Triple-Speed Ethernet Intel® FPGA IP User Guide

ID 683402
Date 5/30/2022
Document Table of Contents 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 being identical to the CRC is very low, 0.532%.

If the MAC function detects a collision while transmitting the preamble or start frame delimiter (SFD) field, it sends the jam pattern only after transmitting the SFD field, which subsequently results in a minimum of a 96-bit fragment.

If the MAC function detects a 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 14. Frame Retransmission

The backoff time is a multiple of slot times. One slot is equal to a 512 bit times period. The number of the delay slot times, before the Nth retransmission attempt, is chosen as a uniformly distributed random integer (r) in the following range:

0 r < 2k

k = min( n , N ), where n is the number of retransmissions and N = 10.

For example, after the first collision, the backoff period, in slot time, is 0 or 1. If a collision occurs during the first retransmission, the backoff period, in slot time, is 0, 1, 2, or 3.

The maximum backoff time, in 512 bit times slots, is limited by N set to 10 as specified in the IEEE Standard 802.3.

If a collision occurs after 16 consecutive retransmissions, the MAC function reports an excessive collision condition by setting the EXCESS_COL bit in the command_config register to 1, and discards the current frame from the transmit FIFO buffer.

In networks that violate standard requirements, collision may occur after the transmission of the first 64 bytes. If this happens, the MAC function stops transmitting the current frame, discards the rest of the frame from the transmit FIFO buffer, and resumes transmitting the next available frame. You can check the LATE_COL register (command_config [12]) to verify if the MAC has discarded any frame due to a collision.

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