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BACKGROUND OF THE INVENTION
The invention relates to data transmission systems and particularly to data
transmission between a central office terminal for a telephone system and
remote customer premises, each of which is connected to the telephone
system by conventional telephone lines.
Initial concepts for interconnecting residents of a city concentrated on a
"wired city" concept whereby integrated digital services would be provided
to each subscriber's premises over fiber optic or coaxial cable networks.
In recent years however, emphasis has shifted to integrated voice and data
transmission systems capable of operating over telephone subscriber loops
which are in place today.
Data voice modems are in existence which permit the telephone line to be
selectively used for analog voice transmissions over the telephone handset
or for the transmission of data. Similarly other data transmission schemes
such as FSK, PSK, and AM modulation techniques have been developed. These
technologies have inherent limitations such as cross talk, data quality,
error rate, signal strengths and security.
Spread spectrum technology, which was originally developed for military
uses, has been applied in a few recent developments to overcome the
inherent limitations of previous technologies. U.S. Pat. Nos. 4,425,642,
4,426,661 and 4,475,208, show prior applications of spread spectrum to
data transmissions. It is not believed, however, that any of the prior
developments has been sufficiently integrated into a telephone subscriber
network such that all subscribers on all exchanges can be connected to not
only the telephone utility but any other utility or service organization
including those which transmit various forms of data and information to
subscribers.
SUMMARY OF THE INVENTION
Using modular design, the system polls up to 5,000 subscribers per module
upon command. Transmitting at a 75 baud rate each module polls over 30
subscribers per second. Transmission is half duplex with full error
correcting protocol.
System operation uses the concept of packetized data and forward error
correction techniques throughout the various modules of the system. Signal
formatting and protocol are also optimized to assure that data is
correctly exchanged between the inquiring utility and the remote
subscriber premises. In order to handle 10,000 subscribers per telephone
exchange with the system, various multiplexing techniques are employed as
will be explained in greater detail below.
The various utility host computers preferably sequentially interrogate
subscribers by telephone number. Communication between the utility host
computer and the central office of the telephone company is by modem on a
leased line. A network controller in the telephone central office formats
data that is to be transmitted to or received from subscribers. The
network controller is connected to one or more Central Switching Units,
each consisting of common equipment modules (CEM) card which establishes a
protocol to communicate with up to one hundred central office module (COM)
circuits which are arranged four to a card in a twenty-five card rack or
five to a card in a twenty card rack, using a unique protocol. Status,
control commands and data are transmitted in eight bit words between the
CEM card and each COM circuit. The transmission includes an acknowledge to
be sure that data and control signals are transmitted without error to the
COM card.
The COM cards are connected to solid state electronic cross connects in
groups of fifty which are accessed in the manner of a scanner multiplexer
which interconnect 5000 subscribers to each rack of central office
modules. Thus to communicate with all 10,000 subscribers on a network two
solid state electronic cross connect scanners are utilized. Each
subscriber has a remote terminal unit which is connected to the telephone
line, all of which are wired to the input leads/lines for the solid state
electronic cross connects.
Communications between the COM cards through the solid state interconnects
to the remote subscriber and communications from the remote subscriber are
achieved with spread spectrum signals which include a 7-4-1 Hamming code
for forward error correction as well as bit interleaving so that noise
impulses on the line do not destroy or affect the correct receipt or
transmission of data. Transmission is also achieved by longitudinal
transmission which puts the data signal on both the tip and ring lines in
phase. Since analog voice data is transmitted 180 degrees out of phase,
differential transmission, the data when received on the voice circuit is
rejected by common mode rejection. Data is taken off the telephone lines,
both at the transmitting and the receiving end by summing the signal from
both lines. Since the voice data or other analog transmission is 180
degrees out of phase that information is cancelled and the actual data
transmission, when summed, produces a 6 db gain.
Other unique signalling techniques to achieve the objects of the invention
are set forth in more detail below.
DESCRIPTION OF THE DRAWINGS
FIG. 1 is a basic schematic of the data telemetry or communications network
as used with a meter reading and load management system.
FIG. 2 is a block diagram of the basic units as apportioned between the
subscriber premises, the telephone central office and the service agencies
or utilities.
FIG. 3 is similar to FIG. 2 showing how the various elements are
interconnected by twisted wire pairs, modems and the like.
FIG. 4 shows the various levels of control hierarchy established by the
modules of the invention.
FIG. 5 shows the central switching unit of FIGS. 2 and 3 which is divided
into two essential components, the common equipment module (CEM) and the
central office modules (COM).
FIG. 6 is a perspective view of the Central Switching Unit showing how the
CEM and COM cards are arranged in a twenty-three inch rack in the Central
Office of the telephone office.
FIG. 7 consisting of FIGS. 7a and 7b, 7a--a-7a-d; 7b-a-7b-d are detailed
schematics of the common equipment module (CEM).
FIG. 8 is a block diagram of one of the central office modules (COM).
FIGS. 9a-d are schematic diagrams of a central office module.
FIGS. 10a-b are detailed schematics of one of the solid state electronic
cross connect.
FIG. 11 is a block diagram of a remote terminal unit.
FIGS. 12 and 12a-e are schematic diagrams of the remote terminal unit.
FIG. 13, comprising FIGS. 13a through 13c, are flowcharts of computer
programming used in the data telemetry system; and
FIG. 14, comprising FIGS. 14a through 14d, comprise representations of data
format and data protocol used in communicating information with use of the
invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The disclosed system of this invention is an extremely flexible data
telemetry system which can be used to communicate, both transmit and
receive, with a single telephone subscriber or to all subscribers on a
telephone network. A basic object of the invention is a means and method
to transmit eight bit data packets from one or more sources to any
telephone subscriber having a remote terminal unit 21 and to receive
similar eight bit data packets from that remote subscriber. As disclosed,
the invention is shown in connection with a meter reading and load
management system. It should be understood however, that this is for
purposes of a complete disclosure and a preferred embodiment and that the
communications network or data telemetry system can be utilized in other
ways and for other purposes.
A complete telemetry system according to the invention to service the ten
thousand possible subscribers in a telephone exchange, xxx-0000 to
xxx-9999 would include the following component modules, each discussed in
more detail below. A network controller 25 (FIGS. 2,3) connects to the
host computer 28 (FIGS. 1,3) to receive and transmit telemetry data
packets 30. The basic telemetry packet, in an eight byte data block 30,
consists of three bytes of pay load, three bytes of address, one byte
which is a status control character and a one byte check sum. These
packets are communicated to and from remote terminal units 21 for each
subscriber. Two central switching units 34 (FIGS. 2,3, 5-9) are connected
to the network controller 25 with a common equipment module 38 (FIG. 7)
and one hundred central office module circuits 40 (FIGS. 8,9). Two hundred
solid state electronic cross connect 44 circuits (FIG. 10) are used, each
scanner consisting of fifty solid state electronic cross connect relays 48
(FIGS. 3,10), each solid state electronic cross connect circuit 44
connected to one central office module 40 circuit and to fifty
subscribers. Each of the ten thousand subscribers has a remote terminal
unit 21 (FIG. 12) which is programmed by the user or utility to receive
the payload and address data and respond as programmed into the remote
terminal unit 21.
FIG. 4 shows the four layer control hierarachy for half duplex exchange of
data between a utility company computer 28 and remote terminal units 21
through the central office of a telephone exchange. The data transmissions
between the utility 28 and the subscriber 21 via the telephone loop are
eight byte messages 74. The raw data burst 74, in its most fundamental
form, is sixty-four bits long (8 bytes).
The data paths for the entire system generally is the core raw data burst
74 surrounded by routing, addressing and error correcting information.
The remote terminal unit 21 is connected to the peripherial equipment 60
and polls the equipment 60 by means of the eight byte messages 74
including three bytes of address and three bytes of information which are
established by the utility company.
The remote terminal unit 21 and the central office modules 40 have a
specific half duplex data exchange. This exchange is initiated by the
central office module 40 selecting a given subscriber through the
channel's solid state electronic cross exchange interface 44. The central
office module 40 then sends the down loaded eight byte block 74 to the
remote terminal unit 21. If the subscriber remote terminal unit 21
receives the block correctly, the remote terminal unit 21 responds with
the requested data 74 again in its eight byte block. At this point the
central office module 40 verifies the incoming data 74 and issues an eight
byte acknowledgment block. The central office module 40 then flags that
the data exchange has taken place and data from the remote terminal unit
21 is then transmitted back to the utility 28 through the network
controller 25.
The common equipment module 40 multiplexes and demultiplexes the data 24
between each subscriber channel on the central office module cards 40 and
the network controller 25. The common equipment module 40 buffers data 74
for transmission to and from the remote terminal unit 21 and the network
controller 25. The common equipment module 40 also generates diagnostic
and problem indications for access by the network controller 25.
The common equipment module card 40 handles all bus control and data
transfer. ALE transfers are always two cycles for each data transfer. The
ALE (Address Latch Enable) line determines if the data bus contains
address/control 150,160 or data words 162. The address/control 160
transfer is a write to the bus only. As indicated, the data 162 can be
bi-directional transfer.
The solid state electronic cross connects 44 are located at the telephone
company central office. They connect the central office central switching
units 34 one common equipment module 38 and one hundred common office
modules 40 to the telephone company main distribution frame (MDF). There
are one hundred solid state electronic cross connects 44 per central
switching unit 34, one per channel 40. Each solid state electronic cross
connect 44 interfaces at the main distribution frame with fifty subscriber
telephone lines.
Since the remote terminal units 21 interconnect to the subscriber line
prior to the telephone hand set, data transfers can occur regardless of
whether or not the telephone hand set is on or off hook and, therefore,
regardless of whether or not the telephone line is being used for analog
exchange of voice information.
Referring to FIGS. 1-3, the use of the data telemetry system of the present
invention, and the component elements of the system are shown in block
diagram form. When used as a meter reading and load management system, the
originator of services, electric, water and gas utilities can each
communicate with a network controller 25 through conventional high speed
modems 54, preferrably on a leased or dedicated line 56. The subscriber is
identified by his or her unique subscriber address and while an eight byte
block of data is described in connection with the preferred embodiment,
the system is currently configured to access up to 32 bytes of transmitted
and received data for remote peripherals 60 to be read or controlled.
As represented on the left side of FIG. 1, peripherial equipment 60 which
might be utilized on such a system includes LCD displays to display data
or other information from the utility company, meter readings such as
electric, gas, water and the like using pulse counting or digital encoding
techniques. The reading of temperatures and voltages or other analog
inputs, switching control and status for high power consuming appliances
such as the water heater, air conditioner, pool pump, dryer and the like.
Any information which is susceptible of being converted to digitized data
reading or switchable components lend themselves to operation and control
according to the present invention.
The utility host computer 28 is connected to the data modem system through
one or more network controllers 25 such as a Digital Equipment Corporation
PDP 11/73 computer. More than one network controller 25 can be used for
high loading of the system. Additional network controllers 25 may also be
used in a redundant manner to assure proper operation of the controller 25
wherein the controllers 25 compare receive and transmit information and a
correctly operating controller 25 assumes control in the event of
disfunction of any of the controllers 25.
As shown in FIGS. 1-4, a fully developed system configured to serve 10,000
subscribers on a telephone exchange would include a single network
controller 25 for each exchange and two central switching units 34, one
for each 5000 subscribers.
Shown in FIG. 5, is a single shelf 34 showing twenty central office module
cards each card having five central office module circuits 40 all of which
are controlled by a single common equipment module or controller card 38
which is interconnected to each of the central office module cards along a
back plane bus.
Referring now to each of the drawings the structure, function and operation
of the components of the system can be described and understood.
The utility host computer or computers 28 interconnect with the network
controller 25 using high speed modems 54 and RS 232 interface connections
to be sure the data has passed between the computers 25,28 without
interruption. The network controller 25 receives the subscriber
identification, the address, and in the eight byte data packet, the
particular peripherial equipment 60 that is going to be read or
controlled. From an internal data table in the memory associated with the
network controller computer 25, the network controller assembles and
formats data to be transmitted to the appropriate central switching unit
34.
The format of the data is set forth in FIG. 14a. As illustrated in FIG. 14a
each block of data consists of eight bit bytes 74 which are sequentially
transmitted to the appropriate central switching unit 34. The bytes
include a start byte 76, the shelf number 78 which identifies one of the
central switching units 34, the channel number 80 which identifies one of
the central office module circuits 40 on the shelf 34, the user number 82
which is one of the 50 lines connected to the associated solid state
electronic cross connect 44, and then sequential bytes of data 74. These
data bytes 74 when transmitted include in addition to data to be
transmitted, polling instructions such as control words to read meters 60
or to perform switching functions at the peripherial equipment 60 or
remote subscriber location or to display data.
When received from the remote terminal units 21, data is passed by the
central office module circuit 40 in the same format to the network
controller 25, with the eight bytes of user information 74 including the
meter readings, acknowledgement of switch or control information, status
and the like. Following the user data a status byte 86 is included for
control purposes as well as, a check sum 88 to be sure that the data and
information has been transmitted correctly between the network controller
25 and the central office module 40. If the check sum 88 does not indicate
correct transmission or the receipt of the correct information from the
utility, the data 74 is retransmitted.
The common equipment module (CEM) 38 comprises a single card, a detailed
schmematic of which is shown in FIGS. 7a and 7b, and a back plane of the
shelf 34 which interconnects the CEM card 38 to each of the twenty or
twenty-five central office module (COM) cards 40. The CEM card 38 operates
as a master control in a master slave relationship for all communications
with the remote terminal units 21. The CEM card 38 multiplexes and
demultiplexes the data between each subscriber channel and the network
controller 25. The CEM 38 also buffers data for transmission to and from
the RTU's 21. The CEM 31 is also used to generate diagnostic and problem
indications for access by the network controller.
The CEM card 38 consists of an 8085 microprocessor 104, programming in an
erasable programable read only memory 106, a multiplexer/demultiplexer 108
to switch between addressable registers on the card and between control
and data transmissions, and a static random access memory 110 for the
storage and manipulation of data received and to be transmitted. The
circuitry also contains two counters 112,114, one 112 which is part of a
watch dog circuit and the other 114 of which is based on the clock to
generate the baud rate for the transmission and receipt of data. Two
input/output ports 118,120 are also utilized, one 118 for addressing and
the other 120 for memory manipulation. A dual universal asyncronous
receiver/transmitter chip (DUART) 125 is used with one UART 128 connected
to interface with the network controller and the other UART 129 connected
to interface with the solid state electronic cross connect circuits 44.
Referring to FIG. 7a, the various registers are shown. A first latch 130 is
used as a control register which is interconnected to a pair 132,133 of
one of sixteen decoders to strobe specific COM cards 40 when a
communication is to take place on that card 40.
A failure analysis circuit 135 is interconnected with the microprocessor to
indicate programming problems and circuit or line failures. Two latches
142,143 are interconnected to form a data bus register. A fourth latch 145
is used as a switch register for a DIP switch 148 into which is manually
programmed the shelf 74 number and the baud rate for communications.
The mode of operation or protocol of operation of the CEM card 38 is
essentially as follows. The network controller 25 provides data to the CEM
card 38 in the form shown in FIG. 14a. The data is received by the CEM
card 38 and stored in the static ram 110. The check sum 88 is verified to
be sure the data has been received accurately. The dip switch 148 is read
through the switch register 145 and the shelf 34 number 78 transmitted by
the network controller 25 is verified by the microprocessor 130. The
microprocessor 130 calculates the COM card 40 to be strobed through the
control register 130 and one of the one of sixteen decoders 132,133 by
dividing the channel number 80 by four or five depending upon the number
of COM circuits 40 on each card. The user number 82 is connected through
the DUART 129 to set up the solid state electronic cross connect 44 which
will be discussed below. Thereafter data is transmitted to the COM cards
40 using a sequential protocol which transmits command words followed by
data words in the following manner.
Initially status of the particular channel, that is whether or not it is
busy, is checked using the eight bit transmission 150 shown in FIG. 14b.
As shown in FIG. 14b, the protocol of communication between the CEM card
38 and the COM cards 40 is initiated with the first eight bit byte control
word 150. The first control word 150 is a status inquiry. Thereafter,
transmissions are alternated with the use of the ALE line 156 with two
cycles for every data transfer. The ALE line determines if the data bus
contains address/control words 160 or data 162. The data can be a
bi-directional transfer. All transfers are controlled by three control
lines, the ALE line 156, the ready line 165, and individualized card
strobes 168.
With control or status words 150,160, the most significant bit, bit 7 is
set to a zero or one with zero being a "read" command and one being a
"write" command. The least significant bit is the control bit and bits one
through three indicate the channel, one through five on a twenty card rack
and one through four on a twenty-five card rack. Alternate bytes 162
contain eight bit data to or from the user-subscriber as indicated in FIG.
14b.
All data outputs 150,160,162 must be stable until the ready line 165 goes
low. All data 162 reads must not be read until the ready line 165 goes
low.
Therefore, data transfers take place as follows.
The status control word 150 is sent to the COM card 40 which inquires
whether or not the channel is busy. If not busy, signified by an
acknowledge, a first command word 150 is latched into the data bus
register 142,143. The particular COM card 40 to be accessed is latched
into the control register 130 to create a low strobe 168 from the one of
sixteen decoders 132,133 to that card 40. The ALE line 156 is sent low to
indicate a command word 150,160 is to be transmitted and after the bus is
stabilized, the ready line 165 is clocked low. This transmits the command
150,160 word indicating the data transfer to take place and the channel
upon which it is to take place. After the particular COM circuit 40 has
been prepared to receive the data word 162, the data word 162 is then
clocked into the COM module 40 using the same process and procedure. As
shown in FIG. 13a, all transmissions 150,160,162 to and from the CEM card
38 to any COM card 40 are acknowledged 170 for purposes of error-free
transmission so the data does not get lost while communicating between
modules.
When the data 162 has been transmitted fully from the CEM card 38 to the
selected COM channel 40, a data transmission "33" takes place signifying
to the COM card 40 that the data 162 can be and should be transmitted by
the COM card 40 through the solid state electronic cross connect 44 to the
addressed remote terminal unit 21.
Referring to FIGS. 8 and 9, the method of achieving the transmission with
the central office modules 40 can be understood. As shown in FIG. 9, a
microprocessor 190 with internal memory receives alternately each command
word 160 and data byte 162 on the eight line data bus 195, the
microprocessor 190 being conditioned to do so by the ready line 165, the
ALE line 156 and the strobe line 168 to the card. The programming for the
microprocessor, shown in FIG. 13b, formats the data for transmission in a
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