7-4 Cisco BPX 8600 Series Reference
Traffic Management Overview
Standard ABR notes:
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 7-2 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, i.e., 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), Variable
Bit Rate (VBR), Available Bit Rate (ABR with VS/VD, ABR without VS/VD, and ABR using
Foresight), and Unspecified Bit Rate (UBR). ABR with VS/VD supports explicit rate marking and
Congestion Indication (CI) control.
