The question on how to interconnect existing (sub-)systems to create a larger or global system is a traditional one and has been studied in the literature before. Within the context of computer networks, for example, the embedding of heterogeneous, especially incompatible sub-networks (e.g., a reliable and an unreliable sub-network) into a global network was such a question. Two approaches have been identified to solve this question in [3]. One of them is to interconnect available sub-networks just as they are, so without building addi- tional functionality on top of these sub-networks. The other one is to use the sub-networks as building blocks, and adapt their functionality by applying “wrapping” functions such that their services become compatible with each other (e.g., both sub-networks became reliable). The motivation behind this harmonization approach is that interconnection will be easier if all sub- networks exhibit a uniform service level. The latter approach is also followed by the ISO 8648:1988 standard that defines how “real networks” should be interconnected [4]. HConsumer HPayRequest HPayRequest HPayRequest HPS HProvider HPayConfirm HPayConfirm time X
Inspired by the approaches in [3], we discuss two similar approaches to solve the micropayment system interconnection problem:
• ad-hoc interconnection of payment systems; • interconnection of uniform payment systems.
5.3.1 Ad-hoc interconnection of payment systems
The first interconnection approach is to take the different payment systems and interconnect them in an ad-hoc manner.
Figure 5.6 depicts the ad-hoc interconnection of two different payment systems. The HConsumer is represented within the HPS by the Payer entity, which uses payment system A. The HProvider is represented within the HPS by the Payee entity, which uses payment system B. Hence, Payer and Payee are the actual users of the existing payment systems on behalf of the hybrid system users, and their functionality depends on the underlying payment system. The Payer and Payee entities realise the mapping between the hybrid payment serv- ice and the existing payment services. Payment systems A and B are intercon- nected via a Payment Gateway (PG) that incorporates the Payee and Payer entities specific to system A and B, respectively.
Figure 5.6 Ad-hoc interconnection of payment systems
Payment Gateway
Payment system A Payment system B
HProvider
Hybrid payment system
Payee Payer
HConsumer
Payer interconnection Payee
INTERCONNECTIONAPPROACHES
This approach requires the definition of mappings between each pair of inter- connected payment systems. This actually means that two mapping rules must be defined for each pair of interconnected payment systems, because the inter- connection must be bi-directional and the PG should be able to function as payer and payee for each system. The PG must determine, generate for and receive from each underlying payment system information structured in differ- ent formats. Additionally, the PG must also store this information to provide support for auditing (see Section 4.5.1). These operations require a complex functionality and huge data storage capacity from the PG.
In case a new payment system is incorporated in the hybrid system built on an ad-hoc manner, mapping rules between the new and already incorporated systems, and the hybrid system need to be defined.
Looking at the number of current micropayment systems and at their relatively short longevity (i.e., their availability on the electronic payments market), and considering future developments (e.g., appearance of new systems, increasing volume of micropayments), the ad-hoc interconnection would only work if the number of payment systems is small, so interconnections could be developed on an individual basis. Despite of the strong point of this approach that allows existing systems to be interconnected without building new functionality on top of them (except the PG), a more generic approach is required which can be applied in case of many payment systems.
5.3.2 Interconnection of uniform payment systems
The second interconnection method is to harmonize or (de-)enhance the payment services of existing systems to a uniform level, which we call the uniform payment service and interconnect these uniform payment services [5]. A prerequisite for this method is that the harmonization or (de-)enhancement of existing (and future) micropayment systems to the uniform payment service is possible. We call a system that provides the uniform payment service a uniform payment system, and a money transfer that is performed by such a system a uniform payment.
Figure 5.7 depicts the interconnection of two uniform payment systems that wrap the existing micropayment systems A and B from Figure 5.6. The Payer
entity is decomposed into (i) an HPayer entity, which is a user of a Uniform payment system (1 or 2) and provides the hybrid payment service to the HCon- sumer or supports the PG’s interconnection function, and (ii) a UPayer entity, which is a user of an existing payment system (A or B) and provides the uniform payment service to the HPayer. Similarly, the Payee is decomposed into enti- ties HPayee and UPayee such that the HPayee provides the hybrid payment service to the HProvider or supports the PG’s interconnection function, and UPayee, which is a user of an existing payment system provides the uniform payment service to the HPayee. The composition of internal entities HPayee, HPayer and interconnection of the Payment Gateway is called the Hybrid Payment Gateway (HPG) in the sequel. The HPG also coordinates the process- ing of hybrid payments by coordinating and contributing to the initiation of uniform payments.
Figure 5.7 Interconnecting uniform payment systems
An advantage of this approach over the first one is that only one set of mapping rules needs to be defined for each existing payment system. Another advantage is that the mapping rules between the services of the uniform and the hybrid payment systems (realized by the HPayer and HPayee) have to be defined only
HProvider
Hybrid payment system HConsumer
Payment system A
Uniform payment system 1
Payment system B
Uniform payment system 2
Payment Gateway HPayer UPayer A UPayee A HPayee UPayee B UPayer B
HPayee interconnection HPayer HPG
HSAP 1 HSAP 2
USAP 12
USAP 11 USAP 21 USAP 22
P a y e r P a y e e
FUNCTIONALREQUIREMENTSOFTHEUNIFORMPAYMENTSYSTEM
once. Moreover, if a new payment system is incorporated in the hybrid payment system, only two mapping rules need to be defined. In conclusion, in this interconnection approach, we have seriously reduced the number of mapping rules compared to the previous one.
A generally applicable uniform payment service, which can be provided by existing payment systems, enables a generic and systematic method to inter- connect micropayment systems [5]. Moreover, the hybrid system seems to be easier to be designed and realized. Therefore, in the remainder of this thesis, we consider the interconnection of uniform payment systems.