Notes on the Text

Part I

1. Digital networks may consist of one or more routers that route data to the correct user. An analog network may consist of one or more switches that establish a connection between two or more users.

数字网络由一个或多个路由器组成，路由器把数据发送给恰当的用户。模拟网路由一个或多个交换器组成，交换器在两个或多个用户间建立连接。

2. This is to combat attenuation that can render the signal indistinguishable from noise.

（中继器）用来抗击衰减，再现噪声中难以分辨的信号。

3. A channel is a division in a transmission medium so that it can be used to send multiple streams of information.

信道是传输媒介中的一部分，可用于使传输媒介传送多个信息流。

4. Alternatively, one could allocate each channel a recurring segment of time over which to broadcast — this is known as time-division multiplexing and is sometimes used in digital communication.

另一种方法是给每个信道分配重复的时间段，在这种时间段中进行广播，称为时分多路技术，有时用于数字通信中。

Part Ⅱ

1. This method of transmission is used when data is sent intermittently as opposed to in a solid stream.

当数据是间歇地发送而不是连续流时，使用这种传输方式。

2. Synchronous transmission uses no start and stop bits but instead synchronizes transmission speeds at both the receiving and sending end of the transmission using clock signals built into each component.

同步传输不使用起始和结束位，而是用插入各数据单元中的时钟信号使接收端和发送端传输速度同步。

3. Due to the absence of start and stop bits the data transfer rate is quicker although more errors will occur as the clocks will eventually get out of sync, and the receiving device would have the wrong time that had been agreed in the protocol for sending/receiving data, so some bytes could become corrupted by losing bits.

由于没有起始和结束位，数据传输速率较快，尽管会出现更多差错，因为时钟最后将失去同步，接收设备就要发生时间错误，这种时间关系是由发送/接收数据的协议所规定的，因此一些字节因丢失数据比特而被破坏。

4. How the sending device indicates that it has finished sending a message, e.g., in a communications port a spare wire would be used, for serial (USB) transfer start and stop digits may be used.

发送设备如何表示已完成一个消息的发送，例如在通信端口可用一根空闲的导线，对于串行（USB）传输用开始和结束字来表示。

5. Handshaking is the process by which two devices initiate communications, e.g., a certain ASCII character or an interrupt signal/request bus signal to the processor along the control bus.

握手是两个设备开始建立通信的过程，例如沿控制总线送给处理器的某个ASCII字符或是中断信号/请求总线信号。

Part Ⅲ

1. Information theory is a branch of applied mathematics and engineering involving the quantification of information to find fundamental limits on compressing and reliably communicating data.

信息论是应用数学和工程学的一个分支，涉及信息的定量，用以确定数据压缩和可靠通信的基本界限。

2. Intuitively, entropy quantifies the uncertainty involved in a random variable. For example, a fair coin flip will have less entropy than a roll of a die.

直观地说，熵定量地表示一个随机变量所包含的不确定性。例如，掷硬币比掷骰子的熵小。

·a fair coin flip：这里fair表示公平，硬币两面出现的概率相等。

3. Information theory, however, does not involve message importance or meaning, as these are matters of the quality of data rather than the quantity of data, the latter of which is determined solely by probabilities.

然而，信息论不涉及消息的重要性或者意义，因为这些是关于数据的质而不是数据的量，数据的量完全取决于概率。

4. The most fundamental results of this theory are Shannon's source coding theorem, which establishes that, on average, the number of bits needed to represent the result of an uncertain event is given by its entropy; and Shannon's noisy-channel coding theorem, which states that reliable communication is possible over noisy channels provided that the rate of communication is below a certain threshold called the channel capacity.

这个理论最基本的结论是香农信源编码定理：在平均意义上表示一个不确定事件的结果所需要的比特数由其熵给出，以及香农有噪信道编码定理：只要通信速率低于某个阈值，即信道容量，就可能在有噪信道上进行可靠通信。

5. Coding theory is concerned with finding explicit methods, called codes, of increasing the efficiency and reducing the net error rate of data communication over a noisy channel to near the limit that Shannon proved is the maximum possible for that channel.

编码理论是有关寻找一些明确的方法，称为编码，这些方法会提高效率，降低有噪信道数据通信的净误码率，以接近香农所证明的该信道最大可能达到的极限。

6. The former quantity indicates how easily message data can be compressed while the latter can be used to find the communication rate across a channel.

前者（信息熵）指出消息数据压缩的难易程度，而后者（互信息）用来确定信道的通信速率。

7. Lossy data compression allocates bits needed to reconstruct the data within a specified fidelity level measured by a distortion function. This subset of Information theory is called rate-distortion theory.

有损数据压缩是在给定保真度条件下分配重建数据所需比特数，保真度由失真函数来度量。信息论的这个分支称为率-失真理论。