Ethernet Framing
The physical layer helps you get a string of bits from one device to another. The framing of the bits allows the receiving device to interpret the bits. The term framing refers to the definition of the fields assumed to be in the data that is received. Framing defines the meaning of the bits transmitted and received over a network.
The framing used for Ethernet has changed a couple of times over the years. Each iteration of Ethernet is shown in Figure 28-6, with the current version shown at the bottom.
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Figure 28-6 Ethernet Frame Formats
The fields in the last version shown in Figure 28-6 are explained further in Table 28-3.
Table 28-3. IEEE 802.3 Ethernet Field Descriptions
|
Field |
Field Length in Bytes |
Description |
|
Preamble |
7 |
Synchronization |
|
Start Frame Delimiter (SFD) |
1 |
Signifies that the next byte begins the Destination MAC field |
|
Destination MAC address |
6 |
Identifies the intended recipient of this frame |
|
Source MAC address |
6 |
Identifies the sender of this frame |
|
Length |
2 |
Defines the length of the data field of the frame (either length or type is present, but not both) |
|
Type |
2 |
Defines the type of protocol listed inside the frame (either length or type is present, but not both) |
|
Data and Pad |
46–1500 |
Holds data from a higher layer, typically a Layer 3 PDU (generic), and often an IP packet |
|
Frame Check Sequence (FCS) |
4 |
Provides a method for the receiving NIC to determine whether the frame experienced transmission errors |