What is the purpose of writing a topic sentence

What is wi-fi infrastructure? a metropolitan area network that uses radio signals to transmit and receive data a communications technology aimed at providing high-speed wireless data over metropolitan area networks a means by which portable devices can connect wirelessly to a local area network, using access points that send and receive data via radio waves includes the inner workings of a wi-fi service or utility, including the signal transmitters, towers, or poles and additional equipment required to send out a wi-fi signal