Reliability (computer networking): Difference between revisions

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The [[Asynchronous Transfer Mode]] (ATM), the [[Avionics Full-Duplex Switched Ethernet]] (AFDX), and [[Time Triggered Ethernet]] (TTEthernet) are examples of packet-switched networks protocols where the timeliness and reliability of data transfers can be assured by the network. AFDX and TTEthernet are also based on IEEE 802.3 Ethernet, though not entirely compatible with it.

ATM uses connection-oriented [[virtual channel]]s (VCs) which have fully deterministic paths through the network, and [[UPC and NPC|usage and network parameter control]] (UPC/NPC), which are implemented within the network, to limit the traffic on each VC separately. This allows the usage of the shared resources (switch buffers) in the network to be calculated from the parameters of the traffic to be carried in advance, i.e. at system design time. That they are implemented by the network means that these calculations remain valid even when other users of the network behave in unexpected ways, i.e. transmit more data than they are expected to. The calculated usages can then be compared with the capacities of these resources to show that, given the constraints on the routes and the bandwidths of these connections, the resource used for these transfers will never be over-subscribed. These transfers will therefore never be affected by congestion and there will be no losses due to this effect. Then, from the predicted maximum usages of the switch buffers, the maximum delay through the network can also be predicted. However, for the reliability and timeliness to be proved, and for the proofs to be tolerant of faults in and malicious actions by the equipment connected to the network, the calculations of these resource usages cannot be based on any parameters that are not actively enforced by the network, i.e. they cannot be based on what the sources of the traffic are expected to do or on statistical analyses of the traffic characteristics (see [[network calculus]]).<ref>{{cite journal| first1=Y. J. | last1=Kim | first2=S. C. | last2=Chang | first3=C. K. | last3=Un | first4=B. C. | last4=Shin | title=UPC/NPC algorithm for guaranteed QoS in ATM networks | journal=Computer Communications | volume=19 | number=3 | date=March 1996 | pages=216–225 | publisher=[[Elsevier Science Publishers]] | ___location=Amsterdam, Thethe Netherlands | doi=10.1016/0140-3664(96)01063-8 }}</ref>

AFDX uses frequency ___domain bandwidth allocation and [[Traffic policing (communications)|traffic policing]], that allows the traffic on each virtual link (VL) to be limited so that the requirements for shared resources can be predicted and [[congestion prevention|congestion prevented]] so it can be proved not to affect the critical data.<ref>AFDX Tutorial, {{cite web |url=http://www.techsat.com/fileadmin/media/pdf/infokiosk/TechSAT_TUT-AFDX-EN.pdf |title=Archived copy |access-date=2015-02-03 |url-status=dead |archive-url=https://web.archive.org/web/20150618140031/http://www.techsat.com/fileadmin/media/pdf/infokiosk/TechSAT_TUT-AFDX-EN.pdf |archive-date=2015-06-18 }}</ref> However, the techniques for predicting the resource requirements and proving that congestion is prevented are not part of the AFDX standard.