Motorola PowerQUICC II MPC8280 Series Reference Manual page 1215

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. Figure 34-1 provides a simple illustration of ATM Inverse
Multiplexing technique in one direction. This technique applies in the opposite direction.
Single ATM
Cell Stream
from ATM
Layer
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
MOTOROLA
Freescale Semiconductor, Inc.
IMA Group
TC
Physical Link #1
+
PHY
TC
Physical Link #2
+
PHY
TC Physical Link #3
+
PHY
TC
Physical Link #4
+
PHY
Figure 34-1. Basic Concept of IMA
Chapter 34. Inverse Multiplexing for ATM (IMA)
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IMA Protocol Overview
IMA Group
TC
+
PHY
Original ATM
Cell Stream to
ATM Layer
TC
+
PHY
TC
+
PHY
TC
+
PHY
34-5