The second chapter, data communications infrastructure
1. Nyquist theorem and Shannon
[Analysis] test method Nyquist theorem is ideally incense data rate requirements in the agricultural theory is subject to noise interference, the data rate request
Points [analysis]
To see whether the reference to the title or the like noise SNR Image
If the reference noise or signal to noise ratio, it is necessary to use the Shannon $ C = Wlog_2 (1+ \ cfrac {S} {N}) $
Wherein the signal to noise ratio to use to obtain the equation: $ dB = 10lg (\ cfrac {S} {N}) $, the problem will be given in decibels
It does not mention the use of the Nyquist theorem $ R = 2Wlog_2N $
Remarks [coaching]
Shannon theory, is generally given in decibels 30dB, then the corresponding signal to noise ratio $ \ cfrac {S} {N} $ to 1000.
Then the formula for the Shannon theory is C = Wlog $ 21001 $ ≈ $ Wlog 21024 10W $ $ $ =
2. Modulation
Test method [analysis]
This test method is a test center give waveforms corresponding allow you to distinguish what kind of technology is common modulation. Or name given by a modulation technique, which requires a clear number of types of bits with the symbol
Points [analysis]
1, a common modulation techniques familiar waveform diagram ASK, FSK, PSK
2, the number of known symbols and bit number of the type commonly owned modulation techniques, such as ASK, FSK, PSK symbols the number of types is two, the number of bits is 1.4DPSK, QPSK symbols the number of types is four, the number of bits is 2
Remarks [coaching]
Memory, familiar with points
3. Coding
Test method [analysis]
Test often present test center is 1. NRZ (NRZ), Manchester encoding and differential Manchester encoding and 2. The coding scheme corresponding to the coding efficiency and usage scenarios
Points [analysis]
1, the coding technique, the need to know the sampling, quantization and coding process 3
2, NRZ coding features: a representative signal inversion level 1, level 0 indicates no change, and represents the completion of one symbol, without the voltage back to 0.
Another definition is not NRZ: 1 indicates a high level, a low level represents 0.
3, Manchester coding features: Low voltage represents 1, otherwise 0. The course represents definitions may be reversed
4, differential Manchester encoding features: Manchester encoding and the like in the middle of each bit time interval, a signal transition occurs. The difference is that at the beginning of each time interval. 0 enable signal transitions at the beginning of the time interval. And a signal will retain its previous value interval tail. Thus, depending on the initial value signal, the low level 0 will jump from the high level signal, or a jump from LOW to HIGH. Analyzing the order from right to left
5, Manchester coding efficiency and differential Manchester code is 50%, 4B / 5B and 8B / 10B encoding efficiency 80%
6, Manchester codes to classical 10M Ethernet, 4B / 5B applied to 100Base-FX, 100Base-TX, FDDI, 8B / 10B applied Gigabit (fiber) ethernet
Remarks [coaching]
Memory knowledge, carried out by the graphics memory combined concept
4. Reuse
Test method [analysis]
Workers in the net test, the test sites often test sites in E1 and T1 carrier carriers. But recommendations also need to become familiar with the multiplexing of expression and usage scenarios
Points [analysis]
1, the carrier rate is 2.048Mbps E1, a total of 32 sub-channels (rate 64kbps), wherein two control channels CH0 to CH16 position
Rate 2, T1 carrier is 1.544Mbps, a total of 24 subchannels
Remarks [coaching]
Memory, familiar with points
The error control - parity
Test method [analysis]
This is the test center test method involves the concept of parity
Points [analysis]
1, the configuration of the error detection code: = error detection code check field information field +
2, information field and a check field, a number of odd parity is an odd number, the number 1 is the even parity is an even number
3, parity is error detection code, not only error detection correction
Remarks [coaching]
General information will be given the title field, and given the need for odd or even parity enable candidates to determine the checksum field fill 0 or 1
6. The error control - Hamming parity
Test method [analysis]
This test method is a test center concept involves calculating the Hamming check
Points [analysis]
1, m + k + 1 <= 2k, m represents the number of data bits, the number of bits k check bits, the title will be given the value of m, so find the value of k
2, each data bit is determined by the positional relationship of check bits to verify
3, the parity bit is placed on the power of position 2, which is on the first place 1,2,4,8,16 ......
4, for example, bit 3 is the data bit, the third bit (3) is represented by binary 0011, the third bit of data is 2 bits of parity check bits.
5, Hamming code is an error correction code, not only error correction, error detection can also
Remarks [coaching]
Hamming parity slightly more general concept questions, problems arise frequency calculation is not so high. Key to remember the formula is m + k + 1 <= 2k
7. Cyclic Redundancy Check error control -CRC
Test method [analysis]
This test method is a test center concept relates to a cyclic redundancy check CRC calculation and
Points [analysis]
1, to calculate the CRC checksum, CRC polynomial generated according to need, the title will be given. For example: an original packet is 11001010101, which is a generator polynomial X4 + X3 + X + 1.
2, the maximum power of the generator polynomial, such as the example is 4, then 0 in the original 4 up message back packets to obtain new data.
3, corresponding to the divisor polynomial is obtained by generating, as in the example is 11011
4, the last two of 11011 modulo division with the new data, obtained after the replacement remainder four new data 0, i.e., obtain the CRC
5, CRC checksum error detection code is only error detection without correction
6, Ethernet checksum using CRC is cyclic redundancy check
Remarks [coaching]
NM exam not yet calculations involving only through the respective test concept, i.e. point analysis Article