It is recommended to pay attention to the collection and continue to update...
table of Contents
1. Data communication method
Communication direction: simplex (TV), full-duplex (mobile phone), half-duplex (intercom).
Transmission mode: asynchronous transmission (with start bit and stop bit mark, suitable for long distance, small data volume, start-stop type), synchronous transmission (one or more SYN, short distance, large data volume, synchronous type).
2. Data exchange method
Circuit switching:
advantages: exclusive, real-time, suitable for transmitting a large amount of data;
disadvantages: a physical connection needs to be established, and the utilization rate is low, such as the early telephone system.
Message exchange:
Advantages: No dedicated channel is required, line utilization is high, and store and forward nodes can check and correct errors.
Disadvantage: There is a communication delay. Such as logistics packages.
Packet switching:
advantages: higher utilization, optional path, data rate conversion, priority support.
Disadvantages: delay and high overhead. For example, the post office sends a letter. It can be divided into datagram and virtual circuit.
Packet datagram: one-way transmission, connectionless. Rupu Communications.
Grouped virtual circuit: interactive, logical connection. Such as registered mail.
3. Multiplexing technology
Frequency division multiplexing FDM: different frequencies, sub-channel isolation frequency bands to prevent crosstalk, such as CATV, WIFI.
Time-division multiplexing TDM: different time, occupied by turns, such as pistols, mobile phones. Divided into synchronization time and minutes T1, E1; statistical time and minutes, such as ATM.
WDM: Different wavelengths, such as optical fiber.
4. Digital Transmission Standard
T1 standard: 1.544Mbps, 125us=8000 times. 1.544Mbps=[24*(7+1)+1]*8000. T2=4T1 T3=7T2 T4=6T3 (United States and Japan)
E1 standard: 2.048Mbps, 125us=8000 times. 2.048Mbps=[32*(7+1)]*8000. CH0 and CH16 control signaling, 30 voice data. E2, E3, E4 formula: E444 (China and Europe) E2=4E1, E3=4E2, E4=4E3.
SONET standard (United States) and SDH standard (international): used in optical fiber networks. 155.520.
5. Data error detection and correction
Error detection code: For parity check, the number of 1s is odd/even. Can only detect errors but not correct them. Mobile communications are widely adopted.
Hamming code: Add a redundant check bit k after the data bit m to form information m+k, then one bit error can be corrected when m+k<2^k-1 is satisfied.
Code distance d: Also called Hamming distance, the smallest number of bits (number of bits) that are different between two code words.
How many errors can be detected: <=d-1.
How many errors can be corrected: <d/2.
CRC code: Redundant cyclic check code, a cyclic code, through cyclic shift to achieve error detection, easy to implement in hardware, and widely used in local area networks. Can only detect errors but not correct them.
Calculate the CRC check code: polynomial division, no carry addition. If the generator polynomial is G(X)=x^4+x+1 and the information codeword is 10111, find the CRC check code.
- Write divisor
- Write dividend
- Modulo 2 calculation