Flow control (data): Difference between revisions

Revision as of 18:02, 29 August 2018

Revision as of 21:57, 10 November 2018

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The sender needs to wait for the ACK after every frame it transmits. This is a source of inefficiency, and is particularly bad when the [[propagation delay]] is much longer than the [[transmission delay]].<ref name="PC-Radio Flow Control" />

Stop and wait can also create inefficiencies when sending longer transmissions.<ref name=ak2 /> When longer transmissions are sent there is more likely chance for error in this protocol. If the messages are short the errors are more likely to be detected early. More inefficiency is created when single messages are broken into separate frames because it makes the transmission longer.<ref name=lwilliam />

== Sliding Window ==

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* For PC-to-modem and similar links, in the case of DTR flow control, DTR/DSR are raised for the entire modem session (say a dialup internet call where DTR is raised to signal the modem to dial, and DSR is raised by the modem when the connection is complete), and RTS/CTS are raised for each block of data.

An example of hardware flow control is a [[Half-duplex]] radio modem to computer interface. In this case, the controlling software in the modem and computer may be written to give priority to incoming radio signals such that outgoing data from the computer is paused by lowering CTS if the modem detects a reception.

* Polarity:

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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 [[Asynchronous Transfer Mode|ATM]] in its [[Constant Bit Rate|CBR]], [[Variable bitrate|VBR]] and [[Unspecified Bit Rate|UBR]] services (see [[traffic contract]] and [[congestion control]]).<ref name=ATM-Traffic-Management />

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.

== 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 [[Available Bit Rate|ABR]] (see [[traffic contract]] and [[congestion control]]).<ref name=ATM-Traffic-Management /> Transmit flow control described above is a form of closed-loop flow control.

This system incorporates all the basic control elements, such as, the sensor, transmitter, controller and the regulator. The sensor is used to capture a process variable. The process variable is sent to a transmitter which translates the variable to the controller. The controller examines the information with respect to a desired value and initiates a correction action if required. The controller then communicates to the regulator what action is needed to ensure that the output variable value is matching the desired value. Therefore, there is a high degree of assurance that the output variable can be maintained at the desired level. The closed-loop control system can be a feedback or a feed forward system:

A feedback closed-loop system has a feed-back mechanism that directly relates the input and output signals. The feed-back mechanism monitors the output variable and determines if additional correction is required. The output variable value that is fed backward is used to initiate that corrective action on a regulator. Most control loops in the industry are of the feedback type.

In a feed-forward closed loop system, the measured process variable is an input variable. The measured signal is then used in the same fashion as in a feedback system.

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<ref name="Go Back N">[http://www.credoreference.com/entry/bhfidt/go_back_n_arq Focal Dictionary of Telecommunications, Focal Press ] last accessed 3 December 2012.</ref>

<ref name="Selective Repeat">[https://docs.google.com/viewer?a=v&q=cache:rp6GPovdFiIJ:facta.junis.ni.ac.rs/eae/fu2k02/fu06.pdf+&hl=en&gl=us&pid=bl&srcid=ADGEESiFJNDv8E5p9LS_AHD-zZHMUUrWx5FRTU7xolQL_D58JT3mHXPwldc0CQu32LFShXmnc0MleeH6GUvw0qL3jGuxlwlh_SYoA2h0NZqOruQA3mUXAcEK7YW7_lbx_FSKP1ou5473&sig=AHIEtbTxR21FrNmWYX-vN8cV0S3hi4yg2A Data Transmission over Adpative HF Radio Communication Systems using Selective Repeat Protocol ] last accessed 3 December 2012.</ref>

<ref name=ak2>[{{cite web|url=http://www.angelfire.com/ak2/wireless/flowctrl.html]|author=arun|website=angelfire.com|title=Flow lastControl accessedTechniques |date=20 November 2012.|accessdate=10 November 2018}}</ref>

<ref name=lwilliam>[{{cite web|url=http://people.bridgewater.edu/~lwilliam/Chapter%2005/sld053.htm] |website=people.bridgewater.edu|title=last accessed 1 December 2012.|date=1 December 2012|accessdate=10 November 2018}}</ref>

<ref name="PC-Radio Flow Control">[{{cite web|url=http://www.theparticle.com/cs/bc/net/flowctrl.pdf]|date=28 lastSeptember accessed2005|title=ERROR 23CONTROL |accessdate=10 November 2012.2018}}</ref>

}}