3Com 7600 manual 1Example of a source to destination ABR control loop

13-2CHAPTER 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 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 ABR flow control occurs between a sending end-system (source), such as for ABR 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 13-1Example 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: