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TDM (Time-Division Multiplexing) is a technology to stream multiple channels of data over a single medium by
splitting it into time slots for each data channel. The shared medium typically uses a constant bitrate, often with fixed
size multiplexed time slots, but not always, see the Examples.

TDM is used with many technologies:



ISDN S0 uses a raw bitrate of 192 kbps, which is splitted into 48 bit frames, 4000 each second, 250 μs each frame.
Each of this 4000 S0 frames is divided into:

  • 2 x 8 = 16 bits B-Channel #1 x 4000 = 64 kbps
  • 2 x 8 = 16 bits B-Channel #2 x 4000 = 64 kbps
  • 2 x 2 = 4 bits D-Channel x 4000 = 16 kbps
  • 2 x 6 = 12 bits sync/control x 4000 = 48 kbps

The bits are interleaved like this:

  • 8x B1 + 8x B2 + 2x D + 6x sync/control + 8x B1 + 8x B2 + 2x D + 6x sync/control = 48 bits

This is a good example of using TDM to multiplex mixed bitrates in fixed size frames. The interleaving preserves a constant
8-bit sample rate for the B-Channels, which reduces buffering overhead and protocol latency, important for realtime audio.


Most router and network SoCs today have dedicated audio DSPs and TDM hardware to multiplex audio streams, ie. for VoIP.


The stream router pcmlink.ko uses the TDM subsystem of these SoCs:

This list is gathered from the presence of xxxx_* symbols in pcmlink.ko.

The isdn_fbox_fon5.ko stack references the pcmlink_ul_codectdmbus symbol in pcmlink.ko.

Piglet_noemif.ko contains the strings gpio_avm_tdm_dcl and gpio_avm_tdm_fsc.
DCL is the Data Clock (bitrate), FSC is the Frame Synchronization Clock (time slot sync).


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