to a visual challenge. This conclusion can be exported by reading and processing the SIP messages' headers. The disadvantage of this mechanism is that the recipient domain is respon- sible to produce and judge the challenge test. Therefore, this domain is vul- nerable to Denial of Service attacks. If a spammer, who 'owes' a lot of zombie PCs, starts calling a large number of subscribers of a specific SIP provider, then the provider would create a large number of tests and consume a lot of his resources. The other disadvantage of this technique is that the caller has to support this technique on his client. Otherwise he will get some undesired messages whenever he tries to call. 23.4.1.6 Audio Analyzer The Audio Analyzer module is intended to be used for fighting SPIT generated by automats. This implies that the audio message which is heard by the receiver is always the same for the same SPIT wave. So a calculated checksum of the audio data that are sent by the SPITter is the same. In a normal VoIP communication, the audio checksums of two different calls are never the same. If a checksum is seen more than once the probability of SPIT is very high for this signature (so also for the call). The audio signature must be calculated before the call reaches the receiver. For that reason, all calls from unknown callers will be handled by an Audio Analyzer. The latter will receive the call, calculate the checksum and redirect the call to the receiver if the audio checksum was not seen before. The analyzer has similarities with an Interactive Voice Response (IVR). It answers a call and plays a pre-recorded audio for human callers. This audio informs the caller of a short delay while processing his call. This does not only inform human callers, it also simulates a human receiver for SPITters, which are waiting for an audio answer before sending their messages. While playing the information the Audio Analyzer starts recording the caller audio. The recorded audio data will be feed to a checksum calculating algorithm and the c