In a Passive Optical Network, which multiplexing technique is used for the upstream channel?

• A. The uplink uses wavelength-division multiplexing (WDM)
• B. The uplink uses time-division multiplexing (TDM)
• C. The uplink uses code-division multiplexing (CDM)
• D. The uplink uses amplitude modulation multiplexing

Answer: (B)

In the upstream of a Passive Optical Network, sharing the same fiber among many users has to be done without collisions, and time-division multiplexing makes that possible. The central office coordinates access and assigns separate time slots to each Optical Network Unit, so each ONU can transmit only during its slot. This burst-style, slot-based approach (driven by dynamic bandwidth allocation) efficiently uses the shared medium while keeping hardware simple and cost-effective.

WDM would force separate wavelengths for every user, adding cost and complexity with no inherent need on the uplink. Code-division multiplexing would require coding and correlation that aren’t standard in PONs. Amplitude modulation multiplexing isn’t a recognized method for organizing multiple upstream users in these networks.