difficult and time consuming. Please note that the function may be affected by laws and regulations in a country. For example, a digitally signed document may be valid as a proof while another country does not accept it. Figure 6.9 shows an example of the problem. If an offer is sent between Alice and Bob in traditional format (paper), the offer is written on paper and a person signs it. The signature as well as a date can be used as proof of the sent offer. In a world with only ones and zeros, this has to be solved in another way by the security system. Denial of Service Denial of service is a threat against the availability of a computer system. By sending storms of messages (for example, web page requests or e-mails), a server becomes busy handling these faked messages and no 'useful' tasks can be done by the system. Ordinary users cannot use the system in a normal way. There are several ways of doing this: One user lets many computers send heavy traffic to one and the same destination. The origin can thus be difficult to trace as the actual traffic is generated by other computers. Offer to Bob 1 week later Alice 'Is your offer still valid' 'I did not send you an offer' Figure 6.9 Example of non-repudiation Understanding Changing Telecommunications - Building a Successful Telecom Business An application sends thousands of e-mails to a specific user. Bugs in e-mail software can be utilized in a way that e-mails multiply themselves and overload the system/server. 6.4 NON-AVAILABILITY FOR NON-SECURITY REASONS This is a large area. All the traditional reasons for non-availability remain, but the suc- cessive reduction of physical volume decreases the number of elements, which is an advantage for reliability and availability. Other factors make the network more vulnerable, with the introduction of web services as one contributing factor. 6.5 CONNECTING SECURITY TERMS INTO TELECOMMUNICATION So far the vocabulary has mainly been security oriented. It has also been service oriented. What is needed is to apply or even 