大话通信-通信基础知识 第二章 笔记
香农的“信息论”， 把通信系统抽象成如下模型：
发信 -- 信源编码器 -- 信道编码器 -- 传输媒介 -- 信道译码器 -- 信源译码器 -- 收信
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噪声
编码问题，可以归纳为“信息用什么格式传送到目的地” 的问题的集合，包括信息论中的信源编码和信道编码过程，包括数模，模数转换、抽样、复用解复用，也包括各种数据帧、分组、信元的封装格式。
每种原始信号的情况千差万别：人发出的声音、计算机保存的文档、大自然的景色，这些信息本省都是有特点的，对传送的要求也完全不同，如实时性、准确度、信息量的大小、压缩的可能性，他们决定了网络技术的体制、网络拓扑结构、宽带等的规划和设计。这都造成了编码形式的多样性。
人耳朵听到的范围是20Hz-20kHz，
波的两种转换方式：以信号的基带信号表示或者另一种频率表示。另一种方式是，将信号与另一频率更高的信号（载波）结合，然后在不同的频率上传输这些合成的信号。载波是工作在预先定义的单一频率的连续信号。改变载波以便它能以合适传输的形式表示数据，就是我们常说的调制（modulation）。
传播时候，信号的衰减，需要中继放大器来放大信号，会导致失真的叠加，需要数字技术来解决。
Pulse-code modulation
Pulse-code modulation (PCM) is a method used to digitally represent sampled analog signals, which was invented by Alec Reeves in 1937. It is the standard form for digital audio in computers and various Blu-ray, Compact Disc and DVD formats, as well as other uses such as digital telephone systems. A PCM stream is a digital representation of an analog signal, in which the magnitude of the analogue signal is sampled regularly at uniform intervals, with each sample being quantized to the nearest value within a range of digital steps.
PCM streams have two basic properties that determine their fidelity to the original analog signal: the sampling rate, which is the number of times per second that samples are taken; and the bit depth, which determines the number of possible digital values that each sample can take.
经历了抽样量化以后，就开始另外一个通信过程-信道编码，这是一个将数字数据转换为可以在线路上传送的数字信号的过程。
要是数字信号能够在线路中传送，就要是信号的传递方式适应线路的要求。一次必须进行信号的信道编码。用于完成这项任务的3中常用编码技术是曼彻斯特编码，查分曼彻斯特编码和翻转不归零（NRZI）码
Manchester code
In telecommunication and data storage, Manchester code (also known as Phase Encoding, or PE) is a line code in which the encoding of each data bit has at least one transition and occupies the same time. It therefore has no DC component, and is self-clocking, which means that it may be inductively or capacitively coupled, and that a clock signal can be recovered from the encoded data.
Manchester code is widely used (e.g. in Ethernet; see also RFID or Near Field Communication). There are more complex codes, such as 8B/10B encoding, that use less bandwidth to achieve the same data rate but may be less tolerant of frequency errors and jitter in the transmitter and receiver reference clocks.
Differential Manchester encoding
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It has been suggested that Conditioned Diphase be merged into this article or section. (Discuss) Proposed since January 2009. |
Differential Manchester encoding, also called biphase mark code (BMC) or FM1, is a line code in which data and clock signals are combined to form a single 2-level self-synchronizing data stream. It is a differential encoding, using the presence or absence of transitions to indicate logical value. It has the following advantages over some other line codes:
- A transition is guaranteed at least once every bit, allowing the receiving device to perform clock recovery.
- Detecting transitions is often less error-prone than comparing against a threshold in a noisy environment.
- Unlike with Manchester encoding, only the presence of a transition is important, not the polarity. Differential coding schemes will work exactly the same if the signal is inverted (wires swapped). (Other line codes with this property include NRZI, bipolar encoding, coded mark inversion, and MLT-3 encoding).
- If the high and low signal levels have the same voltage with opposite polarity, coded signals have zero average DC voltage, thus reducing the necessary transmitting power and minimizing the amount of electromagnetic noise produced by the transmission line.
The symbol rate is twice the bitrate of the original signal. Each bit period is divided into two half-periods: clock and data. The clock half-period always begins with a transition from low to high or from high to low. The data half-period makes a transition for one value and no transition for the other value. One version of the code makes a transition for 0 and no transition for 1 in the data half-period; the other makes a transition for 1 and no transition for 0. Thus, if a "1" is represented by one transition, then a "0" is represented by two transitions and vice versa, making Differential Manchester a form of frequency shift keying. Either code can be interpreted with the clock half-period either before or after the data half-period.
Non-return-to-zero
In telecommunication, a non-return-to-zero (NRZ) line code is a binary code in which 1's are represented by one significant condition (usually a positive voltage) and 0's are represented by some other significant condition (usually a negative voltage), with no other neutral or rest condition. The pulses have more energy than a RZ code. Unlike RZ, NRZ does not have a rest state. NRZ is not inherently a self-synchronizing code, so some additional synchronization technique (for example a run length limited constraint, or a parallel synchronization signal) must be used to avoid bit slip.
For a given data signaling rate, i.e., bit rate, the NRZ code requires only half the bandwidth required by the Manchester code.
When used to represent data in an asynchronous communication scheme, the absence of a neutral state requires other mechanisms for bit synchronization when a separate clock signal is not available.
NRZ-Level itself is not a synchronous system but rather an encoding that can be used in either a synchronous or asynchronous transmission environment, that is, with or without an explicit clock signal involved. Because of this, it is not strictly necessary to discuss how the NRZ-Level encoding acts "on a clock edge" or "during a clock cycle" since all transitions happen in the given amount of time representing the actual or implied integral clock cycle. The real question is that of sampling--the high or low state will be received correctly provided the transmission line has stabilized for that bit when the physical line level is sampled at the receiving end.
However, it is helpful to see NRZ transitions as happening on the trailing (falling) clock edge in order to compare NRZ-Level to other encoding methods, such as the mentioned Manchester code, which requires clock edge information (is the XOR of the clock and NRZ, actually) and to see the difference between NRZ-Mark and NRZ-Inverted.
复用和解复用
时分复用技术TDM：
在同一条线路上按照时间未知均匀分片，每个时间分片被一个用户的信息流占用。这就是“时分复用” - 把时间拆分，然后共享。
频率分片复用技术FDM:
Frequency-division multiplexing (FDM) is a form of signal multiplexing which involves assigning non-overlapping frequency ranges to different signals or to each "user" of a medium.
先通过TDM为代表进行分析，之前提到的PCM - pulse-code modulation,即 脉冲编码调制，是时分复用的典型应用。通信技术往往比现实生活的例子要复杂，通信设备中并没有这样的时间概念，大家采用信息内容本省来获取相互间的一致性，这就是同步。
在复用中，引入同步的概念。TDM提出了同步码的概念，来保持若干路信号步调一致的在一条线缆上复用。
国际电信联盟（ITU）将话音PCM复用做了两种建议：一种叫做PCM30/32制式，一种叫做PCM24。两种系列的路数不同，一个是30路话音，一个是22路话音，每种的另外两路分别被同步和信令占用），而且因为历史的原因，帧结构及压扩律也不同。TDM中每个时间片切片被称为一个“时隙” 。
1. 以太网帧
IEEE802.3标准给出了以太网的技术标准，他对ingle包括物理层的连线，电信号和戒指访问曾协议的内容。
以太网的标准拓扑结构为总线型，但是目前的快速以太网和吉比特网以太网为了最大程度的减少冲突，并最大程度的提高网络速度和使用效率，使用交换机来进行网络连接和组织。、这样，以太网的拓扑结构就成了星型；但是逻辑上以太网仍然使用总线型拓扑结构和CSMA/CD（carrier Sense Multiple Access/Collision Detect ）的总线征用技术。