Ethernet (ETH): media access control (MAC) with DMA controller RM0033
850/1381 RM0033 Rev 9
The MAC sublayer performs the following functions associated with a data link control
procedure:
• Data encapsulation (transmit and receive)
– Framing (frame boundary delimitation, frame synchronization)
– Addressing (handling of source and destination addresses)
– Error detection
• Media access management
– Medium allocation (collision avoidance)
– Contention resolution (collision handling)
Basically there are two operating modes of the MAC sublayer:
• Half-duplex mode: the stations contend for the use of the physical medium, using the
CSMA/CD algorithms.
• Full duplex mode: simultaneous transmission and reception without contention
resolution (CSMA/CD algorithm are unnecessary) when all the following conditions are
met:
– physical medium capability to support simultaneous transmission and reception
– exactly 2 stations connected to the LAN
– both stations configured for full-duplex operation
28.5.1 MAC 802.3 frame format
The MAC block implements the MAC sublayer and the optional MAC control sublayer
(10/100 Mbit/s) as specified by the IEEE 802.3-2002 standard.
Two frame formats are specified for data communication systems using the CSMA/CD
MAC:
• Basic MAC frame format
• Tagged MAC frame format (extension of the basic MAC frame format)
Figure 323 and Figure 324 describe the frame structure (untagged and tagged) that
includes the following fields:
• Preamble: 7-byte field used for synchronization purposes (PLS circuitry)
Hexadecimal value: 55-55-55-55-55-55-55
Bit pattern: 01010101 01010101 01010101 01010101 01010101 01010101 01010101
(right-to-left bit transmission)
• Start frame delimiter (SFD): 1-byte field used to indicate the start of a frame.
Hexadecimal value: D5
Bit pattern: 11010101 (right-to-left bit transmission)
• Destination and Source Address fields: 6-byte fields to indicate the destination and
source station addresses as follows (see Figure 322):
– Each address is 48 bits in length
– The first LSB bit (I/G) in the destination address field is used to indicate an
individual (I/G = 0) or a group address (I/G = 1). A group address could identify
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