Freescale Semiconductor MPC8260 - Manual (Page 1107)

Inverse Multiplexing for ATM (IMA)
MPC8260 PowerQUICC II Family Reference Manual, Rev. 2
Freescale Semiconductor 33-5
Figure 33-1. 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.
33.2.2 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 33-2 ). 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
TC
+
PHY
TC
+
PHY
TC
+
PHY
TC
+
PHY
Physical Link #4
Cell Stream to
ATM Layer
IMA Group
TC
+
PHY
TC
+
PHY
TC
+
PHY
TC
+
PHY