28236-DSH-001-B Datasheet, PDF (205/443 Pages) M/A-COM Technology Solutions, Inc. – ATM ServiceSAR Plus with xBR Traffic Management

CN8236
ATM ServiceSAR Plus with xBR Traffic Management
6.0 Traffic Management
6.3 ABR Flow Control Manager
6.3.7.4 Forward RM Cell
Transmission Decisions
6.3.7.5 ACR Change
Notification
The CN8236 also adjusts the transmission rates of an ABR in-rate cell stream
before sending a Forward RM cell. This adjustment utilizes a Rate Decision
Vector selection process. As stated above, four of the eight ARDB vectors are
used for CI/NI rate adjustment. The other four are used for Forward RM cell rate
adjustments.
Instead of the CI/NI bits, the Forward RM cell vector selection is based upon
the ADTF timer and the CRM counter. These internal measures may force the
connection to decrease its rate when sending an RM cell.
The ER for Forward RM cells on any channel is set in word 18 of the VCC
table entry (FWD_ER). The normal value for FWD_ER is PCR. When this field
is initialized in the VCC table entry, it must be set to the rate specified by the
exponent table entries so that the resulting selected rate is ≥ PCR. This is because
when a Forward RM cell is eventually received as a Backward RM cell, the
CN8236 maps the available cell rate (ACR) to a rate specified by the exponent
table. If PCR falls between two exponent table rates and FWD_ER is set to PCR,
the ACR of the connection is limited to the lower of the two exponent table rates,
thereby lowering the rate below PCR.
An ACR change notification mechanism per ATM Forum AF-SAA-0108*
Appendix D is implemented for both source and destination. Five fields in the
SEG VCC table entry (S_EN_NCR, S_NCR_LO, S_NCR_HI, S_NCR_TRIG,
and S_NCR_DIR) are used for source ACR change notification. Six fields in the
RSM VCC table entry (D_EN_NCR, D_NCR_LO_, D_NCR_HI,
ND_CR_TRIG, D_NCR_DIR, and ACR_NOT_ER) are used for destination
ACR/ER change notification. x_EN_NCR enables the ACR change notification
mechanism. x_NCR_LO is the low threshold value, and x_NCR_HI is the high
threshold value. x_NCR_TRIG indicates that a notification has been triggered.
x_NCR_DIR indicates which threshold was crossed last, logic high for HI, logic
low for LO. Finally, ACR_NOT_ER allows the user to choose between
destination ACR or ER change notification. A special status queue entry is
written when the following conditions occur:
• x_NCR_LO triggers a notification when
– The new value of ACR⁽¹⁾ is less than or equal to NCR_LO, AND
– Either this is the first notification OR the last notification was triggered
from NCR_HI.
• x_NCR_HI triggers a notification when
– The new value of ACR⁽¹⁾ is greater than or equal to NCR_HI, AND
– Either this is the first notification or the last notification was triggered
from NCR_LO.
NOTE: ⁽¹⁾For the destination change notification, this can be either ACR or ER
depending on ACR_NOT_ER value.
The special segmentation status queue (see Section 4.3.6) containing the
current ACR value and is indicated by the NCR bit set to a logic 1. The
SRC_NOT_DEST bit indicates whether the notification is source- or
destination-generated. In the case of a destination change notification the current
ER value (TA_ER) is also provided.
28236-DSH-001-B
Mindspeed Technologies™
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