(66) is a one-way function that takes an arbitrarily long piece of plaintext and from it computes a fixed,length bit string. (67) is a protocol that most USENET machines now use it to allow news articles to propagate from one machine to another over a reliable connection. (68) is an approach preferred by cable TV providers. In this solution the fiber terminates at an opt~electrical converter and the final segment is a shared coaxial cable. (69) is a device used to connect two separate network that use different communication protocol. (70) is a digital-to-digital polar encoding method in which the signal level is always either positive or negative.

A．Router

B．Gateway

C．Bridge

D．hub