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Flow ControlThe flow control mechanism is used for controlling the flow of data in a network under well-defined conditions, while congestion control is used for controlling the flow of data when congestion has actually occurred 1. Flow control mechanisms are classified by whether or not they have feedback. Open-loop flow control The Open Loop flow control mechanism is characterized by having no feedback between the receiver and the transmitter. This simple means of control is widely used. The allocation of resources must be a prior reservation or hop-to-hop type. The Open Loop flow control has inherent problems with maximizing the utilization of the ATM network resources. Resource allocation is made at connection setup using a CAC (Connection Admission Control) and this allocation is made using information that is already old news during the lifetime of the connection. Often there is an over-allocation of resources. Open-Loop flow control is used by the CBR, VBR and UBR services (see traffic contract and congestion control) 1. Closed-loop flow control The Closed Loop flow control mechanism is characterized by the ability of the network to report pending network congestion back to the transmitter. This information is then used by the transmitter in various ways to adapt its activity to existing network conditions. Closed Loop flow control is used by ABR (see traffic contract and congestion control) 1. See Also References 1 Network Testing Solutions, ATM Traffic Management White paper, http://scholar.google.com/scholar?hl=en&lr=&safe=off&q=cache:FAB0FGEX85cJ:www.parallaxresearch.com/dataclips/pub/infotech/protocols/ATM/ATM_Traffic_Mgmt.PDF+comparison+GCRA+algorithms, last accessed 15 March 2005.
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