Observer Analyzer : Analyzer : Expert Analysis : Ethernet Expert Explanations
  
Ethernet Expert Explanations
Created: 2015-08-17
Ethernet Align Errors
This event is triggered when the number of alignment errors per second is higher than the value configured in the Expert Thresholds setup.
Ethernet Alignment errors are detected when a packet is not "aligned" on a phase boundary. For timing purposes, the network adapter card assembles and sends a "preamble" for Ethernet packets. Then timers on both Ethernet adapters (sending and receiving) synchronize (agree) on phase timing, and calculate a phase position to begin the actual packet. This phase position is used so that the receiving adapter can know when the packet begins, and how the packet should correspond to the actual signal wave.
Alignment errors can be caused by a number of factors. Typically, they are caused by a previous collision. When a collision occurs, either a CRC error or an Alignment error almost always results. In the case of an Alignment error, if the collision occurs during a transmission after the preamble, the position of the resulting signal with respect to the phase of the wave is incorrect. The receiving adapter acknowledges this, and the packet is discarded.
Possible reasons for the event:
bad cabling between station and hub/switch, a cable that is too long or electrical interference on the cable
a bad Ethernet card or router connection NIC
Ethernet CRC Errors
This event is triggered when the number of CRC errors per second is higher than the value configured in the Expert Thresholds setup.
There are two types of Ethernet CRC errors:
1) MAC frame CRC errors, and
2) Internal protocol CRC errors
 
MAC Frame CRC Errors
These CRC errors are the most common, and are what most devices and analyzers are referring to when they claim a CRC error has occurred. Ethernet packets are encapsulated in a MAC frame that contains a preamble, and a post-envelope CRC check. The Ethernet adapter on the sending station is responsible for creation of the preamble, the insertion of the packet data (addressing, protocol, data, etc.) and then calculating a CRC checksum and inserting this at the end of the packet. The receiving station uses the checksum to make a quick judgment if the packet was received intact. If the checksum is not correct, the packet is assumed to be bogus and is discarded.
MAC frame CRC errors can be caused by a number of factors. Typically they are caused by either faulty cabling, or as the result of a collision. If the cabling connecting an Ethernet Adapter or hub is faulty the electric connection may be on and off many times during a transmission. This "on and off" state can interrupt parts of a transmission, and "damage" the signal.
If a collision happens during packet transmission, the signal for the specific packet will be interrupted, and the resulting received packet will be damaged.
If the signal is interrupted partially during transmission, the CRC checksum that was calculated by the network adapter will no longer be valid and the packet will be flagged as a CRC error and discarded.
CRC errors are common on a busy network, and a small percentage does not reflect a network problem. When the percentage is large, or when a single station shows a larger percent CRC errors there is probably a problem that needs to be addressed.
 
 
Internal protocol CRC Checksums
Some protocols (TCP/IP for example), have a second (in addition to the MAC frame CRC checksum) checksum for data integrity purposes. This checksum is calculated on only a portion of the internal data of each packet, and can give a second and independent check for the validity of the packet's contents. Observer calculates this checksum independent of the MAC layer CRC and displays the results in the decode display.
These CRC errors are very rare and can be caused by malfunctioning software or protocol drivers.
Possible reasons for the event:
a bad Ethernet card or router connection NIC
bad cabling between station and hub/switch, a cable that is too long or electrical interference on the cable
 
Ethernet too big errors
This event is triggered when the rate of oversize frames exceeds the value set in the Expert Thresholds settings.
Ethernet Frame Too Big errors are detected when a packet exceeds the Maximum Frame Size defined for the network.
Possible reasons for the event:
a device on the network is configured to transmit larger frames than the network allows.
a malfunctioning Ethernet card or router connection NIC can also generate frames too large for the network.
Ethernet too small errors
This event is triggered when the rate of oversize frames exceeds the value set in the Expert Thresholds settings.
Ethernet Frame Too Small errors are detected when a packet fails to meet the Minimum Frame Size defined for the network.
Possible reasons for the event:
a device on the network is configured to transmit smaller frames than the network allows.
a malfunctioning Ethernet card or router connection NIC can also generate frames too small for the network.