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The invention relates to a telephone system comprising: a satellite
arranged to relay a plurality of one-way signalling channels and one-way
speech transmission channels, the latter being paired into two-way speech
transmission channels; a plurality of earth stations comprising a
plurality of line equipments each of which is connected to a subscriber
and has an input for incoming speech signals, a first output for signals
indicating the condition of the subscriber and a second output for
outgoing speech signals and number information from the subscriber, a
signalling equipment connected to the first output of the line equipments
and adapted to transmit reports about the conditions of the subscribers
and to receive commands about the allocation of calling and called sides
of said two-way speech transmission channels to calling and called
subscribers over the signalling channels, and a frequency synthesizer
equipment connected to the input and the second output of the line
equipments and arranged to connect the calling and called subscribers to
their allocated sides of the two-way speech transmission channels under
control of said commands; and a central control station in which a second
signalling equipment is arranged to receive said reports and transmit said
commands and a control equipment, connected to the second signalling
equipment, for generating the commands and address them to the earth
stations.
The publication INTELSAT/IEE Conference on Digital Satellite Communication,
No. 59, 1969 describes telephone systems which comprise a synchronous
satellite and in which a plurality of earth stations are connected to
their respective transit stations and comprise respective control
processors arranged to distribute transmission channels from a pool of
transmission channels between the earth stations in proportion to their
actual need for transmission capacity. The object of this so-called
DA-technique, DA being an abbreviation for demand assignment, is to
achieve an effective utilization of the satelite's limited number of
transmission channels. It is for example possible to take advantage of
those variations in the traffic density between the earth stations due to
differences in the local time. A number of transmission channels of
so-called PA-type, PA being an abbreviation for preassigned, are
permanently distributed between the earth stations to give them sufficient
transmission capacity for their average traffic so that the transmission
channels of the DA-type from said pool will only be needed during traffic
peaks.
An extension of a telephone system of the above mentioned type to comprise
also a plurality of unsophisticated earth stations which are connected
directly to subscribers offers a solution to the problem of giving
telephone service to such sparsely populated areas where it is difficult
to establish reliable terrestrial networks.
An object of the invention is which a telephone system that comprises a
satellite and a plurality of earth stations connected directly to
subscribers shall have a low total cost.
Another object is that such a telephone system shall have the same
behaviour relative the subscribers as a conventional telephone system.
The invention, the characteristics of which appear from the appended
claims, will now be described more in detail with reference made to the
accompanying drawing where:
FIG. 1 shows signalling and speech transmission paths in a telephone system
according to the invention;
FIGS. 2 and 3 show signalling frames for the signalling between a plurality
of earth stations connected directly to subscribers and; a central control
station in the telephone system of FIG. 1 and
FIGS. 4 and 5 show block diagrams over the internal structure of the earth
stations and the control station respectively.
FIG. 1 shows the signalling and speech transmission paths in a telephone
system comprising: a synchronous satellite (not shown) arranged to relay a
plurality of one-way signalling channels and one-way speech transmission
channels, the latter being paired into two-way speech transmission
channels; a plurality of low traffic earth stations 1 comprising a
plurality of line equipments each of which is connected to a subscriber
and has an input for incoming speech signals, a first output for signals
indicating the subscriber condition, and a second output for outgoing
speech signals and number information from the subscriber, a signalling
equipment connected to the first output of the subscriber equipment and
arranged to transmit reports about the subscriber conditions and to
receive commands about the allocation of calling and called sides of the
two-way speech transmission channels to calling and called subscribers
over said signalling channels, and a frequency synthesizer equipment
connected to the input and to the second output of the line equipment and
arranged to connect the calling and called subscribers to their allocated
sides of said two-way speech transmission channels under control of said
commands; and a central control station 2 in which a second signalling
equipment is arranged to receive the reports and to transmit the commands,
and a control equipment connected to the second signalling equipment, for
generating the commands and address them to the low traffic earth stations
1.
In the central control station 2 in FIG. 1 a number information decoding
means is connected to a data input of the control equipment, and a
frequency synthesizer equipment is arranged to connect the number
information decoding means to the called sides of said two-way
transmission channels.
In FIG. 1 a small number of high traffic earth stations 3 are connected to
their respective transit stations in an extensive conventional telephone
system and are connected with each other and with the central control
station 2 partly via a terrestrial network and partly via the synchronous
satellite. According to the example, the central control station 2 is also
connected to a transit station in said telephone network. Among the
two-way speech transmission channels of the synchronous satellite, a first
number of channels, the transmission paths of which are symbolized by
continuous lines, are permanently distributed between the earth stations
3, and a second number of channels, the transmission paths of which are
symbolized by dashed lines, can temporarily be allocated to any one of
these earth stations under control of the central control station 2 in a
known manner, as described e.g. in the U.S. Pat. No. 3,848,093.
According to an example shown in FIG. 1 it is assumed that a subscriber 4
lifts a handset from a telephone set arranged for voice frequency keying
and the earth station 1 to which the subscriber 4 is connected transmits
via a first signalling channel, the transmission path of which is
symbolized by a dotted line, a report about a calling condition at the
subscriber 4 to the central control station 2. In response thereto,
station 2 generates a command about allocation of a two-way speech
transmission channel, that temporarily is free, to the subscriber 4 and
transmits this command via a second signalling channel, the transmission
path of which is symbolized via a dash-dotted line, to the earth station 1
of the subscriber 4. The frequency synthesizer equipment of the earth
station associated with subscriber 4 then connects the allocated
subscriber 4 to a calling side of the speech transmission channel. At the
same time the frequency synthesizer equipment of the central control
station 2 connects the number information decoding means to a called side
of the allocated speech transmission channel to enable the transmission of
number information regarding a called subscriber 5 from the subscriber 4
to the control equipment of the central control station 2 via the speech
transmission channel, the transmission path of which is symbolized via a
dashed line.
The control equipment of the central control station 2 localizes now, by
means of the number information, that particular earth station 1 to which
the called subscriber 5 is connected, generates a command about allocation
of the called side of the speech transmission channel to the called
subscriber 5, and transmits this command via the second signalling channel
to the earth station 1 of the subscriber 5. The frequency synthesizer
equipment of this earth station 1 connects the subscriber 5 to the called
side of the speech transmission channel. Then a speech circuit between the
subscribers 4 and 5, symbolized with a dashed line, is established and
will be supervised by the control equipment of the central control station
2 which periodically receives reports about the conditions of the
subscribers 4 and 5 transmitted from their respective earth stations 1.
It is to be noed that the control station 2 will transmit only speech
circuits established over its own transit station. When e.g. a speech
circuit is established via the synchronous satellite between a calling
subscriber that has a terrestrial connection to an earth station 3 via a
local station and a transit station, and a called subscriber connected to
an earth station 1, the control station 2 will not transit it although it
takes part in the establishment of the speech circuit.
FIGS. 2 and 3 show signalling frames for the signalling between the earth
stations 1 and the central control station 2 in FIG. 1. According to the
example the earth stations 1 are divided into 10 groups each of the order
of 100 earth stations. In each group the individual earth stations are
allocated a common first signalling frame that has a length of 1000 ms and
respective time slots within the frame according to line a in FIG. 2 to
transmit via a first 4 kHz signalling channel report words RW to the
central control station 2 in the form of bursts. These bursts comprise in
a sequential order shown on line b in the same figure information
regarding carrier synchronism CR, bit synchronism BR, word synchronism UW,
transmitting station TI, message type H, and actual message M, whereafter
a guarding interval G follos to inhibit overlapping between the bursts
from the earth stations 1 because of minor errors in their mutual
synchronism.
The division of the earth stations 1 into groups each of which is allocated
a signalling frame that has a length of 1000 ms implies that an extension
of the telephone system of FIG. 1 to comprise more earth stations 1 can be
accomplished simply by adding one signalling frame of the same length for
each ground station to be added, all the signalling frames allocated the
earth stations 1 being transmitted within separate 4 kHz signalling
channels.
The central control station 2 is allocated a separate second signalling
frame according to line a in FIG. 3 of the same length as the first
signalling frame, 1000 ms, to transmit via a second 4 kHz signalling
channel partly time and synchronizing information in the form of a
reference burst RB and partly command words CW as soon as these are
required by the earth stations 1. The control station 2 is, furthermore,
arranged to trasmit in pauses occurring between the command words CW
continuous synchronizing information S in the form of a bit pattern which
according to the example consists of alternately occurring binary ones and
zeros. When a command word CW is to be transmitted the control station 2
is arranged to indicate this in advance by a temporary change S in the
synchronism information S obtained simply by a repetition of e.g. the
binary digit zero in said bit pattern.
The reference burst RB comprises in a sequential order shown on line b in
FIG. 3 information regarding carrier synchronism CR, bit synchronism BR,
word synchronism UW, transmitting station TI, addressed station ES,
message type H and actual message M. The information regarding the
addressed station ES may refer to all the earth stations as well as to a
selected group among them in order to enable the transmission of a common
command. The command words CW comprise information regarding an earth
station 1 that is selectively addressed, the message type H and the actual
message M.
The information regarding the word synchronism UW comprises a word that is
unique in the signalling frame in FIG. 3 and is detected by the earth
stations 1. A time signal is generated upon the completed detection and is
utilized to insure that the respective burst from the earth stations 1
will arrive at the transponders in the synchronous satellite and at the
central control station within time slots whose time positions are fixed
in relation to the time position of the reference burst RB.
The extra synchronism information S is used to maintain during the pauses
between the reference burst RB and the command words CW the most accurate
synchronism possible in the modems in the earth stations 1 as well as in
the transponders in the synchronous satellite in the telephone system.
FIG. 4 shows a block diagram of the internal structure in any of earth
stations 1 in FIG. 1 and the connection to a subscriber 6 that, according
to the example, has a telephone set equipped for voice frequency keying. A
line equipment 7, that has a two-wire connection to the subscriber 6 and
comprises a transmission bridge of two matched transformers to provide an
intermediate link to a four-wire connection, is provided with an input 8
for incoming speech signals, a first output 9 for signals indicating the
condition of the subscriber 6, which information is received via a line
relay in the line equipment 7, and a second output 10 for outgoing speech
signals and number information from the subscriber 6. A transmitting
signalling equipment 11 has an input 12 connected to said first output 9
of the line equipment 7 and is arranged to transmit the report words RW
via a modem 13, and a receiving signalling equipment 14 is arranged to
receive the command words CW via the same modem 13. A transmitting
frequency synthesizer equipment 15 and a receiving frequency synthesizer
equipment 16 are connected to the second output 10 and to the input 8,
respectively, of the line equipment 7, and are arranged to connect the
subscriber 6 to calling or called sides of an allocated two-way speech
transmission channel via the modem 13 and under control of said command
words CW.
A synchronism detector 17 is connected to a first output 18 of the
receiving signalling equipment 14 to detect the unique word UW in the
signalling frame according to FIG. 3 and to generate the time signal in
the form of a short pulse upon completion of the detection. The pulse is
fed to an input of an edge triggered flip-flop 19 to set it and thereby
activate an AND-gate 20 to supply high frequency pulses from a crystal
controlled pulse generator 21 to a counter 22. A decoder 23 is connected
to the counter 22 and is arranged to supply an activation signal to an
activation input 24 of the transmitting signalling equipment 11 when the
counter 22 has counted an appropriate number of pulses from the pulse
generator 21 to insure that the burst of the report word RW will arrive at
the transponders in the synchronous satellite and at the central control
station 2 in FIG. 1 within a time slot whose time position is fixed in
relation to the time position of the reference burst RB, taking into
consideration also the transmission time from the actual earth station 1.
The activation signal from the decoder 23 is also used for resetting the
counter 22 and the flip-flop 19.
An AND-gate 25 is connected in series with an OR-gate 26 between two input
registers 27 and 28 provided for the transmitting frequency synthesizer
equipment 15 and the receiving synthesizer equipment 16 respectively, and
a second output 29 of the receiving signalling equipment 14. The second
output 29 is arranged for supplying a first control code to the input
registers 27 and 28 obtained via said command words CW and indicating a
called side of a two-way speech transmission channel allocated to the
subscriber 6. An inhibition input of the AND-gate 25 is connected to said
first output 9 of the line equipment 7 to prevent the influencing of the
frequency synthesizer equipments 15 and 16 by said control code if the
subscriber 6 should be occupied, in which case the hand handset is
off-hook and a binary one-signal is supplied on the inhibition input. By
this arrangement time is saved during the establishment of connections
because the reporting from the earth station 1 to the control station 2
can be made after that the connection is established.
The input registers 27 and 28 of the frequency synthesizer equipments 15
and 16 are also arranged to be fed via the OR-gate 26 and a third output
30 of the receiving signalling equipment 14 with a second control code
obtained from the command word CW and indicating a calling side of a
two-way speech transmission channel allocated to the subscriber 6. The
command word CW is in this case generated in such a way that the
transmitting signalling equipment 11 has, via its input 12, connected to
said first output 9 of the line equipment 7, sensed that the subscriber 6
has gone off-hook and has then transmitted a message about the calling
condition at the subscriber 6 to the control station 2 via said report
word RW.
The line equipment 7 is allotted: an AND-gate 31 that has a control input
connected to said first input 9 of the line equipment 7 in order to
beceome activated upon a calling condition at the subscriber 6; a tone
detector 32 that has an input connected to second output 10 of the line
equipment 7 and an output arranged to supply an output signal upon the
occurrence of said number information; an edge triggered flip-flop 33 that
has a set input connected to said third output 30 of the receiving
signalling equipment 14 via the AND-gate 31 and the OR-gate 26 to become
set by said second control code indicating a calling side of a two-way
speech transmission channel allocated to a subscriber 6 and a reset input
connected to the output of the tone detector 32 to become reset by said
number information; and a relay 34 that has a control input connected to
an output of the flip-flop 33 and is arranged to connect a keying tone
voltage KT to said input 8 of the line equipment 7 when the flip-flop 33
is set. The purpose of the arrangement is to interrupt the keying tone to
the subscriber 6 immediately when the number information occurs. If the
interruption of the number information should in known manner be connected
for its decoding which takes place in the control station 2, the keying
tone should be interrupted with a delay of 1/2 second.
The tone detector 32 comprises two tone detector circuits 35 and 36 are
tuned to a first frequency band covering 697-941 Hz and a second frequency
band covering 1209-1477 Hz respctively allocated to said number
information and are connected in parallel between the input and the output
of the tone detector 32 via a respective threshold circuit 37 and 38 and a
respective input of an OR-gate 39. The output of the tone detector 32 is
in addition connected to an activation input 40 of the transmitting
frequency synthesizer equipment 15 to provide transmission therefrom only
for number information or outgoing speech. Because the tone detector 32
reacts selectively on the number information and the speech signals and is
relatively insensitive to background noise it is possible to economize the
total instantaneous transmitting power on the speech transmission channels
in the transponders of the synchronous satellite in the telephone system.
The line equipment 7 is also allotted a flip-flop 41 that has a set input
connected to a fourth output 42 of the receiving signalling equipment 14
arranged for supplying a set signal to the flip-flop 41 obtained from said
command words CW and representing the information that a subscriber called
upon by the subscriber 6 is idle and receiving a ring signal, which
information the control station 2 has obtained by said report words RW
from that earth station 1 to which the called subscriber is connected. A
relay 43 has a control input connected to an output of the flip-flop 41
and is arranged to connect a ringing tone voltage RT to input 8 of the
line equipment 7 when the flip-flop 41 is set. A reset input of the
flip-flop 41 is connected to a fifth output 44 of the receiving signalling
equipment 14 arranged for supplying a reset signal to the flip-flop 41
obtained by said command words CW when the central control station 2 has
received a report from that earth station 1 to which the called subscriber
is connected that this has answered.
A sixth output 45 of the receiving signalling equipment 14 supplies a set
signal to a flip-flop 46 when the command words CW indicate that the
control station 2 has received a report indicating that the called
subscriber is busy, whereupon a relay 47 that has a control input
connected to an output of the flip-flop 46 feeds a busy tone voltage UT to
said input 8 of the line equipment 7. When the subscriber 6 replaces the
handset a binary zero signal is obtained on said first output 9 of the
line equipment 7 which zero signal is inverted by an inverter 48 to form a
reset signal that is fed to a reset input of the flp-flop 46.
The reset signal that is fed to the flip-flop 46 from the sixth output 45
of the receiving signalling equipment 14 is in addtion fed via an OR-gate
49 as a reset signal to the reset inputs of the input registers 27 and 28
for the transmitting and receiving frequency synthesizer equipment 15 and
16 respectively. A reset signal to these input registers 27 and 28 is in
addition obtained from a seventh output 50 of the receiving signalling
equipment 14 when the command word CW indicates that a conversation has
been terminated. This command word CW is generated in such a way that the
transmitting signalling equipment 11 has sensed that the subscriber 6 has
replaced his handset via its connection 12 to said first output 9 of the
line equipment 7 and has transmmitted a report on this to the central
control station 2.
If on the other hand the subscriber 6 should be called upon and be
allocated a called side of a two-way speech transmission channel via the
control code from the second output 29 of the receiving signalling
equipment 14 the same control code is used for setting a flip-flop 51 via
an AND-gate 52 that has a control input connected to the inverter 48. A
relay 53 that has a control input connected to an output of the flip-flop
51 supplies a ringing voltage RS to the input 8 of the line equipment 7
when the flip-flop 51 is set. The flip-flop has a reset input connected to
the first output 9 of the line equipment 7 and is consequently reset when
the subscriber 6 off-hooks the handset.
FIG. 5 shows a block diagram of the internal structure of the central
control station 2 in FIG. 1 which comprises: a modem 54; a transmitting
signalling equipment 55 arranged to transmit said reference burts RB,
synchronism information S and command words CW via the modem 54; a
receiving signalling equipment 56 arranged to receive said report words RW
via the same modem 54; and a control equipment 57 that contains a central
processor unit CE, a program memory PM and a data memory DM and is
connected to the transmitting and receiving signalling equipment 55 and 56
respectively, and is also arranged to generate said command words CW and
address them selectively to the earth stations 1.
In the central control station 2 an operator 58 at a switch board 59 and a
decoder 60 for said voice frequency keyed number information are connected
to a respective data input 61 and 62 of the control equipment 57. The
decoder 60 is in addition connected to the switchboard 59 to provide the
operator 58 with a visual display of the decoded number information. A
receiving frequency synthesizer equipment 63 is arranged to connect the
operator 58 at the switchboard 59 and the decoder 60 to called sides of
two-way speech transmission channels allocated to calling subscribers in
the telephone system. The receiving frequency synthesizer equipment 63 is
then controlled by a control code written into an input register 64 by the
control equipment 57. The latter can through the described arrangement
receive verbal as well as voice frequency keyed number information from
the calling subscribers via their allocated speech transmission channels.
A transmitting frequency synthesizer equipment 65 is arranged to provide
the operator 58 at the switchboard 59 with a wo-way connection to the
calling subscribers under control of a second control code written into a
second input register 66 by the control equipment 57. The operator 58 at
the switchboard 59 has in addition a two-way connection 67 to the control
equipment 57 in order to be able to take an active part in the
establishment of speech circuits between the subscribers in the telephone
system. This in combination with the fact that the operator 58 is able to
talk with the calling subscribers makes the telephone system in FIG. 1
very flexible and also gives it the capacity to serve illiterates.
It is to be noted that if the number information is supplied from the
calling subscribers in the form of dialling pulses, these can be converted
to a voice frequency code for transmission via the speech transmission
channels allocated to the subscribers in such a way that they are arranged
to key a tone voltage by means of a relay in the line equipment 7 in FIG.
4. The tone code that the decoder 60 in FIG. 5 shall decode is in this
case received in the form of a number of tone bursts corresponding to the
number of dialling pulses.
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