Flow control
Encyclopedia
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 . 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.
Windowing Flow control
data buffer
.
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
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:
is usually referred to as software flow control.
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) .
(see traffic contract
and congestion control) . Transmit Flow Control described above is a form of Closed-loop flow control.
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.
Types of flow control
Network congestion- A prevention mechanism that provides control over the quantity of data transmission that enters a device.
Windowing Flow control
- mechanism used with TCPTransmission Control ProtocolThe Transmission Control Protocol is one of the core protocols of the Internet Protocol Suite. TCP is one of the two original components of the suite, complementing the Internet Protocol , and therefore the entire suite is commonly referred to as TCP/IP...
.
data buffer
- A prevention control mechanism that provides storage to contain data- bursts from other network devices, compensating for the variation of data transmission speeds.
Transmit flow control
An earlier version of this section was based on Federal Standard 1037CFederal Standard 1037C
Federal Standard 1037C, titled Telecommunications: Glossary of Telecommunication Terms is a United States Federal Standard, issued by the General Services Administration pursuant to the Federal Property and Administrative Services Act of 1949, as amended....
.
Transmit flow control may occur:
- between data "on and off" terminal equipmentTerminal equipmentIn telecommunication, the term terminal equipment has the following meanings:* Communications equipment at either end of a communications link, used to permit the stations involved to accomplish the mission for which the link was established....
(DTEData terminal equipmentData Terminal Equipment is an end instrument that converts user information into signals or reconverts received signals. These can also be called tail circuits. A DTE device communicates with the data circuit-terminating equipment...
) and a switching center, via data circuitElectrical networkAn electrical network is an interconnection of electrical elements such as resistors, inductors, capacitors, transmission lines, voltage sources, current sources and switches. An electrical circuit is a special type of network, one that has a closed loop giving a return path for the current...
-terminating equipment (DCEData circuit-terminating equipmentA data circuit-terminating equipment is a device that sits between the data terminal equipment and a data transmission circuit. It is also called data communications equipment and data carrier equipment...
), the opposite types interconnected straightforwardly, - or between two devices of the same type (two DTEs, or two DCEs), interconnected by a crossover cableCrossover cableA crossover cable connects two devices of the same type, for example DTE-DTE or DCE-DCE, usually connected asymmetrically , by a modified cable called a crosslink...
.
The transmission
Transmission (telecommunications)
Transmission, in telecommunications, is the process of sending, propagating and receiving an analogue or digital information signal over a physical point-to-point or point-to-multipoint transmission medium, either wired, optical fiber or wireless...
rate may be controlled because of network
Telecommunications network
A telecommunications network is a collection of terminals, links and nodes which connect together to enable telecommunication between users of the terminals. Networks may use circuit switching or message switching. Each terminal in the network must have a unique address so messages or connections...
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
- either stop-and-go,
- or use a sliding window.
Flow control can be performed
- either by control signal lines in a data communication interface (see serial portSerial portIn computing, a serial port is a serial communication physical interface through which information transfers in or out one bit at a time...
and RS 232), - or by reserving in-band control characters to signal flow start and stop (such as the ASCIIASCIIThe American Standard Code for Information Interchange is a character-encoding scheme based on the ordering of the English alphabet. ASCII codes represent text in computers, communications equipment, and other devices that use text...
codes for XON/XOFFXon/XoffSoftware flow control is a method of flow control used in computer data links, especially RS-232 serial. It uses special codes, transmitted in-band, over the primary communications channel. These codes are generally called XOFF and XON . Thus, "software flow control" is sometimes called...
).
Hardware flow control
In common RS 232 there are pairs of control lines:- RTS flow control, RTS (Request To Send)/CTS (Clear To Send) and
- DTR flow control, DTR (Data Terminal ReadyData Terminal ReadyData Terminal Ready, abbreviated as DTR, is a control signal present inside an RS-232 serial communications cable that goes between a computer and another device, such as a modem...
)/DSR (Data Set Ready),
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:
- In case of RTS control flow, DTE sets its RTS, which signals the opposite end (the slave end such as a DCE) to begin monitoring its data input line. When ready for data, the slave end will raise its complementary line, CTS in this example, which signals the master to start sending data, and for the master to begin monitoring the slave's data output line. If either end needs to stop the data, it lowers its respective "data readyness" line.
- For PC-to-modem and similar links, the case of DTR flow control, DTR/DSR are raised for the entire modem session (say a dialup internet call), and RTS/CTS are raised for each block of data.
Software flow control
Oppositely, XON/XOFFXon/Xoff
Software flow control is a method of flow control used in computer data links, especially RS-232 serial. It uses special codes, transmitted in-band, over the primary communications channel. These codes are generally called XOFF and XON . Thus, "software flow control" is sometimes called...
is usually referred to as software flow control.
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 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
Asynchronous Transfer Mode
Asynchronous Transfer Mode is a standard switching technique designed to unify telecommunication and computer networks. It uses asynchronous time-division multiplexing, and it encodes data into small, fixed-sized cells. This differs from approaches such as the Internet Protocol or Ethernet that...
in its CBR, VBR
Variable bitrate
Variable bitrate is a term used in telecommunications and computing that relates to the bitrate used in sound or video encoding. As opposed to constant bitrate , VBR files vary the amount of output data per time segment...
and UBR services (see traffic contract
Traffic contract
If a service wishes to use a broadband network to transport a particular kind of traffic, it must first inform the network about what kind of traffic is to be transported, and the performance requirements of that traffic...
and congestion control) .
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 ABRAvailable Bit Rate
Available Bit Rate is a service used in ATM networks when source and destination don't need to be synchronized. ABR does not guarantee against delay or data loss. ABR mechanisms allow the network to allocate the available bandwidth fairly over the present ABR sources...
(see traffic contract
Traffic contract
If a service wishes to use a broadband network to transport a particular kind of traffic, it must first inform the network about what kind of traffic is to be transported, and the performance requirements of that traffic...
and congestion control) . Transmit Flow Control described above is a form of Closed-loop flow control.
See also
- Software flow control
- Computer networking
- Traffic contractTraffic contractIf a service wishes to use a broadband network to transport a particular kind of traffic, it must first inform the network about what kind of traffic is to be transported, and the performance requirements of that traffic...
- Congestion control
- Teletraffic engineering in broadband networksTeletraffic engineering in broadband networksTeletraffic engineering is a well-understood discipline in the traditional telephone network, where traffic patterns are established, growth rates can be predicted, and vast amounts of detailed historical data are available for analysis. However, for modern broadband networks, the teletraffic...
- Teletraffic engineeringTeletraffic engineeringTelecommunications traffic engineering, teletraffic engineering, or traffic engineering is the application of traffic engineering theory to telecommunications...
- Ethernet flow controlEthernet flow controlEthernet flow control is a mechanism for temporarily stopping the transmission of data on Ethernet family computer networks.-Description:Ethernet is a popular family of computer network protocols. Flow control can be implemented at the data link layer...
- HandshakingHandshakingIn information technology, telecommunications, and related fields, handshaking is an automated process of negotiation that dynamically sets parameters of a communications channel established between two entities before normal communication over the channel begins...