SFD has the value of 171 (10101011 in binary notation), which is transmitted with least-significant bit first as 213 (0xD5 hexadecimal). :section 4.2.5 The SFD is immediately followed by the destination MAC address, which is the first field in an Ethernet frame. The SFD is designed to break the bit pattern of the preamble and signal the start of the actual frame. The SFD is the eight-bit (one-byte) value marking the end of the preamble, which is the first field of an Ethernet packet, and indicating the beginning of the Ethernet frame. For Ethernet variants transmitting single bits instead of larger symbols, the on-the-wire bit pattern for the preamble together with the SFD portion of the frame is 10101010 10101010 10101010 10101010 10101010 10101010 10101010 10101011 :sections 4.2.5 and 3.2.2 since octets are transmitted least-significant bit first, the corresponding hexadecimal representation is 0x55 0x55 0x55 0x55 0x55 0x55 0x55 0xD5. The preamble of an Ethernet packet consists of a 56-bit (seven-byte) pattern of alternating 1 and 0 bits, allowing devices on the network to easily synchronize their receiver clocks, which is followed by the SFD to mark a new incoming frame. 802.3 Ethernet packet and frame structure LayerĮthertype (Ethernet II) or length (IEEE 802.3)Īn Ethernet frame inside an Ethernet packet, with SFD marking the end of the packet preamble and indicating the beginning of the frame.Īn Ethernet frame starts following a seven-octet preamble and one-octet start frame delimiter (SFD), both of which are part of the Ethernet packet enveloping the frame. Some implementations of Gigabit Ethernet and other higher-speed variants of Ethernet support larger frames, known as jumbo frames. The table below shows the complete Ethernet frame, as transmitted, for the payload size up to the MTU of 1500 octets.
The internal structure of an Ethernet frame is specified in IEEE 802.3-2012. Data on Ethernet is transmitted with most-significant octet (byte) first within each octet, however, the least-significant bit is transmitted first, except for the frame check sequence (FCS). A data packet on the wire and the frame as its payload consist of binary data.
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