What determines the length of a time division multiplexing (TDM) frame?

The length of a time division multiplexing (TDM) frame is determined by the number of input channels to the multiplexer. In TDM, multiple signals are combined into a single signal by allocating a specific time slot for each input channel within each frame. The total length of the frame is a direct function of how many channels are being multiplexed together. Each channel gets its own time slice, and the frame length must accommodate all active channels to ensure that each one can transmit its data during its designated time slot.

The other options don't define the frame length in TDM. The number of modulation techniques used pertains to how the individual signals are processed and sent, but it doesn’t affect the size of the TDM frame. The transmission speed of the channels might influence how quickly data can be sent, but it does not change the frame structure itself. Similarly, while available bandwidth is crucial for determining overall system capacity and may influence how many channels can be multiplexed, it does not directly dictate the frame length, which is purely a factor of the number of channels involved.