A method and apparatus for producing and maintaining a repository of service level test points for network based VPNs is presented. For each segment of the current version of the subscription file, the server computer searches an earlier version of a signature list for an old segment signature which matches a new segment signature corresponding to the segment. The signature is compared to a reference signature. Only a thread portion of the screw is capable of horizontally advancing and retreating in the longitudinal groove. The second comparator receives the periodic signal and compares the periodic signal with a first threshold voltage to generate a second output signal. The DB and the channel are formed by one half tone mask and the GB and the pixel electrode are formed by another half tone mask.