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Contents of Recommendation Q.1032
SIGNALLING REQUIREMENTS RELATING TO ROUTING OF CALLS
TO MOBILE SUBSCRIBERS
1. Introduction
2. General routing rules
3. General requirements for the fixed network
4. Signalling aspects on routing a call to a mobile managed
by a home PLMN situated in the same country
5. Routing a call to a foreign mobile subscriber
6. Alternative solution: re-routing of the call after clearing
the previous connection
7. Unsuccessful call set-up
*
* *
Recommendation Q.1032
SIGNALLING REQUIREMENTS RELATING TO ROUTING OF CALLS
TO MOBILE SUBSCRIBERS
1. Introduction
When a subscriber wants to call a mobile subscriber, the fixed network
needs to know the actual location of the MS in order to route the connection to
the relevant Mobile Services Switching Centre (see Recommendation Q.1003 on
location registration). This contribution tries to present the signalling
requirements the fixed network has to comply with for that purpose. The document considers
the different assumptions concerning the capabilities of the fixed exchanges to
perform some signalling procedures prior to call set-up.
This Recommendation assumes that the routing analysis requirements
specified in Recommendation Q.107bis are fulfilled.
This Recommendation assumes that the ISDN number of the mobile contains
a specific National Destination Code. The cases where the mobile numbering plan
is fully integrated in the fixed numbering plan are for further study.
2. General routing rules
The number dialled by the calling subscriber contains no indication
concerning the actual location of the called MS. Therefore, to set-up the complete
connection, it is necessary to know the location of the MS and the routing
address to be used, i.e. the Mobile Station Roaming Number. The only equipment able to
provide this information is the Home Location Register. Therefore to route the
call to the Mobile Services Switching Centre where the MS is located, it is
necessary to interrogate the HLR.
The preferred procedure with regard to signalling is the following:
1) When a subscriber wants to call a mobile station he dials the ISDN
number of that station.
2) The local exchange (or a transit exchange) analyzes the number
dialled and recognizes the mobile service National Destination Code indicating that
the call is destined to a mobile subscriber. In general this complete routing
analysis can be made for the national calls only: when the outgoing exchange
recognizes that the calling subscriber dialled the international prefix, it routes
the call directly to the outgoing International Switching Centre (ISC) without any
further analysis. This ISC can then recognize the mobile national destination
code.
3) If the result of routing analysis shows that it is necessary to get
additional information to set-up the complete connection to the MSC where the
called station is located, then this information must be obtained from the HLR in
charge of the mobile subscriber. If the interrogation procedure is implemented
in an exchange referred to in 2) above, this exchange then performs the
interrogation of the Home Location Register. The HLR sends back the roaming number of the
called MS. This procedure is supported by the Transaction Capabilities of
Signalling System No. 7.
4) The connection is then set-up in the fixed network to the MSC
according to the roaming number of the MS.
3. General requirements for the fixed network
To route a call up to a mobile subscriber, an interrogation of the HLR
must be performed in order to get the roaming number allocated to that MS. This
interrogation procedure is supported by the Transaction Capabilities of
Signalling System No. 7. The preferred solution is that the local exchanges be adapted to
TC, and able to perform this interrogation: then they can route the call
directly to the called mobile according to the roaming number they obtain from their
interrogation of the HLR. The following section of this document shows possible
solutions if this assumption is not fulfilled.
As it is described below, in the case where there are no interrogation
facilities in the fixed network, on recognition that a call is destined to a
mobile subscriber, the routing is first performed to a Gateway MSC. The
interrogation of the HLR is then performed by the MSC and the call proceeds according to the
Roaming Number received.
Section 5 deals with the routing of calls to foreign mobile stations:
usually, in this case, the local exchange does not analyze the national part of
the called address and routes directly to the outgoing International Switching
Centre which then performs the correct routing of the call.
4. Signalling aspects on routing a call to a mobile managed by a home PLMN
situated in the same country
4.1 The originating exchange is adapted to the interrogation procedure
(Figure 1/Q.1032)
If the originating local exchange is able to perform the interrogation
procedure, the call set-up occurs as it is specified in section 2 of this
document.
4.2 The originating exchange is not adapted to the interrogation procedure
If the originating exchange is unable to use TCAP, the following cases
can be considered:
- the interrogation procedure is performed by a transit exchange;
- the call is re-routed by a Gateway MSC.
4.2.1 The interrogation is performed by a transit exchange (Figure 2/Q.1032)
If the originating exchange is unable to perform the interrogation of
the HLR, the connection is set-up to a transit exchange. This exchange analyzes
the address received (the ISDN number of the subscriber) and notices that the call
is destined to a mobile subscriber. It then performs the interrogation of the
HLR and routes the call as it is described in section 2.
4.2.2 The call is re-routed by a Gateway MSC (Figure 3/Q.1032)
If the fixed network is unable to interrogate the HLR in order to route
the call to the actual location of the MS, the connection is set-up to a
Gateway MSC.
The Gateway MSC interrogates the HLR of the called MS (using MAP in
general cases). It receives back the roaming number of the subscriber. With this
address, the GMSC set-up a connection via the telephone (or ISDN) network to the
MSC where the mobile is located. If the called subscriber is abroad, the
connection is normally set-up via the international network.
5. Routing a call to a foreign mobile subscriber
As for a normal telephone call, the calling subscriber, when he wants to
join a foreign mobile subscriber, dials the international access prefix first.
His local exchange, according to this prefix, routes the call directly to the
outgoing International Switching Centre without any further analysis of the number
dialled.
The routing of the call is then performed by the outgoing international
Switching Centre. Two assumptions can be envisaged:
- the outgoing International Switching Centre recognizes that the
called party is a mobile subscriber and can perform the interrogation
of the HLR;
- the outgoing International Switching Centre is unable to perform
the interrogation of the HLR.
5.1 The outgoing ISC can perform the interrogation of the HLR
(Figure 4/Q.1032)
When the outgoing International Switching Centre receives the call, for
routing purposes it analyzes the digits of the country code and the first digits
of the national significant number of the called party address. It can then
notice that the call is destined to a mobile subscriber and needs a preliminary
interrogation transaction prior to setting up the connection.
With the roaming number, the ISC then routes the call to the MSC where
the mobile is actually located. The connection is set-up via the international
network if the MS is not in the same country as the calling subscriber.
5.2 The outgoing International Switching Centre is unable to perform the
interrogation of the HLR (Figure 5/Q.1032)
If the outgoing International Switching Centre is unable to perform the
interrogation procedure, it routes the call to the incoming ISC of the country
where the Home PLMN of the called mobile is situated according to the telephone
(or the ISDN) number dialled by the calling subscriber.
The incoming ISC receiving the call notices that it is destined to a
mobile. The following assumptions can be envisaged:
- this ISC can perform the interrogation;
- this ISC is unable to perform the interrogation: therefore the
interrogation has to be made either by a national transit exchange or
by a Gateway MSC.
In this assumption where the actual routing has to be made in the home
country of the mobile, the connection may comprise two international links in
tandem if the subscriber is roaming abroad. Therefore it would be better that the
interrogation is performed in the outgoing country; this method would limit the
length of the complete connection. The worst case will appear when the called
mobile is roaming in the country of the calling subscriber: the complete connection
comprises two international links in tandem instead of a simple national
routing.
5.3 The International Switching Centre recognizes that it is a call to an MS
but cannot perform the interrogation
In this case, the International Switching Centre routes the call to a
Gateway MSC which performs the interrogation:
- if the GMSC is accessed by the outgoing ISC, see Figure 6/Q.1032.
- if the GMSC is accessed by the Incoming ISC, see
Figure 7/Q.1032.
6. Alternative solution: re-routing of the call after clearing the previous
connection (Figure 8/Q.1032)
The ISUP provides a backward message to indicate that the call should be
re-routed and containing the new address. This facility may be used in the case
where a foreign MS is called and no interrogation functions are available in the
fixed network to get the Roaming Number from the HLR. A long international
connection may be established before the location of the MS is determined but this
facility could allow the call to be "dropped back" to the suitable MSC.
7. Unsuccessful call set-up
7.1 Roaming not allowed
If the MS is roaming in an area where it is not allowed to have calls,
the location is not stored in the HLR and an indication is set. When a call is
set-up to this subscriber, the HLR will return an unsuccessful indication to the
originating exchange.
7.2 Restart of the HLR
After a restart, the HLR considers that the information coming from the
back up is still valid. If an interrogation is related to a subscriber whose
information is not yet restored, the HLR gives back the Roaming Number it has in
its tables. If there is a mistake, the restoration procedure specified in
Recommendation Q.1004 will re-establish the correct information.
7.3 Mobile station roaming number unallocated
If the incoming MSC receives a call which roaming number is declared
unallocated by the VLR, it sends back an unsuccessful call set-up indication to the
outgoing exchange. This situation may occur after a restart of the HLR or of the
VLR (see Recommendation Q.1004).