Freescale Semiconductor MPC8260 - Manual (Page 1158)

Inverse Multiplexing for ATM (IMA)
MPC8260 PowerQUICC II Family Reference Manual, Rev. 2
33-56 Freescale Semiconductor
33.4.9 APC Programming for IMA
Dynamically adding and dropping links from a group changes the overall bandwidth of the group. The
bandwidth of a particular ATM channel is programmed as a percentage of the overall bandwidth, up to
100% in the case of APC pace of 1. In the case of IMA, it is desirable to allow for the dynamic addition
and deletion of links without changing the bandwidth of individual ATM channels, so that ATM traffic
contracts will not be violated. To do this without requiring updates of the APC parameters of all of the
ATM channels, the APC algorithm must be modified to consider the number of links in the group.
To accomplish this for channels which will be used with IMA groups, the APC parameters should be
programmed to define the bandwidth of a channel as a percentage of one link of the IMA group. As such,
the APC pace can be greater than 100%, in the case that a channel uses more bandwidth than a single link
can provide. The APC pace will be scaled automatically by the IMA group transmit parameter
TNUMLINKS. After scaling, if the pace required of the group is still greater than 100% of the group
bandwidth, the channel will be rescheduled at 100% of the group bandwidth. Refer to the examples in
Table 33-28 .
6 INTO1/
GRLI INTO1: Interrupt queue overflow 1. See INTOx above.
GRLI: Global red-line interrupt. GRLI is set when a free buffer pool’s RLI flag is set. The RLI
flag is also set in the free buffer pool’s parameter table.
7 INTO0/
GBPB INTO0: Interrupt queue overflow 1. See INTOx above.
GBPB: Global buffer pool busy interrupt. GBPB is set when a free buffer pool’s BUSY flag is
set. The BUSY flag is also set in the free buffer pool’s parameter table.
Table 33-28. Examples of APC Programming for IMA
Example Description
1 [The simplest case.] For an IMA group consisting of one 2Mbps link, with one CBR channel consuming the
full bandwidth of that link (i.e. 2Mbps), TNUMLINKS for the group should be programmed to 1 and the APC
pace of the channel should be programmed to 1 (PCR=1, PCR_Fraction=0).
2 Another 2Mbps link is added to the IMA group in Example 1. The overall bandwidth of the group is now
4Mbps. However, TNUMLINKS is now 2, and therefore the programmed PCR will be scaled by 2. [Note that
the scaling is done automatically internally; the PCR programmed into the channels transmit connection
table entry is not modified.] A scaled PCR of 2 indicates that the channel uses 50% of the bandwidth of the
group, or 2Mbps. Comparing to example 1 above, we see that the bandwidth of the channel has not
changed, even though the bandwidth of the group has changed.
3 Consider an IMA group consisting of five 2Mbps links, over which a 6Mbps CBR channel is to be sent.
6Mbps is 300% of what a single 2Mbps link can provide, so its pace should be programmed as 1/3 (PCR=0,
PCR_Fraction=85). Scaling the pace by TNUMLINKS results in 5/3 (PCR=1, PCR_Fraction=169), which
indicates that the channel will use 60% of the bandwidth of the group, or 6Mbps.
Table 33-27. IDSR/IDMR Field Descriptions (continued)
Bits Name Description