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Description  |
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BACKGROUND OF THE INVENTION
The invention relates to a cordless telephone unit for making and receiving
calls over a telephone network; and more particularly to an improvement in
such units which minimizes the cost and effort of verifying that the
handset is within range of a fixed station.
The cord between a telephone set and the telephone network connection is
generally several meters long. The cord between the handset or receiver
and the telephone set, such a desk set or wall-mounted set, is still
usually considerably shorter. Consequently, a user is bound to a certain
spot while initiating a call, and can move only a short distance from that
spot during the conversation.
Greater freedom is provided by so-called handsfree facilities, such as
Speakerphone set having a loudspeaker and a sensitive microphone, enabling
the user to move about freely for several meters in the room during a
telephone conversation. However, it is still necessary to touch the fixed
unit in order to initiate or receive a call.
To increase the user's range even more, cordless telephones are
increasingly used in various countries. With these telephones, the
transmission of information between a fixed station and a portable
handset, together forming a cordless telephone unit, is effected through a
pair of radio transmission channels, forming a duplex pair. The fixed
station is connected via connector box to a main station or a private
branch exchange, to provide access to the public telephone network. Since
the fixed station and handset are connected to each other over radio
transmission channels, the range of free movement is increased to as much
as 200 meters or more from the associated fixed station or connector box.
In the Federal Republic of Germany there are 40 channels available for
transmission of information, lying the 900 MHz. frequency range. In this
frequency range, and with the limited transmission distance, the same
radio channel is not apt to be seized by mutuallly adjacent cordless
telephones. If a user wishes to initiate a call, and operates the "cradle
switch" of the handset, or the fixed station receives a ringing signal
over the network connection, then a station searcher which is part of the
handset receiver, or part of the fixed station, will commence search for
an idle radio channel in the assigned band, and upon finding such a
channel will seize the duplex pair. To minimize the possibility that
messages transmitted between the fixed station and handset of one cordless
telephone can be overheard by another cordless telephone, to permit
correct allocation of charges, and to ensure that the allocation of pairs
of channels is unambiguous, an identification number is assigned to each
cordless telephone independent of the subscriber's telephone number.
A method of searching for a radio channel and exchanging identification
numbers, when a radio connection between handset and fixed station is set
up, is further described and explained in European Patent Specification
EP-B No. 10 074 940. According to this method, an idle transmission
channel is seized by energizing the transmitter of the set from which
communication is to be established, and a data packet containing the
identification number is transmitted over the channel. This set then waits
for an acknowledgement data packet, containing the same identification
number to be transmitted by the corresponding opposite set. Upon receiving
the identification number, on acknowledgement message, from the opposite
station this number is compared by an arrangement in the initating station
to is own identification number. If the two numbers are identical, the
connection is switched through to the telephone network by a control
arrangement, the exchange of identification numbers is stopped and the
existing radio connection is monitored.
An article in ntz, Vol. 38 (1985), No. 7, pages 468-471, describes a
cordless telephone system for the 900 MHz. frequency band. The automatic
management of the 40 transmission channels is described on page 468. When
the fixed station is in a rest position, all 40 channels are searched
consecutively, to detect any request from a portable handset to establish
a connection to the telephone network. The channels are searched
cyclically, to determine if a tested channel is occupied; if it is not
occupied, this channel is noted as idle, and the receiver switches to the
next channel in the cycle.
Further monitoring, during the course of a conversation, is described on
page 470 as an exchange of identification numbers which takes place every
15 seconds. During his exchange of numbers, speech transmission is
interrupted for a short time interval, and the transmission of
identification number can be heard softly during this interval.
Another method of monitoring a cordless telephone connection is described
in Technische Mitteilungen PTT, No. 4, 1986, pages 152-165. In this
monitoring method, identification numbers are exchanged to safeguard the
assigning of the two stations. Monitoring is further described and
explained at page 157: if the channel quality drops below a
signal-to-noise ratio of 20 dB, for more than 500 ms., a user is informed
by a warning tone emitted by the handset. If the receiving quality does
not improve during the next 10 seconds, for example as a result of a
change in the user's position, then the set automatically attempts to
establish a new radio contact. After a connection has been set up, the
identification number as described above is transmitted every 15 seconds
and acknowledged automatically by the opposite station. If the
acknowledgement signal contains an error twice within 35 seconds, or is
not received, a new radio contact is again automatically attempted.
Operation of the systems described above, to establish connections and
monitor transmission quality, presupposes that a connection has been made
between the subscriber's fixed station and that telephone network.
However, these systems do not provide a way for a user to verify whether
he is still within the coverage area of the fixed station when the user
moves a considerable distance while carrying the handset without utilizing
the public telephone network or at least occupying a line selector.
SUMMARY OF THE INVENTION
An object of the invention is to provide a cordless telephone system in
which a user can verify a simple manner whether he is still within radio
coverage area, without necessarily establishing connection to the
telephone network.
A further object is to provide this verification capability while keeping
the cost of additional circuitry as low as possible.
According to the invention the cordless telephone unit includes a circuit
in the handset, controllable by a user, for activating transmission of an
identification number and a code; and the fixed station comprises a
circuit for comparing the receive code with a code signal stored in the
fixed station, and a response to an identity between the received code and
the stored code signal, transmitting an acknowledgement signal to the
handset to acknowledge receipt of the code.
According to one embodiment of the invention, upon receipt of the
acknowledgement signal, the handset automatically renews radio contact
with the fixed station.
According to another preferred embodiment, the handset has an annunciator
which automatically produces a user-detectable sight or sound signal if an
acknowledgement signal is not received following transmission of the
identification number and code by the handset.
In a still further preferred embodiment of the invention, the handset
includes user-controllable means for transmitting an identification
number, to cause the fixed station to attempt to access a telephone
network; and a user-controllable circuit for causing the handset to
transmit an identification number and code for verifying that the handset
is still withn radio coverage area, without establishing connection to the
telephone network. In this embodiment, upon receiving the identification
number and code, and determination that the code matches a code signal
stored in the fixed station, rather than accessing the telephone network
the fixed station transmits an acknowledgement signal which comprises the
same identification signal and code to the handset. The handset provides a
warning signal to the user if the acknowledgement is not received.
BRIEF DESCRIPTION OF THE DRAWING
The sole FIGURE is a block diagram of the fixed station and handset
circuits of a cordless telephone unit in accordance with the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
A cordless telephone unit is formed by a fixed station BS, including
circuits for connection to a public telephone network (not shown), and a
handset for sending and receiving messages over the telephone network via
the fixed station.
The handset MS and fixed station BS each include a station searcher
arranged as part of a respective receiver DMS and EBS, for selecting an
idle radio transmission channel from a plurality of predetermined duplex
pairs of radio transmission channels in response, respectively, to an
attempt by a user to initiate a call from the handset, or receipt of a
ringing signal by the fixed station from the telephone network. To
establish radio contact between the fixed station and handset, the
transmitter SMS or SBS in the respective portion of the unit, from which
radio contact is to be initiated, transmits an identification number K
over an idle radio transmission channel to the respected receiver EBS or
EMS of the opposite station. This identification number K is stored in the
respective identification number store SK arranged in the fixed station BS
and in the handset MS. Receivers EMS and EBS are respectively connected to
arrangements EVMS and EVBS, in the handset MS and fixed station BS, to
scan and compare the identification number K received from the other
station. When the identification numbers correspond, the connection to the
public telephone network is switched through.
To enable the verification of radio contact, according to the invention the
handset MS and fixed station BS each further include a respective code
store SCMS and SCBS respectively controlled by control circuits STMS and
STBS. Signals from the code stores are fed respectively to the
arrangements EVMS and EVBS and the respective transmitters SMS and SBS.
To verify that he is within radio coverage area, without establishing a
connection to the telephone network, a user depresses a switch key T in
the handset MS. Upon receipt of the switch signal, the control STMS causes
the handset transmitter SMS to transmit the identification number K and
the code C through any automatically selected idle radio transmission
channel to the receiver EBS of the fixed station. The identification
number K and code signal C received in the fixed station are then compared
in the arrangement EVBS with the stored values of K and C from the fixed
station stores SCBS and SK. If the received number and code correspond to
the stored number and code signal, a signal from the arrangement EVBS
causes the control circuit STBS to initiate transmission by the
transmitter SBS of the identification number K and the code C.
In the handset MS, the signals received by the signal EMS are provided to
the arrangement EVMS for comparison of the received signals with the
stored values of identification number K and code C. If these values do
not correspond, or if no signal is received from the fixed station BS, an
out of range signal is provided from the arrangement EVMS to the control
STMS. An optical or acoustic signal is then produced by an annunciator AMS
in the handset, to warn the user that the handset is not within the radio
coverage area of the fixed station.
With the cordless telephone unit according to the invention, no battery
power is wasted by unnecessary automatic attempts to verify radio
coverage; but a user can at all times quickly and economically verify if
the handset is still within range of the fixed station, so that the user
will be able to be aware of and receive any incoming calls from the
switched telephone network. With this circuit, the amount of battery power
required to verify that the handset is within radio range, is less than
that required if a user attempts to verify radio contact by seeking to
obtain a dial tone from the network. Further, very little additional
circuitry is required: an extra code store in each station of the unit,
and an increased comparison and logic capability in the arrangements EVBS,
EVMS and associated control circuits.
Verification according to the invention also obviates the possibility that
an incoming call may be blocked from reaching the fixed station, because
it is attempted to be placed at the same time that the user is verifying
coverage. From the standpoint of the telephone utility, cordless units
according to the invention further offer the advantage that line selectors
will not be occupied by users merely seeking to determine if they still
have radio contact with their fixed stations.
It will be clear that transmission of K+C signals and acknowledge signals
can also be used automatically during the time a connection is
established, to verify the channel quality.
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