CA 02203~38 1997-04-23
WO96/15609 PCT ~ 5/00616
Overload prevention in a telecommunications network node
Field of the Invention
The invention relates generally to traffic
control in telecommunication networks. More
specifically, the invention relates to a method and an
arrangement for preventing an overload in a
telecommunication network.
The invention is intended especially for so-
called Intelligent Networks (IN) that are being
developed at present, but the same principle can be
applied in any network wherein two or more nodes are
interconnected in such a way that at least one of the
nodes can be loaded by one or several other nodes.
Background of the Invention
An Intelligent Network usually refers to a
network comprising more intelligence (i.e. a better
ability to utilize information stored in the network)
than the present public (switched) networks. Another
characteristic of the Intelligent Network is that the
network architecture somehow draws a distinction
between, on the one hand, the operations concerning the
switching itself and, on the other hand, the stored data
and its processing. Such a division makes it possible
that, in principle, the organization providing network
services can be different from the organization managing
the physical network in which the services are provided.
Conceptually, an Intelligent Net:work can be divided into
three parts. The first part comprises the nodes
switching traffic (performing connections), the second
part contains the services prov:ided by the network, and
the third part consists of the internodal communication
protocol, i.e. the "language" the machines use to
communicate with one another. Since all services must
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be presented as a sequence of messages conforming with
the protocol, the protocol defines the "intelligence"
of the network.
In order to facilitate the underst~n~;ng of the
present invention, reference is first made to a simple
basic situation illustrated in Figure l, wherein two
~rh; n~ ( or network nodes) l and 2 are shown, the
m~ch;nec being interconnected by means of a sign~ll;ng
link 3. Machine l comprises a database DB, and m~r.hi ne
2 is a client asking questions from machine l by
transmitting messages to machine l over the link 3. When
m~.hi n~ l receives a question, it initiates a
transaction resulting in an answer after a certain
processing period. When the answer is ready, machine l
transmits it to machine 2 over the link 3. Each answer
costs machine 2 a certain sum.
A theoretical omnipotent machine l would answer
each question ;m~;ately so that the correlation
between the questions rate (questions per time unit) and
the answering rate (answers per time unit) would look
like the description in Figure 2a. However, there is in
practice a limit to how fast machine l can provide
answers. Taking this into account, the response curve
of the omnipotent machine l becomes like the one shown
in Figure 2b. When the questions rate exceeds a certain
threshold Amax corresponding to the highest possible
answering rate, the latter rem~; n~ constant, i.e. some
of the questions will not be answered. However, this
situation does not correspond to a real situation,
either. In practice, the situation is such that as the
questions rate ~xcP~ a certain threshold value for a
long period of time, machine l b~r,o~ overloaded so
that the increasing questions rate further reduces the
answering rate. This situation is illustrated in Figure
2c. The decreasing answering rate is due to the fact
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that the ~ h;ne starts wasting its resources, for
example in such a way-that it reserves more and more
free memory for storing the questions, so there will be
correspon~l; ngly less and less memory available for
computing the answers. The t;hreshold value of the
questions rate at which an overload situation occurs is
not constant, but it depends on how much of the capacity
of machine l is dedicated to answering. For example, the
threshold value is lower than usual when the database
DB of machine l is being updated.
The purpose of any overload prevention method
is to make the curve (Figure 2c) describing a real
situation resemble as closely as possible the curve
(Figure 2b) describing an ideal situation. On the other
î5 hand, it is reasonable to provide the overload
prevention of m~r.h;n~3 1 partly in machine 2, so that
machine 2 would not have to load the transmission
connection between the machines by transmitting messages
that would be discarded by machine l.
Suppose that in order to protect itself, the
overloaded machine l transmits to m~.h;ne 2 a
restriction or filtering request with which it requests
machine 2 to reduce the number of questions to be
transmitted. Such a request -typically contains two
restriction parameters: the upper limit of the questions
rate U (i.e. the upper limit of the number of questions
performed per time unit) and the duration of the
filtration T (i.e. of the restriction). When machine 2
receives such a request, it begins to filter (restrict)
the questions traffic so that the questions rate will
be at most U, so that part of the questions will fail
(they will not even reach machine l). Machine 2
- continues this restriction operation for the period of
time T indicated in the restriction request. If machine
2 receives a new request during this period, the upper
-
CA 02203~38 1997-04-23
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limit of the questions rate and the interval will be
updated to correspond to the new values. Instead of the
upper limit of the questions rate, the parameter U may
also indicate the proportion of all service request
messages ~ch;ne 2 should transmit to marh;ne 1. For the
sake of clarity, only the former meaning (upper limit
of the questions rate) will be used hereinafter for the
parameter U, unless otherwise mentioned.
When m~Chi rle 2 uses the above-described
overload prevention mechanism, it has two problems.
The first problem is how to select the
aforementioned parameters U and T. A long filtration
time T and a low value of the parameter U ~;min;sh the
overload, but they also entail a clearly lower revenue
for ~rh;ne 1. On the other hand, a short filtration
time and a higher value for the parameter U do not
necessarily reduce the number of questions sufficiently
for the overloading situation to be cleared up, and an
overloading situation also means a lower revenue.
A simple way of eliminating this problem is to
divide the response characteristic into consecutive
overload regions Ln (n=0,1,2..) according to Figure 3,
each of the regions having its own values for the
parameters U and T. If, at all times, machine 1 is able
to determine its own load level, then the restriction
parameters can be stored in the machine in a format (Ln:
T, U), so that the machine can retrieve the required
values of the parameters T and U on the basis of the
load level Ln. However, this does not quite eliminate
the aforementioned problem, but shifts the trouble of
selecting the parameters to the operator. There are also
methods by means of which the parameters can be selected
automatically, based on the utilization ratio of the
machine.
-
-
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The other problem relates to when to send and
when not to send restriction requests. Machine l should
transmit the first restriction request when it is close
to b~rnm;ng overloaded. It should then send a
restriction request either when the restriction period
T expires (if the overload condition is still on) or
when the restriction parameters change. Machine l should
not transmit new restriction requests if machine 2
restricts the questions correctly (with the right
threshold value for the questions rate and the right
filtration time T). However, since there is no feedback,
machine l cannot know if and how machine 2 restricts the
questions. If machine 2 is the only source of questions,
then machine l can solve the problem by monitoring the
questions rate and by transmitting a new restriction
request when the rate of the incoming questions Pxr~
the allowed threshold value U. If there are several
machines transmitting questions, then efficient book-
keeping is required to monitor the traffic and this
makes the arrangement complicated.
This second problem is thus of the
synchronization type, since machine l must keep up to
date (i.e. synchronize) the restriction entity that is
in a remote machine according to the loading situation
of machine l at each moment.
The overload preventi~n in the Intelligent
Network operates in a manner that is very similar to the
above-described example. The Intelligent Network
architecture is based on service switching points (SSP)
and service control points (SCP) that make decisions
concerning for example the routing and the charging of
the calls. The Service Control Points, which are
typically clearly fewer in number than the SSPs, contain
knowledge of what the different services do and how to
access the data that the services need. In an
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Intelligent Network, a Service Control Point is like the
~ch;ne l of the above-described example, cont~n~ng a
database, and the SSP is like the ~ch;ne 2 that asks
questions. The above-described synchronization is also
a problem in the Intelligent Network since the
communication protocol between the nodes is not reliable
in this respect.
The above-described example ~onc~rned a network
that was as simple as possible with respect to its
topology. For example an Intelligent Network is a
network with a typically star topology. A star network
basically comprises two kinds of nodes: central nodes
and peripheral nodes. Peripheral nodes generate traffic
that flows towards the central node. When the
Intelligent Network comprises more than one SCP, the
architecture corresponds to several superimposed star
networks sharing peripheral nodes. Figures 4a to 4c
illustrate the above-described alternatives with
reference CN (in the IN: SCP) denoting a central node
and reference PN (in the IN: SSP) denoting a peripheral
node. Figure 4a shows a star network having one central
node CN and three peripheral nodes PN. Figure 4b shows
a star network in its simplest form corresponding to the
example of Figure l (one central node and one peripheral
node), and Figure 4c shows two star networks sharing
peripheral nodes PN.
In addition to the Intelligent Network, many
other networks have a star topology. Examples of such
networks include a network formed of a satellite and
earth stations, wherein the satellite switches traffic
generated by the earth stations, or a network consisting
of a base station controller and base stations of a
cellular network.
In some known (intelligent) networks, the
above-described synchronization is implemented with a
_
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broadcasting method, wherein the~ central node transmits
a restriction request to all the peripheral nodes
~onnpcted thereto whenever its loading situation changes
(or the restriction parameters change for some other
reason, for example when the operator changes them) and
the peripheral nodes respond to each restriction request
they receive with an acknowledgment. The central node
keeps a record of the acknowledgement messages, and if
some nodes have not transmitted an acknowledgement
message within a certain control period, the central
node retransmits the restriction request to these nodes.
The broadcast is repeated again to all the nodes as the
restriction period (T) expires i the overload condition
is still on. However, it is difficult to implement such
a method in a typical network comprising several nodes,
and in addition the method is not reliable, since a
peripheral node may be damaged ~or example ;~m~; ately
after it has transmitted the ac:knowledgement message,
in which case the central node will not be informed of
the situation. Another drawback of such a method is that
the central node also transmits a restriction request
in vain to nodes which cause an insignificant loading
on the central node (this could only be avoided by
monitoring separately the traffic from each peripheral
node, which, however, is a complicated and therefore
undesirable solution).
The latter problem has been solved in some
known Intelligent Networks in such a way that as the
restriction parameters change, a restriction request is
always transmitted in response to a service request
message (which may be for example the question of the
example provided in the beginning) sent by a peripheral
node. Therefore, the peripheral nodes with light traffic
will correspo~;ngly receive fewer restriction requests.
The drawback of this method is, however, that it causes
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a great deal of traffic over the signalling link between
the central node and a peripheral node. It also causes
a large number of updates in the peripheral node.
Summary of the Invention
The purpose of the present invention is to
provide a new kind of arrangement by means of which the
above-described drawbacks can be el;~;nAted and
synchronization can be implemented in a simple and
sufficiently reliable manner (i.e. that the loading
machine operates as well as possible in accordance with
the current load level of the loaded machine). This
objective is achieved with the method according to the
invention, the first embodiment of the invention being
characterized in that the service node transmits a
restriction request to a predetermined proportion of the
total number of the service request messages fulfilling
the given criteria, so that each individual service
request message has a predetermined probability of
triggering the tr~n~m;~sion of a restriction request,
the probabilities being selected so that the total
number of the restriction requests is smaller than the
total number of the service requests. The second
embodiment is in turn characterized in that the service
node retransmits the restriction request to all the
nodes connected thereto in such a way that the
retransmission is performed to a predetermined
proportion of the total number of service requests
received by the service node, so that said transmission
is provided to an individual service request with a
predetermined probability that is less than one. The
arrangement according to the invention is in turn
characterized in that the network service node comprises
at least one random number generator means and at least
one comparing means, whereupon the comparing means
-
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compare a number stored beforehand in the node with a
random number generated by the random number generator
means, and control the transmission of the restriction
request message on the basis of the result of the
comparison.
The idea of the invention is to transmit a
restriction request from a network service node in such
a way that it is transmitted only to a predetermined
proportion of the total number of the service request
messages, so that a response to an individual service
request message is provided with a predetermined
probability that is less than one at least part of the
time, but typically for most of the time. Therefore the
nodes loading the service node the most also have the
greatest probability of receiving a restriction request.
It is also possible that the principle is applied only
to a certain part of the traffic (to service request
messages fulfilling predetermined criteria).
A restriction request is provided whenever the
restriction parameters have changed or the restriction
period expires and there is still ~n overloading
situation in the service node. ~owever, the latter
alternative is only true for th~ ~ct~Ye ~thod, since
there is no need to monitor t~e ~xpiry of the
restriction period in the "reactiv~ method.
Due to the arrangement according to the
invention, synchronization can be implemented in a
reliable and simple manner so that the connections
between the nodes are not overloaded. The arrangement
according to the invention also guarantees that no
unnecessary restriction re~uests are transmitted to
nodes causing light loading. The saved bandwidth can be
used for the transmission of other messages in the
network.
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A farther advantage of the arrangement
according to the invention over the known arrangements
is that it requires changes only in the service node of
the network, and that these changes are minor. No
changes are required, for example, in the internodal
protocol.
It should also be noted that in an individual
overloading situation, the first restriction request can
be transmitted either automatically (without an incomi.ng
service request) or in response to an incoming service
request. Furthermore, the restriction request may either
be an individual message or it may be contained in a
message that would in any case be provided in response
to a service request message.
Brief Description of the Drawings
The invention and its preferred embodiments
will be described in greater detail below with reference
to the examples according to the ~rrompanying drawings,
in which
Figure 1 illustrates the questions traffic
between two machines,
Figure 2a illustrates a response of a
hypothetical m~ch;ne to service requests,
Figure 2b illustrates a response of an
omnipotent m~ch;ne to service requests,
Figure 2c illustrates a response of a real
machine to service requests,
Figure 3 illustrates the division into
different load levels, performed in a node,
Figure 4a shows a star network comprising four
nodes,
Figure 4b shows a star network in its simplest
form,
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Figure 4c shows a star network comprising two
superimposed star networks sharing peripheral nodes,
Figure 5 shows an intelligent network
comprising two central nodes and three peripheral nodes,
Figure 6 illustrates t:he communication between
the nodes in an intelligent network,
Figure 7a illustrates an intelligent network
and the formation of its central node from functional
blocks situated at different hierarchical levels,
Figure 7b shows the division of one block shown
in Figure 7a with respect to the call restriction
function,
Figure 8 illustrates the operation of a known
traffic restriction method,
Figures 9 and 10 illu.strate the operation of
an intelligent network in a loading situation,
Figure 11 is a flow chart illustrating the
operation of the method according to the invention in
a network central node,
Figure 12 shows means to be added to the
network central node,
Figures 13a and 13b show two alternatives for
a parameter used in the method according to the
invention when the parameter is periodic, and
Figures 14a, 14b and 14c show three different
alternatives for the parameter used in the method
according to the invention when the parameter is
adaptive.
Detailed Description of the In~ention
In the following, the invention will be
described in greater detail by using as an example a
(star) Intelligent Network wherein calls are
transmitted. As described above, the architecture of the
Intelligent Network is based on service switrh;ng points
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(SSP) and service control points (SCP). These nodes are
interconnected by means of a network SN according to the
signAl 1 ;ng system number 7 (SS7, described in greater
detail in the CCITT Blue Book Specifications of
Signalling System No. 7, Melbourne 1988), in the manner
shown in Figure 5. In mutual communication the SSP and
the SCP utilize the Intelligent Network application
protocol (INAP) described in the ETSI (European
Telecommunications St~ rd Institute) st~n~rd ETSI IN
CSl INAP Part l: Protocol Specification, Draft prETS 300
374-l, November 1993. In the SS7 protocol stack
illustrated in Figure 6, the INAP is the upmost layer
situated on top of the Transaction Capabilities
Application Part (TCAP), the Signalling Connection
Control Part (SCCP) and the Message Transfer Part (MTP).
The SSP is generally a commercial telephone exchange
with a modified call control software, and the SCP
comprises the service control logic and has access to
the services database. Call traffic passes through the
SSPs. The Service Control Points make some of the
decisions concerning the routing and the charging of the
calls. During a call in the Intelligent Network, there
may be one or more INAP dialogues between the SSP and
the SCP. Each of these dialogues begins with a
predetermined message (initial detection point message)
hereinafter referred to as an initial message.
When the network traffic is heavy, the SCP may
become overloaded. In order to prevent this, the
Intelligent Network has a decentralized load control
system that uses a so-called call gapping method to
restrict messages arriving towards the SCP (the term
"call gapping" is used in several international
standards, for example in the CCITT Blue Book,
Recsmm~ tion E.412, 3.l.l.2 and R~o~?ndation Q.542,
5.4.4.3). The call gapping method is a known traffic
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control method that is based on the frequency of call
occurrence (rate of arrival), in which method the number
of calls is limited in such a way that at most a certain
m~x; r~ number of calls per time unit are allowed to
pass. In addition to the aforementioned st~nA~rds, such
a method is also described for example in US Patent
4,224,479. The SCP monitors the loading situation and
the SSPs restrict the traffic, if necessary, by
rejecting some of the calls before the related dialogue
is started.
Assume that the network comprises, in the
manner shown in Figure 7a, two nodes SSPl and SSP2, and
one SCP. The SCP can be considered to contain a
hierarchy of functional blocks A to E. Each block can
be considered to comprise, according to Figure 7b, a
gapping gate 70 operating according to the call gapping
method, and a subsystem SS located behind the gapping
gate. All telecommunication wit:h the subsystem passes
through the gapping gate, and the gapping gate gathers
statistics about the traffic, the condition of the
subsystem, and the condition of the other parts of the
SCP. From this data the gapping gate calculates the load
level of the subsystem in question.
The normal load level of the subsystem is LO
(cf. Figure 3). When the load level changes from LO to
Ll, the gapping gate will try t:o limit the traffic by
sending a call gapping request to both SSPs. Such a
request typically comprises the following groups of
parameters: (l) gap criteria, (2) gap indicators, and
(3) gap treatment. The gap criteria identify the portion
of the traffic that is the object of the call gapping
operation, for example, only calls starting with 800 can
be limited. Gap indicators define the m~X; mum number of
initial messages (calls) U allowed in a time unit (in
fact the gap indicators define the shortest allowable
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. 14
interval I = 1/U between two successive initial
messages, which, in principle, amounts to the same
thing) and the gap duration T, whereupon the rate of
initial messages between the arrival of the call gapping
request and the end of the duration can be at most the
aforementioned maximum. The operation of this call
gapping method is illustrated in Figure 8. When the
traffic rate (shown on the horizontal axis) offered by
the network is less than the aforementioned maximum U,
there is no call gapping. When the offered traffic rate
exceeds this value, the SSP rejects some of the calls
so that the rate of the transmitted traffic (shown on
the vertical axis) will be U. An ideal case is described
by a broken line, and a real situation by a continuous
line. In practice, the characteristic is a continuous
approximation of the piecewise linear characteristic of
the ideal case. This is due to the fact that the offered
traffic is not divided evenly on the time axis.
The gap treatment parameters determine what to
do with rejected calls. For example, the speech channel
of a rejected call can be connect~ to a voice
announcement or to a busy tone. ~n ~d~;t~on, the call
gapping request contains a contr~l fl~'d ~t~lch indicates
whether the call gapping reques~ com~s f rom an automatic
overload prevention mechanism or from an SCP operator.
The above-described groups of parameters are disclosed
in the aforementioned standard ETSI IN CSl INAP Part 1:
Protocol Specification, Draft prETS 300 374-1, November
1993, Item 7.3.6, which is referred to for a more
detailed description.
When a call gapping request arrives at an SSP,
the SSP creates, based on the information it has
received, an image of the sending gapping gate (i.e. the
subsystem controlled by the gapping gate). This is
illustrated in Figure 9, wherein the overloaded block
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(C) is denoted by hatching and the call gapping reguest
transmitted by the SCP by the reference CG. By means of
the gap criteria and this image, the SSP identifies the
traffic that is directed to the overloaded subsystem and
restricts the rate of this traffic. When the period of
time indicated in the call gapping request expires, the
SSP de~tloy~ the image of the subsystem from its memory.
The gapping gate in the SCP is "static", i.e.
it exists all the time. The image of the gapping gate
(or the corresponding subsystem) in the SSP is
temporary; the SSP creates the image when it receives
a call gapping request and destroys it when the duration
T specified in the call gapping request has expired.
When the SSP receives a call gaE)ping request containing
the same gap criteria as an already existing image, the
other parameters of that image will be updated to
correspond to the new ones.
Another approach is to view the images (copies)
in the SSP as objects with t:wo states: active and
passive. When an image receives a call gapping request,
it turns active and starts to restrict traffic. When the
image is in the active state it can receive several call
gapping requests from the SC:P. When the duration
specified in the last call gapping request expires, the
image turns passive again.
When two of the subsystems in the SCP are
simultaneously overloaded, there is correspondingly an
image (copy) of each gate in the SSP. As more and more
subsystems become overloaded, the logical structure of
the images in the SSP starts to resemble the hierarchy
. of the gapping gates in the SCP. This process is
illustrated in Figure lO.
The aforementioned ETSI standard (Item
7.3.l9.l.l) also defines a special "call gap
encountered" indicator, which the SSP adds to the
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initial message if the call has passed through the
gapping gate. This indicator thus informs the SCP that
the concerned SSP performs call gapping. However, the
SCP cannot be certain that the SSP performs the call
gapping with the correct parameters, wherefore the 5CP
cannot trust this indicator in making decisions about
whether to send a call gapping request or not. An
example of this is a network which comprises one SCP and
several SSPs, and in which network one of the SCP
subsystems is on the load level L1 having a
corresponding upper limit U, to be indicated to the SSP,
of e.g. 10 initial messages (10 calls) per second. If
the load level now changes from L1 to L2, having a
correspn~;ng upper limit of e.g. 5 initial messages (5
calls) per second, the SCP transmits a call gapping
request CG containing a new upper limit to each SSP. In
this situation, if the data of some SSPs will not be
updated, for example due to faults, then these SSPs
continue restricting the traffic with the old (higher)
value of U until the duration indicated in the call
gapping request expires. Due to this, the concerned
subsystem may move further to the next load level L3.
The SCP cannot distinguish between updated and non-
updated SSPs, since the same indicator is received from
all SSPs.
There have been efforts to solve the problem
in the manner described in the beginning, so that the
same call gapping request is repeated after each initial
message arriving from the SSP. However, this arrangement
produces (a) more traffic over the signalling link
between the SCP and the SSP, and (b) repeated updating
of the information (subsystem images), concerning the
SCP, in the SSP.
The operation according to the present
invention proceeds in such a way that a call gapping
CA 02203~38 1997-04-23
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request is transmitted in response to only a
predetermined proportion of the initial messages (or
corresponA~ng service requests). This predetermined
proportion will be described below with the letter p
(O<pSl). As it is apparent from the above, the objective
of the overload prevention mechanism is to set an upper
limit U for the traffic rate from an individual SSP. The
inverse of the product of the incom;ng traffic rate and
the parameter p thus represents an average time interval
between two successive call gaFIping requests (updating
interval). When the traffic rate from an SSP exceeds the
value U, i.e. when a call gapping request should be
transmitted to the SSP, the average updating interval
is less than l/(pU).
The operation accordi~g to the invention is
preferably implemented in such a way that for each
;n~nming initial message, a random number R, for which
0 S R < 1 holds, is generated in the central node SCP.
If this number is smaller than the parameter p, a call
gapping request is transmitted. The decision-making can
thus be described in the following manner:
send the call gapping request, if R < p,
wherein 0 5 R S 1, and 0 < p S 1.
Figure 11 illustrates the operation according
to the invention within each subsystem of the central
node. In phase 111 a random number R is generated,
whereafter it is ~X~m; ned in phase 112 whether the
random number is smaller than the parameter p. If the
result of the comparison is positive, a call gapping
request is transmitted to the node which transmitted the
service requests (phase 113). If the result of the
comparison is negative, no transmission is done, but
normal operation is continued (I?hase 114).
As shown in Figure 12, random number generator
means 120 generating a random number R, and comparing
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18
means 121 comparing the random number with the parameter
p are added to the central node. The romr~ring means
control transmitting means 122, which transmit the call
gapping request CG forward. In the case of a central
node of an Intelligent Network, each subsystem may have
its own incorporated random number generator and
comparing means, or all subsystems may share the same
means.
Instead of the random number generator, it is
also possible to use counters counting initial messages.
For example, if p = 1/10, every tenth initial messaye
would trigger the transmission of a call gapping
request. The drawback of counters is, however, that the
distribution of the call gapping requests to be
transmitted is too regular with respect to the
distribution of the ;nCom;ng initial messages.
The selection of the parameter p is discussed
in greater detail below.
Assume that the traffic to the central node
arrives from a source having a Poisson distribution that
is known per se and transmitting on average r initial
messages per second (the average speed is r). The
objective is to select the parameter p in such a way
that the source receives one call gapping request every
d seconds (whereupon d is the average updating interval
of the restriction parameters). Since the source
transmits rd initial messages within d s~con~, the
objective can be achieved by selecting p = l/(rd)
(assuming that rd ~ 1). When the traffic arrives from
N identical Poisson sources, the central node receives
Nrd initial messages in d seconds. In order to achieve
the above goal, the central node must correspondingly
transmit N call gapping requests during that time (p =
N/(Nrd) = l/(rd)).
CA 02203~38 1997-04-23
WO96/lS609 PCT ~ 5/00616
19
The receiver of the central node does not
thereby distinguish between the different sources
(peripheral nodes), but when the parameter p is
constant, each initial message will trigger the
transmission of a call gapping request with an equal
probability p ((0 < p ' 1). If p 2 1/(rd), each
transmitting process the rate of which PX~ Pds r
receives on average at least one response within d
seconds. If the transmitting process is not Poisson-
distributed but arbitrary, the number of messages it
sends within d seconds is also arbitrary. A reply cannot
thereby be guaranteed within d seconds, but within 1/p
messages.
Since the average interval between call gapping
requests is 1/(rp) seconds, the parameter p can be
determined according to the worst possible case (r = U),
whereupon the interval is the longest. (The interval is
naturally even longer when the rate of the source is
less than U, but in such a case the call gapping
requests are not intended to affect the source rate. The
worst case is thus when the source rst~ 1~ equal to the
threshold U.) The parameter p c~n thus ~l~ determined by
first choosing the average upd3t~ng ~nt~rval d on the
basis of the source rate threshold U ~nd ~y calculating
then the value of p from the eql~ation
p = l/(Ud)- (1)
If possible, the value d should be at least one
order of magnitude higher than l/U, and it should also
be less than the gap duration, i.e. (10/U) < d < T.
For example, if U = 10 :initial messages/s, the
gap duration T - 24 seconds, and the chosen value of d
is 2 seconds, then the parameter p will be p = 1/20.
This means that in order to keep the peripheral nodes
(SSP) updated with an accuracy of 2 seconds, only 5
.
CA 02203~38 1997-04-23
W O96/15609 PCTirJ~ al6
percent of the messages must be responded to with a call
gapping request.
It should also be noted that, strictly
speaking, the parameter p is not constant, but it
depends on the restriction parameters U and d. However,
it can be said that p has a constant value whenever the
aforementioned restriction parameters are fixed. (The
restriction parameters change when the load level
changes, as described above. There may also be more load
levels than described above, so that in practice the
parameters can change almost continuously).
If the parameter U indicates the proportion of
the traffic a peripheral node should forward to the
central node, and if the traffic rate from the
peripheral node to the central node is for example r
(initial messages/s) and the peripheral node restricts
the traffic correctly, the traffic rate after the call
gapping is rU (initial messages/s). The formula of the
parameter p will then be p = l/(fUd), wherein f is the
traffic rate selected by the operator for each load
level Ln or selected by the node automatically, and
wherein fUd > 1.
The parameter p can be selected as a periodic
function of time p(t) with a constant period Tp that is
at most the gap duration T and with a mean value E[p(t)]
that is, according to what is described above, at least
l/(Ud), i.e.
p(t) = p(t+nTp), where Tp < T;
E[p(t)] ~ l/(Ud).
n is an integer (n = 0,1,2,.. ) determining the
period in question.
Two alternatives for such a periodic function
are shown in Figures 13a and 13b, wherein time is shown
on the horizontal axis and parameter p on the vertical
axis.
CA 02203~38 1997-04-23
W O96/15609 PCTn~95~ 16
In the case of Figure 13a, the parameter p
varies according to a square wave, so that the period
length Tp is divided into two parts. In the first part
p = a, and in the second part p = b.
p(t) { a~ f nTp-'.t~Tl+nTp
E[p(t)] = Tl(a-b) + b
In particular, when a = 1 and b = 0, the
behaviour of the central node is completely
deterministic. The central node alternates between two
different states in such a way that in the first part
of the period (length T1) it transmits a call gapping
request in response to each initial message and in the
second part of the period (length~Tp-Tl) it transmits no
call gapping requests. The condition set for the mean
value E[p(t)] 2 1/(Ud) then translates into the
condition for the ratio of the parts of the period:
(T1/Tp) 2 1/(Ud).
In the case of Figure 13b, the parameter p
varies like a saw-tooth wave:
E[p( t) ] = 1---2P
p ( t) = { 1 C( f nTp) ,Tl f nTp~ t~ (n+l ) Tp
wherein n is an integer (n = 0,1,2,...), determining the
period in question.
=
CA 02203~38 1997-04-23
W O96/15609 PCT/~ 'u~616
In this case, the closer p is to its peak
value, the faster the response (i.e. call gapping
request) to a source with an excessive rate is obtA;ne~.
When p is periodic, the period Tp is preferably
selected in such a way that the gap duration T
corresponds to an integral number of periods Tp (i.e. T
= kTp, k - 1,2,3...). The beginning of the gap duration
(or the end of the preceding call gapping period) is
then likely to coincide with the moment the parameter
p is also at its peak value.
The parameter p can also be selected to be
adaptive so that it depends on the rate of the total
traffic to the node. Such alternatives are described in
Figures 14a to 14c, wherein the total traffic rate to
the central node is shown on the horizontal axis and the
parameter p on the vertical axis.
In the case of Figure 14a, the central node SCP
starts transmitting call gapping requests only when the
total traffic rate ~ce~ a certain threshold Ra. In
the case of Figure 14b, the central node SCP also starts
transmitting call gapping requests only when the total
traffic rate ~cee~ a certain threshold Ra, but
furthermore, the proportion (i.e. p) of the call gapping
requests to be transmitted increases stepwise as the
total traffic rate exceeds the thresholds Rb and Rc. In
the case of Figure 14c, the value of the parameter p
increases linearly from zero to one between the
thresholds Ra and Rb. In the case of Figure 14a, the
advantage is that no unnecessary call gapping requests
are sent to nodes causing light loading. In the cases
of Figures 14b and 14c, another advantage is that even
the most difficult overloading situations can be
effectively handled.
As it was mentioned above, the receiver of the
central node does not distinguish between the different
CA 02203~38 1997-04-23
WO96/15609 PCT ~ ~/00~16
sources (peripheral nodes). This also means that the
central node only has to mon:Ltor the total rate of
messages arriving from n sources, without having to
monitor the rate of messages from individual nodes. (The
initial message does contain a field indicating the
transmitter, so that the traffic can be monitored
specifically for each node, but it is a far more
complicated solution because it requires a great deal
of book-keeping in the central node.)
The principle according to the invention can
also be applied in the above-described known
broadcasting method in such a way that the central node
does not maintain a record of which node has transmitted
an acknowledgement message, but the central node
performs a rebroadcast randomly according to the
invention, in response to a predetermined proportion of
initial messages. Each broadcast consists of N call
gapping requests, where N is the number of peripheral
nodes SSP connected to the cent:ral node, so that each
peripheral node will receive one call gapping request.
In this case, the parameter p is selected on the basis
of the above-described formula (l) by dividing it by N,
i.e.
p = l/(NUd),
wherein d is the average updating interval of
the restriction parameters of the peripheral node, when
the total traffic from the peripheral nodes is NU. For
example, if N = 5, U = lO initial messages/s, T = 24,
and d has the chosen value of 2 s~on~s, then p = l/lOO
(i.e. rebroadcast is performed on one percent of all
; ncom; ng service requests).
In the ~nn~r described above, the book-keeping
concerning acknowledgement messages can be el;~;n~ted,
since it is this feature that makes the known
broadcasting method complicated.
CA 02203~38 1997-04-23
W O 96/15609 PCTn~WS/00616
24
In this embodiment according to the invention,
the central node may transmit the first call gapping
request message co~c~rning an overloading situation
automatically as soon as the restriction parameters
change, but it is simpler, however, to provide even the
first call gapping request to each peripheral node in
response to the initial messages transmitted by the
peripheral nodes.
Even though the invention is described above
with reference to the examples according to the
~ompanying drawings, it is clear that the invention
is not restricted thereto, but it may be modified within
the scope of the inventive idea disclosed above and in
the appended claims. The method according to the
invention can be applied for example to a certain part
of the traffic, as it is disclosed above. Since the
arrangement according to the invention is in principle
applicable in any telecommunications network having the
basic situation according to Figure 1, the central node
iS referred to in the appended claims as a service node
(not restricted to an Intelligent ~etwork) and the
peripheral node in turn as a node ~no~ restricted to a
star network). The service requ~s~ must also be
understood to relate generally to any services the
performance of which loads the service node.
