1. Introduction
Spam is defined as unsolicited bulk commercial messages generally. E-mail spam has spread widely and caused much damage. In theses days, as adoption of VoIP service increases, concern about spreading of VoIP spam is increasing. VoIP spam can be propagated more easily comparing with that of PSTN due to the following characteristics of VoIP service.
Price of VoIP service is lower than that of PSTN. Discount rate of international VoIP service is much greater that that of domestic VoIP services.
VoIP service can provide various additional features of IP services as well as real-time call connection service.
Bulk spam delivery is easy owing to the allocation of IP address to telephone number.