Freescale Semiconductor MPC8260 - Manual (Page 1159)

Inverse Multiplexing for ATM (IMA)
MPC8260 PowerQUICC II Family Reference Manual, Rev. 2
Freescale Semiconductor 33-57
Per the above explanation and examples, it is seen that TNUMLINKS is the only parameter which needs
to be modified by software when a link is added or dropped from an IMA group. All other APC parameters
need not be modified. [Note, however, that if links are dropped such that the total scheduled bandwidth of
the ATM channels is greater than 100% of the IMA group bandwidth, this will result eventually in APC
overruns, and should therefore be corrected and/or avoided.
NOTE
Software should ensure that the length of the APC scheduling table is
increased if links are added to the IMA group. When incrementing the
number of links in an IMA group, the user might exceed the length of the
APC scheduling table; if this happens, the ATM channel is mapped outside
of the APC scheduling table and the channel stops.
33.4.9.1 Programming for CBR, UBR, VBR, and UBR+
All APC parameters for CBR, UBR, VBR, and UBR+ channels which will be transmitted over IMA
groups should be scaled by the intended steady-state value of TNUMLINKS. Their values should be
divided by TNUMLINKS, as they will be scaled by TNUMLINKS when the pacing algorithms are
performed. The parameters which must be scaled include: PCR, SCR, OOBR, BT, MCR, and MDA.
33.4.9.2 Programming for ABR
ABR channels are a special case, in that they are not programmed as a percentage of the physical line rate
as inferred from the period of requests from the PHY layer. Instead, the rate of an ABR channel is
programmed as a percentage of an explicitly-provided parameter, the LINE_RATE_ABR, which is
programmed in the APC parameter table for each APC. Therefore, when links are added or dropped from
an IMA group which carries ABR traffic, the parameter LINE_RATE_ABR must also be scaled to reflect
the change in bandwidth of the group. For example, if TNUMLINKS increases from three to four, then
LINE_RATE_ABR must also be multiplied by 4/3.
4 Assume that one link is dropped from the IMA group in Example 4. The overall bandwidth of the group is
now 8Mbps, and TNUMLINKS becomes 4. Therefore, the pace for the 6Mbps CBR channel described
above scales to 4/3 (PCR=1, PCR_Fraction=84), indicating that the channel will use 75% of the bandwidth
of the group, which is still 6Mbps.
5 Assume that two links more are dropped from the IMA group in Example 4. The overall bandwidth of the
group is now 4Mbps, and TNUMLINKS becomes 2. Therefore, the pace for the 6Mbps CBR channel
described above scales to 2/3 (PCR=0, PCR_Fraction=170). This is less than 1, which is not possible to
support, so it is rounded up to 1. The channel originally programmed for 6Mbps now consumes 100% of
the 4Mbps IMA group.
Table 33-28. Examples of APC Programming for IMA
Example Description