<P> Conversely, XON / XOFF is usually referred to as software flow control . </P> <P> 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 . </P> <P> Open - loop flow control has inherent problems with maximizing the utilization of 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 and reserved but unused capacities are wasted . Open - loop flow control is used by ATM in its CBR, VBR and UBR services (see traffic contract and congestion control). </P> <P> Open - loop flow control incorporates two controls; the controller and a regulator . The regulator is able to alter the input variable in response to the signal from the controller . An open - loop system has no feedback or feed forward mechanism, so the input and output signals are not directly related and there is increased traffic variability . There is also a lower arrival rate in such system and a higher loss rate . In an open control system, the controllers can operate the regulators at regular intervals, but there is no assurance that the output variable can be maintained at the desired level . While it may be cheaper to use this model, the open - loop model can be unstable . </P>

Concept of flow control and buffering in transport layer