Motorola PowerQUICC II MPC8280 Series Reference Manual page 1265

Table 34-28. Examples of APC Programming for IMA (continued)
Example
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.
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.
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.
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.
34.4.9.1 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.
34.4.9.2 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,
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Freescale Semiconductor, Inc.
Description
NOTE
Chapter 34. Inverse Multiplexing for ATM (IMA)
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IMA Programming Model
34-55