Cisco BPX 8650, Installation And Configuration Manual

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Cisco BPX 8600 Series Installation and Configuration
Release 9.3.30, Part Number 78-12907-01 Rev. E0, May 2005
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. which is 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.
VS/VD Description
ABR sources and destinations are linked through bidirectional 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).
Figure 21-2 ABR VS/VD Flow Control Diagram
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.
Forward flow data cells
Forward RM cells
Backward RM cells
Source Destination
Node
NE
Node
NE
Node
NE
RM cell control loop for forward flow data
Only the flows for forward data cells and their associated RM cell
control loop are shown in this diagram. The flows for backward
flow data cells (destination to source) and their associated RM
cell control loop are just the opposite of that shown for the forward
flow data cells.
NE = Network element
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