An optical semiconductor relay comprises a light emitting element converting an electrical signal into an optical signal, a first photodiode array receiving the optical signal from the light emitting element. A conductor strip for carrying this current is formed on the semiconductor substrate via a plurality of insulating layers so as to extend around the primary semiconductor region of the Hall-effect device. The variable duty cycle circuit adjusts the duty cycle of the clock signal by an amount dependent on a duty cycle index value that it receives. In response to whether the first subset of addresses results in session establishment, the proxy server may then request a second subset from the location server.