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Description  |
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TECHNICAL FIELD
This invention relates to an interactive two-way data service network for
conveying synchronously timed digital messages point to point throughout
the network, and more particularly it relates to local area base station
cell sites subdivided into zones for processing communications within the
zones from subsets of subscriber units of a configuration for integrating
communications into a nationwide network of interconnected base station
cell sites for point to point communications with identified remote
subscriber units, wherein the subscriber units comprise low energy,
stationary and mobile, digital transceivers, which may be battery
operated.
BACKGROUND ART
A wireless interactive video system disclosed in U.S. Pat. No. 4,591,906,
May 27, 1986, Fernando Morales-Garza, et al. provides for real time
interactive digital communication from a large audience of subscribers in
urban areas in the vicinity of a central television transmitting station.
The Federal Communications Commission (FCC) has now established in the
U.S.A. communication standards for such interactive video data service
allocating wireless transmissions in the 218-219 MHz band for FCC
licensing for public use in assigned local base station areas authorizing
low power subscriber interaction units of maximum effective radiated power
under twenty watts.
There has been no known interactive video data service system available
heretofore that has the capability of servicing an assigned base station
area with subscriber units transmitting in a milliwatt power range. With
such an improved system, battery powered, portable subscriber units,
suitable for such functions as meter reading, would become feasible with
low battery drain, permitting interactive digital communication in local
areas or nationwide.
Wireless interactive video data service is provided without telephone lines
or cable systems over a nationwide network of base stations in the manner
disclosed in U.S. Pat. No. 5,101,267, Mar. 31, 1992, Fernando Morales, by
way of satellite transmissions between local area base stations and a data
center.
This nationwide communication capability permits live video programs viewed
nationwide, such as world series baseball games, to become interactive for
individual subscriber participation. Thus, mass communications over a
substantially real time communication system with such large urban area
audience participation that would jam any existing public telephone
switching network capability are made feasible.
Each local base station in such a nationwide communication system must be
capable of interacting within designated license restrictions in the
presence of peak local audience participation without significant
switching delays to establish substantially real time interactive two-way
connections over a network processing an audience of very large numbers of
participants wishing to communicate substantially simultaneously.
Prior art two-ay radio transmission network technology, as represented for
example by portable telephone communication systems, is generally
incompatible with efficient substantially real time communication in the
presence of heavy subscriber activity. This occurs because in telephone
systems switching and connection operations must be made compatible with
switching instructions from subscriber instruments with coded audio tones
at audio frequencies accompanying analog audio messages. Thus with long
numeric identification numbers for nationwide long distance connections,
typically of ten decimal digits, which must be manually entered while
busying lines to complete point-to-point connections as a part of the
interconnecting signal data, switching circuits are engaged for very long
periods of time inconsistent with substantially real time connections or
heavy traffic conditions. Accordingly busy signals are encountered often
to restrict the size of a participating audience for immediate connection
and the follow-up contention for a line requiring re-dialing is
frustrating to the potential using audience. Thus, interactive response
that requires telephone exchange communications tends to be delayed and
discouraging to participants, and introduces the critical problem of
identifying and communicating interactively between subscribers in real
time without jammed exchanges and the frustration of encountering busy
signals and starting over with a new attempt to communicate.
Similarly, even with the restricted amount of digital data that might be
transferred in digital paging system messages, where typically some
messages only indicate a short fixed length message such as a calling
telephone number, there is little possibility of approaching real time
communications in the presence of heavy traffic because of the
complexities of the necessary telephone switching networks employed for
conveying messages.
In order to process digital information accurately, efficiently and
privately it is necessary to precisely time and organize the digital data
and accompanying commands. For real time two-way digital communications
with large audiences wanting prompt access to the message conveyance
system or network, synchronous signal timing becomes critical and
absolutely necessary for real time interactive communication. In general,
audio telephone communications are of an analog nature not critical to
timing and are conveyed asynchronously. Thus, prior telephone art signal
communication systems are unsuited for adoption in interactive video data
systems that convey private point to point digital messages on a real time
basis for large audiences.
Typical patents relating to nationwide communications employing such prior
art telephone switching techniques are now briefly referenced as
representative of the present state of the are utilizing analog (voice)
telephone communication networks and cellular technology to accommodate
low-cost mobile battery-operated subscriber units operable within local
cellular subdivisions.
In the telephone arts: Freeburg, U.S. Pat. Nos. 4,481,670, Nov. 6, 1984 and
4,525,861, Jun. 25, 1985 and 4,550,443, Oct. 29, 1985 provide for handing
off best signals from portable radio sets in two-way audio analog
communications between overlapping zones served by different fixed
location cellular transceivers, which in some cases use different
frequency bands for isolating adjacent zones.
In the paging arts, modems are used for connection with a telephone system
for communication and switching over a national network as typically set
forth in Andros, et al. U.S. Pat. Nos. 4,870,410, Sep. 26, 1989 and
4,875,039, Oct. 17, 1989, and therefore are subject to the same switching
system bottlenecks previously described even when short digital only
communication is desired.
It is accordingly an objective of this invention to improve the state of
the art by effectively using licensed interactive communication channels
to provide substantially real time, synchronously timed digital
communications of variable length between geographically separated base
station subscribers of an interactive video data service system. Capacity
for heavy audience participation without substantial delays during peak
loading conditions is essential in a manner compatible with the FCC
licensing conditions for interactive video data service.
It is a further objective of this invention to introduce into interactive
video data service a system providing effective two-way interactive
communications with simplified low-cost subscriber units transmitting in
milliwatt peak power ranges under parameters compatible with FCC licensing
restrictions.
Another object of the invention is to introduce portable digital
communication subscriber units into an interactive video data service
system adapted for local and national communications.
Other objects, features and advantages of the invention will be found
throughout the following description, the drawings and the claims.
BRIEF DESCRIPTION OF THE INVENTION
A base station configuration for interactive data service provides several
interrelated features for improving effectiveness of digital
communication. Such features include (1) a system employing portable
subscriber units of milliwatt transmitting power capacity, and (2)
increasing substantially the number of subscriber units operable at the
base station. Thus, typically 4000 subscriber units at a base station may
be processed for point to point nationwide communication at 5.16 Kbaud
data rate per unit.
A significant advantage of the invention is the capacity to rapidly connect
a very large number of individually identified subscribers at each base
station for parallel communications and connecting new subscribers into
awaiting communication slots without significant delay.
In one embodiment, the features of the present invention are embodied in a
subscriber multiplexing system at the base station that relates
synchronously with a base station carrier signal or the television frames
of a master TV channel. Thus, the communications and switching connections
are synchronized throughout a nationwide network for more efficiently and
promptly processing point-to-point real time communications. Even more
significantly is the corresponding freedom to multiplex digital messages
of variable length from a large number of transmitting subscriber units at
the base station, with the assurance that little access waiting time will
be encountered by subscribers to complete switching connections, even for
nationwide communications.
The base station comprises a central transmitter and data processing site
for processing transmitted digital data to subscriber units within the
base station designated area. A plurality of receive only stations
distributed throughout the region and connected by wire, cable, microwave
link or radio to the central data processing site then process and relay
transmitted digital data from subscriber units within subdivided zones in
the base station designated area. Thus, the base station serves a gridwork
of receiver sub-cell sites distributed at locations permitting reliable
response by subscribers transmitting with milliwatt digit signal levels in
the FCC authorized 218-219 MHz band. Provision is made to process fringe
signals between the different subdivided zones so that low-cost portable
battery-operated milliwatt transmitter subscriber units may be moved
throughout the base station geographical area for reliably performing such
functions as meter reading and data transfer.
The base station system is adapted for communication in a nationwide
network of base stations over a satellite communication network such as
that of U.S. Pat. No. 5,101,267. Thus, the base station data processor
locally segregates, accumulates and formats the messages from individual
subscribers for retransmission over the satellite network to a switching
hub and data processing center with the capacity to locate individual
subscribers in remote base stations over a nationwide network.
BRIEF DESCRIPTION OF THE DRAWINGS
In the accompanying drawings:
FIG. 1 is a block system diagram of a nationwide interactive video data
satellite system embodiment of the invention that provides point-to-point
communications between subscriber response units in local service areas
and with various vendors of goods and services;
FIG. 2 is a diagrammatic view of a local area base cell site system
embodiment of the invention for communications with very low powered local
subscriber units, including mobile or portable units;
FIG. 3 is a fragmental sketch of r-f signal protocol at the base cell site
for permitting communications from a significant number of subscriber
units simultaneously in real time;
FIG. 4 is a diagrammatic sketch of message frames for illustrating the
maximization of data processed at local cell sites;
FIG. 5 is a block diagram of a base cell site to satellite communication
link afforded by the invention to process fixed length frames and
formulate variable length messages from subscribers during an active
connection interval;
FIGS. 6A and 6B respectively are a block system diagram of communication
channels at the base cell site, and a corresponding diagrammatic system
flow diagram for transmitted messages between local subscribers the cell
data center and the satellite connected network of cell sites;
FIGS. 7A and 7B respectively are a block diagram illustrating transit time
characteristics of messages at a base cell site, and a diagrammatic view
of typical communication frames showing relative times at different cell
site communication stations;
FIGS. 8A and 8B are respectively frequency band charts for FCC allocated
frequency bands and subchannel bands for interactive video data services
as employed by this invention; and
FIGS. 9A and 9B are respective block circuit diagrams of subscriber units
for digital only fixed or mobile communication services and integrated
video and digital data service embodiments of the invention.
THE PREFERRED EMBODIMENTS
In FIG. 1 the nationwide interactive network embodying this invention and
setting out in perspective support services and equipment is illustrated
in block diagram format. Thus a set of subscribers at response units 4
communicate over the wireless 218-219 MHz r-f links 5 to either a set of
local remote receivers 20, each connected by a link 21 such as a telephone
line to repeater cell 3, or to a local area base station repeater cell 3,
one of a set of such repeater stations in different geographic locations
for communicating via satellite 1 under control of a data and switching
control center 2. Regional and local service or product participants 7
also communicate with the local area cells 3 and the control center 2. The
basic operation of this system is set forth in said U.S. Pat. Nos.
4,591,906 and 5,101,267. Details of point-to-point switching and
communication throughout the system identified at switch control center 14
and accompanying terminal directory 13, downloading of data and software
from the control center 15, and the processing of billings and
transactions 16, and the corresponding interaction of the memory and
software at the subscriber unit 17 are set forth in co-pending application
Ser. No. 07/889,626, May 28, 1992 for Software Controlled Interactive
Video Network and Ser. No. 07/932,241, Aug. 19, 1992 for Interactive
Satellite Broadcast Network, which are incorporated hereinto by reference
as background material to the extent necessary for providing a full
disclosure of operating details.
In this system therefore, simplified low cost subscriber response units 4
are universally applicable to a wide range of interactive functions by
means of software control facilities. The system furthermore in its r-f
processing system efficiently handles mass data for accommodating very
large system peak load capacity substantially in real time through the
switch control center 14 with typically stores for each subscriber
currently updated information that need not be transmitted with every
transaction such as the directory identification code, name, address,
telephone number and credit card, etc.
An explicit cell site 3 embodiment utilizing a local base station repeater
cell 3 afforded by this invention, which expands the interactive
capabilities and functions of the subscriber response units 4 while
improving performance and reducing cost, is diagrammatically shown in FIG.
2. The outer dotted ring 19 outlines the limits of a local area cell site,
such as may be licensed by the FCC, for interactive video data service.
The cell site embodiment utilizing local base station repeater cell 3
communicates with the satellite system via directed dish antenna 3A, and
transmits digital communications and TV video overlay trigger signals to a
set of subscribers X throughout the assigned territory by way of antenna
8.
A set of typically ten remote, receive-only, fixed-location relay stations
20A-20N are positioned at strategic locations within the cell area. Each
local remote receiver station 20 is connected by cable, microwave or
leased telephone line 21 to the cell site utilizing a local base station
repeater cell 3. Thus, transceiving subscriber units X 4, 4', etc. located
within the subdivided response zones 22 communicate with the local remote
receivers 20 over a significantly reduced transmission path distance
within the subdivided response areas 22, as compared with direct
transmission from a local base station repeater cell to transceiving
subscriber units X 4, 4', etc. This subdivision feature, accordingly for
the first time in interactive video data system provides for reliable
transmission at radiated power levels in the milliwatt region. Distinct
advantages result including less chance for external interference and long
life battery operated portable subscriber units 4 which can be moved
throughout the cell territory (19).
Accordingly, this invention encourages such additional interactive services
in the network as typified by meter reading, and inventory control in soft
drink dispensing machines, etc. in a manner saving so much manpower and
expense as to be viable economically in this type of interactive video
data service system. In the latter two examples, very simple digital
communication subscriber units 4 may be provided without the necessity for
video displays, in the manner later described. Other examples are site
alarms for remote monitoring of open doors, fires, failure, temperature,
etc. Two-way paging services are also thus made available, or telemeter in
location or condition of delivery trucks, etc. Furthermore, with full
service video display TV installation at a subscriber station 4, the
feasibility of moving such remote units to different locations in a house,
office, or car is established. Accordingly this invention is in part
directed to the provision of portable or mobile interactive subscriber
stations and communication units for interactive video data service
systems compatible with FCC standards. With the lower power transmitters
provided, adjustments of power in the subscriber units may be avoided by
simple AGC at the remote receiver terminals. Smaller and portable home
units are also possible. There is considerable advantage of longer battery
life for portable units.
A further substantial advantage to the invention is the ability to handle
point-to-point connections nationwide under peak traffic conditions with
very little subscriber waiting time for access to the system. The system
protocol for reception of messages and response at the subscriber units in
the chart form of FIG. 3 illustrates the number, typically 640, of
subscribers X that can be simultaneously using the system at any cell site
5. With reference again to FIG. 2, thus assume that each of ten fixed
remote receiver stations 20A-20N within the cell area (19) is capable of
processing 64 on-air subscriber units X. This results because the
milliwatt powered subscriber units X are adapted for transmission in a
single one of the ten subdivided areas or zones 22, with provisions
preventing interference with adjacent zones 22A, 22B, etc.
Other system advantages are: (1) that low power subscribers use the system
at outer cell boundaries, thereby reducing chances for inter-cell
interference, (2) that the expansion of the system may occur by adding
subdivided zones as the subscriber base grows, (3) that the passive local
remote receive-only receives have no problems in meeting FCC interactive
video data service conditions, (4) and that capital, power and operating
costs substantially decrease.
With reference now to FIG. 3, to transmit with ten subscriber units X in
the respective zones 22 of FIG. 2, the protocol assigns in a timed
broadcast period 30 a home unit (HU) response time interval 31, at an
accumulated 5.1 kbaud response rate. Each of these switched-in user home
units then transmits a digital message superimposed by modulation on the
218-219 MHz band subcarrier. The broadcast time interval 32 permits the
cell site transmitter 8 of FIG. 2 to broadcast a message including a
(ringing) signal that may include an address code number for activating a
single home unit within the cell area 19 of FIG. 2. Each home unit has a
built in address code that must be used to activate that unit 14 of FIG.
2, and the central data switch control unit maintains a directory of all
such numbers in the nationwide network. The broadcast time interval 33
provides a time gap for checking errors and for providing desirable
control signals. Guard gaps 36 are supplied between successive broadcast
periods 30, also identifiable as r-f frames.
With reference again to FIG. 3, the r-f frames permit transmission at 5.1
kbaud for each of say ten subdivisions 22 of FIG. 2. Thus the cell stores
in a buffer the ten multiplexed home unit data rates to load the buffer at
a 51 kbaud rate. A total data rate for the main cell area 19 of FIG. 2 is
51 kbaud from the ten simultaneous responses from the separate
subdivisions 22 of FIG. 2. Assuming no errors and 1000 bit messages from
each home unit, with 3,000 home units trying to get their message through
ten channels, the waiting time for a "line" would be less than one minute
without contention, thereby minimizing the necessity of "redialing".
The legends in the right hand column show that each remote receiver station
20A-20N is assigned a corresponding communication frequency bandwidth
f.sub.1 -f.sub.n, thereby isolating the communications from subscriber
units X in each subdivision 22 within the cell area 19 as shown in FIG. 2.
The length of the r-f frames 30 is 12.4 milliseconds including a guard
band 36 of about 120 microseconds.
Typical message protocol is illustrated in FIG. 4 for fixed frame message
lengths of 30 bytes of eight bits. This fixed length is important in
minimizing access time to the system under peak load conditions, since
there will be substantially no need air time incurred while a subscriber
is awaiting to be connected or disconnected. Various functional categories
are typically included in the broadcast interval 32 as shown in the
blocks. Of note is the home unit ID section which addresses the unit to be
activated (similar to a telephone number), and the Packet ID byte for
accumulating a sequence of home unit response frames into a packet. All
messages and protocols are consistent with the transmission of data
implicitly as part of a video message during the vertical blanking
interval or transmission over a digital r-f link parallel to a video
channel. However, as will be more particularly set forth later, it is
pertinent to synchronize timed data within the nationwide system, even
taking into account differences in travel time of radio waves (see U.S.
Pat. No. 4,591,906), and for this reason the technique described in U.S.
Pat. No. 4,755,871, Jul. 5, 1988 for Control of RF Answer Pulses in a TV
Answer Back System may be used to synchronize transmissions with the TV
carrier signal from the cell site transmitter and to organize all the
multiplexed timing slots for avoiding idle on-air time. Thus, this system
departs from any former telephone switching system art which is
asynchronously switched.
As seen from FIGS. 1, 2, 4 and 5, the nationwide transmission of messages
from the individual subscriber home units 4, (X) longer than 240 bits
require several frames, with accumulation into packets, identifiable in
the broadcast frame 32. The cell site transmission system 40 thus
processes a set of packets in the manner shown in FIG. 5 to accumulate
subscriber messages of variable length in a set of serial transmissions
for transmitting to the satellite at higher transmission frequency.
Accordingly packet builders 41, 41A, etc. are individually assigned to a
responding one of simultaneously active subscribers until the subscriber's
variable length message of n 592 bit frames is completed, and after
pricing 42 the messages are accumulated 43, synchronously timed 45 and
transmitted to the satellite 44. These accumulated messages are received
at the central data station 2 for switching, adding pertinent subscriber
data and a receiving address and retransmitting over the satellite at a
receiving point, such as a further subscriber or a service provider.
Now FIGS. 6A and 6B relate to the communication sequences within the local
base station repeater cell area (19, FIG. 2) between home units 4, the
cell site utilizing local base station repeater cell 3 and local remote
stationary receivers 20A-20N. Note that the home unit 4 is also designated
as an interactive data appliance (IDA), a general term including
subscriber video stations, digital alarms, or the like, and portable
units.
The data flow chart of FIG. 6B relates to a "set-up" and response sequence
of intercommunications between the respective subscriber units 4, (IDA)
local remote fixed station subdivision receivers 20A-20A, (RR) and the
cell site utilizing local base station repeater cell 3. Synchronization is
controlled by the carrier frequency Tx.sub.a of the cell transmitter upon
which the subscriber unit 4 locks. Then the subscriber unit 4 initiates a
response which includes both the subscriber ID and the cell ID for the
purpose of handoff between cells with portable units or fringe area cells.
The remote receiver 20 receives the subscriber's transmission on its
frequency RX.sub.su, and passes an acknowledgement to the base station
repeater cell 3 for sampling transmission and auditing the transmission
routing. Thus base station repeater cell 3 selects the local remote
receiver 20A-20N, etc. that receives the best subscriber signal. Note that
the local remote receiver 20 receives both the transmissions from the cell
transmitter frequency at Rx.sub.a1 and the communications at its assigned
frequency Rx.sub.su, and similarly the subscriber unit transmits on two
alternative frequencies, one tuned to a particular remote receiver 20
frequency.
The base station repeater cell 3 then relays the best frequency back to the
subscriber unit 4 for tuning in and finishing communications with the best
and only local remote receiver 20. This is the end of the "set up" period
and the start of the transmission period, during which the message bits
are relayed to the base station repeater cell 3 by the local remote
receiver 20 tuned in, and are at the base station repeater cell 3
processed and relayed into the network to the central data hub via the
VSAT link. Note that the gap 33 between the base station repeater cell 3
broadcast interval 32 and the home unit response interval 31 is used for
the set up function so that a single frame period covers the procedure of
FIG. 6B through the sending of a single frame of the message from the
subscriber unit. If transmission conditions change, a succeeding frame of
the subscriber's message thus could be transmitted from a different remote
receiver at a different frequency. Thus the packet ID byte portion of FIG.
4 is significant for reassembling the message frames into a single message
packet (also identified). The arbitrary cell identification umber 486 is
similar to a telephone exchange area code designation in the
identification of the cell or the subscriber's complete ID address.
This set up procedure is important for "hand-off" of a portable unit from
one stationary local remote receiver site 22 to another as fringe areas
are encountered, such as at borders 25 between two local remote receiver
activity sites 22 (FIG. 2). Similarly the portable units can move from
cell to cell when adjacent cells are present such as in urban areas,
requiring similar hand-off procedure. The hand-off may be initiated in
different ways.
As above described, the base station repeater cell 3 may initiate the
hand-off of a subscriber 4 from a local remote receiver 20 in one zone to
another in a different zone within the subdivided cell. Thus a signal
strength (RSSI) measurement may serve as a criterion for handoff, with the
cell directing the subscriber into a set-up routine when signals below a
threshold, -80 dBm for example, are encountered. Since the subscriber unit
4 stores the message data, it is retained until the set-up procedure is
completed in about 50 milliseconds.
Alternatively the subscriber unit software may cause the subscriber unit 4
to place itself in a set-up routine when the RSSI goes below a chosen
threshold value, so that the home unit response is transmitted only after
set-up with a satisfactory cell or cell sub-division zone (22) connection
of proper signal strength.
When the subscribers 4 are transportable from cell to cell, the packets
(FIG. 5) should be stored at the data processing center 2 rather than at
the base station repeater cell 3 level. Each packet carries an
identification of the subscriber for this purpose and the packet ID is
carried in the broadcast frame (FIG. 4) for such processing. Thus at the
central hub (2) a packet of three frames could be derived from two
different cells, generally adjacent in geographical relationship. Note the
cell ID in the subscriber's transmissions (FIG. 6B), which is used for
control purposes.
Also with reference to FIG. 2, the possibility of fringe hand off errors or
interfering signals between cells is avoided by the allocation of
different transmission frequencies for communicating with the
geographically adjoining local remote receiver stations (20) in the
adjacent base station repeater cell areas (19, 26). Thus, in the vicinity
of overlapping base station repeater cell regions 19 and 26, the related
frequencies f.sub.x, f.sub.y assigned to adjacent local remote receivers
20X and 20Y may avoid interference problems between local remote receiver
stations 20 in different adjacent base station repeater cell territories.
Critical timings in the messages processed within the base station repeater
cell site (19, FIG. 2) are discussed in relationship to FIGS. 7A and 7B.
For keeping the message bits accurately synchronized within the system,
the delays in transit time of r-f transmissions must be accounted for.
Those transit times are noted in FIG. 7A, and the transmitted message
frame timings are set forth in FIG. 7B. The frames are sequentially
separated by a 120 microsecond guard band. The approximate 2.7 microsecond
delay within the cell area (19) of 2 miles diameter is encountered between
the subscriber (IDA) 4 and the closest local remote receiver station 20 of
approximately ten such stations distributed about the base station
repeater cell. This is of no significance since by the use of fifty
microsecond pulse widths in the communications that is less than 6% of the
pulse width and thus no range adjustment is needed for that propagation
induced delay. The base station repeater cell 3 thus adjusts its
synchronization with system timing of the received IDA responses after
accounting for the approximately two times 10.6 microsecond (average)
delay time for the transmissions to IDA 4 and back to base station
repeater cell 3.
FIGS. 8A and 8B set forth the FCC approved bands for licensed interactive
communications, thus allocating fifteen channels of bandwidth capable of
carrying the messages under the conditions described herein.
FIGS. 9A and 9B respectively illustrate portable subscriber units afforded
by this invention for interactive two-way wireless communications in a
cell compatible with FCC standards for interactive video data services of
the simplified digital appliance type (9A) and the more comprehensive
video display type (9B).
In the simplified version of FIG. 9A, the transceiver 50 permits two-way
wireless communications in the 218-219 MHz bands set forth in FIG. 8, and
compatible with the functions hereinbefore set forth such as in connection
with FIG. 6A. The double headed arrow notation for the radiowaves at
antenna 49 signifies two way wireless communication. For digital
communications, an input register 51 for received digital data is supplied
and an output register 52 for retaining interactive subscriber entered
messages from transducer 53, typically a manual keyboard or a digital
sensing instrument. Digital display means may be provided for subscriber
viewing of either or both register contents. Thus the data processor 54,
by way of suitable software controls the system with different modes of
operation such as the manual control 55 suitable to keyboard input of data
from a subscriber, or an automatic monitoring control mode 56 for relaying
an alarm or inventory reading at a subscriber's coin operated vending
machine. The frequency control section 57 serves to monitor and set the
transmission carrier frequency during set up procedures for transmission
to a most favorable fixed local remote receiver (20) station. Also it
serves as the system clock to synchronize the transmission frequency of
digital data pulses with the system by means of locking to a TV station
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