Note: Descriptions are shown in the official language in which they were submitted.
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An implementation of call drop-back
in a telecommunication network
FIELD OF THE INVENTION
The present invention relates to a method for performing a
call drop-back in a telecommunication system and to a
corresponding control device as well as to accordingly
adapted telecommunication network elements provided with
the same.
BACKGROUND OF THE INVENTION
Recently, the features offered by telecommunication
networks in connection with so-called value added services
have remarkably increased in number.
Among such features, there has been established a feature
permitting a calling subscriber_ C, calling a called
subscriber A (who, at the moment of being called, is not
reachable for whatever reason), to leave a message on a
message server for the called subscriber A. A message thus
left on a message server is also referred to as a message
registered to said message server. The called subscriber A
may listen to the message left for him (and/or thus
registered to the message server) at an arbitrary point of
time later. Thus, such a message server has a function of
an answering machine provided at the telecommunication
network side.
For the purpose of the present invention, the following
should be noted. A telecommunication system means any
telecommunication in which at least one exchange for
switching calls between at least two subscribers or between
one of said subscribers and a message server is provided,
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and wherein said message server is adapted to maintain a
record of a plurality of previous messages intended for a
called subscriber and registered to the message server
record from a respective calling subscriber. Also, although
the term "exchange" is used in this specification as a term
having its origin in the field of circuit switched
networks, the term "exchange" is also intended to cover
corresponding devices known as "routers" in the field of
packet switched networks and in the filed of "Voice over IP
calls" and/or Internet Telephony. Also, the term "call" as
used herein is intended to be understood in its broadest
sense such that it covers any kind of connection (circuit
switched, packet switched, or IP (Internet Protocol) based
connections as well as real time, non-real time, constant
bit rate or variable bit rate connections).
In this connection, it is to be understood that the present
invention is not limited to any particular type of
telecommunication system, and the present invention may be
applied to a PSTN telephone system such as the ISDN
network, or to a mobile radio telecommunication system such
as the GSM telecommunication system or to IP telephony
systems. Still further, the principles as set out in this
specification may also be applied to a telecommunication
system still under development such as the UMTS system (3ra
generation mobile telecommunication system) as long as any
of such aforementioned systems fulfills the above noted
prerequisites of an "exchange" and a "message server".
In addition, a message server means any means and/or
network element adapted to keep a record of a plurality of
previous messages intended for a called subscriber (and/or
a plurality of subscribers registered to the network) and
registered to (i.e. stored in or recorded to) the message
server record from a respective calling subscriber. In this
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connection, the expression "message" has to be understood
in its broadest sense and it may comprise voice mail
messages (recorded spoken speech messages) as well as text
messages such as SMS messages and any other type of
messages such as data messages comprising images and/or
text and/or voice (for example also referred to as
"multimedia messages") in whatever combination as long as
the message has a content which is defined by the calling
subscriber.
Also, the term "call drop-back feature" as used herein
means a feature (service) offered by the telecommunication
network which enables a called subscriber listening to the
messages registered for him to make a direct call to a
respective calling subscriber who has left the listened
message for the called subscriber.
According to the prior art, several related services in
connection with such a service also named call return
service have been described.
The U.S. patent number US-A-4,930,152 by Miller (assigned
to Bell Communications Research), of May 29, 1990,
discloses a call return telephone service. According to the
disclosed arrangement, only a list of calling subscribers'
telephone numbers is kept in the switch (exchange) of the
network for the purpose of calling back the previously
calling subscribers.
However, this arrangement does not disclose that any
specific messages having a content defined by the calling
subscriber can be left for the called subscriber. Moreover,
as a result, also no message server is provided according
to this prior art arrangement.
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The European patent application number EP-A2-0 795 992 by
Hanson & Rudrapatna (assigned to AT&T Corporation)
discloses a method and apparatus for a prepaid return call
in connection with a message server.
According to this prior art arrangement, the called
subscriber as the recipient of messages stored on the
message server is not enabled to initiate a listening
operation for learning the contents of the messages stored
for him. Instead, according to this prior art arrangement,
the called subscriber is contacted upon initiation of the
message server, which periodically tries to contact the
called subscriber in order to deliver the message.
However, due to periodically contacting the called
subscriber by the message server and only a maximum number
of such "tries" being permitted, there exists the risk that
the message left for the called subscriber may never be
delivered to him. Also, as the called subscriber can not
influence when the left messages are delivered to him, he
may feel uncomfortable if the messages are delivered at an
inappropriate time of the day or night. Only if a message
could be delivered by the message server to the called
subscriber successfully, the called subscriber is offered a
possibility to initiate a callback to the calling
subscriber (who has prepaid this callback call).
Still further, the European patent application EP-A2-0 897
239 by Roth et al. (assigned to Siemens AG) discloses a
program controlled communication system having access to a
voice mail server. According to his prior art, a called
subscriber can retrieve messages intended for him from a
voice mail server VMS. A calling subscriber may
automatically be called back upon retrieval of the
corresponding message from the VMS.
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However, such a callback mechanism does not allow a direct
callback from the cal.lF~d to the (previously) calling
subscriber, since in connection with the callback service,
only an exchange of another stored message (from the called
subscriber to the previously calling) subscriber is
enabled. Stated in other words, only pre-stored messages
are exchanged with an interaction of the message server
being involved between the called and the calling
subscriber or vice versa.
Thus, prior art systems known in connection with call back
services suffer from a variety of drawbacks and
inconveniences for the users (subscribers).
SUMMARY OF THE INVENTION
Consequently, it is an object of the present invention to
enable a called subscriber A to make a callback call to
several different calling subscribers during a single
message server session.
According to the present invention, this object is achieved
by a method for performing a call drop-back in a
telecommunication system, said telecommunication system
comprising at least at least one exchange for switching
calls between at least two subscribers or between one of
said subscribers and a message server, wherein said message
server is adapted to maintain a record of a plurality of
previous messages intended for a called subscriber and
which previous messages were recorded to the message server
record from a respective calling subscriber, wherein said
method comprises the steps of: invoking a callback call
from said called subscriber to said calling subscriber
based on the information recorded in said message server
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record via at least one of said at least one exchange, and
upon invoking said callback call, setting the system into a
state enabling said called subscriber to be connected back
to said message server after the termination of said
invoked callback call.
Advantageous further developments of the method according
to the present invention are set out in the dependent
claims.
Still further, the object underlying the present invention
is solved by a control device for performing a call drop-
back in a telecommunication system, said control device
being adapted to carry out the method according to the
present invention. Such a control device may even be
distributed over at least two telecommunication network
elements.
Also the present invention relates to a telecommunication
network element comprising a control device as
aforementioned.
Stated in other words, according to the present invention,
when a called subscriber A makes a listening call to a
message server it is possible to invoke the call drop-back
feature (i.e. to make a callback call to the subscriber who
has left a message (to which message subscriber A listens
when making the listening call) to the message server. The
callback call will be dropped to some or at least one of
the previous exchanges (or even to the originating
exchange) from where the call will be routed to the
previously calling subscriber C. The connection between the
exchange and the message server will be released
immediately and if the re-routed call (callback call) does
not succeed or is terminated, the exchange-to-message-
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server connection (MSC-VMS/SN) will be re-established. When
the call / connection between called subscriber A and
previously calling subscriber C will be released, it is
enabled that the connection between subscriber A and the
message server is re-established and that the subscriber A
may listen to a next message which has been left at the
message server and to make a new callback call to a person
(subscriber) who has left that message.
In order to accomplish this, needed information will be
transmitted between the message server and the exchange in
a new parameter, the transmission being based on the usage
of ISUP (ISDN user part) signaling. The name of the new
parameter also referred to as transmitted "indication" may
be "VMS service indicators'.'. The parameter / indication
includes at least a call drop back type indicator (CDB TYPE
INDICATOR), indicating if a re-establishment of the
listening call to the message server (VMS/SN) is needed
after the callback call is terminated, a call drop back
release reason indicator (CDB RELEASE REASON INDICATOR),
indicating whether the callback call to said calling
subscriber (C) has been successful or not, and a message
server session indicator (VMS/SN SESSION ID), containing a
subscriber identification of the listening called
subscriber A and a message identification identifying the
message listened to, these identifications enabling to
resume the listening call after the callback call has
terminated.
Accordingly, with the present invention as briefly outlined
above, it becomes advantageously possible that a called
subscriber, listening to his messages left for him at the
message server, is enabled to carry on listening to his
messages after a callback call to the subscriber who has
left that message, without the need to make a new call to
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the message server afterwards. Thus, a called subscriber
can make different calls to several different numbers
during one message server session.
Also, the drawbacks mentioned earlier herein before as
attributed to the prior art arrangements are removed by
virtue of the present invention being implemented.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will be more readily understood upon
referring to the following description of preferred
embodiments when read in conjunction with the accompanying
drawings. In the drawings,
I5
Fig. 1 shows a schematic block diagram of a
telecommunication network architecture and the relevant
components required for the present invention to be carried
out;
Fig. 2 (composed of Figs. 2A and 2B) illustrates a
schematic signaling scenario of signals relevant for the
present invention which are transmitted between subscribers
and telecommunication network elements depicted in Fig. 1;
Fig. 3 shows a basic example of a message record provided
in the message server and the data stored therein;
Fig. 4 shows an example of a data format usable for
transmission of the VMS service indication; and
Fig. 5 shows an actual format of the VMS service
indication.
DETATT~FI~ I~F~C'RTPTTCWI f~F PRF'FF.RRF'fl F'MR~IIITMF'nlTC
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Subsequently, the present invention will be described in
detail with reference ~o the drawings.
Fig. 1 shows a schematic block diagram of a
telecommunication network architecture and the relevant
components required for the present invention to be carried
out. For the specific example described, the architecture
of a mobile telecommunication network has been chosen, such
as for example, the GSM network.
However, also other network architectures may be applied,
as mentioned above. Namely, a similar architecture may be
applied in connection with packet switched networks and/or
Voice over IP networks (Internet Telephony Networks), with
the exchange being replaced by a router. Nevertheless, for
the purpose of the present invention, the term exchange is
intended to cover both meanings, an exchange as such as
well as a router.
In order to simplify the explanation, the following
description focuses on the GSM network architecture as an
example.
A calling subscriber C, when calling a called subscriber A,
communicates via an air interface AIR-I/F with a base
station subsystem BSS, which in turn communicates via an
interface A-I/F with a network switching subsystem NSS. The
base station subsystem BSS comprises at least one base
station (not shown) and at least one base station
controller (not shown).
The base station subsystem and/or the at least one base
station controller (not shown) communicates via the
interface A-I/F with a network switching subsystem NSS,
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particularly, with one of at least one exchange such as a
mobile service switching center MSC. Of course, in a
network covering a big area and /or a great number of
subscribers, plural exchanges, i. e. MSC's are required and
provided for. However, for the sake of keeping the
description simple and understandable, the present
invention is described with reference to only one exchange
MSC.
The exchange MSC communicates via an interface VMS-I/F with
a message server such as, for example, a voice mail server
/ service node VMS/SN.
Thus, when a calling subscriber C calls a called subscriber
A, the signal flow can be depicted as C -> BSS -> MSC ->
BSS -> A, in case that subscriber A can be reached and
answers his phone. In contrast thereto, in case that the
called subscriber A does not answer his phone / terminal
either to some defect, due to being busy with another call,
or because he has switched off his terminal, this may be
informed to the subscriber C. The subscriber C may then
initiate to leave a message via the exchange MSC at the
message server VMS/SN which is intended for the called
subscriber A. The signal flow in this case can be
summarized as C -> BSS -> MSC -> VMS/SN. Alternatively, the
leaving of a message may be implemented to be performed
automatically at the side of the exchange MSC upon
detection that the called subscriber is not reachable.
At an arbitrary point of time later, the previously called
subscriber may then interrogate the message server VMS/SN
to learn whether any message has been left for him. In this
case, the information flow is A -> BSS -> MSC ->
VMS/SN -> MSC -> BSS -> A. The subscriber A thus listening
to his voice mail may initiate a direct callback call to
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the calling subscriber who has left the message listened
to, without the need for dialing a number. Namely, when A
wants to call back C, the call is "dropped back" from the
message server to a previous exchange MSC involved in
connecting the subscriber A to the message server, from
where the callback call is then connected further to the
previously calling subscriber C.
Previously, a connection between the exchange MSC and the
message server VMS/SN was released as soon as the call
dropped back to the MSC, so that a new connection to the
message server had to be established by the subscriber A
after such a callback call, if the subscriber A wanted to
continue to listen to his voice mails.
According to the present invention and to the new signaling
between the exchange and the message server as presented in
the present specification, it is enabled to a previously
called subscriber listening to his messages such as voice
mails, to continue to listen thereto after having made a
callback to a calling subscriber without the need to
establish a new call to the message server.
This will be set out in still greater detail in connection
with Figs. 2 to 4.
Fig. 2 (composed of Figs. 2A and 2B) illustrates a
schematic signaling scenario of signals relevant for the
present invention which are transmitted between subscribers
and telecommunication network elements depicted in Fig. 1.
In order to keep the Figure simple and to simplify the
description, the base station subsystem BSS indicated in
Fig. 1 has now been omitted in the drawing. Nevertheless,
actually, it is still present and involved in the "real"
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signaling. Moreover, in Fig. 2, the network elements and
subscribers involved in the signaling as well as the
signaling itself are illustrated in the horizontal
direction, while the time axis is intended to be
represented in vertical direction.
In Fig. 2A, a calling subscriber C, in step S1 tries to
call a called subscriber A via the exchange MSC (and the
base station subsystem BSS(not shown). In a step Sla, it is
detected that the called subscriber A is not reachable.
In response thereto, this is either signaled to the
subscriber C in order to decide whether he wants to leave a
message for the subscriber A (not shown), or at the side of
the exchange MSC there is automatically provided the
possibility to the calling subscriber C to leave a message
for the called subscriber at the message server VMS/SN, as
shown in step S2. Since the way of leaving a message in the
message server is not within the scope of the present
invention, the corresponding description is somewhat
abbreviated and kept short by merely outlining the signal
flow in principle.
At an arbitrary point of time later, and upon initiation of
the previously called subscriber A, in step S3, the
subscriber A initiates a listening call via the exchange
MSC to the message server VMS/SN in order to listen to one
or more messages which have been left for him at the
message server VMS/SN. In this way, a VMS/SN session is
opened, and the call drop back feature (CDB feature) is
invoked. (Note that the listening is assumed to be started
from a message number 1 onwards, one message after the
other, as will be set out later in greater detail).
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Then, in a following step S4, it is detected whether any
messages) is (are) recorded for the listening subscriber
in a record at the ~ne~~age server VMS/SN. Although the
detection is illustrated as being performed at the message
server side, this can also be performed at the exchange
side. In the former case, a corresponding control device
would then be distributed over a plurality of at least two
network elements, while in the latter case, the control
device would be located at and/or form part of the exchange
MSC.
Then , in a following step S5, the listening subscriber A
obtains the message information of the stored message(s).
Stated in other words, the message information of one such
message (if any) is forwarded to the called subscriber A.
The message contains for example a voice mail message
together with an identification of the previously calling
subscriber C, for example his telephone number MSISDN, or
already his name. Having received this information, i.e.
having listened to the voice mail, the called subscriber A
may personally decide (e.g. dependent on the contents of
the listened voice mail and/or dependent on the identity of
the subscriber C who has left this message for the called
subscriber A) whether he wants to call back the previously
calling subscriber C.
If so, in step S6, the called subscriber invokes a callback
call, the callback call invocation being forwarded via the
exchange MSC to the message server VMS/SN. At the message
server side, a pointer points to the entry in the record of
recorded messages which has immediately been listened to by
the called subscriber A. If a callback call is thus
invoked, at the message server side an indication, i.e. a
VMS service indication is generated and transmitted, step
S7, via the VMS-I/F interface to the exchange MSC. This
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indication, i.e. the VMS service indication is, for
example, transmitted using ISUP (ISDN User Part) signaling.
Upon having received the transmitted indication, at the
exchange side, the listening call is released. More
precisely, from step S3 onwards, there exists a connection
from subscriber A to the exchange MSC, and also further
from the MSC to the message server VMS/SN. Releasing the
listening call now means, that only the connection between
the exchange MSC to the message server is released.
Then, in a step S9 (step S9a and S9b) based on the (still)
existing connection between the subscriber A and the
exchange MSC (step S9a), a connection to the previously
calling subscriber C is established (step S9b). This
connection between subscribers A and C is also referred to
as callback call.
Thus, in case the subscriber C can be reached, the
subscriber A can directly call back the subscriber C and
talk to him. Having finished the conversation between
subscribers A and C, the callback call is released, step
510. However, there are other reasons for releasing the
callback call. For example, if the callback call can not be
established due to the subscriber C being not reachable
(e.g. because of being busy in another call, etc., as
mentioned above), also in this case the callback call
(attempt) is released and the release reason is returned in
step S10 to the exchange MSC.
In step 511, at the exchange side, the transmitted release
reason is set and/or written in a call drop-back release
reason indicator (CDB release reason indicator) in the VMS
service indication (to be described later in greater
detail). Also, the VMS session indicator will be written to
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the VMS service indication parameter. Thereafter, in step
512, the message server session (VMS/SN session)is resumed
(if the CDB type indicator in the VMS service indication
previously transmitted in step S7 was set to a value
indicating indicated "YES"), and in step 513, the record of
messages kept at the message server is updated. That is,
the CDB release reason transmitted in the indication in
step S12 is written into a corresponding message record.
In step 514, it is detected whether there is/are one or
more other messages recorded at the message server record
within the partition assigned to the called /listening
subscriber which are not labeled as having served as a
basis for a successful callback call to the calling
subscriber who has originated the call/ left the message at
the message server VMS/SN.
If such a message is present, the sequence returns to step
S5 and the steps from S5 through S14 are repeated.
Otherwise, the message server session is terminated.
Thus, the called subscriber A can make a call to several
different numbers of previously calling subscribers who
have called him, during one message server VMS/SN session.
There is no longer a need to make a new call to a message
server after a callback call has been made to a previously
calling subscriber.
The data exchanged between the message server VMS/SN and
the exchange MSC form one aspect of the present invention.
Therefor, to enhance still further the understanding of the
present invention, those data and an example of a possible
data format are described with reference to Figs. 3 and 4.
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Fig. 3 shows an example of a message record MESSAGE RECORD
provided in the message server VMS/SN and the data stored
therein.
The entire message record is partitioned in subscriber
partitions. In Fig. 3, a partition for subscriber A is
shown. A partition may be identified by the subscribers
telephone number MSISDN (e. g. MSISDN A). In a more secure
manner, it may be identified by a personal identification
code such as a PIN number (personal identification number)
and or a combination thereof, also in a ciphered form, if
desired.
Within each such partition, the following data are kept in
respective data fields:
l.) a call number (CALL No.):
This indicates the number of a call. In a most simple
implementation, the call number is associated with the the
time the message has been recorded in the record. For
example, call number #1 represents the oldest message,
while call number #N represents the most recently recorded
message.
Alternatively, the order of the messages can be
arranged such that they are arranged in an order of
priority of the registered message. For example, there may
be provided a possibility for a calling subscriber to
indicate that his message is urgent (high priority), of
normal importance (medium priority) or of subordinated
importance (low priority). Then, the messages can be sorted
according to the indicated priority thereof, if necessary
in combination with the time of being recorded.
2.) calling subscriber identity
(CALLING SUBSCR. ID (MSISDN) No.)
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This indicates the identity of the calling subscriber,
for example by his telephone number (C1, C2, ..., Cn),
which is used for establishing the callback call. Also, the
MSISDN number can be stored together with the subscriber's
name in order to simplify the recognition of the calling
subscriber to the called subscriber.
Also, the messages can be sorted according to the
subscribers' telephone numbers and/or names, also in
combination with the above mentioned sorting according to
time of registration and/or priority.
3.) message (MESSAGE)
This indicates a memory area of the message record
allotted for storing the message left by the calling
subscriber C for the called subscriber A. The message may
for example be a voice mail message (VMl, VM2, ..., VMn),
or any other message as mentioned earlier above.
4.) call drop back release reason
(CDB RELEASE REASON INDICATION)
For this entry, at least one indication indicating
whether a callback call has been established and performed
with success is to be provided. That is, a flag indicating
whether subscriber A, upon listening to his voice mail from
subscriber C, was successful in calling back subscriber C,
is to be provided in a minimum solution. Therefore, in a
most general implementation, one bit is already sufficient
for encoding this information (e. g. "YES" - l, representing
a normal termination after successful callback, "NO" = 0
representing all other error reasons for non-successful
callback).
However, in more sophisticated implementations,
depending on the number of reasons, a higher number of bits
has to be provided for coding this information. Generally,
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in order to code 2" reasons, n bits have to be provided for
coding the CDB REELEASE REASON.
The data entries in a subscriber partition mentioned under
items 1 to 3 herein above are written at the time of
recording the message by the calling subscriber C who can
not reach the called subscriber A at that time, while the
data entry mentioned under item 4 is written in step S13 in
Fig. 2B, when the message record is updated upon a callback
call establishment.
Also, in an alternative implementation, an entry such as
"CDB RELEASE" _ "YES" (flag being set to "1") need not be
written to the record, but a corresponding message could be
deleted from the respective subscriber partition in the
message record at the message server, thereby continuously
decreasing the number of messages in the record after
having performed a callback call successfully.
Fig. 4 shows a basic example of a data format usable for
transmission of the VMS service indication.
As illustrated, the VMS service indication comprises three
fields, which are partly again divided in sub-fields.
A) VMS/SN SESSION ID
The VMS/SN SESSION ID identifies the session at the
message server to which -after a callback call has been
performed- is to be returned. To this end, the VMS/SN
SESSION ID field includes a subscriber ID field, e.g.
including the subscriber's telephone number MSISDN, such as
the MSISDN A. This subscriber ID represents a pointer to
the subscriber partition in the message record.
Additionally, the VMS/SN SESSION ID includes a message
ID, i.e. an identification of the message that has been
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listened immediately before. For example, the message ID is
represented by the call number (mentioned in connection
with the Fig. 3 description). The message ID thus
represents a pointer to a specific message having this
number within the respective subscriber partition (pointed
to by the subscriber ID).
B) CDB RELEASE REASON INDICATION
This field is similar and/or identical to the one
discussed above in connection with Fig. 3 in so far as its
contents is concerned.
However, it should be noted that in step S7, this
field CDB RELEASE REASON INDICATION of the VMS SERVICE
INDICATION does not contain a data entry, or at least one
which is commonly known as "don't care" and which has no
meaning. In Fig. 5 to be described below, such a data entry
is referred to as "No Indication"
C) CDB TYPE INDICATOR
This call drop-back type indicator indicates whether a
return to the message server VMS/SN is necessary and/or
possible after a callback call has been established and
thereupon released. Thus, also the CDB TYPE INDICATOR may
be coded, in a most general implementation, by using one
bit indicating "return: yes" (e.g. "1") or "return: no"
(e.g. ..0,.) .
Nevertheless, in a practical implementation, the CDB TYPE
INDICATOR may assume four values (coded by 2 bits) referred
to as "NO indication", "Unsupervised CDB", "Supervised CDB"
or "CDB return" (the latter being also referred to as
"intelligent CDB"). Upon receipt of the corresponding
information, the exchange MSC then initiates an appropriate
action depending on the received information. Further
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details are set out in connection with the description of
Fig. 5.
The setting of the CDB TYPE INDICATOR in connection with
the above practical implementation may already be performed
at the time of manufacturing when implementing the method
feature in the network device(s).
Nevertheless, it can also be provided for that the CDB TYPE
INDICATOR will be set in the VMS/SN according to the
instructions of the subscriber A listening to the messages
left for him at the VMS/SN when the subscriber A configures
his services.
However, it is also conceivable to adopt a kind of
"dynamic" setting of the CDB TYPE INDICATOR, which may then
be performed at the message server side, prior to step S7.
Namely, the VMS/SN may be adapted to make the decision
concerning the CDB TYPE INDICATORS depending an the
services offered by the VMS/SN to the subscriber A and
depending on whether there are messages left in the message
server VMS/SN.
In this connection, for example, the setting of the CDB
TYPE INDICATOR may be performed based on the following
examples of a logic evaluation of data items described
before:
CDB TYPE INDICATOR =
"YES" if current call number < maximum call number n
OR
"YES" if any CDB RELEASE REASON INDICATION in the
record (of the partition) indicates "not successful".
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(In the latter case ("the OR clause"), a retry for a
callback call for a previously unsuccessful callback call
attempt can result in tine flow).
CDB TYPE INDICATOR =
"NO" if current call number = maximum call number
AND
no CDB RELEASE REASON INDICATION in the record of the
partition indicates "not successful".
Of course, various modifications of the above mentioned
logic evaluations are further conceivable. Also, the
sequence of indicators within the indication is not limited
to the one presented and another data arrangement may be
adopted. Also, in step 512, the CDB TYPE INDICATOR need not
be transmitted back to the message server, as it has been
evaluated before, so that in such a case, also the value
"NO indication" may be transmitted.
Fig. 5 shows an actual format of the VMS service indication
which shows the subfields of the VMS service indication in
greater detail.
As shown in Fig. 5, the VMS service indication comprises
spare fields and /or spare bits which are used to provide
for a possibility to expand the indicators in the future,
if necessary. For example, it could become necessary to
code more CDB release reasons etc., for which coding more
bits are required.
The CDB type indicator is composed of two bits, thus
enabling the coding of four contents and/or meanings,
namely:
"00" representing "No Indication"; the value "No
indication" is used in the CDB type indicator in the IAM
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(Initial Address Message) with which the MSC-VMS/SN
connection will be re-established (see step S12 in Fig.
2B). If there is the value "No indication" in the REL
(RELease) message which invokes the call dropback
functionality, the exchange MSC is caused to operate like
in case of the Unsupervised CDB (see below), where there is
no return to the VMS/SN connection.
"0l" representing an "Unsupervised CDB", according to
which setting the call does not return to the VMS/SN in any
case after a callback call has been initiated to the
(previously) calling subscriber C. This is an option for a
setting in case a network operator does not wish to have
the present invention being implemented in his network,
e.g. for cost reasons,
"10" representing a "Supervised CDB", according to
which setting the call is routed back to the VMS/SN after a
callback call to the calling subscriber C has not been
successful (e.g. if the CDB release reason does not
indicate a normal termination cf the CDB call), and
"11" representing an "Intelligent CDB", according to
which setting the call returns to the VMS/SN if the
callback call to the calling subscriber has not succeeded
and if the callback call to the calling subscriber C has
succeeded.
Furthermore, the CDB release reason is coded using three
bits, thus enabling the coding of up to eight CDB release
reasons. Five values are used in connection with the
illustrated actual format of the VMS service indication,
namely:
"000" representing "NO Indication" (transmitted for
example in step S7, when a CDB release reason at the
attempt of a callback call is yet not determined),
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"001" representing that the previously calling party C
(no being called back) is not reachable, e.g. due to the
terminal being switched off,
"O10" representing that the calling party C being
busy, i.e. occupied by another call and/or data
transmission,
"100" representing a normal termination of the CDB,
i.e. that the callback call has been established successful
and that the call has been normally terminated, e.g., after
the subscribers A and C have talked to each other, and
"111" representing that subscriber C does not reply to
the callback call attempt although he is reachable while
not being busy.
Finally, the VMS Session Indicator consists of 8 to 28
bytes having a contents as discussed above, i.e. containing
the subscriber ID and a message ID needed to resume the
VMS/SN session after the (new) callback call has finished
or after an unsuccessful establishment of the callback
call.
Herein above, the present invention has mainly been
described with reference to a method. Nevertheless, the
present invention also aims to a corresponding control
device adapted to carry out the method. To this end, a
control device comprises (not shown) accordingly adapted
means for listening / session opening / CDB feature
invocation, detection, obtaining, callback call invocation,
indication transmission, listening call release, callback
call establishment and release, setting session resumption
and indication transmission, as well as for updating.
Also, those means may be provided in a single network
element such as the exchange (MSC), but where appropriate,
the means constituting the control device may also be
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distributed over at least two telecommunication network
elements e.g. over the exchange MSC and the message server
VMS/SN.
Accordingly, as has been described herein above, the
present invention proposes a method for performing a call
drop-back in a telecommunication system, said
telecommunication system comprising at least at least one
exchange (MSC) for switching calls between at least two
subscribers (A, C) or between one of said subscribers and a
message server (VMS/SN), wherein said message server
(VMS/SN) is adapted to maintain a record of a plurality of
previous messages intended for a called subscriber (A) and
which previous messages were recorded to the message server
record from a respective calling subscriber (C), wherein
said method comprises the steps of: invoking (S6) a
callback call from said called subscriber (A) to said
calling subscriber (C) based on the information recorded in
said message server record via at least one of said at
least one exchange (MSC), and upon invoking said callback
call, setting the system into a state enabling said called
subscriber (A) to be connected back to said message server
(VMS/SN) after the termination of said invoked callback
call. Also, a corresponding control device as well as a
network element comprising the same are proposed.
It should be understood that the above description and
accompanying figures are merely intended to illustrate the
present invention by way of example only. The preferred
embodiments of the present invention may thus vary within
the scope of the attached claims.
