Ethernet Frame Formats

Ethernet Frame Formats:

Ethernet (IEEE 802.3) Frame Format?

• PREAMBLE Ethernet frame starts with a 7-byte Preamble. This is a pattern of alternating 0s and 1s, which indicates the start of the frame and allows the 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 a 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 the upcoming bits are the start of the frame, which is the destination address. Sometimes SFD is considered part of PRE; this is the reason the 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 the Source Address is always individual (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 limitations of Ethernet.

• Data, this is the place where actual data is inserted, also known as? Payload. Both the 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 the 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 a 4 Byte field. This field contains a 32-bit hash code of data, which is generated over Destination Address, Source Address, Length, and Data field. If the checksum computed by the destination is not the same as the sent checksum value, the data received is corrupted.

Brief overview of the Extended Ethernet Frame (Ethernet II Frame) :

The 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 a 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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