Part I physical layer

Packet switching and circuit switching *

Circuit switching

Every phone is connected to a switch , The switch uses the switching method , Let telephone users communicate with each other ：

1. When the user connects the phone , A connection is established from the calling end to the called end , That is, a dedicated Physical path .
2. This connection guarantees the communication resources required by both parties to talk , When communicating Will not be occupied by other users .
3. After the call , The switch releases this dedicated physical path .

This has to go through “ Establishing a connection $\to$ conversation $\to$ The three-step switching method of releasing the connection is called circuit switching .

Circuit switching is connection oriented .

When using circuit switching to transmit computer data , The transmission efficiency of the line is low , Because computer data appears suddenly on the transmission line .

Packet switching

Its principle （ Store and forward ） yes ：

1. At the sending end , First, divide the longer messages into shorter ones 、 Fixed length data segments ;
2. Each data segment is preceded by a header to form a group ;
3. Packet switching network “ grouping ” As a data transmission unit, each packet is sent to the receiving end in turn ;
4. After receiving the packet, the receiver strips the header and restores it to a message ;
5. Finally, the received data is restored to the original message at the receiving end .

The characteristics of packet switching are tag based . This is a connectionless way .

The advantages of packet switching ： Efficient 、 flexible 、 rapid 、 reliable .

The disadvantages of packet switching ： Store and forward cause delay , Carrying the head causes overhead .

Transverse comparison

In exchange for	Group	Whether to store	Data characteristics
Circuit switching	no	no	Continuous transmission of large amounts of data
Message switching	no	yes	Transmit intermittent small amounts of data
Packet switching	yes	yes	Transmission burst 、 Intermittent data

Time delay *

Delay refers to the time required for a message or a packet to pass from one end of a network to the other end . It consists of several different parts ： transmission delay 、 Propagation delay 、 Processing delay .

transmission delay ： When sending data , The time required for data blocks to enter the transmission medium from the node $$Hairsendwhendelay=\frac{CountAccording\; to\; theblockLongdegree(Than,)}{Pass\; ontransportspeedrate(Than,/second)}$$

Propagation delay ： electromagnetic wave （ light wave ） The time it takes to travel a certain distance in the channel . Note that transmission rate and propagation rate are not the same concept .$$Pass\; onseedingwhendelay=\frac{LetterAvenue(rice)}{LetterNumberstayLetterAvenueOnOfPass\; onseedingspeedrate(rice/second)}$$

Processing delay ： The time taken by the switching node to do some necessary processing for storage and forwarding . The length of processing delay depends on the current traffic in the network . Sometimes the queuing delay can be used as the processing delay .

Queuing delay ： When packets are transmitted over the network , It's going through a lot of routers . But after the packet enters the router, it has to wait in the input queue for processing .

For high speed network links , The increase is only the transmission rate of data, not the propagation rate of bits on the link .

$$totalwhendelay=Hairsendwhendelay+Pass\; onseedingwhendelay+It\text{'}s\; aboutThe\; reason\; iswhendelay+rowteamwhendelay$$
$$whendelaybeltwideproduct=Pass\; onseedingwhendelay\times beltwide$$

Round trip delay RTT： Indicates that data is sent from the sender , The total time delay of receiving the acknowledgement from the receiving end to the sending end .

The basic concept of the physical layer

The role of the physical layer ： Try to shield the differences between these transmission media and communication means ;

Physical layer tasks ： Identify some features related to the interface of the transmission media （ mechanical properties 、 Electrical characteristics 、 features 、 Process characteristics ）; Complete the conversion of transmission mode （ Parallel transmission inside the computer 、 Serial transmission on transmission media ）.

Basic knowledge of data communication

The model of data communication system

The data communication system is divided into three parts ： Source system （ Source point 、 transmitter / Modulator 、 Receiver / Demodulator 、 End ）、 Transmission system 、 Purpose system .

Signals can be divided into two categories ：

1. analog signal / Continuous signal ： The subscriber line between the modem in the user's home and the telephone terminal office ;
2. digital signal / Discrete signal ： Computer to modem or telephone network trunk line .

The basic waveforms of different discrete values are called symbols .

Some basic concepts about channel

A channel is generally used to represent In one direction A medium for transmitting information . Therefore, a communication line includes a transmission channel and a reception channel .

According to the way information is exchanged between the two sides , There are three types of communication ：

1. One-way communication / Simplex communication ;
2. Two way alternating communication / Half duplex communication ;
3. Two way simultaneous communication / Full duplex communication .

The signal from the source is called baseband signal , It contains more low-frequency components , Even DC component , However, the channel cannot transmit directly , Therefore, modulation is required ：

1. Baseband modulation / code ： Only the waveform of the signal is transformed , After transformation, it is still baseband signal ;
2. Bandpass modulation ： Transfer the frequency range of baseband signal to higher frequency band , And converted to analog signal .

The basic band-pass modulation methods are ： amplitude modulation 、 frequency modulation 、 Phase modulation .

Common coding methods *

Manchester code ： For symbols 1（ Jump down ）, The front half of the center is high , The rear half of the center is low level . For symbols 0（ Jump up ）, On the contrary .

problem ： In digital communication , If the receiver does not know the transmission rate of the sender, it can only be determined by the symbol width . however , Baseband signals cannot do this . for instance , I don't know what it came from 11 Is it one symbol or two symbols .

Differential Manchester code ： For symbols 1, The level of the continuation boundary in the first half of the center , The second half jumps . For symbols 0, Both the front half and the rear half of the center need to be switched .

The limit capacity of the channel *

The cause of waveform distortion ：

1. High symbol transmission rate ;
2. Long signal transmission distance ;
3. High noise interference ;
4. Poor transmission media quality .

conceptually , There are two factors that limit the transmission rate of symbols on the channel ：

The range of frequencies a channel can pass through

The high frequency component of the signal is attenuated during transmission , The waveform loses the clear boundary between symbols , This phenomenon is called intersymbol crosstalk .

Nyquist criterion ： Under the assumed ideal conditions , To avoid code to code crosstalk , The upper limit value of the transmission rate of the symbol .

At a bandwidth of $W(Hz)$ In the low-pass channel , If the influence of noise is not considered , Then the maximum rate of symbol transmission is $2W(codeelement/second)$. If the transmission rate exceeds this upper limit, there will be a serious problem of inter symbol crosstalk , Making it impossible for the receiver to decide the symbol .

explain ：

1. The maximum symbol rate that can be transmitted in the actual channel is significantly lower than the upper limit value given by the Nyquist criterion .
2. Symbol and bit are two different concepts , But there is a certain relationship between the transmission rate of the two .

Signal-to-noise ratio

The influence of noise is relative . If the signal is relatively strong , Then the effect of noise is relatively small .

SNR is the ratio of the average power of the signal to the average power of the noise , It is often recorded as $S/N$.

People usually use decibel as the unit of measurement , namely $$LetterThe\; noiseThan(dB)=10{\log }_{10}(S/N)(dB)$$

Shannon's formula points out that , Channel limit information Transmission rate C by $$C=W{\log }_2(1+S/N)(b/s)$$

explain ：

1. The greater the bandwidth of the channel or the SNR in the channel , The higher the limit transmission rate of information ;
2. As long as the information transmission rate is lower than the limit information transmission rate of the channel , There must be some way to achieve error free transmission ;
3. In fact, the information transmission rate that can be achieved is much lower than Shannon's limit transmission rate ;

The combined action of Nyquist criterion and Shannon formula

Transmission media under the physical layer

Transmission medium refers to the physical path between the transmitter and the receiver in a data transmission system . It is divided into guided transmission media and non guided transmission media .

In guided transmission media , Electromagnetic waves travel along solid media ; Unguided transmission media refers to free space , Electromagnetic wave transmission is wireless transmission .

Guided transport media

（1） Twisted pair ： Lay two insulated copper wires together , Then twist it in a regular way .

The method of increasing stranding degree and electromagnetic shielding can improve the anti electromagnetic interference ability of twisted pair and reduce the crosstalk between different twisted pairs in the cable .

（2） Coaxial cable .

（3） optical cable

When light is emitted from a medium with a high refractive index to a medium with a low refractive index , Its refraction angle will be greater than the incident angle . therefore , If the incident angle is large enough , Total reflection occurs , That is, when the light touches the cladding, it will fold back to the core . The process repeats itself , Light also travels along optical fibers .

Unguided transmission media

1. Wireless transmission uses a wide range of frequencies ;
2. Shortwave communication mainly depends on the ionosphere , But the communication quality of short wave channel is very poor ;
3. Microwave propagation in space is mainly linear ： Ground microwave relay communication ; Satellite communications .

Channel multiplexing technology *

The role of reuse ： The data of multiple users are transmitted on one backbone without interference .

Frequency division multiplexing FDM And time division multiplexing TDM

All users occupy different bandwidth resources at the same time . Early mobile phones used frequency division multiplexing technology .

All users occupy the same bandwidth at different times .GSM Mobile phones use time division multiplexing technology .

FDM、TDM Explain whether it is multiplexed in frequency domain or time domain , It is not emphasized whether the multiplexed channel is used for multiple users or one user .

FDMA（ Frequency division multiple access ）、TDMA（ Time division multiple access ） It is emphasized that this multiplex channel can allow multiple users to access .

PCM system

PCM Pulse code modulation can convert analog signals into digital signals .

at present , Most long-distance trunk lines use time division multiplexing PCM Digital transmission mode .

Early digital transmission systems had many disadvantages ：

1. Rate standards are not uniform ; The rate system of multiplexing has two incompatible international standards ： North America and Japan T1 Speed and European E1 rate .
2. It's not synchronous transmission .

China adopts the European system , With E1 Is a group of degree ：E1 A time division multiplexing frame of （125$\mu s$） It is divided into 32 A time slot , Each slot bit 8bit. So every second there is 8000 frame , So the transmission rate is $8000\times 8\times 32=2.048Mbit/s$.

T1 The data rate of the primary group is $1.544Mb/s$.

Statistical time division multiplexing STDM

The defect of time division multiplexing ： It may cause a waste of line resources .

STDM The frame is not a fixed allocation slot , Instead, time slots are allocated dynamically on demand .

STDM	describe
advantage	Improve the utilization of the line
shortcoming	The time slots in statistical time division multiplexing are not allocated to a user , Therefore, there must also be the address information of the user in each time slot , Such statistical time division multiplexing must have some overhead

Wavelength division multiplexing WDM

Wavelength division multiplexing is the frequency division multiplexing of light .

Code division multiplexing CDM

When the code division multiplexing channel is shared by multiple users with different addresses , It's called code division multiple access CDMA.

Each user can use the same frequency band for communication at the same time . Because each user uses different code types specially selected , So there is no interference between users .

send out ：

1. Use CDMA Each of the stations is assigned a unique $m$ bit Chip sequence .
2. If a station wants to send bits 1, Then send your own chip sequence ; If you want to send 0, The binary inverse code of the chip sequence is sent .

EXAMPLE：

Chip sequence	1	0
00011011	00011011	11100100

For convenience , Generally will 0 Write it down as -1, take 1 Write it down as +1.

CDMA Another important feature of the system is that the chip sequences allocated to each station must not only be different , They must also be orthogonal to each other .

that $S\cdot S=1,S\cdot T=0$.

receive ：
X The signal received by the station is the sum of chip sequences sent by each station .

According to the above publicity and superposition principle , So the result of inner product is ： Signals from all other stations are filtered out , And the rest S Signal sent by station . If the result of inner product is 1, So what you send is 1, Instead of 0.

Broadband access technology

ADSL technology

ADSL Technology uses digital technology to transform the existing subscriber line of analog telephone ：

1. Although the frequency band of standard analog telephone signals is limited to 300～3400Hz Between , But the subscriber line itself can pass through more than 1MHz;
2. ADSL Technology leaves unused high-end spectrum for users to access the Internet .

HFC network

It is a residential broadband access network developed on the basis of the cable TV network with a wide coverage at present .

and ADSL The same way of thinking , As long as users connect their computers to cable modems, they can easily access the Internet .

advantage ： Than ADSL Higher data rates ;
shortcoming ： If there are many users , The actual Internet speed will be very low （ Fixed bandwidth ）.