Physical layer Overview
本质上，物理层就是怎样把数字信号通过指定传输媒介传递出去。
The physical layer involves signals and bit streams that travel on media, thus the layer 1 standards define the technologies including the following specifications: Mechanical (e.g. materials, dimensions, pinouts), Electrical (e.g. voltage levels), Functional, and Procedural.
物理层考虑的是怎样才能在连接各种计算机的传输媒体上传输数据比特流，而不是指具体的传输媒体。
The physical layer (always associated with data link layer) technologies are defined by organizations such as:
The International Organization for Standardization (ISO)
The Institute of Electrical and Electronics Engineers (IEEE)
The American National Standards Institute (ANSI)
The International Telecommunication Union (ITU) （通信相关运营商）
The Electronics Industry Alliance/Telecommunications Industry Association (EIA/TIA)
National telecommunications authorities, such as the Federal Communication Commission (FCC) in the USA. （无线相关）
每一个标准化组织来源不一样，标准化不一样。
Function
The physical layer is responsible for moving bits of data through physical media. Data, representing the bits in each frame, are turned into electrical signals on copper cables, or pulses of light on optical fibers, or microwave signals on wireless connections.
Operations
数模 The physical layer accepts a complete frame (帧） from the data link layer, encodes the binary digits as a series of signals and transmit these signals across the local physical media. （把frame转成二进制传出去）
模数 The physical layer also retrieves these individual signals from the media, restore them to their bit representations, and passes the bits up to the data link layer as a complete frame.
Moreover, for frame identification, the physical layer must distinguish where one frame ends and the next frame begins when receiving the bits on a particular medium, therefore, the physical layer may add particular bit patterns to indicate the start and end of a frame.
Encoding
把sinal转成data。
Encoding is a method of converting binary data 1s and 0s into a form (predefined code) that can travel on a physical communications link. These codes are groupings of bits used to provide a predictable pattern that can be recognized by both the sender and the received. Using predictable patterns helps to distinguish data bits from control bits and provide better media error detection.
Two encoding methods are:
Transistor-Transistor Logic (TTL) (Level driven) encoding is characterized by a high signal and a low signal (often +5 or +3.3 V for binary 1 and 0 [zero] V for binary 0 [zero]). （高电平表示1）
Manchester (Edge driven) encoding has the bits encoded as transitions. The Manchester encoding results in 1 being encoded as a low-to-high transition and 0 (zero) being encoded as a high-to-low transition. （上跳是1，下跳是0）
信噪比和香农公式
• 噪声是随机产生的，它的瞬时值有时会很大。因此噪声会使接收端对码元的判决产生错误。但是如果信号相对较强，那么噪声的影响就相对较小。
• 所以，信噪比就是信号的平均功率和噪声的平均功率之比。
香农公式表明：
• 信道的带宽或信道中的信噪比越大，则信息的极限传输速率就越高。
• 只要信息传输速率低于信道的极限信息传输速率，就一定可以找到某种办法来实现无差错的传输。
• 若信道带宽 W 或信噪比 S/N 没有上限（当然实际信道不可能是这样的），则信道的极限信息传输速率 C 也就没有上限。S为信道内所传信号的平均功率。N为信道内部高斯噪声功率。
• 实际信道上能够达到的信息传输速率要比香农的极限传输速率低不少。
对于频带宽度已确定的信道，如果信噪比不能再提高了，并且码元传输速率也达到了上限值，那么还有办法提高信息的传输速率。
• 用编码的方法让每一个码元携带更多比特的信息量。
Modulation
Modulation uses a carrier frequency as the basic frequency of a communication, then encodes information onto the carrier wave. A carrier frequency is an electronic wave that is combined with the information signal and carries it across the communications channel. There are three aspects of the basic carrier wave that can be modulated by:
▪Amplitude Modulation (AM)
▪Frequency Modulation (FM)
▪Phase Modulation (PM)
带通调制：把基带信号的频率范围搬移到较高的频段，并转换为模拟信号,调制的过程称之为载波。
• 调幅(AM)：载波的振幅随基带数字信号而变化。(通过幅度变化携带01信息）
• 调频(FM)：载波的频率随基带数字信号而变化。（抗干扰能力较强）
• 调相(PM) ：载波的初始相位随基带数字信号而变化。（0相位不变，1变180度）
载波调制好处：可以把数据调到好的频段。提高利用率。
