Inverse Multiplexing Over Atm (Ima) - Alcatel-Lucent 7750 Configuration Manual

Interface Configuration

Inverse Multiplexing Over ATM (IMA)

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 L2 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, etc.). 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.
7750 SR Interface Configuration Guide Page 101