The Physical Layer is the lowest layer(1)of the OSI/TCP model representing the literal physical means of data transmission via bits. 1’s and 0’s, on and off. Every bit transmitted and received by a NIC, data cable, and antenna are represented by this layer. Layer 1 defines these devices and mediums along with the standards in which they physically communicate. It’s focuses on pure data transmission at it’s rawest form, like a radio wave or fiber pulse, not the logical elements.

Physical Layer (Layer 1)

• The lowest layer of the OSI model responsible for transmitting raw bits (1s and 0s) across physical media
• Defines the electrical, mechanical, and procedural specifications for activating, maintaining, and deactivating physical connections
• Key concept: Focuses on how bits are transmitted physically, not what they mean (that’s Layer 2’s job)
• Operates with voltage levels, light pulses, radio frequencies, and electrical signals

Physical Transmission Methods

• Electrical signals: Copper cables use voltage differences to represent bits
  • For example: +5V = 1, 0V = 0 (simplified - actual encoding is more complex)
• Optical signals: Fiber optic cables use light pulses
  • Light on = 1, light off = 0 (or vice versa depending on encoding)
• Radio waves: Wireless transmission uses electromagnetic frequencies
  • Different frequencies, amplitudes, or phases represent different bit patterns

Common Physical Media Types

Encoding Methods

• Manchester Encoding: Each bit period split in half - transition represents the bit value
  • Used in older Ethernet (10BASE-T)
  • Self-synchronizing but requires double the bandwidth
• 4B/5B Encoding: Maps 4 data bits to 5 transmitted bits
  • Used in 100BASE-TX (Fast Ethernet)
  • Provides error detection and maintains signal timing
• 8B/10B Encoding: Maps 8 data bits to 10 transmitted bits
  • Used in Gigabit Ethernet and Fibre Channel
  • Better DC balance and error detection than 4B/5B

Ethernet Physical Standards

Vocabulary

• Bit: Binary digit (1 or 0) - the fundamental unit of data at Layer 1
• Baud rate: Number of signal changes per second (not always equal to bit rate)
• Attenuation: Signal strength loss over distance
• EMI (Electromagnetic Interference): External electrical signals that corrupt data transmission
• Crosstalk: Signal interference between adjacent wire pairs in the same cable
• Duplex: Communication direction capability
  • Half-duplex: One direction at a time (like walkie-talkies)
  • Full-duplex: Both directions simultaneously (like phone calls)

Notes

• Critical rule: Layer 1 problems affect everything above it - always check physical connections first when troubleshooting
• Cable length limits are hard limits - exceeding them causes signal degradation and packet loss
• Fiber optic cables are immune to EMI but require specialized equipment and training to terminate
• When designing networks, consider future bandwidth needs - running Cat 6A costs little more than Cat 5e during installation but provides significant upgrade path
• Auto-negotiation happens at Layer 1 - devices automatically determine the highest common speed and duplex settings
• Use show interfaces command to verify physical layer status (up/up means Layer 1 and 2 are both operational)
• Remember: “Physical layer = can you ping? No? Check the cable first”