The present invention discloses an etchant for etching at least two different metal layers, the etchant comprising hydrogen peroxide and one of carboxylic acid, carboxylate salt, and acetyl group. A cell phone terminal is connected to the home gateway apparatus via an IP network. When the client management module detects that a process of the subset has terminated or become inoperable, the client management module can restart the process, as well as notify a remote system manager.