(56) is a one-way function that takes an arbitrarily long piece of plaintext and from it computes a fixed-length bit string.

(57) 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.

(58) is an approach preferred by cable TV providers. In this solution the fiber terminates at an opto-electrical converter and the final segment is a shared coaxial cable.

(59) is a device used to connect two separate network that use different communication protocol.

(60) is a digital-to-digital polar encoding method in which the signal level is always either positive or negative.

（57）是（）

A.RTP

B.RTTP

C.FTP

D.NNTP