www.ti.com
EMAC Functional Architecture
2.4Ethernet Protocol Overview
Ethernet provides an unreliable, connectionless service to a networking application. A brief overview of the ethernet protocol follows. For more information on the carrier sense multiple access with collision detection (CSMA/CD) access method (ethernet’s multiple access protocol), see the IEEE 802.3 standard document.
2.4.1Ethernet Frame Format
All the ethernet technologies use the same frame structure. The format of an ethernet frame is shown in Figure 6 and described in Table 6. The ethernet packet is the collection of bytes representing the data portion of a single ethernet frame on the wire (shown outlined in bold in Figure 6).
The ethernet frames are of variable lengths, with no frame smaller than 64 bytes or larger than RXMAXLEN bytes (header, data, and CRC).
Figure 6. Ethernet Frame
|
|
|
|
|
| Number of bytes |
|
|
| ||
7 | 1 | 6 | 6 | 2 |
| 46 − (RXMAXLEN - 18) | 4 |
| |||
|
|
|
|
|
|
|
|
|
|
|
|
| Preamble |
| SFD | Destination |
| Source | Len |
| Data | FCS |
|
|
|
|
|
|
|
|
|
| |||
Legend: SFD=Start Frame Delimiter; FCS=Frame Check Sequence (CRC) |
|
| |||||||||
|
|
|
|
| Table 6. Ethernet Frame Description |
|
| ||||
Field | Bytes |
|
|
|
|
| Description |
|
| ||
Preamble | 7 |
| These 7 bytes have a fixed value of 55h. They wake up the receiving EMAC ports and | ||||||||
|
|
|
|
| synchronize their clocks to that of the sender’s clock. |
|
| ||||
Start of Frame | 1 |
| This field with a value of 5Dh immediately follows the preamble pattern and indicates | ||||||||
Delimiter |
|
|
| the start of important data. |
|
|
| ||||
Destination | 6 |
| This field contains the Ethernet MAC address of the intended EMAC port for the frame. | ||||||||
address |
|
|
| It may be an individual or multicast (including broadcast) address. If the destination | |||||||
|
|
|
|
| EMAC port receives an Ethernet frame with a destination address that does not match | ||||||
|
|
|
|
| any of its MAC physical addresses, and no promiscuous, multicast or broadcast | ||||||
|
|
|
|
| channel is enabled, it discards the frame. |
|
| ||||
Source | 6 |
| This field contains the MAC address of the Ethernet port that transmits the frame to the | ||||||||
address |
|
|
| Local Area Network. |
|
|
| ||||
Length/Type | 2 |
| The length field indicates the number of EMAC client data bytes contained in the | ||||||||
|
|
|
|
| subsequent data field of the frame. This field can also be used to identify the data type | ||||||
|
|
|
|
| carried by the frame. |
|
|
| |||
Data | 46 to (RXMAXLEN - 18) | This field carries the datagram containing the upper layer protocol frame (the IP layer | |||||||||
|
|
|
|
| datagram). The maximum transfer unit (MTU) of Ethernet is (RXMAXLEN - 18) bytes. | ||||||
|
|
|
|
| Therefore, if the upper layer protocol datagram exceeds (RXMAXLEN - 18) bytes, the | ||||||
|
|
|
|
| host must fragment the datagram and send it in multiple Ethernet packets. The | ||||||
|
|
|
|
| minimum size of the data field is 46 bytes. Thus, if the upper layer datagram is less | ||||||
|
|
|
|
| then 46 bytes, the data field must be extended to 46 bytes by appending extra bits after | ||||||
|
|
|
|
| the data field, but prior to calculating and appending the FCS. |
|
| ||||
Frame Check | 4 |
| A cyclic redundancy check (CRC) is used by the transmit and receive algorithms to | ||||||||
Sequence |
|
|
| generate a CRC value for the FCS field. The frame check sequence covers the 60 to | |||||||
|
|
|
|
| (RXMAXLEN - 4) bytes of the packet data. Note that the | ||||||
|
|
|
|
| included as part of the packet data, depending on the EMAC configuration. | ||||||
24 | Ethernet Media Access Controller (EMAC)/Management Data Input/Output (MDIO) | SPRU975B |
