Note: Descriptions are shown in the official language in which they were submitted.
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TRANSPORT METHOD FOR THE MOBILE APPLICATION PART (MAP), AND APPARATUS
THEREFOR
Field of the Invention
s The present invention relates to a Mobile Application Part transport
mechanism for use in
a Public Land Mobile Network.
Background to the Invention
In existing Public Land Mobile Networks (PLMN), signalling information, e.g.
relating to
call set-up procedures, management, and teardown. is generally carried between
signalling
points by a Signalling System No.7 (SS7) based transport mechanism. SS7 is a
widely
used transport protocol involving multiple protocol layers.
Information is exchanged between Mobile Switching Centres (MSCs). Home
Location
Registers (HLRs), and Visitor Location Registers (VLRs) using messages defined
by the
standardised Mobile Application Part (MAP) protocol, messages which are
carried by the
SS7 transport mechanism. More particularly, the SS7 layers involved in the
transport of
the MAP messages are:
3o a Message Transport Part (MTP) which handles inter- alin message
separation,
error detection and correction. as well as an interface to the physical data
link:
a Signalling Connection and Control Part (SCCP) which is responsible for
controlling signalling connections in the SS7 network as well as for routin~
between
signalling points; and
?> a Transaction Capabilities Application Part (TCAP) which facilitates the
use of
advanced Intelligent Network (IN) services by providing for the exchange of
information
between signalling points using a connectionless service of the SCCP.
The complexity of the conventional transport mechanism will be readily
apparent, but
.so such complexity is required in order to ensure the correct routing and
error free
transmission of signalling data between the numerous signalling points of a
PLMN.
Summarv of the Present Invention
~~ It has been recognised by the inventors of the present invention that the
complex SS7
transport mechanism is not necessary for transporting MAP messages between
signalling
points of a PLMN which are co-located. i.e. which are located in close
proximity to one
another. Thus, it is possible to employ a "lightweight" transport mechanism
which
CONFIRMATION COPY
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reduces or eliminates the coding and decoding requirements inherent in the SS7
transport
mechanism.
According to a first aspect of the present invention there is provided a
method of
transmitting signalling information between two signalling points of a Public
Land Mobile
Network, which signalling points are co-located, the method comprising;
formulating said signalling information into messages according to the Mobile
Application Part (MAP) protocol; and
transmitting the messages between the two signalling points using a packet
switched data network.
As the signalling points to which the method of the present invention is
applied are co-
located. it is possible to couple the two signalling points using a direct
connection.
Embodiments of the present invention remove the requirement for processing
signalling
information using TCAP and SCCP when the information is to be transmitted
between co-
located signalling points.
Preferably, the above method is used in a network after a signalling point
wishing to send
3o a MAP message has determined whether or not the message is destined for a
co-located
signalling point. In the event that the destination signalling point is not co-
located, then a
network other than said packet switched data network may be used to transport
the
message. One such alternative network is an SS7 network.
?a Preferably, the co-located signalling points of a PLMN coupled by the said
packet
switched data network include two or more of a Mobile Switching Centre (MSC),
a
Gateway Mobile Switching Centre (GMSC), a Home Location Register (HLR), and a
Visitor Location Register (VLR). The signalling points may also include one or
more
Intelligent Network (IN) nodes.
~o
Preferably, the packet switched data network is an IP network, where the MAP
sits on top
of the IP layers (including an IP protocol layer, and a TCP and/or UDP layer)
at each of
the co-located signalling points. Where the co-located signalling points have
access to an
SS7 network, the MAP at the signalling points may also sit on top of the SS7
protocol
layers. More preferably, an adaptation layer is provided between the MAP and
the IP and
SS7 layers, the adaptation layer responding to a MAP dialogue initiation by
determining
whether or not the destination address (e.g. Global Title) for the dialogue
corresponds to or
is associated with a co-located signalling point. If the destination address
(e.g. Global
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Title) for the dialogue does correspond to a co-located signalling point, then
the adaptation
layer determines the IP address corresponding to the destination address.
According to a second aspect of the present invention there is provided
apparatus for
transmitting signalling information between two signalling points of a Public
Land Mobile
Network, which signalling points are co-located, the apparatus comprising;
first signal processing means for formulating said signalling information into
messages according to the Mobile Application Part (MAP) protocol;
second signal processing means for formulating MAP messages according to a
packet switched data transport mechanism; and
transmission means for transmitting the formulated packet switched messages
between the two signalling points over a packet switched network.
According to a third aspect of the present invention there is provided a
signalling point
within a Public Land Mobile Network (PLMN), the signalling point comprising:
first signal processing means for formulating said signalling information into
messages according to the Mobile Application Part (MAP) protocol;
second signal processing means for formulating MAP messages according to a
packet switched data transport mechanism; and
?o transmission means for transmitting the formulated packet switched messages
to a
second co-located signalling point over a packet switched network.
The signalling point may also act as a signalling transfer point for messages
received from
non-co-located signalling points, whereby the signalling transfer point relays
MAP
?a messages to a co-located signalling point over the packet switched network.
Preferably, the signalling point comprises means for determining whether or
not a
destination signalling point for a MAP message is co-located and, if so, for
passing the
MAP messages to said second signal processing means and, if not, for passing
the MAP
3o messages to an SS7 transport mechanism to provide for transmission of the
messages over
an SS7 network.
Brief Description of the Drawings
_ss For a better understanding of the present invention and in order to show
how the same
may be carried into effect reference will now be made, by way of example, to
the
accompanying drawings, in which:
Figure 1 illustrates schematically a part of a Public Land Mobile Network;
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Figure 2 illustrates the transport mechanism protocol stack implemented at
certain
signalling points ofthe PLMN of Figure 1; and
Figure 3 is a flow diagram illustrating a method of transmitting Mobile
Application
Part messages between signalling points of the PLMN of Figure 1.
s
Detailed Description of Certain Embodiments
In Figure 1 there is illustrated schematically a Public Land Mobile Network
(PLMN)
comprising two Gateway Mobile Switching Centres (GMSCs) 1,2 which provide the
interface between the PLMN and a Public Switched Telephone Network (PSTN) 3.
The
GMSCs 1,2 "represent" the PLMN from the view point of the PSTN 3 and
signalling
communications therebetween are carried using the ISDN User Part {ISUP)
protocol. The
GMSCs may also provide the interfaces between the PLMN and other networks
although
this is not shown in Figure 1.
t5
Within the PLMN, it is necessary to communicate signalling information between
signalling points of the network for the purpose of call set-up, management.
and teardown.
Figure 1 illustrates a number of signalling points within the PLMN including:
the GMSCs
1,2, Mobile Switching centres (MSCs) 4,5 which are responsible for routing
calls within
?o the PLMN; Visitor Location Registers (VLRs) 6,7 which maintain a record of
the
subscribers registered with associated MSCs at any given time; and Home
Location
Registers (HLRs) 8,9 which maintain a permanent record of the PLMN subscribers
together with a dynamic record of the location of those subscribers at any
given time.
These signalling points are in close proximity to one another, i.e. they are
"co-located". In
3a some circumstances the co-located signalling points may be located in the
same room.
Signalling information is conveyed between the various signalling points of
the PLMN
using the Mobile Application Part (MAP) interface protocol. Conventionally,
MAP
messages are transported using the SS7 transport mechanism. This mechanism
will not be
~o described in detail here (reference should be made for example to
"Understanding
Telecommunications", Vols 1 & 2, Studentlittaratur, Lund, Sweden, ISBN 91-44-
00214-
9), although Figure 1 does illustrate the provision of an SS7 network in which
SS7
signalling links (illustrated by broken lines) may be routed via a Signalling
Transfer Point
(STP) 10. The SS7 transport mechanism is also used to convey ISUP signalling
messages
3 ~ between the GMSCs 1,2 and the PSTN 3.
In Figure 1, the solid lines indicate Ethernet connections between signalling
points. These
Ethernet connections are used to carry IP datagrams encapsulating MAP
messages,
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providing an alternative to the SS7 transport mechanism described in the
preceding
paragraph. It is noted that only direct ethernet connections (point-to-point)
are provided
between signalling points, i.e. there is no requirement for routers in the IP
network.
5 Figure 2 shows the protocol which is implemented at each of the PLMN
signalling points.
The MAP 11 sits on top of a so-called "adaptation layer" 12 which in turn sits
on top of
two distinct protocol stacks. A first of these stacks 13 provides the
conventional SS7
transport mechanism, whilst the second 14 provides for the IP transport
mechanism. As
far as the MAP 11 is concerned, the adaptation layer 12 behaves identically to
the TCAP
io so that no modifications to the MAP 11 are required.
Initialisation of a MAP dialogue commences with a dialogue request message
being
passed from the MAP 11 to the adaptation layer 12. The adaptation layer uses a
user
reference contained in the dialogue request message to determine the
associated Sub-
System Number (SSN). If the SSN does not indicate that the user is a HLR, VLR,
or
MSC, the dialogue is designated for SS7 and the normal SS7 procedures utilised
(i.e.
protocol stack 13).
If, on the other hand, the SSN does indicate that the user is a HLR, VLR, or
MSC, the
2o called address, i.e. Global Title (GT), is checked by the adaptation layer
12. Each of the
MSC/VLRs, GMSCs, and HLRs which are co-located with the originating signalling
point
is associated with a Global Title series. These series are pre-recorded in an
address table,
accessible to the adaptation layer 12. The address table contains a mapping
between
Global Title series and IP addresses. If the check determines that the called
Global Title is
2s not a member of one of the recorded Global Title series, the dialogue is
again transported
over SS7. However, if the check confirms that the called Global Title is a
member of a
recorded Global Title series, then, providing that the TCP/IP link towards
that address is
active, a request granted message returned to the MAP 11 by the adaptation
layer. In the
event that the identified link is not active, then the dialogue is once again
transported over
3o SS7. Subsequent messages relating to the same dialogue are transported over
IP or SS7
depending upon the initial checks performed by the adaptation layer.
To limit the processor load required for checking an address called by a MAP
dialogue
request message, the number of addresses contained in the address table is
restricted to a
35 relatively small number, i.e. the number of co-located signalling points
which can make
use of the lightweight IP transport mechanism is restricted. A suitable number
of
signalling points may be six or less.
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Figure 3 is a flow diagram illustrating the processing steps carried out at a
signalling point
following initiation of a MAP dialogue. Typically, these steps are carried out
by a suitably
programmed computer, or by one or more Digital Signal Processors (DSPs),
although
other suitable implementations will be readily apparent.
It will be appreciated by the person of skill in the art that various
modifications may be
made to the above described embodiment without departing from the scope of the
present
invention. For example, rather than using the TCP routing protocol above the
IP protocol,
a simpler routing protocol may be used, e.g. Point to Point Protocol (PPP). In
a further
modification, the invention may be employed to relay MAP messages received at
a
signalling transfer point from an originating signalling point, not co-located
with the
receiving signalling point, to a destination signalling point which is co-
located with the
signalling transfer point. In this case, the signalling transfer point will
check whether or
not the destination address of the received MAP messages is contained within
the address
~ ~ table already described and, if so, determine the associated IP address.
