 The foregoing description shows only one preferred embodiment of the present invention. Various modifications are apparent to those skilled 2 in the art without departing from the scope of the present invent;on which 3 is only limited by the appended claims. Therefore, the embodiment 4 shown and described is only illustrative, not restrictive.
19 VVhen cordless station 41 activates its tone ringer (steps 102, 103) in response to the ringing signal, goes off hook (step 104) and deactivates 21 the tone ringer (step 105). An off-hook signal is then sent (step 106), 22 which is received by the base station (step 62) and a turn-on signal is sent23 to the cordless station (step 63) to allow it turn on its speech circuit.
24 It is seen that, while the cordless station is switched successively to an idle channel if each channel test reveals a busy condition, the base station 26 iS switched successively to the same channel in response to the end-of-27 switching signal from the cordless station signaling that the switched 28 speech channel is also idle on the cordless station side of the system.
29 The foregoing description shows only one preferred embodiment of the present invention. Various modifications are apparent to those skilled 2 in the art without departing from the scope of the present invent;on which 3 is only limited by the appended claims. Therefore, the embodiment 4 shown and described is only illustrative, not restrictive.
Claims (8)
1. A cordless telephone system having a control channel and a plurality of speech channels, comprising: a base station coupled to a switched telephone network, and a cordless station, said base station including means for scanning said speech channels during a standby mode and detecting idle channels from the scanned channels, means responsive to a call request from the cordless station or an incoming call from the network for transmitting a channel identification signal to the cordless station over said control channel identifying the detected idle channels as possible candidate channels, and means for selecting a first one of the candidate channels and waiting for receipt of an end-of-switching signal from the cordless station, and for selecting a second one of the candidate channels if the end-of-switching signal is not received within a prescribed period of time following the selection of the first candidate channel; and said cordless station including means for receiving said channel identification signal over said control channel, and means for selecting and testing said first candidate channel identified by the received channel identification signal for availability at the site of the cordless station and transmitting said end-of-switching signal over the selected first candidate channel if same is determined as being idle, and for selecting and testing said second candidate channel for availability at said site if the selected first candidate channel is determined as being busy and for transmitting said end-of-switching signal over the selected second candidate channel if same is determined as being idle.
2. A cordless telephone system as claimed in claim 1, wherein said base station comprises means for waiting for receipt of the end-of-switching signal during a second prescribed period of time after the base station is switched to the second candidate channel and selecting a third one of the idle channels if said end-of-switching signal is not received during said second prescribed period of time, and wherein said cordless station comprises means for selecting and testing said third candidate channel if the selected second candidate channel is determined as being busy during said second prescribed period of time and for transmitting said end-of-switching signal over the selected third channel if same is determined as being idle.
3. A cordless telephone system as claimed in claim 1, wherein said base station comprises memory means for storing busy/idle status of said scanned channels and means for periodically updating the busy/idle status of said scanned channels with results of the scanning means during said standby mode.
4. A cordless telephone system as claimed in claim 1, wherein said cordless station comprises memory means for storing data identifying the possible candidate channels contained in the received channel identification signal and selecting one of the candidate channels according to the data stored in said memory means.
5. A cordless telephone system as claimed in claim 1, wherein said base station comprises means for transmitting a proceed-to-talk signal to said cordless station in response to receipt of said end-of-switching signal when said call request is originated from said cordless station and wherein said cordless station comprises means for establishing a talking connection in response to said proceed-to-talk signal.
6. A cordless telephone system as claimed in claim 1, wherein said base station comprises means for transmitting a ringing signal to said cordless station in response to receipt of said end-of-switching signal when said incoming call is received from the network and means for transmitting a proceed-to-talk signal to said cordless station in response to receipt of an off-hook signal from the cordless station, and wherein said cordless station comprises a tone ringer for alerting users in response to receipt of said ringing signal, means for transmitting said off-hook signal when an off-hook condition is detected in said cordless station, and means for establishing a talking connection in response to said proceed-to-talk signal.
7. In a cordless telephone system having a control channel and a plurality of speech channels, a method comprising the steps of: A) scanning said speech channels during a standby mode and detecting idle channels from the scanned channels; B) receiving, at a base station, a call request from a cordless station or an incoming call from a switched telephone network; C) transmitting a channel identification signal from said base station to the cordless station over said control channel identifying the detected idle channels as possible candidate channels, and receiving said channel identification signal over said control channel at said cordless station; D) switching the base station to a first one of the candidate channels and waiting for receipt of an end-of-switching signal from said cordless station and switching the cordless station to said first candidate channel and testing said first candidate channel for availability at the site of the cordless station and transmitting an end-of-switching signal if said first candidate channel is determined as being idle; and E) switching the base station to a second one of the candidate channels if the end-of-switching signal is not received from the cordless station within a prescribed period of time following the selection of the first candidate channel and switching the cordless station to the second candidate channel and testing said second candidate channel for availability at the site of the cordless station if the first candidate channel is determined as being busy and transmitting said end-of-switching signal over the second candidate channel if same is determined as being idle.
8. A method as claimed in claim 7, further comprising the steps of: A) waiting for receipt of the end-of-switching signal during a second prescribed period of time after the base station is switched to the second candidate channel; B) switching the base station to a third one of the idle channels if said end-of-switching signal is not received during said second prescribed period of time, switching said cordless station to the third candidate channel if the second candidate channel is determined as being busy during said second prescribed period of time and testing the third candidate channel for availability at the site of said cordless station; and C) transmitting said end-of-switching signal from the cordless station over the selected third channel if same is determined as being idle.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP2-108351 | 1990-04-24 | ||
| JP2108351A JP2595758B2 (en) | 1990-04-24 | 1990-04-24 | Cordless telephone system |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2041030C true CA2041030C (en) | 1995-02-07 |
Family
ID=14482510
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002041030A Expired - Fee Related CA2041030C (en) | 1990-04-24 | 1991-04-23 | Cordless telephone system capable of quickly establishing connection during call setup phase |
Country Status (8)
| Country | Link |
|---|---|
| US (1) | US5384827A (en) |
| EP (1) | EP0454080B1 (en) |
| JP (1) | JP2595758B2 (en) |
| KR (1) | KR950013161B1 (en) |
| AU (1) | AU636476B2 (en) |
| CA (1) | CA2041030C (en) |
| DE (1) | DE69128658T2 (en) |
| HK (1) | HK1002543A1 (en) |
Families Citing this family (16)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JPH04293325A (en) * | 1991-03-22 | 1992-10-16 | Nec Corp | Radio telephone set |
| US5260988A (en) * | 1992-02-06 | 1993-11-09 | Motorola, Inc. | Apparatus and method for alternative radiotelephone system selection |
| WO1995032591A1 (en) * | 1994-05-19 | 1995-11-30 | Airnet Communications Corp. | System for dynamically allocating channels among base stations in a wireless communication system |
| US5870678A (en) * | 1994-12-22 | 1999-02-09 | Matsushita Electric Industrial Co., Ltd. | Cordless telephone apparatus |
| KR100319309B1 (en) * | 1995-01-25 | 2002-04-24 | 다치카와 게이지 | Mobile communication system |
| JPH09130853A (en) * | 1995-10-26 | 1997-05-16 | Uniden Corp | Communicating method, communication equipment and cordless telephone system |
| EP0786917B1 (en) * | 1996-01-23 | 2013-08-21 | Ntt Mobile Communications Network Inc. | Mobile communication system, network and mobile station |
| US6160805A (en) * | 1996-09-30 | 2000-12-12 | Motorola, Inc. | Method for synchronizing a session timer |
| DE69732481D1 (en) * | 1996-11-29 | 2005-03-17 | Koninkl Philips Electronics Nv | A TELECOMMUNICATION SYSTEM, A CHANNEL SELECTION PROTOCOL AND A RADIO STATION |
| JPH10164660A (en) * | 1996-11-29 | 1998-06-19 | Nec Corp | Radio communication equipment |
| US6212396B1 (en) | 1997-03-27 | 2001-04-03 | Nortel Networks Limited | Generic handset programming and configuration |
| US6078821A (en) * | 1998-02-25 | 2000-06-20 | Motorola, Inc. | Cordless radiotelephone system having an extendable geographic coverage area and method therefor |
| US7251488B2 (en) * | 2002-06-28 | 2007-07-31 | Interdigital Technology Corporation | Method and system for coordinating services in integrated WLAN-cellular systems |
| JP4207005B2 (en) * | 2005-01-28 | 2009-01-14 | ブラザー工業株式会社 | Cordless equipment |
| US8125887B2 (en) * | 2006-03-01 | 2012-02-28 | Nec Corporation | Mobile communication system, its scramble code assigning method, mobile station, and base station |
| CN108809545B (en) | 2017-05-04 | 2023-01-06 | 华为技术有限公司 | Method and device for transmitting uplink control information |
Family Cites Families (14)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JPS5930333A (en) * | 1982-08-13 | 1984-02-17 | Nippon Telegr & Teleph Corp <Ntt> | Method for setting radio line |
| JPS61177040A (en) * | 1985-01-31 | 1986-08-08 | Nec Corp | Radio circuit control system |
| JPH0352048Y2 (en) * | 1985-04-30 | 1991-11-11 | ||
| JPS61274531A (en) * | 1985-05-30 | 1986-12-04 | Nec Corp | Channel selection system |
| US4802200A (en) * | 1985-08-27 | 1989-01-31 | Nippon Telegraph And Telephone Corporation | Radio telephone system control apparatus and method |
| US4672657A (en) * | 1985-12-17 | 1987-06-09 | Motorola, Inc. | Multichannel telephone system |
| GB8531492D0 (en) * | 1985-12-20 | 1986-02-05 | Racal Res Ltd | Cordless telephones |
| JP2580129B2 (en) * | 1986-07-18 | 1997-02-12 | ソニー株式会社 | Transceiver |
| JPS6348926A (en) * | 1986-08-19 | 1988-03-01 | Fujitsu Ltd | Multi-channel access system for cordless telephone |
| DE3643004A1 (en) * | 1986-12-17 | 1988-06-30 | Philips Patentverwaltung | METHOD AND CIRCUIT FOR CONNECTING AND MONITORING A CORDLESS TELEPHONE APPARATUS |
| FI97010C (en) * | 1987-05-29 | 1996-09-25 | Nec Corp | Control station and radio communication network system including switching device units operating in different operating modes |
| JP2642380B2 (en) * | 1988-02-16 | 1997-08-20 | 日本電信電話株式会社 | Wireless communication device |
| JPH02228137A (en) * | 1989-03-01 | 1990-09-11 | Mitsubishi Electric Corp | Radio circuit setting method |
| JPH02249326A (en) * | 1989-03-23 | 1990-10-05 | Iwatsu Electric Co Ltd | Idle channel designation system |
-
1990
- 1990-04-24 JP JP2108351A patent/JP2595758B2/en not_active Expired - Lifetime
-
1991
- 1991-04-23 AU AU75308/91A patent/AU636476B2/en not_active Ceased
- 1991-04-23 CA CA002041030A patent/CA2041030C/en not_active Expired - Fee Related
- 1991-04-23 KR KR1019910006495A patent/KR950013161B1/en not_active IP Right Cessation
- 1991-04-24 DE DE69128658T patent/DE69128658T2/en not_active Expired - Fee Related
- 1991-04-24 US US07/690,417 patent/US5384827A/en not_active Expired - Lifetime
- 1991-04-24 EP EP91106579A patent/EP0454080B1/en not_active Expired - Lifetime
-
1998
- 1998-02-26 HK HK98101477A patent/HK1002543A1/en not_active IP Right Cessation
Also Published As
| Publication number | Publication date |
|---|---|
| HK1002543A1 (en) | 1998-08-28 |
| AU7530891A (en) | 1991-11-07 |
| KR910019469A (en) | 1991-11-30 |
| JP2595758B2 (en) | 1997-04-02 |
| DE69128658T2 (en) | 1998-05-14 |
| KR950013161B1 (en) | 1995-10-25 |
| EP0454080B1 (en) | 1998-01-14 |
| EP0454080A3 (en) | 1993-04-14 |
| US5384827A (en) | 1995-01-24 |
| DE69128658D1 (en) | 1998-02-19 |
| EP0454080A2 (en) | 1991-10-30 |
| AU636476B2 (en) | 1993-04-29 |
| JPH046919A (en) | 1992-01-10 |
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