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| A [[data packet]] on an [[Ethernet]] link is called an '''Ethernet frame'''. A frame begins with preamble and start frame delimiter. Following which, each Ethernet frame continues with an Ethernet header featuring destination and source [[MAC address]]es. The middle section of the frame is payload data including any headers for other protocols (e.g. [[Internet Protocol]]) carried in the frame. The frame ends with a 32 & 64 -bit [[cyclic redundancy check]] which is used to detect any corruption of data in transit.
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| ==Structure==
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| A data packet on the wire is called a frame and consists of binary data. Data on Ethernet is transmitted most-significant octet first. Within each octet, however, the least-significant bit is transmitted first.<ref>{{cite web |url=http://www.infocellar.com/networks/ethernet/frame.htm |title=Ethernet Frame |quote=Ethernet transmission is strange, in that the octet order is big-endian (leftmost octet is sent first), but bit order little-endian (rightmost, or LSB (Least Significant Bit) of the octet is sent first). |accessdate=2012-03-20}}</ref>
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| The table below shows the complete Ethernet frame, as transmitted, for the payload size up to the [[maximum transmission unit|MTU]] of 1500 [[octet (computing)|octets]].<ref group=note>The bit patterns in the preamble and start of frame delimiter are written as bit strings, with the first bit transmitted on the left (''not'' as octet values, which in Ethernet are transmitted least significant bit(s) first). This notation matches the one used in the IEEE 802.3 standard.</ref> Some implementations of [[Gigabit Ethernet]] (and higher speed ethernets) support larger frames, known as [[jumbo frame]]s.
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| The internal structure of an Ethernet frame is specified in IEEE 802.3-2012. <ref>IEEE 802.3 Clause 4.2.7.1</ref>
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| {| class="wikitable" style="text-align: center;"
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| |+ 802.3 Ethernet frame structure
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| |-
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| ! Preamble !! Start of frame delimiter !! MAC destination !! MAC source !! [[802.1Q]] tag (optional) !! [[Ethertype]] ([[Ethernet II|Ethernet II]]) or length ([[IEEE 802.3]]) !! Payload !! [[Frame check sequence]] (32‑bit [[cyclic redundancy check|CRC]]) !! [[Interframe gap]]
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| | 7 [[Octet (computing)|octet]]s || 1 octet || 6 octets || 6 octets || (4 octets) || 2 octets || {{nowrap|42{{#tag:ref |42 octet minimum applies when 802.1Q is present. When absent, 46 octet minimum applies.<ref name=8023C35>IEEE 802.3-2005 Clause 3.5</ref>|group=note}}–1500 octets}} || {{nowrap|4 octets}} || 12 octets
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| |-
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| | colspan="2" | || colspan="6" style="background:#fdd;"| {{nowrap|← 64–1518 octets (68-1522 octets for 802.1Q tagged frames) →}} ||
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| |-
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| | colspan="9" style="background:#fdd;"| {{nowrap|← 84–1538 octets (88-1542 octets for 802.1Q tagged frames) →}}
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| |}
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| ===Preamble and start frame delimiter===
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| {{see also|Syncword}}
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| A frame starts with a 7-octet preamble and 1-octet start frame delimiter (SFD).<ref group=note>Preamble and start frame delimiter are not displayed by [[packet sniffing]] software because these bits are stripped away at OSI Layer 1 by the [[network interface controller]] before being passed on to the [[OSI layer 2]] which is where packet sniffers collect their data. There are layer-2 sniffers which can capture and display the preamble and start frame delimiter but they are expensive and mainly used to detect physical related problems.</ref> Prior to [[Fast Ethernet]], the on-the-wire bit pattern for this portion of the frame is 10101010 10101010 10101010 10101010 10101010 10101010 10101010 10101011.<ref>IEEE 802.3 Clause 4.2.5</ref> Since octets are transmitted least-significant bit first the corresponding hexadecimal representation is 0x55 0x55 0x55 0x55 0x55 0x55 0x55 0xD5.
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| [[PHY (chip)|PHY transceiver chips]] used for [[Fast Ethernet]] feature a 4-bit (one [[nibble]]) [[Media Independent Interface]]. Therefore {{clarify|reason="Therefore" implies the previous sentence led into this one. As written it does not. How does a X-bit MII lead the reader to understand why the preamble is Y instances of Z?|date=January 2014}} the preamble will consist of 14 instances of 0x5, and the start frame delimiter 0x5 0xD. [[Gigabit Ethernet]] transceiver chips use a [[Gigabit Media Independent Interface]] that works 8-bits at a time, and 10 Gbit/s ([[XGMII]]) PHY works with 32-bits at a time.
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| ===Header===
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| The header features destination and source MAC addresses which have 6 octets each, the EtherType protocol identifier field and optional [[IEEE 802.1Q]] tag.
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| ====802.1Q tag====
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| The [[IEEE 802.1Q]] tag is an optional 4-octet field that indicates [[Virtual LAN]] (VLAN) membership and [[IEEE 802.1p]] priority.
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| ====EtherType or length====
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| This two-octet field can be used for two different purposes. Values of 1500 (0x05DC) and below indicate that it is used as the size of the payload in octets while values of 1536 (0x0600) and above indicate that it is used to represent [[EtherType]]. EtherType is used to indicate which protocol is encapsulated in the payload of an Ethernet Frame.
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| ===Payload===
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| The minimum payload is 42 octets when an 802.1Q tag is present and 46 octets when absent.<ref name=8023C35/><ref group=note>Minimum payload size is dictated by the 512-bit slot time used for [[Carrier sense multiple access with collision detection|collision detection]] in the Ethernet LAN architecture.</ref>{{#tag:ref |Both 42 and 46 octet minimums are valid when 802.1Q is present.<ref>IEEE 802.1Q-2011, Annex G</ref>|group=note}} The maximum payload is 1500 octets. Non-standard [[jumbo frame]]s allow for larger maximum payload size.
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| ===Frame check sequence===
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| The [[frame check sequence]] is a 4-octet cyclic redundancy check which allows detection of corrupted data within the entire frame.
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| ===Interframe gap===
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| [[Interframe gap]] is idle time between frames. After a frame has been sent, transmitters are required to transmit a minimum of 96 bits (12 octets) of idle line state before transmitting the next frame.
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| ==Ethernet frame types==
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| There are several types of Ethernet frames.
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| *Ethernet II frame, or Ethernet Version 2,<ref group=note>A version 1 Ethernet frame was used for early Ethernet prototypes and featured 8-bit MAC addresses and was never commercially deployed.</ref> or DIX frame is the most common type in use today, as it is often used directly by the Internet Protocol.
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| *[[Novell]] raw [[IEEE 802.3]] non-standard variation frame
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| *[[IEEE 802.2]] [[Logical Link Control]] (LLC) frame
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| *[[IEEE 802.2]] [[Subnetwork Access Protocol]] (SNAP) frame
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| The different frame types have different formats and [[MTU (networking)|MTU]] values, but can coexist on the same physical medium. Differentiation between frame types is possible based on the table below.
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| {| class="wikitable" style="text-align: center;"
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| |+ Ethernet frame differentiation
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| |-
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| ! Frame type !! Ethertype or length !! Payload start two bytes
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| | Ethernet II || ≥ 1536 || Any
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| | Novell raw IEEE 802.3 || ≤ 1500 || 0xFFFF
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| |-
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| | IEEE 802.2 SNAP || ≤ 1500 || 0xAAAA
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| |-
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| | IEEE 802.2 LLC || ≤ 1500 || Other
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| |}
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| In addition, all four Ethernet frames types may optionally contain an IEEE 802.1Q tag to identify what VLAN it belongs to and its priority ([[quality of service]]). This encapsulation is defined in the [[IEEE 802.3ac]] specification and increases the maximum frame by 4 octets.<!--802.3as subsequently increased maximum frame size still further-->
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| The IEEE 802.1Q tag, if present, is placed between the Source Address and the EtherType or Length fields. The first two octets of the tag are the Tag Protocol Identifier (TPID) value of 0x8100. This is located in the same place as the EtherType/Length field in untagged frames, so an EtherType value of 0x8100 means the frame is tagged, and the true EtherType/Length is located after the Q-tag. The TPID is followed by two octets containing the Tag Control Information (TCI) (the IEEE 802.1p priority ([[quality of service]]) and VLAN id). The Q-tag is followed by the rest of the frame, using one of the types described above.
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| ===Ethernet II===
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| '''Ethernet II framing''' (also known as '''DIX Ethernet''', named after [[Digital Equipment Corporation|DEC]], [[Intel]] and [[Xerox]], the major participants in its design<ref>{{cite book |title=Drew Heywood's Windows 2000 Network Services |author=Drew Heywood |author2=Zubair Ahmad |publisher=Sams |year=2001 |isbn=0-672-31741-9 |page=53}}</ref>), defines the two-octet [[EtherType]] field in an Ethernet [[Frame (telecommunications)|frame]], preceded by destination and source MAC addresses, that identifies an [[upper layer protocol]] [[Encapsulation (networking)|encapsulating]] the frame data. For example, an EtherType value of 0x0800 signals that the frame contains an [[IPv4]] datagram. Likewise, an EtherType of 0x0806 indicates an [[Address Resolution Protocol|ARP]] frame, 0x8100 indicates an IEEE 802.1Q frame and 0x86DD indicates an [[IPv6]] frame.
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| [[Image:Ethernet Type II Frame format.svg|thumb|center|700px|The most common Ethernet Frame format, type II]]
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| As this industry-developed standard went through a formal [[IEEE]] standardization process, the EtherType field was changed to a (data) length field in the new 802.3 standard.<ref group=note>Original Ethernet frames define their length with the framing that surrounds it, rather than with an explicit length count.</ref> Since the recipient still needs to know how to interpret the frame, the standard required an [[IEEE 802.2]] header to follow the length and specify the type. Many years later, the 802.3x-1997 standard, and later versions of the 802.3 standard, formally approved of both types of framing. In practice, both formats are in wide use,{{Citation needed|date=March 2011}} with original Ethernet framing the most common in Ethernet local area networks, due to its simplicity and lower overhead.
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| In order to allow some frames using Ethernet v2 framing and some using the original version of 802.3 framing to be used on the same Ethernet segment, EtherType values must be greater than or equal to 1536 (0x0600). That value was chosen because the maximum length of the payload field of an Ethernet 802.3 frame is 1500 octets (0x05DC). Thus if the field's value is greater than or equal to 1536, the frame must be an Ethernet v2 frame, with that field being a type field.<ref>{{cite book|author=LAN MAN Standards Committee of the IEEE Computer Society|title=IEEE Std 802.3x-1997 and IEEE Std 802.3y-1997|publisher=The Institute of Electrical and Electronics Engineers, Inc.|date=20 March 1997|pages=28–31}}</ref> If it's less than or equal to 1500, it must be an IEEE 802.3 frame, with that field being a length field. Values between 1500 and 1536, exclusive, are undefined.<ref>IEEE Std 802.3-2005, 3.2.6</ref> This convention allows software to determine whether a frame is an Ethernet II frame or an IEEE 802.3 frame, allowing the coexistence of both standards on the same physical medium.
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| ===Novell raw IEEE 802.3===
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| Novell's "raw" 802.3 frame format was based on early IEEE 802.3 work. Novell used this as a starting point to create the first implementation of its own [[IPX]] Network Protocol over Ethernet. They did not use any LLC header but started the IPX packet directly after the length field. This does not conform to the IEEE 802.3 standard, but since IPX has always FF at the first two octets (while in IEEE 802.2 LLC that pattern is theoretically possible but extremely unlikely), in practice this mostly coexists on the wire with other Ethernet implementations, with the notable exception of some early forms of [[DECnet]] which got confused by this.
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| [[Novell NetWare]] used this frame type by default until the mid-nineties, and since NetWare was then very widespread, while IP was not, at some point in time most of the world's Ethernet traffic ran over "raw" 802.3 carrying IPX. Since NetWare 4.10, NetWare now defaults to IEEE 802.2 with LLC (NetWare Frame Type Ethernet_802.2) when using IPX.<ref>{{cite newsgroup | author=Don Provan | title=Ethernet Framing | date=17 September 1993 | newsgroup=comp.sys.novell | id=1993Sep17.190654.13335@novell.com | url=http://groups.google.com/group/bit.listserv.novell/browse_thread/thread/d00a24530625714c }} — a classic series of Usenet postings by Novell's Don Provan that have found their way into numerous FAQs and are widely considered the definitive answer to the Novell Frame Type usage.</ref>
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| ===IEEE 802.2 LLC===
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| {{Main|IEEE 802.2}}
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| Some protocols, those designed for the [[OSI stack]], operate directly on top of IEEE 802.2 LLC encapsulation, which provides both connection-oriented and connectionless network services.
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| IEEE 802.2 LLC encapsulation is not in widespread use on common networks currently, with the exception of large corporate [[NetWare]] installations that have not yet migrated to NetWare over [[Internet Protocol|IP]]. In the past, many corporate networks used IEEE 802.2 to support transparent translating bridges between Ethernet and [[Token Ring]] or [[FDDI]] networks.
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| There exists an [[Internet standard]] for encapsulating IPv4 traffic in IEEE 802.2 LLC SAP/SNAP frames.<ref>
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| {{cite web
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| | url = http://tools.ietf.org/html/rfc1042
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| | title = RFC1042: A Standard for the Transmission of IP Datagrams over IEEE 802 Networks
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| | publisher = Network Working Group of the IETF
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| | date = February 1988
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| }}
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| </ref> It is almost never implemented on Ethernet, although it is used on [[FDDI]], [[Token Ring]], [[IEEE 802.11]] {{Clarify|reason=it's not clear whether this refers to the 802.11 family, or just legacy 802.11|date=June 2012}} and other [[IEEE 802]] LANs. IP traffic cannot be encapsulated in IEEE 802.2 LLC frames without SNAP because, although there is a LLC SAP protocol type for IP, there is no such type for [[Address Resolution Protocol|ARP]], which is required for operation of any medium to large network. IPv6 can also be transmitted over Ethernet using IEEE 802.2 LLC SAP/SNAP, but, again, that's almost never used.
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| ===IEEE 802.2 SNAP===
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| {{Main|Subnetwork Access Protocol}}
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| By examining the 802.2 LLC header, it is possible to determine whether it is followed by a SNAP header. The LLC header includes two additional eight-bit address fields, called ''service access points'' (SAPs) in OSI terminology; when both source and destination SAP are set to the value 0xAA, the SNAP service is requested. The SNAP header allows EtherType values to be used with all IEEE 802 protocols, as well as supporting private protocol ID spaces. In IEEE 802.3x-1997, the IEEE Ethernet standard was changed to explicitly allow the use of the 16-bit field after the MAC addresses to be used as a length field or a type field.
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| [[Mac OS]] uses IEEE 802.2 LLC SAP/SNAP encapsulation for the [[AppleTalk]] v2 protocol suite on Ethernet (“EtherTalk”).
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| ==Maximum throughput==
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| We may calculate the [[protocol overhead]] for Ethernet as a percentage
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| :<math>\text{Protocol overhead} = \frac{\text{Frame size} - \text{Payload size}}{\text{Frame size}}</math>
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| We may calculate the ''protocol efficiency'' for Ethernet
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| :<math>\text{Protocol efficiency} = \frac{\text{Payload size}}{\text{Frame size}}</math>
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| Maximum efficiency is achieved with largest allowed payload size and is:
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| :<math>\frac{1500}{1538} = 97.53%</math>
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| for untagged frames, since the frame size is maximum 1500 octet payload + 8 octet preamble + 14 octet header + 4 octet trailer + minimum interframe gap corresponding to 12 octets = 1538 octets. The maximum efficiency is:
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| :<math>\frac{1500}{1542} = 97.28%</math>
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| when 802.1Q VLAN tagging is used.
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| The [[throughput]] may be calculated from the efficiency
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| :<math>\text{Throughput} = \text{Efficiency} \times \text{Net bit rate}\,\!</math>,
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| where the physical layer [[net bit rate]] (the wire bit rate) depends on the [[Ethernet physical layer]] standard, and may be 10 Mbit/s, 100 Mbit/s, 1 Gbit/s or 10 Gbit/s. [[Maximum throughput]] for 100BASE-TX Ethernet is consequently 97.53 Mbit/s without 802.1Q, and 97.28 Mbit/s with 802.1Q.
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| [[Channel utilization]] is a concept often confused with protocol efficiency. It considers only the use of the channel disregarding the nature of the data transmitted – either payload or overhead. At the physical layer, the link channel and equipment do not know the difference between data and control frames. We may calculate the [[channel utilization]]:
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| :<math>\text{Channel utilization} = \frac{\text{Time spent transmitting data}}{\text{Total time}}</math>
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| The total time considers the round trip time along the channel, the processing time in the hosts and the time transmitting data and acknowledgements. The time spent transmitting data includes data and acknowledgements.
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| ==Runt frames==<!--[[Runt frame]] redirects here-->
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| {{Wikiversity|Topic:Web Science/Part1: Foundations of the web/Internet Architecture/Ethernet}}
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| A runt frame is an Ethernet frame that is less than the IEEE 802.3<!-- or IEEE 802.2?? --> minimum length of 64 octets. Possible causes are collision, underruns, a bad network card or software.<ref>
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| {{cite web
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| | url = http://www.cisco.com/en/US/docs/internetworking/troubleshooting/guide/tr1904.html
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| | title=Troubleshooting Ethernet
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| | publisher = Cisco Systems
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| }}
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| </ref>
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| ==Notes==
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| {{Reflist|group=note}}
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| ==References==
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| {{Reflist}}
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| ==Further reading==
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| <gallery>
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| File:How to build an Ethernet Frame.webm|Video which explains how to build an Ethernet Frame
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| File:Minimum Frame Length in Ethernet explained.webm|Minimum Frame Length in Ethernet explained
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| </gallery>
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| {{Ethernet}}
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| {{DEFAULTSORT:Ethernet Frame}}
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| [[Category:Ethernet|Frame]]
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| [[de:Datenframe]]
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