1 user plane functions performed by microcode, 2 transmit architecture, User plane functions performed by microcode -11 – Freescale Semiconductor MPC8260 User Manual
Page 1113: Transmit architecture -11
Inverse Multiplexing for ATM (IMA)
MPC8260 PowerQUICC II Family Reference Manual, Rev. 2
Freescale Semiconductor
33-11
33.3.1.1
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 .25
µm (HiP4) rev B silicon and forward)
33.3.1.2
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
33.3.2
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.