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Claims  |
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We claim:
1. A message interceptor for a communications signalling network of the
type comprising a plurality of signalling points interconnected by
bi-directional point-to-point links over which messages are passed to
transfer data between the signalling points, the passing of messages over
each said link being effected in accordance with a link-level protocol;
the message interceptor being intended for insertion in a link between a
first and a second signalling point and comprising:
a first link interface for interfacing with a first portion of said link
that connects with said first signalling point,
a first link-level protocol engine connected to said first link interface
for implementing said link-level protocol in respect of messages exchanged
with said first signalling point over said first link portion, said first
link-level protocol engine having means for extracting data carried in
messages received from said first signalling point and means for
incorporating other data into messages for sending to said first
signalling point,
a second link interface for interfacing with a second portion of said link
that connects with said second signalling point,
a second link level protocol engine, connected to said second link
interface for implementing said link-level protocol in respect of messages
exchanged with said second signalling point over said second link portion,
said second link-level protocol engine having means for extracting data
carried in the messages received from said second signalling point and
means for incorporating other data into messages for sending to said
second signalling point,
first transfer means for transferring data extracted by said first
link-level protocol engine to said second link-level protocol engine for
incorporation into messages, and
second transfer means for transferring data extracted by said second
link-level protocol engine to said first link-level protocol engine for
incorporation into messages at least one of said transfer means including
storage means for storing predetermined selection criteria, and
selective-action means for effecting at least one of the following
actions:
modification of particular data being transferred between protocol engines,
inhibiting transfer of particular data between said protocol engines, said
selective-action means being responsive to the data to be transferred by
transfer means to effect said action only on data meeting a corresponding
selection criterium held in said storage means.
2. A message interceptor according to claim 1, wherein said
selective-action means of said at least one transfer means can effect both
the actions of modification and inhibiting, each said selection criterium
having one of said actions associated therewith, which action is effected
by the selective-action means upon said criterium being met.
3. A message interceptor according to claim 1, wherein only said first
transfer means includes said storage means and selective-action means and
said selective-action means is effective to perform only said suppression
action.
4. A message interceptor according to claim 1, wherein both said first and
second transfer means include said storage means and selective-action
means and said selective-action means is effective to perform at least
said modification action.
5. A message interceptor according to claim 1, wherein said data extracted
by the said protocol engines from at least some of the messages received
thereby comprises at least one of the following data items:
identity of a signalling point from which the message originated;
identity of an intended destination signalling point of the message;
identity of a communications user number being called;
identity of a communications user number of a calling party;
a data type indicator indicating a type of data contained in other data
items extracted from the message, said selective-action means performing
said suppression action on data extracted from messages with data items
meeting selection criteria based on at least one of the following:
(a) a pre-selected value or range of values of said data item,
(b) a combination of data items with respective preselected values or range
of values;
(c) a preselected threshold number of messages received in unit time with
data items meeting criteria according to one of (a) and (b) above,
(d) a preselected threshold ratio determined over unit time between a
number of messages meeting first criteria according to one of (a) and (b)
and a number of messages meeting second criteria according to one of (a)
and (b) above.
6. A message interceptor according to claim 1, wherein said
selective-action means performs said suppression action on data
constituting a call set-up request whilst allowing transfer of data
relating to on-going calls, the selective-action means suppressing said
call setup request data only upon such data meeting at least one other
said selection criterium.
7. A message interceptor according to claim 1, where said data carried by
certain of said messages is request data specifying a request to which a
response is expected, the request data including an identifier of a
signalling point placing the request data on the signalling network, said
selective-action means effecting said suppression action upon one of said
selection criterion being met that relates to said request data, said
selective-action means including means operative in an event of said
request data being suppressed, to pass response data to a link-level
protocol engine providing the request data, the response data indicating
that the request has not been met and including said identifier, and said
protocol engine receiving the response data and incorporating it in a
message, the destination of which is set to the signalling point
identified by said identifier.
8. A message interceptor according to claim 7, wherein said request data is
call set-up request data.
9. A message interceptor according to claim 1, wherein said data carried by
said messages includes link control data relevant to operation of the link
carrying the messages, said transfer means being such that said link
control data is passed between said protocol engines without modification
or suppression.
10. A message interceptor according to claim 4, wherein the modification
action effected by said selective-action means of the first transfer means
is encryption and the modification action effected by said
selective-action means of the second transfer means is decryption.
11. A message interceptor according to claim 10, wherein the selection
criteria associated with the first and second transfer means are such that
data relating to link control and message routing is not subject to
modification action by said selective-action means.
12. A message interceptor according to claim 1, wherein the said
selective-action means effects a modification action involving data syntax
translation in respect of data identified by said selection criteria.
13. A message interceptor according to claim 1, wherein said storage means
is programmable and includes an external interface enabling said selection
criteria to be downloaded into the storage means together with an
indication of the associated action to be taken, where more than one
action is performable by said selective-action means.
14. A message interceptor according to claim 1, wherein said transfer means
includes means storing a signalling point code identifying the message
interceptor as a signalling point of said signalling network, and means
for providing network-level functions to the message interceptor.
15. A message interceptor according to claim 14, wherein the interceptor is
intended to operate with a signalling network compliant with at least SS7
level 3, said means for providing network-level functions comprising
level-3 means for providing SS7 level 3 functions, said data extracted by
each said link-level protocol engine being passed to said level-3 means
and the latter serving to pass on to the selective action means data
concerned with SS7 levels above level 3.
16. A communications signalling system of the type referred to in claim 1,
at least one said signalling point having a respective associated message
interceptor according to claim 1, inserted in each of the links connecting
with that signalling point.
17. A communications signalling system according to claim 16, wherein the
selection-criteria storage means of message interceptors are programmable,
the system including a common programming means for programming the same
selection criteria into the storage means of all the message interceptors
associated with a same signalling point. |
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Claims |
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Description |
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TECHNICAL FIELD
This invention relates to a message interceptor for a communications
signalling network, and in particular, but not exclusively, to a message
interceptor for use with a signalling network operating substantially in
accordance with Signalling System No.7, whether as specified by the CCITT,
ANSI, ETSI (for GSM), Bellcore or similar body, such a network being
herein referred to as an SS7 network. The CCITT Signalling System Number 7
is specified in Recommendations Q.700-Q.716 CCITT Volume VI-Fascicle VI.7,
Geneva 1989, ISBN 92-61-03511-6 which is herein incorporated by reference.
BACKGROUND ART
In modern communications technology it has become common practice to
provide two related but separate network infrastructures: a transmission
network for carrying enduser data traffic, and a signalling network for
controlling operation of the transmission network in accordance with
control signals transferred through the signalling network. In practice
such signalling networks comprise high-speed computers interconnected by
signalling links; computer programs control the computers to provide a set
of operational and signalling functions in accordance with a standardized
protocol. One example of such a signalling protocol is the
afore-mentionned Signalling System No. 7 (SS7) which is being extensively
deployed for control of telephone and other data transmission networks. An
SS7 network basically comprises various types of signalling points,
namely, signalling end points (SEPs) and signalling transfer points (STPs)
interconnected by signalling links, the SEPs being associated for example
with respective service switching points (SSPs) of the transmission
network, and service control points (SCPs). Congestion may arise in the
signalling network as a result, for example, of a number
of SEPs simultaneously wishing to pass messages to another SEP (such as an
SCP providing a database resource to the network). In this case, the links
to the target SEP may not be able to handle the concentration of message
traffic. To manage such possible congestion, the SS7 protocol provides a
congestion control mechanism by which when a message is received in the
outgoing buffer of a link causing the buffer to be filled to an
upper-threshold level, a choking message is sent back to the SEP that
generated the message, temporarily requiring it not to send any more
messages to the same destination. When the buffer level falls below a
lower threshold, the link is taken as no longer congested. This congestion
control mechanism is primarily operated in the signalling transfer points.
A drawback with such a congestion control mechanism is that it is
non-selective in nature--once congestion occurs in a link, choking
messages are issued in response to all subsequently received messages
until the link becomes non-congested, regardless of the origin,
destination or content of the messages.
One possible way of reducing congestion would be to introduce a selective
restriction mechanism in each existing signalling point that restricted
the flow of certain predetermined types of messages, such as messages
originating from a particular signalling point or concerning a particular
called party. Such a mechanism would, for example, prevent more than a
given number of messages from an originating signalling point from passing
through a particular network node in unit time. A drawback of this
mechanism is that it requires analysis of the messages and this causes
processing overhead in the signalling points. Such overhead is
particularly undesirable where the existing signalling point is a network
resource such as an SCP because it is such resources that are the likely
bottlenecks in the system.
Indeed, it would be useful generally to reduce the processing load of
signalling points such as SCP, as this would enable them to handle more
messages per unit time and improve overall network performance.
It is therefore an object of the present invention to provide a selective
message restriction apparatus that does not require processing overhead in
the existing signalling points. A separate objective of the present
invention is to provide apparatus that can remove certain processing tasks
from the existing network signalling points, particularly signalling end
points.
SUMMARY OF THE INVENTION
According to the present invention, there is provided a message interceptor
for a communications signalling network of the type comprising a plurality
of signalling points interconnected by bi-directional point-to-point links
over which messages are passed to transfer data between the signalling
points, the passing of messages over each said link being effected in
accordance with a link-level protocol; the message interceptor being
intended for insertion in a said link between a first and a second said
signalling point and comprising:
a first link interface for interfacing with a first portion of said link
that connects with said first signalling point,
a first link-level protocol engine connected to said first link interface
for implementing said link-level protocol in respect of messages exchanged
with said first signalling point over said first link portion, said first
link-level protocol engine having means for extracting data carried in the
messages received from said first signalling point and means for
incorporating other data into messages for sending to said first
signalling point,
a second link interface for interfacing with a second portion of said link
that connects with said second signalling point,
a second link level protocol engine, connected to said second link
interface for implementing said link-level protocol in respect of messages
exchanged with said second signalling point over said second link portion,
said second link-level protocol engine having means for extracting data
carried in the messages received from said second signalling point and
means for incorporating other data into messages for sending to said
second signalling point,
first transfer means for transferring data extracted by said first
link-level protocol engine to said second link-level protocol engine for
incorporation into messages thereby, and
second transfer means for transferring data extracted by said second
link-level protocol engine to said first link-level protocol engine for
incorporation into messages thereby,
at least one of said transfer means including storage means for storing
predetermined selection criteria, and selective-action means for effecting
at least one of the following actions:
modification of particular data being transferred between said protocol
engines,
suppression of the transfer of particular data between said protocol
engines, the said selective-action means being responsive to the data to
be transferred by the transfer means to effect a said action only on data
meeting a corresponding said selection criterium held in said storage
means.
Thus, the message interceptor is inserted directly in a link and carries
out its selective action functions independently of the signalling point
at the ends of the link: these are therefore relieved of processing
overhead for carrying out these functions. This overhead could be
substantial particularly in respect of data modification such as
encryption/decryption or syntax translation for database access.
It may be convenient to give the message interceptor its own signalling
network identity (its own signalling point code) in order to facilitate
certain monitoring and management functions. Although in this case the
message interceptor itself forms a signalling point, it is still
appropriate to view the message interceptor as being inserted in a link
between two other signalling points and the message interceptor still
gives the noted advantages for the signalling points between which it is
inserted.
The message interceptor can simply be arranged to effect messages
suppression for selected messages received in one direction along the
link. Alternatively, the message interceptor can be arranged to
selectively effect suppresion and modification actions on messages passing
in both directions along the link. Other combinations of action capability
and direction of application are also possible.
In signalling networks, such as an SS7 network, where sequence numbers are
applied to messages at the link level and acknowledgements are returned
based on these numbers, it is the task of the link-level protocol engines
of the message interceptor to ensure that the integrity of the sequence
number and acknowledgement flows is maintained. However, it should be
noted that this is done separately between the first signalling point and
message interceptor and between the message interceptor and second
signalling point. The suppression of a message by the interceptor thus
causes no problems as the message is acknowledged as received over the
link portion over which it reached the message interceptor, whilst it
makes no appearance on the other link portion.
Typically, the data extracted by the interceptor's protocol engines from at
least some of the messages received thereby comprises at least one of the
following data items:
the identity of the signalling point from which the message originated;
the identity of the intended destination signalling point of the message;
the identity of the communications user number being called;
the identity of a communications user number of a calling party;
a data type indicator indicating the type of data contained in other data
items extracted from the same message,
In this case, where the selective-action means performs a suppression
action, this will be on data extracted from messages with data items
meeting selection criteria based on at least one of the following:
(a) a pre-selected value or range of values of a said data item,
(b) a combination of data items with respective preselected values or range
of values,
(c) a preselected threshold number of messages received in unit time with
data items meeting criteria according to one of (a) and (b) above,
(d) a preselected threshold ratio determined over unit time between the
number of messages meeting first criteria according to one of (a) and (b)
and the number of messages meeting second criteria according to one of (a)
and (b) above.
Thus, for example, the message interceptor can be set to allow through only
a given number of call set up messages in unit time from a particular
source.
Where the message interceptor is arranged to suppress messages carrying
request data specifying a request (such as call set up) to which a
response is expected, the selective-action means preferably includes means
operative in the event of said request data being suppressed to pass
response data to the link-level protocol engine providing the request
data, the said protocol engine receiving the response data incorporating
it in a message the destination of which is set to the signalling point
originating the request. In this way, a rapid and clean request refusal
can be achieved, minimising processing within the signalling points.
Advantageously, where the data carried by the messages includes link
control data relevant to operation of the link carrying the messages, the
transfer means is arranged to pass the link control data between the
protocol engines without modification or suppression. In this manner,
overall link control is unaffected by the presence of the message
interceptor.
In respect of the data modification action capability of the message
interceptor (such as encryption), the selection criteria associated with
the first and second transfer means are preferably such that data relating
to link control and message routing is not subject to modification action
by said selective-action means.
Advantageously, the storage means storing the selection criteria is
programmable and includes an external interface enabling the selection
criteria to be downloaded into the storage means together with an
indication of the associated action to be taken where more than one action
is possible by said selective-action means.
Generally, multiple message interceptors will be deployed in a single
network. A typical application would be to insert a respective message
interceptor in each of the links connecting with a particular signalling
point in order either to prevent overloading of that point (by message
suppression) or to effect a processing task (encryption/decryption; syntax
translation) on messages exchanged with that signalling point.
In this case, common programming means are preferably provided for
programming the same selection criteria into the storage means of all the
message interceptors associated with the signalling point.
Advantageously, the interceptor is intended to operate with a signalling
network compliant with at least SS7 level 3. Indeed, the transfer means
can advantageously include SS7 level 3 functionality for handling
network-level issues, though in this case the message interceptor needs to
be allotted a signalling point code as already envisaged above.
BRIEF DESCRIPTION OF DRAWINGS
A message interceptor embodying the present invention will now be
described, by way of non-limiting example, with reference to the
accompanying drawings, in which:
FIG. 1 is a schematic diagram of a part of an SS7 signalling network;
FIG. 2 is a diagram illustrating the overall SS7 architecture;
FIG. 3 is a diagram showing the general form of a signal unit used for
transferring information across links in an SS7 network;
FIG. 4 is a diagram illustrating the deployment of several message
interceptors embodying the invention to protect/assist an SCP such as
illustrated in the FIG. 1 network;
FIG. 5 is a functional diagram of a first form of the message interceptor
intended to remove selected signal units passing in one direction along a
link;
FIG. 6 is a functional diagram of a second form of the message interceptor
intended to modify selected signal units passing in one direction along a
link;
FIG. 7 is a functional diagram of a third form of the message interceptor
intended to selectively remove or modify signal messages passing in either
direction along a link; and
FIG. 8 is a functional diagram of a fourth form of the message interceptor
provided with SS7 level 3 functionality.
BEST MODE FOR CARRYING OUT THE INVENTION
Referring to FIG. 1, an SS7 network 10 is shown inter-communicating three
signalling end points constituted by two service switching points SSPs 11
(between which pass speech circuits 12 of a transmission network not
further illustrated) and a service control point SCP 13 that can control
the operation of the SSPs to provide special services. The SS7 network 10
includes two pairs 14 of signalling transfer points STPs, and a plurality
of link sets 18 interconnecting the SSPs, SCP and STPs into a redundant
network. Each signalling link set 18 is made up of one or more individual
signalling links, the number of signalling links in a link set being
chosen to provide appropriate capacity for the level of signalling traffic
expected. The redundancy provided in respect of the STPs and links is to
ensure that the failure of a single component of the network core does not
cause the whole network to fail.
It should be noted that an SS7 network will typically comprise more STP
pairs, SSPs and SCPs than illustrated.
Messages traversing the links of the network may be any of a large number
of different types, depending on the nature of the call to which the
message relates and the function specified by the message.
In order to facilitate an understanding of the present invention, a brief
review will first be given of the layered structure of the SS7
architecture and of the messages passed over the links of the network 10
to implement the SS7 architecture.
FIG. 2 illustrates the SS7 architecture. Levels 1 to 3 (referenced 21, 22,
23) form the message transfer part (MTP) 24. The MTP 24 is responsible for
transferring signalling information between signalling points in messages.
Level 4 (not referenced as a whole) comprises circuit-related user parts,
namely ISDN User Part 26, Telephone User Part 27, and Data User Part 28.
These user parts define the meaning of the messages transferred by the MTP
24 and provide functionality to the users of SS7 (block 29).
The user parts 26, 27, 28 are specific to particular types of
circuit-related applications as indicated by their names. Level 4 also
includes functional elements defining a general protocol for
non-circuit-related information, such as operations, maintenance and
administration information or network database information (provided, for
example, from an SCP 13). The main functional element in this Level 4
protocol is the Transaction Capabilities (TC) 30 which sits on top of a
Signalling-Connection-Control Part (SCCP) 31 and beneath a TC Users
element 32. The SCCP 31 actually forms part of the transfer mechanism for
non-circuit-related applications, combining with MTP 24 to provide a
transfer mechanism meeting the OSI Layer 3/4 boundary requirements.
Considering the MTP 24 in a little more detail, Level 1 (reference 21)
defines the physical, electrical and functional characteristics of the
transmission path for signalling. MTP Level 2 (reference 22) defines the
functions and procedures for the transfer of signalling messages over a
link between two directly-connected signalling points. MTP Level 3
(reference 23) provides functions for the reliable transfer of signalling
information from one signalling end point to another. Thus, Level 3 is
responsible for those functions that are appropriate to a number of
signalling links, these being separable into signalling-message handling
functions and signalling-network management functions.
When considering the passing of messages over a single link, it is the
combination of Levels 1 and 2 that provides for the reliable transfer of
signalling information. The Level 2 functions provide a framework in which
the information is transferred and performs error-detection and
error-correction processes; the Level 2 functions are carried out afresh
on a link-by-link basis. At Level 2, information is seen as being
transferred between signalling points in messages known as "signal units".
The general form of a signal unit 40 is shown in FIG. 3. As can be seen, a
field 41 carrying message/data is encapsuled in a Level 2 framework
comprising the following fields:
an 8-bit flag field;
a 7-bit backward sequence number field (BSN);
a backward-indicator bit (BIB);
a 7-bit forward sequence number field (FSN);
a forward-indicator bit (FIB);
a 6-bit length indicator field (LI);
a spare 2-bit field (SP);
a 16-bit check field; and
an 8-bit terminating flag field.
The FSN, FIB, BSN, BIB and check fields provide error correction
functionality at link level in a manner well understood by persons skilled
in the art.
There are three types of signalling unit:
MSU--the Message Signal Unit--MSUs carry all service/application data sent
on the SS7 network. The amount of data per MSU is limited to 273 octels
maximuln.
LSSU--the Link Status Signal Unit--LSSUs carry information relating to the
status of the link and are therefore concerned with Level 2 functions.
Normally, LSSUs are only seen during the initial alignment procedure when
a link is brought into service but are used at other times, for example,
to stop the flow of signal units when processors are busy.
FISU--the Fill-In Signal Unit--When no MSUs or LSSUs are to be sent, a
signalling point continually sends FISUs. FISUs carry basic Level 2
information only, for example, the acknowledgement of the last MSU (field
41 is empty).
The length indicator (LI) within each message indicates the signal unit
type:
LI=0 means FISU
LI=1 or 2 means LSSU
LI=3 or more means MSU.
FIG. 3 illustrates at 42 the basic format of an MSU; as can be seen, it
comprises a service information octet SIO of 8 bits and a signalling
information field SIF of 8n bits, where n is a positive integer. The SIO
field includes a Service Indicator sub-field that defines the user part or
equivalent appropriate to the message. The SIF contains the information
being transferred and will generally include a routing label 43 comprising
a 14-bit destination point code indicating the destination signalling end
point, a 14-bit originating point code indicating the originating
signalling end point, and a 4-bit signalling link selection field for
specifying a particular link in cases where two signalling points are
linked by a multiple-link link set. The MTP 24 is not aware of the
contents of the SIF other than the routing label.
As an example of the information that may be borne by an MSU, where a call
is being set up, the first message to be sent is an initial-address
message (IAM) which will contain the required address (e.g. the digits
dialled by the calling customer). Other MSUs may contain the address of
the calling party.
Turning now to a consideration of the present invention, FIG. 4 shows an
SCP 50 (such as the SCP 13 of FIG. 1) to which four links 51 A,B,C,D are
connected. Links 51A and 51B belong, for example, to a link set connecting
SCP 50 with an STP 48 whilst links 51C and 51D may belong to further link
set connecting SCP 50 with an STP 49. Inserted in each link 51 A,B,C,D is
a respective message interceptor 52 embodying the present invention. Each
message interceptor 52 is operative to monitor the messages on the link in
which it is inserted, and to take predetermined action on detecting
messages that meet pre-specified selection criteria. The selection critera
are programmed into the message interceptor 52 from a remote station 53
over a LAN 54.
If the purpose of the message interceptors 52 is to protect the SCP 50 from
overload, the action taken by each interceptor 52 will be to selectively
suppress messages from the corresponding link before they reach the SCP.
On the other hand, if the purpose of the message interceptors 52 is to
relieve the SCP of particular processing tasks (such as decryption of
incoming messages and encryption messages), then each message interceptor
52 will act to modify each message it receives that meets the
predetermined selection criteria.
Each message interceptor may, in fact, be operative to effect both a
message suppression action and a message modification action, each action
being triggered by a different set of criteria.
FIG. 5 is a functional block diagram of a first form of message interceptor
52 that is intended simply to suppress particular messages sent in one
direction along a link.
The message interceptor 52 of FIG. 5 comprises two interfaces 60, 61
interfacing with respective portions 62, 63 of the link in which the
interceptor is inserted (here assumed to be link 51A for convenience).
Each link portion 62, 63 comprises two unidirectional channels 62A, B and
63A, B. In the present case, it is assumed that the channel 62A and 63A
are inbound toward the SCP 50 and the 62B and 63B are outbound from the
SCP.
Interfaces 60, 61 provide an MTP Level 1 interface to the corresponding
link portions.
Associated with each interface 60, 61 is a respective Level 2 protocol
engine 64, 65. The protocol engines 64, 65 act generally in the same
manner as standard Level 2 protocol engines for MTP and will typically
each be a hardware/firmware implementation of an appropriate state
machine. Each protocol engine 64, 65 includes a respective data extraction
circuit 66, 67 for extracting the Level 3 information from each MSU
received at the corresponding interface. Each protocol engine 64, 65 also
includes a data insertion circuit 68, 69 for inserting Level 3 information
supplied to it into a MSU for outward transmission through the
corresponding interface.
In addition, each protocol engine 64, 65 is operative to output a signal
indicative of each LSSU received over the link portion 62A, 63B
respectively, notwithstanding that normally such LSSUs generally have no
direct visibility beyond Level 2.
A transfer circuit 70 (dotted outline) serves to transfer data extracted by
the data extraction circuit 66 of the protocol engine 64 to the data
insertion circuit 67 of the protocol engine engine 65. As will the more
fully described hereinafter, this transfer process is selective in nature.
A second transfer circuit 80 non-selectively transfers data extracted by
the data extraction circuit 69 of the protocol engine 65 to the data
insertion circuit 68 of the protocol engine 64.
It can thus be seen that in principal the message interceptor extracts the
Level 3 information from MSUs received on the link channel 62A and
selectively transfers that data to the link 63A for transmission to the
SCP 50. In addition, Level 3 information contained in an MSU received over
link channel 63B from the SCP, is transferred across for onward
transmission over link channel 62B. The operation of the link portion 62
is maintained at link level (MTP Level2) by the protocol engine 64, this
operation including the tracking of sequence numbers and reponsiveness to
link status signal. Similarly, the operation of the link portion 63 is
maintained at the link level by the protocol engine 65. LSSUs are also
generally transferred across between the link portions by the transfer
circuits 70 and 80.
Considering the transfer circuit 70 in more detail, the data extracted by
the circuit 66 is supplied to a data register 71, this data in fact
comprising both the data in field 41 of an MSU and the length indicator LI
of the MSLI. The protocol engine 64 indicates the presence of new data in
register 71 by the supply of a signal NEW to a selective action control
circuit 72 of the transfer circuit 70. The selective action circuit 72
then examines the contents of the data register 71 and compares these
contents against criteria pre-stored in a criteria store 73. These
criteria which have been input into the store 73 over the LAN 54 from the
work station 53 (see input "PROG" in FIG. 5), serve to identify particular
messages that are not to be passed on to the SCP over the link channel
63A. Thus, if the contents of the data register 73 meet any one of the
criteria stored in the store 73, the selective action control circuit 72
does not transfer these contents onward; in other words, the control
circuit 72 acts to suppress the contents of selected MSUs. If none of the
criteria stored in store 73 are meet, the control circuit 72 causes the
contents of the register 71 to be passed to a buffer 74 for transfer to
the data insertion circuit 67.
The selection criteria stored in store 73 may, for example, relate to one,
or a combination, of the following data items:
the identity of the signalling point from which the message originated;
the identity of the intended destination signalling point of the message;
the identity of the communications user number being called;
the identity of a communications user number of a calling party;
a data type indicator indicating the type of data contained in other data
items extracted from the same message,
Thus the selection criteria may be based on at least one of the following:
(a) a pre-selected value or range of values of a said data item,
(b) a combination of data items with respective preselected values or range
of values,
(c) a preselected threshold number of messages received in unit time with
data items meeting criteria according to one of (a) and (b) above,
(d) a preselected threshold ratio determined over unit time between the
number of messages meeting first criteria according to one of (a) and (b)
and the number of messages meeting second criteria according to one of (a)
and (b) above.
By way of example, a simple message suppression criteria would be to
suppress all calls directed to a particular party; in this case, the
selection criteria would be the intersection of a data type indicator
indicating a call set-up (IAM) message with a data item having
predetermined call party digits. In other words, the control circuit 72
would suppress any IAM message containing the specified called party
digits.
A more complicated selection criterion would be to limit the number of new
calls initiated in a moving time window from a particular party. For such
more complicated selection criteria, the selective action control circuit
72 must keep various running totals. The keeping of such counts and any
subsequent calculations based on such counts can be treated as actions
consequent on certain basic criteria relating to data type and/or content
being met, these consequential actions being stored along with the basic
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