Ethernet Frame (Ethernet II type)
| 7 | 1 | 6 | 6 | 2 | 2 | 2 | 42-1496 | 4 bytes |
| Preamble | SFD | DA | SA | TPID | TCI | Type Length | Data | CRC |
- Preamble (PRE)- 7 bytes. The PRE is an alternating pattern of ones and zeros that tells receiving stations that a frame is coming, and that provides a means to synchronize the frame-reception portions of receiving physical layers with the incoming bit stream
- Start-of-frame delimiter (SFD)- 1 byte. The SOF is an alternating pattern of ones and zeros, ending with two consecutive 1-bits indicating that the next bit is the left-most bit in the left-most byte of the destination address
- Destination address (DA)- 6 bytes. The DA field identifies which station(s) should receive the frame
- Source addresses (SA)- 6 bytes. The SA field identifies the sending station
- TPID- defined value of 8100 in hex. When a frame has the EtherType equal to 8100, this frame carries the tag IEEE 802.1Q / 802.1P
- TCI - Tag Control Information field including user priority, Canonical format indicator and VLAN ID
|
| 3 | 1 | 12bits |
| User Priority | CFI | Bits of VLAN ID (VIDI) to identify possible VLANs |
- User Priority- Defines user priority, giving eight (2^3) priority levels. IEEE 802.1P defines the operation for these 3 user priority bits
- CFI- Canonical Format Indicator is always set to zero for Ethernet switches. CFI is used for compatibility reason between Ethernet type network and Token Ring type network. If a frame received at an Ethernet port has a CFI set to 1, then that frame should not be forwarded as it is to an untagged port
- VID- VLAN ID is the identification of the VLAN, which is basically used by the standard 802.1Q. It has 12 bits and allow the identification of 4096 (2^12) VLANs. Of the 4096 possible VIDs, a VID of 0 is used to identify priority frames and value 4095 (FFF) is reserved, so the maximum possible VLAN configurations are 4,094.
- Length/Type- 2 bytes. If the value of this field is less than or equal to 1500, then the Length/Type field indicates the number of bytes in the subsequent MAC Client Data field. If the value of this field is greater than or equal to 1536, then the Length/Type field indicates the nature of the MAC client protocol (protocol type)
| 0×0800 | IP Internet Protocol (IPv4) |
| 0×0806 | Address Resolution Protocol (ARP) |
| 0×8035 | Reverse Address Resolution Protocol (RARP) |
- Data- Is a sequence of nbytes (48=< n =<1500) of any value. The total frame minimum is 64bytes.
- Frame check sequence (FCS)- 4 bytes. This sequence contains a 32-bit cyclic redundancy check (CRC) value, which is created by the sending MAC and is recalculated by the receiving MAC to check for damaged frames.
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