MPC8280 datasheet(1152/1386 Pages) FREESCALE

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Inverse Multiplexing for ATM (IMA)

MPC8280 PowerQUICC II Family Reference Manual, Rev. 1

34-6

Freescale Semiconductor

Figure 34-2. 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.

ATM

ICP2

ATM ATM

ICP1

ATM

ICP0

Link 0

IMA Frame Length = M cells

ICP2 F

ATM

ATM ICP1

ICP0

ATM

Link 1

ATM

ICP2

ATM

ICP1 ATM

ATM

ICP0

ATM

Link 3

M-1

ATM: ATM Layer Cell

ICPx: IMA Control Protocol Cell

F: Filler Cell

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