In order to get a more flexible handling of the telephone traffic destined to the NMT network an additional procedure (Routing Enquire/Response) to request and obtain the position of a mobile station is required.

All calls destined for a national NMT-number is always routed in the PSTN to the nearest MTX. This MTX then acts as a Gateway MTX (MTXG) which interrogates the MTXH in order to fetch the routing information for routing the call directly to the MTXV.

For calls destined to an international NMT-number, the call will be routed to the International Switching Centre (ISC) in the normal way.

If the ISC is able to analyse the digits beyond the country code, and identifies the Called Party Address as an NMT-number, it routes the call to its International MTXG (IMTXG).

The IMTXG will then interrogate the MTXH in the normal way and forward the call to the MTXV.

This procedure may change according to the solution chosen for the GSM system.

The requirements described in this section is not a part of the MUP specification, and should only be looked upon as guidelines for the national implementation.

The telephone/ signalling network is responsible for routing all calls destined to one of its national mobile stations to the nearest MTX in the network. This MTX then acts as the MTXG.

When all transit exchanges in the telephone/ signalling network are programmed to route all traffic to national MS’s to its nearest MTX, there is a risk that when a call/ message is sent from the MTXG to either MTXV or MTXH, any transit nodes may route this call/ message to its nearest MTX.

How this is avoided is a national matter. However the following methods are identified:

All transit exchanges in the network carrying traffic between MTX’s is able to analyse when a message is generated in an MTX and when it is not. A message generated in an MTX should not be routed to the nearest MTX.

When an MTX sends a message to another MTX, an over-decadic digit is placed in the beginning of the address information. All transit exchanges carrying traffic between MTX’s must be able to analyse this digit, and when present the call/ message is not routed to the nearest MTX.

The international transit exchange must be able to insert this digit when receiving a call/message to one of its national MTX’s from the international network, and it must be able to remove it when receiving a call/ message from its national network.

When a No. 7 IAI message is sent to an MTX, the originating or Gateway MTX sets the "Nature of Address" indicator in the "Message Indicators" field to the value 01 (this value is reserved for national use). The normal value generated in the PSTN is 10 (national message).

The message may then logically be regarded as belonging to a NMT network, and any transit exchange should not perform routing to its nearest MTX when this value is set to 01, only when set to 10.

The international exchange must be able to change this indicator to 01 when receiving a call/ message to one of its national MTX’s from the international network, and it must be able to change the value from 01 to 11 (international number) when receiving a message from its national network.

The MSRN may not be a NMT number but belong to the numbering plan of the PSTN.

The full implementation of the Routing Enquire/ Response procedure is regarded as a national matter.

In the minimum MUP configuration any MTX must be able to receive a REM and respond with a RIM with the following fixed content:

• The Address Indicators set to:
• Bit A: 0 (national number)
• Bit B: 0 (MSRN is not included)

• No MSRN is transferred. When receiving this message the MTXG will route the call to the MTXH, using the dialled MS directory number.MTXH will handle the call in the same way as if no Gateway MTX was involved in the call (the MTXG will, seen from the MTXH, act as a normal transit exchange in the PSTN).

When a call arrives at an MTXG, which could also be the MTXH or the MTXV, the MTXG takes the actions described in the following paragraphs.

The action depend on whether the MTXG has the direct knowledge of the position of the called mobile station (MTXG acts as the MTXH) or if it has to make a request to the MTXH to get this information.

If the MTXG acts as the MTXH for the called mobile station, the call will be handled in the normal way for an MTXH receiving a call, e.g.:

Set up the call if the position of the MS is MTXH.

Forward the call to MTXV if the position of the MS is MTXV.

Set up the connection to a C-number according to the call transfer services activated.

If the MTXG does not act as the MTXH, the gateway exchange sends a Routing Enquire Message (REM) to MTXH including the MS directory number dialled by the calling subscriber. When sending REM a timer T6 is started.

Depending on which message the MTXG receives and the rerouting address included in this message, different actions are taken.

• The MTXG acts as the MTXV. If the routing information in the RIM contains the address of the MTXG, the MTXG is the MTXV. The MTXG then initiates the normal call set-up, e.g.:
• In the case where the mobile station is free it is paged. When answering the call is set up.
• In the case where the mobile station is busy, MTXG performs the necessary signalling towards the PSTN, or it may perform a call forwarding service if conditional call transfer on busy is activated (see section 3.8.2).
• In the case where the mobile station is unavailable (no call acknowledgement from the MS), MTXG performs the necessary signalling towards the PSTN, or it may perform a call forwarding service if conditional call transfer on no page response is activated (see section 3.8.2).

• The MTXG does not act as the MTXV.
• If the routing information in the RIM does not contain the address of the MTXG, MTXG forwards the call to the address given in the RIM (using the MSRN as address).
• The MTXG receives a Routing Information Reject Message (RIR), or T6 expires before RIM/RIR is received.If the MTXG receives the (RIR), it may, depending on the reason in the RIR, send an appropriate announcement and then perform the necessary signalling towards the PSTN. The procedure is then terminated.
• The same procedure is followed if T6 expires before RIM/ RIR is received.

When the MTXH receives a REM, it examines the MS directory number in the message. If the number is incomplete or unallocated it returns the RIR with the appropriate reason set.

If the number is accepted it then examines if the position of the MS is MTXH. Depending on position, different actions are taken.

• The position of the MS is MTXH
• If the position of the MS is MTXH, MTXH allocates a MSRN (which now will act as a "homing" number), and returns the information in the RIM. When sending RIM a timer T7 is started.
• When MTXH later on receives the call using this MSRN, it stops the timer T7 and sets up the call. The MSRN is then released and may be used for another call.
• If T7 expires before the call is received, the MSRN is released and may be used for another call.

• The position of the MS is not MTXH
• If the position of the MS is not MTXH, MTXH checks if conditions in MTXH requires routing via MTXH. This may be the case e.g. if the circuit between MTXH and MTXV is to be extra charged, if MTXV has not MUP implemented or if the subscriber has activated any of the unconditional call transfer services. If routing via MTXH is required, MTXH

• If this is not the case, MTXH performs the procedure described in section 3.7 for fetching a MSRN from the MTXV by sending the RNE to MTXV.
• If an RNM is received as a response, the MSRN is copied to the RIM and sent to the MTXG. MTXH then terminates the procedure.
• If an RNR is received as a response, a RIR is sent to the MTXG using the reason received in the RNR. MTXH then terminates the procedure.

As a national option, the procedure in 3.9.3. and 3.9.4 may be modified in the following way:

• The signal REM sent from MTXG to MTXH is in all procedures replaced by REM2 which in addition to the information of REM contain information whether the call already has been re-routed to IN.
• As a national option a special coding of RIR may be used to contain rerouting information. In this case RIR will replace IRI in the following description.

When a call arrive in the MTXG it will send a request for routing information to MTXH, using the REM2 message with "No suppression". If the MTXG acts as MTXH the procedure will be replaced by request within the MTX.

When MTXH receives the REM2 message with "No suppression" the MTXH will check if the subscriber has any terminating IN-services active. The check may be based on permanent subscriber data and information about where the subscriber is roaming.

If there is no active terminating IN-service the MTX will respond to the REM2 message in the way described in 3.9.3 and 3.9.4.

If there is an active terminating IN-service, the MTXH will respond to the REM2 message with IRI, which contain information on where to re-route "IN-pointer " and where the subscriber is roaming (B-location).

When the MTXG receives the IRI it will re-route the call according to the "IN-pointer" and send the "B-location" to the IN-node. How this is done is

a national option but it may be done by adding "IN-pointer" and "B-location" as prefix to the originally received B-number.

After the rerouting of the call to IN, one of the possible outcomes of the treatment in «IN» is that the call shall be terminated at the originally called mobile station. In this case rerouting to IN (described in 3.9.5.1) must be suppressed. This procedure is described below.

When a call arrives in the MTXG, MTXG may recognise the call as a call that already has been treated in "IN". How this is done is a national option.

If the call has been treated by IN, MTXG will send a REM2 message with "Suppress terminating IN-category".

When receiving REM2 with "Suppress terminating IN-category", MTXH will ignore a possible active terminating IN-service and the call will from this point be handled as described in 3.9.3 and 3.9.4 after that MTXH has received a REM message.

The following figures illustrates the Gateway procedures.

Figure 3.9.5a: Routing Enquire/ Response procedure, MS updated in MTXH, successful procedure.

Figure 3.9.5b: Routing Enquire/ Response procedure,

unsuccessful procedure in MTXH.

Figure 3.9.5c: Routing Enquire/ Response procedure,

MS updated in MTXV, successful procedure.

Figure 3.9.5d: Routing Enquire/ Response procedure, MS updated in MTXV, unsuccessful procedure.

Figure 3.9.5e: Rerouting of terminating call to IN, MS has active

terminating IN service, successful procedure.