The physical layer is the lowest layer of a computer network; it is mainly responsible for the physical transmission of data.

The physical layer provides services to the data link layer, so that the data link layer only needs to consider how to encapsulate its protocol data units (PDUs), without worrying about how the data is actually transmitted.

Note: for introductory material on computer networks, see the following:

• Overview of Computer Networks

Important basic concepts of computer networks

• Components of a network:
  • Network edge: hosts (Host) and terminal devices (Terminal Device)
  • Network core: routers (Router), switches (Switch) and communication links
• Access networks: optical fiber (FTTH, fiber to the home), DSL (telephone line), coaxial cable, wireless access (WiFi, 4G/5G)
  • Media: wired (twisted pair, coaxial cable, optical fiber), wireless (radio waves, microwaves, infrared, laser)
• Router: a device that connects multiple networks and is responsible for forwarding packets between them
• Switch: connects multiple devices and is responsible for forwarding frames within the same network (LAN)
• Switching methods:
  • Circuit switching: a dedicated physical connection must be established before data transmission, and the connection is used exclusively
  • Message switching: no connection needs to be established; store-and-forward
  • Packet switching: data is split into many small packets, each transmitted independently; high additional overhead
• Protocol: the rules that both communicating parties must follow, including:
  • Syntax: the structure or format of data and control information
  • Semantics: what control information to send, what actions to perform and what responses to give
  • Timing: a detailed specification of the order in which events occur
• Service: under the control of a protocol, communication between two peer entities enables a layer to provide services to the layer above it, independently of the layers below.
• Interface: the entry point through which an upper layer uses the services of the lower layer, and the exit through which the lower layer provides services to the upper layer.
• Connection-oriented: connect first, then transmit. Connectionless: transmit directly.
• Entity: any hardware or software process that can send or receive information.
• The OSI seven-layer model:

• The TCP/IP four-layer model:

• Service access point: the logical interface through which the entities of two adjacent layers in the same system exchange information is called a service access point (SAP).
  • Data link layer: the data link layer SAP is the “header” and “trailer” of a frame
  • Network layer: the network layer SAP is the IP address
    • IP address: a network-layer address, used to identify a node in the network
  • Transport layer: the transport layer SAP is the port
    • Port: a transport-layer address, used to identify a process
• Service primitives:
  • PDU: protocol data unit; a packet transferred between peer entities is called a protocol data unit.
  • SDU: service data unit; a packet transferred between adjacent layers is called a service data unit.
• Connection uniqueness: the 4-tuple (source IP address, source port, destination IP address, destination port) uniquely identifies a connection

1. Basic concepts of the physical layer

1.1 Functions of the physical layer

• Defining the characteristics of physical interfaces
  • Mechanical characteristics: the shape and size of the interface, the number and arrangement of pins
  • Electrical characteristics: the interface’s voltage range, current strength, impedance matching, transmission rate, etc.
  • Functional characteristics: the functions of the interface’s signal lines
  • Procedural characteristics: the procedures of the interface
• Defining the characteristics of the transmission medium
• Defining the data transmission rate
• Bit synchronization and clock synchronization
• Defining the bit encoding scheme

1.2 Basic concepts of data communication

• Data: the entity that carries information
• Signal: the electrical or electromagnetic representation of data
• Analog signal: a continuous signal
• Digital signal: a discrete signal

2. Transmission media

2.1 Guided transmission media

1. Twisted pair

   • Unshielded twisted pair (UTP)
   • Shielded twisted pair (STP)
   • Commonly used in Ethernet

2. Coaxial cable

   • Baseband coaxial cable
   • Broadband coaxial cable
   • Strong resistance to interference

3. Optical fiber

   • Single-mode fiber
   • Multi-mode fiber
   • Long transmission distance, high bandwidth

2.2 Unguided transmission media

• Radio waves
• Microwaves
• Infrared
• Laser

3. Transmission modes

3.1 Serial vs. parallel transmission

• Serial transmission: data is transmitted one bit at a time, in sequence
• Parallel transmission: multiple bits of data are transmitted simultaneously, in sequence

3.2 Synchronous vs. asynchronous transmission

• Synchronous transmission: data blocks of fixed length are transmitted one after another
  • External synchronization: a separate clock signal line is added
  • Internal synchronization: the clock information is embedded in the data
• Asynchronous transmission: data blocks of variable length are transmitted one after another

3.3 Simplex, half-duplex and full-duplex

• Simplex: one-way transmission — radio broadcasting
• Half-duplex: two-way alternating transmission — walkie-talkies
• Full-duplex: two-way simultaneous transmission — telephone

4. Channels and channel capacity

4.1 Basic concepts of channels

• Simplex communication: one-way transmission
• Half-duplex communication: two-way alternating transmission
• Full-duplex communication: two-way simultaneous transmission

4.2 Channel capacity

• Nyquist’s theorem: the maximum symbol rate of an ideal low-pass channel
  • Formula:
  • : bandwidth
  • : number of signal levels
• Shannon’s theorem: the maximum data transmission rate of a channel
  • Formula:
  • : bandwidth
  • : signal-to-noise ratio
• The relationship between signal-to-noise ratio and bandwidth

5. Digital transmission systems

5.1 Encoding and modulation

1. Baseband transmission

   • Non-return-to-zero (NRZ) encoding: has synchronization problems
     • A positive level represents 1, a negative level represents 0
   • Return-to-zero (RZ) encoding: self-synchronizing, but low coding efficiency
   • Manchester encoding
     • There is a transition in the middle of each symbol; a negative transition represents 1 and a positive transition represents 0
   • Differential Manchester encoding

2. Bandpass transmission

   • Amplitude modulation (AM)
   • Frequency modulation (FM)
   • Phase modulation (PM)

5.2 Multiplexing techniques

• Frequency-division multiplexing (FDM)
• Time-division multiplexing (TDM)
• Wavelength-division multiplexing (WDM)
• Code-division multiplexing (CDM)

6. Physical-layer devices

6.1 Repeaters

• Function: amplify the signal
• Characteristics: cannot connect networks of different types
• The 5-4-3 rule

6.2 Hubs

• Function: a multi-port repeater
• Characteristics: shared bandwidth, a single collision domain
• Operating mode: broadcast

Translated from the Chinese original.