Wednesday, March 2, 2011

Troubleshooting Your Ethernet? Look for Physical Frame Corruption!

When troubleshooting your Ethernet network, the first thing to look for is physical frame corruption. Provided an organization is using coaxial Ethernet, below are four possible causes of physical frame corruption in an Ethernet network, each one different in the way it corrupts the frame and therefore recognizable (Note: Twisted-pair Ethernet implementations will not manifest these types of corruption patterns).

1) Collisions
Generally when a collision occurs, several bytes of the preamble of the colliding frame will be read into your analyzer's buffer before the signal is completely destroyed. You will see these bytes in the hexadecimal decode of the packet as either several bytes of AA's or several bytes of 55's at the very end of the frame (remember, AAh=1010b, 55h=0101b. Depending on where the collision occurred, the preamble could be perceived as either of these). If you see more than 8 bytes of AA or 55, then the corruption was not caused by a collision and more investigation is necessary.

2) Signal Reflections
One cause of signal reflection is an un-terminated cable. Electrons travel down the wire until they reach the cable's end, where, with no resistor to absorb the voltage potential, they reflect back from the open end of the cable. Another cause of signal reflections is mixing cables with different impedances. Impedance can be thought of as the "rate of flow" of the wire. When electrons from the higher impedance wire attempt to travel through the lower impedance wire, some of them can't make it and are reflected back, destroying the signal. The final cause of signal reflections is exceeding the maximum allowable bend radius of the cable. The copper media is deformed, causing reflections.

3) Electrical Noise
Physical frame corruption caused by electrical noise is similar in appearance to corruption caused by reflections in that there is no preamble in the frame -- the frame just seems to stop short, but is different in that the frames are generally cut off at random lengths.

4) Malfunctioning Hardware
Frame corruption caused by hardware malfunctions is potentially the hardest to diagnose because of the large number of ways that hardware can malfunction. Generally, hardware malfunctions will occur either randomly or constantly, but not regularly. The type of frame corruption is impossible to predict, generally manifesting as random "garbage" in the frame, but some common signs are: 
      •  A stream of ones or zeros. A transceiver has malfunctioned and is "jabbering" on the wire. Most transceivers have jabber detection circuitry that prevents the adapter from transmitting for longer than a certain preset time.
      • Gigantic frames (greater than 1500 bytes). Same as above. 
With these four main causes dissected above, troubleshooting an Ethernet network doesn't have to be confusing. While these tips are universal, specific analyzer's behavior might differ and an organization should determine what's best in terms of troubleshooting its own specific network.

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