Ethernet Collision - Nortel RLC Installation And Administration Manual

June 2005

Ethernet Collision

The yellow Ethernet Collision LED indicates a collision on the RLC's Ethernet
interface (half-duplex only). Half-duplex Ethernet connections only allow their
nodes to either send or receive packets at any given time. Collisions occur when
two nodes on a half-duplex Ethernet connection attempt to transmit information
simultaneously. Before transmitting, the Network Interface Card (commonly
referred to as the NIC card) monitors the line, or listens, for transmissions. A
NIC card listens to the line for the amount of time that it takes to transfer a
minimum-sized packet the maximum length of the cable. If the NIC card senses
no transmission from the destination node, it proceeds with its own
transmission.
If a NIC card detects a collision, it waits for a period of time determined by the
back-off algorithm, then re-transmits the packet. Ethernet nodes keep track of
how many times they must re-transmit a packet with a maximum collision re-try
counter. In previous versions of Remote Gateway 9100 Series software, the
maximum collision re-try counter had a limit of 15. After 16 unsuccessful
attempts to transmit a packet (the original attempt plus 15 re-tries) the Remote
Gateway 9100 Series unit dropped the packet. In an effort to decrease delay and
improve QoS, the limit of the maximum collision re-try counter is now eight.
After nine unsuccessful attempts to transmit a packet (the original attempt plus
eight re-tries) the Remote Gateway 9100 Series unit drops the packet and begins
attempting to transmit the next packet.
Note: When you configure the RLC IP interface for full-duplex Ethernet, the
Ethernet Collision LED remains on solid (constantly lit). For configuration
information, refer to "Full Duplex Ethernet" on page 185.
RLC Installation and Administration Guide
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