GPRS billing: getting ready for UMTS In his first article about UMTS, Lucas Baugé looks into the key challenges of GPRS billing. He seeks to show how solving these challenges will help operators succeed in the not so distant future of UMTS. It is widely accepted that GPRS, and potentially EDGE, offers a smooth technology migration from GSM to UMTS. GPRS has been designed to integrate itself with the GSM system as a complement to the existing circuit-switched service. At the same time GPRS introduces major concepts on which UMTS will rely. However, UMTS is more than just a higher-bandwidth medium giving access to the pre-existing mobile services. In terms of billing, it will require more than just the addition of an IP billing module to the existing GSM billing system. Nowadays any new billing system should be flexible enough to cope with coming standards and parameters. Those who keep prepared for changes so as to provide appropriate charging policies as soon as it becomes possible may well become the 3G winners. In order for UMTS to succeed and operators to reap a return on investment for the Billions that has been exchanged for licences, UMTS must offer: A technology that delivers; Innovative services that attract customers; An acceptable and profitable billing & pricing environment. Billing for UMTS services becomes a strategic issue and one that operators cannot afford to get wrong. In trying to find the winning formula an operator must understand: 1. How 3G technologies and standards affect the billing scenario; 2. How emerging services will be billed (what does the customer expect); 3. The impact of billing and vice versa on the now de facto mobile Internet world. These issues cannot be explained in brevity and so this article is the first of three instalments and will focus on the impact of technology migration on the billing processes from a 2G to 3G environment. This first article addresses the issue from a telecom perspective. The second article will discuss the products and services migration including the charging policy from the customer perspective, while the final article will offer an insight into the framework for accessing content on the Internet. M O B I L E Copyright SITICOM page 1/6
Taking the first steps Solving the challenges raised by a 2½G GPRS implementation is thus not a waste of time for operators that envision UMTS operation. These challenges include: the ability to handle a greater amount of information from which to retrieve appropriate rating parameters; the ability to manage multiple sources of information that exceed the mobile network coverage; the ability to address GPRS roaming. Dealing with the greater amount of information A GSM voice customer generates in average 3 CDRs per day and a data user is expected to generate at least ten times as many, which raises collection and mediation issues. Most of the information that will be used for billing purposes in GPRS is generated from the PDP (Packet Data Protocol) Context which is the underlying concept supporting the always on feature. This feature is regarded as one of the major steps toward UMTS being accomplished with GPRS. The PDP Context establishes a virtual path (GTP Tunnel) between the Serving GPRS Service Node (SGSN) and the Gateway GPRS Service Node (GGSN) and defines the different characteristics of the data session. The mobile user is attributed an IP address, connects to the appropriate GGSN via an Access Point Name (APN) giving him access to the appropriate content or service, and negotiates a Quality of Service (QoS) profile for the session. Different types of Call Detail Records (CDR) will be generated by the SGSN reporting for SMS usage, mobility management usage or GPRS resources usage associated with the PDP context. The GGSN will generate CDRs reporting for the PDP context usage in relation with the external network accessed. To help us dealing with all these CDRs the GPRS standards introduce a new functional entity, part of the GPRS Core Network, known as the Charging Gateway (CG) so as to perform a kind of pre-collection of CDRs. From the information provided by these CDRs, several rating parameters can be retrieved: Time: although it is not the appropriate rating parameter to be used on its own in packet-based sessions, time can help assessing the content or QoS level usage. Data volume: obviously the first rating parameter that comes to mind in packet based sessions. GPRS standards allow differentiation from uplink and M O B I L E Copyright SITICOM page 2/6
MSC/VLR BTS BSC SGSN Access GPRS backbone CG SS Mediation Rating GGSN Billing Billing System System downlink data volumes. QoS: GPRS standards introduce parameters describing the QoS profile used for a given session: Reliability, Precedence, Delay, Peak Throughput and Mean throughput. These parameters are negotiated at PDP context activation. They give the opportunity to the operator to set up QoS differentiated subscriptions. APN: this parameter is actually made of two domain names, e.g. <vodafone.gprs vizzavi.com>, WAP Gateway External IP networks Content usage Figure 1. GPRS rating information sources Web server WAP server corresponding respectively to the home mobile network and the server or remote network accessed. This can give an idea of the content accessed by the user but not in a great deal of details. Dealing with multiple sources More appropriate information about content usage or m- commerce transactions is to be retrieved from elements potentially outside the operator s network. Such elements could be a WAP gateway, a WAP server or Web server or even a firewall. The log files of these elements can give for example the URLs that have been accessed by the user and for how long. It becomes then possible to assess the usage of the content accessed. Figure 1 gives an overall picture of the different information sources that can be potentially involved in 2½G mediation. An issue raised by these external elements is the aggregation of all the records generated by one user during a specific data session. In the GSM world, the user is identified by the International Mobile Subscriber Identity. GPRS standards define the charging ID that identifies one PDP context irrespective of the nodes involved (useful in hand-over cases). Outside the GPRS network the user can be identified by the IP address he was allocated for the session. The mapping between the IMSI and the temporary IP M O B I L E Copyright SITICOM page 3/6
DNS Home Home Mobile Mobile Network H-GGSN H-SGSN GPRS/UMTS backbone 1 2 BG External Packet Network Roaming Network (GRX) GTP Tunnel 1 GPRS root DNS address allows the aggregation of the records and could be performed by a security server. Another issue comes from the fact that a 2½G user can access external data networks using either the GPRS bearer or GSM circuit-switched data bearer. It means that there should be interactions between the GSM legacy time-based billing system and the new GPRS billing system so as to provide a consistent billing for data services. GPRS Roaming GPRS re-uses existing GSM intelligent entities such as the Home Location Register (HLR), Visited Visited Mobile Mobile Network V-GGSN Figure 2. GPRS/UMTS Data roaming BG GTP Tunnel 2 V-SGSN DNS the Visited Location Registers (VLRs) and the different authentication elements. However, it also introduces GPRS-specific concepts related to roaming. When a mobile user roaming on a visited network initiates a data session, two scenarios can occur as illustrated in Figure 2: 1. the user accesses the external data network via the visited SGSN and the home GGSN. 2. the user accesses the external data network via both the visited SGSN and the visited GGSN. The first scenario is the most problematic as records related to a single user and a single PDP context will be generated in two different networks. In such a case, the home network giving access to the external data network via its gateway could be in charge of the content usage and the visited network in charge of the access usage. The access records are then conveyed via TAP3 files. In this scenario, a roaming network interconnecting home and visited mobile networks needs to be used. A standard is still under work, known as GPRS Roaming exchange (GRX). It should address the issues such as the QoS agreements between operators (more about this in the third article). M O B I L E Copyright SITICOM page 4/6
UMTS: an even bigger challenge Fundamental changes have been introduced by GPRS technology compared to 2G networks which results in new fundamental requirements for billing systems. The framework for UMTS has been set by GPRS (Charging gateway, SGSN and GGSN CDRs) and the issues surrounding GPRS roaming scenarios will remain roughly the same. However, a bigger challenge may well come with the implementation of UMTS. Handling the service boom However, GPRS should allow a rather limited range of services (mainly e-mails, remote LAN access and WAP based services) compared to UMTS that will offer real-time multimedia services. Value-added location-based services, emerging with GPRS, should also boom in the UMTS environment involving new entities and functions. Generally speaking UMTS will extend the amount of information to be treated by mediation. Introducing wider QoS principles The QoS principles introduced by GPRS remain rather basic and relatively vague. Moreover, due to radio access limitation, GPRS should only provide best effort QoS, which gives little place for QoS differentiation when rating and billing data services. Hence, a charging policy based on QoS will only truly become applicable with UMTS thanks to higher bandwidth, newly defined traffic classes (conversational, streaming, interactive, background) and the multiple PDP context capabilities (e.g. one for voice, one for data and one for video in a videoconference session). The Virtual Home Environment A major impact on billing processes will come from the Virtual Home Environment (VHE). The idea underlying VHE is that the user should be able to seamlessly access his personalised set of services regardless of the access medium and regardless of the serving network. It implies that a 3G user may start a data session while covered by a home network cell, then continue it at the airport via a wireless LAN (or Bluetooth technology) and finish it abroad served by another 3G mobile network, without encountering major difficulties. For the 3G billing system this requires that data from several different access technologies are aggregated (Bluetooth, Wireless LANs, and other 3G networks). This environment will create a greater number of record events, more complex than those handled in GPRS. The VHE framework should incorporate a Service Creation Environment allowing the operator, third parties and even the end-user to modify and create M O B I L E Copyright SITICOM page 5/6
3G services. It means that the 3G billing system should be flexible enough to manage this multipleentry service environment. Voice over IP In the longer term the circuitswitched features in the core network are meant to gradually disappear in favour of IP based ones giving finally birth to the all-ip version of UMTS. The architecture should then be based on packet technologies for simultaneous real-time and non real-time services, allowing 3G users to make VoIP calls. UMTS standards recommend the use of the Session Initiation Protocol (SIP) as call set up signalling protocol for multimedia sessions over IP. Mobile users will still have to perform an Attach procedure followed by a PDP Context activation before being able to use the service. However, in addition they will have to register with a Call State Control Function (CSCF) entity which is similar to a Gatekeeper in the VoIP world. For 3G billing systems this means that new rating information will become available from gatekeepers or SIP proxies in addition to the existing one. This information should be properly integrated with existing billing information to ensure consistency between circuit switched voice calls and voice over IP data calls. shown that billing for UMTS services will become a highly strategic issue. We have also seen that getting GPRS billing right will offer a good step in the right direction towards UMTS. A large number of components introduced by GPRS can be reused under UMTS and the basic principles for data roaming will be. In the next article we will see how operators should price and bill for services under UMTS. This will lead to our third article where we will describe how operators will benefit from the emerging standards that allow billing for content on the Internet. ABOUT THE AUTHOR Lucas Bauge is a consultant with SITICOM Group where he has specialised in the areas of WAP, GPRS, UMTS and Billing for Mobile Services. Mr. Bauge has previously worked for British Telecom where he looked into seamless provisioning under UMTS. Contact: lbauge@siticom.com or consulting@siticom.com Web: www.siticom.com In summary, this article has M O B I L E Copyright SITICOM page 6/6