DocID018909 Rev 11
RM0090
Ethernet (ETH): media access control (MAC) with DMA controller
1232
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)