igital photos has started to increase. Traffic in the interactive class has higher priority in scheduling than background class traffic, SO background applications use transmission resources only when interactive appli- cations do not need them. 5.4.5 TRANSACTIONAL CLASS Transactional means financial transactions B2B and B2C where security and information accuracy are crucial. 'Micro payments' for parking and cinema tickets must be possible. Transaction-based services have caught the interest of stakeholders who are eyeing business opportunities. The development depends a lot on trust and security. The mobile phone becomes a personal trusted device (PTD). Typical transaction-based services include supporting infrastructure services throughout the various stages of the service life-cycle, such as issuing the WAP identity modules (WIM), certificate authority (CA) services and payment-clearing services. Banks are strong players here. 5.4.6 MESSAGING CLASS Messaging can be integrated into the background class (or interactive class when inter- active). However, it is a revenue-wise important type of service and it requires special equipment. This could justify its own service class for the network designer. Messaging offers user-to-user communication via storage units with a store-and-forward mailbox, and/or message handling, such as information editing, processing and conversion func- tions. See Section 5.3.6. 5.5 VALUE-ADDED SERVICES The meaning of the term value-added service (VAS) is not very clear. In Understanding Telecommunications, book 1, a distinction was proposed between IN and VAS services, where IN was network oriented and labelled advanced fundamental technical plans, and VAS was in principle implemented outside the network and more equal to information services. In any case VAS is interpreted as something more than ordinary telephony. The examples below show that service enablers play a key role for VAS. Service enablers are further described in Chapter 8. Let us take location as a VAS example. By using