Motorola PowerQUICC II MPC8280 Series Reference Manual page 1216

IMA Protocol Overview
this required behavior, the receiver can detect the defensible delays be measuring the arrival
times of the IMA frames on each link.
IMA Frame Length = M cells
ATM ATM ATM
F F ICP2 F
ATM ICP2 F
ATM: ATM Layer Cell
At the transmitting end, the cells are transmitted continuously. If there are no ATM layer
cells to be sent between ICP cells within an IMA frame, then the IMA transmitter sends
filler cells to maintain a continuous stream of cells at the physical layer. The insertion of
Filler cells provides cell rate decoupling at the IMA sublayer. The Filler cells should be
discarded by the IMA receiver.
A new OAM cell is defined for use by the IMA protocol. The cell has codes that define it
as an ICP or Filler cell.
The data multiplexing performed by IMA is cell-based, where cells are distributed among
the links in the IMA group in a round-robin cycle. In order to compensate for different clock
rates, IMA must periodically insert 'stuff' cells into faster links in order to maintain a
consistent average data rate over the links of the group. Furthermore, IMA must
compensate for potential differences in delay between the links of the group. Per the IMA
specification, the allowable delay differential for DS1/E1 links is 25ms, which at E1 rates
is equivalent to approximately 118 cells. The IMA microcode allows the user to define the
allowable delay differential via a delay compensation buffer of programmable length.
34-6
Freescale Semiconductor, Inc.
F ICP2 F ATM ATM
ATM ATM ATM ICP1
F ATM F ICP1 ATM F
ICPx: IMA Control Protocol Cell
Figure 34-2. Illustration of IMA Frames
MPC8280 PowerQUICC II Family Reference Manual
For More Information On This Product,
Go to: www.freescale.com
Link 0
F ICP1 F F
Link 1
M-1 3
F F F F
Link 3
ATM ATM
ICP0
F: Filler Cell
ATM F ICP0
2 1 0
ICP0 F ATM
ATM F F
MOTOROLA