Ethernet Frames

Started by VelMurugan, Nov 04, 2008, 11:52 PM

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VelMurugan

Ethernet Frames

There are various types of data frames such as Ethernet frames, Ethernet jumbo frames and snap frames etc. The following illustrates the frame fields associated with both Ethernet and Ethernet IEEE 802.3 jumbo frames.

Various fields exist for both Ethernet frames and Ethernet jumbo frames:

SDF = Start Frame Delimiter

FCS = Frame Check Sequence

The Ethernet frames fields illustrated in the table are as follows:

    * Preamble: The alternating pattern of one and zeros tells receiving stations that an Ethernet or Ethernet jumbo frame is coming. The Ethernet frame includes an additional byte that is equivalent of the Start of Frame (SOF) field specified in the Ethernet IEEE 802.3 jumbo frames.

    * Start of Frame (SOF): The IEEE 802.3 delimiter byte ends with two consecutive 1 bits, which serve to synchronize the frame reception portions of all stations on the LAN. SOF is explicitly specified in Ethernet. It is also known as Start Frame Delimiter (SFD).

    * Source and Destination Address: The first 3 bytes of the addresses are specified by the IEEE on a vendor dependent basis. The last 3 bytes are specified by the Ethernet or IEEE 802.3 vendor. Source address should be unicast address which is the address of a single device while destination address could be unicast multicast of broadcast address of all nodes.

    * Type (Ethernet): After Ethernet processing is completed this upper layer receive data.

    * Length (Ethernet jumbo frame): The length indicates the number of bytes of data that follows this field.

    * Data (Ethernet): When processing at physical layer and link layer is completed , frame data is forward towards upper layer protocols for processing which is show in Type Field. Although Ethernet version 2 does not specify any padding (in contrast to IEEE 802.3), Ethernet expects at least 46 bytes of data.

    * Data (Ethernet jumbo Frame): After physical layer and link layer processing is complete, the data is sent to an upper layer protocol, which must be defined within the data portion of the frame, if at all. When frame data is not enough and it should be minimum 64 bytes frame.

    * Frame Check Sequence (FCS): It contains 4 bytes of CRC value which sending device has created and it is then calculated again by receiving device to find out damaged frames.

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