Note: Descriptions are shown in the official language in which they were submitted.
93~23g62 PCr/GB93/00972
Method of controll~ng trafftc ~n a ~1ecommunicat10n network
This inven~ion relates the control of a
telecommunications networ~ which includes at least one
local exchanae connec~ed by routes to one or more parent
S trunk excnanges each being one of a plurality of trunk
exchanges interconnected by trunk routes and to
telecommunications networks so controlled. It is
particuiar~y concerned with the detection of focused
overloads in such networks.
Commonly a local exchange is associated with a home
exchange, through which incoming calls to the local
exchange are routed, and a security exchange, through which
outward calls from the local exchange are routed, in normal
operation. The security exchange is so called because it
can also be used to~ route incoming calls to the local
exchange if the home exchange fails. The security and home
exchanges are collectlvely referred to as the parent
excha~ges of the local exchange. Exchanges other than the
pa.rent exchanges are referred to as far-end trunk exchanges
of the local exchanges.
Near real-time networ~ traffic management (NTM) is an
essential component of network management if optimal
traffic performance in terms of call throughput is to be
ensured. To ~ive an indication of the volume of traffic
which may be involved, BT's tru~k network in the United
Kingdom currently handles approximately six million call
att~mpts per hour during the busy periods which is
e~uivaIent to l,700 cal~ attempts per second. Given such
a volume of traffic it is essential that any networ~
difficulties are detected and controlled as quickly as
possible. For example, difficulties are often encountered
by network traffic managers due to abnormal traffic
patterns which can be caused by events such as ~phone-ins,
tele-votes and public holidays (for example Christmas Day
35 and New Year' s Eve/Day). In all these cases traffic in the
W093/23962 ~13 S ~ 3 3 PCT/GB93/~97 ~
network varies widely from the normal le~el, some~imes
quite spectacularly, and the network must be controlled to
maintain the best o~erall networ~ performance.
With the introduction of digital switches such as
- System X it is ~ossible to monltor closely the performance
of each exchange and the routes between them and to the
subscribers. BT~s Network Traffic Management System (NTMS)
currently receives statis.ics on upwards of 37, 000 routes
from 490 exchanges in the UR every fi~e ~inutes, which
10 measurement period was chosen to be a long enough period to
be able to obtain a statistically reliable measureme~t of
- the network performance whilst being short enough to allow
effective real-time control of the network.
The information received by the NTMS is proces~ed to
provide CCITT recommen~ed parameters. The parameter value~
are compared with thresholds to determine if any
dif~iculties exist on the monitored network elements.
Usually the first indication of a network problem is
when an 'exception' is displayed on a wall-board, or on a
graphical interface at an individual manager~s workstation,
a~ a Traffic Management Centre. Exceptions are those
parameter values, calculated from network element
measurements, which deviate suf~iciently from a
predetermined threshold for that value. The exceptions are
ranked in a priority order with the top 20 displayed.
However, due to the manner in which the thresholds are set
by the networ~ traffic manaqers, some exceptions do not
necessarily indicate a difficulty as thres~olds are
percentage-based and set a value which ensures all
potential difficulties are captured. This results in
exceptions being displayed that are oc~asionally spurious
or insignificant. The exceptions therefore need to be
ex~mined in more detail to determine if a real difficulty
exists and whether it warrants any action. To help in this
activity several information sources are currently used by
the network traffic managers.
~) 93/23962 ~ 13 5 ~ 3 ~ PC~/GB93~00972
-- 3
The NTMS provides near real-time surveillance and
monitoring or the network' s s.atus and perfor~ance. It
provides the network traffic mana~ers with information to
enable them take prompt action to control the flow of
traffic to ensure the maximum utilization of the network in
all situations. The NTMS allows network tra~fic managers
to look at the raw statistics as well as derived ge~eric
parameters and to compare traffic patterns over the last
few measurement periods to isolate any trends.
An On-Line Traffic Information System (OTIS) takes the
measur~ment of statistics from the NTMS system and
~ processes them to provide summarised historical d~ata for
daily and weekly traffic patterns. This system allow~ the
networ~ traffic managers to examine his~orical traffic
patterns to detect afi~ radical shifts in traff -
A data management system provides the network trafficmanagers with an up-to-date copy of ths routing tables at
all trunk exchanges. This infor~ation is used to check the
routes to which ~alls can be routed, which controls are in
for~e and the routing algorithms b~ng used.
There is also a broadcast speaker facility which
connects the world-wide network mana~ement centre to all
the regional centres.
Once a potential difficulty has been detected,
acknowledged and analyzed, it is characterised and a
decision made whe~her to control it using the available
range of expansive and restrictive controls to either allow
alternative traff~ic paths through the network or to
restrict and block call attempts to particular areas,
respectively. The situation must then be'monitored to
ensure the controls are having the desired effect and that
they are removed as soon as a problem has been dealt with
eff~ctively.
It is important that the controls applied are
sufficient to suppress the problem yet no so severe that
WO 93/23962 ~. ~ 3 5 ~ 3 3 pcr/GB93/oog7~
they cause their own pro}:lems by congesting other parts of
the networ3~ or by bloc3~ing potentially success ful calls.
One class of exception associated with
telecommunications networks is the focused overload. The
5 All Circuits Engaged (ACE ) CCITT parameter may be monitored
in order to determine when a focused overload occurs but
this has been found to be not very satisfactory at present
and will be less so in ~he near future when call failures
will only be shown (on a wallboard~ with respect to the
last of several routes tried. Currently, all the routes
tried are shown and focusad overloads give rise to a
- graphic "star" effect o~ a wallboard.
Accordlng to a first aspect of the pres~nt invention,
a method of controlling a telecommunications networ~, which
15 includes at least one~local exchange connected by routes to
one or more parent trunk exchanges each being ons of a
plurality of trunk exchanges interconnected by tru~k
routes, is characterised in that the method comprisa~
applying focused overload controls to the trunk exchanges
of the network when the Bids per Circuit per Hour (BCH)
along the primary traffic routes from a parent trunk
exchange to the local exchange exceed a first threshold and
the magnitude of a de~rease in the average Answer Seizure
Ratio tASR) on the trunk primary traffic routes to the
parent exchange over a measurement period exceeds a second
thresho~d.
A primary traffic route is the route first tried for
a call.in a network, all alternative routes then being
!
secondary.
The BCH parameter gives a normalised indicatian of the
number of call attempts down a particular route and will ~ 7
generally stay at a normal value well below the maximum BCH
that can be handled by the exchanges under normal
conditions but will tend to move to and above that value
during a focused overloadO These normal values provide a
maximum expected BCH range of the routes in the networ~.
7 ;~ 3 ~
r ;;.; ~ 93/23962 PCI /GB93/00972
~ _ 5
The BCH will also rise due to an increase in country-
wide traffic .o a large number of subscribers served by a
given lo~al exchange, for example when a disaster befalls
an area, but this will not necessarlly result in a focused
overload if a surficiently high number of calls continue to
be connected~ ~he inclusion of the test of the ASR in the
present invention serves to distinguish between these two
scenarios. In the event of a rocused overioad it has been
dPtermined by the applicant that the average ASR on the
trunX primary traffic routes into the home exchange drops
very sharply and c~n remain low for a number of five minute
measurement periods before slowly returning to its normal
value whereas a general increase in traffic to a local
exchange does not have this effect.
The first thr~shQld can be set according to the normal
maximum expected value of the BCH parameter on the route
that ca~ be properly handled by the network. It has been
found that in a focused overload the BCH parameter rises
very sharply reaching a peak in the first five minutes
20 followed by an exponential drop to the normal level over
the following forty m~nutes. The ma~ximum exp~cted value of
Seizures per Circuit per Hour ( SC~ ) is approximately 60
divided by the average call length which at 2 to 3 minutes
gi~es a normal maximum expected BCH for BT' s UX network of
about 20 to 30 with the normal value at most about 10.
To provide for the proper application of control to
the parent exchanges it is preferable that the first
threshold is towards the high end of the above maximum
expected BCH ran~e. In this case a value' of 30 can be
used.
The ASR has.been found to drop very sharply in the
event of a sudden focused overload and it remains low for
about 15 minutes before slowly returnlng to its normal
~alue. The second threshold can therefore be conveniently
set equal to the normal daily excursion which for BT's UX
network is approximately 10%. A 10~ drop in the average
WO 93J23962 ~ 13 S 6 3 3 PCI /GB93/00972
Answer Sei~ure Ratio (ASR~ on the trunk primary traffic
routes into the home exchange over a single measurement
period is very siynificant and can therefore be used to
detect the focused overload condition.
If it is desired to apply focused overload controls
only when there is a national problem, caused by a national
telephone appeal rather than a local announcement for
example, ~ocused overload controls c n be applied to the
trunk exchanges of the network only when the increase in
the average value of the BCH along the trunk primary
traffic routes into the respective parent exchange over a
- measurement period exceeds a third threshold.
Analysis of focused-oYerloads in BT's UK networ:k shows
that a third threshold value of 40% greater than the long
term BCX average over~all the trunk prima~y traffic routes
into the home exchange.
will generally be satisfactory but exceptions may occur
occasionally at moments of normal traffic increase for
exa~ple between 08.00 and 09.00 hours and just before the
cheap rate period. The third threshold is preferably set
to a value large enough ~o usually avoid false indications
of a problem or raised at times of a normal increase in
traffic to increase the sensitivity and reliability of the
test generally.
A networ~ may be controlled according to the present
invention by an apparatus to which is input the network
parameters and which provides control i~formation to the
network controller when the foc~sed overload condition has
been deteeted so that the network manager can apply the
method of the present invention to the network.
Alternatively the network can be controlled automatically
on detection of the focused overload to apply the method of
the present invention without the intervention of a network
manager.
The focu~ed overload controls to be applied to the
network may be call gapping applied to the home trunk
i
~ 93/23962 ~ t 3 ~ ~ 3 3 PCT/GB93/~972
. 7
exchanges or to one or more of the far-end trunk exchang~s
of the locai exchange, for example. :~
It is also useful if a determination can be made
readily or when the focused overload controls can be
removed~ ie that the focused overload problem has been
overcome. I~ has been found that it ls satisfactory to
remove the focused overload controls once the BCH from the
home exchange to the local exchange falls below a fourth
threshold, which threshold is pre~erably 5. Once it has
been determined that the focused o~erload controls can be
removedj the focused overload controls applied to the
- exchanges of the network are removed either by a network
manager or automatically by a network controlling
apparatus. In this way local focused overload corltrols can
be overidden once it- has been determined globally that the
problem has been solved.
According to a second a~pect of the prese~ invention,
there is provided a me.hod of con~rolling a network, which
includes at least one local exchange connected by routes to
one or more parent trunk exchanges each being one of a
plurality of trunk exchanges interconnected by trunk
routes~ in which call gapping is applied to one or more of
the exchanges of the networ~ on detection of a focused
overload at an exchange, the method being characterlsed in
~5 that call gapping of numbers of a local exchange i5 applied
8i ther:
a) to an associated parent exchange whenever the Bids
per Circuit per Hour (BCH~ from the parent exchange to the
, . . .
local exchange is greater than a fifth threshold and the
Answe~ Seizure Ratio (ASR) on primary traffic routes from
the parent exchange to the local exchange has stayed below
a sixth threshold for the duration of a measure~ent period;
or
b) to a far-end trunk exchange of the local exchange
whenever the ASR from the far-end trunk exchange to a
parent trunk exchange of the local exchange has stayed
., ~ .. .... ... .. .
W093/23962 ~13 ~i 6 3 3 - 8 - PCT/GB93/0097~
below a seventh threshold for the duration of a measuremeI~t
peri od.
A method according to the second aspect ~f the present
invention can be used advantageously, whether or not in
combination wlth a method according to the first aspect of
the present invention. The ad~rantage lies in the increased
flexibility available. If call gapping is applied to the
home trunk exchange, it has effect on all routes in to that
home trunk exchange for the local exchange. If it is
applied at the far-end trunk exch nges, it is poss.ible tO
select one or more of the far-end trunk exchanges which is
or are contributing heavily to the focused o~erload, and
call gap only that one exchange, or those selected trunk
exchanges.
When applying call-gapping to one or more of the far-
end trunk exchanges, a less sensitive test than that used
for the application of local focused overload controls has
been found to be appropriate. The seventh threshold can
conveniently be set to be about equal to the averasz normal
ASR for exchanges in the network so the exchangQs are
regulated to allow the associated ASRs to move back to this
value.
Clearly, the fifth, sixth and seventh thresholds will
in general need to be determined by analyzing the normal
25 values for the particular networ~ to be controlled in
accordance with the above general principles.
The local controls are preferably removed
progressively as the problem is resolved at di f f erent parts
of the ne~wor~. In particular it is preferable that the
call gapping is removed from a parent trunk exchange once
the ASR and Percentage Occupancy (OCC) have stayed above an
eighth threshold and below a ninth threshold, respectively,
for the duration of a measurement period. (The percentage
occupancy is the total traffic in Erlangs, divided by the
num~er of in-ser~ice circuits on a route.) The eighth and
ninth thresholds can be set to equal the highest normal ASR
93/23962 ~ l. 3 ~ 6 3 3 PCT/GB93/00972
and OCC for exchanges in the network whi~h ror BT~s UK
network are in the order oî ~o% and 60%, respectively.
The call gapping is pxeferably applied to a far-end
trunk exchange until the ASR to the home exchange has
- stayeà above ~he eighth threshold for the duration of a
measurement period.
The extent of call gapping applied can be a fixed
amoun~, for example 5 second call gapping, but will
preferably be selected according to the prevailing
conditions based on prior knowledge of the call handling
capability of the subscri~er ~eing called or from general
exper~ence of ~he network managex.
Various values of the thresholds can be selected
according to the characteristics of the particular network
to which the present.invention is to be applied. If the
normal time variation of the parameters being monitor*d is
known, the threshold values could be adjusted accordingly
to provide a more consistent alert of a focused overload
a~d to reduce the chance of a normal increase in traffir
triggering a false indication of a problem.
Embodiments of the invention will now be described by
way of example only with reference to the accompanying
drawings of whi ch:
Figure 1 is a schematic representation of a network
controllable by the method of the present invention;
Figure 2 is a schematic representation of a control
system for implementing the method of the present
invention; and
Figure 3 is a schematic representation of a contxol
system for implementing the method of the pres.ent invention
using direct control of a network by a computer.
Referring to Figure 1 a telecommunications network
comprises a number of digital main switch units {DMSUs) -
txunk exchanges - of which only five are shown for clarity
and are referenced 102, 104, 106, 108 and 110.
Subscr~bers' customer premises e~uipment, of which only two
W093/23962 ' PCT/GB93/0097
~ 10 -
are shown referenced 112 and 114, are connected to the
trunk exchanges iO~ to iiO vla respective digital local
exchange (DLE) 116 znd 118.
The ~LE 116 is connec~ed to the D~5U 102 through which
incoming calls to the subsc-:~ers attached to the DLE 116,
including subscriber 112, are routed. ~he DMSU 102 is
commonly referred to as the home trunk exchange for thP DLE
116. The DLE 116 is also connected to the DMSU 104 through
which outgoing calls from the DLE are routed. This is
usually called a securitY exchange as incoming calls to the
exchange 116 can be routed through it should the DMSU 102
- fail.
Similarly, the DMSU 108 is the home exchange for the
digital local exchange 118.
The home and sec~urity exchanges associated with ~ DLE
are collectively reférr~d to as the pare~t exchanges of
~hat DLE.
Those exchanges other than the parent exchanges are
referred to as the far-end trunk exchanges of a gi~en
digital local exchange. For the networ~ of Figure 1 all
exchanges other than trunk exchanges 102 and 104 are
regarded as far-end trunk exchanges for the digital local
exchange 116, for example.
If a large number of calls are attempted to a
subscriber 112 within too short a period the digital local
exchange 116 may not be able to handle the calls and in
extreme cases the DLE might also go into overload. This
will block unrelated traffic in the area around the
destination exchange~. This is what is called a focused
overload which needs to be controlled if congestion is to
be avoided.
A particular method of controlling a
teiecommunications network such as that illustrated in
Figure 1 will now be described with additional reference to
Figure 2 which shows a network control system implementing
the method of the present invention.
-
_. ,. , . . . . , ~ .
~i.~93/~3962 .7 1 3 S ~-3 3 pcr/GB93J~97~
-- 11
Referring to Figure 2 the network of Figlre 1 is
denoted by box 202. Every five minutes, which is the
measurement period of the network of Figure 1, a set of
statistics is generated by the digital exchanges of the
ne~work 202 whicn is processed by an NTMS system 204 to
provide generic measurement values including those of the
CCITT recommendation. These ?arameters are input to a run-
time system 206 which applies rules to the received
parameters from the NTMS 204 by means of an appropriately
coded expert system. The run-time system 206 provides
recommendations to aid a network traffic manager ~08 detect
~ and control focused ovexload problems in the network 202
according to embodiments of the present invention.
The run-time system 206 employs a three phasP cycle in
which recommendations for focused overload control actions
are passed to the network traffic manag~r 20B.
The run-time system 206 monitors each local ex~hange
and determines when the BCH along the route from the home
trunk exchange of a local exchange exceeds 30 and in which
the BCH a~eraged over trunk primar~ tr-ffic routes to that
home exchange have increased by 40% since the last
measuring period. When these conditions are satisfied the
run-time system supplies a recommendation to the network
traffic manager 208 to apply focus overload controls and in
paxticular, in this embodiment, call gapping to the parent
trunks of the local exchange or to far-end trunks of the
local exchange affected - assumed for the purposes of the
following description to be local exchange 116 of Figure 1.
It might be noted that it is unlikely that focused
overload controls would be applied to both a far-end trunk
exchange and a parent exchange since application of a
control to the parent exchange makes any control at a far-
end trunk exchange rednn~nt.
The rur.-time system 206 can provide recommendations to
the network traffic manager 208 on applying a specific,
.
W093~23962 213 5 6 3 3 PCT/GB93/~97 ~ '
- 12 -
local focused overlaod controls to the trunk exchangeR 102
to ilO to the network 202 according to the pres nt
invention. In particular the parent trunk exchange 102 of
the local exchange 116 will be subjected to call gapping to
numbers of the local exchanse 116 once the ASR from the
pare~t trunk exchange to the local exchange 116 has
remained at less than 20% for the duration of a fi~e minute
- measuring period and the BCH along the route from the
parent trunk exchange to the local exchange 116 is greater
than 10.
The run-time system 206 can alternatively recommend
call gapping to numbers of ~he local exchange 116 at a far-
end trunk exchange 106 ~o 110 of the locaL exchange 116
- when the ASR along the route from the far-end trunk
exchange to a parent exchange 102, 104 of the local
excha~ge 116 falls below 45%.
The run-time system 206 will also recommend to the
network traffic manager 208 when the call yapping at a
~iven trunk exchange should cease. Advice is given to
re~ove call gapping from a parent trunk exchange 102 of the
local exchange 116 once the ASR to the local ex~hange 116
is greater than 50% and the OCC along the route from the
parent trunk exchange to the local exchanqe 116 is less
than 80% for the measuring period, and to remove the call
gapping of the far-end trunk exchanges 106 to 110 of the
local exchange 116 once the ASR along the route from the
far-end trun~ exchange to the parent exchange 102 of the
local exchange 116 once a~aln rises above 45%.
The run-time system 206 can alternatively also
recommend that all focused overload controls are removed
once the BCH along the route from a local exchange's home
excha~ge to that local èxchange is less than ~ive. This
ensures that aLl controls will be removed under a relevant,
overriding condition.
It is envisaged that the run-time system 206 may
directly control the network to apply the method of the ,r
~-} ~13~533
'~ 93/23962 PCrJGB93/00972 .
-- 13 --
present invention as shown in Figure 3 but at present it is
expec~ed that it will be necessary to use a network traffic
manager 208 to implement of the method of the present
invention to allow overriding of the recommendations at his
or her discretion.
The method of the present invention has been simulated
on a computer representation of BT's UK telecommu~lcations
network in which the run-time system 206 comprises a
PROLOG-based expert system coded with the rules necess ry
to provi~e the recommendations described above in response
to the appropriate CCITT parameters ~rom the NTMS 20~.
The particular system employed was a QUINTUS PROLO~
expert system run on a Sun Sparc station. Parameters
generated during real network activity were recorded for
lS the entire GB netwo~k for four, 5-minute measure~e~t
periods a~d stored as a data file on the computer.
A C-language program provide an interface between the
raw parameters in the computer file and the expert syste~
in that as the expert system required information about the
network, the C-language program ca}culated the data from
the raw parameters.The particular thresholds adopted in
order to determine when a focused overload condition is
likely to occur and when the various call gapping controls
are to be applied and relaxed will be set according to the
particular network 202 to which the method is to be
applied. The present invention is not restricted to a~y
particular values of threshold adopted in the above
described specific embodi~ment.
