13-2 CHAPTER 13: TRAFFIC MANAGEMENT IN THE ATM NETWORK
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 loc al 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 termi nation 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. Figure13-1 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.
Figure13-1 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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