技術類文章

BDC CH3: Physical Layer

CH3: Physical Layer

Digital-to-Digital Conversion

  • Non-return to Zero Level (NRZ-L)
    • 1: negative voltage
    • 0: positive voltage
    • synchronization problem:
      • transmitting a long sequence of 1s or 0s
  • Non-return to Zero Invert (NRZ-I)
    • 1: voltage change
    • 0: no change
    • synchronization problem:
      • transmitting a long sequence of 0s
  • Manchester
    • 1: low to high
    • 0: high to low (Z-shape)
    • ex. 10BaseT
  • Differential Manchester
    • 1: no change at the begining
    • 0: change at the begining
    • ex. Token RingAmplitude

Analog-to-Digital Conversion

  • Pulse Code Modulation (PCM，脈衝編碼調變)

    • A scheme for converting analog singnal to digital data(類比訊號編碼為數位訊號)

  • Pulse Amplitude Modulation (PAM，脈衝振幅調變)

    • 將原本的波型轉變為脈衝訊號(類比訊號取樣)
    • Measuring the amplitude of the signal at equal intervals
    • Using PAM (Pulse Amplitude Modulation) to digitalize

  • PAM(取樣)->PCM(編碼)

  • Nyquist Theorem(奈奎斯特取樣定理)

    • To ensure the accurate reproduction of an original analog signal using PAM, the sampling rate must be “at least twice” the highest frequency of the original signal
    • 為避免訊號失真，信號在取樣時所使用的頻率，必須要為原訊號頻率的二倍以上

  • When we want to digitize the human voice (0 to 4000 Hz), what is the bit rate, assuming 8 bits per sample?

  • Answer:

    • Simple rate = 4,000 * 2 (nyquist theorem) = 8,000
    • Bit rate = 8,000 * 8 = 6,4000 bps = 64 Kbps

  • [補充] Ethernet Physical Layer

    • Transfer speed = lanes * bits per hertz * spectral bandwidth
    • 1Gbps = 4 X 4 X 62.5
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