Note: Descriptions are shown in the official language in which they were submitted.
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TITLE
DIGITAL CONCENTRATOR FOR ~SE WITH A
DIGITAL TEL~PHONE OFFICE
BACKGROUND OF THE INVENTION
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(l~_Field of the Invention
The present invention relates to digital
telephone switching and more particularly to a digital
concentrator for use with a digital telephone office.
The digital concentrator allows the number of sub-
scriber stations connected to a Tl line to exceedthe capacity of the Tl line based on the probability
of occurrence of simultaneous requests for service~
Remote subscriber stations are thus provided with
the full range of customer features associated with
a digital telephone office. Thus an economical means
is provided for connecting remote subscriber stations
to a class 5 telephone office.
(2)_ Description of the Prior Art
Concentration systems are old and well
known. Typically they require a concentrator located
near the subscriber stations and an expander located
in a central office. Such concentrators and expanders
are of an analog nature and require one appearance
on the output of the expander for each appearance
on the input of the concentrator. Thus the connection
between these subscriber stations and the telephone
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office are minimized, but such an arrangement requires
the use of an expansion stage. Further/ such analog
concentration systems are not adaptable to digital
telephone offices unless the analog signals from the
output of the expansion stage are converted to digital
format for use by the digital telephone office.
Accordingly, it is an object of the present
invention to provide a digital concentrator which
can be connected directly to a digital telephone
office without the use of an expansion stage or an
analog to digital interface.
SUMMAR~ OF THE INVENTION
The present invention provides for digital
concentration of connections between remote subscriber
stations and a Tl channel connected to a digital
telephone office. Thus it extends the full range
of features of a digital telephone office to rural
subscribers by allowing the number of subscriber
stations, connected to a Tl line, to exceed the capacity
of the Tl line. Based on probability of occurrence
of simultaneous requests for service a 4 to 1 con-
centration stage is practical. Thus 96 subscriber
stations can be connected to one Tl line via the
digital concentrator.
The digital concentrator includes a Pulse
Amplitude Modulated (PAM~ bus connected to a plurality
of line circuits via associated sample and hold circuits,
filter circuits and hybrid circuits. This PAM bus
is further connected to a non-linear A/D-D/A converter
which converts the PAM signals to Pulse Code Modulated
(PCM) signals. A digital trunk interface is connected
to the A/D-D/A converter and provides transmission
of the PCM signals to an associated digital telephone
office. Each sample and hold circuit samples the
analog output of an associated filter and extends
the sample through it so that the A/D converter has
sufficient real time to make the conversion. An analog
gate is enabled by a decoder which decoded data from a
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memory which was loaded by a processing unit. The
decoder operates to selectively enable the connection
of the various line circuits to the digital trunk
interface.
The line circuits are connected to the PAM
bus according to the sequence of addresses read from
the memory. When a subscriber station goes off-hook,
its associated line circuit extends a signal to the
processing unit which detects this request for service
and communicates the line circui~ number to the tele-
phone office via the digital trunk interface. The
telephone office responds by transmitting the Tl line
channel number to be assigned to this line circuit
and the processing unit loads the designated memory
slot with the line circuit address. The memory se-
quentially displays the line circuit addresses which
are decoded and used to enable the corresponding line
circuit connection to a non-linear A/D and D/A con-
verter which converts them from PAM format to PCM
ormat. These signals are then transferred to the
digital trunk interface which inserts them in the
Tl channel data stream and transmits them to an as-
sociated digital telephone office.
When the next line circuit address is read
out of the memory the decoder again enables signals
from the associated subscriber station to be gated
to the A/D-D/A converterO Thus each subscriber station
which is busy is given a time slot in the sequence
defined by the position of its address in the memory
during which its signals are transmitted to the tele-
phone office. A clock circuit is used to control
the rate at which addresses are read out of the memory
and thus also determines the duration of the time
period during which each subscriber station is enabled
through the PAM bus. The digital trunk interface
thus inserts the PCM signals associated with the time
slot for each subscriber station in a predetermined
position in a Tl line. A Tl line is a repetitive
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transmission of 193 bits during each 125 microseconds.
These 193 bits include a framing bit and 24 signals
represented by an 8-bit code of each signal.
Thus 96 subscriber stations are connected
to a digital telephone office via a single Tl line
and are provided access to a channel on a request
for service basis.
DESCRIPTION OF T~E DRAWING
The single figure of the accompanying drawing
is a block diagram of a digital concen'crator for use
with a digital telephone office in accordance with
the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to the accompanying drawing,
the diyital concentrator of the present invention
is shown connected between a plurality of subscriber
stations and a digital telephone office. Line circuits
are shown connected between associated subscriber
stations and their associated sample and hold circuits
(15-17) which are connected to analog gate circuits
23, 25, 27. The line circuits are further connected
to analog gate circuits 24, 26, 28. Non-linear A/D-D/A
converter 30 is shown connected between PAM bus 29
and digital trunk 40. Data link 50 is connected be-
tween digital trunk 40 and unipolar/bipolar converter60 which is connected to an associated digital tele-
phone office. Processing unit 100 i~ shown connected
to data link 50 and the line circuits. Line circuits,
sample and hold circuits, analog gate circuits, A/D-D/A
converters, digital trunks and unipolar/bipolar con-
verters are well known in the field of digital tele-
phone offices. Data link 50 is a bit stream merging
circuit. Such circuits are old and well known and
have long been used to merge supervisory data into
the supervisory channels of a Tl frame, i.e. the eighth
bit of each channel in every 6th frame. Data link
50 can also be a novel circuit which is disclosed
in our copending application SN 366,695-6 titled,
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Digital Satellita Telephone Office, which was filed
on December 12, 1980. Processing units are also old
and well known and any suitable compute~, mini-computer
or microprocessor can be used.
Processing unit 100 initially detects a
request for service from an off-hook subscriber station
via its associated line circuit, particularly the
make contacts of relay 1. Processing unit 100 then
con~unicates the identity of the off-hook subscriber
station to an associated telephone office via data
link 50 which sequentially inserts each bit of this
communication sequentially into the framing pattern
of the Tl line data stream generated by digital trunk
40. Each Tl line data frame includes 193 bits which
comprise 24 channels of 8-bits each plus a framing
bit. Since the terminal framing bit is only used
in every odd numbered frame, the framing bit position
is available for supervisory signaling in every even
numbered frame and data link 50 inserts the bits
transmitted between processing unit 100 and the digital
telephone office in that spare framing bit position.
The telephone office responds by transmitting
the Tl channel number to be assiged to the off-hook
subscribar station. In this manner, the digital tele-
phone office knows which subscriber sta~ion is trans-
mitting in which Tl channel and thus there is no need
for an expansion stage to provide an appearance for
each subscriber station at the telephone office.
Processing unit 100 then loads memory 22
witb the address of the off-hook line circuit to be
connected ~o PAM bus 29. Clock 20 then causes the
addresses to be read out of memory 21 sequentially
and decoder 22 operates to generate an individual
enable signal for each sequential address decoded
from memory 21. Analog gating circuits 23 through
28 thus becoma enabled when their connection to decoder
22 is enabled.
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For example, when the subscriber station
connected to line equipment 11 goes off-hook it ex-
tends a loop signal to line circuit 11 which converts
it to a 4-wire signal via hybrid 2, amplifies it via
S amplifier 3 and filters it with filter 5. This signal
i5 then sampled and held by sample and hold circuit
15 which transfers it as sampled to gate circuit 23.
However gate circuit 23 is not enabled until clock
cireuit 20 causes the address associated with line
equipment 11 to be read out of memory 21, and decoded
by decoder 22, thus enabling gate circuit 23. Pro-
cessing unit 100 previously loaded the address of
the off-hook line circuit in the memory slot identified
by the telephone office. Upon being enabled gate
circuit 23 transfers the signal from line equipment
11 to PAM bus 29. The signal from line circuit 11
thus remains extended via gate circuit 23 and PAM
bus 29 to A/D-D/A converter 30 as long as its asso-
ciated address is decoded by decoder 22.
When the clock circuit 20 generates the
next clock pulse, memory circuit 21 displays the next
sequential line circuit address which is decoded by
decoder 22 and thus causes the associated gate circuit
to become enabled while gate circuit 23 is disabled.
Thus the signal lead from line circuit 11 will no
longer be extended tG A/D-D/A converter 30 but the
signal lead from the next sequentially enabled line
eircuit will be extended via an associated enabled
gate eircuit and PAM bus connection 29 to A/D-D/A
eonverter 30.
In this manner each connected subscriber
s~ation generating a request for serviee is given
a time slot as defined by the duration of the clock
pulses generated by clock circuit 20 during which
they have access to A/D-D/A eonverter 30 via an asso-
ciated gate circuit and PAM bus connection 29. A/D-D/A
eonverter 30 then eonverts the Pulse Amplitude Modula-
tion (PAM) signals received from PAM bus connection
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29 to Pulse Code Modulation (PCM) signals which are
extended to digital trunk 40. Digital trunk 40 then
inserts each 8-bit PCM signal from A/D-D/A converter
30 into one of the 24 8-bit channels defined on the
Tl line. Therefore clock circuit 20 can enable a
maximum of 24 subscribers stations during each Tl
line time period. Data link 50 transmits these signals
to unipolar/bipolar converter 60 which converts them
from the unipolar ~ormat used in the digital concen-
trator to the bipolar format used for transmissionbetween telephone offlces.
Similarly the associated digital telephone
office can transmit PCM signals to the digital con-
centrator which will be received by the digital trunk
interface. A/D-D/A converter 30 then converts these
PCM signals into PAM signals and applies them to PAM
bus 29. Again PAM bus 29 must be enabled in order
to transmit signals and therefore when clock circuit
20 causes memory 21 to display a line circuit address
decoder 22 enables the decoded analog gate circuit
(e.g. 24, 26, or 28), which transmits these signals
to the connected line circuit and consequently to
the connected subscriber station.
The digital concentrator makes this con-
nection economical since it allows the number ofsubscriber stations connected to a Tl channel to e~-
ceed the capacity of the Tl channel based on the low
probability of simultaneous requests for service.
Rural tel~phone subscribers are thus provided
with full feature telephone service through use of
a digital concentrator connected to a remote digital
telephone office.
It will be obvious to those skilled in the
art that numerous modifications of the present invention
can be made without departing from the spirit of the
invention which shall be limited only by the scope
of the claims appended hereto.
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