在生产生活中有许多网络,如电网、供水网、原油管道 运输网、交通运输网、通讯网、国际互联网等.以供水网络 为例,设仅有一个出水口和一个进水口网络每段管道都有 一个容量(单位时间内通过管道的最大水量).水由出水口 流出经过水管网络后流入进水口,这就形成一个水的实际的 稳定的有向的流动,称之为流

6.1循环语句概述 循环:反复执行称为“循环体”的程序段。 循环控制常用于数学迭代、对象遍历等问题的求解,几乎所有实用程序 都包含循环。特别是在现代多媒体处理程序(图像、声音、通讯)中,循 环更是必不可少。 Intel公司为了加快循环程序的执行,在CPU硬件中加入 多媒体扩展指令MM(Multi-Media-eXtension--);amd在cpu中加入 3DNow!指令。 循环结构是结构化程序三种基本结构之(顺序结构、分支结构)

什么是信息?信息是“含义”,是一种“代码”,它被用来精确地描述物体或事 件(以消除不确定性)。信息是独立于物质和能量以外的自然的第二属性。例 如,生物基因包含了物种遗传所需的生物信息,语言是人们用来表达思想、情感 和传递知识的人类信息。计算机和网络通讯中的二进制编码是现代信息技术的 基础。当今世界是信息社会,信息已经成为与材料、能源同等甚至更为重要的资 源,成为促进和发展科学、教育、政治、经济的基础

Information content of a random variable Random variable x Outcome of a random experiment Discrete R V takes on values from a finite set of possible outcomes PMF: P(X=y)=Py) How much information is contained in the event X=y? Will the sun rise today Revealing the outcome of this experiment provides no information Will the Celtics win the NBa championship? Since this is unlikely, revealing yes provides more information than revealing Events that are less likely contain more information than likely events

Sampling Sampling provides a discrete-time representation of a continuous waveform Sample points are real-valued numbers In order to transmit over a digital system we must first convert into discrete valued numbers

The signal suffers an attenuation loss l Received power PR= PT/L Received snr=E,/No, Eb=Pr/Rb Antennas are used to compensate for attenuation loss Capture as much of the signal as possible

Noise is additional\unwanted \signal that interferes with the transmitted signal Generated by electronic devices The noise is a random process Each\sample \ of n(tis a random variable Typically, the noise process is modeled as\Additive White Gaussian noise”(AWGN) White: Flat frequency spectrur Gaussian noise distribution

Cyclic Codes a cyclic code is a linear block code where if c is a codeword, so are all cyclic shifts of c E.g., 1000, 110, 101,011 is a cyclic code Cyclic codes can be dealt with in the very same way as all other lbc s Generator and parity check matrix can be found a cyclic code can be completely described by a generator string G All codewords are multiples of the generator string In practice, cyclic codes are often used for error detection(CRc) Used for packet networks When an error is detected by the received, it requests retransmission

CSMA/CD and Ethernet Two way cable wsws Ws W CSMA with Collision Detection(CD)capability Nodes able to detect collisions Upon detection of a collision nodes stop transmission Reduce the amount of time wasted on collisions Protocol