METHOD FOR SIGNALING IN A SIGNALING TRANSFER POINT

PROCEDE DE SIGNALISATION DANS UN POINT DE TRANSFERT DE SIGNALISATION

English Abstract

Routing loops may occur in signaling networks so that messages are routed to a
destination in a loop without ever reaching said destination. The invention
shows how such loops can be detected and eliminated as speedily as possible.

French Abstract

Dans des réseaux de signalisation, il peut y avoir des boucles d'acheminement, faisant que des données sont acheminées dans une boucle vers une destination à laquelle elles ne parviennent jamais. L'invention indique comment reconnaître et éliminer aussi vite que possible de telles boucles.

Claims

5
Claims
1. Method for the signalling in a signalling transfer point, in accord
wherewith
-- signalling messages stemming from source signalling points are routed in
the direction toward destination signalling points,
-- the presence of a loop or, respectively, the possibility of the presence of
a
loop over a departing linkset to a destination signalling point (SP X) is
checked by a routing test (MRVT) and/or by a real-time method,
-- given a positive check result, signalling messages are automatically
prevented from continuing to be sent to the destination signalling point
(SP X) via the appertaining linkset.
2. Method according to claim 1, characterized in that
-- given said positive check result, test messages are first sent via a
linkset to
destinations that can be reached via said linkset;
-- in the case of returning test messages, signalling messages are then
automatically prevented from being sent to a destination that had returning
test messages.
3. method according to claim 1 or 2, characterized in that signalling
messages are prevented (downstream) from being sent to the appertaining
destination
via the appertaining linkset in that the specific, departing linkset to this
destination is
blocked in the routing table of the signalling transfer point.
4. Method according to claim 1 or 2, characterized in that signalling
messages are prevented (upstream) from being sent to the appertaining
destination via
the appertaining linkset in that the signalling transfer point (STP A) sends a
transfer
prohibited message regarding the destination signalling point (SP X) to the
preceding
signalling transfer point (STP B), whereupon the preceding signalling transfer
point
(STP B) will reroute or, respectively, stop the traffic to the destination
signalling point
(SP X).
5. Method according to one of the claims 1 through 4, characterized in that
said interruption of the loop is controlled by the operations maintenance and
administration part (OMAP).

6. Method according to one of the claims 1 through 5, characterized in that
said interruption of the loop is controlled by the message transfer part
(MTO).
7. Method according to one of the claims 1 through 6, characterized in
that, after blocking the linkset contained in the loop, the new, current route
is in turn
immediately checked for freedom from loops in the signalling transfer point
(STP A).
8. Signalling system of a signalling transfer point that routes signalling
messages to destination signalling points, characterized in that
-- it checks the presence of a loop or, respectively, the possibility of the
presence of a loop over a departing linkset to a destination signaling point
(SP X) by a routing test (MRVT) and/or by a real-time method, whereby
-- when a positive check result is obtained, signalling messages are
automatically prevented from continuing to be sent to the destination
signalling point via the appertaining linkset.
9. Signalling system of a signalling transfer point according to claim 8,
characterized in that it verifies the detected possibility of the presence of
a loop by
sending test messages to destinations reachable via said linkset before it
automatically
prevents signalling messages from continuing to be cent via the appertaining
linkset to
a destination for which said test messages return.

Description

CA 02331706 2000-11-10
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METHOD FOR SIGNALLING IN A SIGNALLING TRANSFER POINT
It can occur in signalling networks according to signalling system No. 7
that routing loops occur in the network on MTP level 3 due to incorrect
plamaing or
operating errors, so that messages are routed to one or more destinations in a
loop
without ever reaching their destination. Of particular interest here are loops
:having a
length greater than 2 ("length of a loop" means the plurality of signalling
points
participating in a loop) and, in particular, how such loops can be eliminated
when they
are recognized.
If loops potentially present in the tables ~~re in fact used for routing, thus
represents a serious problem for the network since rrtessages, on the one
hand, do not
arrive at their destination and, on the other hand, use valuable resources in
the
network. It should therefore be eliminated as fast as possible.
Loops having the length 2 (so-called ping-gong loops) cannot occur given
a functioning protocol in the MTP (message transfer point). Should they
nonetheless
arise, these loops can be easily recognized in real time in a signalling
transfer point in
that a check is carried out to see whether a message i,s to be routed over the
same
linkset on which it was received. They are just as easy to correct in that the
unsuccessful protocol actions (sending transfer prohiibited -- TFP -- messages
to the
cooperating party) are repeated.
2 0 Loops having a length >2 are more difficult to recognize. A check can in
fact be performed with every message in a specific STP as to whether this
message
derives izom precisely this STP (by comparing the CiPC contained in the
me:csage to
the PointCode of the STP). When this is the case, there is a loop in the
network.
STPs, however, do not necessarily generate messages or, respectively, do not;
2 5 necessarily generate messages to the destination or destinations to which
there is a
loop.
This problem can be solved by a real-time method that recognizes the
possibility of a loop, for example due to a lasting overload on a linkset.
When said
method recognizes the possibility of a loop, the operating personnel can be
informed
3 0 so that corrective measures can be initiated.

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The standard (Q.753, Q.754) defines another solution of the problem, what
is referred to as the M~'F' route verification test (MRVT) that checks all
possible paths
in an MTP network between two given points for correctness, including freedom
from
loops. Upon occurrence of errors such as, for example, loops, the operating
;personnel
is informed in order to undertake corrections. MRVT in fact has the advantage
over a
real-time method that it ~;.an also recognize loops bei:ore they are actually
used, since
all possible paths are checked, not only the current ones. The disadvantage,
however,
is that a separate protocol is required for it. When this is not realized in
the entire
network, the check is not possible or is only possible in incomplete form.
This
l0 situation is specifically established in the international signalling
network. I)ue to the
load that it generates, moreover, the MRVT cannot constantly check all routes
between all points in the network.
The invention is based on the object of overcoming the aforementioned
disadvantages.
This object is achieved by a method according to claim 1.
The invention is explained in greater detail below with the assistance of
the drawing, whereby the drawing comprises 3 Figures.
Figure 1 shows an example of a loop.
Figures 2 and 3 shows methods for parting a loop.
2 0 The present invention particularly reveals how, given real-time
recognition of loops having a length > 2 and/or upon recognition of loops by
the
MRVT, the loops can be broken by automatic, real-time, protocol-compatible
methods that are simple to realize. The time elapsing before the operating
personal
takes action can thus be bridged.
2 5 It must thereby be mentioned that it is advantageous, given possible loops
that were recognized by the MRVT or a real-time method for a linkset, to check
before having potential recourse to automatic correction measures (the MRV'T,
namely, does not supply any statements whether a possible loop is also being
employed at the time and, under certain circumstances, the real-time method
cannot
3 0 make any statements about the destination to which a possible loop is
present). Said
check ensues by sending otherwise unemployed MT'P network management messages

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to the destinations that c;an be reached (according to the routing) via the
appertaining
linkset. When such test messages return to the STP, these messages are
detf;cted by
comparing the OPC contained in the message to the point code of the STP, and a
loop
or, respectively, several loops are recognized. Correction measures can
thereby
remain limited to loops being currently employed.
Said check vvith the assistance of test messages is already useful when it is
realized in only one STP since all loops that run through this STP can be
recognized.
The check method can also always be active.
Another possibility is comprised in making the initiation of correction
1 o measures dependent of the evaluation of the (relative) probability that
the possible
loop could be employed. These information can be made available by the M:RVT
in
the form of priorities of the individual paths constituting the loop.
When a loop to a destination X is recognized in an STP A by the MRVT
or by real-time methods, one can proceed in the following way for breaking the
loop:
a) Breaking the loop "downstream" in that the speciific departing path to this
destination is blocked in the routing table in A. This step can, in
particular, be
implemented when other paths to X are also available proceeding from A. In
this
case, it is recommendable to also check the route employed as an alternative
for the
occurrence of a loop. Although the lack of a detection of a loop is no
guarantee that
2 0 there is not some other loop that no longer contains A, there is at least
a probability
that the problem has been eliminated.
b) Alternatively, or if, fo:r example, there no longer happens to an alternate
(loop-free)
route proceeding from A, the loop can be broken "upstream", i.e. to the
preceding
STP B on the loop, in that A sends B a transfer prohibited message with
respect to X.
2 5 In response thereto, B will reroute or, respectively, stop the traffic to
X. Since B will
subsequently periodically review the availability of the route to X via A with
what are
referred to as route set messages, it must be assured that A does not answer
these
messages with a transfer allowed, since B could otherwise re-close the loops.
After final correction of the routing tables by the operating persormel, the
3 0 actions automatically undlertaken by the MTP or the operations maintenance
and
administration part (OMAP) can be in turn reversed by the operating personnel
(Note:

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OMAP comprises higher-ranking SS7 management functions, for example MRVT,
screening functions and measurements. "Informing the operating personnel" is
also
(partially) part of the OMAP)
A special characteristic of the invention is comprised in the mechanism for
breaking loops having the length > 2 with automatic measures that are simple
to
realize upon utilization of existing protocol features. In particular, the
method can
already be employed and is useful when it is realized in only a single STP.
One possibility for realizing the alternative b) is to automatically activate
what is referred to as ILS/DPC screening (ILS = incoming linkset; see Q.705,
~8) in
A for messages from ~ to X. However, a linking of the ILS/DPC screening into
the
MTP management network is needed for this purpose such that an illegal message
is
answered with a TFP message and the route set test messages are also correctly
handled.

Representative Drawing