Note: Descriptions are shown in the official language in which they were submitted.
~L23~D~
SUPERVISORY CONTROL NETWORK PROTOCOL FOR COMMUNICATION SYSTEM
This invention relates to a supervisory control network
protocol for a communication system in which both solicited and
unsolicited messages can be transmitted along a single path to provide
information on the status of the communication system.
Lund of the Invention
Multi channel transmission systems which are used to
provide communication links generally require a supervisory control
network to monitor the status of the various stations along the
system. In prior systems, the soliciting of information on the status
of each of the communications channels in the transmission system has
been done on both a continuous and intermittent basis. It is
important that this status information be transmitted with a minimum
delay when it is used to signal the need for switching in of a standby
channel whenever a fault develops on one of the working channels ox
the communication system. One approach to minimizing this delay is to
transmit a digital signal in such a way that status information about
each of the channels of the system can be updated one bit at a time
at each station along the network. This approach results in a delay
of about one bit time as the signal passes through each station over
and above propagation delay. Systems which modify data on a per
character basis will experience a one character delay ( 10 bits) at
each station.
However, a disadvantage of polling is that Mach
station must wait until a request for status in-formation is made in
order that the receiving station can accurately interpret the response
,.
~3~5~
to it. In general, such status information need only be solicited on
an occasional basis providing each station can initiate an error
(or unsolicited) message should a failure be detected in one of its
working channels of the communication system. Thus, it is
advantageous to have a single channel network protocol in which both
solicited and unsolicited messages can be handled at any station
utilizing a relatively simple hardware configuration. One difficulty
arising from such a unidirectional protocol is that upstream stations
cannot be made immediately aware when a downstream station has
initiated an unsolicited message. Consequently, a clash or overlap of
two messages may result so that an upstream message is either lost
entirely or only partially received as a garbled message,
Statement of the Invention
The present invention overcomes this problem by
providing a protocol for transmitting either solicited or unsolicited
status messages over a single channel network so that either an
overlap does not occur, or if it does the message from the upstream
station is blocked so that it does not result in a garbled message
being communicated to the receiver. Hence, only whole solicited or
unsolicited messages are communicated to the receiver. The
information lost when an upstream unsolicited message is deleted by a
downstream message, will eventually be recovered in a solicited
message generated by a background poll (auditing activity).
Thus in accordance with the present invention there is
provided a method of transmitting supervisory signals along a single
path of a supervisory network comprising intermittently transmitting a
~3~65~
solicited message signal of digital bits from a transmitting station
through each of a plurality of intermediate stations to a receiving
station for soliciting status information from at least one of the
intermediate stations, modifying the bits of the solicited message
signal at the intermediate station to provide said information for the
receiving station, and in response to selected conditions at one of
the intermediate stations, transmitting therefrom an unsolicited
message signal of digital bits of status information through the
downstream intermediate stations to the receiving station.
The method also includes preceding each transmission
from the transmitting station of said solicited message signal by an
inhibit (hold-off) signal having a bit length at least equal to that
of the unsolicited message signal and in response to the inhibit
signal, inhibiting the transmission of an unsolicited signal from each
of the intermediate stations until after reception of the subsequent
solicited message signal, thereby avoiding overlap between a solicited
and an unsolicited message signal.
In a particular embodiment, the method includes
transmitting an inhibit signal from said one intermediate station for
the non-overlapping period of the two signals in response to the
reception of an unsolicited message signal from an upstream station
overlapping the transmission of the unsolicited message signal -from
said one intermediate station, whereby the entire signal from the
upstream station is prevented from reaching the receiver and only
complete unsolicited message signals reach the receiver.
~3q~
grief Description of the Drawings
An example embodiment of the invention will now be
described with reference to the accompanying drawings in which:
Figure 1 is a block diagram of a supervisory control
network for a communication system,
Figure 2 illustrates typical message signals generated
at various stations along the network illustrated in Figure 1, and
Figures 3, PA, 4B, illustrate flow charts For the
various header packets which can be transmitted as part of the message
sigrlals by the transmitter and intermediate stations respectively of
the network illustrated in Figure 1.
Description of the Preferred Embodiment
Referring to Figures 1 and 2, the supervisory control
network comprises a transmitter station To connected to a receiver
station Rx through a plurality of intermediate stations 11, 12
through In. With the exception of the receiver Rx, each of these
stations To, 11, It, through In is connected via status
lines to the controlled system which is illustrated as a fiber optics
transmission system. Receiver RX9 which is typically co-located
with the last station In, is the controlling, decision-making,
processor. Stations To, 11...1n are structurally identical but
station To uses a slightly modified algorithm. In a typical
installation, the fiber optics (or alternately radio transmission
system FORTS having d plurality of repeater stations Fit, F1, F2,--Fn,
would include both working We, Winnie and standby S channels which
would be switched in and out under control of the control information
- 4 -
transmitted Jo the respective switch controllers Cut, C1, C2--Cn.
Each of the stations To, If, Inn, Rx of the
supervisory control network includes a programmed microcontroller.
The microcontroller in the receiver Rx, in response to status
information received from the other stations of the network, transmits
switching control information to the controllers Cut, C1, C2 and On
via a control line 20 which is utilized to switch in and out the
various standby S and working We, Winnie channels of the fiber
optics transmission system in a well known manner. Any control
message solicits a response which conveys the success or failure of
the respective station. In addition, the receiver microcontroller
Rx intermittently transmits polling information to the transmitter
microcontroller To in response to which the latter transmits a
solicited message signal for status information of the stations along
the network. This auditing activity is used to recover information
lost in deleted unsolicited messages, or in messages lost due to
transmission errors. Any of the intermediate stations 11, Inn
and the transmitter station To, in response to selected status
information such as a failed working channel in the fiber optics
transmission system, can immediately initiate an unsolicited message
providing a message is not being currently received from an upstream
station.
Referring more specifically to Figure 2, the signaling
protocol of the network is divided into five-bit packet intervals
US which have been numbered for reference purposes only. The output
signal waveforms from each of the stations To, 11, 12, and In, are
:
I
identified by corresponding reference characters. During the idle
state of the network, the transmit station To continuously transmits a
null packet of digital bits having a "01110" bit pattern as
illustrated in packet interval #1. This pattern is repeated by each
of the intermediate stations It, 12, and In with a half-bit delay
being introduced at each of these stations by the algorithm together
with any propagation delay. This half-bit delay is because the
sampling time is delayed from the bit transitions so that the value of
the input bit on which the output is to be based, is reliably known.
Sampling occurs at the center of the bit time. However for ease of
understanding, neither the half-bit delay nor the propagation delay is
shown in Figure 2.
Whenever a failure occurs at a site of the transmission
system, an unsolicited message signal is initiated at the corresponding
station. In this example, an unsolicited packet header HER having a
"01101" bit pattern is initiated at station 12 as shown during
interval #2. this is immediately followed by five packets of
UNSOLICITED DATA giving channel and status information of the
corresponding station F2 in a well known manner. As this data is
transferred through intermediate station In to receiver Rx, station In
modifies the data with additional information on the status of the
channels at station Fun. However, upstream stations such as To and 11
are not aware that station 12 has initiated an unsolicited message
signal. A fault therefore could develop which would result in
either of these stations To and 11 initiating their own unsolicited
message signal. As shown during interval I this has occurred at
- 6 -
I
intermediate station 11 which has initiated an unsolicited packet
header HER that is followed by its own UNSOLICITED DATA during
intervals #6 to #10. In order to avoid a partially garbled signal from
station 11 being transferred to the receiver Rx, station 12 blocks the
transmission from the upstream station whenever it is transmitting
its own UNSOLICITED DATA. In addition, the microcontroller in station
12 which monitors the incoming data from station If has determined that
the overlap commenced three packets after initiating its own
unsolicited packet header (during interval I and consequently it
transmits three INHIBIT packets having a "01010" bit pattern (the same
as the HOLD OFF packets) during intervals #8, #9, and ~10 in order to
totally block the unsolicited message signal from station Il. As a
result, partially garbled signals from upstream stations are inhibited
from reaching the receiver Rx.
At any time the receiver Rx can initiate a poll for
status request. This poll for status request is transmitted via the
control line 20 to the microcontroller in the transmitting station
To. This microcontroller then transmits a SOLICITED HOLD-OFF
pattern for at least five consecutive packets as shown during
intervals I to ~12. This pattern "01010" is the same as that
transmitted from station 12 to provide blocking of the signal from
station 11 following the transmission of its unsolicited message
signal This HOLD-OFF pattern from the transmitter To insures that
none of the intermediate stations If, It or In will initiate
transmission of an unsolicited message signal and also provides
sufficient time for the network to clear any overlapping unsolicited
~23~
message signals, such as is occurring between output signals from
stations To and 11 during intervals #8, I and loo Following the
transmission of the five HOLD-OFF packets, the transmitter To then
transmits a solicited packet header HER having a "01001" pattern
followed immediately thereafter by the solicited data as illustrated
during intervals #13 and ~14. As with the unsolicited data, this
solicited data may be altered as it is passed through each of the
intermediate stations 11, It, and In to provide appropriate status
information for the receiver Rx. As the content of this data is not
JO of significance to the operation of the present invention, it is only
shown in block form and has not been specifically defined.
Referring to the flow charts illustrated in Figures 3
and PA, 4B, there are Four possible five-bit header packets which can
be transmitted by the transmitter station To, and by the intermediate
stations 11, 12---ln along the network. These header packets are
"Oily" null, "Oilily" unsolicited, "Oilily" solicited, and "Oilily"
hold-off. The first two incoming bits of each header with a "O-1"
transition, are always used for bit synchronization. These first two
bits are normally not modified by the local station. If the third
received bit of each header is a "0" solicited data or a hold-off
packet is indicated. Conversely, if bit 3 is a "1" unsolicited or a
null packet is indicated. The fourth bit if a "O" advises that a
25-bit message will follow (solicited or unsolicited) and if a "1" it
is a hold-off or null packet. Bit 4 is repeated, inverted, in bit 5
to minimize the chance of an error causing a false message start
recognition.
I
As shown in Figures PA, 4B, if the local station is in
an "inhibit phase" (which is used to block an unsolicited message
signal), the last three bits of the header packet are overwritten and
a "0, 1, 0" sequence is unconditionally transmitted during bits 3, 4,
5, respectively. However if the local station is to transmit
unsolicited data it will modify the incoming unsolicited data, if
bits 3, I, 5 of the header are "1, 0, 1" and will transmit its own
unsolicited data if these incoming header bits are "1, 1, 0",
respectively. Thus, if two stations originate unsolicited messages at
the same time (ignoring propagation and half-bit delays) then the
downstream station will alter the message of the upstream station.
At each of the intermediate stations (during the
transmission of an unsolicited message originated at that station)
input data is sampled at bits 4, 9, 14, 19 and 24 (i.e. the Thea bit of
every 5-bit packet). If a "0" bit is detected, then the station
enters the 'inhibit phase' and 1, 2, 3, 4, or 5 inhibit packets are
added at the end of transmission of the unsolicited data (depending
upon which of the five packets had the first "0") to inhibit the
non-overlapped portion of the up-stream transmission.
In the figures of this illustrated embodiment, there is
considerable overlap between the transmission of the various message
signals. In practice however, the solicited messages are widely
spaced while the unsolicited messages are only transmitted on demand.
Since the failure rate of any contemporary transmission system is
generally very low, the likelihood of such an overlap occurring
between the various packets is very small. Consequently, when a
3q3~
failure does occur the transmission of an unsolicited packet signal
will generally commence immediately so that there is a minimum time
loss before the signal reaches the receiver ox which in turn transmits
switching control signals back to the stations, thus the total time
required to transmit an unsolicited message after a fault is detected
will be the sum of: 5 bits/header latency (Max), n x 1/2 bit station
delay (where n = number of downstream stations), propagation delay,
and 30 bit serialization delay (one 5-bit header packet plus five
5-bit data packets), Solicited messages additionally include at
least a 25 bit inhibit signal. Such a protocol minimizes the overall
delay in initiating a transfer from a working channel to a standby
channel in the fiber optics or other transmission system,
- 10 -
