MC92501 Datasheet, PDF (12/52 Pages) Motorola, Inc – ATM Cell Processor

SECTION 5. AVAILABLE BIT RATE (ABR) SUPPORT
5.1. Overview and Features
The MC92501 provides a full Available Bit Rate (ABR)
solution for switch behavior relative rate marking and EFCI
marking in accordance with ATM Forum TM 4.0. It also
provides the switch fabric with an interface to increase the RM
cellsÕ trafÞc priority. Following is a list of features:
Â¥ Performs Relative Rate (RR) marking on Forward
Resource Management (FRM) and/or Backward Resource
Managment (BRM) cells, on selected connections. This
feature is enabled by either setting the ATMC CFB
ConÞguration RegisterÕs (ACR) VP RM Cell PTI (NPRP) bit
or by setting the PTI Þeld in the cellÕs header to Ò110BÓ.
Â¥ Performs EFCI marking on non-RM cells whose PTI[2] =
0, on selected connections. This feature is enabled by
either control registers or by Þelds that it gets from the
overhead of cells which are received from the switch fabric.
Â¥ Resets EFCI on non-RM cells whose PTI[2] = 0, on
selected connections.
Â¥ Checks CRC on received RM cells and generates CRC for
transmitted RM cells.
Â¥ Provides different priority to RM cells.
Â¥ Can copy RM cells to the microprocessor or remove them
from the ßow.
5.2. RM Cell Definition
A cell is an RM cell if and only if at least one of the following
conditions is met:
Â¥ The cell belongs to a VC connection and its PTI = 6.
Â¥ The cell belongs to a VP connection, its VCI = 6, and its
PTI = 6.
Â¥ The cell belongs to a VP connection, its VCI = 6, and the
ATMC CFB ConÞguration Register has theVPRPÑVP RM
Cell PTI bit set.
5.3. RM Cell Fields
Header = 5 bytes
Payload = 48 bytes
8 8 2x8 2x8 2x8
4x8
4x8
GFC/ VPI VCI PTI CLP HEC PID
ER CCR MCR
QL
SN
VPI
30 x 8 + 6
Reserved
10
CRC-10
DIR BN
CI
NI
RA
Reserved
1
1
1
1
1
3
NOTES:
PID = 1
DIR = Direction
Â¥ 0 = Forward RM cell
Â¥ 1 = Backward RM cell
BN = Backward Explicit Congestion
Â¥ 0 = Generated by source
Â¥ 1 = Generated not by the source
CI = Congestion Indication
NI = No Increase Bit
ER = Explicit Rate
CCR = Current Cell Rate
MCR = Minimum Cell Rate
CRC - 10
Figure 9. RM Cell Fields
MC92501
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