A data packet on an Ethernet link is called an Ethernet frame. A frame begins with preamble and start frame delimiter. Following which, each Ethernet frame continues with an Ethernet header featuring destination and source MAC addresses. The middle section of the frame is payload data including any headers for other protocols "Internet Protocol" carried in the frame. The frame ends with a 32-bit cyclic redundancy check which is used to detect any corruption of data in transit. A data packet on the wire is called a frame and consists of binary data. Data on Ethernet is transmitted most-significant byte first, within each byte, however, the least-significant bit is transmitted first.
Preamble and start frame delimiter
A frame starts with a 7-octet preamble and 1-octet start frame delimiter (SFD). Prior to Fast Ethernet, the on-the-wire bit pattern for this portion of the frame is 10101010 10101010 10101010 10101010 10101010 10101010 10101010 10101011. Since octets are transmitted least-significant bit first the corresponding hexadecimal representation is 0x55 0x55 0x55 0x55 0x55 0x55 0x55 0xD5.
PHY transceiver chips used for Fast Ethernet feature a 4-bit (one nibble) Media Independent Interface. Therefore the preamble will consist of 14 instances of 0x5, and the start frame delimiter 0x5 0xD. Gigabit Ethernet transceiver chips use a Gigabit Media Independent Interface that works 8-bits at a time, and 10 Gbit/s (XGMII) PHY works with 32-bits at a time.
Header
The header features source and destination MAC addresses which have 6 octets each, the EtherType protocol identifier field and optional IEEE 802.1Q tag.
802.1Q tag
The IEEE 802.1Q tag is an optional 4-octet field that indicates Virtual LAN (VLAN) membership and IEEE 802.1p priority.
EtherType or length
EtherType is a two-octet field in an Ethernet frame. It is used to indicate which protocol is encapsulated in the payload of an Ethernet Frame.
Payload
The minimum payload is 42 octets when 802.1Q tag is present and 46 octets when absent. and the maximum payload is 1500 octets. Non-standard jumbo frames allow for larger maximum payload size.
Frame check sequence
The frame check sequence is a 4-octet cyclic redundancy check which allows detection of corrupted data within the entire frame.
Interframe gap
Interframe gap is idle time between frames. After a frame has been sent, transmitters are required to transmit a minimum of 96 bits (12 octets) of idle line state before transmitting the next frame.
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Preamble and start frame delimiter
A frame starts with a 7-octet preamble and 1-octet start frame delimiter (SFD). Prior to Fast Ethernet, the on-the-wire bit pattern for this portion of the frame is 10101010 10101010 10101010 10101010 10101010 10101010 10101010 10101011. Since octets are transmitted least-significant bit first the corresponding hexadecimal representation is 0x55 0x55 0x55 0x55 0x55 0x55 0x55 0xD5.
PHY transceiver chips used for Fast Ethernet feature a 4-bit (one nibble) Media Independent Interface. Therefore the preamble will consist of 14 instances of 0x5, and the start frame delimiter 0x5 0xD. Gigabit Ethernet transceiver chips use a Gigabit Media Independent Interface that works 8-bits at a time, and 10 Gbit/s (XGMII) PHY works with 32-bits at a time.
Header
The header features source and destination MAC addresses which have 6 octets each, the EtherType protocol identifier field and optional IEEE 802.1Q tag.
802.1Q tag
The IEEE 802.1Q tag is an optional 4-octet field that indicates Virtual LAN (VLAN) membership and IEEE 802.1p priority.
EtherType or length
EtherType is a two-octet field in an Ethernet frame. It is used to indicate which protocol is encapsulated in the payload of an Ethernet Frame.
Payload
The minimum payload is 42 octets when 802.1Q tag is present and 46 octets when absent. and the maximum payload is 1500 octets. Non-standard jumbo frames allow for larger maximum payload size.
Frame check sequence
The frame check sequence is a 4-octet cyclic redundancy check which allows detection of corrupted data within the entire frame.
Interframe gap
Interframe gap is idle time between frames. After a frame has been sent, transmitters are required to transmit a minimum of 96 bits (12 octets) of idle line state before transmitting the next frame.
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