Physical Layer

What concerns the physical layer in general?

• Tranmission of bits over a com-channel

  • mechanical aspects (plugs, sockets, ...)

  • elecromagnetic characteristics (cable quality, voltage level/light colour)

  • temporal characteristics (frequencies, sync -> both sides need to be in sync)

  • other characteristics of the transmissions and paths

How can the transmission media of the physical layer be grouped?

• Wired media = guided

  • Fiber: Mono/Multi-mode

  • Copper cable: Twisted Pair/Coax

• Wireless = unguided

  • Radio/Satellite

  • Microwaves

  • Infrared

  • (Visible) Light

Give examples for non-networked transmissions

• Semiconductor memory like flash storage

• "Magnetic media" such as data tapes or hard disks

On what parameters does the tranmission quality depend in the physical layer?

• Wire/fiber length

• Wire/fiber quality (e.g. purity of copper)

• Frequency (fiber: "colour")

• Connectors, "crimping"/"fusing" of connections between plug and cable must meet standards too

What are the parameters that determine the bandwith in the physical layer?

• Specific bandwidth - length product of fiber cable

• Expressed as <cable-length> * <bandwidth>

• Typically constant for a given cable, depends on material and other properties

How can attenuation (loss of signal strength) form?

• General "attenuation": put in a lot, get out less

• Two types

  • Insertion loss

    • Easy to counteract: send in more power

  • Transmission loss

• Should be as low as possible but

  • depends on cable quality

  • longer cable -> higher attenuation

  • higher frequency -> higher attenuation

Attenuation \text{[dB]} = 20 \cdot log_{10} \frac{V_{in}}{V_{out}}

What is propogation delay in the context of the physical layer?

• Time it takes for a signal to travel over a cable

• Time of flight measurement

  • typically min. 60% of c

  • spiral/twisting of cable means that there is usally a little bit more distance to cover

Propagation speed = \frac{1}{\sqrt{wire capacitance * wire inductance}}

What causes delay skew in a network cable?

• Occurs when different signal paths in a cable have different lengths or travel times

• Can be caused by

  • Different twists in wire pairs within a cable

  • Different routes or materials in optical fibers