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Network flow control manages traffic congestion in communication networks using techniques for both circuit-based and packet-switched networks. Connection admission control is used for circuit networks, while packet-switched networks use buffer storage and window scaling in TCP networks. RS-232 discusses software-based XON-XOFF flow control and hardware-based transmission control lines.
Network flow control is a means for communication networks to manage traffic congestion on the network. Various techniques can be used for both circuit-based networks and packet-switched networks such as those used on the Internet. Through the use of flow control techniques, the network is able to provide better overall quality of service and reduce the frequency of lost communications.
Circuit networks, such as time division multiplexing networks for some types of digital communication, typically use dedicated circuits for communication. In these cases, the network may lose the use of all its available channels for bandwidth. If a channel is idle while other traffic is waiting to be transmitted, network flow control techniques can be implemented to balance the array of available communication circuits.
With such circuit networks, a technique known as connection admission control (CAC) is used for proper network flow control. Algorithms are used to observe the network and make decisions about the use of available resources. In general, the algorithms work to allow the use of a circuit according to the evaluation of some variables, such as the type of service required, the quality of service to be satisfied and other traffic information such as peak rates and sustainable rates. One drawback is that some CAC methods also prioritize available circuits, thus reserving some circuits for occasional high-priority connections, which defeats the purpose since those circuits still remain idle if there is no high-priority communication in transit.
With packet switched networks, a myriad of possible network flow control techniques can be implemented depending on the type of network or the software or hardware available. In most cases, a packet switched network will use some sort of buffer, which is a storage mechanism to capture data packets as they are received from a sender. The packets go into the buffer and the receiving application extracts them for use. If the sending transmission is faster than the receiver’s ability to process data from the buffer, it can fill up quickly.
In Transmission Control Protocol (TCP) networks, network flow control can be managed through a technique known as window scaling. The three-way handshaking that occurs during a TCP connection transmits information about the size of the recipient’s window. This way, the recipient can let the sender know how much buffer space is available so that the sender can avoid sending too much data.
The recommended Telecommunications Standard 232 (RS-232) article also discusses provisions for managing network flow control in serialized networks. With RS-232, it is divided into a software-based solution or a hardware-based solution. The software technique is also known as XON-XOFF flow control, where an XON signal is sent by the receiver claiming to be able to receive data and XOFF when it has a full buffer. Errors can occur if the communication signal is bad, whereby the sender may not properly receive an XOFF signal and continue sending packets to the recipient. Flow control signals are also sent over the same channel as data communication, which uses a small amount of bandwidth.
Hardware-based network flow control, on the other hand, is much more reliable at the additional expense of implementing additional physical lines. In this case, separate transmission control lines are set up: one is called the request to send (RTS) line and the other is the clear send (CTS) line. The sending end announces its intention to transmit on the RTS line and the receiver announces its ability to receive on the CTS line.
