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21-5
Cisco BPX 8600 Series Installation and Configuration
Release 9.3.10, Part Number 78-11603-01 Rev. D0, July 2001
Chapter 21 Configuring ATM Connections
Traffic Management Overview
Standard Available Bit Rate
Standard Abr uses RM (Resource Management) cells to carry feedback information back to the
connection’s source from the connection’s destination.
Abr sources periodically interleave RM cells into the data they are transmitting. These RM cells are
called forward RM cells because they travel in the same direction as the data. At the destination these
cells are turned around and sent back to the source as Backward RM cells.
The RM cells contain fields to increase or decrease the rate (the CI and NI fields) or set it at a particular
value (the explicit rate ER field). The intervening switches may adjust these fields according to network
conditions. When the source receives an RM cell it must adjust its rate in response to the setting of these
fields.
VSVD Description
Abr sources and destinations are linked via bi-directional connections, and each connection termination
point is both a source and a destination; a source for data that it is transmitting, and a destination for
data that it is receiving. The forward direction is defined as from source to destination, and the
backward direction is defined as from destination to source.
Figure 21-2 shows the data cell flow in the forward direction from a source to its destination along with
its associated control loop. The control loop consists of two RM cell flows, one in the forward direction
(from source to destination) and the other in the backward direction (from destination to source).
The data cell flow in the backward direction from destination to source is not shown, nor are the
associated RM cell flows. However, these flows are just the opposite of that shown in the diagram for
forward data cell flows.
A source generates forward RM cells which are turned around by the destination and returned to the
source as backward RM-cells. These backward RM-cells may carry feedback information from the
network elements and/or the destination back to the source.
The parameter Nrm is defined as the maximum number of cells a source may send for each forward RM
cell, that is, one RM cell must be sent for every Nrm-1 data cells. Also, in the absence of Nrm-1 data
cells, as an upper bound on the time between forward RM cells for an active source, an RM cell must
be sent at least once every Trm msecs.
BXM Connections
The BXM-T3/E3, BXM-155, and BXM-622 cards support ATM Traffic Management 4.0.
The BXM cards are designed to support all the following service classes:
• Constant Bit Rate (Cbr)
• real time Variable Bit Rate (rt-Vbr)
• non-real time Variable Bit Rate (nrt-Vbr)
• Available Bit Rate (Abr with VSVD
• Abr without VSVD, and Abr using ForeSight)
• Unspecified Bit Rate (Ubr)
Abr with VSVD supports explicit rate marking and Congestion Indication (CI) control.