Note: Descriptions are shown in the official language in which they were submitted.
NETWORK SIGNALLING PROTOCOL FOR COMMUNICATIONS SYSTEM
This invention relates to a network signalling
protocol and more particularly to one for establishing and main-taining
communication between a plurality of s-tations ~"hen connected to a
common transmission lineO
Background of the Invention
In communication systems it is often necessary to
transmit signalling and control information from each node or station
to one or more of the other stations over a common transmission line.
In one arrangement, each station is allocated a unique time slot in
which to transmit its pertinent signalling information. In general,
such systems must be capable of adding stations to or removing them
from the common transmission line without disrupting the operation of
the balance of the network.
The capacity of such a system is determined by the
maxilnuln number of time slots which have been allocated by the network
protocol. However if much o-f the time this maximum capacity is not
being utilized then it is more efficient to eliminate the time slots
of the unused nodes or stations. This can also help to alleviate many
of the problems in coupling two independently operating portions of
the network together, which can occur when a fault or break in the
transmission line has separated major portions of the network from
each other. After this break occurs, communications must continue
between the functioning portions of the network. However, when the
fault is rectified, the two portions of the network must be rejoined
with minimal disruption to each other.
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If the operating sequence has been compacted so as to
eliminate unused time slots, it is necessary to in-form all oF the
stations when one or more stations wants to join the network so that
the required time slots can be allocated. When a single station joins
the network it can first monitor the operation of the other stations
on the transmission line. However, operating sub-sec-tions which join
each other cannot initially monitor each other without interference
occurring on the common transmission line. The key therefore is to
provide a protocol in which all of the stations on the network can be
made aware that one or more stations will be joining the network or
portions of the network will be joined to each other.
Statement of the Invention
The present invention provides an improved network
signalling protocol for communicating between a plurality of sta-tions
when connected to a common transmission line. The protocol at each
station comprises ini-tializing the network by transmitting in a
commonly recogni7ed sequence, at least a header signal from each
station in a uniquely allocated time slot which commences a
corresponding number of time slots following the previously
transmitted signal. The protocol includes marking each time slot in
which no signal has been transmitted and thereafter transmitting at
least the header signal for the station in the next time slot
following that of the immediately preceding transmitted signal
so as to eliminate the marked time slots in which no signal has
been transmitted.
In order to inform each of the stations that the
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network is to be reinitialized, the protocol provides for allocating
one additional time slot, commonly referred to as a null slot, in
which any one station can insert an initializing request signal
(i.e. a break signal for a preselected time in-terval) in response to
selected conditions at that station. This occurs whenever one or more
stations are to join the network. When opera-ting portions of the
network are initially joined together, interference or clashing o-f the
signals occurs on the line. This is recognized by each of the
stations on the network and the result is that all stations then cease
transmission. Thereafter, the network is reinitialized by following
the same sequence as the first initialization.
Brief Description of the Drawings
An example embodiment of the invention will now be
described with re-ference to the accompanying drawings in which:
Figure 1 is a block and schematic diagram of a network
encompassing the signalling protocol of the present invention in which
one of the s-tations is illustra-ted in further detail, and
Figures 2A and 2B illustrate the typical time slots
of the network illus-trated in Figure 1 both during and after
initialization.
Description of the Pre-Ferred_Embodiment
Referring to Figure 1, the network comprises 15 nodes
or stations of which stations 9 to 12 are shown in block form while 10
is illustrated in further detail. It will be evident tha-t the
principles could be readily extended to large networks with hundreds
of nodes.
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The network uses a common transmission line 20 having
two unidirectional channels 21 and 22 transmitting in easterly and
westerly directions respectively.
Referring more particularly to station IO which is
typical of each of the stations in the network~ there is i11ustrated a
microprocessor control uni-t 30 in which signalling information such as
the status of the station is coupled to and -from the control unit 30
on data signalling lines 31 and 32. Each station can function as an
end terminal for portions of the network so that information can be
IO received and transmitted in one direction only. Alternately, the
station IO can be effectively bypassed so that all information on
the common transmission line 20 passes directly there-through.
Information then received from station 9 passes through an OR gate 23,
the output of which is coupled to station 11. Similarly, signalling
information from station 11 is passed through an OR gate 24 to
station 9.
All received information on the transmission line 20
is coupled from the output of OR gates 23 and 24 through OR gate 25
to the control unit 30. This arrangement insures that all signalling
information on the line 20 is received by the station and that the
information transmitted to the line 20 from the local station IO is
the same as that appearing on the line. Any discrepancy can indicate
interference from another station which will result in the ne-twork
being reinitialized.
Figure 2A illustrates the sequential transmission from
each station in its allotted time slot during initialization. Each
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closed segment indicates signal transmission -From that station during
its alloted time slot, while an open segmen-t indicates no transmission
by the station. The latter time slots are marked by each of the o-ther
stations so that the system can be compacted during subsequent cycles.
The null slot 0 is used for requesting initialization of -the network.
To achieve this a break signal from any one or more oF the stations is
transmitted for at least a preselected time interval during this null
slot so as to cause the network to immediately expand to its maximum
capacity as shown in Figure 2A. This minimum time interval is chosen
so that spurious bits or spikes on the line 20 will not cause
reinitialization of the network. Each station on the network then
transmits at least its header signal after a minimum time-out
interval. However this can be followed by additional signalling
information and ends with a trailer signal, which extends the length
of the time slot up to a preselected maximum time-out interval.
Although each station transmits in a unique commonly recognized
seguence, it is common practice For the header signal to include the
address of the transmitting station as well as that of the receiving
station or stations. This ensures that no errors have developed in
the transmitting sequence. When a station fails to transmit during
its allotted time slot, a time interval of twice the transmission time
for the system occurs before the next sta-tion commences transmission
of its header signal. This ensures that each station notes the
absence of transmission and marks the blank time slot. After three
repeated failures of a station to transmit its header signal, the
network will further compact to remove that station from the
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transmission sequence as shown in ~igure 2B. The addresses of each of
the stations are arranged so that the odd-numbered stations transmi-t
in ascending order followed by the even-numbered stations in
descending order. Assuming the sta-tions have been numbered in
increasing order this minimizes the variations in propaga-tion delay
between consecutive stations.
In the illustrated embodiment, stations 4, 5, 13,
and 15, are currently disconnected from or are not transmitting on the
network. When initialization takes place, the lowest numbered station
on the network, (in this case station 1) will immediately after the
null slot commence transmission of its header signal followed if
necessary by additional status or other signalling information and a
trailer signal. This is recognized by station 3 which then commences
transmission immediately thereaFter. Since station S is disconnected
from the network, no transmission takes place immediately following
the end of transmission from station 3. However after a further
minilnum time-out interval has elapsed, station 7 commences
transmission of its header signal and other signalling information.
This is followed by stations 9 and 11. Since both stations 13 and 15
are disconnected from the network two time slots will elapse before
station 14 commences transmission oF its header signal. This is
followed by sta-tions 12, 10, 8, and 6, and after another time slot
interval for station 4, which is also disconnected from the network,
station 2 transmits its header signal. The one additional null slot 0
is then automatically inserted by all stations prior to the repe-tition
of the sequence.
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During the initial sequence each sta-tion marks the -time
slots 4, 5, 13, and 15 in which no address has been transmitted.
Thereafter, with the exception of null slot 0, each sta-tion as shown
in Figure 2B, transmits its header signal right after the reception oF
the signal from the station which transmitted immediately preceding it
so as to eliminate the marked -time slots in which no header signal was
transmitted. Thus continued transmission takes place from each of the
stations on the network with the exception of null slot 0 in which no
transmission normally takes place.
However, should one of the stations 4, 5, 13, or 15,
attempt to join the network, an initialization request (or break)
signal is stuffed into the null slot 0 by this station alerting all
other stations they must go through a reinitialization sequence.
Reinitialization can also take place if any station detects the
signalling information being transmitted to the transmission line 20
is not the same as that appearing on the line. This can occur
whenever a -Fault develops on the line 20. That station will then
insert a break signal in time slot 0 which also results in a
reinitialization of the network. Thus any station connected to the
network, whether currently transmitting or not, can trigger
reinitialization of the network. The transmission sequence then
expands to its maximum number of nodes or stations and af-ter marking
each of the time slots in which no header signal has been transmitted,
contracts to only the active stations with the exception of the
additional null slot 0. Each station recognizes a minimum time slot
as equal to twice the maximum propagation time between alternate
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numbered nodes on the system.
One advantage of compacting the network to only ac-tive
stations is that interference will occur immediately upon opera-ting
portions o-f the network being joined to each other. If the -two
portions remain in an expanded state they could conceivably be
superimposed without interference occurring for some time. While they
would eventually clash with each other, there is a prolonged period in
which there is no controlled flow of information through the network.
It is therefore desirable that the network reinitialize immediately
upon the sections of the network being joined to each other.
When two operating portions of the network are
initially coupled together signal clashing will be immediately evident
to the two stations transmitting at that instant. Each station will
then attempt to retransmit its entire signal up to its maximum
time-out interval at which time they will cease transmission. At this
point all of the other stations on the network will be aware of the
garbled nature of the transmission and once transmission of both
stations has ceased, each station will recognize it as the beginning
of a null slot prior to reinitialization o-f the network. Each station
in turn will then commence to transmit at least its header address in
the assigned order.
