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BACKGROUND OF THE INVENTION
The present invention relates to a route selection method for
telecommunications networks and, more particularly, to an adaptive routing
control method which permits optimum routing according to the network
status (trunk usage, offered traffic volume, or congestion conditions).
In telecommunications networks with a plurality of switching nodes routes
for interconnecting them usually include a first route which achieves the
most economical call connection between each originating-terminating node
pair. When the first route is not busy, the first route is used to
interconnect the originating and terminating nodes, whereas when the first
route is busy, alternates routes can be established via one or more other
switching nodes. With such a conventional route selection algorithm,
however, switching nodes through which alternate routes can be established
are limited and the order of their selection also is fixed because of
technical restrictions inherent to the call-connection control system
employed.
With the recent introduction of switching nodes of a stored program control
system and a common channel signaling inter-office system for an
inter-office signal transfer, it has become possible to utilize, in place
of the above-mentioned route selection algorithm, a dynamic routing method
which affords flexible routing based on the distribution of idle trunks in
the network.
The dynamic routing method may be classified into time-dependent routing
and state-dependent routing (see B. R. Hurley, et al., "A Survey of
Dynamic Routing Methods for Circuit Switched Traffic," IEEE COMMUNICATIONS
MAGAZINE, Vol. 25, No. 9, pp. 13-21, September 1987, for example).
The time-dependent routing is a method in which a suitable routing pattern
is preset for each predetermined time slot, i.e. a method in which a set
of alternate routes and the order of their selection are preset for each
first route and a call originating in a switching node is connected to the
intended destination node, following the routing pattern preset for the
time slot concerned. A typical example of the time-dependent routing is a
DNHR (Dynamic Nonhierarchical Routing) system proposed by AT & T, Inc. of
the United States (see G. R. Ash, et al., "Design and Optimization of
Networks with Dynamic Routing," BSTJ, Vol. 60, pp. 1787-1820, October
1981, for instance).
The state-dependent routing is a method which performs a call connection
while updating the routing pattern in real time in accordance with the
network status such as trunk usage in the network. This method is
implemented by centralized or distributed control.
In the state-dependent routing method by centralized control a network
control center collects data about the trunk usage throughout the network,
calculates a routing pattern between each originating-terminating node
pair, and indicates the routing pattern to each switching node in real
time. An example of this state-dependent routing method by centralized
control is a TSMR system proposed by AT & T, Inc. of the United States and
a DCR system by Northern Telecom of Canada (see the afore-mentioned
literature by B. R. Hurley, et al., for instance).
In the state-dependent routing method by distributed control each switching
node independently detects the network status and autonomously searches
for an alternate route based on the network status information, thereby
setting an appropriate routing pattern between an origin-destination node
pair. Examples of this method are those proposed by British
Telecommunications of Great Britain and Centre National D'etudes des
Telecommunications of France (commonly known as "CENT"). Both methods are
common in basic principle, and the method by British Telecommunications is
called a DAR system (see B. R. Stacey, et al., "Dynamic Alternative
Routing in the British Telecom Trunk Network," International Switching
Symposium, ISS-87, B12.4.1-B.12.4.5, 1987, or Hennion B., "Feedback
Methods for Calls Allocation on the Crossed Traffic Routing,"
International Teletraffic Congress, ITC-9, pp. HEENNION-1 to HENNION-3,
1979, for example).
Some proposals have been made so far for the dynamic routing as mentioned
above but they have the following problems yet to be solved for practical
use.
(i) The time-dependent routing of the aforementioned DNHR system, for
instance, would work well in a country like the United States where a
plurality of standard times are used, the traffic busy hour differs
sharply with regions, an appropriate routing pattern for each time slot
can be forecast, and updating of the routing pattern can be scheduled.
Where the traffic busy hour is common almost all over the country as in
Japan, however, the time-dependent routing, if used singly, would not be
so effective. In a country like Japan it is of prime importance to
efficiently handle offered traffic, quickly responding to an excess or
shortage of the trunk-number of transit links which is caused by
restrictions on the management of trunk resources such as the trunk
assignment interval, the trunk modularity, etc. or unpredictable traffic
variations, and the state-dependent routing is more effective rather than
the time-dependent routing.
(ii) In general, the state-dependent routing by centralized control permits
efficient routing, because a routing pattern can be indicated based on the
optimization of the entire network through observation of its status, for
example, the trunk usage in the network. However, in the case where the
observation cycle is long or an information transfer delay occurs, that
is, where a time lag is great between the observation and the execution of
a call connection by a routing pattern based on the observation, the state
of the network varies during this time resulting in an increase in the
probability of effecting erroneous control. This will not produce the
intended effect and will lower the call-connection quality.
To avoid such a problem and hence achieve the intended effect, it is
necessary to reduce the network status observation cycle and the switching
node control cycle. The aforementioned TSMR or DCR system, for example,
premises that both cycles are within 10 seconds. In a large-scale
telecommunications network in which the number of switching nodes to be
controlled is several hundreds and the number of links to be measured is
as large as tens of thousands, however, such a high-speed observation and
control are difficult. In other words, the amount of data to be processed
by the network control center, the amount of data to be transferred
between the switching nodes and the network control center, and
measurements in the switching nodes and the amount of data to be
transmitted and received among them are enormous and the facilities
therefor are also vast, resulting in an uneconomical system. In addition,
a failure in the control center of such a large-scale network will throw
the network into disorder.
(iii) With the a aforementioned DAR system and the self-routing system in
the state-dependent routing by distributed control, no network control
center is employed and each switching node checks the status of alternate
routes by a signal handled in its call-connection procedure and
autonomously changes an alternate route accordingly, thereby implementing
a preferably routing pattern throughout the telecommunications network.
Consequently, the problem mentioned above in (ii) can be avoided. In a
large-scale telecommunications network, however, the number of alternate
routes for each origin-destination node pair becomes appreciable,
incurring various disadvantages. For instance, in a telecommunications
network which forms a mesh by 100 switching nodes the number of alternate
routes via two transit links between each origin-destination node pair
alone is as large as 98.
In such an instance, (a) alternate routes are rechecked through a search by
trial and error prior to a call-connection procedure, and consequently,
when the number of available alternate routes is unnecessarily large, the
search is repeated inevitably many times until a routing pattern updated
according to temporary traffic variations is restored to its initial
state. Similarly, when a traffic pattern throughout the network changes or
transmission equipment breaks down, the search is repeated many times
until each switching node shifts to a new favorable routing pattern. This
will deteriorate the call-connection quality and increase the amount of
data to be processed by each switching node. (b) An increase in the amount
of data managed by each switching node calls for an increase in the number
of tables for processing data and the number of counters for counting the
number of calls. That is to say, the amount of data which is managed for
each origin-destination node pair or each first route increases, and
consequently, alternate route tables are required and the state of
alternate routing must be monitored from the viewpoint of network
management. This necessitates a number of counters for counting the number
and the traffic volume of alternate calls and the transit-call-completion
probability in each alternate route. Moreover, (c) an increase in the
number of counters used will cause an increase in the computer running
time to be processed for measurement by the counters.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide an adaptive
routing control method which obviates the above-mentioned defects of the
prior art, enables an optimum alternate route to be selected in accordance
with real time traffic variations and the current network conditions
(which consist of a network topology and a matrix of the number of trunks
between each node pair), and affords the reduction of the amount of data
to be managed by each switching node and the number of tables and counters
used even in a large-scale telecommunications network.
To attain the above objective, in the telecommunications network to which
the adaptive routing control method of the present invention is applied, a
plurality of switching nodes are interconnected via links each composed of
a plurality of trunks, one or more routes each composed of a set of one or
more links are present between each node pair, and at least one network
control center is connected via a control signal link to each switching
node. According to the present invention, the network control center
adaptively determines, for each node pair, a set of available routes each
composed of one or more routes which are set available in accordance with
the traffic volume in the telecommunications network and the number of
trunks set for each link. The network control center sends the sets of
available routes to each switching node and, at a predetermined time,
updates the set of available routes and resends them to each switching
node. Each switching node responds to a call-connection request to select
one of the available routes and performs a required call-connection
procedure.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram illustrating an example of the
telecommunications network to which the adaptive routing control method of
the present invention is applied;
FIG. 2 is a function-block-chart of a network control center NC in the
telecommunications network depicted in FIG. 1;
FIG. 3 is a table I showing sets of available alternate routes for
respective first routes and currently assigned routes, provided to a
switching node N1 in the telecommunications network depicted in FIG. 1;
FIG. 4A is a flowchart showing a call-connection procedure in an
originating node;
FIG. 4B is a flowchart showing a call-connection procedure in a terminating
node;
FIG. 4C is a flowchart showing a call-connection procedure in a transit
node;
FIG. 5 is a table II showing available or unavailable status of assigned
alternate routes;
FIG. 6 is a flowchart showing another example of the call-connection
procedure in the originating node;
FIG. 7 is a table III showing the numbers of idle trunks recorded for
respective alternate routes and their choice probabilities determined in
accordance with them;
FIG. 8 is a flowchart showing another example of the call-connection
procedure in the originating node according to the routing control method
of the present invention;
FIG. 9 is a function-block-chart of a network control center of the
telecommunications network;
FIG. 10 is a schematic diagram showing an overflow traffic volume or the
margin of traffic volume calculated for each link on the basis of the
end-to-end traffic volume in the telecommunications network so as to
determine a set of available alternate routes for each link;
FIG. 11 is a flowchart showing an example of the procedure for determining
the sets of available alternate routes;
FIGS. 12A through 12F are schematic diagrams showing an example of the
procedure for determining the sets of available alternate routes;
FIG. 13 is a flowchart showing another example of the procedure for
determining the sets of available alternate routes;
FIG. 14 is a flowchart showing another example of the procedure for
determining the sets of available alternate routes;
FIG. 15 is a flowchart showing still another example of the procedure for
determining the sets of available alternate routes;
FIG. 16 is a graph showing the number of available alternate routes in each
set and the call-completion probability, for explaining the effect of the
present invention;
FIG. 17 is a graph showing the relationship between calculated traffic
forecasting errors and the call-completion probability, for explaining the
effect of the present invention;
FIG. 18 is a schematic diagram showing a telecommunications network
including a communications satellite link to which the routing control
method of the present invention can be applied; and
FIG. 19 is a schematic diagram for explaining the relationship between a
transmission network and communication links in the telecommunications
network.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In FIG. 1 there is shown the general constitution of the telecommunications
network embodying the adaptive routing control method of the present
invention. A plurality of switching nodes N1 to N6 are interconnected via
solid-line links L12, L23, . . . to form various routes. The links L12,
L23, . . . each include a preset number of trunks. A network control
center NC is provided in association with these switching nodes N1 to N6.
The switching nodes N1 to N6 are connected to the network control center
NC via broken-line control signal links C1 to C6, respectively. The
switching nodes N1 to N6 each possess the functions of an originating node
which originates a call, a transit node which relays the call, and a
terminating node which is the destination of the call.
Now, definitions will be given of some terms which will be used in the
following description of embodiments of the present invention.
First Route: A predetermined route which connects two arbitrary switching
nodes for a call-connection. When there is one link which directly
connects the two switching nodes, it is used as the first route, and when
such a link is not found, a predetermined route is used as the first route
which connects them via one or more other switching nodes (i.e. transit
nodes).
Possible Routes: Routes through which two arbitrary switching nodes can be
connected in the communications network shown in FIG. 1. In the
description of the present invention they are defined as routes, each
formed by a maximum of two links.
Set of Available Routes: One or more routes selected by the network control
center from all the possible routes.
Alternate Routes: Possible routes except the first route.
Outgoing Link: A link from which a call is sent out from each switching
node.
First and Second Alternate Links: A link which connects an originating and
a transit node over an alternate route will be referred to as a first
alternate link. A link which connects the transit node and a terminating
node will be referred to as a second link.
Set of Available Alternate Routes: One or more alternate routes preselected
by the network control center from all alternate routes for the first
route which connects two arbitrary switching nodes.
In the embodiment of the present invention which is applied to the
telecommunications network depicted in FIG. 1 the network control center
NC predetermines, for each time slot, sets of available alternate routes
which are used by the switching nodes N1 to N6, respectively, and
transfers the predetermined sets of available alternate routes to the
switching nodes N1 to N6 at predetermined times. The switching nodes N1 to
N6 each respond to a call-connection request to preferentially search the
first route for an idle trunk, and when no idle trunk is found in the
first route, the switching node tries a call-connection via an alternate
route adaptively selected, in accordance with the trunk usage, from the
set of available alternate routes transferred from the network control
center NC. In the following description a link which directly connects two
arbitrary switching nodes Ni and Nj (where i and j are positive integers,
which are not equal to each other) will be identified by Lij and a route
which connects the two switching nodes via transit node Nk (where k is a
positive integer, which is not equal to the integers i and j) will be
identified by Rikj.
Switching Node
FIG. 2 is a function-block-chart of each of the switching nodes N1 to N6 in
the telecommunications network shown in FIG. 1. The switching node Ni
performs network-control-data transfer operations 21, call-connection
signal processing 22, call processing 23 and routing data management 24.
The network-control-data transfer operations 21 includes an operation 21a
of receiving routing data, i.e. sets of available alternate routes from
the network control center NC via the control signal link Ci and an
operation 21b of transmitting network data to the network control center
NC via the control signal link Ci. The call-connection signal processing
22 includes: a trunk-status-information transfer operation 22a of
receiving trunk status information from other switching nodes or
transmitting trunk status information in the switching node Ni via the
links Li1, Li2, . . . , Lij, . . . ; a transit-call-blocking signal
transfer operation 22b of sending a transit-call-blocking signal back to
an originating node in the case of a failure in the transit-call
connection because of no idle trunk being found in the outgoing link of
the switching node Ni when it acts as a transit node, or receiving the
transit-call-blocking signal from a transit node when the switching node
Ni acts as an originating node; and a completion/blocking signal transfer
operation 22c of sending the call-completion signal or call-blocking
signal to an originating node when the switching node Ni acts as a
terminating node, relaying the call-completion signal or call-blocking
signal to an originating node when the switching node Ni acts as a transit
node, or receiving the call-completion signal or call-blocking signal when
the switching node Ni acts as an originating node. The call processing 23
includes: an outgoing trunk selecting operation 23a for connecting a call
to an idle trunk of a desired link in response to a call-connection
request; a trunk holding operation 23b for performing a call-connection
procedure when receiving the call-completion signal from a terminating
node; a call-information transfer operation 23c for selecting an
appropriate route to the terminating node in response to the
call-connection request and a call-blocking operation 23d for performing a
call-blocking procedure when the call connection to the intended
terminating node in response to a call-connection request has finally been
blocked. The routing management 24 has databases 24A and functions 24B.
The databases 24A include: available alternate routes 24a, i.e. the
aforementioned sets of available alternate routes received from the
network control center NC; currently assigned alternate routes 24b
selected from the set of available alternate routes 24a; unavailable
alternate routes 24c selected from the currently assigned alternate routes
24b; outgoing-trunk-status information 24d indicating the number of trunks
provided in each outgoing link of the switching node Ni; and trunk-status
information 24e indicating the busy/idle status of the trunks of each
link. The functions 24B includes an assigned alternate route
initialization/updating function 24f of determining and updating the
assigned alternate routes, a function 24g of setting the assigned
alternate routes available/unavailable and a trunk-status observing
function 24h.
Let it be assumed that the switching nodes, for example, N1 and N4 are an
originating and a terminating node in the telecommunications network shown
in FIG. 1. In general, the most economical route L14 is selected as the
first route, and when no idle trunk is found in the link L14, an alternate
route is used. In this instance, possible alternate routes are R134, R164,
R124, and R154, but the network control center NC specifies and indicates
in advance to the switching node N1 a set of available alternate routes
for each first route as shown in Table I of FIG. 3. The available
alternate routes to the switching node N4 are routes R134, R154 and R164
which pass through transit nodes N3, N5 and N6, respectively. Based on
trunk status information of each outgoing link of the transit nodes N3, N5
and N6 (i.e. the second link of each available alternate route) the
switching node N1 selects in advance from the set of available alternate
routes at least one route which is expected to be high in the
call-completion probability, the alternate route or routes thus selected
being assigned as shown in Table I. The switching node N1 selects one of
the assigned alternate routes and tries a call connection.
FIGS. 4A, 4B and 4C are flowcharts showing call-connection procedures which
each switching node performs, FIG. 4A showing a process flow primarily for
an originating node, FIG. 4B a process flow for a terminating node, and
FIG. 4C a process flow for a transit node.
In FIG. 4A, upon detection of a call, the switching node identifies the
type of the call in step S.sub.1, and if it is a terminating call to the
switching node, the process shifts to the process flow shown in FIG. 4B.
The switching node checks in step S.sub.B1 whether or not a trunk to a
subscriber or local node is idle which is the destination of the call, and
if the trunk is idle, the switching node connects the call to the
subscriber (or local node) in step S.sub.B2 and then sends a
call-completion signal back to the originating node in step S.sub.B3.
Where the trunk to the subscriber or local node (hereinafter referred to
as a subscriber trunk, for the sake of brevity) is busy in step S.sub.B1,
the switching node sends a call-blocking signal back to the originating
node in step S.sub.B4.
Where it is determined in step S.sub.1 in FIG. 4A that the call is an
alternate call, the switching node performs the processing as a transit
node, shown in FIG. 4C. In step S.sub.C1 it is determined whether there is
an idle trunk in the outgoing link to the terminating node which is the
destination of the call, and if the idle trunk is found, the call is
connected to the terminating node through the idle trunk in step S.sub.C2.
Thus the call is sent to the terminating node, which performs the
processing shown in FIG. 4B; namely, the terminating node sends a
call-completion or call-blocking signal back to the transit node in step
S.sub.B3 or S.sub.B4. The transit node receives the call-completion or
call-blocking signal from the terminating node in step S.sub.C3 and, in
step S.sub.C4, sends the received signal to the originating node together
with trunk-status information of the aforementioned outgoing link of the
transit node. Where no idle trunk is found in the outgoing link in step
S.sub.C1, a call-blocking signal and a transit-call-blocking signal (also
referred to as trunk-busy signal) indicating the occurrence of call
blocking in the transit node are sent back to the originating node in step
S.sub.C5. The transit-call-blocking signal is used as trunk status
information.
Where it is detected in step S.sub.1 in FIG. 4A that the call is an
originating call, the switching node performs the following processing as
an originating node. The following description will be given on the
assumption that the switching nodes N1 and N4 are an originating and a
terminating node, respectively, as in the above. It is checked in step
S.sub.2 whether or not there is an idle trunk in the outgoing link L14
which forms the first route to the terminating node, and if an idle trunk
is found, the call is connected to the next node via the first route L14
in step S.sub.3. Thus the call is sent to the terminating node N4, which
performs the processing shown in FIG. 4B and from which a call-completion
or call-blocking signal is sent back to the originating node N1 in step
S.sub.B3 or S.sub.B4. The originating node N1 receives the call-completion
or call-blocking signal in step S.sub.4 in FIG. 4A, and it is determined
in step S.sub.5 which signal was received. Where the received signal is
the call-completion signal, the originating node N1 transfers
call-information to the terminating node N4 in step S.sub.6 and completes
the call-connection procedure. Where it is determined in step S.sub.5 that
the received signal is the call-blocking signal, the process terminates
with a call-blocking operation in step S.sub.7. When no idle trunk is
found in step S.sub. 2, the process proceeds to step S.sub.8, wherein an
available alternate route, for instance, R134 is selected from the
currently assigned alternate routes R134, R154 and R164 for the first
route L14, shown in Table I of FIG. 3. Then it is checked whether or not
there is an idle trunk in the first alternate link L13 of the selected
alternate route R134 in step S.sub.9.
In step S.sub.8, one of the assigned alternate routes is selected randomly,
cyclically, or on a predetermined order basis out of currently assigned
alternate routes. There are two methods to determine busy/idle trunk
status. One method permits the use of the trunk when there is at least one
idle trunk. The other one permits the use of the trunk only when there is
a predetermined number of two or more idle trunk. The latter method is
employed to give the connection of a call using the link as the first
route (which call will hereinafter be referred to as a basic call) high
priority over the connection of an alternate call.
If an idle trunk can be found in step S.sub.9, the process proceeds to step
S.sub.10, wherein the call is connected to the next node, e.g. a transit
node N3. Thus the call is sent to the transit node N3, wherein the process
shown in FIG. 4C is performed. The signal sent back from the transit node
N3 in step S.sub.C4 or S.sub.C5 is received by the originating node N1 in
step S.sub.11, and it is checked in step S.sub.12 whether the signal
received in step S.sub.11 is a call-connection or call-blocking signal. In
the case of the call-blocking signal, the call-blocking operation is
performed in step S.sub.13, and it is checked in step S.sub.14 whether or
not the call-blocking signal is appended with a transit-call-blocking
signal, i.e. a trunk-busy signal. The transit-call-blocking signal means
that no idle trunk was found in an outgoing link L34 of the transit node
N3, and the assigned route R134 which passes through the transit node N3
is set unavailable in step S.sub.15. Then it is checked in step S.sub.16
whether or not the currently assigned alternate routes need to be updated,
and if so, the currently assigned alternate routes are updated in step
S.sub.17.
The updating of the currently assigned alternate routes in step S.sub.16 is
required in the case (a) where all the currently assigned alternate routes
are unavailable, (b) where the number of currently assigned alternate
routes set available is smaller than a predetermined value, or (c) where
at least one of the currently assigned alternate routes is unavailable. In
the case (a), all the currently assigned alternate routes are updated in
step S.sub.17. In the case (b) or (c), all the currently assigned
alternate routes or unavailable ones of them need only to be updated in
step S.sub.17. Where it is determined in step S.sub.16 that no updating is
needed, the procedure ends.
When it is determined in step S.sub.12 that the received signal is the
call-completion signal, this means that the call has been connected to an
idle trunk of the outgoing link L34 in the transit node N3. In this
instance, the call-information is transferred to the terminating node N4
via the transit node N3 in step S.sub.18, and on the basis of the
trunk-status information of the outgoing link L34 in the transit node N3,
appended to the received signal, it is checked in step S.sub.19 whether or
not the alternate route R134 needs to be set unavailable. That is to say,
in the case where, as a result of the connection of the call to an idle
trunk of the outgoing link L34, no more idle trunks exist the number of
remaining idle trunks becomes smaller than a predetermined value, or the
idle trunk ratio becomes smaller than a predetermined value, the alternate
route R134 is set temporarily unavailable in step S.sub.15, and then the
process proceeds to step S.sub.16. Even if it is determined in step
S.sub.19 that the alternate route R134 need not be set temporarily
unavailable, it is checked in step S.sub.16 whether or not the currently
assigned alternate routes need to be updated, because there is the
possibility that the number of currently assigned alternate routes becomes
smaller than a predetermined value.
When it is determined in step S.sub.9 that no idle trunk is found in the
first alternate link L13 of the alternate route R134, the currently
assigned alternate route R134 is set unavailable temporarily in step
S.sub.20. Then it is checked in step S.sub.21 whether or not there still
remain any other currently assigned alternate routes which are available,
and if yes, the process returns to step S.sub.8, repeating the processing
of steps S.sub.8 to S.sub.21. When it is determined in step S.sub.21 that
the currently assigned alternate routes are all unavailable, they are all
updated in step S.sub.22 and the procedure ends after the call-blocking
operation in step S.sub.23. Incidentally, the updating of the currently
assigned alternate routes in step S.sub.22 is performed by the same
operation as used in step S.sub.17.
When the currently assigned alternate routes are all unavailable in step
S.sub.21, there is another method. In this method, it is possible to keep
the call call-waiting in the broken-linked step S.sub.24, all the
currently assigned alternate routes are updated in step S.sub.22 and then
it is determine in the broken-line step S.sub.25 whether to retry the
connection of the call held call-waiting. If it is determined to retry the
call-connection, the process goes back to step S.sub.8 as indicated by the
broken line, trying the call-connection to one of the updated currently
assigned alternate routes. If it is determined in step S.sub.25 not to
retry the call-connection, the call-blocking operation is carried out in
step S.sub.23. This improves the call-completion probability. The return
of the process from step S.sub.25 to S.sub.8 for retrying the
call-connection is limited to only once, for example.
There are two methods of setting the selected alternate route of the
currently assigned ones routes temporarily unavailable in step S.sub.15 in
FIG. 4A. First, the currently assigned alternative routes are set
unavailable for a predetermined time period from the time set in step
S.sub.15 in the process flow of the originating node (in FIG. 4A) or for a
time period determined according to the trunk-status information received
from the transit node. Second, the transit node sends back the
trunk-status information to the originating node together with information
of its observation time in step S.sub.C4 in the process flow of the
transit node (in FIG. 4C) and the originating node sets the currently
assigned alternate routes unavailable for a predetermined time period from
the trunk-status observation time or for a time period determined
according to the trunk-status information. In either, case, the time at
which each alternate route is released from the unavailable status is
calculated in step S.sub.15 and is stored as shown in Table II of FIG. 5.
In step S.sub.8 one of the alternate routes which have already been
released from the unavailable status at the current time is selected by
referring to Table II of FIG. 5.
The aforementioned trunk-status information which determines the
unavailable-status period of the currently assigned alternate routes is,
for instance, the number of idle trunks, and the smaller the number of
idle trunks, the longer the unavailable-status period is set. For example,
when the number of idle trunks is zero, the unavailable-status period is
set to 15 seconds, and when two or more trunks are idle, the
unavailable-status period is zero seconds. Since the trunk status of links
is usually ever-changing, the method of setting the unavailable-status
period on the basis of the aforementioned trunk-status observation time is
advantageous in that the unavailable-status period of the alternate routes
can be set independ | | |
