Note: Descriptions are shown in the official language in which they were submitted.
1~7~34;3
This application is a division of ~anadian
~pplication Serial number 37~,0~8, filed May 22, 1981.
F`IELD O~ THE INVENTION
The present invention relates in general to data
communication systems, and more particularly, to a multiple
services system using telephone transmission lines to link a
subscriber data subsystem to a cc-ntral office data subsystem,
over which various data services rnay be supplied to the sub-
scriber on request, information may be obtained from the
subscriber on demand, and signaling may be transmitted to
energizing and security services in response to alarm
conditions, without interference with normal telephone
services.
BAC KGROVND OF TH E INVENT I ON
Numerous systems have been developed over the years
which utilize telephone lines, power lines and CATV lines for
carrying signals of 211 types representing alarm conditions,
data readings, survey data, and various types of control
signals to initiate control functions. Such systems are
attractive from an economical point of view in that they take
advantage of existing transmission media and, therefore, like
radio and television, make possible long-range communications
and transmission of control and data signals without large
expenditures for the transmission media itself. However, such
systems have to date been used for the most part for the
transmission of alarm, survey and control signals and have been
generally unavailable to the individual subscriber for handling
a broader range of data communications and services, and those
systems which are in use require dedicated access to the data
service, which is expensive and inefficient.
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Television has greatly increased the communication
of data to the home over that provided by radio in that it
involves both a visual and aura~ communication. However,
commercial television, like radio, provides a unilateral
service in that the viewer basically is capable of receiving
only that which others wish to ransmit at any given time.
Prior services systems which utilized the telephone, power and
CATV lines were also, for the most part, unilateral systems in
that such systems were designed to transmit information only in
a single direction, i.e., from the subscriber to power,
security and other service companies. In those systems
designed for remote meter reading and audience survey, control
signals were transmitted to individual subscribers and in
response thereto the data was transmitted automatically to the
collecting agency; however, in these systems the subscriber had
little or no control over the information being transmitted and
certainly had no power or capability of envoking the receipt of
data on request.
The world is presently at the doorstep of the next
great advance in the extension of data services to the private
sector of the community. Data processing systems have already
become the indispensable tool of the business and scientific
communities, providing word processing, data retrieval, systems
analysis, reservations control and many other services on
various levels of sophistication. However, such services can
also be made available to the individual subscriber in his home
by way of available telephone lines. The provision of multiple
services to the telephone subscriber represents a fascinating
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prospect for the future, which encompasses a broad range of
technologies for delivering such services, including satellite,
coaxial cable and fiber optic systems.
However, in systems designed to handle both alarms
and data communication, problems are encountered in handling
both types of signals witho~t undue complexity and without
having to buffer the data and interleave it with the alarms. In
addition, where multiple services are to be made available to
the su~scriber, some means must be provided to access different
types of terminals at the customer premises on a selective
basis in a way which is compatible with the reporting of alarms
and other conditions as desired.
It therefore a broad object of the present invention
to provide a multiple service broadband system for existing
telephone local loop service.
It is a further object of the present invention to
provide a system of the type described which will be trans-
parent to normal telephone service while providing a
multiplicity of new services including data service commu-
nication, alarm communication and energy management.
It is a further object of the present invention toprovide a system of the type described which includes a modular
data service switching arrangement integrated into the basic
security system switch to permit faster switchover and inter-
leaving of data service and alarm and automatic meter readingdata.
It is still another object of the present invention
to provide a system of the type described in which data service suppor
is independent from data rate code, format and protocol of the data
service system.
It is still a further object of the present invention
to provide a system of the type described which operates on the
basis of a sophisticated communications protocol with error
detection and retransmission to minimize false alarms and
erroneous data.
It is another object of the present invention to
provide a system of the type described having increased
flexibility and expandability to allow for system growth in
terms of both services and the number of subscribers served.
BRIEF DESCRIPTION OF THE INVENTION
The present invention provides a multiple services
broadband system which expands the use of the twisted-pair
telephone distribution network, immediately allowing the
integration of many future services with conventional telephone
service over existing and familiar transmiss~on media in such a
way as to allow both services to be used simultaneously. The
system in accordance with the present invention comprises a
security and data system which is capable of serving a large
number of both residential and commercial subscribers,
providing a unique integration of both alarm and data handling
functions in a single system.
One of the features of the present invention relates
to the provision of a subscriber security system supporting
self-test and monitor verification, with continuous monitoring
and central reporting of (a) automatic intrusion detection; (b)
automatic heat, smoke and cold dctection and manual fire alert;
(c) manual police alert; (d) manual medical alert; (e) main-
tenance features; and (f) stand-alone operation. Services such
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as automatic utility meter reading and energy management load
shedding are also provided.
A second aspect of the present invention relates to
the provision within the system of a central data service
interface, distribution control and subscriber data service
interface which provides for the extension of a broad range of
data services to the subscriber on request. Data services such
as provided by Videotex in the ~nited States and Teledon in
Canada cover a wide range of data services and provide
unlimited possibilities for data communication to the residen-
tial subscriber on a scale not heretofore possible or
practical. For example, both local and distant telephone
directory service could be provided to the individual
residential subscriber by way of his home television set,
eliminating the need for telephone directories, incl~ding
classified directories, and thereby eliminating the huge costs
involved in the printing and distribution of such publications.
All types of information and statistics, including educational
materials, can also be made available through the data service,
along with the availability of centralized data processing
equipment which would otherwise be far beyond the economic
reach of the individual subscriber.
Two different system approaches are available in
accordance with the present invention to permit services to be
provided to both new subscribers, in locations where the
serving area concept of telephone service is being employed,
and to existing subscribers served by a conventional local loop
system. In this regard, where the serving area concept is
being employed, the communication system uses the spare pair
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local loop from the subscriber data system to the junction wire
interface box and a multiplexed Tl data channel from the
junction wire interface to the local exchange office. On the
other hand, for existing subscribers which are served by a
conventional local loop system, a data-above-voice approach is
used to make possible multiple services communication which is
transparent to the normal telephone transmission. The present
invention is therefore available for use in both urban and
rural areas for homes and business office applications.
The system in accordance with the present invention
includes central office computer control for the automatic
monitoring and control of the various services to be trans-
mitted, and it features a microprocessor controlled subscriber
unit for ~lexibility in the transmission of such services.
Thus, the system's basic function is to expand the use of the
twisted pair telephone distribution network, while additional
services are integrated with the basic telephone service over
the same transmission media in such a manner that they are
transparent to the telephone service, permitting the add-on
services and the telephone to be in use simultaneously.
Master control over the system is exercised by a
central control system whose task includes performing recog-
nition, verification and reporting of alarms received from the
subscribér; meter reading; relaying energy management and load
shedding commands from utilities to subscribers; and network
testing and control. Reporting of alarms is handled in a
number of ways including aural/visual indicators, printers and
display devices located within the central office area and by
data links extending to such devices remotely located at
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emergency centers. The system also provides a two-way data
path to urban residential subscribers over the conventional
tcopper pair) telephone exchange plant which allows simul-
taneous use of the telephone (signaling, supervision, talking)
and the home data services without interference between the two
services. However, the data signals provided by the system are
not directed through the telephone exchange switches provided
for voice message traffic, but use physically- separate paths
for voice-band data. This is provided by 1200-baud half-duplex
information transmission between the subscriber and the local
exchange office, which provides for data transmission at
reduced cost.
A remote multiplexer, which is located adjacent to
the jumper wire interface accepts 1200-baud data from the
subscribers lines and encodes it into a Tl rate (1.544 mbps)
bit stream. In the opposite direction of transmission, the
demultiplexer processes the bit stream and a Tl-type line
transmits the 1.544 mbps bit stream between the remote
- multiplexer and a similar multiplexer located in a local
exchange office.
Security, control and data services are provided to
the home subscriber through various panels, interfaces and
alarm sensors attached to a s'ubscriber data system located on
the subscriber's premises. The heart of the subscriber data
system is a microprocessor-controlled subscriber control
processor. Security services are available to the subscriber
through manual panels and alarm sensors whose inputs are
detected'by the subscriber data system and communicated to the
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central control system. Manual-alerting panels (with push-
buttons) enable the subscriber to alert an appropriate
emergency service. Distinct alert messages for police, fire,
medical and the like are received at the central control center
to expedite the appropriate response.
Through the operation of the microprocessor-
controlled subscriber control processor, the subscriber data
system provides a self-test and subsystem testing capability to
assure that failure of critical services (fire, intrusion,
emergency alert) are brought to the attention of the central
control system. Additionally, an enunciator system warns the
subscriber when automatic or manual devices in the home are
activated (for verification) from the central control system.
A switching communications controller provided at the
central exchange maintains communication with the various
subscriber data systems to detect alarms, failure conditions or
requests for data service. Any change in status at a sub-
scriber data system is reported to the central control system.
~pon request from a subscriber data system, the switching
communications controller simply switches the subscriber's
communication channel to one of a plurality of available data
service ports, but on a non-dedicated line basis, so that
services are time-shared between subscribers. Each data
service interface port supports asynchronous ASCII communi-
2~ cations at a data rate of 1200-baud.
The data service is preferably made available at
several locations in the subscriber's home or business by means
of a subscriber data bus. Access to the data bus is gained
through plug-in jack connections, with data bus and jack
3~3
locations being provided in parallel with the telephone wiring
for simultaneous access to telephone and data service at each
location.
Security functions and meter reading are performed
under control of a central control system computer, which
exercises master control over the system, including recog-
nition, verification and reporting of subscriber alar~s, meter
reading, network testing and control. On a per-subscriber
basis, the central control system maintains constant commu-
nications with the switching communications controller;receives and logs all alarm indications; reports emergency and
maintenance alarms to a selected, manned terminal, sorts all
emergency alarms and routes them to a 911-center; provides
detailed information to complete an alarm printout; reads
meters at preset intervals or on demand and flags readings
outside a predetermined range.
Finally, communications within the system are
effected on the basis of a predetermined protocol, i.e., rl~les
governing message transactions between the various elements of
the system, to permit orderly and efficient use of the
communication facilities. All communications are in
asynchronously-transmitted characters which are combined into
units referred to as packets. Each packet will contain a link
control field which includes information relative to the format
and validation of the packet itself, as well as sequence
control of communications between units, to ensure error-free
transmission of data.
With the system of the present invention, not only
may alarm, control and other signaling be transmitted to and
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11743~3
01 from the subscriber's own premises using the existing telephone
02 lines, but a broad range of data services may be made available
03 to the subscriber in his home in a unique and efficient way.
04 This not only includes access to available data services, but
05 also interconnection with other subscribers over the data lines
06 and connection to packet switching systems for long-range
07 electronic mail service.
08 According to a preferred characteristic of the
09 invention, a microprocessor-controlled switching communication
controller for effecting selective communication between first,
11 second and third systems, is comprised of a first service
12 interface unit having a first input for connection to the first
13 system and including a first interface microprocessor, at least
14 one first line switch normally connecting the first lnput to the
first interface microprocessor, and a first bus switch connected
16 to the first line switch. A second interface unit has a second
17 input for connection to the second system and includes a second
18 interface microprocessor, at least one second line switch
19 normally connecting the second input to the second interface
microprocessor, and a second bus switch connected to said second
21 line switch. A plural conductor service bus is connectable to
22 the first and second line switches via the first and second bus
23 switches under control of the first and second interface
24 microprocessor, respectively; and service controller
microprocessor circuit is connected via the service bus to the
26 first and second interface microprocessors and to the third
27 system for effecting communication between the first and third
28 systems via the first line switch and the first interface
29 microprocessors for effecting communication between the first
and second systems by interconnecting the first and second
31 inputs via the first and second line switches, the first and
32 second bus switches and the service bus.
33 These and other ob]ects, features, and advantages of
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01 the present invention will become more apparent from the
02 following detailed description of preferred embodiments of the
03 invention when taken in conjunction with the accompanying
04 drawings.
05 BRIEF DESCRIPTION OF THE DRAWINGS
06 Figure 1 is a schematic block diagram of the
07 communication system in accordance with the present invention;
08 Figure 2 is a schematic diagram of the subscriber's
09 subsystem portion of the system of Figure l;
Figure 3 is a schematic diagram of the central office
11 subsystem of the system of Figure l;
12 Figure 4 is a schematic block diagram showing the
13 data-above-voice communication scheme available with the system
14 of Figure l;
Figure 5 is a schematic diagram of the separate data
16 path approach available with the system of Figure l;
17 Figure 6 is a schematic diagram of the system of
18 Figure 1 showing somewhat more detail of the subscriber data
19 system and the central control system;
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Figure 7 is a schematic circuit diagram showing
details of the interconnections of the subscriber, data bus in
the subscriber subsystem;
Figure B is a schematic diagram of the data service
switch arrangement shou~ing part of the switching communications
controller, subscriber subsystem and data service system;
Figure 9 is a diagram showing the format of an infor-
mation character forming the basic message unit of
communication in the system;
Figure 10 is a diagram of the packet format for com-
munications in the system;
Figure ll is a diagram showing the allocation of the
link control bits in the header of each packet;
Figures 12a - 12e are operational diagrams showing a
typical data transfer operation;
Figures 13a - 13d are operational diagrams showing a
data transfer operation involving a transmission error;
Figures 14a - 14c are operational diagrams showing
one example of the sequence reset operation in accordance with
the present invention;
Figures 15a and 15b are operational diagrams showing
another example of the sequence reset operation; and
Figures 16a and 16b are operational diagrams showing
still another example of the sequence reset operation.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The system in accordance with the present invention,
which is illustrated in its most-basic form in Figure 1, is
implemented as a distributed processing network with serial
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data links between the various processors. Each subscriber,
either residence or business, will have its own subscriber data
system 1, which is a microprocessor controlled unit capable of
handling the security, control and data services at the sub-
scriber premises.
Associated with the subscriber data system is asubscriber data bus 2 into which the subscriber may plug
various peripheral units throughout the subscriber premises,
such as a television set 4 and terminal key pad 5, via a data
service terminal /bus interface unit 3. The unit 3 communi-
cates with the subscriber data system 1 over the control
communications lines of the subscriber data b~s to request and
receive permission to use the subscriber data system link Tl to
the central exchange via the junction wire interface
multiplexer 6.
The central office end of the Tl link is connected by
way of a multiplexer 7 to a switching communication controller
8, which maintains communication with the subscriber data
systems 1 to detect alarms, failure conditions or requests for
data service, selectively interconnecting a subscriber data
system 1 with a central control system 9 for transmission of
alarm signals and energy management control signals, while
connecting the s~bscriber data system 1 to a data service
system 10 upon request to provide for communication of data
services to the subscriber, to a packet switching system 18 for
transmission of messages to distant locations, or to other
communication and data processing systems at the request of the
subscriber. ~hus, the system in accordance with the present
invention is basically made up of two subsystems, i.e., a
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subscriber subsystem linked through the available telephone
lines to a central office subsystem.
Figure 2 illustrates the basic arrangement of the
subscriber subsystem, the heart of which is the subscriber data
system 1 linked to the common data bus 2. The subscriber data
system 1 is based on an intelligent microprocessor, such as
provided in the Intel 8085 microprocessor family, for
formatting and coordinating communicatlon with the central
office subsystem. Dedicated wiring connects fixed location
sensors, such as smoke and heat detectors 13 and intrusion
detectors 14, throughout the subscriber's premises to the
subscriber data system 1. In addition, various
manually-operable alert buttons 15 for police, fire and medical
alert are connected to the subscriber data system 1, these
alert buttons 15 being preferably disposed at convenient
locations in the subscriber's premises for emergency use.
Meter readings may be reported to the central office
subsystem on a periodic basis or on demand from the central
office, and energy management commands received from the
central office are routed by the subscriber data system 1 to
the appropriate control interface, via the meter reading and
energy management interface 11.
Data services are made available at several locations
in the subscriber's home or business by means of the common
data bus 2, which makes allowance for the portable nature of
such devices as the television 4, key pad 5 and data terminal
12, as well as for flexibility in adding or changing services
at the subscriber premises.
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The central office subsystem is illustrated in
functional block diagram form in Figure 3. The key to the
system's flexibility is found in the microprocessor-controlled
switching communication controller 8, which provides the
switching and interface functions between the data service
system 10 and the central control computer 19 in the central
control system 9 and the subscriber subsystem 20 via the
communication link. The central control computer 19 performs a
multiplicity of functions including telephone usage reporting,
alarm reporting, meter reading and energy management control,
network testing and control, and polling/survey reporting. In
this regard, the central control computer 19 exercises master
control over the system, maintaining constant communications
with the switching communication controller 8, receiving and
logging all alarm indications, reporting emergency and
maintenance alarms, sorting all emergency alarms, providing
detailed information to complete alarm printouts, reading
meters at preset intervals or on demand and flagging readings
outside a predetermined range. The central control computer 19
also automatically tests the entire network to ensure
troublefree operation and error-free communications.
Two different technical approaches are available in
accordance with the present invention for effecting communi-
cation between the subscriber subsystem 20 and the central
office subsystem 30 using the avallable telephone lines. One
approach involving data-above-voice techniques is illustrated
in Figure 4. This approach uses only one twisted pair to link
the subscriber to the central office for both the telephone and
services communication. The subscriber's subsystem 20 is
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connected via a modem and filter 22 to the twisted pair
subscriber loop at the subscriber's premises and the
communication received at the central office is applied to the
central office subsystem 30 via a similar filter 22 and modem
21 arrangement which provides band separation and transmission
of the data in the 5 to 10 KHz band.
The second approach available in accordance with the
present invention, as illustrated in Figure S, takes advantage
of the Serving-Area concept in local telephone service dis-
tribution in which feeder cables are run to a distributioncabinet serving a few hundred subscribers in a local area. It
is common practice to provide two distribution pairs to each
subscriber in such a system, one for normal telephone service
and one as a spare. Signaling in accordance with the present
invention is ~herefore effected using the spare distribution
pair between the distribution point and the subscriber. The
signals received from the subscriber subsystem 20 are applied
via a driver/receiver or modem 16 over the spare distribution
pair to a similar driver/receiver or modem 17 for application
to the multiplexer/demultiplexer 6 in the distribution cabinet
of the junction wire interface. These signals are then carried
over the Tl link using screened pairs to the central office
where the signals are demultiplexed by multiplexer/
demultiplexer 7 and applied to the central office subsystem 30.
While Figure 1 discloses the system in accordance
with the present invention on the basis of the separate data
path approach to communication over the telephone lines, it is
apparent from the discussion in connection with Figures 4 and5
that either the data-above-voice approach or the separate data
7L1 ~ L~? 3
path approach may be implemented in the system of the present
invention, or both approaches may be incorporated into the same
system. In this way, the system is available for use in both
new and old areas, finding application in both rural and urban
areas for homes and business office applications.
Figure 6 illustrates the system of Figure 1 with
further details of the subscriber subsystem and the manner in
which it interfaces in the system, as well as some of the
various peripheral devices which may be connected to the
central control system 9 for monitor and control purposes. As
seen in the figure, each subscriber data system 1 is made up of
a subscriber interfàce unit 120 controlled by the subscriber
control processor 110, which controls the functions to be
performed within the subscriber subsystem. Security services
are available to the subscriber through the manual panels and
alarm sensor equipment, with inputs from these panels being
detected and forwarded to the central control system 9 via the
switching communication controller 8, which acts as a concen-
trator for such purpose. Manual alerting panels for alerting
the appropriate emergency services are also provided for each
subscriber to cause distinct alert messages at the central
control system 9 for police, fire and medical emergencies to
permit different operational reaction procedures.
In addition to its primary function of controlling
the su~scriber subsystem, the subscriber control processor lln
has a self-test and subsystem testing capability to assure that
failures in the subscriber subsystem are brought to the atten-
tion of the central control system 9, insofar as
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critical services, such as fire, intrusion and emergency alert,
are concerned. On one of the panels, an indicator
lamp (not shown) is provided to indicate communication
activity with the central control system 9 to assure the
subscriber that the system is functioning. The siren/alarm
system is activated when automatic or manual devices in the home are
activated. An annunciator is activated upon verification from the
central office system 9 that the alarm has been received.
The switching communications controller 8 recognizes the
loss or absence of communications to each subscriber subsystem
and generates an alarm message at the central control system 9
for each such loss. When communications to a subscriber's
subsystem are interrupted, and polling has stopped, an
indicator lamp lS provided to inform the subscriber;
however, the subscriber's subsystem continues to operate on its
own without connection to the central control system 9,
providing local alarm and meter 3ccumulation functions.
Security functions and meter reading are performed
under control of the central control system 9. In addition,
meter-reading data will be gathered by the system 9 and
transferred onto magnetic tape in a magnetic tape data storage
unit 93 for processing by an external computer. The central
control computer 19 which forms the heart of the central
control system 9 may be provided in the form of a DEC PDP-1134A
Computer, which will exercise master control over the system.
In addition, a secondary communication port in the service interface
controller provides alarm and network status reports to an
emergency alarm Logger 81 or secondary control system in the event `
of loss of communication with the central control system.
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When an alarm condition is reported to the central
control system 9, via the switching communications controller
8, the system will provide the d~mographic information to
complete an alarm printout on ~he hardcopy alarm printer 92.
One line of up to 64 characters of information will also be
printed, normally containing the subscriber's name and address.
In addition, up to two other lines of demographic information
are available for each specific a].arm condition, such as fire,
assault, intrusion, cold, etc. 'rhese lines will be printed
when the corresponding type of alarm occurs and usually contain
information such as the type of structure, medical problems, or
friends' and relatives' phone numbers on the basis of infor-
mation stored in the mag tape data storage unit 93 or system
disc storage unit 94.
The central control system 9 will receive all alarm
indications from the subscriber through the switching communi-
cations controller 8 and will report emergency and maintenance
alarms via the guard's console 91 with appropriate demographic
data retrieved from the disc unit 94, identifying pertinent
alarm and subscriber specific information. All emergency
alarms will be acknowledged by the guard's console 91 to
inhibit continuous activation of the console's audio alarm
function.
Automatic meter-reading data will be requested auto-
matically at preset intervals or by operator demand via theoperator's console 95. Commands generated through the
switching communications controller 8 will cause the accumu-
lated totals in the subscriber data system 1 to be transmitted
upstream to the central control system 9, where the new
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readings will be checked for range and roll-over and these
readings will be used to update the master meter-reading files
stored in the system disc 94. New reading increments o~tside a
predetermined range will be flag~ed for further investigation
and correction as necessary.
The line or character printer unit 96 will be used
for the output of statistical data accumulated by the system,
the output of automatic meter-reading data for review, as well
as a backup for the hardcopy alarm printer 92.
Figure 7 illustrates in greater detail the config-
uration of the subscriber data bus 2 and its interconnection to
the subscriber data system 1. The subscriber data bus 2 serves
to provide data service access via the subscriber data system 1
through multiple pre-wired data jacks distributed throughout
the subscriber's premises. The subscriber data bus 2 origi-
nates at the subscriber data system 1 and provides for
multi-dropped serial data links between the subscriber data bus
and either bus-compatible data service terminals or the
combination of a bus interface unit 3' and compatible data
service terminal 3'', as illustrated.
The bus 2 consists physically of four twisted pairs
wired in parallel, which may be distributed in four star-wired
legs up to a maximum of 100 feet on each leg. Each leg can be
daisy-chain wired to a plurality of mini-jacks, as illus-
trated in Figure 7. The four twisted pairs which make up thebus are designated DBLC DBDO DBDI and DBPF. The twisted pair
designated DBLC provides the data bus link control circuit,
which is a half-duplex multi-dropped link provided to
communicate control data between the subscriber data system 1
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~1.'o'~ 3
and multiple bus interface units 3' in the standard data link
control protocol for multi-point data links, as will be
described in greater detail hereinafter, with the subscriber
data system 1 operating as the primary station. The twisted
pair DBDO represents the data bus data-out circuit, which is
used to carry data from the data service system 10 via a line
switch 125 in the subscriber data system 1 under control of the
subscriber control processor 110 to a bus interface unit 3'.
The twisted pair DBDI represents the data bus data-in circuit,
which is used to carry data from a bus interface unit 3' via
the line switch 125 to the data service system 10. The fourth
twisted pair DBPF in the bus 2 is used to provide power to
small, low-power devices that may be attached to the bus, such
as a low-power key pad device 126.
Link control over the subscriber data bus 2 is
exercised through the standard system multi-point protocol with
the subscriber data system 1 acting as the primary station
operating over the DBLC circuit. Information communicated over
this link between the subscriber data system 1 and a bus inter-
face unit 3' is used to determine and control the status of the
terminal and to identify the type of terminal and other
information with respect thereto, as will be described more
fully hereinafter. For data service terminals, the subscriber
data system 1 takes action to connect the DBDO and DBDI
circuits of the bus 2 through the line switch 125 to the
communication link which extends to the central office upon
receipt of a request to send (RTS) message. When the line
switch 125 has been set up to connect the transmit line to the
data service system 10, thé subscriber data system 1 indicates
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3~3
the clear-to-send (CTS) status to the bus interface unit 3'.
On receipt of the CTS signal from the subscriber data system 1,
the bus interface unit 3' may then connect the transmitted data
and received data circuits directly to the subscriber data bus
lines DBDI and DBDO, respectively, through the appropriate line
drivers and receivers which form part of the subscriber
interface unit 120, as seen in Figure 7. The data service
switch control allows for the transmission of one complete
upstream message followed by one complete downstream message.
The downstream message will be monitored by the subscriber data
system, and a command canceling the clear-to-send (CTS) status
will be sent to the bus interface unit 3' following completion
of the downstream transaction. To avoid risk of contention
with a downstream message, a data service terminal 3'' must not
attempt a second upstream transmission following the dropping
of the clear to send status, but should proceed through another
RTS/CTS cycle. As is apparent from the foregoing description,
all communications over the subscriber data bus 2 are
asynchronous and do not require the use of timing interchange
circuitry.
The detailed configuration of the switching communi-
cation controller 8 in accordance with this invention is
illustrated in Figure 8. Beginning with the subscriber data
system 1, the subscriber control processor 110 controls access
to the subscriber data bus 2 by individually polling the
various terminals connected thereto. All subscriber's
terminals use a bus interface unit 3', or equivalent circuitry
within the terminal, to communicate with the snbscriber data
system 1 over the bidirectional serial control communications'
-21-
line DBLC of the bus 2. The subscriber data system 1 also
maintains control of a line switch 125 which allows it to
communicate upstream to the rest of the network, or to turn the
line over for use by the data service system 10, a packet
switching network 18 or other communication or data handling
system. Since the subscriber data system 1 will not
eavesdrop on the data service system 10 when the line is
switched over, a line-activity monitor 128 is provided to allow
the subscriber control processor 110 to detect when the data
service system 10 has completed a data transfer and to
recognize fault modes.
The switching communication contr~ller 8 is made up
of a plurality of subscriber service interfaces 200 and a
plurality of data service interfaces 225 interconnected by way
of an internal data bus 210. Although a plurality of
subscriber service interface circuits 200 are provided in the
switching communications controller 8, a single circuit 200 is
illustrated in detail in Figure 8 highlighting one subscriber's
path.
Within the subscriber service interface 200 there are
two switches involved in providing data service to each
subscriber ùnder control of a subscriber service interface
microprocessor 205. One switch in the form of a line switch
201 allows the processor 205 to communicate with a particular
subscriber data system 1 for normal communications via the
telephone transmission lines in a manner transparent to normal
telephone service or to select data service via the bus switch
202 under control of a service interface controller
microprocessor 250. The bus switch 202 in conjunction with the
-2~-
1174343
line switch 201 serves to connect the subscriber's lines
directly to one of the paths in the internal data bus 210, each
path including two data lines which extend to the data service
interface circuits 225. Again, since the microprocessor 205
S will not eavesdrop on the data service applied through the bus
switch 202, a line activity monitor 203 is used to sense an
inactive line following transmissions and exception conditions.
Alarm, meter reading and various control signals which are
passed between the subscriber data system 1 and the central
control system 9 follow a path including line switch 201,
microprocessor 205, the control line 215 of the data bus 210,
and the service interface controller microprocessor 250. This
path represents the normal communication path through the
switching communication controller 8, which also acts as a
concentrator for this purpose.
The counterpart of the subscriber service interface
circuit 200 to the data service system 10 is the data service
interface circuit 225, which has the same construction and
basically the same operation as its counterpart. Switch
control of the switches 231 and 232 within the data service
interface circuit 225 is effected by the data service interface
microprocessor 235 in response to the service interface
controller microprocessor 250 to connect the appropriate pair
of the internal data bus 210 carrying the subscriber's data to
an available one of eight data service lines extending to the
data service system 10. The data service lines each consist of
a send-data line and a receive-data line with modems 236, 238
being provided at the respective ends thereof. As seen in
Figure 8, the switches 231 and 232 are operated together to
-23-
1:~7~3~3
provide a tandem connection of the bus 210 to the lines of the
data service system lO; however, only the switch 231 is
essential in this path.
For transmission of meter reading, alarm and control
signals, the system operates to transfer information from the
subscriber control processor llO to the subscriber service
interface microprocessor 205 via the telephone lines, fr-om
which the information is passed to the service interface
controller microprocessor 250 via data bus 215 for forwarding
to the central control system 9, and the flow of information in
the other direction follows the reverse path. Such
communication is effected in accordance with the system
protocol, which will be described in more detail hereinafter.
However, for data service to be extended to the subscriber, the
communication link from the subscriber data system l must be
switched by the switching çommunication controller 8.
The operation of the switching communication
controller 8 for a data service operation will now be
described. From this description it will be noted that the
half-duplex link between the switching communication controller
8 and the subscriber data system l feeds back all data on the
transmit circuit to the receive circuit. At the subscriber
data service 1, feedback of data from the bus interface unit 3'
onto the data bus 2 is inhibited within the subscriber data
system 1. However, any data transmitted by the data service
system 10 will be fed back by the switching communication
controller 8 over the full duplex link between the switching
communication controller 8 and the data service system lO to
the data service system receive circuit. This feedback may be
-2~-
li74343
inhibited or used for error chec~ing by the data service system
10.
A request for data service is initiated with the user
entering his command through the terminal key pad 5, which
raises a reguest-to-send (RTS) condition through the data
service terminal 3 " to the bus interface unit 3'. The bus
interface unit 3' goes into a wait state until it is next
polled by the subscriber data system 1, which will generate a
poll requesting any pending commands from the bus interface
unit 3'.
The bus interface unit 3' will respond to a poll from
the subscriber data system 1 by transmitting a request-to-send
(RTS) control message on the subscriber data bus 2. ~s part of
its communication protocol, the subscriber data system 1 will
acknowledge successful receipt of the message and using the
line contention protocol established for its communications
with the switching communication controller 8, will immediately
relay the request-to-send control message to the microprocessor
205 in the subscriber service interface 200 via line switch
201. The subscriber service interface microprocessor 205 will
acknowledge successful receipt of the message, queue the
message for transmission to the service interface controller
microprocessor 250 and will wait until it is polled by the
microprocessor 250, which is continuously scanning the
2S subscriber service interface circuits 200 and data service
interface circuits 225 which share its common communication bus
210. When the requesting subscriber service interface 200 is
polled to determine if any upstream messages are pending, the
interface circuit 200 will transmit the request-to-send control
`<~3
message, which now includes both the subscriber identification
and the identification of the particular subscriber service
interface circuit 200, to the microprocessor 250 via lines 215
of the bus 250.
The subscriber interface controller microprocessor
250 will acknowledge successful receipt of and process the
request-to-send control message by scanning the current data
service line allocation table in its memory to determlne the
availability of one of the twenty-four bus lines to the
appropriate data service interface circuits 225. Assuming a
line is available, it is assigned to the requesting subscriber
and the corresponding switch control commands are forwarded as
switch control messages to the requesting subscriber service
interface microprocessor 205 and data service interface
microprocessor 235 of the interface circuit 225 specified from
the allocation table.
The service interface contro~ller microprocessor 250
now waits until its polling sequence reaches the specified data
service interface 225, at which time it will transmit its poll,
with the switch control message embedded therein, to the
designated interface circuit 225, which will acknowledge
successful receipt of the message and execute the specified
switch command to connect the designated data service line onto
the specified lines of the internal data bus 210. When the
polling sequence of the service interface controller
microprocessor 250 reaches the requesting subscriber service
interface 200, the circuit 250 will transmit its poll, with the
switch control message embedded therein to the requesting
subscriber service interface 200. Using the line contention
-26-
~ 3
protocol, the subscriber service interface 200 will at that
time transmit a control message to the subscriber data system 1
which indicates successful assignment of a data service line
and directs the subscriber data service to switch its incoming
line to the subscriber data bus 2.
The subscriber data system 1 will acknowledge
successful receipt of the message and after the acknowledge
message has been transmitted to the subscriber service
interface 200, it will switch the line over so that it
interfaces with the bus interface unit 3' over the subscriber
data bus 2.
Receipt of the acknowledged message by the subscriber
service interface 200 initiates the switching of the
subscriber's line onto the specified line on the internal data
bus 210 via the switches 201 and 202. This should occur at
about the same time as the subscriber data system 1 switches
its upstream line to the telephone link, so that the transmit
data line of the subscriber data bus 2 is now directly linked
from the bus interface unit 3' to a port in the data service
system 10.
The subscriber data system will then forward a
clear-to-send control message to the bus interface unit 3'
which will acknowledge successful receipt of the message and
forward it to the data service terminal 3''. Upon receipt of
its clear-to-send signal, the terminal 3'' transmits its
waiting command directly to the data service system 10. The
data service terminal 3'' should then immediately begin looking
for the response from the data service system lG.
3 ~ 3
The data service system 10 will recognize the end of
the upstream message request from the format of the message
received from the data service terminal 3'', and this will
cause the data service system 10 to react to the request by
locating and formatting the requested display data. This
processing may require the data service system 10 to link a
subscriber's requests to prior requests through the
subscriber's identification data passed with the request.
Since the line has already been switched over, the data service
system 10 will transmit the downstream response through the
same port through which the request was received.
The subscriber service interface microprocessor 205
will be the first to recognize the end of the downstream
response when its downstream line activity monitor 203 times
out. The subscriber data system 1 will also monitor the line
activity to determine the end of the downstream transmission.
Once this has occurred, the subscriber service interface
microprocessor 205 will control the bus switch 202 to switch
the subscriber's line off the internal data bus 210 and control
the line switch 201 to connect the line from the subscriber
data system 1 back to the subscriber service interface
microprocessor 205. At this time, the subscriber data system 1
also switches the communication line off the subscriber data
bus 2 and reestablishes communications with the subscriber
service interface 200 for purposes of sending any pending
upstream alarms or meter readings for forwarding through the
service interface controller micropr~cessor 250 via line 215 to
the central control system 9. If no alarm or meter reading
messages are pending, a simple poll message is sent to notify
-28-
~ .3~3
the subscriber service interf2ce that it may transmit any
pending downstream messages from the central control system 9
for the subscriber data system 1.
Since there are far fewer data service ports than
subscribers, there is a possibility that a subscriber request
cannot be serviced due to all links being allocated. The data
service system 10 will therefore be considered busy when all
available data links are currently allocated. Under such
circumstances, the service interface controller microprocessor
250, when it detects the busy condition upon reference to its
data service line allocation table, will send a busy message
through the subscriber service interface 200 to the subscriber
data system 1 from which the message is forwarded on the
subscriber data bus 2 to the bus interface unit 3'. The bus
interface unit 3' could immediately retry by sending a
request-to-send message to the subscriber data system on the
next poll, or it may terminate the transaction and require the
subscriber to cause a retry.
The system in accordance with the present invention
is a computer-based data communications system which requires a
data link control, i.e., a line protocol, to permit orderly and
efficient use of the communications facilities available. All
communications are in asynchronously-transmitted characters
which include one start bit, eight data bits, one parity bit,
and one stop bit, as seen in Figure 9. Data bits are
designated 0 through 7 from the least-significant bit to the
most -significant bit and are transmitted in that order.
Character parity may, for example, be even, so that the parity
bit is set to force an even number of "1" bits when taken with
-29-
1:~'7i~ 3
the data bits. With eight data bits, this will ensure that at
least two bits (including the start bit) will be 0 for each
character. This will serve to reduce the number of undetected
errors from sp~rrious characters received due to noise
occurring on the data links.
All communications will be units known as packets,
each packet being divided into three parts including a header,
the information field, and a terminator as seen in Figure 10.
The packet header consists of three one character fields
representing a flag, an address field, and a link control
field.
The start of a packet shall be identified by the
receipt of a unique flag character followed by a non-flag
character. Thereafter all characters with the same value as
the flag are treated in context and not as flag characters
until after receipt of the packets terminator. The address
field is used to identify elements within the system, the
address field for downstream packecs indicating the destination
of the packet, and for upstream packets, the address field
showing the station originating the packet. For example, a
subscriber service interface circuit 200 will identify itself
in the address field for packets sent to the service interface
controller microprocessor 250.
The link control field in the header will contzin
information relative to the format and validation of the packet
itself. The common bit assignments for this link control field
are indicated in Figure 11 and comprise a priority packet bit,
an initiate/response bit, an ACh/NAK bit, a variable length
-30-
.. . . , .. ,.. _ ..... . .. ........ ..
3-~3
packet flag, a piggy-back packet flag, a se~ence bit and a
seq~ence reset bit.
The initiate/response bit of the link control field
(l for initiate; 0 for response~ is used to designate when a
response is required. All packets in the system will require
an acknowledge or negative acknowledge (ACK/NAK) except for
ACX/NAK packets themselves. Th~s, a packet with this bit set
for response implies that the packet contains only an ACK/NAK
and no further information.
The ACK/NAK bit (1 for ACK; O for NAK) is used by a
receiving station to return the status of the received packet
to the transmitting station. The ACK ("good" packet) or NAK
("bad" packet) information is interpreted to terminate the
transaction or cause retransmission on poll, respectively. The
initiate/response (I/R) and the ACK/NAK bits are used together
to show the contents of the information field, so that even the
one message per packet format permits a packet to carry an ACK
from the last transmission and a new message in the information
field. For example, when the I/R and ACK/NAK bits are both
zero, it is an indication that the packet contains an NAK
response only and that the information field is to be
disregarded. Where the I/R bit is zero and the ACK/NAK bit is
one, the packet contains an ACK response only and the infor-
mation field is to be disregarded. Where the I/R bit is one,
and the ACK/NAK bit is zero, the packet contains a new message
with no ACK/NAK implied or the message relates to a poll with a
NAK. Finally, if the I/R bit is one and the ACK/NAK bit is
one, the packet contains a new message and an ACK to the
last-received packet.
-31-
~ 3~3
The sequence bit in the link control field of the
header is used to identify the packet sequence, odd or even, in
recognizing lost or garbled packets. A definite respO~se (ACK
or NAK) is reguired for all packets except those containing
only an ACK~NAK, i.e. t where the I/R response bit is zero. The
station initiating the transaction will update its transmit
sequence bit, as stored in memory, upon receipt of an ACK from
the destination. This means that each processor must retain
two sequence bits (transmitted packets and received packets)
for each station with which it communicates, these sequence
bits being stored in memory at the particular station. The
received packet sequence bit is updated at the station when the
ACK is issued ~nless there has been a sequence bit error. This
particular sequence control will be described by way of example
with reference to Figures 12a through 12e.
In multipoint data links, the station designated as
the primary initiates all transactions. That station initiates
a packet transmission with the sequence bit as updated from its
last transaction to poll the secondary (receiving) station as
illustrated in Figure 12a. Since the receiving station was
expecting the received sequence bit (and the packet was
otherwise valid), it updates its received sequence bit and then
generates its response. If it has no message to return, it
responds with only the ACK, as illustrated in Figure 12b. The
primary station recognizes the ACK, and updates its transmit
sequence bit since the secondary station response was an ACK
only, its sequence bit will be disregarded.
At the next transaction, the primary station will
once again poll the secondary station in the manner described
3~
above, as seen in Figure 12c. This time the sequence bit is
"1". Once again, the secondary station will recognize the
sequence bit as that which was expected and will acknowledge
the packet receipt to the primary station. If the secondary
station has data to return, it ~ill simultaneously initiate a
second transaction, as seen in Figure 12d. The receipt of the
acknowledge (ACK) will cause the primary station to update its
transmission sequence bit, and at the same time, the sequence
bit received being the same as that expected, the primary
station also updates its received sequence bit and transmits an
acknowledge (ACK). Receipt o~ an ACK by the secondary station
will cause it to update its transmission sequence bit as
illustrated in Figure 12e.
The sequence bit allows detection of an error con-
dition where an acknowledge response is lost and the
originating station retransmits. The receiving station detects
the retransmission and, expecting a new packet because it had
acknowledged the previous packet, recognizes the fact that it
was duplication because of the "incorrect" sequence bit. These
received duplications would be ignored and the acknowledge
(possibly containing a message as well) would be retransmitted.
An example of the sequence of events which occur during such an
erroneous transmission is illustrated in Figures 13a - 13d.
Assuming that the operation described in connection
with Figure 12 continues and that the primary station forwards
a packet including data to the secondary station, as indicated
in Figure 13a, the secondary station will validate the packet
and recognize the expected sequence bit. It will then update
its received packet sequence bit and issues an acknowledge as
-33-
:~:Q'7~3~3
indicated in Figure 13b. However, if the ACK response is
garbled or otherwise lost before the primary station receives
it, the primary station will time out and assume that its
transmission was lost and that it must retransmit the original
packet as seen in Figure 13c. The secondary station receives a
seguence bit equal to zero in this packet transmission while
expecting a one. This tells the secondary station that the
packet is a repetition of the last packet and should be
discarded. The receive packet sequence bit is not updated, but
the acknowledge response is retransmitted to the primary
station, as illustrated in Figure 13d. The received ACK causes
the primary station to update its transmitted packet sequence
bit. This problem does not arise if an NAX response is lost
since the transmitting station time out has the same effect as
lS receipt of the lost N~K.
Initialization and resynchronization of the sequence
bit are handled by use of a sequence reset bit in the link
control field of the header. For normal operations, as in the
examples described in connection with Figures 12 and 13, this
bit is zero; however, when a packet is the first transmitted
after processor powerup (initialization) or after lost communi-
cations are reestablished (resynchronization), the initialized
station will set its own sequence bits to zero and set the
sequence reset bit to one. ~ one in this bit position directs
the receiving station to accept the packet regardless of
sequence bit value and to reset its own sequence bits to zero,
i.e., the next packet initiated by each station will have a
sequence bit equal to zero. A sequence reset is considered an
-34-
initiate packet and requires a response, even though the I/R
bit may not be set.
Three examples of situations where the sequence reset
bit is used will be described in connection with Figures 14, 15
and 16. In all three cases, the transmissions are simple
poll/responses; however, it should be clear that cases where
messages are passed in the informatisn field are also handled
in the same way.
In the first example, the normal recovery/
initialization sequence occurs when both the primary and
secondary station recognize that they have lost communications.
In this case, each station will attempt to control recovery
from its end by sending a sequence reset bit packet. Since the
primary station must initiate any transactions, its poll will
be the first transmission. Since it has recognized the lack of
communications, it has already reset to zero both of its
sequence bits and has set t~e sequence reset bit for all polls
while attempting to reestablish communication, as seen in
Figure 14a. Since the secondary station also recognized the
fact that communications have been broken, it has reset both of
its sequence bits and is ready to respond to the sequence reset
bit set to the first poll from the primary station. The
received sequence reset bit causes it to reset those sequence
bits (no change in this case), as seen in Figure 14b, and
acknowledge the packet because it is both a poll and a sequence
reset packet. The primary station first recognizes that the
sequence reset bit is set, and therefore resets its sequence
bits. It then recognizes the acknowledge indication from the
secondary station, updating its transmit sequence bit and then
transmits an acknowledge for the sequence reset packet,
updating its received sequence bit, as seen in Figure 14c. In
response, the secondary station updates its transmit sequence
bit. The initialization process is now complete with both
stations having the same sequence bit configuration and
therefore ready for the next transaction using the normal
operations already described.
In the second example, if the primary station assumes
that the secondary station has gone off-line, but the secondary
station assumes it has had continuous communications available,
then only the primary station will send a seguence reset
packet. This case can arise, for example, if the primary
processor fails and automatically restarts within the secondary
stations poll time-out interval. In this case, the primary
station will recognize its need to resynchronize the sequence
bit and will respond, as in the first example, by sending a
poll with the sequence reset bit set, as seen in Figure l5a.
The secondary station will process the sequence reset bit flag
by resetting both of its sequence bits and issuing an acknow-
ledge back to the primary station, as seen in Figure 15b. Thereceived acknowledge causes the primary station to update its
transmitted sequence bit. This terminates the transaction with
both stations holding the same sequence bit configuration.
In the third example, the primary station shows the
secondary station as on-line, but the secondary station assumes
that it has lost communications. Such a case, which is the
converse of the situation described in the second example,
could arise if the secondary station should fail and recover
between polls from the primary station. In this case, the
-36-
.. .... ,.. . _ ... .
~ ~ 7~
primary station would send a poll as if everything were normal,
as seen in Figure 16a. As in the first example, the secondary
station has gone into recove~ operations having reset its
sequence bits and is waiting to transmit a sequence reset bit.
Since the secondary station has reset both of its sequence
bits, it does not know what the expected sequence bit from the
primary should be, and it must ignore the received sequence
bit. As seen in Figure 16b, the secondary station acknowledges
the poll with the waiting seauence reset packet. In response
to this sequence reset packet, the primary station will reset
its sequence bits, at which point the acknowledge to the poll
is recognized, resulting in the update of the transmit sequence
bit. Finally, the primary acknowledges the sequence reset
packet and the secondary station concludes the transaction by
updating its transmit sequence bit on receipt of the
acknowledge. Again, both stations are now ready for normal
operations.
The terminator field of each packet will provide an
additional level of confidence that the packet was correctly
received. The terminator for all communications shall consist
of a one character longitudinal redudancy check (LRC). The LRC
shall be defined as the bit-wise modulo-2 sum of all characters
except the flag character. This may be implemented as a result
of applying an EXCLUSIVE-OR logical operation on the partial
result for each successive character. The receiving station
must correctly receive a valid LRC before performing any
further processing on a packet.
The priority packet bit which forms part of the link
control field as seen in Figure 11 is used to flag high-
~ ~7~3~3
precedence packets for priority processing (one for priority
packets; zero otherwise). The use of this bit may be reserved
for emergency-category alarm messages, such as smoke, heat,
intrusion, police, fire and medical alarms.
The variable length packet flag which is also
included in the link control field has a fixed length for each
link and use of this flag (one for variable length packets;
zero for structured packets) allows the protocol to support
variable length packets where the first character of the
information field specifies a packet length. Such variable
length packets may include one or more messages in the
information field.
The piggy-back packet flag which is included in the
link control field is used in a structured packet (fixed
length) to identify the length of a variable length packet
which follows and contains the message itself. This format
calls for two packets to transmit a message which does not fit
into the structure format, the second of which has the
"variable length packet flag" set as described above. Use of
this format offers greater protection because the length must
be transmitted twice and once acknowledged before a prolonged
message could be transmitted. This protects against the case
where the length of the character is incorrectly received and
results in long waits for additional characters which may not
be coming, causing system hangups.
The system in accordance with the present invention
utilizes two types of communications links, multi-point where
one primary station communications with multiple secondary
stations on the same link, and point-to-point involving a
-38-
... . - .. , . . _, . ... . .
1~o~ 3
direct link between two stations. Multi-point data links
operate with one primary (or master) station which sequentially
invites the secondary (or slave) stations to send it messages,
otherwise known as polling. In some systems, the converse of
this, where the master station selects a station to which it
sends data, is known as a "select" operation. No differen~
tiation between these terms is required in the standard
protocol since a "select" sequence also implies a "poll"
sequence, following which the secondary station may respond
with just an acknowledge, or with a message for the primary
station.
The only point-to-point link between the major system
elements involving the standard protocol is the switching
communication controller to subscriber data system link. Full
advantage of the dedicated nature of this link is taken through
the use of a point-to-point protocol with contention. Instead
of the switching communication controller repea~edly polling
the subscriber data system, both alarm response times are
minimized by allowing either station to initiate a message
transaction. This will invariably lead at some time or another
to "contention", where both stations attempt to transmit on the
link simultaneously. However, the risk of this`occurrence is
minimized through the elimination of nonessential polling, and
rapid recovery from this condition is assured through the use
~5 of different time-out periods by each station.
In the event that a message is lost or not recognized
by a receiving station, it is the primary station's responsi-
bility to recover from the failure by use of a response
time-out. The response time-out periods commence with the
-39-
~ ;3
completion of transmission of the last character of each packet
transmitted by the primary station and is canceled when a
complete packet is received from the addressed secondary
station (where it is an error packet or not). If the response
time-out period is not canceled within the allotted time, a
time-out occurs. This is treated by the primary station in the
same manner as the receipt of a NAK packet, in which case no
response is required and the time causes an increment of the
NAK counter. The primary station has the option of reissuing a
poll packet to the secondary station or continuing on to
another secondary station, and returning later.
Each secondary station on a multi-point link utilizes
a poll time-out sequence which serves to detect when the
primary station has lost contact with that secondary station.
The poll time-out sequence commences with station initiali-
zation and is retriggered upon receipt of a poll packet with
the expected (alternating sequence) bit (or with the sequence
reset bit set).
While the switching communication controller 8 has
been described in Figure 8 as connecting a data service system
10 to selected subscriber data systems, it is within the scope
of the present invention to provide connection of the sub-
scriber data systems to other services in a similar way, such
as to the packet switching system 18 shown in Figure 1, as well
as other types of communication systems. Indeed, the invention
is not to be limited to the details of the exemplary
embodiments specifically disclosed herein, but encompasses all
of those changes and modifications which fall within the
general principles set forth in this description.
--~0--
', ,. ~.
