Description of functions
User manual 58185_TREE M4/6-TX PN IP20_hdb_en_10
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4.1.2
Media Access Controller (MAC)
IEEE- 802.3 functions
Each Ethernet port has a separate MAC that is capable of performing every
function required under IEEE- 802.3, e.g., frame formatting, frame stripping,
CRC checking, CSMA/CD enforcement, collision handling.
Monitoring of incoming
packets
The Media Access Controllers check the validity of all received packets.
They discard:
packets with CRC errors,
packets with a length below 64 bytes,
packets with a length greater than 1536 bytes.
They forward valid packets to the Switching Fabric Unit (SFU)
(see section 4.1.3 "Frame switching").
Sending packets
Full-duplex mode:
In full-duplex mode, the transmission line is always avail-
able.
Half-duplex mode:
In half-duplex mode, the transmission line may be
blocked.
Interrupting a transfer when the transmission line is blocked.
The transmitter delays the transfer until the line is available.
Transmission after an interruption:
The transmitter sends the following in the following order:
A 56-bit preamble,
an eight-byte start-of-frame delimiter,
the payload.
The minimum interpacket gap is 96 bits.
Preamble regeneration
Bit synchronization may result in the switch receiving packets that have a pre-
amble with fewer than 56 bits. If this occurs, the switch restores the bits that
are missing from the preamble to the full 56.
Collision detection
Full-duplex mode:
No collisions occur in full-duplex mode.
Half-duplex mode:
The switch monitors the collision signal during the trans-
fer.
Interrupting a transfer when a collision occurs:
If a collision is detected at one of the ports, the MAC sends a jam signal to
all connected devices.
Transmission is interrupted.
Transmitting after the interruption:
The switch delays transmission by a random interval in accordance with
the back-off algorithm specified in IEEE 802.3.
