Inverse Multiplexing

An inverse multiplexer (often abbreviated to "inverse mux" or "imux") allows a data stream to be broken into multiple lower data rate communications links. An inverse multiplexer differs from a demultiplexer in that each of the low rate links coming from it is related to the other ones and they all work together to carry the same data. By contrast, the output streams from a demultiplexer may each be completely independent from each other and the demultiplexer does not have to understand them in any way.

A technique that is the inverse, or opposite, of multiplexing. Traditional multiplexing folds together multiple low-speed channels onto a high-speed circuit. Inverse multiplexing spreads a high-speed channel across multiple low-speed circuits. Inverse multiplexing is used where an appropriately high-speed circuit is not available. A 6-Mbps data stream, for example, might be inverse multiplexed across four (4) T1 circuits, each running at 1.544 Mbps. Inverse multiplexing over ATM (IMA) fans out an ATM cell stream across multiple circuits between the user premises and the edge of the carrier network. In such a circumstance, multiple physical T1 circuits can be used as a single, logical ATM pipe.The IMAcompliant ATM concentrator at the user premises spreads the ATM cells across the T1 circuits in a round robin fashion, and the ATM switch at the edge of the carrier network scans the T1 circuits in the same fashion in order to reconstitute the cell stream.There is a similar implementation agreement (IA) for Frame Relay. Multilink point-to-point protocol (PPP) serves much the same purpose in the Internet domain.

Note that this is the opposite of a multiplexer which creates one high speed link from multiple low speed ones.

This provides an end to end connection of 3 x the data rate available on each of the low rate data links. Note that, as with multiplexers, links are almost always bi-directional and an inverse mux will practically always be combined with its reverse and still be called an inverse mux. This means that the "de-inverse mux" will actually be an inverse mux.

Inverse muxes are used, for example, to combine a number of ISDN channels together into one high rate circuit, where the DTE needs a higher rate connection than is available from a single ISDN connection. This is typically useful in areas where higher rate circuits are not available.

An alternative to an inverse mux is to use three separate links and load sharing of data between them. In the case of IP, network packets could be sent in round robin mode between each separate link. Advantages of using an inverse mux over separate links include

  • lower link latency (one single packet can be spread across all links)
  • fairer load sharing
  • network simplicity (no router needed between boxes with high speed interfaces)