sting commercial mobiles. The compatibility between new network element SW releases is verified by testing with a set of mobiles against the existing verified network element SW release. The selection of mobile devices is based on available market share information, mobile feature support and user level competence. Terminal Interoperability This testing is done in cooperation with mobile vendors to verify and maintain the compatibility between new and existing network element SW releases with different new mobiles being launched in the market. Terminal interoperability testing is executed separately for different systems like ETSI or ANSI, which are present in different markets. A.3.9 System Acceptance The system acceptance procedure is conducted on all sites. The number and type of tests involved are agreed beforehand as this activity is directly related to invoice payments. The vendor's target is to raise an invoice and get the payment released as soon as possible. The operator tries to slow it down in order to hold their finances for as long as possible. The acceptance testing procedure can be a bulky document including all details regarding prerequisites, software level, hardware level, environment setup, testing results, signalling charts and diagrams. The vendors specify these tests in order to prove their equipment capability, depending on what kind of system compliance they provide to the operators' RFP (request for proposal) and system requirements. Meanwhile, the operators also create their own tests, based on what kind of system performances they expect when they order or buy the equipment. It is always a trade-off between the two, at the end of which all the acceptance criteria are defined and documented. These acceptance criterias define when the operator accepts the respective acceptance test result and the test is considered to be 'successfully completed' from the vendor's side. Figure A.9 describes the various stages of the acceptance procedure before the final acceptance is achieved. The upper pa