Editing Frame check sequence

{{short description|Error-detecting code used in communications protocols}}

{{Use American English|date = March 2019}}

[[File:Ethernet frame.svg|thumb|Structure of an Ethernet packet, including the FCS that terminates the [[Ethernet frame]]<ref>{{Citation
 | url = https://ecfsapi.fcc.gov/file/1050839507018/IEEE%20Standard%20for%20Ethernet.pdf
 | title = 802.3-2012 - IEEE Standard for Ethernet
 | date = 2012-12-28 | access-date = 2015-07-05
 | publisher = [[IEEE Standards Association]] | section = 3.1.1 Packet format
}}</ref>]]

A '''frame check sequence''' ('''FCS''') is an [[error-detecting code]] added to a [[Frame (networking)|frame]] in a [[communication protocol]].  Frames are used to send [[payload data]] from a source to a destination.

== Purpose ==
All frames and the bits, bytes, and fields contained within them, are susceptible to errors from a variety of sources. The FCS field contains a number that is calculated by the source node based on the data in the frame. This number is added to the end of a frame that is sent. When the destination node receives the frame the FCS number is recalculated and compared with the FCS number included in the frame. If the two numbers are different, an error is assumed and the frame is discarded.

The FCS provides error detection only. Error recovery must be performed through separate means. [[Ethernet]], for example, specifies that a damaged frame should be discarded and does not specify any action to cause the frame to be retransmitted. Other protocols, notably the [[Transmission Control Protocol]] (TCP), can notice the data loss and initiate retransmission and error recovery.<ref>{{cite book |last1=Odom |first1=Wendell |title=CCNA 200-301 Official Cert Guide, Volume 1 |date=2019-10-10 |publisher=Cisco Press |isbn=9780135792735 |url=https://books.google.com/books?id=LyCtDwAAQBAJ}}</ref>

==Implementation==
[[File:Ethernet Frame.png|thumb|Detailed structure of an [[Ethernet frame]]]]

The FCS is often transmitted in such a way that the receiver can compute a running sum over the entire frame, together with the trailing FCS, expecting to see a fixed result (such as zero) when it is correct. For [[Ethernet]] and other [[IEEE 802]] protocols, the standard states that data is sent least significant bit first, while the FCS is sent most significant bit (bit 31) first. An alternative approach is to generate the bit reversal of the FCS so that the reversed FCS can be also sent least significant bit (bit 0) first. Refer to {{section link|Ethernet_frame|Frame check sequence}} for more information.

==Types==
By far the most popular FCS algorithm is a [[cyclic redundancy check]] (CRC), used in Ethernet and other IEEE 802 protocols with 32 bits, in [[X.25]] with 16 or 32 bits, in [[HDLC]] with 16 or 32 bits, in [[Frame Relay]] with 16 bits,<ref name="cisco-technote-fr">{{cite web
 | url = http://www.cisco.com/c/en/us/support/docs/wan/frame-relay/47202-87.html
 | title = Frame Relay Glossary
 | date = 2009-04-30 | access-date = 2015-07-05
 | publisher = [[Cisco Systems]]
}}</ref> in [[Point-to-Point Protocol]] (PPP) with 16 or 32 bits, and in other [[data link layer]] [[communications protocol|protocols]].

Protocols of the [[Internet protocol suite]] tend to use [[checksum]]s.<ref>{{cite IETF |rfc=1071 |title=Computing the Internet Checksum}}</ref>

== See also ==
* [[Syncword]]

== References ==
{{Reflist}}

{{DEFAULTSORT:Frame Check Sequence}}
[[Category:Link protocols]]
[[Category:Logical link control]]
[[Category:Packets (information technology)]]