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1. Physical Layer

The physical layer defines the means of transmitting raw bits over a physical data link connecting network nodes.

The bitstream may be grouped into code words or symbols and converted to a physical signal that is transmitted over a transmission medium. The physical layer provides an electrical, mechanical, and procedural interface to the transmission medium. The shapes and properties of the electrical connectors, the frequencies to broadcast on, the line code to use and similar low-level parameters, are specified by the physical layer.

  • Cables
  • Repeaters
  • Hubs

Cables​

Ethernet (X-Base-T)​

  • X: speed
    • BASE-T: megabit/s
    • GBASE-T: gigabit/s
  • Base: base-band signaling, means only 1 signal at a time. The contrary of broadband signaling (that we have at home).
  • T: twisted-pair, 4 pairs of wired twisted to form that cable.
    • Category 3, 4, 5, etc, means most twists per meter of wire, which means more copper and cleaner signal.

Flavors (Straight & Crossover)​

X-Based T cables come in two flavors, similar device types will use a crossover cable between them.

  • Straight
  • Crossover

So and end-host/router and a switch will use a straight cable, but if you connect two routers or two switches together, or an ethernet computer to ethernet router, you'll have a crossover cable

Types (Shielded & Unshielded)​

Also, there are two types of X-Based T cables:

  • Unshielded Twisted Pair (use at home for ethernet connections)
  • Shielded Twisted Pair (only used in places of electromagnetical interference, for example if you run an unshielded cable behind a refrigerator or an air condition unit, you may have signal degradation)

Others​

Other important type of cable is the Serial or V.35 cable, which is connected to a serial port on a router (between router and router).

The last important cable to remember is the Console or Rollover Cable, that are used from the console port of a CISCO device to the communication/COM of your PC. This is the cable that enables you to directly connect to a router.

Devices​

Repeater​

An X-Based T cable is capable of carrying a signal for total of 100 meters, after that, there is no guarantee that the signal will not degrate.

Repeater will repeat signal and extend it for another 100 meters.

Usually repeaters mean that the network wasn't properly designed.

Hub​

Multi-port repeater.

A Hub take signal from one port and flood the signal out to every other port (devices) except the port that originated the signal.

Router Boot Sequence​

  1. Router performs a Power ON self test to verify all hardware function.
  2. Program in ROM (called Bootstrap) looks for the IOS (default Flash) and loads it.
  3. IOS is expanded into RAM and startup config in NVRAM is loaded.
  4. Config from NVRAM is loaded into RAM as the running-config.
  5. If start-up config not present look for TFTP server.
  6. If TFTP not found go into config dialogue.