Note: Descriptions are shown in the official language in which they were submitted.
WO 94124801 PCT/GB93/00800
21CO~G7
TELECOMMUNl~:ATIONS SYSTEM
This invention relates to telecommunications, e.g. telephone
systems and in particular to systems incorporating one or more
private network.
BACKGROUND OF THE INVENTION
A conventional telecommunications private network comprises a
number of private exchanges or switches (PBX's) interconnected
via private telephone lines and each of which serves a number of
telephone or user terminals. The PBX's of the private system are
interconnected by private leased lines which are installed-by the
appropriate telephone service supplier. Call routing and call
handling within the private network are controlled via the PBX's.
In ~ddition to a basic telephony service (POTS) a private network is
generally required to provide additional features such as call
forwarding, call transfer, ring-back when free and conference calls.
These features are not in general provided on a public network as
they are specific requirements of business rather than domestic
subscribers.
A further service that is finding increasing usage is that of providing
a direct dialling facility to telephone extensions attached to a PBX.
For example the CENTREX system provides such a service.
The provision of private leased lines represents a significant capital
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investment by the telephone service suppiier. Furthermore these
leased lines represent an underused asset as they are in significant
use, for only a part, typically about one third, of the twenty four hour
cycle. However, during their idle period, these lines are not
available to carry telephone traffic, e.g. to reduce an overload, for
subscribers other than the private user to whom the lines have
been leased. Expansion of the network is also difficult, as new
de~ic~ted lines have to be provided for new business subscribers.
,,
OBJECT OF THE INVENTION
An object of the invention is to minimise or to overcome the above
disadvantage.
It is a further object of the invention to provide a system in which
calls between PBX's are routed via a public network.
It is a further object of the invention to provide a system that is fully
compatible with the above mentioned direct dialling facility.
SUMMARY OF THE INVENTION
According to the invention there is provided a telecommunications
system incorporating a public network and one or more private
networks, each said private network having a plurality of nodes
bel~rJ~cn which communication is, in use, effected via the public
network, and wherein the public network has means for providing a
transparent communications path between the nodes of each said
private network.
According to the invention there is further provided a
telecommunications system incorporating a public network, and one
or more private networks each comprising a plurality of private
exchanges (PBX's) and between which communication is, in use,
effected via the public network, the system having means for
mapping call signalling information from a said private network to
the public network and inverse mapping call signalling information
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from the public network to that private network whereby to provide a
transparent communications path via the public network between
the private exchanges of each said private network.
.. . .
According to another aspect of the invention there is provided a
method of providing telecommunications signalling between a
plurality of nodes or private exchanges (PBX's) constituting a
private network, the signalling being effected via a public network,
the method including mapping signalling information associated
with each private network message to corresponding signalling
intormation associated with the public network, routing the message
across the public network, and inverse mapping the signalling
information associated with the public network to the signalling
information associated with the private network whereby to provide
a transparent communications path between the exchanges of the
private network.
By providing a transparent signalling path between the nodes of the
private network, all the call handling features of the private network
are available to users of that network even though these features
may not be provided by the public network itself. The private
network calls are routed via the public network rather than via
conventional dedicated leased lines or 'tie lines' making more
efficient use of the public network capacity. The public network in
effect provides virtual tie line connections between the PBX's of the
private network.
The technique is of particular application to system employing a
digited private network signalling system e.g. the DPNSS (RTM)
private network common channel signalling protocol, but it will be
appreciated that the technique is in no way limited to that particular
signalling protocol. The technique is also applicable e.g. to the
QSIG European PBX signalling standard. The public network also
has a common channel signalling capability and may for example
employ the CCS7 (common channel signalling system) protocol
such as the DMS (RTM) digital multiplex system, although it will
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again be appreciated that the invention is not limited to this
particular public switch. The DPNSS protocol is a common channel
signalling system, primarily for use within private networks, to
provide telephony, data services, and a range of supplementary
services. DPNSS contains circuit related ('Real Call') procedures,
and circuit independent ('Virtual Call') procedures to provide these
services. In order to provide DPNSS transparency over CCS7 we
support equivalent mechanisms. DPNSS 'Real' calls are
progressed across CCS7 using the integrated Signalling Digital
Network User Part (ISUO), and DPNSS 'Virtual' calls are
progressed using the CCS7 Transaction Sub-Layer (TSL) and the
Signalling Connection Control Part (SCCP).
We achieve this transparent carriage by reconstituting the DPNSS
information at the CCS7 end nodes so that full DPNSS functionality
is provided. In order to provide the required functionality, the
system operates according to the following principles.
Any information that can be mapped uniquely between DPNSS and
CCS7 messages, and therefore can be reconstituted completely, is
interworked at a CCS7 end node. I.e. this information will not need
to be carried transparently within the CCS7 message as well as
being mapped. Any DPNSS information that can not be mapped
uniquely, but can be used to set appropriate CCS7 parameters is
utilise~. but is also carried transparently within the CCS7 mess~ge.
Any DPNSS information that has no relevance at all to the CCS7
message is carried transparently within the CCS7 message.
BPllEF DESCRIPTION OF THE DRAWINGS
An embodiment of the invention will now be described with
reference to the accompanying drawings in which:-
Fig. 1 is a schematic design of a telecommunications systemin which private network traffic is carried via a public network;
Fig. 2 illustrates the manner in which calls are routed across
the network of Fig. 1;
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Figs. 3 and 4 illustrate respectively the high level mapping of
message and parameter information in the system of Fig. 1;
Fig. 5 illustrates the call set-up process;
Fig. 6 illustrates the pre-conversation phase of a call;
Fig. 7 illustrates the speech phase of a call;
Figs. 8 and 9 illustrate respectively forward and backward
release of a call;
Fig. 10 illustrates the support structure of a virtual call in the
system of Fig. 1;
Fig. 11 illustrates the high level mapping of virtual messages;
Fig. 12 illustrates the transmit mechanism of virtual calls;
Fig. 13 illustrates the handling of additional messages in a
virtual call;
Fig. 14 illustrates premature release of a virtual call; and
Fig. 15 illustrates processing of a virtual call at a terminating
node.
DESCRIPTION OF PREFERRED EMBODIMENT
In the following description of the preferred embodiment, particular
reference is made to the DPNSS private signalling system and to
the DMS public switching system. It will be appreciated that the
desc, i~.tion of the invention with those two systems is given by way
of example only and that exploitation of the invention is in no way
limited to the use of both or either of these particular systems.
Referring to Fig. 1, the public network incorporates a number of
nodes 11 each of which may comprise e.g. a switching office or a
signal transfer point. A switching office is a node which is host to a
number of system users and acts as a source and sink of user
generated messages. A signal transfer point is a node which
tandems messages between switching offices. Selected nodes of
the public network may comprise e.g. gateway switches 100 to
pemmit communication with other networks.
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The public network is accessed by a plurality of subscribers some
of which comprise single telephones 12 and others of which
comprise a plurality of private exchanges (PBX's) 14, the private
exchanges forming the nodes of a private network.
Signalling within the system of Fig. 1 is effected via a common
channel signalling (CCS) protocol. This technique partitions the
control of a call from the voice path so that inter-office
communication is based on a request data exchange capability.
This contrasts with conventional per-trunk signalling (PTS) in which
a call's control signal and voice signals are multiplexed on to the
same trunk.
In the system of Fig. 1, calls between PBX's are routed across the
public network using quasi-associated signalling, this technique
being illustrated schematically in Fig. 2. Messages relating to a
particular signalling relation between a pair of PBX's 14a, 14b are
conveyed over two or more linksets in tandem passing through one
or more nodes 11a, 11b of the public network. For calls between
PBX's, the public system nodes function as signalling transfer
points to provide physical communication between the PBX's 14a,
14b, to provide in effect a virtual communications path directly
between the PBX's of the private network.
To achieve transparency of a communications path between PBX's,
we provide mapping of both message information and parameter
information between the private network DPNSS information and
the integrated signalling digital network user part (ISUP) ~ssoci~ted
with the public network. The high level mapping of message and
parameter information are illustrated respectively in Figs. 3 and 4 of
the drawings.
The private network calls routed via the public network are of two
types; real calls and virtual calls. A real call requires establishment
of a connection between two parties and can carry speech or data.
A virtual call is a mechanism for communicating information
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between nodes when a physical circuit connection is not required,
e.g. when making a call-back when free request.
Real Calls
For 'Real' calls the signalling messages and related procedures are
sectioned into the following call phases:
Successlul Call Set-up Phase-
This phase begins with an initial address message, and ends with
. the receipt of an address complete message, indicating all digits
have been successfully received at the terminating end.
Uns~cce3sl.l1 Call Set-up Phase
This phase begins if any unsuccessful backward signal is received
in response to an initial address message. The action taken on
receipt of such a signal may be to reattempt the call on-another
circuit, or to enter the Release Phase, and clear the call attempt.
Pre-Conversalion Phase
This phase begins with the receipt of the address complete
message, and ends when an answer mess~ge is received.
Speech Phase
This phase begins with the receipt of an answer message, and
continues until the call is released.
Release Phase
This may be initiated from any other phase during the call. The call
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will be released, and trunks will be passed to the Idle Phase
Idle Phase
Interaction between trunk maintenance and call processing will
occur in this phase.
For each phase a description of the mapping required is given
below. - -
In the successful call set-up phase the following procedures occur.
The first message to arrive at the originating public system node is
a DPNSS ISRM(C). The ISRM(C) message is the first message to
be sent in the call set-up phase, it is only applicable in this phase,
and can only pass in the forward direction. This message contains
all the selection information needed to progress the call, and may
also contain information relating to supplementary service requests.
All this information is carried in the DPNSS Selection Block, which
may be up to 44 octets in length.
When the DPNSS ISRM(C) message is received at the public
system node, the calling line category (CLC) is examined for
compatibility with the outgoing route and the SIC field is screened
to determine whether or not the call is allowed to continue. The
procedure is illustrated in Fig. 5 of the drawings.
Unsuccessful Call Set-up Phase
This phase is initiated in one of the following ways:
An internal DMS treatment is set. A DPNSS message is received
with an invalid SIC code. The call will be handled as specified in
reference 3.
An ISUP Release Message (REL) is received.
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If the failure occurred in the terminating DPNSS network the ISU
P REL message contains DPNSS Indication Block information,
which
is passed to DPNSS, and a DPNSS CRM is generated (see Figure
12 on page 38). If the failure occurred in the DMS ISUP network
the ISUP REL message is mapped to a DPNSS CRM.
An ISUP Blocking Message (BLO)/ Reset Circuit Message (RSC)/
or an Unrecognised Message is received, from the DMS ISUP
network. The DMS network isolates the related circuit and repeats
the call attempt on another circuit.
Pre-Conversation Phase
This phase begins with the receipt of a backward DPNSS Number
Acknowledge Message, and generally ends with the receipt of an
DPNSS Call Connected Message (CCM).
On receipt of the DPNSS NAM message the DMS generates an
ISUP ACM, from the information received within the DPNSS
Indication block, and from information held at the DMS. The
DPNSS SM message is always accepted, as 64Kbit/s clear
capability is always assumed. On receipt it is mapped into an ISUP
PAM message and sent over the preceding link.
Any other DPNSS message received in the backward direction,
other than CCM or unrecognised or invalid link-by-link messages
are mapped into an ISUP PAM and sent over the preceding link. In
the case of EEM(l)'s and EEM(C) all the information will be
collected at the node before being mapped into an ISUP PAM. This
will also be true for LLM(l)s and LLM(C).
On receipt of a DPNSS CCM message, an ISUP ANM is generated
and is sent over the ISUP link, the procedure being illustrated in
Fig. 6.
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Call Duration (Speech) Phase
This phase, illustrated in Fig. 7, begins with the receipt of a
backward DPNSS CCM Acknowledge Message, and ends when
release is initiated.
r~elea,Q Phase
,
The Release Phase can be initiated from any other phase during
the call. Release is activated on a link-by-link basis. On receipt of
a DPNSS CRM message, an ISUP REL message is generated and
sent over the ISUP link. Likewise, on receipt of an ISUP REL
message, a DPNSS CRM is generated by the DMS, and sent over
the DPNSS link. The DPNSS indication block information will be
mapped into the ISUP REL message and passed to DPNSS
respectively.
In response to the CRM message, a DPNSS CIM is generated and
sent over the DPNSS link. In response to an ISUP REL message
an ISUP RLC is generated and sent over the ISUP link.
Figs 8 and 9 illustrate the release procedure for forward direction
release and backward direction release respectively.
Idle Phase
This phase begins when the RLC/CIM message appropriate to the
circuit being idled is received.
The Virtual Call
The DPNSS 'Virtual Call' is a mechanism for communicating
information between nodes, when a physical circuit is not required,
for instance when registering and cancelling a Diversion
supplementary service, or when making a Call Back When Free
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request. The equivalent functionality is provided by the ANSI
Transaction Sub-Layer (TSL) and the ANSI Signalling Connection
Control Part (SCCP). A limited portion of the ANSI Component
sub-layer is also supported to handle error conditions.
As a general principle the DPNSS 'Virtual' call signalling messages
and procedures can be considered as the same as those for a
'Real' call. The DPNSS 'Virtual' call can be considered as a '
Query'rResponse' cycle, performed with the DPNSS ISRM and
CRM messages respectively, this maps well to the Transaction
Sub-Layer (TSL) 'Query'/'Response' transaction. The capabilityto
pass on any DPNSS message received during a 'Virtual' call is
supported across the CCS7 network to ensure that the principle of
DPNSS transparency can be followed if thus functionality is
required at any time.
The structure used for the support of 'Virtual' calls is shown
schematically in Fig. 10. A new Transaction User (TR-User) is
created called 'Signalling Transparency'. This application assigns
transaction identifiers (TRID) and encapsulate/extract the received
DPNSS messages to and from TSL-primitives. It utilises the TSL
services to provide a connection-oriented association between the
two peer TR-Users at the originating and terminating CCS7 nodes.
At the transport level the SCCP connectionless service is used to
route the TSL messages across the CCS7 network.
Figure 11 shows a high level mapping of DPNSS 'Virtual' mess~ges
to and from TSL messages. Within the description any lefelence to
the TSL QUERY message should be interpreted as QUERY with
permission, and any reference to the TSL CONVERSATION
message should be interpreted as CONVERSATION with
permission.
.,
The DPNSS 'Virtual' call, as mentioned previously, can be thought
of as a single transaction. Within the following section the
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signalling messages and procedures for such a transaction will be
defined. The specific procedures relating to each of the three
possible scenarios ('Transit', 'Gateway', and 'End' node capabilities)
will be discussed separately. This is illustrated schematically in Fig.
12. As for 'Real' calls, the underlying principle in the transit case is
to pass on any information received at the CCS7 end nodes. A
number of TSL messages are used between the originating and
terminating nodes to convey the DPNSS information. A TSDL
Query message is issued to begin a transaction, a TSL
Conversation message is issued to allow a continuation of the
transaction, and a TSL Response message is issued to end the
transaction. TCAP protocol errors are reported via a Reject
component within a Response or Unidirectional message.
Application errors are reported via a Return Error component within
a Response or Unidirectional message.
On receipt of all the DPNSS Selection Block information (obtained
from an ISRM(C) or a sequence of ISRM(I), SSRM(l)s ands
SSRM(C)), the DMS node attempts to assign a TSL Transaction
Identifier (TRID) to identify the transaction at the CCS7 originating
node.
If successful, the DMS node generates a TSL QUERY message,
from information within the DPNSS Selection Block, and information
held at the DMS. The DPNSS message(s) is carried transparently
within the QUERY message. The message will be sent to the
succeeding node, via the CCS7 SCCP connectionless service. An
association between the assigned TRID and the DPNSS virtual
channel identifier is maintained by the DMS for the duration of the
transaction, in order to route correctly any further messages.
If the attempt to assign a TRID is not successful, the DMS node will
fail the call with a DPNSS CRM indicating 'Congestion'.
At any DMS transit nodes the SCCP message received will be
progressed as a normal connectionless SCCP message.
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On receipt of a TSL QUERY request at the DMS terminating node,
the QUERY message is validated, and the DMS node attempts to
allocate a TRID.
,. .
If successful an 'Empty' TSL CONVERSATION message, i.e..
containing no user data) is generated by the DMS and sent to the
originating node to acknowledge the received message. This
acknowledgement procedure must be carried out for the DPNSS
transparency application, so that any further DPNSS messages
received at the CCS7 originating node can be progressed over the
CCS7 network, (i.e. the 'terminating nodes' TRID is required at the
CCS7 originating node before any other message can be sent in
the forward direction). Once the TSL QUERY message has been
validated and a TRID assigned, the DPNSS message is extracted
from the QUERY message, and processed as normal in order to
determine the DPNSS outgoing route. A timer to supervise the
outgoing DPNSS 'virtual' call is started on sending the 'virtual' ISRM
message. An association between the CCS7 terminating nodes
TRID and the DPNSS virtual channel identifier is maintained by the
DMS node for the duration of the transaction, in order to route
correctly any further messages.
If the attempt to assign a TRID is unsuccessful, a return error
component is generated by the DMS and sent within a
Unidirectional message to the originating node to terminate the
attempted transaction.
Any further DPNSS messages received for the 'Virtual' call are held
at the DMS until the TSL CONVERSATION message, providing the
TRID of the CCS7 terminating node arrives. Once received any
DPNSS messages being held at the DPM, other than CRM, are
encapsulated in TSL CONVERSATION messages and sent over
the CCS7 link as illustrated in Fig. 13. Messages, such as the
EEM(l)s that have to be buffered at the DMS before being sent over
each link, can be collected together and sent within a single TSL
CONVERSATION message. The limit on the number of messages
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being determined by the DPNSS Indication Block limit of 135
octets.
On receipt of any further TSL CONVERSATION messages from the
CCS7 terminating node, the DMS extracts the DPNSS message,
processes it as normal, and sends it over the preceding DPNSS
link.
A DPNSS CRM message received in the forward direction indicates
that the DPNSS 'Virtual' call should be released, and hence is
mapped to a TSL RESPONSE message to initiate the termination
of the TSL transaction. A DPNSS CIM message is generated in
response to the CRM, and sent over the preceding DPNSS link.
The TSL TRID is then released. This procedure is illustrated in Fig.
14 of the drawings.
On receipt of a TSL RESPONSE message, the DPNSS CRM
message is extracted and sent, and the TSL TRID is released. If a
TSL Response or unidirectional message is received containing a
Reject or Return Error component, the DMS fails the 'Virtual Call'
with a DPNSS CRM. At any DMS transit nodes the SCCP
mess~ge received is progressed as a normal connectionless SCCP
message.
At the DMS terminating node, any backward DPNSS message
received for the 'Virtual' call, other than CRM, is mapped into a TSL
CONVERSATION message and sent over the CCS7 link.
A DPNSS CRM message received in the backward direction
indicates that the DPNSS 'Virtual' call should be released, and
hence is mapped to a TSL RESPONSE message to initiate the
termination of the TSL transaction. A DPNSS CIM message is
generated in response to the CRM, and sent over the preceding
DPNSS link as illustrated in Fig. 15.
On receipt of any TSL CONVERSATION message from the CCS7
originating node, the DMS extracts the DPNSS message,
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processes it as normal, and sends it over the succeeding DPNSS
link. On receipt of a TSL RESPONSE message, the DPNSS CRM
message is extracted and sent and the TSL TRID is released.
. .
It will be appreciated that adaptation of the technique to telephone
systems other than those described above may be achieved by the
provision of suitab!e mapping and inverse mapping ~unctions
between the public and private systems.
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