信元是信元交换的单位。为控制差错，在信元中包括CRC校验和，其生成公式为(6)，校验和对(7)进行校验。信元交换采用(8)技术进行复用。在交换过程中，当实施VP交换时，其中VPI、VCI的变化情况是(9)。若在交换过程中出现拥塞，该信息被记录在信元的(10)中。

(159)

(158)

(157)

For each blank, choose the best answer from the four choices and write down on the answer sheet.

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

(72)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.

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

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

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

(156)

(153)

(154)

(152)

帧中继网的虚电路建立在()

(151)