Overview of Physical Layer
The physical layer address how to transmit binary data over a transmission medium different computers connected together (eg: 010101) bitstream, is the manner in which, which form transmission.
Some characteristics of the interface of the transmission medium: mechanical characteristics, electrical characteristics, functional characteristics, process characteristics
Mechanical properties: the interface shape, size, number of leads, a number of popular talk is predetermined design crystal head cable and the like
Electrical characteristics: a predetermined voltage range when the voltage range (-5V ~ + 5V) and the like in the network transmission line used in the
Features: indicates the significance of a voltage level of an article that appears online
Process characteristics: also known protocol feature provides for the establishment of each connection member associated work steps
Physical communication basics
Data communication model
The picture shows the correspondence between the source point, a transmitter, a receiver, end point, the source system, a transmission system, the purpose of the system
The PC to send the data into 010101, a digital bit stream to pass on behalf of 010,101 modems, modem digital bit stream to an analog signal, transmitted through the public telephone network to a destination far away. Then came resolved to reverse the original data on the line
Mentioned digital bit stream, after the analog signal will explain to probably understand the model of data communication, data is converted to know what kind of situation reached the target.
Common Terms
1) the purpose of communication: transmitting information
2) Data: Entity shipping information
3) Signal: electrical or electromagnetic data show, popular talk is to represent our data through some form of electrical or electromagnetic, and this is what we call signals, electrical, electromagnetic (such as some electromagnetic waves, etc.).
数字信号：代表消息的参数的取值是离散的，下面就是数字信号，通过一高一低，不连续的波。
模拟信号：代表信息的参数的取值是连续的，下面就是模拟信号， 有高有低，但是是连续的波
4) symbols: when a time domain waveform representation using a digital signal representative of the discrete values of the basic waveform becomes different symbols. Too theoretical language, looked like did not see, in fact, very simple symbols, see figure below, in the digital signal, with 0 or 1 waveform is a symbol
码元长度：每一个码元都是通过一段时间间隔来表示， 这个时间间隔就是码元长度Note: 1 can carry the amount of information nbit symbol, not a symbol representative of the amount of information can only 1bit, which can be nbit, that is above 1 symbol represents the amount of information such 1bit 0 or 1.
Transmission channel and
Generally represents channel information to the transmission medium in one direction, so that said common communication line for transmitting information often contains a channel and a channel to receive information
(1) simplex communication: that is one-way communication, the communication only in one direction and the opposite direction without interaction
(2) half-duplex communication: that is, can two-way communication, but can not communicate, transmit data at a party, the other party must accept the data, waiting for the other finished, and then send themselves can, for example, a television inside the military walkie-talkie, both is only waiting for the party finished speaking, the other party to speak.
(3) Full-duplex communication: with respect to the half-duplex communication is concerned, it is able to simultaneously and bi-directional communication, e.g., a telephone.
Baseband signal and a bandpass signal
1) baseband signal: i.e. base band signal from the source signal, the image data signals representing a variety of text or image file output from the computer belongs to a baseband signal, the baseband signal is directly expressed sent the signal information to be transmitted, For example: we speak of sound waves is the baseband signal
2) bandpass signal: The baseband signal after carrier modulation, the high-frequency range is moved in the channel frequency band for transmission
Coding and modulation
1) coding: it converts the digital data into coded digital signal is called
2) Preparation of: converting the digital data into an analog modulated signal is called
The physical layer transmission medium
Popular talk: Earlier we know what form the data is transmitted, but we do not know what kinds of patterns in the above transfer it? In two ways:
And transmission type media guide non-guided transmission medium
Type transmission media guide
1) twisted pair
屏蔽双绞线STP
无屏蔽双绞线UTP
Type commonly combined bandwidth of the strand
2) coaxial cable
50Ω coaxial cable for digital transmission, as used for baseband transmission, also called baseband coaxial cable
75Ω coaxial cable for analog transmission, i.e., broadband coaxial cable
LAN early stages of development, has been widely used coaxial cable, with advances in technology, local area network gradually replaced by twisted pair, now mainly used in the residential area of the cable, the bandwidth depending on the cable quality.
Two common network cable (cross-line and straight line)
Straight line: the specific line-sequential production methods are: twisted-pair sequence is the same on both sides of cramping, are unified: 1: White Orange, 2: orange, 3: white and green, 4: blue, 5: white, blue, 6: green, 7: white brown, 8: brown.
Note that the same at both ends of the line-sequential one correspondence. This is done 100M standard network cable line, ie 568B standard, which is what we usually call the main line or the standard line, direct line
Straight lines most widely used, this type of Ethernet cable connections used to achieve the following:
主机到交换机或集线器
路由器到交换机或集线器
交叉线：就相当于直通线是一一对应，而交叉线就不一样，1对3，2对4等
3）光缆
光纤长什么样子
工作原理：就是通过光的折射，在里面传播，直到光出来。
具体光在其中是怎么传播的，看下图就知道了。
光纤分多模光纤和单模光纤
区别：单模光纤指只能传输一种电磁波模式，多模光纤只可以传输多个电磁波模式，实际上单模光纤和多模光纤之分，也就是纤芯的直径之分。单模光纤细，多模光纤粗。
在有线电视网络中使用的光纤全是单模光纤，其传播特性好，带宽可达10GHZ，可以在一根光纤中传输60套PAL- D电视节目
光纤的优点
（1）传输损耗小，中继距离长，对远距离传输特别经济。
（2）抗雷电和电磁干扰性能好。这在有大电流脉冲干扰的环境下尤为重要
（3）无串音干扰，保密性好，也不易被窃听或截取数据。
（4）体积小，重量轻。这在现有电缆管道已拥塞不堪的情况下特别有利。
非导引型传输媒体
非导向传输媒体就是指自由空间，其中的电磁波传输被称为无线传输。 通俗来讲，就是通过无线，不用那种固态的媒体，在空气中自由传播，在空气中传播的波很多，如何分别呢，就是通过每个波的频率不一样
信道复用技术
复用：通过下图，就可以很容易知道什么是复用，就是同时公用一条信道来进行传输信息。
信道复用技术：频分复用、时分复用、统计时分复用、波分复用
频分复用
用户在分配到一定的频带后，在通信过程中自始至终都占用这个频带(这个是一个范围，例如下图)，所有用户在同样的时间占用不同的带宽资源（请注意，这里的“带宽”是频率带宽而不是数据的发送速率）
分析：
发送数据
接受数据
时分复用
按时间轮流分配带宽资源给不同的用户，每个用户只在分配的时间里使用线路传输数据。
通俗来讲：就是每个用户都有自己的时间段来传输数据，没到自己时间就需要等待，直到属于自己的传输时间段的到来，周期性的周转
特点：在信道中，每个资源都有先后顺序，并且不会乱，一直是按照一定的顺序传输数据。
缺点：计算机数据的突发性质，用户对分配的子信道的利用率不高，因为不知道什么时候通道就会发送数据，可能别的通道都不发数据，就一个通道需要发送，但也还是要等待一定的时间，即使信道是空的。
统计时分复用
是对时分复用的一种改进，它能完善时分复用对信道的利用率不高这个缺点
原理：就是在每个要发送的数据上面做一个特殊的标记，而不是通过一个多路复用器，周期性的发送数据， 给每个要发送的数据，放入STDM帧中，让STDM帧带着数据发送过去，而接受的话，只需要分析STDM帧就行了。
注意：时分复用 又称同步时分复用，统计十分复用 称为 异布时分复用。 因为某一个用户所占用的时间间隙并不是周期性的出现。
波分复用
波分复用就是光的频分复用
数字传输系统
PCM(脉冲编码调制)技术：将模拟电话信号转换为数字信号的一种技术。
有两种PCM技术：北美的24路PCM(T1) 1.544M/s、 欧洲的30路PCM(E1) 我国采用的是E1 2.048M/S
宽带接入技术
电话网线拨号接入
老式的拨号联网方式，因为速度最高只能达到56kb/s
工作原理：通过将计算机发出的信号转换为音频信号，因为要通过电话网线来传播数据，所以这样转换来转换去，达到的上网速率太低，虽然方便，但实用性不强， 不能同时上网和打电话。
数字用户线接入
DSL：数字用户线
xDSL：用数字技术对现有的模拟电话用户线进行改造，在DSL上加了x 表示不同的数字用户线技术。
这个其实就是改善了电话网拨号接入的缺点，能同时上网和打电话，并且网速提高了很多，
工作原理：xDSL 技术就把 0~4 kHz 低端频谱留给传统电话使用，而把原来没有被利用的高端频谱留给用户上网使用。这样一来，就互不干扰了。通过频率的高低来区分是电话还是网络数据。
举例：ADSL的接入网
上行和下行带宽做成不对称的。指的是上行信道和下行信道，具体看DMT调制技术中的解释。
ADSL 在用户线的两端各安装一个 ADSL 调制解调器。目的就是为了区分低频和高频
我国目前采用的方案是离散多音调 DMT (Discrete Multi-Tone)调制技术
DMT 调制技术采用频分复用的方法，把 40 kHz 以上一直到 1.1 MHz 的高端频谱划分为许多的子信道，其中 25 个子信道用于上行信道，而 249个子信道用于下行信道。
Each subchannel occupies a bandwidth of 4 kHz, and using different carriers (i.e., different tones) for digital modulation. This approach is equivalent to using many small pair of the subscriber line modem transmit data in parallel.
Access Network operating principle
Access HFC Hybrid Fiber Coax
CATV: a coaxial cable network tree topology, the analog frequency division multiplexing technology way transmission of TV programs,
HFC CATV on the basis of the transformation, in order to develop a residential broadband access network.
Features:
HFC network trunk lines using optical fiber, coaxial cable original CATV network trunk portion of the transducer in an optical fiber, and an optical fiber using analog technology, each family required to install a user interface box
HFC network having a wider spectrum than the CATV network, and has a bidirectional transmission function
Fiber nodes (optical distribution node), the coaxial cable are those, see figure
Optical Access
Of FTTx (Fiber To ...): broadband access network resident program, where the letter x may represent different meanings
FTTH: Fiber to the Home, has been laying fiber to the home user access network may be a resident of the final solution (155Mb / s)
FTTB: Fiber to the Building, after the optical fiber into an electrical signal into the building, then partitioned cable or twisted pair to each user.
FTTC: each user from the roadside to the star structure may be used as the twisted-pair transmission medium (155Mb / s)
Ethernet Access
LANs, college campuses, large enterprises, government through various internal LAN, then let LAN access to the Internet.
Wireless network access
In front of all cable access, many of them now use wireless networking, mobile phones, laptops with wireless connections and so on, it is easy from 1G to 2G to 3G to 4G era now, each generation characteristics are not the same, going out of the 5G .
1G: analog cellular mobile communications can only voice communication, with the telephone network by dialing the like, speed very slowly
2G: digital voice communications mainly, but also can provide SMS, e-mail, browse the Web data communication function
3G: 3G era, remember previously used the web version of QQ, it is to be able to accept information has been refreshed, ha ha,
4G: speed quickly, can reach speeds between 1M ~ 2M.