DocID018909 Rev 11 1125/1731
RM0090 Ethernet (ETH): media access control (MAC) with DMA controller
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33.5 Ethernet functional description: MAC 802.3
The IEEE 802.3 International Standard for local area networks (LANs) employs the
CSMA/CD (carrier sense multiple access with collision detection) as the access method.
The Ethernet peripheral consists of a MAC 802.3 (media access control) controller with
media independent interface (MII) and a dedicated DMA controller.
The MAC block implements the LAN CSMA/CD sublayer for the following families of
systems: 10 Mbit/s and 100 Mbit/s of data rates for baseband and broadband systems. Half-
and full-duplex operation modes are supported. The collision detection access method is
applied only to the half-duplex operation mode. The MAC control frame sublayer is
supported.
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
33.5.1 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)
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