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transfer delay. The ABR service does not require bounding the delay or
the delay variation experienced by a given connection.
The UBR service category is inherently open-loop. UBR is not subject to a
specific traffic contract but may be subject to a local policy in individual
switches and end-systems. The UBR service category is intended for
non-real-time applications, i.e. those not requiring tightly constrained
delay and delay variation.
Flow Control Model
for ABR
ABR flow control occurs between a sending end-system (source), such as
a 7600, and a receiving end-system (destination). Sources and
destinations are connected via bi-directional connections. For a
bi-directional ABR connection, each connection termination point is both
a source and a destination. For the sake of simplicity, only the information
flow from the source to the destination with its associated RM-cell flows
is considered. The forward direction is the direction from the source to
the destination, and the backward direction is the direction from the
destination to the source. Figure 13-1 shows the data flow and the
RM-cell flow over a VCC connection between the two stations.
Corresponding to the forward data flow from the source to the
destination, there is a control loop consisting of two RM-cell flows, one in
the forward direction and one in the backward direction.
Figure 13-1
Example of a source to destination ABR control loop
A source generates forward RM-cells which are turned around by the
destination and sent back to the source as backward RM-cells. These
backward RM-cells carry feedback information provided by the network
elements and/or the destination back to the source. A network element
may:
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