FCS and alignment erros

Question

Hi,
What is the difference between FCS errors and alignment erros ? Both have a bad CRC ?
Thanks,
Navid

Answer 1

Hi,
"An Alignment Error indicates a received frame in which both are true:
* The number of bits received is an uneven byte count (that is, not an integral multiple of 8)
* The frame has a Frame Check Sequence (FCS) error.
Alignment Errors often result from MAC layer packet formation problems, cabling problems that cause corrupted or lost data, and packets that pass through more than two cascaded multiport transceivers.
Frame Check Sequence (FCS) Errors, a type of CRC, indicate that frames received by an interface are an integral number of octets long but do not pass the FCS check. The FCS is a mathematical way to ensure that all the frame's bits are correct without having the system examine each bit and compare it to the original. Packets with Alignment Errors also generate FCS Errors.
Both Alignment Errors and FCS Errors can be caused by equipment powering up or down or by interference (noise) on unshielded twisted-pair (10BASE-T) segments. In a network that complies with the Ethernet standard, FCS or Alignment Errors indicate bit errors during a transmission or reception. A very low rate is acceptable. Although Ethernet allows a 1 in 108 bit error rate, typical Ethernet performance is 1 in 1012 or better.
A Cyclic Redundancy Check (CRC) Error is an RMON statistic that combines "FCS Errors" and "Alignment Errors". These errors indicate that packets were received with:
* A bad FCS and an integral number of octets (FCS Errors)
* A bad FCS and a non-integral number of octets (Alignment Errors)
CRC Errors can cause an end station to freeze. If a large number of CRC Errors are attributed to a single station on the network, replace the station's network interface board. Typically, a CRC Error rate of more than 1 percent of network traffic is considered excessive."
for more infos:
http://support.3com.com/infodeli/tools/netmgt/tncsunix/product/091500/c11ploss.htm#17795
HTH
Andrea