The physical layer deals with how to transfer a stream of data bits over a transport medium that connects various computers, rather than referring to a specific transport medium.

2.1 Basic concepts of the physical layer

The primary task of the physical layer is described as determining some characteristics of the interface with the transport media

  • Mechanical properties :(what the interface is like) specifies the shape and size of the connectors used for the interface, the number and arrangement of leads, securing and locking devices, etc.
  • Electrical characteristics :(the number of volts used) indicates the range of voltages that occur on each line of the interface cable.
  • Functional characteristic :(the characteristic of the battery voltage on a line) indicating the meaning of the voltage present at a particular level on a line.
  • Discipline characteristics :(order in which signals are emitted to achieve different functions) indicates the order in which various possible events occur for different functions.

2.2 Basic knowledge of data communication

  1. Model of data communication system

    A few terms:

    • Data — An entity that transports information.
    • Signal — the electrical or electromagnetic representation of data during transmissionform.
      • Digital signal: Indicates a message. The value is yesdiscrete.
      • Analog signal: indicates a message. The value is yescontinuous.
    • Source (origin) : The source that generates and generates data.
    • Destination: The destination of receiving data.
    • Channel: The medium through which signals are transmitted. Generally used to refer to the medium that transmits information in one direction, so a communication line usually contains a sending channel and a receiving channel

  2. Three ways of communication:

    • Unidirectional communication (simplex communication) – there can be communication in one direction and no interaction in the other direction.
    • Two-way alternating communication (half duplex communication) – both communicating parties can send messages, but not both at the same time (and therefore not both at the same time).
    • Two-way simultaneous communication (full duplex communication) – the two communicating parties can send and receive messages simultaneously.

  3. Two types of data transmission

    • Serial transmission: slow speed, low cost, suitable for long distance

    • Parallel transmission: fast speed, high cost, suitable for short distance

2.3 Code source, Baud, Rate, and Bandwidth

  1. Code source: a signal waveform with a fixed length

  2. Rate: Data transfer rate, the amount of data transferred per unit time. Can be expressed in terms of symbol transfer rate and information transfer rate

    • 4. The number of symbols transmitted per unit of time, in Baud.
    • Information transmission rate: The number of bits transmitted per unit of time, in bits per second
    • Relation: IF a symbol carries N bits of information, the symbol transmission rate of M Baud corresponds to the information transmission rate of M x N bit/s

  3. Bandwidth: The highest rate of data that can pass from one point of a network to another per unit of time, often used to indicate the capacity of a network’s communication lines to transmit data. The units are b/s.

Problem sets:

2.4 Nye’s criterion and Shannon’s theorem

  1. The distortion

    Factors affecting distortion:

    • Symbol transfer rate
    • Signal transmission distance
    • noise
    • Transmission media quality

  2. Intersymbol interference

    The loss of a clear boundary between symbols in the waveform of a signal received by a receiver

  3. Nye’s guidelines

    In ideal low-pass (noise-free, bandwidth-limited) conditions, in order to avoid inter-symbol crosstalk, the limiting symbol transmission rate = 2W Baud, W is the ideal low channel bandwidth, in Hertz (Hz).

    Problem sets:

  4. Shannon’s law

    In a bandwidth constrained and noisy channel, to avoid error, the highest symbol transmission rate = W Baud, W is the ideal low channel bandwidth, in Hertz (Hz)

2.5 Encoding and modulation

  1. Baseband signal and Broadband signal

    Channel: The medium through which signals are transmitted. Generally used to refer to the medium that transmits information in one direction, so a communication line usually contains a sending channel and a receiving channel

    Signals transmitted over channels: Baseband signals and broadband signals

    • Baseband signal is the digital signal 1 or 0 directly represented by two different voltages, and then sent to the line for transmission.
    • Wideband signal is a frequency division multiplexing analog signal modulated by baseband signal.

    Ps: When the distance is relatively close, the signal is not easy to lose, you can use the baseband signal. However, when the distance is long, the baseband signal can be converted to broadband signal, so that it is not easy to lose, and then restored to the baseband signal after reaching the receiver.

  2. Encoding and modulation

    • Modulation — The process of converting a digital signal into an analog signal.
    • Demodulation — the process of converting analog signals into digital signals.

  3. Modulation is the transformation of waveform (spectral transformation). The basic binary modulation methods are as follows:

    • Amplitude modulation (AM) : The amplitude of the carrier varies with the baseband digital signal.
    • Frequency modulation (FM) : The frequency of the carrier varies with the baseband digital signal.
    • Phase modulation (PM) : The initial phase of the carrier varies with the baseband digital signal.

2.6 Channel multiplexing technology

Multiplexing refers to the process of synthesizing multiple channels along a transmission path and then reverting to the original mechanism or removing the multiplexing technology of various channels at the terminal

  • Frequency division multiplexing (FDM) : All users use different bandwidth resources at the same time.
  • TDM: All users occupy the same bandwidth at different times.
  • Wavelength Division multiplexing (WDM) : Frequency division multiplexing of light.
  • Code division multiplexing CDM: Differentiating according to transmitted data codes at the same time and frequency

2.7 Physical layer transmission media

Transmission medium

  • Twisted-pair cable
    • Shielded Twisted Pair (STP)
    • Unshielded Twisted Pair UTP
    • Making standard
      • 568B: Orange white, orange, green white, blue, blue white, green, brown white, brown
      • 568A: Green white, green, orange white, blue, blue white, orange, brown white, brown
  • Coaxial cable – obsolete

  • Optical fiber

    • Single-mode fiber

    • Multimode fiber

  • Wireless — Wireless signal frequency IEEE802.11

2.8 Physical Layer Devices

  1. repeater

  2. Hub (multi-port repeater)