Physical layer and Numerical transmissions物理层与数据传输 2

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Formats and Standard 2

• 多种协议 different protocols

Code NRZ (Non Return to Zero) 不归零码

Binary Séquence  10000101111

As picture showed below, after a +level, the votage inverted as a -level, rather than back to 0 level.

Code NRZI (Non Return to Zero Inverted)

NRZ-I编码中，编码后电平只有正负电平之分，没有零电平，是不归零编码。

Utilisation : Fast Ethernet (100BaseFX), FDDI

Principe : transition for a one bit and nothing for 0. 翻转代表0，不变代表1。

注：上图中第一个电压信号有错误，第一个应为负。

基带传输技术 Codage MLT3 （MLT3基带码是一种归零编码RZ）

Utilisation : Fast Ethernet (100BaseTX, 100BaseT4), ATM,

Principle: In this encoding, only 1  change the status signal. The coded 0 retaining the value previously transmitted. The one are encoded successively in three states: + V, 0 and-V.

1.如果下一比特是0，则输出值与前面的值相同；

2.如果下一比特是1，则输出值就要有一个转变：如果前面输出的值是+V或-V，则下一输出为0；如果前面输出的值是0，则下一输出的值与上一个非0值符号相反

Codage 2B1Q (双二进制对一四进位)

Utilisation : RNIS/ISDN, HDSL

Principle: The 2B1Q code matches a group of two bits 

symbol can assume four different values ​​depending on the table below:

                                        

电压值多，抗干扰能力下降，对传输线路要求高

Code MANCHESTER 曼彻斯特码

Utilisation : Ethernet 10Base5, 10Base2, 10BaseT, 10BaseFL

Principle in the Manchester encoding, the basic idea is to provoke a transition signal for each transmitted bit. A 1 is represented by the transition from V + to V, a 0 is represented by the transition from-V to + V.

由电压阶跃表示码值，+-为1，-+为0

Codage Manchester différentiel 差分曼彻斯特码

Utilisation : Token Ring

Principle: it is the presence or absence of transition at the beginning of the interval of the clock signal which carries the coding. 

1 is encoded by a lack of transition, a 0 is encoded by a transition at the beginning of clock cycle.

一种简单的识别差分曼彻斯特编码的方法：

主要看两个相邻的波形，如果后一个波形和前一个的波形相同，则后一个波形表示0，如果波形不同，则表示1.

FM or FSK (Frequency Shift Keying) 频移键控编码

幅度恒定不变的载波信号的频率随着输入码流的变化而切换，FSK是二进制信号的频移键控的英文缩写，它是指传号（指发送"1"）时发送某一频率正弦波，而空号（指发送"0"）时发送另一频率正弦波。

In frequency modulation, the logic levels are represented by the change in carrier frequency. For example: HART 1200 Hz and 2200 Hz

根据Bell202的建议，来电显示的数据传送采用连续相位的二进制频移键控，比特率是1200bps，而"1"对应的频率是1200Hz，"0"对应的频率是2200Hz。

FSK modulation is used for low-speed transmissions on the public switched telephone network.

     

Phase modulation or PSK (Phase Shift Keying) 相移键控

定义 ：移相键控方法是通过改变载波信号的相位值来表示数字信号 1，0的。如果用相位的绝对值表示数字信号1，0，则称为绝对调相。如果用相对偏移值表示数字信号1.0，则成为相对调相。

      

Amplitude Modulation  or ASK (Amplitude Shift Keying) 幅移键控

Modulation QAM (Quadrature Amplitude Modulation) 正交振幅调制

幅度和相位同时变化，属于非恒包络二维调制。QAM是正交载波调制技术与多电平振幅键控的结合 。

The QAM (Quadrature Amplitude Modulation) or quadrature amplitude modulation phase is a technique that uses a combination of phase modulation and amplitude.

正交振幅键控是一种将两种调幅信号（2ASK和2PSK）汇合到一个信道的方法，因此会双倍扩展有效带宽，正交调幅被用于脉冲调幅。

Example coding 3 bits per period can be used in the following table

             

           

100： 初相为0+kπ，末相 π+kπ       

001： 初相为0+kπ，末相 π+kπ ，相差为0 +kπ

011： 初相为π/4 +kπ，末相 5π/4 +kπ，相差为π/4 +kπ

110： 初相为0+kπ，末相π +kπ，相差为 8π/4 -5π/4 + kπ= 3π/4 + kπ

There is also a variant of QAM, Trellis Coded Modulation TCM (Trellis Coded Modulation).  This type of modulation is used to speed modems (V32, V34, V90).

Frequency Division Multiplexing :MRF  频分复用技术

Allows division of the total bandwidth to allocate channels

Eg telephone12 voices every telephone 4000 Hz (3000 Hz can be used more than 2 spaces between bands 500 Hz) which gives a bandwidth of 48 kHz divided between 60 and 108 kHz.

例子: ADSL

(Asynchronous Digital Subscriber Line) phone line bandwidth of 1 MHz is used only 4 KHz. We can therefore use the rest to convey more informationA 4 kHz band is reserved for voice (POTS: Plain Old Telephone Service)A band is reserved for the flow of data to network users. (Data Upstream: Upstream).A band is reserved for the stream network to users. (Dowstream Data: Downstream)Different filters are used to direct signals to the phone or computer equipment. (using more technology for QAM signals info)

TDM (Time Division Multiplexing)

WDM / DWDM (Wavelength Division Multiplexing)

XDSL

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