Formats of the Ethernet data transfer blocks and the type field

Historical formats

There are four types of Ethernet data blocks (English frames ethernet):

• Ethernet version I (no more does not use, definition 1980 by consortium DEC, Intel, Xerox)
• The Ethernet version 2 or Ethernet II data block (English ethernet II frame), the so-called DIX Frame (definition 1982 by the consortium DEC, Intel, and Xerox).

Since 1983 developed the standard IEEE 802,3. Ethernet is quasi a synonym for this standard. IEEE 802,3 defines two Frameformate:

• IEEE 802,3 3.1.a basic MAC frame
• IEEE 802,3 3.1.b Tagged MAC frame

Original Xerox version a 16-bit-Feld had 1 Ethernet data block, in which the length of the data block was deposited. Since this length for the transmission of the Frames is not important, it was used by the later Ethernet II standard than Ethertype field. The format of Ethernet I with the length field is now part of the standard 802.3.

The Ethernet II format uses the bytes 13 and 14 in the framework as Ethertype. Without a length field as in the Ethernet I framework one does. The length of a Frames is conveyed not by a numerical value but by bit-exact signaling of the end of transmission. The length of the data field remains as with Ethernet I on 1500 limited. Also the Ethernet II format is now part of the standard 802,3 - only the Ethertypen with numerical values smaller than 1500 was omitted, because the numerical values are interpreted now small 1500 in this field as length and against the actual length are examined.

IEEE 802,3 defines the 16-bit-Feld after the MAC addresses as type/length field. As the convention that values between 0 and 1.500 pointed on the original Ethernet format, and higher values the EtherType it indicates, the coexistence of the standards on the same physical medium was made possible. The permissible values for Ethertype are administered by IEEE. This administration is limited to the assignment of new Ethertype of values. IEEE gives consideration to already Ethertype of values assigned for Ethernet II with the new assignment, documents these however not. Thus it comes that the value 0x0800 for IP data in the IEEE documentation of the Ethertype values is e.g. missing! Ethertype describes the format and/or minutes for the interpretation of the data block. The LLC field and a possible SNAP field are already part of the MAC Frame data field. Into the Tagged MAC Frame 4 bytes with the QTAG prefix are slid after the pour Macadresse. This field is defined and made possible by the standard 802.1Q up to 4096 Vlans on a physical medium. The permitted overall length of the Mac Frames is extended to 1522 bytes, the length of the data field remains specified to 1500.

IEEE 802,3 Tagged MAC Frame

The structure

The preamble and SFD

The preamble consists 7 of byte is enough for alternating bit sequence (101010"… 1010), which serves the synchronisation of the network devices. So the devices involved in the network can prepare for dealing a data communication and synchronize with the clock of the signal. Subsequently, a byte large start Frame delimiters SFD follows. This field (10101011) continues the preamble, the two last bits stands on 1.

A goal and pouring MAC AdresseDie destination address identify the target computer, which is to receive the data. This address can be also a multicast/an Broadcast address. The source address identifies each MAC address of the two fields has a length of six byte.

VLAN TagNur in the Tagged MAC Frame follow additionally 4 bytes as VLAN day. First the two contain the constant of 0x8100 (=802.1qTagType] some Tagged MAC Frame as such mark. From the position in the basic MAC Frame the field Ethertype would stand here. One can regard the value 0x8100 thereby also as Ethertype for Vlan data. In the next two bytes 3 bit values are then located for the Vlan Priority, 1Bit Canonical format indicator and 12 bits for the Vlan ID.

Information type-field-gives the next higher layer over used minutes within the utilizable data. The values are ever larger than 0x0600 (otherwise that is a Ethernet I frame with length field in this position). The special value 0x08100 for the marking VLAN tags is reserved in the value supply of type. Values in the type field (EtherType) for some important minutes:

In Ethernet 802,3 Frames the length of data contents can be indicated in DATA the part (length field) to the compatibility as Ethernet I in place of the type field. Since the data field may be longer in no Ethernet Frame than 1500 bytes, the values can be used over 1500 as minutes types (Ethertyp).

The utilizable data The utilizable data can be long per data block between 0 and 1500 byte. They are the actual information, which will transfer is. The utilizable data are interpreted by minutes indicated by type.

To bring that used around the Ethernet framework on the necessary minimum size of 64 byte. This is important, in order to recognize collisions surely! Preamble and SFD (8 byte) are not taken in account with the necessary minimum length of the Frames, probably however a VLAN day. A PAD field necessarily if as utilizable data less than 46 (with a Tagged Frame 42) byte will be transferred are. 6-Byte-Zieladresse + 6-Byte-Quelladresse + 4-Byte-VLAN-TAg + 2-Byte-Typfeld + 42-Byte-Nutzdaten + 4-Byte-CRC = Minutes indicated in type must ensure that these bytes added as PADs are not interpreted.

FCS (Frame check Sequence) the FCS field represents a The FCS contains the examination of the entire Frames, starting from destination address. The preamble, which is not SFD and the FCS in the FCS contained. If a package is provided with the transmitter, a (carriage return character) computation is accomplished over the entire bit sequence except the preamble and the check total is attached to the data block. The receiver implements the same computation after the receipt and compares its result with contents of the FCS field. If the values do not agree, the receiver proceeds from an incorrect transmission and the data block is rejected. For the computation of the CRC-32 the first 32 bits of the Mac address are inverted (to the recognition of missing zeros in the first bits) and the result likewise inverts (avoidance of the zero-problem).

Transformation into a data stream

The bytes starting from the MAC address up to the end of the utilizable data are first transferred with the niederwertigsten bit. The (carriage return character) against it with the bit first one transfers. One or more bits are coded thereafter by a cable code, mainly for the clock recuperation and decrease of the DC voltage portion. With old 10 MBit the Ethernet a simple Manchester coding was used for the clock recuperation and standing dc component suppression, late 4B5B-Code (clock recuperation) in combination with MLT-3-Code (DC voltage portion) for instance with 100-MBit-Ethernet over Twisted Pair cables.

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