1.5Data transmission and the physical layer

The physical layer is the bottom layer, handling "how to convert bits into a physical signal for transport," but in practice it directly connects to design decisions such as how efficiently to multiplex a limited transmission medium (bandwidth) and which multiplexing scheme to choose for long distance or higher speed. This section builds an understanding of transmission coding and multiplexing mechanisms, then covers selecting technology to fit requirements.

1.5.1Transmission coding

• Transmission coding converts the internal 0/1 digital data of a computer into an electrical, optical, or radio signal on the transmission medium. A scheme that represents 0/1 using only high/low voltage has the weakness that when the same value repeats, the signal does not change, making it hard for the receiver to synchronize its clock (timing).
• To counter this weakness, encodings that embed bit-value information in the signal transition itself are used (e.g., Manchester encoding, where the signal always transitions at the midpoint of each bit period), making it easier to recover the clock from the signal transitions (self-synchronization) so that synchronization does not break down even with long runs of identical bits.

1.5.2Multiplexing (TDM/FDM/WDM)

• Multiplexing carries multiple communications (channels) simultaneously over a single transmission medium. TDM (Time Division Multiplexing) slices time into small units and assigns each channel a time slot in turn. FDM (Frequency Division Multiplexing) divides the frequency band and assigns each channel a different frequency range for simultaneous transmission.
• WDM (Wavelength Division Multiplexing) applies the FDM principle to optical fiber communication using the wavelength (color) of light, achieving greater capacity by simultaneously carrying multiple optical signals at different wavelengths on a single fiber. It is widely used in long-distance, high-capacity backbone links.

Suppose you are a network designer at a telecom operator given the requirement to increase the capacity of an inter-city backbone link without laying any additional optical fiber. The existing single optical fiber already carries multiple customer circuits multiplexed via TDM, but because TDM shares time within a single wavelength (a single optical signal), there is a physical ceiling to multiplexing along the time axis alone. To increase capacity without adding fiber, introducing WDM (Wavelength Division Multiplexing)—simultaneously sending multiple optical signals at different wavelengths superimposed on the same single fiber—effectively creates a state where "multiple independent transmission paths exist within one fiber." Within each wavelength's channel, customer circuits can still be multiplexed via TDM, so in practice WDM and TDM are not mutually exclusive but are combined. If, instead, the requirement is only a small number of channels between a few short-distance sites newly being connected, the cost of WDM's optical components may not be justified, and simply dividing the frequency band with FDM or plain TDM can be sufficient. The constraint of "new fiber is impractical to lay, yet greater capacity is needed" is precisely the judgment basis for adopting WDM—WDM is not chosen merely because "multiplexing is desired."

1.5.3Section summary

• Transmission coding relies on self-synchronization, recovering the clock from signal transitions, as a countermeasure for long runs of identical bits
• TDM = time division, FDM = frequency division, WDM = wavelength division of light (the optical analog of FDM). WDM and TDM are used in combination
• The judgment basis for adopting WDM is the constraint that new fiber is impractical yet greater capacity is essential, not simply a general need for multiplexing