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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to multi-channel access (MCA) cordless telephones of
the type generally described in copending application Ser. No. 403,477,
filed Sept. 6, 1989 and assigned to the same assignee as the present
invention; and, more particularly, to an MCA-type cordless telephone
system in which several remote stations, such as handset units, may be
queried simultaneously for the purpose of establishing a communication
channel with one of them, within a brief period of time.
2. Description of the Prior Art
Cordless telephone systems of the multi-channel access type which rely upon
relatively low electrical power requirements have been proposed
heretofore. In one such proposal, eighty-seven separate communication
channels and two separate control channels are provided, each channel
being a "cordless" communication path having a particular carrier
frequency onto which information and data are modulated. To expand the
functions of an MCA cordless telephone system, a so-called
"multi-cordless" arrangement has been introduced in which a master station
is comprised of plural base units, each including a set of transmitting
and receiving circuits adapted to communicate with one of several remote
stations, such as handset units.
An example of the aforementioned multi-cordless telephone arrangement is
illustrated in FIG. 1 in which one master station, such as base unit 2, is
adapted to communicate with plural remote stations, such as the eight
handset units 1A, 1B, . . . 1H that are illustrated. Base unit 2 is
connected to a conventional telephone line 3 and is adapted to communicate
with the handset units by way of radio transmission between the antenna
provided at the base unit and each of the antennas provided at the handset
units.
In the arrangement shown in FIG. 1, when an incoming telephone call is
received from telephone line 3 to base unit 2, the base unit signals all
of handset units 1A-1H at the same time, whereupon one of the handset
units responds and, thus, answers the incoming telephone call. Typically,
the following techniques are used to signal the handsets and complete a
telephone call:
In one technique, base unit 2 signals each of handsets 1A-1H individually
and in sequence. At the completion of the entire sequential signalling
operation, that is, after the last handset has been signalled, the base
unit awaits the response from a handset which wishes to answer the
telephone call.
Another technique relies upon simultaneous signalling of all of the
handsets, with the expectation that only one handset, that is, the desired
handset unit, will respond.
As yet another technique, all of handset units lA-1H are signalled at the
same time, but the signalling identifies a particular handset unit as a
"representative handset unit" which is the only unit enabled to respond.
Thus, although other "non-representative" units receive the signal from
base unit 2, they are inhibited from responding thereto and, thus, are not
able to answer the incoming telephone call.
All of the aforementioned techniques suffer from various disadvantages and
drawbacks. The first technique, which signals individual handset units in
sequence, requires an unusually long period of time to complete the
sequential signalling and to answer the incoming telephone call. This is
because the handset units are not maintained in a steady receive-enable
mode of operation so as to detect base unit signalling. Rather, in an
effort to minimize the power requirements of the handset units (due to the
fact that these units typically are energized by a battery), a handset
unit typically is disposed in a quiescent or "sleep" mode from which it is
"awakened" to its receive mode only periodically. For example, and as
shown in FIG. 2A, in a 2-second period, the handset unit may be disposed
in its receive mode for only 0.2 seconds, while remaining in its
quiescent, power-down mode for the remaining 1.8 seconds. Thus, a handset
unit is enabled only briefly to receive the signalling transmitted from
base unit 2. Furthermore, the periodicity illustrated in FIG. 2A, which
illustrates the changeover between the quiescent and receive modes of a
handset unit, is not synchronized in all of handset units 1A-1H. Although
the repetition rates of these changeover operations may be about the same,
the instantaneous times at which the handset units are changed over from
their quiescent mode to their receive mode are random.
Accordingly, to ensure that a signal from base unit 2 will be detected by
each of handset units 1A-1H, the base unit transmits this signal
periodically over a predetermined interval to handset unit 1A, then to
handset unit 1B, and so on. As illustrated in FIGS. 2A and 2B, the signal
transmitted from the base unit may exhibit a duration of only 120
milliseconds, but this signal is repealed continuously for a period of
approximately 2.5 seconds to make certain that the signal is present when
handset unit 1A is disposed briefly in its receive mode. It is expected
that the handset unit receives the signal from base unit 2 at a time t2,
whereafter this handset unit does not return to its quiescent mode but,
rather, remains in its receive mode, as represented by the broken line
shown in FIG. 2A. Following the end of the 2.5 second signalling interval,
which occurs at time t3, the signalled handset unit responds at, for
example, time t4, as illustrated in FIG. 2C. This "answer back" signal is
delayed by an amount & following the end of the signalling interval. The
"answer back" signal is detected at base unit 2 to establish a
communication channel between the base unit and the responding handset
unit (assumed herein to be handset unit 1A).
In a typical MCA cordless telephone system, the signalling from base unit 2
to a handset unit and the "answer back" from that handset unit are carried
out over a control channel which, typically, is separate and distinct from
the communication channel. Since the use of the control channel for other
purposes generally is not permitted, a handset unit is inhibited from
responding to the signalling from the base unit until all of the handset
units have been signalled. Once this signalling is completed, the base
unit awaits the receipt of an "answer back" from one of the handset units,
whereafter a call indicator at that handset unit, such as a ringing
device, is energized. It is appreciated that if 2.5 seconds are needed to
signal one handset unit successfully, twenty seconds (2.5 seconds .times.8
handset units) are needed to call all of the handset units. This unusual
time delay is unacceptable, especially since an incoming telephone caller
may discontinue his telephone call long before it is completed on the
assumption that the called party (i.e. base unit 2 in combination with
handset units 1A-1H) is not available to "answer".
In the second signalling technique mentioned above, wherein all of the
handset units are signalled simultaneously, the time needed to establish a
communication channel between the base unit and one handset unit is
substantially reduced from that just described. However, it is not unusual
for two or more handset units to transmit "answer back" signals to the
base unit at time t4. As a result, radio interference between these
simultaneously transmitted "answer back" signals prevents the base unit
from determining which handset unit wishes to establish the communication
channel. Hence, communication is prevented.
In the third of the aforementioned techniques for establishing
communication between the base unit and a handset unit, wherein only the
representative handset unit is permitted to response to the signalling,
the drawbacks generally attending the other two techniques are avoided.
However, if the representative handset unit is unable to receive the
signal transmitted from base unit 2, or if the representative handset unit
is unable to communicate with the base unit, for example, if its battery
power is low, if it has malfunctioned or if another cordless telephone in
close proximity therewith monopolizes the control channel, or if it is
located beyond the effective communication range with the base unit, the
signalling process is stopped. That is, base unit 2 is unable to establish
communication with the representative handset unit or with any other
handset unit. Consequently, an incoming telephone call cannot be
"answered" if there is no reply from the representative handset unit,
regardless of the reason for that lack of reply.
OBJECTS AND SUMMARY OF THE INVENTION
Therefore, it is an object of the present invention to provide an improved
multi-channel access cordless telephone system which avoids the aforenoted
disadvantages and drawbacks.
Another object of this invention is to provide an MCA cordless telephone
system which avoids cross-modulation when a base unit signals several
remote units, without requiring special circuitry to suppress
cross-modulation.
A further object of this invention is to provide an MCA cordless telephone
system in which several remote stations may be signalled from a master
station to establish a communication channel between the master and a
remote station in a brief period of time.
An additional object of this invention is to provide an MCA cordless
telephone system in which a communication channel may be established
quickly between a master station and one of several remote stations,
without undesired cross-modulation, and wherein only one remote station at
a time is permitted to communicate with the master station.
Yet another object of this invention is to provide an MCA cordless
telephone system of the aforementioned type in which initial signalling,
such as in response to an incoming telephone call, is carried out between
the master and remote stations by way of a separate control channel or,
alternatively, by way of an available one of the multi-channels.
Various other objects, advantages and features of the present invention
will become readily apparent from the ensuing detailed description, and
the novel features will be particularly pointed out in the appended
claims.
In accordance with this invention, a communication channel is established
between a master station and one of plural remote stations, such as
handset units, in a multi-channel access (MCA) cordless telephone system.
The master station transmits to the remote stations, at the same time,
access data including station identifying data for designating a selected
remote station as a representative station and calling data which seeks a
response from the representative station. In the event that a response to
the calling data is not received from the previously designated
representative station, the station identifying data is changed to
designate a new remote station as the representative station. When a
representative station responds to the calling data, the master station
polls the remote stations, in sequence, inviting a reply from each polled
station. A communication channel is established between the master and the
polled station which replies to the polling operation.
In accordance with one aspect of this invention, the access data is
transmitted over a control channel simultaneously to the remote stations,
and the response to the calling data is returned to the master station
over that control channel. The communication channel, when established,
differs from the control channel.
In accordance with another aspect of this invention, the access data is
transmitted over an available one of the multi-channels included in the
MCA system, and this available channel also is used to return the response
from the representative station to the master station, to poll the remote
stations and to define the communication channel over which the master
station and the poll-replying remote station communicate.
When a separate control channel is used, the access data includes
identifying data for selecting the communication channel to which the
master and poll-replying remote stations are connected for communication.
In accordance with a feature of this invention, a ringing signal
representing an incoming telephone call is transmitted from the master
station to the remote stations if the representative station has responded
to the access data. The ringing signal is used to energize incoming call
indicators, such as ringing circuits, located at each remote station.
As an aspect of the aforementioned feature, an acknowledgment is returned
to the master station from a remote station, when that remote station is
polled, indicating that the remote station wishes to answer the incoming
telephone call. For example, a talk key may be operated at the remote
station. Consistent with this aspect, when the master station receives the
aforementioned acknowledgment, it terminates the ringing signal which had
been transmitted to the remote stations.
As a result of the present invention, if a remote station designated as the
representative station does not respond to the access data transmitted
from the master station, that is, it does not answer the incoming
telephone call, a new representative station is designated. This process
continues until, eventually, a designated representative station responds,
whereafter all of the remote stations which are capable of communicating
with the master station (for example, they are not one of the
non-responding stations which had been designated as a representative
station) are polled, in sequence, to permit one of the polled stations to
"answer" the call.
BRIEF DESCRIPTION OF THE DRAWINGS
The following detailed description, given by way of example, will best be
understood in conjunction with the accompanying drawings in which:
FlG. 1 is a schematic representation of an MCA cordless telephone system in
which the present invention finds ready application;
FIGS. 2A-2C are timing diagrams which are helpful in understanding the
manner in which a communication channel is established between a master
station and a handset unit in the system shown in FIG. 1;
FIG. 3 is a block diagram of a cordless telephone which uses the present
invention;
FIG. 4 is a schematic representation of the COMMAND signal structure used
with the present invention;
FIG. 5 is a schematic representation of the sequence in which signals are
transmitted when initiating an outgoing telephone call with the cordless
telephone shown in FIG. 3;
FIG. 6 is a schematic representation of the manner in which an outgoing
telephone call is terminated by the cordless telephone shown in FIG. 3;
FIGS. 7A and 7B are schematic representations of one example of the
sequence in which signals are transmitted in accordance with the present
invention to "answer" an incoming telephone call;
FIG. 8 is a schematic representation of another example of the sequence in
which signals are transmitted in accordance with the present invention;
FIGS. 9A and 9B represent a flow chart which explains the manner in which
the master station shown in FIG. 3 operates to "answer" an incoming
telephone call;
FIGS. 10A and 10B represent a flow chart which explains the manner in which
the remote station of FIG. 3 operates to "answer" an incoming telephone
call; and
FIGS. 11A-11C are schematic representations of a cordless telephone system
which operates in accordance with the present invention without separate
control channels.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
Referring now to the cordless telephone represented by the block diagram
shown in FIG. 3, this telephone is adapted to be used in the system shown
in FIG. 1 wherein a master station 2 is adapted to receive and transmit
telephone calls via a telephone line 3 and communicate with any one of
several remote handset units 1A-1H. FIG. 3 illustrates one embodiment of a
typical handset used with this invention, such as handset unit 1A (also
referred to herein as a remote station), and also illustrates in greater
detail the embodiment of a base unit 21A and the embodiment of an
interface circuit 23A provided at master station 2. The cordless telephone
is provided with, for example, eighty-seven separate communication
channels, or sets of carrier frequencies, and also is provided with two
control channels, of which only one control channel is described. For
increased flexibility and enhanced functions, master station 2 is provided
with four base units 21A, 21B, 21C and 21D, of which base unit 21A is
shown in greater detail, as mentioned above. It will be appreciated that
each base unit is adapted to communicate with any one of handset units
1A-1H.
Still further, although master station 2 is adapted to process incoming and
outgoing telephone calls over a single telephone line, the aforementioned
desirability of improved flexibility and enhanced functions is aided by
connecting two telephone lines to the master station, as represented by
lines 3A and 3B.
Since the construction of handset units 1A-1H are substantially identical,
only handset unit 1A is illustrated in detail. This handset unit includes
a transmitting circuit 110, a receiving circuit 120, a keypad 131, a talk
key 132, several function control keys 133-137 and a microcomputer 140.
Transmitting circuit 110 is coupled to an antenna 100 to supply radio
frequency energy thereto. In particular, the transmitting circuit
functions to convert audio signals, such as audio signals St generated by
a microphone 111, to suitable radio frequency energy. In this regard,
transmitting circuit 110 preferably includes a frequency modulator to
supply FM signals to antenna 100. The transmitting circuit also functions
to convert command signals CMND supplied from microcomputer 140 to radio
frequencies for transmission via antenna 100. As will be described, the
command signals are digital signals adapted to be frequency modulated by
conventional frequency shift keying (FSK) techniques. The FM audio and FM
command signals are transmitted from antenna 100 to master station 2 as up
channel FM signals Su.
Receiving circuit 120 is coupled to antenna 100 and is adapted to receive
down channel FM signals Sd which are transmitted from master station 2.
The received FM signals may be audio signals which are demodulated and
supplied as audio signals Sr to a speaker 121. The received FM signals
also may be command signals received as FSK signals; and receiving circuit
120 functions to demodulate and supply these command signals CMND to
microcomputer 140.
Keypad 131, which may be analogous to a conventional keypad such as
normally provided on tone-dialing telephones, is coupled to microcomputer
140. Keypad 131 may be operated in a conventional manner to generate
telephone numbers and other identifying codes.
Talk key 132 may be a conventional nonlocking push switch which, when
operated, supplies a suitable indicating signal to the microcomputer. As
will be described, microcomputer 140 responds to the operation of talk key
132 to change the operating mode of handset unit 1A. For example, if the
handset unit is disposed in a standby mode capable of receiving and
processing command signals, the microcomputer responds to the activation
of talk key 132 to establish an active mode, known as the "talk mode",
whereby audio signals may be transmitted to and from the handset unit,
thereby enabling a user to carry on a telephone conversation. Once in this
talk mode, the subsequent activation of talk key 132 once again changes
the operating mode of the handset unit to the standby mode. It will be
appreciated that, in the standby mode, the microcomputer establishes
either the receive mode or the quiescent (or sleep) mode for the handset
unit, as discussed above in conjunction with FIG. 2A. That is, the
microcomputer periodically changes over the handset unit from its
quiescent mode to its receive mode, the latter being established for a
brief interval of time (e.g. about 200 milliseconds).
Function control keys 133-137 are similar to talk key 132 in that they are
nonlocking push switches. These keys function as internal keys to carry
out various operations, such as to establish an intercom connection, to
place a telephone call on "hold", to re-dial the last-dialed telephone
number, to dial predetermined telephone numbers, and the like.
In addition to speaker 121, which functions as, for example, the earpiece
of a telephone instrument, handset unit 1A includes a speaker 139 which is
coupled to an oscillating circuit 138 which, in turn, is controlled by
microcomputer 140. The oscillating circuit is adapted to drive speaker 139
to generate a ringing tone which represents an incoming telephone call.
Microcomputer 140 includes a microprocessor and, in one embodiment, may be
implemented by Model MSM-80C51, produced by Oki Electric Company, Ltd. of
Japan. The microcomputer is adapted to generate the command signal CMND
which is transmitted to master station 2 as an up channel signal via
antenna 100. Microcomputer 140 also is adapted to receive and interpret
the command signal received from the master station as a down channel
signal via antenna 100. In addition, transmit and receive control signals
TCTL and RCTL are applied to transmitting circuit 110 and receiving
circuit 120, respectively, to selectively enable or inhibit the operations
of these circuits. These transmit and receive control signals also
establish the particular communication channel over which handset unit 1A
communicates with master station 2. That is, the particular transmit and
receive carrier frequencies are determined by these control signals.
A random access memory (RAM) 141 is coupled to microcomputer 140 and is
adapted to store various codes, control signals, identifying signals and
the like which are used by the microcomputer, handset unit 1A and the
illustrated cordless telephone system. For example, to distinguish the
cordless telephone illustrated in FIG. 3 from another, similar cordless
telephone, a system identification code is used to identify this system.
As a numerical example, a 25-bit system identification code SYID is stored
in RAM 141 to distinguish this cordless telephone system from others.
Likewise, the particular handset with which RAM 141 is associated is
distinguished from other handsets in the illustrated system. For the
example wherein eight handset units are provided, a handset identification
code HSID may be assigned to each unit such as, for simplification,
HSID[1] may be assigned to handset unit 1A, HSID[2] may be assigned to
handset unit 1B, and so on. It will be appreciated 11 that a handset unit
will respond only when an identification code 12 HSID identifies that
particular handset unit. To enable all handset units to be accessed
simultaneously, a particular identification code may be used as a
"universal" code, to which all of the handset units respond. For example,
HSID[0] may be used as the universal identification code.
Referring to master station 2, base units 21A-21D are of similar
construction and, in the interest of brevity, only base unit 21A is
illustrated in detail. The base unit includes a transmitting circuit 210
coupled to an antenna 200 to transmit radio frequency signals to the
handset unit. Audio signals Sr which are received from, for example,
telephone line 3A, are frequency modulated (although other forms of
modulation may be used) by transmitting circuit 210 and transmitted via
antenna 200. In addition, a command signal CMND may be applied to the
transmitting circuit for modulation and transmission to the remote handset
units. Modulated command signals are transmitted as down channel FM
signals Sd.
Receiving circuit 220 is adapted to receive and demodulate audio and
command signals transmitted from the remote handset units. The demodulated
audio signals St are supplied generally to a telephone line, such as line
3A, to carry on a telephone conversation with a calling or called party.
The received command signals, which are transmitted as up channel FM
signals Su, are supplied to a microcomputer 24 which controls the
operation of master station 2.
Transmit and receive control signals TCTL and RCTL are supplied to
transmitting circuit 210 and receiving circuit 220, respectively, by
microcomputer 24. These transmit and receive control signals are similar
to those described above in conjunction with handset unit 1A and
selectively enable and inhibit the operations of transmitting circuit 210
and receiving circuit 220, as well as establish the communication channel
(i.e. the carrier frequencies) for transmitting between base unit 21A and
one of the remote handset units. It will be appreciated that the carrier
frequency to which transmitting circuit 210 is tuned is equal to the
carrier frequency to which receiving circuit 120 is tuned and, similarly,
the carrier frequency to which receiving circuit 220 is tuned is equal to
the carrier frequency to which transmitting circuit 110 is tuned.
The transmitting and receiving circuits included in base unit 21A are
coupled through a switch circuit 22 to one or the other of interface
circuits 23A and 23B. Switch circuit 22 is controlled by microcomputer 24
to select the appropriate interface circuit. In addition, and as will be
described, the switch circuit functions to connect base unit 21A to any
one of base units 21B and 21D to permit an intercom-type communication
between two handset units.
Microcomputer 24 is coupled to base units 21A-21D, switch circuit 22 and
interface circuits 23A and 23B for the purpose of controlling these
respective units and circuits. The microcomputer may, for example, be a
Model MSM-80C154, produced by Oki Electric Company, Ltd. of Japan. The
manner in which the microcomputer operates will be described in detail
below.
Interface circuits 23A and 23B are adapted to connect telephone lines 3A
and 3B to switch circuit 22 and, thence, to a base unit selected by
microcomputer 24. The interface circuits are substantially the same, and
only interface circuit 23A is shown in detail. As illustrated, a 4-line to
2-line converter 231 is provided to match telephone line 3A with any one
of base 23 units 21A-21D. This provides compatibility between 2-wire and
4-wire lines.
A switch 232, which is analogous to a conventional telephone hook switch,
couples telephone line 3A to converter 231. Microcomputer 24 is coupled to
switch 232 to control its on-hook or off-hook mode.
A signal forming circuit 233 is coupled to converter 231 and is controlled
by microcomputer 24 to generate dialing signals which are applied to the
telephone line. The dialing signals may be conventional tone signals, such
as dual tone multi-frequency (DTMF) signals, or pulse signals. The mode
(tone or pulse) and content (telephone number) of these dialing signals
are determined by the microcomputer.
A ring detecting circuit 234 is coupled directly to telephone line 3A and
is adapted to detect an incoming ringing signal. The output of detecting
circuit 234 is coupled to microcomputer 24 to provide an indication
thereto when an incoming telephone call has been placed to master station
2.
Microcomputer 24 is coupled to a read only memory (ROM) 241 which, inter
alia, stores the system identification code SYID that identifies this
particular cordless telephone system. The ROM also provides operating
instructions to control the operation and function of microcomputer 24.
Before describing the manner in which the cordless telephone system
operates, reference is made to FIG. 4 which is a schematic representation
of the signal structure of the command signal CMND that may be transmitted
over the down channel from master station 2 to a remote handset unit, or
transmitted over the up channel from a handset unit to the master station.
The command signal includes a bit synchronizing signal BSYN of, for
example, twenty-four alternating bits, such as [101010. . . ], followed by
a frame synchronizing signal FSYN formed of, for example, sixteen bits of
a particular pattern for transmission over an up channel (i.e. from a
handset unit to the master station) and exhibiting a different,
distinctive pattern for transmission over a down channel from the master
station to one or more remote handset units. Thereafter, the command
signal includes a system identification code SYID of, for example,
twenty-five bits, followed by an error correcting code (ECC) which is used
in conjunction with the identification code SYID. The error correcting
code may be formed of, for example, twelve bits, and this code is followed
by a dummy bit DBIT of, for example, three bits. Finally, a 5-byte control
code CTRL is provided.
The contents of control code CTRL are described in greater detail below.
Suffice it to say that the control code is used to represent an incoming
telephone call, to identify a handset unit for communication, to identify
a communication channel over which information is communicated between the
master and remote stations, to indicate an answer or response from a
handset unit, to control the turn-on or turn-off of a ringing indication,
to represent a polling of handset units, to represent a response to
polling, to represent the actuation of particular switches or keys, etc.
When a command signal CMND of the type shown in FIG. 4 is applied to
handset microcomputer 140 or to master station microcomputer 24, the
system identification code SYID and handset identification code HSID are
examined to determine if the respective codes are valid. That is, the
identification code SYID must identify this particular cordless telephone
system and the identification code HSID must identify the proper handset
unit for which the command signal is intended. Only valid command signals
(i.e. those having the proper SYID and HSID codes) are accepted and
processed.
The manner in which an outgoing call is initiated from a remote handset
unit 1A, 1B, . . . 1H now will be described in conjunction with the
sequence schematic diagram of FIG. 5. To best understand FIG. 5, it should
be noted that a solid vertical line represents a channel over which
information is transmitted and received by a handset unit and by a base
unit, a control channel is represented by the vertical line C, a
communication channel is represented by the vertical line V, a single
solid line indicates the standby mode of a handset unit (which, as
discussed above, periodically is changed over from a "sleep" mode to a
receive mode), a double solid line represents the receive mode of a
handset unit or base unit and a hatched double line indicates a talk mode
such that the handset unit or the base unit, as the case may be, may
receive and transmit. Progress in the vertical direction from top to
bottom corresponds to a progression in time. It will be understood that
this time relationship is neither linear nor drawn to any representative
scale.
As illustrated, a handset unit which, for the purpose of the present
description, is assumed to be handset unit 1A, normally is disposed in its
standby mode and periodically enters its receive mode, as represented by
the periodic pulses. A base unit, such as base unit 21A, normally is
disposed in its receive mode and, thus, is capable of receiving command
signals from the handset units. In response to the operation of talk key
132, microcomputer 140 supplies transmitting circuit 110 and receiving
circuit 120 with transmit and receive control signals TCTL and RCTL,
respectively, thereby enabling up channel signals Su to be transmitted and
down channel signals Sd to be received. In response to the operation of
the talk key, handset unit iA is disposed in its talk mode.
In addition, microcomputer 140 supplies a command signal CMND whose control
code CTRL represents a request to make an outgoing telephone call and also
identifies, with identification code HSID, the handset unit which seeks to
make the outgoing telephone call. This command signal CMND is transmitted
from handset unit 1A to base unit 21A over the up control channel.
Command signal CMND is received by antenna 200 at master station 2 and
supplied to, for example, base unit 21A which presently is in its standby,
receive mode. The received FM signal Su is demodulated by receiving
circuit 220 to recover the command signal which is supplied to
microcomputer 24. The microcomputer confirms that the identification code
SYID identifies the proper cordless telephone system by, for example,
comparing the received SYID code to the SYID code stored in ROM 241.
Assuming that the received command signal CMND is valid, microcomputer 24
supplies the transmit control signal TCTL to transmitting circuit 210
which is enabled thereby to transmit signals Sd over the down control
channel to handset unit 1A.
Microcomputer 24 also forms a command signal CMND whose control code CTRL
represents confirmation that the requesting handset unit may initiate an
outgoing telephone call. The control code also includes a handset
identification code HSID to identify handset unit 1A as the unit
authorized to initiate the outgoing telephone call, as well as a channel
number code to assign a particular communication channel over which
information is communicated between the base unit and the handset unit.
This command signal CMND is supplied to and transmitted by transmitting
circuit 210 and is illustrated in FIG. 5 as the "answer back" signal
transmitted as an FM down control channel signal Sd.
When this FM signal Sd is received by receiving circuit 120 of handset unit
1A, the command signal CMND is demodulated and supplied to microcomputer
140. Here, the identification codes SYID and HSID included in the received
command signal CMND are verified, whereupon microcomputer 140 sets the
carrier frequencies of the transmitting and receiving circuits to the
communication channel assigned by the received command signal.
At the time that microcomputer 24 supplies the command signal to
transmitting circuit 210, it also supplies transmit and receive control
signals TCTL and RCTL to transmitting and receiving circuits 210 and 220,
respectively, to set the carrier frequencies therein to those which
correspond to the channel number assigned by the microcomputer for
communication. In addition, base unit 21A is disposed in its talk mode.
The step increase illustrated in FIG. 5 represent the changeover from the
control channel to the assigned communication channel in both the handset
unit and the base unit. Thus, a communication channel is extended, or
opened, between handset unit 1A and base unit 21A. Although the remaining
handset units 1B-1H receive the frequency modulated command signal from
the base unit, these remaining handset units are maintained in their
standby mode of operation because the handset unit identification code
HSID received at each unit does not match its respective HSID code. Thus,
the microcomputers included in such remaining handset units do not
initiate any change in the operating conditions thereof.
Following the reception at handset unit 1A of the command signal which
authorizes the handset unit to initiate an outgoing telephone call and
which assigns the selected communication channel thereto, microcomputer
140 forms a command signal CMND in which the control code CTRL identifies
the carrier frequencies to which transmitting and receiving circuits 110
and 120 have been set. This command signal is transmitted to base unit 21A
as an FM up channel signal Su over the assigned communication channel.
Confirmation that the handset and base units are connected to the same
communicati | | |
