Inverse Multiplexing for ATM (IMA)

Inverse Multiplexing for ATM (IMA) is a method which provides a modular bandwidth for user access

to ATM networks and for connection between ATM network elements, at rates between the traditional

order multiplex level. An example is to achieve rates between the T1/E1 and T3/E3 levels in the

asynchronous digital hierarchies. T3/E3 links are not necessarily readily available throughout a given

network, and therefore the introduction of ATM Inverse Multiplexers provides an effective method of

combining the transport bandwidths of multiple links (such as T1/E1 links) grouped to collectively

provide higher intermediate rates.

The ATM Inverse Multiplexing technique involves inverse multiplexing and de-multiplexing of ATM cells

in a cyclical fashion among links grouped to form a higher bandwidth logical link whose rate is

approximately the sum of the link rates. This is referred to as an IMA group.

The picture above provides a simple illustration of the ATM Inverse Multiplexing technique in one

direction. The same technique applies in the opposite direction.

IMA groups terminate at each end of the IMA virtual link. In the transmit direction, the ATM cell stream

received from the ATM layer is distributed on a cell by cell basis, across the multiple links within the IMA

group. At the far-end, the receiving IMA unit recombines the cells from each link, on a cell by cell basis,

recreating the original ATM cell stream. The aggregate cell stream is then passed to the ATM layer.

The IMA interface periodically transmits special cells that contain information that permit reconstruction

of the ATM cell stream at the receiving end of the IMA virtual link after accounting for the link differential

delays, smoothing CDV introduced by the control cells, and so on. These cells, defined as IMA Control

Protocol (ICP) cells, provide the definition of an IMA frame.