IMA is a cell based protocol where an ATM cell stream is inverse-multiplexed and de-multiplexed in a cyclical fashion among ATM-supporting channels to form a higher bandwidth logical link where the logical link concept is referred as an IMA group. By grouping channels into an IMA group, customers gain bandwidth management capability at in-between rates (for example, between E-1/DS-1 and E-3/DS-3 respectively) through addition/removal of channels to/from the IMA group.
In the ingress direction, traffic coming over multiple ATM channels configured as part of a single IMA group, is converted into a single ATM stream and passed for further processing to the ATM Layer where service-related functions, for example Layer 2 TM, or feeding into a pseudowire are applied. In the egress direction, a single ATM stream (after service functions are applied) is distributed over all paths that are part of an IMA group after ATM layer processing takes place.
An IMA group interface compensates for differential delay and allows only for a minimal cell delay variation. The interface deals with links that are added, deleted or that fail. The higher layers see only an IMA group and not individual links, therefore service configuration and management is done using IMA groups, and not individual links that are part of it.
The IMA protocol uses an IMA frame as the unit of control. An IMA frame consists of a series of consecutive (128) cells. In addition to ATM cells received from the ATM layer, the IMA frame contains IMA OAM cells. Two types of cells are defined: IMA Control Protocol (ICP) cells and IMA filler cells. ICP cells carry information used by IMA protocol at both ends of an IMA group (for example IMA frame sequence number, link stuff indication, status and control indication, IMA ID, TX and RX test patters, version of the IMA protocol, and so on). A single ICP cell is inserted at the ICP cell offset position (the offset may be different on each link of the group) of each frame. Filler cells are used by the transmitting side to fill up each IMA frame in case there are not enough ATM stream cells from the ATM layer, so a continuous stream of cells is presented to the physical layer. Those cells are then discarded by the receiving end. IMA frames are transmitted simultaneously on all paths of an IMA group and when they are received out of sync at the other end of the IMA group link, the receiver compensates for differential link delays among all paths.