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06.3 Ethernet Frame Structure
9/11/2016 Cisco ELearning for ICND1 v2.0 https://ondemandelearning.cisco.com/ciscosc/icnd1#/sections/6/pages/3 1/2 Ethernet Frame Structure Section 6: Understanding Ethernet and Switch Operation In Ethernet terminology, the container into which data is placed for transmission is called a frame. The frame contains header information, trailer information, and the actual data that is being transmitted. There are two types of Ethernet frames, the Ethernet II frame and the IEEE 802.3 frame. As shown in the tables below, the structure of the two frame types is similar. Ethernet II Frame Field Length, in Bytes Ethernet 8 6 6 2 46 1500 4 Preamble Destination Address Source Address Type Data FCS IEEE 802.3 Frame Field Length, in Bytes IEEE 802.3 7 1 6 6 2 461500 4 Preamble SOF Destination Address Source Address Length 802.2 Header and Data FCS This table shows the fields of the MAC layer for both types of Ethernet frame. The table offers a description for each field. MAC Layer Fields for Ethernet II and IEEE 802.3 Frames Field Description 9/11/2016 Cisco ELearning for ICND1 v2.0 https://ondemandelearning.cisco.com/ciscosc/icnd1#/sections/6/pages/3 2/2 Field Description Preamble The Preamble field consists of 7 bytes of alternating 1s and 0s, which are used to synchronize the signals of the communicating computers. Destination Address The Destination Address field contains the address of the NIC on the local network to which the packet is being sent. Source Address The Source Address field contains the address of the NIC of the sending computer. Type In the Ethernet II standard, the Type field contains a code that identifies the network layer protocol. In the 802.3 standard, the Length field specifies the length of the data field. Therefore, the protocol information is contained in 802.2 fields, which are at the LLC layer that is contained in the 802.2 header and data field. Data The Data and Pad field contains the data that is received from the network layer on the transmitting computer. This data is sent to the same protocol on the destination computer. If the data is shorter than the minimum length of 46 bytes, a string of extraneous bits is used to pad the field. FCS The FCS field includes a checking mechanism to ensure that the packet of data has been transmitted without corruption. SOF The SOF delimiter field (for 802.3 only) contains bits that signal the receiving computer that the transmission of the actual frame is about to start and that any data that follows is part of the packet. 802.2 Header and Data The Data and Pad field contains the data that is received from the network layer on the transmitting computer. This data is sent to the same protocol on the destination computer. If the data is shorter than the minimum length of 46 bytes, a string of extraneous bits is used to pad the field. Up Next: Ethernet Addresses