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Inverse Multiplexing for ATM (IMA)

MPC8280 PowerQUICC II Family Reference Manual, Rev. 1

Freescale Semiconductor

34-5

Figure 34-1. Basic Concept of IMA

In the transmit direction (near-end), 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 protocol enables the

demultiplexing/deconstruction (transmit) of an ATM cell stream into multiple links. When receiving, the

IMA protocol multiplexes/reconstructs incoming cells from multiple links into the original ATM cell

stream. The IMA protocol must compensate for differences in clock rate and delay over the multiple links.

34.2.2

IMA Frame Overview

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. The receiver end reconstructs the ATM

cell stream after accounting for the link differential delays, smoothing CDV introduced by the control

cells, etc. These cells, defined as IMA Control Protocol (ICP) cells, provide the definition of an IMA

frame. The transmitter must align the transmission of IMA frames on all links (shown in Figure 34-2). This

allows the receiver to adjust for differential link delays among the constituent physical links. Based on this

required behavior, the receiver can detect the defensible delays be measuring the arrival times of the IMA

frames on each link.

Original ATM

Single ATM

Cell Stream

from ATM

Layer

Physical Link #1

Physical Link #2

Physical Link #3

IMA Group

PHY

Physical Link #4

Cell Stream to

ATM Layer

IMA Group

PHY

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