The physical layer, hubs, and ethernet

The main functions of the physical layer is to send bits and receive bits, as well as state transitions.

• Bits come in only values of 1 or 0 (morse code with numerical values).
• State transitions – changes in voltage from high to low and vice-versa.

Hubs at the physical layer

What is a hub?

• A hub is really a multiple port repeater.

Hubs in a network

• All devices in the same collision domain.
• All devices in the same broadcast domain.
• Devices shares some bandwidth.

What is a Physical Star Network?

• A physical star network is where the hub is a central device and cables are extended in all directions from it.

Note: Hubs and repeaters can be used to enlarge the area covered by a single LAN segment, however, this is NOT RECOMMENDED.

Ethernet at the physical layer

• Created by a group called DIX (Digital, Intel, Xerox)
• There are three types of ethernet
  • 802.3u – Fast Ethernet
  • 802.3ab – Gigabit Ethernet on Category 5
  • 802.3ae – 10Gbps over fiber and coax

Electronics Industries Association and The Newer Telecommunications Industry Alliance (EIA/TIA)

• The standard body that creates the physical layer/specifications for ethernet.
• EIA/TIA specifies that ethernet use a registered jack (RJ) connector with a 4 5 wiring sequence on unshielded twisted pair (UTP) cabling (RJ45).

Cables

• Cables are measured in decibals (dB)
• Cabling in corporate and home markets is measured in categories.
  • A higher quality cable will have a higher rated category and lower attenuation.
    • ex. category 5 > category 3

Key term:

• Crosstalk – the unwanted signal interference from adjacent pairs in the cable.