2-01 What problems does the physical layer need to solve? What are the main characteristics of the physical layer?
1) Problems to be solved:
The physical layer should shield the differences in transmission media and communication means, so that the data link layer above the physical layer cannot feel these differences, so that the data link layer only needs to consider how to complete the protocols and services of this layer, and does not have to consider the network Specific transmission media and means of communication.
2) Main features
① Before OSI, many physical regulations or protocols have been formulated, and in the field of data communication, these
physical regulations have been adopted by many commercialized equipment. In addition, the physical layer protocol covers a wide range, so it has not been adopted so far. The abstract model of OSI
formulates a new set of physical layer protocols, but follows the existing physical procedures, and defines the physical layer as describing the mechanical, air, functional and process characteristics of the interface with the transmission medium.
②Because there are many ways of physical connection and there are many types of transmission media, the specific physical protocol is quite complicated.
2-02 What is the difference between a procedure and an agreement?
In computer networking, procedures are protocols. For example, the protocol of the physical layer is also often referred to as the procedure of the physical layer, but the term "protocol" was used first before the term "protocol" appeared.
2-03 Try to give a model of a data communication system and explain the functions of its main components.
As shown in the figure below, a data communication system can be divided into three parts, namely, source system (or sender, sender), transmission system (or transmission network) and destination system (or receiver, receiver).
The source system generally consists of two parts:
Origin:
The source device generates the data to be transmitted, for example, input Chinese characters from the keyboard of the computer, and the computer generates the output digital bit stream. The source is also called the source station or the source.
Transmitter:
Usually the digital bit stream generated by the source can be transmitted in the transmission system after being encoded by the transmitter, which is typically a modulator, and many computers now use built-in modems.
The target system generally also includes two parts:
receiver:
Receive the signal transmitted by the transmission system and convert it into information that can be processed by the destination device. A typical receiver is a demodulator.
end:
The endpoint device takes the transmitted digital bit stream from the receiver and outputs the information. The terminal is also called the destination station or the destination.
Transmission system:
It is responsible for transmitting data from the source system to the density system. The transmission system can be a simple transmission line or a complex network system connected between the source system and the destination system.
2-04 Try to explain the following nouns: 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.
① Data: The purpose of communication is to transmit messages. Voice, text, image, video, etc. are all messages. Data is the entity that conveys messages. Data is information expressed in a specific way, usually a meaningful symbol sequence. The representation of this information can be processed by computers or humans.
②Signal: A signal is an electrical or electromagnetic representation of data
③Analog data: Analog data is also called analog quantity. Compared with digital quantity, it refers to a variable or value whose value range is continuous. Analog data refers to continuous values generated in a certain interval, such as sound, image, temperature and pressure, etc. Analog data is typically represented by an analog signal, such as a series of varying electromagnetic waves (as in radio and television broadcasts), or a voltage signal (as in audio voltage signals in telephone transmissions).
④Analog signal: also called continuous signal—the value of the parameter representing the message is continuous. For example, the analog signal is transmitted on the subscriber line between the modem in the user's home and the telephone end office.
⑤Baseband signal: The signal from the source is called baseband signal (basic frequency band signal). Data signals representing various text or image files output by a computer are all baseband signals.
⑥Band-pass signal: The signal after carrier modulation is called a band-pass signal (that is, it can only pass through the channel within a certain frequency range).
⑦Digital data: It is also called digital quantity in data communication. Compared with analog quantity, it refers to a variable or value whose value range is discrete.
⑧Digital signal: also known as discrete signal - the value of the parameter representing the message is discrete. For example, digital signals are sent from the computer in the user's home to the modem or on the trunk line of the telephone network.
⑨Symbol: When using time domain (or simply referred to as time domain) waveforms to represent digital signals, the basic waveforms representing different discrete values are called symbols. When using binary encoding, there are only two different symbols, one representing the 0 state and the other capable of representing the 1 state.
⑩ Simplex communication: Also known as one-way communication, that is, there can only be communication in one direction without interaction in the opposite direction. Radio broadcasts or priority telecasts and television broadcasts are of this type.
⑪Half-duplex communication: also known as two-way alternate communication, both parties can send and receive information to each other, but both parties cannot send and receive at the same time.
⑫Full-duplex communication: also known as two-way simultaneous communication, that is, both communicating parties can send and receive information at the same time.
⑬Serial transmission: The method in which data is transmitted sequentially on a single bit-wide transmission line, 1 bit by 1 bit, is called serial communication.
⑭Parallel transmission: refers to the simultaneous transmission of data on multiple parallel channels in groups. It is the transmission of multiple data bits between devices at the same time during the transmission. It is commonly used to form a character. Several binary codes are transmitted on several parallel channels at the same time.
2-05 What are the characteristics of the physical layer interface? What does each contain?
1) Mechanical characteristics: specify the shape or size of the connector used for the interface, the number and arrangement of pins, fixing and locking devices, etc. Common connectors of various specifications have strict standardization regulations.
2) Electrical characteristics: Indicate the range of voltage appearing on each line of the interface cable.
3) Functional characteristics: Indicate the significance of a certain level of voltage appearing on a certain line.
4) Process characteristics: Indicate the sequence of occurrence of various possible events for different functions.
2-06 What factors limit the transmission rate of data in the channel? Can the signal-to-noise ratio be increased arbitrarily? What is the significance of Shannon formula in data communication? What is the difference between "bits/s" and "symbols/s"?
Conceptually, there are two factors that limit the transmission rate of symbols on the channel:
①The frequency range that the channel can pass:
The frequency range that a specific channel can pass is always limited, and many high-frequency components in the signal often cannot pass through the channel. The high-frequency components in the signal will be attenuated during transmission, making it difficult to distinguish high-frequency and low-frequency components, and the boundaries between each symbol become blurred, which is called intersymbol crosstalk. Nay's criterion says: In a low-communication channel with a bandwidth of W (Hz), if the influence of noise is not considered, the highest rate of symbol transmission is 2W (symbol/second). Exceeding this upper limit, there will be severe intersymbol interference, making the information unrecognizable.
②Signal-to-noise ratio: Noise exists in all electronic devices and communication channels. But the influence of noise is relative, if the signal is relatively strong, then the influence of noise is relatively small. Therefore, the signal-to-noise ratio is very important-the ratio of the average power of the signal to the average power of the noise, often recorded as S/N, but generally decibels are used as the unit of measurement (dB):
The meaning of Shannon formula in data communication is:
In 1984, Shannon, the founder of information theory, overturned the famous Shannon formula. Shannon's formula points out that the limit information transmission rate C of the channel is:
The name of the Shannon formula table, the greater the bandwidth of the channel or the signal-to-noise ratio in the channel, the higher the limit rate of information. The significance of this formula is that as long as the transmission rate of information is lower than the limit information transmission rate of the channel, there must be some way to achieve error-free transmission. But Shannon did not give a specific method.
"Bit/second" and "symbol/second": According to different encodings, one symbol can correspond to multiple bits, and one bit can also correspond to multiple symbols. Of course, one symbol can also correspond to one bit, in this case, they are equal in value.
2-07 Assume that the maximum symbol rate of a certain channel limited by Nysser criterion is 20000 symbols/second. If the amplitude modulation is used, and the amplitude of the symbol is divided into 16 different levels for transmission, how high a data rate (bit/s) can be obtained?
16 different levels of amplitude require 4 bits (2^4) to represent, so: 4*20000 = 80000 bit/s
2-08 Assume that a 3 kHz bandwidth telephone channel is used to transmit 64 kbit/s data (error-free transmission), how high should this channel have a signal-to-noise ratio (expressed in ratio and decibels respectively)? What does this result indicate?
1) Expressed by ratio: C = Wlog2(S/N) = 3000Hz*log2(S/N+1) = 64 x 10^3 bit/s
S/N = 2^(64/3)-1;
2) Expressed in decibels: 10log10(S/N) = 10log10(2^(64/3))(dB)
3) Result description: at 64kbit/s transmission rate, the signal-to-noise ratio of the channel is very high
2-09 Use Shannon's formula to calculate the following, assuming that the channel bandwidth is 3100Hz and the maximum information transmission rate is 35kbit/s, then if the maximum information transmission rate is to be increased 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 10 times on the basis of the calculation just now, can the maximum information transmission rate be increased by another 20%?
According to Shannon's formula: 35kbit/s = 3100Hz*log2(S/N+1), get S/N = 2503.5
35*(1+0.6)kbit/s = 3100Hz*log(S/N+1), get S/N = 274131.9
Obtained, 274131.9/2503.5 = 109.0, therefore, S/N should be increased to 109 times
According to Shannon's formula: 3100Hz*log2(10*274131.9+1) /56000 = 1.184, therefore, an increase of 18%, not 20%
2-10 What kinds of transmission media are commonly used? What are the characteristics of each?
Transmission media is divided into guided transmission media and non-guided transmission media:
Guided transmission media:
1) Twisted pair:
It is the oldest and most commonly used transmission medium. Arrange two mutually insulated copper wires side by side, and then twist them in a regular way to form a twisted pair. The current Ethernet (mainstream computer local area network) is basically connected using various types of twisted pair cables. In order to improve the anti-electromagnetic interference ability of the twisted pair and reduce the crosstalk between different twisted pairs in the cable, methods of increasing the degree of twisting of the twisted pair and increasing electromagnetic shielding can be used. A twisted pair with a higher degree of twisting can transmit data at a higher data rate. No matter what kind of twisted pair, the attenuation increases with the increase of frequency. Using thicker wire reduces attenuation but increases the weight and price of the wire.
2) Coaxial cable:
It is composed of inner conductor copper core wire (single-strand solid wire or multi-strand wire), insulation layer, mesh braided outer conductor shielding layer (also can be single-strand) and insulating protective sheath layer. Coaxial cables have good anti-interference characteristics and are widely used to transmit data at a higher rate. The bandwidth of coax depends on the quality of the cable.
In the early days of LAN development, coaxial cables were widely used as the transmission medium, but now twisted-pair cables are basically used as the transmission medium.
3) Optical cable:
Optical fiber communication uses optical fiber (referred to as optical fiber) to transmit light pulses for communication. The presence of light pulses is equivalent to 1, and the absence of light pulses is equivalent to 0. Since the frequency of visible light is very high, about 10^8MHz, the transmission bandwidth of an optical fiber communication system is far greater than the bandwidth of other various transmission media at present. Modern production technology can produce ultra-low loss optical fiber, so that the light can be transmitted for several kilometers in the core without attenuation.
Optical fiber not only has the advantage of very large communication capacity, but also
①Low transmission loss, long relay distance, especially economical for long-distance transmission
②Good anti-lightning and electromagnetic interference performance. This is especially important in environments with high current pulse interference
③ No crosstalk interference, good confidentiality, and it is not easy to be eavesdropped or intercept data
④Small size and light weight, which is especially beneficial when the existing cable ducts are already congested.
Unguided transmission media:
Use frequency band, short-wave communication, microwave communication (there are two main ways of traditional microwave communication: ground microwave relay communication and satellite communication).
Features slightly
2-11 Suppose there is a twisted pair whose attenuation is 0.7dB/km (at 1kHz), if 20dB attenuation is allowed, how long is the working distance of the link using this twisted pair? If the working distance of this twisted pair is to be increased to 100 kilometers, how much should the attenuation be reduced?
Working distance: L = 20dB /0.7dB/km = 28.6 km
The attenuation is reduced to: 20dB/100km = 0.2 dB/km
2-12 Try to calculate the frequency bandwidth of broadcasting operating between 1200 and 1400 nm and between 1400 and 1600 nm. Assume that light travels at a rate of 2 x 10^8 m/s in a ray?
frequency = speed of light/wavelength
Bandwidth = frequency corresponding to short wavelength - frequency corresponding to long wavelength
Therefore, the bandwidth of the broadcast between 1200~1400nm: 2 x 10^8 m/s / 1200 x 10^(-9)m - 2x10^8 m/s / 1400 x 10(-9)m = 2.381 x 10^13 Hz = 23.81 THz
Broadcast frequency bandwidth between 1400~1600nm: 2 x 10^8 m/s /1400 x10^(-9)m - 2 x 10^8 m/s / 1400 x 10^(-9)m = 1.786 x 10^13 Hz = 17.86 THz
2-13 Why use channel multiplexing technology? What are the commonly used channel multiplexing techniques?
In general, the communication channel bandwidth is far greater than the bandwidth required by users. Using channel multiplexing technology can improve channel utilization, share channel resources, and reduce network costs. Commonly used channel multiplexing techniques include frequency division multiplexing, time division multiplexing, statistical time division multiplexing, wavelength division multiplexing, and code division multiplexing.
2-14 Try to write the full names of the following English abbreviations, and give a simple explanation.
FDM，FDMA，TDM，TDMA，STDM，WDM，DWDM，CDMA，SONET，SDH，STM-1，OC-48。
1) FDM (Frequency Division Multiplexing): Frequency Division Multiplexing: Users can occupy different bandwidth resources at the same time.
2) FDMA (Frequency Division Multiple Access): Frequency Division Multiple Access: Using frequency division multiplexing technology, N users can each use a frequency band, or allow more users to use the N frequency bands in turn. This method is called Frequency division multiple access, referred to as frequency division multiple access.
3) TDM (Time Division Multiplexing): Time Division Multiplexing: Users can occupy the same bandwidth resources at different times.
4) TDMA (Time Division Multiple Access): Time Division Multiple Access: Using time division multiplexing technology, each of N users can use one time slot, or more users can use these N time slots in turn. This method is called time division multiple access, or time division multiple access for short.
5) STDM (Statistic TDM): Statistical time division multiplexing: it is an improved time division multiplexing. Multiplexed data is transmitted using STDM frames. But the number of time slots of each STDM frame is less than the number of users connected to the concentrator. Each user sends data to the input buffer of the concentrator at any time, and then the concentrator scans the input buffer in sequence, puts the input data in the buffer into the STDM frame, and skips the buffer without data. When it's full, send it out. Therefore, STDM does not allocate time slots fixedly, but dynamically allocates time slots as needed. Therefore, statistical time division multiplexing can improve the utilization rate of lines.
6) WDM (Wavelength Division Multiplexing): Wavelength Division Multiplexing: It is the frequency division multiplexing of light. Use one optical fiber to simultaneously transmit multiple optical carrier signals with similar frequencies but different ones.
7) DWDM (Dense Wavelength Division Multiplexing): Dense Wavelength Division Multiplexing: With the development of random technology, the number of optical carrier signals multiplexed on one optical fiber is increasing, and now it can be multiplexed on one optical fiber Dozens or more optical carrier signals, so dense wavelength division multiplexing is used.
8) CDMA (Code Division Multiple Access): Code Division Multiple Access: When the code division multiplexing channel is shared by multiple users with different addresses, it is called code division multiple access.
9) SONET (Synchronous Optical Network): Synchronous Optical Network: It is the physical layer standard first proposed by Bellcore in the mid-1980s using optical fiber transmission. SONET defines synchronous and isochronous (time-sensitive data, such as real-time video) information. transmission.
10) SDH (Synchronous Digital Hierarchy): Synchronous digital series: SDH is the digital synchronous transmission system, and SDH is a transmission system protocol. SDH standardizes the frame structure, multiplexing mode, transmission rate level, interface code type and other characteristics of digital signals. Generally, SDH and SONET can be considered synonymous .
11) STM-1: It is a basic transmission module of SDH, with a transmission rate of 155.52Mbps.
12) OC-48: It is a speed standard of SONET, and the corresponding value is - 2.5Gbit/s
2-15 Why can code division multiple access CDMA enable all users to use the same frequency band to communicate at the same time without interfering with each other? What are the pros and cons of this approach to reuse?
Because each user uses a different code pattern that has been specially selected, there is no interference between users.
advantage:
①Strong anti-interference ability; its spectrum is similar to white noise, and it is not easy to be discovered by the enemy.
② It can improve the voice quality of communication and the reliability of data transmission.
③ Increase the capacity of the communication system.
shortcoming:
①Occupy a large bandwidth
2-16 There are four stations for code division multiple access CDMA 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 receive such a chip sequence: (-1 +1 -3 +1 -1 -3 +1 +1). Ask which station sent the data, did the station that sent the data send 0 or 1?
Let the chip sequence be S
For A: A*S = (1 -1 3 1 -1 3 1 1)/8 = 1, so A sends 1
For B: B*S = (1 -1 -3 -1 -1 -3 1 -1)/8 = -1, so B sends 0
For C: C*S = (1 1 3 1 -1 -3 -1 -1)/8 = 0, so C does not send data
For D: D*S = (1 1 3 -1 1 3 1 -1)/8 = 1, so D sends 1
2-17 Try to compare the advantages and disadvantages of ADSL, HFC and FTTx access technologies?
The asymmetrical digital subscriber line ADSL technology uses digital technology to transform the existing analog telephone subscriber line so that it can carry broadband digital services. The biggest advantage of ADSL is that it can use the subscriber lines (copper wires) in the existing telephone network without rewiring. There are many old buildings where telephone lines are pre-existing. However, if the optical fiber is re-laid, it will often cause some damage to the original building. From the consideration of damaging the original building as much as possible, it is very appropriate to use ADSL for broadband access. The disadvantage is that the signal transmission distance is short, the signal attenuation is large, the signal transmission is unstable, easy to be interfered, and the failure rate is high.
Fiber-optic-coaxial hybrid network (HFC network) is a residential broadband access network developed on the basis of the current wide-coverage cable TV network. In addition to transmitting TV programs, it can also provide telephone, data and other broadband interactive services. business. The advantages are wide coverage, high bandwidth, and high transmission rate. The disadvantage is that when using HFC cable modems, the highest data rate enjoyed by users in the coaxial cable section is uncertain, because the data rate that a certain user can enjoy The size depends on how many users are currently transmitting data on this length of cable. Cable TV operators often advertise higher data rates than ADSL (for example, 10Mbit/s or even 30Mbit/s) when surfing the Internet via cable modems, but this may only be true for very few users. However, if a large number of users (for example, hundreds) surf the Internet at the same time, the actual Internet access rate of each user may be unbearably low.
A variety of broadband fiber access methods, called FTTx. Here the letter x can represent different fiber access locations. In fact, FTTx is the place where the photoelectric conversion is performed, extending from the user's home to a place a certain distance from the user's door. Fiber to the home FTTH should be the best choice. The so-called fiber-to-the-home is to lay optical fiber to the user's home. Only after the optical fiber enters the user's home, the optical signal is converted into an electrical signal, so that the user can obtain the highest Internet speed. But fiber-to-the-home FTTH has two problems: first, the current price is not cheap enough; second, ordinary home users do not have such a high data rate demand. To watch video programs smoothly on the Internet, a data rate of several megabits per second is sufficient, and it is not necessary to use a data rate of 100 Mbit/s or higher.
2-18 In ADSL technology, why can the transfer rate be as high as several megabits per second in less than 1 MHz bandwidth?
Using advanced coding technology, each symbol carries multiple bits, that is, transmitting one symbol per second is equivalent to transmitting multiple bits per second.
2-19 What are EPON and GPON?
There are many types of passive optical network PON, but the most popular are the following two, each with its own advantages and disadvantages.
One is the Ethernet passive optical network EPON, which has formed the IEEE standard 802.3ah in June 2004, and the newer version is 802.3ah-2008. EPON uses the Ethernet protocol at the link layer. The Ethernet access is realized by using the topology of the PON. The advantages of EPON are: good compatibility with existing Ethernet, low cost, strong scalability, and convenient management.
The other is gigabit passive optical network GPON, whose standard is ITU-T G.984 approved by ITU in January 2003. It has been updated many times since then, and the latest one is G.984.7 in 2010. GPON adopts the general encapsulation method GEM, which can carry multiple services and provide a potential broadband optical fiber access technology for various types of services.