ATM Pace Control
MPC885 PowerQUICC Family Reference Manual, Rev. 2
Freescale Semiconductor
40-7
For example, again using the previous example with P=51.84Mbps and NCIST=4, assume that a channel
with a bit rate of 100kbps is desired. APC_Pace should therefore be programmed to 51.84Mbps / (4 *
100kbps) = 129.6. This can be approximated by programming APCP=129 and APCPF=39322.
For another example, assume the desired bit rate is 10Mbps. Then APC_Pace should be programmed to
51.84Mbps / (4 * 10Mbps) = 1.296. This can be approximated by programming APCP=1 and APCPF =
19399.
Note that APC_Pace consists of an integer and a fraction. A channel with a non-integer APC_Pace will be
scheduled such that its average pace will be as defined by APCP and APCPF. For example, if APCP and
APCPF were programmed to define a pace of 1.5, then after transmission the channel would be alternately
rescheduled either one time slot following or two time slots following, averaging to 1.5.
For CBR channels, use a high priority APC level (usually the first APC level 1) when activating the
channel with the
TRANSMIT
ACTIVATE
CHANNEL
command.
NOTE
When using AAL5 VBR for transmit, after AAL5 Tx under run situation
(UN bit is set in Interrupt Queue Entry), if the software makes the AAL5
Tx BD ready again (set TxBD[R] bit), the immediate pace between the 1st
cell and 2nd cell of this AAL5 frame becomes OOBR, which should be PCR
according to VBR algorithm. To solve this Tx underrun situation, right
before setting TxBD[R] bit, software should first clear the BNR bit in the
TCT. Then the immediate pace between the 1st cell and 2nd cell of this
AAL5 frame will be PCR.
40.1.6
Programming Rates for VBR Channels
The bit rate for a VBR ATM channel is defined by several parameters: peak cell rate (PCR), sustainable
cell rate (SCR) and burst tolerance (BT), which represents the bucket size in the GCRA (leaky bucket)
algorithm. The MPC885 implements VBR according to the GCRA as defined in ATM Forum T.M-4.0
Figure
shows the credit mechanism which is used in order to generate VBR traffic. The VBR traffic
generation is based the following parameters:
•
SCR - Credit is accumulated according to SCR. This assures that the overall bit rate never exceeds
the sustainable cell rate.
•
BT - Represents the bucket size. This is the maximum number of credit tokens that can be
accumulated.
•
Buffer (data) ready - Represents the gate control. If data is not ready, credits are accumulated. As
soon as data is ready, there is a burst at the PCR rate of a maximum BT size (according to the
available credit). After the burst, if there is still available data to send, it is sent at the SCR rate;
that is, a cell is sent as soon as a credit appears in the bucket.
•
PCR - Represents the barrel limiter. This is the peak cell rate limiter for this channel.
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Страница 1004: ...Buffer Descriptors and Connection Tables MPC885 PowerQUICC Family Reference Manual Rev 2 37 30 Freescale Semiconductor ...
Страница 1022: ...ATM Parameter RAM MPC885 PowerQUICC Family Reference Manual Rev 2 38 18 Freescale Semiconductor ...
Страница 1068: ...ATM Pace Control MPC885 PowerQUICC Family Reference Manual Rev 2 40 22 Freescale Semiconductor ...
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Страница 1162: ...Fast Ethernet Controller FEC MPC885 PowerQUICC Family Reference Manual Rev 2 45 40 Freescale Semiconductor ...
Страница 1172: ...SEC Lite Overview MPC885 PowerQUICC Family Reference Manual Rev 2 46 8 Freescale Semiconductor ...
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Страница 1214: ...SEC Lite Execution Units MPC885 PowerQUICC Family Reference Manual Rev 2 48 38 Freescale Semiconductor ...
Страница 1312: ...Byte Ordering MPC885 PowerQUICC Family Reference Manual Rev 2 A 8 Freescale Semiconductor ...
Страница 1313: ...MPC885 PowerQUICC Family Reference Manual Rev 2 Freescale Semiconductor B 1 Appendix B Serial Communications Performance TBD ...
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