Moving Bits Across the Wire

Physical layer: transfers bits across computer network

A bit is the smallest representation of a data that a computer can understand: it’s a one or a zero.

Modulation: A way of varying the voltage of this charge moving across the cable.

Twisted Pair Cabling and Duplexing

Twisted pair cable: The most common type of cabling used for connecting computing devices. It features pairs of copper wires that are twisted together. (due to nearby magnetic field interference and prevents crosstalk)

Duplex communication: The concept that information can flow in both directions across the cable

One or two cables in the jacket is reserved for one direction and others in the other direction. This allows cables to talk to each other at the same time. (Full duplex)

Half duplex: while it is possible to communicate in both direction only one device can be communicating at a time.

Straight-through cables use 100baseT standards do not use blue and brown cables. Networks using gigabit Ethernet have the option to use blue and brown cables for power over Ethernet.

Straight-through cable key:

• Hubs and switches use:
  • Computers and routers use:
    • Pins 1 & 2 - Orange wires for sending data
    • Pins 3 & 6 - Green wires for receiving data
  • Pins 1 & 2 - Green wires for sending data
  • Pins 3 & 6 - Orange wires for receiving data

Crossover cable

pin positions 1 and 2 on one side of the cable, on the opposite end of the cable, the green wires will appear in the pin position 3 and 6.

uses T-568-B scheme.

Crossover cable key:

• Endpoint 1 of the Ethernet cable:
  • Pins 1 & 2 - Green wires for sending data
  • Pins 3 & 6 - Orange wires for receiving data
• Endpoint 2 of the Ethernet cable:
  • Pins 1 & 2 - Orange wires for sending data
  • Pins 3 & 6 - Green wires for receiving data

Network Ports and Patch Panels

RJ45 most common in computer networking.