Signaling for International Roaming SMS Billing
Ver 0.3 10 August 2006
Revision History 0.1 2006-05-15 0.2 2006-08-03 0.3 2006-08-10
Initial version Expand and update Change MT Return Result OPC/CgPA
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Contents 1. Introduction......................................................................................................................3 2. Existing Signaling............................................................................................................4 3. Proposed Solution............................................................................................................9 4. Glossary.........................................................................................................................17 5. Appendix – MAP Layer Identifier.................................................................................18
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1. Introduction The Voice and SMS Working Group (VSWG) of the CDG IRT is working to ad dress the issues involved with International Short Message Service (SMS) Roaming Billing. Of particular interest has been the need to ensure that the home operator is able to charge its subscribers at a rate which accurately reflects the different intercarrier wholesale rates that may be charged by the operator’s individual roaming partners. There are several broad options possible and/or available for delivery of the necessary information (identification of the serving operator) to the home operator. Based on expressed operator preference (and the fact that some other options are already available), the VSWG has focused on the use of the existing internetwork ANSI-41 signaling to carry the necessary information to the home operator’s Message Center (MC, aka SMSC). The billing record generated by the MC is used for subscriber billing. Given this general approach, there are a number of parameters within the relevant signaling messages that could be used to carry the necessary information. At the June 2006 IRT, operators expressed their preference for the use of a transport-layer identifier (e.g. point code, Calling Party Address) to specify the serving network. This document describes this approach in further detail, including modifications to accommodate different signaling methods (point code routing and Global Title Translation). When point code routing is used, there are two locations for the serving network identifier, and operator feedback on the preferred approach is invited. An appendix describes an alternative approach using ANSI-41 parameters to identify the serving network.
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2. Existing Signaling Most CDMA operators today connect to their international roaming partners via a Roaming Service Provider (RSP). The RSP can represent all remote network elements as a single identity. This arrangement greatly simplifies the configuration requirements in the operator’s network, however in the process the identity of the serving network can be masked from the home operator for SMS.
2.1.
Point Code Routing
Many operators today use Point Code (PC) routing for signaling to and from the RSP. The PCs are meaningful only within a given network/country. The Roaming Service Provider (RSP) is assigned a point code inside each of its customer networks, and uses leased lines to connect back to its own premises. All roaming signaling to and from an operator’s roaming partners is via the single RSP point code. This arrangement is shown in Figure 1 below.
Figure 1 - Existing RSP Signaling
The following subsections show specific scenarios relevant for SMS. In each case there may be one or more Signaling Transfer Points (STPs) present between the RSP and the indicated operator network element – these STPs do not modify the traversing message in any way. The Signaling Connection Control Part (SCCP) layer of the messages is assumed to have Called and Calling Party Addresses either containing PCs that match the Destination and Originating Point Codes (respectively) together with a Subsystem Number (SSN), or SSN only. The SCCP routing indicator indicates that routing is based on the MTP PC and the SSN.
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2.1.1. Subscriber Registration Figure 2 below shows subscriber registration. No indication is available of the actual serving network. The SMS_Address parameter points only to the RSP.
Figure 2 - Subscriber Registration
2.1.2. Mobile-Originated SMS Figure 3 below shows a mobile-originated (MO) SMS. The transport layer identifiers received at the MC only refer to the RSP, and not to the serving operator. The MAP layer contents likewise provide no information about the serving operator.
Figure 3 - Mobile-Originated SMS
2.1.3. Mobile-Terminated SMS Figure 4 below shows a mobile terminated (MT) SMS. The MC sends the SMS to the destination PC received from the HLR in the SMS_Address parameter (the same received by the HLR at registration time). No identification of the serving operator is available at the MC.
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Figure 4 - Mobile-Terminated SMS
2.1.4. SMS Notification Figure 5 below shows a notification from the RSP to the MC (e.g. when a subscriber with a pending message becomes available). The SMS_Address points to the RSP, and the resulting MT-SMS will be as above in Figure 4.
Figure 5 - SMS Notification from RSP
2.1.5. Other Roaming Signaling Figure 6 below shows a general representation of other signaling between the RSP and the operator. All traffic is received from/directed to the single RSP PC.
Figure 6 - Other Signaling
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2.2.
Global Title Routing
Some operators use Global Title Translation (GTT) to route calls through their network, and to and from the RSP. Although the use of GTT can allow the transfer of messages between networks without knowledge of each other’s MTP PC allocations, as used today for CDMA international roaming the RSP is still present as a PC that appears “inside” the operator’s network: GTT is used as a tool to simplify routing tables in nodes of large networks. STPs/SCCP Relay points in the operator’s network route the message, changing the MTP addresses accordingly. The following subsections highlight differences between Point Code and Global Title routing.
2.2.1. General Roaming Signaling Figure 7 below shows a generic ANSI-41 message pair (which cou ld be SMS or other roaming signaling). The MTP layer addresses are modified en route by intervening STPs which also perform Global Title Translation. At the SCCP layer, the routing indicator indicates routing is based on the global title address, and the called and calling party addresses contain Global Titles for the message endpoints. As per IS-807, the Global Titles are in E.212 format. In some cases the Global Title will be the relevant mobile’s IMSI, in other cases an identifier assigned directly to a network element. The RSP may be physically located in a different country to the operator, however as with the point code routing arrangement it is assigned a local point code. For this reason the Global Title for the RSP uses the same Mobile Country Code as the operator.
Figure 7- GTT signaling
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2.2.2. Subscriber Registration and SMS Notification Subscriber registration and SMS Notification use the same signaling techniques as described in §above. The MAP layer SMS_Address parameter contains the RSP GT address instead of a point code, so that mobile-terminated SMS can be addressed u sing GTT.
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3. Proposed Solution As stated above, the approach preferred by operators is for the RSP to modify an address parameter in a transport layer (MTP and/or SCCP) to identify the serving network. The message will appear to the home operator to be coming from (or going to) a different location for each serving operator, rather than just to the single RSP location as today. Many MCs today include the far end transport layer information in their billing record. If this information is set to a value that is specific to the serving operator, a home operator can potentially implement a per-serve charging scheme with no modifications to their MCs. This was the primary reason for the operator preference for this approach (see the meeting minutes for more detail). The differences between the existing point code and GT routing schemes necessitate different approaches to incorporate serving network information for operators who use these schemes.
3.1.
Point Code Routing
For operators using point code routing, the proposed approach is to replace the single RSP point code with a PC that is specific to the serving operator. The set of roaming partner PCs would be unique to each home operator (for example, SMS traffic for Operator 1’s subscribers roaming in Operator 2’s network may appear to Operator 1 to use point code A (a point code assigned to the RSP for this purpose by Operator 1), while SMS traffic for Operator 3’s subscribers roaming in Operator 2’s network appears to Operator 3 to use point code B). This arrangement is illustrated in Figure 8 below.
Figure 8 - Serve-specific Point Codes for SMS
The following subsections show the changes to specific scenarios. Note that Point Codes can be carried in both the MTP and SCCP layers, leading to some options regarding
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where to place the serve-specific value. Except where noted, the SCCP point codes are assumed to either be absent, or the same as their MTP counterparts.
3.1.1. Subscriber Registration Figure 9 below shows the subscriber registering. The regular RSP PC is used at the MTP layer, but inside the ANSI-41 message the SMS_Address parameter (which will be used for subsequent MT-SMS attempts) is set to the new point cod e used by the RSP for SMS roaming between this serving operator and this home operator. Note that some HLRs may store SMS_Address against the MSCID, rather than against the subscriber. In this case another RSP feature may be required to provide a per-serve MSCID in the message. No modifications in the operator’s signaling network are required to support this solution. If differentiated billing for MT-SMS is not required, this modification is not necessary.
Figure 9 - Registration with serve-specific SMS_Address
3.1.2. Mobile-Originated SMS For MO-SMS, the RSP may choose to either modify the MTP Originating Point Code (OPC), or the PC in the SCCP Calling Party Address (CgPA), leaving the OPC as the (generic) RSP PC. SCCP modification may represent a simpler task for the RSPs. For the home operator, there should be little difference between the two. Figure 10 below shows a mobile-originated SMS with the modifications applied at the MTP layer. When sending the SMDPP to the home operator’s MC (indirect SMS routing is assumed here), the RSP sets the MTP Originating Point Code (OPC) to the per-serve value. The MC responds to the same point code. The SCCP Calling Party Address is assumed to contain the MC SSN, and the same serve-specific PC. The home operator must configure its network elements to route the set of serve-specific point codes to the RSP. Modifications to STP Gateway Screening rules may also be necessary to allow incoming traffic from the new OPCs. The RSP accepts the incoming smdpp addressed to the new PC.
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Figure 10 - MO-SMS from serve-specific point code – MTP modifications
As an alternative, the RSP could modify the SCCP CgPA PC, while leaving the OPC untouched. This may represent a simpler development effort for the RSP than the MTP modification discussed above. The resulting messaging is shown in Figure 11 below:
Figure 11 - MO-SMS from serve-specific point code - SCCP modification
The Return Result addressing is identical to the MTP modification case. Although it would be convenient if the Return Result were to use the RSP (generic) PC at the MTP layer (thereby avoiding the need for STP routing tables to include the serve-specific PCs), this is unlikely to be the case for real-world MCs. The reason is that the internal primitive used to pass the received message from the SCCP layer up to the application can only carry a single point code. If, as here, both MTP and SCCP carry a (different) PC, only that from SCCP will be preserved. When the application sends the return message, it will be directed to the PC seen by the application. Both SCCP and MTP will use this PC for their respective destination address parameters. This arrangement is shown in Figure 12 below:
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Figure 12 - Internal Primitives for MO-SMS
For the home operator, the SCCP modification approach may mean that no STP gateway screening changes are required (unless gateway screening / access mediation is used that checks the SCCP CgPA). Although the use of different MTP and SCCP PCs may be somewhat unusual, there should be no problem transporting these messages through the operator’s network, as the SCCP layer is not examined by STPs. The generation of a billing record is presumably a function of the application, rather than the signaling transport layers. The point code included in the billing record should therefore be the one carried in SCCP, which will give the desired effect o f identifying the serving network.
3.1.3. Mobile-Terminated SMS Figure 13 below shows a MT-SMS. The serve-specific point code is received from the HLR in the SMSRequest Return Result message, and used as the Destination Point Code (DPC) by the MC. Serve-specific billing for roaming MT-SMS, if desired, is ach ieved by an assumed inclusion of the DPC in the MC’s billing record. The RSP responds with the serve-specific PC in the OPC/CgPA of the Return Result, in case the MC uses this value to populate its billing record. The alternative SCCP modification is shown in Figure 14.
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Figure 13 - MT-SMS to serve-specific point code, MTP modification
Figure 14 - MT-SMS to serve-specific point code, SCCP modification
3.1.4. SMS Notification Figure 15 below shows an SMS Notification from the RSP. The key parameter is the (MAP-layer) SMS_Address, which will be used as the DPC in the resulting MT-SMS. The normal RSP PC is used at the MTP layer of the SMSNOT message itself.
Figure 15 - SMS Notification with serve-specific SMS_Address
3.1.5. Other Roaming Signaling Other roaming traffic to/from the operator (including SMS traffic where the operator is the serving operator) uses the standard RSP P C assigned for that operator (i.e. no change to existing arrangement).
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3.1.6. Signaling Management The RSP should manage the compete set of serve-specific PCs, and issue TFP/TFA messages concerning these point codes as appropriate to the operator’s network elements.
3.2.
Global Title Translation
For operators using Global Title (GT) routing to signal to/from the RSP, a serve-specific Global Title is used instead of a Point Code. The MC must include the SCCP Called/Calling Party Address Global Title in its billing record to enable MT/MO servespecific billing, respectively. The following subsections discuss changes for individual scenarios.
3.2.1. Subscriber Registration and SMS Notification Both subscriber registration and SMS Notification require no changes to the transport layer arrangements shown in Figure 7 above. At the ANSI-41 layer, the SMS_Address parameter is changed to contain a serve-specific Global Title. While it may be possible to build this (E.212) GT using the serving operator’s true Mobile Country Code (MCC) and Mobile Network Code (MNC), this may impact translation tables if current or future deployments use GT without the RSP. Instead, the recommended approach is to assign a set of addresses from the home operator’s own MCC-MNC range, which will be used to route the message to the RSP’s local presence in the operator’s network.
3.2.2. Mobile-Originated SMS Figure 16 below shows a roamer-originated SMS sent from the RSP to the home operator’s MC. At the MTP layer there is no change from the existing arrangements, while at the SCCP layer the RSP has used a serve-specific E.212 address in the Calling Party Address. If the MC includes the received CgPA in its billing record, then the desired outcome of serve-specific billing can be achieved. For the return result, the MC uses the received CgPA to build the outgoing CdPA. GT translations must be in place for the new serve-specific values to route to the RSP. Depending on the way the range of serve-specific addresses have been chosen, and the internal data configuration in the STPs, this may entail little or no changes to the STP translation tables. Any gateway screening or access mediation that checks the CgPA Global Title against a whitelist must be updated to allow the new values.
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Figure 16 - MO-SMS with serve-specific Global Title
3.2.3. Mobile-Terminated SMS Figure 17 below shows roamer-terminated SMS signaling between the home operator’s MC and the RSP. The MC uses the serve-specific Global Title previously received (in SMS_Address of smsreq or SMSNOT). As in §3.2.2, the operator’s GTT tables must contain data to route the new address to the RSP. The RSP responds to the SMDPP using the serve-specific GT, in case the MC uses this to populate its billing record.
Figure 17 - MT-SMS with serve-specific Global Title
3.2.4. Other Roaming Signaling There are no changes required to other roaming scenarios.
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3.2.5. One-way Global Title Some operators may use Global Title routing only for the Invoke message, but not for the Return Result. This can relive some routing configuration/point code discovery burden on the invoking network element, and allows simple return routing when both source and destination nodes are within the same point code universe. Serve-specific information can be made available at the MC of an operator using oneway GT routing via a combination of the methods discussed above. MO-SMS routing will use the SCCP modification described in §3.1.2. Registration/Notification and MTSMS will use the GT routing described in §§3.2.1 and 3.2.3 respectively.
3.3.
Discussion
The solution described above should meet operators’ needs for serve-specific billing for SMS roaming. Operators using PC routing and those using GT routing require different solutions, both of which must be supported by the RSP. Two options are shown for PC routing. The SCCP modification approach is preferred on the assumption that this is easier for the RSPs to implement (and may also require fewer Gateway Screening changes on the part of the home operator). The requirements on the MC billing record should be the same for either option.
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4. Glossary CDG CdPA CgPA DPC GT GTT HLR IMSI IRT MAP MC MCC MNC MO MSC MSCID MT MTP OPC PC RSP SCCP SMDPP SMS SMS_OA SMSC SMSNOT SMSREQ SSN TFA TFP VSWG
CDMA Development Group Called Party Address Calling Party Address Destination Point Code Global Title Global Title Translation Home Location Register International Mobile Station Identity International Roaming Team Mobile Application Part Message Center (aka SMSC) Mobile Country Code Mobile Network Code Mobile-Originated Mobile Switching Center Mobile Switching Center Identification Mobile-Terminated Message Transfer Part Originating Point Code Point Code Roaming Service Provider Signaling Connection Control Part SMS Delivery Point to Point Short Message Service SMS_OriginatingAddress Short Message Service Center (aka MC) SMS Notification SMS Request Sub-System Number Transfer Allowed Transfer Prohibited Voice & SMS Working Group
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5. Appendix – MAP Layer Identifier At the June 2006 IRT meeting, VSWG participants decided on the transport layer modification approach discussed in the main body of this document. However, for some operators a MAP-layer modification may be better suited. S ee the meeting minutes for discussion of the relative merits of a MAP- and a transport-layer solution. An operator desiring a MAP-based solution as described below would need to approach their RSP on an individual basis. In this approach, the RSP adds a parameter to the SMSDeliveryPointToPoint Invoke (for MO-SMS) or Return Result (for MT-SMS) to indicate the serving system. While “cleaner” from a system impact point of view, this approach may require more modification to the MC, depending on which ANSI-41 parameters are normally included in the MC billing record. There are a number of parameters that may conceivably be used to store the serving system identifier. Ideally, the chosen parameter would have the following characteristics: •
• • • •
Defined in ANSI-41 for inclusion in the SMSDeliveryPointToPoint operation (invoke and/or return Result as appropriate for MO/MT SMS billing) Not populated/examined by existing implementations Allow for sufficient values to cover all CDMA networks Included in MC billing records Intuitively relate somehow to the serving system
The parameters described below meet many but not all of these criteria. For the Invoke, custom values of the SMS_OriginatingAddress (SMS_OA) are used to identify the serving network. Information available at the time of writing indicates that while some operators may populate this parameter, no-one is examining it when received at the MC. Whether it is included on MC billing records is unknown. The exact format of the identifier is largely unimportant, and is left for future specification if required. An MSCID-like value or MCC-MNC would seem to be logical candidates. For the Return Result, ANSI-41 Rev E (following IS-725) allows the presence of the MSCID parameter in the message. Although intended for OTASP usage, the MC could be modified to include this value in the billing record (no further MC action on this parameter is required). A single MSCID per serving operator would be defined and advised by the RSP. The serve-specific values chosen are assumed to be common for all home operators using this solution. This is in contrast to the transport-layer solution in the main body of the document, in which a full set of per-serving roaming partner values is maintained by the RSP for each participating home operator.
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Figure 18 and Figure 19 below show the changes to the SMDPP/smdpp operation for MO- and MT-SMS. PC routing is assumed in the diagram, but the MAP layer changes apply equally to GT routing. All other operations are unchanged from the existing arrangement.
Figure 18 - MO-SMS with serve-specific MAP-layer identifier
Figure 19 - MT-SMS with serve-specific MAP-layer identifier
