A machine tool includes a table slidably supported on a carrier and movable relative to the carrier by a driving device, a telescopic covering device having two or more cover members telescopically slide relative to each other in response to a sliding movement between the table and the carrier. The buffer unit is driven by a first level voltage to buffer a strobe signal, and the sense amplifier is driven by a second level voltage to amplify a signal of an IO line in response to an output signal of the buffer unit. All received communication requests for routing are in the form of the session initiation protocol wherein they are parsed and converted into routing requests processed by the server application and routed to determined destinations and wherein events are established as session events conducted under the session initiation and management protocol. A retaining member has a body including a groove on an outer surface of the body.