Motorola PowerQUICC II MPC8280 Series Reference Manual page 1221

34.3.1.1 User Plane Functions Performed by Microcode
• ATM cell stream splitting and reconstruction
• ICP cell insertion/removal
• Cell rate decoupling (i.e. filler cell insertion/removal)
• IMA frame synchronization
• Stuffing
• Discards cells with bad HECs (available on .25µm (HiP4) rev B silicon and forward)
34.3.1.2 Plane Management Functions Performed by Microcode
As stated above, most plane management functions must be performed in software.
However, certain statistics are intimately related to the lower-level user plane functions,
and are thus best provided by the microcode. These include the following:
• ICP violations
• Transmit stuff events
• Receive stuff events
34.3.2 Transmit Architecture
This section discusses the behavior of the IMA microcode during transmission, focusing
particularly on the independent transmit clock (ITC) mode of IMA. Differences in behavior
when common transmit clock (CTC) mode is used are discussed at the end of this section.
Only one cell scheduler (known as the ATM pace controller or APC) is used per IMA
group. This APC schedules transmission for the IMA group as a single aggregate channel.
The APC hands these cells off to the IMA Tx microcode, which distributes these scheduled
cells to each of the PHYs in the IMA group. To compensate for clocking differences (jitter
and average speed differential), the IMA Tx process distributes ATM cells into N jitter
buffers with a depth of 5 cells. The IMA PHYs take cells from these jitter buffers and
transmit them.
The cell scheduling is triggered by requests from the timing reference link (i.e. assertion of
TxClav from the TRL's PHY). Requests from non-TRL PHYs only interact with the jitter
buffer, and perform the stuffing function as needed (when the jitter buffer becomes too
shallow). Therefore, the microcode tasks performed in response to TRL PHY requests and
non-TRL PHY requests are different.
MOTOROLA
Freescale Semiconductor, Inc.
Chapter 34. Inverse Multiplexing for ATM (IMA)
For More Information On This Product,
Go to: www.freescale.com
IMA Microcode Architecture
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