Freescale Semiconductor MPC8260 - Manual (Page 1108)

Inverse Multiplexing for ATM (IMA)
MPC8260 PowerQUICC II Family Reference Manual, Rev. 2
33-6 Freescale Semiconductor
Figure 33-2. Illustration of IMA Frames
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.
IMA accomplishes these goals by the periodic insertion of special OAM cells, which (among other things)
define M-cell frame boundaries, provide frame sequence numbers, and provide stuffing information. This
framing information is used by the receiver to correlate the received cell streams and extract cells in-order
from the links of the IMA group, thereby reconstructing the original cell stream.
Figure 33-3. IMA Microcode Overview
ATM ATM ATM F ICP2 F ATM ATM F ICP1 F F ATM F ICP0
Link 0
IMA Frame Length = M cells
F F ICP2 F ATM ATM ATM ICP1 F F F F ICP0 F ATM
Link 1
ATM ICP2 F F ATM F ICP1 ATM F ATM
ATM
ICP0 ATM F F
Link 3
0 1 3 2 M-1
ATM: ATM Layer Cell ICPx: IMA Control Protocol Cell F: Filler Cell