LA INDUSTRIA MAQUILADORA EN MÉXICO

GSM (2G) and UMTS (3G) support CS based services including voice, CS data and Short Message Service (SMS). In the CS service domain, the MSC server and the Gateway MSC (GMSC) server are responsible for collecting the charging related information. Through the originating MSC server, all signaling information in a call can be collected even if an inter-MSC handoff occurs (i.e., a call party moves across different MSCs during a call session) [Lin01]. An incoming GMSC is the MSC that can query the Home Location Register (HLR; Figure 6.1(h)) to determine the current location of the called Mobile Station (MS; Figure 6.1(j)). An outgoing GMSC sets

CIRCUIT SWITCHED SERVICE DOMAIN 95

up a call from a GSM network to the Public Switched Telephone Network (PSTN) or other networks.

Figures 6.2 and 6.3 illustrate an offline charging example where MS1 in GSM network 1 makes a CS call to MS2 in GSM network 2 [Lin01, 3GP05a, 3GP05b]. It consists of the following steps:

Figure 6.2 An offline charging example of mobile-to-mobile CS call (Steps 1–3)

Step 1. [Figure 6.2(a)] Upon receipt of the call setup request from MS1, the originat-ing MSC server (i.e., MSC1) creates a Mobile Originated Call (MOC) record for this call.

Step 2. [Figure 6.2(b)] The call is routed to the outgoing GMSC of network 1, and then to the GMSC of network 2 (i.e., GMSC2). An outgoing gateway record is created in the outgoing GMSC and an incoming gateway record is created in GMSC2.

Step 3. [Figure 6.2(c)] GMSC2 interrogates the HLR to determine the current loca-tion (i.e., the Mobile Staloca-tion Roaming Number or MSRN) of MS2. MSRN is a temporary network identity assigned to a mobile subscriber during the call establishment. This number identifies the E.164 number of the terminating MSC. Depending on the operator’s setup, either GMSC2 or the HLR may create an HLR interrogation record.

Step 4. [Figure 6.3(a)] After the MSRN is obtained, GMSC2 routes the call to the terminating MSC server (i.e., MSC2). MSC2 creates a Mobile Terminated Call (MTC) record for this call.

Figure 6.3 An offline charging example of mobile-to-mobile CS Call (Steps 4 and 5)

CIRCUIT SWITCHED SERVICE DOMAIN 97

Step 5. [Figure 6.3(b)] When the call is finished, five CDRs, including the MOC, the MTC, the incoming gateway, the outgoing gateway and the HLR interrogation records, are transferred to the billing systems 1 and 2 via, for example, FTP.

The billing systems then process these CDRs for the charging purposes.

In Taiwan, only the originating party will be charged for a mobile phone call (except for international calls), and only the MOC record is processed for billing. In the US and China, both the calling and the called parties are charged. Therefore, both the MOC and the MTC records should be processed for billing. The outgoing/incoming gateway records and the MTC record are used for settlement of accounts with mobile operat-ors involved in roaming traffic. The HLR interrogation record may not be required for billing and settlement, but it may be used for other purposes such as statistical analysis.

An MOC record (generated by the originating MSC in Figure 6.2(a)) contains the following fields [3GP05b]:

• The Record Type field indicates “Mobile originated’’.

• The Served IMSI and the MSISDN fields specify the International Mobile Subscriber Identity (IMSI) and the Mobile Station ISDN Number (MSISDN; i.e., the telephone number) of the calling party (i.e., MS1 in Figure 6.2).

• The Called Number field specifies the MSISDN of the called party (MS2 in Figure 6.2); i.e., the number dialed by the calling party.

• The Recording Entity field identifies the E.164 number of the originating MSC (i.e., MSC1 in Figure 6.2).

• The Location field indicates the identity of the cell (i.e., Base Station 1 in Figure 6.2) or Service Area Code (SAC) at the time of CDR creation. The SAC is a code of fixed length (two octets) used to identify a service area (for location-based services) within a location area.

• The Basic Service field specifies the bearer of the teleservice employed, e.g., tele-phony (Teleservice No 11), emergency calls (Teleservice No 12), short message mobile terminated (Teleservice No 21), and so on [3GP02].

• The Call Duration field indicates the chargeable duration of the connection.

• The Cause for Termination field specifies the reason for connection release.

• The Call Reference field specifies a local identifier used to distinguish simultaneous transactions for the same MS.

An MTC record (generated by the terminating MSC in Figure 6.3(a)) contains the following fields:

• The Record Type field indicates “Mobile terminated’’.

• The Served IMSI and the MSISDN fields specify the IMSI and the MSISDN of the called party (MS2).

• The Calling Number field specifies the MSISDN of the calling party (MS1). This field is only presented when the calling number is available.

• The Recording Entity field identifies the E.164 number of the terminating MSC (i.e., MSC2 in Figure 6.3).

• The Location field indicates the identity of the cell (i.e., Base Station 2 in Figure 6.3) or SAC at the time of CDR creation. This field is only presented when the location is available.

• The Basic Service, the Call Duration, the Cause for Termination and the Call Reference fields are similar to those in the MOC record.

An outgoing gateway record (generated by the outgoing GMSC in Figure 6.2(b)) contains the following fields:

• The Record Type field indicates “Outgoing gateway record’’.

• The Called Number field specifies the MSISDN of the called party (MS2). This number is used by the outgoing GMSC for routing.

• The Recording Entity field identifies the E.164 number of the outgoing GMSC.

• The Outgoing Trunk Group (TKGP) field indicates the trunk group on which the call leaves the outgoing GMSC.

• The Event Time Stamp fields specify the seizure time and the release time of the outgoing trunk. If the call is successfully established, the answer time for the call is also recorded.

• The Call Duration field indicates the call holding time of the outgoing trunk.

• The Cause for Termination and the Call Reference fields are similar to those in the MOC record.

An incoming gateway record (generated by the GMSC2 in Figure 6.2(b)) contains the following fields:

• The Record Type field indicates “Incoming gateway record’’.

• The Recording Entity field identifies the E.164 number of the incoming gateway (i.e., GMSC2 in Figure 6.2).

• The Incoming TKGP field indicates the trunk group on which the call terminates at GMSC2.

• The Call Duration field indicates the call holding time of the incoming trunk.

• The Called Number, the Recording Entity, the Event Time Stamp, the Cause for Termination, and the Call Reference fields are similar to those in the outgoing gateway record.

An HLR interrogation record (generated by either the HLR or the GMSC in Figure 6.2(c)) contains the following fields:

CIRCUIT SWITCHED SERVICE DOMAIN 99

• The Record Type field indicates “HLR interrogation’’.

• The Served IMSI and MSISDN fields specify the IMSI and the MSISDN of the party being interrogated (i.e., MS2). The IMSI is only presented if the interrogation is successful.

• The Recording Entity field specifies the E.164 number of the HLR/GMSC producing the record.

• The Interrogation Time Stamp field specifies the time when the interrogation occurs.

In the CS service domain, online charging (e.g., for prepaid call service) is imple-mented by using CAMEL. (Appendix E will describe a CAMEL architecture for GPRS online charging.) For a prepaid subscriber, the HLR stores the Originating CAMEL Subscription Information (O-CSI), which includes information such as the gsmSCF address, the prepaid service key and a list of the trigger Detection Points (DPs) such as DP Collected_Info and DP Disconnect. The HLR passes the subscriber’s O-CSI to the Visitor Location Register (VLR) on location update. Based on the concept of O-CSI, we show how CAMEL works. The details are explained in the following steps (see Figure 6.4):

Step 1. [Figure 6.4(a)] A prepaid call is issued from a mobile user (MS1) to a PSTN user (Phone 2). When the call is set up to the originating MSC (i.e., MSC1), this MSC queries the VLR using MS1’s IMSI, and receives an O-CSI from the VLR record.

Step 2. [Figure 6.4(a)] Based on the O-CSI information, MSC1/gsmSSF suspends the call and notifies the gsmSCF to handle the call control when DP Collec-ted_Info is encountered. The gsmSSF passes the CAMEL service key (which indicates the prepaid service) to the gsmSCF to request an instruction (e.g.,

“continue’’ or “release’’) for the prepaid call handling. The gsmSCF retrieves the subscriber’s profile, applies appropriate tariffs, and converts the account balance into the time units for the call. If the subscriber’s balance is depleted,

Figure 6.4 Online charging example for prepaid call service

Billing

the gsmSCF returns the instruction “release’’ to terminate the prepaid call.

If the subscriber’s balance stored in the gsmSCF is sufficient, the gsmSCF returns the instruction “continue’’ to authorize the MSC1/gsmSSF to continue the call setup with the calculated call duration time.

Step 3. [Figure 6.4(b)] MSC1 then generates an MOC record for MS1, and routes the call to Phone 2 through the outgoing GMSC. The outgoing GMSC server creates an outgoing gateway record for accounting with the PSTN.

Step 4. [Figure 6.4(c)] When the prepaid call is terminated, the DP Disconnect is encountered. Then MSC1/gsmSSF notifies the gsmSCF to update the prepaid account balance.

Step 5. [Figure 6.4(c)] The generated records, including the MOC and the outgoing gateway records, are subsequently transferred to the billing system when the call is finished.

An MOC record (generated by the originating MSC in Figure 6.4(b)) contains the following fields:

• The Record Type, the Served IMSI, the Served MSISDN, the Called Number, the Recording Entity, the Location, the Basic Service, the Call Duration, the Cause for Termination, and the Call Reference fields are similar to those for the MOC record in the example of Figure 6.2.

CIRCUIT SWITCHED SERVICE DOMAIN 101

• The gsmSCF Address field identifies the CAMEL server (i.e., the gsmSCF in Figure 6.4) serving the subscriber.

• The Network Call Reference field is used for correlation of call records in different network nodes involving in the call session.

• The MSC Address field contains the E.164 number assigned to the MSC that generates the network call reference.

An outgoing gateway record (generated by the GMSC in Figure 6.4(b)) is the same as that in the example of Figure 6.2, and the details are omitted.

Figures 6.5 and 6.6 illustrate an example for SMS delivery with the following steps:

Figure 6.5 An example of SMS delivery (Steps 1–3)

Figure 6.6 An example of SMS delivery (Steps 4 and 5)

Step 1. [Figure 6.5(a)] When MS1 sends a short message to MS2, MSC1 creates an SMS Mobile Originated (SMS-MO) record and forwards the short message to an MSC called SMS Inter-Working MSC (IWMSC).

Step 2. [Figure 6.5(a)] The IWMSC creates an SMS-MO interworking record and passes this short message to a Short Message Service Center (SM-SC). The SM-SC then forwards the short message to the SMS GMSC of the destination network.

Step 3. [Figure 6.5(b)] The GMSC creates an SMS Mobile Terminated (SMS-MT)