2. Circuit-Switching Circuit-switching network establishes circuit between nodes and terminals. It doesn'd need high technique to establish but It makes some ploblem. For example, when sender and receiver's sending speed is different, it makes delay of sending information.

4. Analog signal and Digital signal Analog signal change date into voltage, current and charge. But Analog signal make noise because turbulence. So error margin gose out when returning it. Digital signal change date into numerical value. There is no error margin and former date can be restored because the numerical value has scattered.

5. Fierwall A firewall is a part of a computer system that blocks unauthorized access when permitting authorized communications. Thanks for it, our computers are protected from some serious trouble.

6. References Wikipedia http://en.wikipedia.org/wiki/Main_Page Network protocol and Internet http://www.atmarkit.co.jp/fwin2k/network/tcpip001/tcpip04.html