1. Communication foundation
Data : The entity that transmits the information
Signal : The electrical or electromagnetic representation of the data (the way the data exists during transmission)
Analog data/signal: continuous
Digital data/signal: discrete
Unit of measurement of digital signal in digital communication: code element
- Symbol : A signal waveform with a fixed duration (the duration is called the symbol width) is used to represent a bit of k-ary system. A symbol can carry n bits of information, which is called a symbol of n baud rate
Data communication system: source, channel and sink
Data transfer rate/data rate:
- Symbol transmission rate: the number of symbols transmitted by the communication system per second, in Baud/s
- Information transmission rate: the number of bits transmitted by the communication system per second, in baud (bit/s)
Bandwidth originally refers to the bandwidth unit of the signal (Hz), and sometimes also refers to the highest data rate (b/s)
Nyquist's theorem / Nyquist criterion:-ideal low communication channel limit symbol transmission rate
- The ultimate symbol transmission rate under the ideal low communication channel = 2W (Baud/s) W is the bandwidth (Hz) of the ideal low communication channel (no noise and limited bandwidth)
Inter-code crosstalk will occur if this limit is exceeded
Shannon's theorem :-The limit data transmission rate of a channel with limited bandwidth and interference from Gaussian white noise
- The limit transmission rate of the channel = W * log2(1+S/N) (b/s) W is the channel bandwidth (Hz) S The average power of the channel transmission signal N is the Gaussian noise power inside the channel SNR=10*log10 (S/N)
Baseband signal/basic band signal: low-frequency original electrical signal without modulation
Broadband signal/modulated signal: It is a high-frequency signal converted from original information after modulation (spectrum shifting and transformation)
Circuit switching : A dedicated physical communication path is established between the two nodes before data transmission. During the transmission, the path is exclusively occupied and released after the communication ends.
Message exchange : the sender delivers the message to the switching device, the switching device first stores the entire message and then selects a suitable idle line to forward the message
Packet switching : same as message switching, it uses storage and makeup, but limits the upper limit of the data block size transmitted each time
Packet switching:
- Connectionless datagram: each datagram packet can take a different path
- Connected virtual circuit: a path connection is established before transmission, and datagram packets follow the same path in sequence
2. Transmission medium
1. Wired media
1. Twisted pair
2. Coaxial cable
3. Optical fiber
Two, wireless media
Radio wave
2. Microwave, infrared, laser
Three, physical layer equipment
1. Repeater: shape and amplify the signal and finally forward it
2. Hub: multi-port repeater