Reliability (computer networking): Difference between revisions

Revision as of 20:37, 16 July 2020

Revision as of 15:34, 9 September 2020

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==Real-time systems==

There is, however, a problem with the definition of reliability as "delivery or notification of failure" in [[real-time computing]]. In such systems, failure to deliver the real-time data will adversely affect the performance of the systems, and some systems, e.g. [[safety-critical]], [[Safety-involved systems|safety-involved]], and some secure [[mission-critical]] systems, must be [[formal methods|proved]] to perform at some specified minimum level. This, in turn, requires that a specified minimum reliability for the delivery of the critical data be met. Therefore, in these cases, it is only the delivery that matters; Notification of the failure to deliver does ameliorate the failure. In [[hard real-time system]]s, all data must be delivered by the deadline or it is considered a system failure. In [[firm real-time system]]s, late data is still valueless but the system can tolerate some amount of late or missing data.<ref name = "Schneider et al 2001">S., Schneider, G.,Pardo-Castellote, M., Hamilton. “Can Ethernet Be Real Time?”, Real-Time Innovations, Inc., 2001</ref><ref name = "Rubenstein et al 1998">Dan Rubenstein, Jim Kurose, Don Towsley, ”Real-Time Reliable Multicast Using Proactive Forward Error Correction”, NOSSDAV ’98</ref><!--[[User:Kvng/RTH]]-->

There are a number of protocols that are capable of meetingaddressing real-time requirements for reliable delivery and timeliness, at least for firm real-time systems (due to the inevitable and unavoidable losses from, e.g., the physical layer [[bit error rate]]s):

[[MIL-STD-1553B]] and [[STANAG 3910]] are well-known examples of such timely and reliable protocols for [[avionics#aircraft networks|avionic data buses]]. MIL-1553 uses a 1 &nbsp;Mbit/s shared media for the transmission of data and the control of these transmissions, and is widely used in federated military [[avionics]] systems (in which "Each [[System#Subsystem|system]] has its own computers performing its own functions".<ref name="Ekman_SAAB">{{citation |author=Mats Ekman, "|title=Avionic Architectures Trends and challenges", {{cite web |url=https://www.kth.se/polopoly_fs/1.146328!/Menu/general/column-content/attachment/3_Ekman_Saab.pdf |title=Archived copy |accessdate=2015-02-03 |url-status=dead |archiveurl=https://web.archive.org/web/20150203164824/https://www.kth.se/polopoly_fs/1.146328!/Menu/general/column-content/attachment/3_Ekman_Saab.pdf |archivedate=2015-02-03 |quote=Each system has its own computers performing its own functions}}</ref>). It uses a [[MIL-STD-1553B#Thebus Bus Controller|Bus Controller]]controller (BC) to command the connected [[MIL-STD-1553B#Theremote Remote Terminals|Remote Terminals]]terminals (RTs) to receive or transmit this data. The BC can, therefore, ensure that there will be no congestion, and transfers are always timely. The MIL-1553 protocol also allows for automatic retries that can still ensure timely delivery and increase the reliability above that of the physical layer. STANAG 3910, also known as EFABus in its use on the [[Eurofighter Typhoon]], is, in effect, a version of MIL-1553 augmented with a 20 &nbsp;Mbit/s shared media bus for data transfers, retaining the 1 &nbsp;Mbit/s shared media bus for control purposes.<!--[[User:Kvng/RTH]]-->

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.