Altera Triple Speed Ethernet MegaCore Function - Manual (Page 42)

Interpacket Gap Insertion
In full-duplex mode, the MAC function maintains the minimum number of IPG configured in the
tx_ipg_length
register between transmissions. You can configure the minimum IPG to any value between
64 and 216 bit times, where 64 bit times is the time it takes to transmit 64 bits of raw data on the medium.
In half-duplex mode, the MAC function constantly monitors the line. Transmission starts only when the
line has been idle for a period of 96 bit times and any backoff time requirements have been satisfied. In
accordance with the standard, the MAC function begins to measure the IPG when the
m_rx_crs signal is
deasserted.
Collision Detection in Half-Duplex Mode
Collision occurs only in a half-duplex network. It occurs when two or more nodes transmit concurrently.
The PHY device asserts the
m_rx_col
signal to indicate collision.
When the MAC function detects collision during transmission, it stops the transmission and sends a 32-bit
jam pattern instead. A jam pattern is a fixed pattern, 0x648532A6, and is not compared to the CRC of the
frame. The probability of a jam pattern to be identical to the CRC is very low, 0.532%.
If the MAC function detects collision while transmitting the preamble or SFD field, it sends the jam pattern
only after transmitting the SFD field, which subsequently results in a minimum of 96-bit fragment.
If the MAC function detects collision while transmitting the first 64 bytes, including the preamble and SFD
fields, the MAC function waits for an interval equal to the backoff period and then retransmits the frame.
The frame is stored in a 64-byte retransmit buffer. The backoff period is generated from a pseudo-random
process, truncated binary exponential backoff.
Figure 4-3: Frame Retransmission
MAC Transmit
Control
MAC Transmit
Datapath
PHY Interface
Retransmission Block
PHY Control
LFSR
Col
Rd _en
Frame
Discard
MAC Transmit
Backoff
Period
Retransmit
Buffer
Control
64 x8
Retransmit
Buffer
Avalon-ST
Interface
WAddr RAddr
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Interpacket Gap Insertion 4-6
2014.06.30