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.

數(shù)字網(wǎng)絡(luò)由一個或多個路由器組成，路由器把數(shù)據(jù)發(fā)送給恰當?shù)挠脩簟ＤM網(wǎng)路由一個或多個交換器組成，交換器在兩個或多個用戶間建立連接。

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

（中繼器）用來抗擊衰減，再現(xiàn)噪聲中難以分辨的信號。

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

信道是傳輸媒介中的一部分，可用于使傳輸媒介傳送多個信息流。

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.

另一種方法是給每個信道分配重復的時間段，在這種時間段中進行廣播，稱為時分多路技術(shù)，有時用于數(shù)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.

當數(shù)據(jù)是間歇地發(fā)送而不是連續(xù)流時，使用這種傳輸方式。

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.

同步傳輸不使用起始和結(jié)束位，而是用插入各數(shù)據(jù)單元中的時鐘信號使接收端和發(fā)送端傳輸速度同步。

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.

由于沒有起始和結(jié)束位，數(shù)據(jù)傳輸速率較快，盡管會出現(xiàn)更多差錯，因為時鐘最后將失去同步，接收設(shè)備就要發(fā)生時間錯誤，這種時間關(guān)系是由發(fā)送/接收數(shù)據(jù)的協(xié)議所規(guī)定的，因此一些字節(jié)因丟失數(shù)據(jù)比特而被破壞。

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.

發(fā)送設(shè)備如何表示已完成一個消息的發(fā)送，例如在通信端口可用一根空閑的導線，對于串行（USB）傳輸用開始和結(jié)束字來表示。

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.

握手是兩個設(shè)備開始建立通信的過程，例如沿控制總線送給處理器的某個ASCII字符或是中斷信號/請求總線信號。

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.

信息論是應(yīng)用數(shù)學和工程學的一個分支，涉及信息的定量，用以確定數(shù)據(jù)壓縮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.

直觀地說，熵定量地表示一個隨機變量所包含的不確定性。例如，擲硬幣比擲骰子的熵小。

·a fair coin flip：這里fair表示公平，硬幣兩面出現(xiàn)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，信息論不涉及消息的重要性或者意義，因為這些是關(guān)于數(shù)據(jù)的質(zhì)而不是數(shù)據(jù)的量，數(shù)據(jù)的量完全取決于概率。

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.

這個理論最基本的結(jié)論是香農(nóng)信源編碼定理：在平均意義上表示一個不確定事件的結(jié)果所需要的比特數(shù)由其熵給出，以及香農(nóng)有噪信道編碼定理：只要通信速率低于某個閾值，即信道容量，就可能在有噪信道上進行可靠通信。

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.

編碼理論是有關(guān)尋找一些明確的方法，稱為編碼，這些方法會提高效率，降低有噪信道數(shù)據(jù)通信的凈誤碼率，以接近香農(nóng)所證明的該信道最大可能達到的極限。

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）指出消息數(shù)據(jù)壓縮的難易程度，而后者（互信息）用來確定信道的通信速率。

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.

有損數(shù)據(jù)壓縮是在給定保真度條件下分配重建數(shù)據(jù)所需比特數(shù)，保真度由失真函數(shù)來度量。信息論的這個分支稱為率-失真理論。