1. What problems does the physical layer need to solve? What are the main characteristics of the physical layer?
(1) The physical layer should shield the physical equipment and transmission media as much as possible, and the different communication methods, so that the data link layer cannot feel these differences and only considers completing the protocols and services of this layer.
(2) Give its service users (data link layer) the ability to transmit and receive bit streams (generally serially transmitted bit streams) on a physical transmission medium. For this reason, the physical layer should solve the problem of physical connections. Establish, maintain and release issues.
(3) Uniquely identify data circuits between two adjacent systems.
Main features of the physical layer:
(1) Since many physical regulations or protocols have been developed before OSI, and in the field of data communication, these physical regulations have been adopted by many commercial equipment. In addition, the physical layer protocols cover a wide range, so so far Instead of formulating a new physical layer protocol based on the OSI abstract model, the existing physical protocols are used to define the physical layer as the mechanical, electrical, functional and procedural characteristics that describe the interface with the transmission media.
(2) Since there are many physical connection methods and many types of transmission media, the specific physical protocols are quite complex.
2. What is the difference between procedures and protocols?
In the early days of data communication, various rules used in communication were called "procedures." Later, computer networks with architectural structures began to use the term "protocol". The previous "procedures" were actually "protocols", but due to It is customary to sometimes still use the old name "procedures" for previously established procedures.
3. Try to give a model of the data communication system and explain the functions of its main components and construction
The source system includes the source point and transmitter, and the destination system includes the receiver and destination
Source point: The source point device generates the data to be transmitted. The origin point is also called the origin station.
Sender: Usually the data generated by the source point must be encoded by the sender before it can be transmitted in the transmission system.
Receiver: Receives the signal transmitted by the transmission system and converts it into information that can be processed by the destination device.
Endpoint: The endpoint device obtains the transmitted digital bit stream from the receiver. The end point is also called the destination station or destination.
4. Try to explain the following terms: data, signal, analog data, analog signal, baseband signal, bandpass signal, digital data, digital signal, symbol, simplex communication, half-duplex communication, full-duplex communication, serial transmission , parallel transmission.
Data: is the entity that carries information
Signal: is the electrical or electromagnetic representation of data
Analog data: Analog signals that carry information
Analog signal: a continuously changing signal
Baseband signal: The signal from the source. Data signals representing various text or image files output by a computer are all baseband signals.
Bandpass signal: After the baseband signal is modulated by the carrier, the frequency range of the signal is moved to a higher frequency band.
Numeric data: data whose values are discontinuous numeric values
Digital signal: a signal that takes on a limited number of discrete values
Symbol: represents a digital signal using a waveform in the time domain, representing basic waveforms of different discrete values.
Simplex communication: communication in one direction without interaction in the opposite direction
Half-duplex communication: That is, both parties in the communication can send information, but both parties cannot send at the same time (and of course cannot receive at the same time). This method of communication is that one party sends and the other party receives, and then vice versa after a period of time.
Full-duplex communication: both parties to the communication can send and receive information at the same time.
Serial transmission: Use a data line to transmit data, one bit at a time, and multiple bits need to be transmitted one after another.
Parallel transmission: using multiple data lines to transmit multiple bits at once.
5. What are the characteristics of the physical layer interface? What does it contain?
Mechanical characteristics:Specify the shape and size of the connector used in the interface, the number and arrangement of pins, and the fixing and locking devices
Electrical Characteristics:Specifies the range of voltages that appear on each line of the interface cable
Functional features:Indicate the meaning of a certain level of voltage appearing on a certain line
Process Characteristics: Specify the order of occurrence of various possible events for different functions
6. What factors limit the data transmission rate in the channel? Can the signal-to-noise ratio be increased arbitrarily? What is the significance of Shannon's formula in data communications? What is the difference between "bits/second" and "symbols/second"?
1. The data transmission rate in the channel is limited by factors such as bandwidth and signal-to-noise ratio.
2. The signal-to-noise ratio cannot be increased arbitrarily
3. The significance of Shannon's formula: 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 undifferentiated transmission.
4. The unit symbol transmission rate of the information transmission rate in bits per second is also called the modulation rate, wave rate or symbol rate. A symbol does not necessarily correspond to a bit.
Calculation formula for this chapter:
信噪比(dB)=10log10(S/N)(dB) //S为信号的平均功率，N为噪声的平均功率。
香农公式：C=Wlog2(1+S/N)(bit/s) //C为信道的极限信息传输速率，W是信道带宽
M进制的码元，码元宽度为T秒：C=1/T×log2(M)
无噪:带宽为BHz。 C=2Blog2(M)
有噪:带宽为BHz。 C=Blog2(1+S/N)
最高码元速率R，码元振幅等级n。 C=Rlog2(n)
频率＝传播速率/波长
频带宽度=最高频率-最低频率
7. Assume that the maximum code element rate of a channel limited by Nysko's criterion is 20,000 symbols/second. If amplitude modulation is used and the amplitude of the symbol is divided into 16 different levels for transmission, what high data rate (b/s) can be obtained? (You can refer to the formula above question 7)
C = RLog2(16) = 20000b/s*4 = 80kb/s
8. Assume that a 3KHz bandwidth telephone channel is used to transmit 64kb/s data (error-free transmission). How high should the signal-to-noise ratio of this channel be (expressed in ratios and decibels respectively? This result shows What's the problem?) (You can refer to the formula above question 7)
C = WLog2(1 + S/N)
Bring in the question information to obtain, S/N = 64.2dB (signal-to-noise ratio)
9. Use Shannon's formula to calculate. Assume that the channel bandwidth is 3100Hz and the maximum channel transmission rate is 35Kb/s. If you want to increase the maximum channel transmission rate by 60%, how many times should the signal-to-noise ratio S/N be increased? If the signal-to-noise ratio S/N is increased to ten times based on the calculation just now, can the maximum information rate be increased by another 20%? (You can refer to the formula above question 7)
From Shannon's theorem C = WLog2(1 + S/N)
The maximum channel transmission rate becomes 35*(1 + 0.6) = 56Kb/s
Then the S/N is expanded to 100 times the original
C1 = WLog2(1 + 10S/N)
At this time, the maximum information rate increased by 18.5%
10.What are the commonly used transmission media? What are the characteristics of each?
Twisted pair:Twisted pair is divided into shielded twisted pair and unshielded twisted pair. It can transmit analog signals or digital signals, with an effective bandwidth of 250kHz. Generally used as telephone lines to transmit sound signals. Twisted pair cables are susceptible to interference from external high-frequency electromagnetic waves and have a high bit error rate.
Coaxial cable:Coaxial cable is divided into baseband coaxial cable and broadband coaxial cable. Due to itshigh bandwidth (up to 300~400Hz) , low bit error rate, high performance-price ratio.
Optical fiber:Optical fiber uses optical fiber carriers to propagate optical signals using the principle of full reverse direction of light. Its advantages are small diameter and light weight: propagation frequency band, large communication capacity: good anti-lightning and electromagnetic interference performance, five crosstalk interference, good confidentiality, low bit error rate
Radio and microwave communications:
Radio microwave communication is divided into terrestrial microwave relay communication and satellite communication. Its main advantages arehigh frequency, wide frequency range, and large communication channel capacity;the signal is less susceptible to industrial interference. Small, high-quality transmission, relatively stable communication; not affected by geographical environment, low construction investment, quick results. The disadvantage is that terrestrial microwave relay communication propagates in a straight line in space, and the transmission distance is limited, generally only 50km, with poor concealment and confidentiality; although satellite communication The communication distance is long and the communication cost has nothing to do with the communication distance, but the propagation delay is large.
11. Assume that the attenuation of a twisted pair is 0.7dB/km (at 1 kHz). If an attenuation of 20dB is allowed, how long is the working distance of a link using this twisted pair? If the working distance of twisted pairs is to be increased to 100 kilometers, how much should the attenuation be reduced?
Working distance of twisted pair: S = 20dB / 0.7dB = 28.6km
100km = 20dB / weakening, then weakening = 0.2dB/km
13. Why use channel multiplexing technology? What are the commonly used channel multiplexing technologies?
Reason: In order to maximize channel utilization through shared channels
Common channel multiplexing techniques:
Frequency Division Multiplexing (FDM):Each signal of frequency division multiplexing occupies different bandwidth resources at the same time
Time division multiplexing (TDM):All users of time division multiplexing occupy the same frequency bandwidth at different times
Wavelength division multiplexing (WDM):Wavelength division multiplexing is frequency division multiplexing of light
Code Division Multiplexing (CDM):Code Division Multiplexing is another method of sharing channels. The Code Division Multiplexing channel is shared by multiple devices with different addresses a>
14. Try to write the full text of the following English abbreviations and give a simple explanation. FDM, TDM, STDM, WDM, DWDM, CDMA, SONET, SDH, STM-1, OC-48.
FDM:Frequency Division Multiplexing, frequency division multiplexing technology, is a method of allocating multiple signals to different frequency bandwidths Technology for transmission.
TDM:Time Division Multiplexing, time division multiplexing technology, is a method of assigning multiple signals to different time periods< /span> technology for transmission.
STDM:Statistical Time Division Multiplexing, statistical time division multiplexing technology, is a dynamic allocation of time slices according to the needs of different signals Technology for transmission.
WDM:Wavelength Division Multiplexing, wavelength division multiplexing technology, is a method of assigning multiple signals to different wavelengths< /span>Technology for transmission
DWDM:Dense Wavelength Division Multiplexing, dense wavelength division multiplexing technology, is a wavelength division multiplexing technology that achieves higher bandwidth by allocating wavelengths more closely. Use technology.
CDMA:Code Division Multiple Access, code division multiple access technology, is a method that encodesdifferent signals into a unique code< /span> to realize the technology of simultaneous transmission of multiple signals.
SONET:Synchronous Optical Network, a synchronous optical network, is a kind oftransmitting data through optical fiber Web Standards
SDH:Synchronous Digital Hierarchy, synchronous digital layering technology, is a kind oftransmitting data through digital signalsInternet standards
STM - 1:Synchronous Transport Module level-1, Synchronous Transport Module level-1, yes One of the transmission rate standards in SDH networks is 155.52 Mbps
OC - 48: Optical Carrier level 48, optical fiber transmission rate 48, is one of the transmission rate standards in SONET networks, which is 2.5 Gbps.
15. Why can code division multiple access (CDMA) enable all users to communicate using the same frequency band at the same time without interfering with each other? What are the advantages and disadvantages of this reuse method?
Each user uses different patterns that are specially selected to be orthogonal to each other so that they do not interfere with each other. The signal sent by this system hasstrong anti-interference ability. Its spectrum is similar to white noise and is difficult to be discovered by the enemy. Occupies a large amount of bandwidth.
16. A total of 4 stations perform code division multiple access communication. The chip sequences of the four stations are A: (-1-1-1+1+1-1+1+1) B: (-1-1+1-1+1+1+1-1) C: (-1+1-1+1+1+1-1-1) D: (-1+1- 1-1-1-1+1-1) Now such chip sequence S is received: (-1+1-3+1-1-3+1+1). Which station sent the data? Is the station sending data sending 0 or 1?
S•A=（＋1－1＋3＋1－1＋3+1+1）／8=1, A sends 1
S•B=（＋1－1－3－1－ 1-3+1-1)/8=-1, B sends 0
S•C= (+1+1+3+1-1-3-1-1)/8=0, C sends nothing
S·D=（＋1＋1＋3－1＋1＋3+1－1）／8=1, D sends 1
17. Try to compare the advantages and disadvantages of xDSL, HFC and FTTx access technologies?
xDSL technology uses digital technology to transform existing analog telephone subscriber lines so that it can carry broadband services. Low cost and easy to implement, but the bandwidth and quality vary greatly. The biggest advantage of the HFC network is that it has a very wide frequency band and can utilize the cable TV network that already has considerable coverage. It requires considerable funds and time to transform the existing 450 MHz one-way transmission cable television network into a 750 MHz two-way transmission HFC network. FTTx (Fiber to...) The letter x here can mean different things. It can provide the best bandwidth and quality, but the line and engineering costs are too high at this stage.
18. Why in ASDL technology, the transmission rate can be as high as several megabits per second in a bandwidth of less than 1MHz?
Relying on advanced DMT coding, frequency division multi-carrier parallel transmission, so that transmitting one symbol per second is equivalent to transmitting multiple bits
19.What are EPON and GPON?
EPON (Ethernet Passive Optical Network) is a new type of optical fiber access network technology, which adopts point-to-multipoint structure, Passive optical fiber transmission provides a variety of services on top of Ethernet. It uses PON technology at the physical layer, uses the Ethernet protocol at the link layer, and uses the PON topology to implement Ethernet access. Therefore, it combines the advantages of PON technology and Ethernet technology: low cost; high bandwidth; strong scalability, flexible and fast service reorganization; compatibility with existing Ethernet; convenient Managementetc.
GPON (Gigabit-Capable PON) technology is the latest generation of broadband passive optical integrated access standard based on the ITU-TG.984.x standard, with< a i=2>High bandwidth, high efficiency, large coverage, rich user interfaces and many other advantages are regarded by most operators as an ideal for realizing broadband and comprehensive transformation of access network services. technology.