In data communications, flow control is the process of managing the pacing of data transmission between two nodes to prevent a fast sender from outrunning a slow receiver. It provides a mechanism for the receiver to control the transmission speed, so that the receiving node is not overwhelmed with data from transmitting node. Flow control should be distinguished from congestion control, which is used for controlling the flow of data when congestion has actually occurred [1]. Flow control mechanisms can be classified by whether or not the receiving node sends feedback to the sending node.
Flow control is important because it is possible for a sending computer to transmit information at a faster rate than the destination computer can receive and process them. This can happen if the receiving computers have a heavy traffic load in comparison to the sending computer, or if the receiving computer has less processing power than the sending computer.
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An earlier version of this section was based on Federal Standard 1037C.
Transmit flow control may occur:
The transmission rate may be controlled because of network or DTE requirements. Transmit flow control can occur independently in the two directions of data transfer, thus permitting the transfer rates in one direction to be different from the transfer rates in the other direction. Transmit flow control can be
Flow control can be performed
In common RS 232 there are pairs of control lines:
which are usually referred to as hardware flow control.
Hardware flow control is typically handled by the DTE or "master end", as it is first raising or asserting its line to command the other side:
Oppositely, XON/XOFF is usually referred to as software 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 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) [2].
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) [3]. Transmit Flow Control described above is a form of Closed-loop flow control.