Ethernet Frame Formats

Ethernet Frame Formats:

Ethernet (IEEE 802.3) Frame Format?

• PREAMBLE Ethernet frame starts with 7-Bytes Preamble. This is a pattern of alternative 0?s and 1?s which indicates the starting of the frame and allows sender and receiver to establish bit synchronization. Initially, PRE (Preamble) was introduced to allow for the loss of a few bits due to signal delays. But today’s high-speed Ethernet doesn’t need Preamble to protect the frame bits.
  PRE (Preamble) indicates to the receiver that the frame is coming and allows the receiver to lock onto the data stream before the actual frame begins.
• Start of frame delimiter (SFD) This is a 1-Byte field that is always set to 10101011. SFD indicates that upcoming bits are starting of the frame, which is the destination address. Sometimes SFD is considered the part of PRE, this is the reason Preamble is described as 8 Bytes in many places.
• Destination Address  This is a 6-Byte field that contains the MAC address of the machine for which data is destined.
• Source Address This is a 6-Byte field that contains the MAC address of the source machine. As Source Address is always an individual address (Unicast), the least significant bit of the first byte is always 0.
• Length is a 2-Byte field, which indicates the length of the entire Ethernet frame. This 16-bit field can hold the length value between 0 to 65534, but the length can not be larger than 1500 because of some own limitations of Ethernet.
• Data  This is the place where actual data is inserted, also known as?Payload. Both IP header and data will be inserted here if Internet Protocol is used over Ethernet. The maximum data present may be as long as 1500 Bytes. In case data length is less than the minimum length i.e. 46 bytes, then padding 0?s is added to meet the minimum possible length.
• Cyclic Redundancy Check (CRC) CRC is 4 Byte field. This field contains 32-bits hash code of data, which is generated over Destination Address, Source Address, Length and Data field. If the checksum computed by destination is not same as sent checksum value, data received is corrupted.

Brief overview on Extended Ethernet Frame (Ethernet II Frame) :

Standard IEEE 802.3 basic frame format is discussed above in detail. Now let?s see the extended Ethernet frame header, using which we can get Payload even larger than 1500 Bytes.

DA [Destination MAC Address] 😕6 bytes
SA [Source MAC Address]:?6 bytes
Type [0x8870 (Ethertype)] 😕2 bytes
DSAP [802.2 Destination Service Access Point]:?1 byte
SSAP [802.2 Source Service Access Point]:?1 byte
Ctrl [802.2 Control Field] 😕1 byte
Data [Protocol Data] 😕> 46 bytes
FCS [Frame Checksum] 😕4 bytes

Although the length field is missing in the Ethernet II frame, the frame length is known by virtue of the frame being accepted by the network interface.

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