Texas Instruments TMS320DM646x manual Ethernet Protocol Overview, Ethernet Frame Format

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Architecture

2.4Ethernet Protocol Overview

Ethernet provides an unreliable, connection-less service to a networking application. A brief overview of the Ethernet protocol is given in the following subsections. For in-depth information on the Carrier Sense Multiple Access with Collision Detection (CSMA/CD) Access Method, which is the 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 4 and described in Table 3. The Ethernet packet, which is the collection of bytes representing the data portion of a single Ethernet frame on the wire, is shown outlined in bold. The Ethernet frames are of variable lengths, with no frame smaller than 64 bytes or larger than RXMAXLEN bytes (header, data, and CRC).

Figure 4. Ethernet Frame Format

			Number of bytes
7	1	6	6	2	46−1500
Preamble	SFD	Destination	Source	Len	Data

Legend: SFD=Start Frame Delimeter; FCS=Frame Check Sequence (CRC)

Table 3. Ethernet Frame Description

4

FCS

Field	Bytes	Description
Preamble	7	Preamble. These 7 bytes have a fixed value of 55h and serve to wake up the receiving
		EMAC ports and to synchronize their clocks to that of the sender’s clock.
SFD	1	Start of Frame Delimiter. This field with a value of 5Dh immediately follows the preamble
		pattern and indicates the start of important data.
Destination	6	Destination address. This field contains the Ethernet MAC address of the EMAC port for
		which the frame is intended. It may be an individual or multicast (including broadcast)
		address. When 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	Source address. This field contains the MAC address of the Ethernet port that transmits the
		frame to the Local Area Network.
Len	2	Length/Type field. 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 type of data the frame is carrying.
Data	46 to	Data field. This field carries the datagram containing the upper layer protocol frame, that is,
	(RXMAXLEN - 18)	IP layer datagram. The maximum transfer unit (MTU) of Ethernet is (RXMAXLEN - 18)
		bytes. This means that if the upper layer protocol datagram exceeds (RXMAXLEN - 18)
		bytes, then the host has to fragment the datagram and send it in multiple Ethernet packets.
		The minimum size of the data field is 46 bytes. This means that if the upper layer datagram
		is less then 46 bytes, the data field has to be extended to 46 bytes by appending extra bits
		after the data field, but prior to calculating and appending the FCS.
FCS	4	Frame Check Sequence. A cyclic redundancy check (CRC) is used by the transmit and
		receive algorithms to 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 this 4-byte field may
		or may not be included as part of the packet data, depending on how the EMAC is
		configured.

SPRUEQ6–December 2007 Ethernet Media Access Controller (EMAC)/Management Data Input/Output (MDIO) 19

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