radio, building security, avionics for automated landing, and many more. A Spectrum Analyzer is needed to determine the frequency characteristics and the magnitude of the noise signals picked up by the cabling links. This laboratory type device can identify the frequencies of the signals (noise) on the link as well as the relative magnitude of the signals at the different frequencies. Alien cross talk is noise induced by other cables in the same pathway. The detection is the same as those mentioned above. Anytime a UTP link is tested in a bundle of cables in which some links are active, the chances are very good that the tester will detect alien cross talk, especially when the traffic is 100BASE-T. The tester will report the “external noise detected” message. Typically, this alien cross talk will not impact the reliability of the network traffic. Excessive cross talk is usually reported in the Near End Cross Talk (NEXT) test results on cable testing devices. NEXT can originate either inside or outside the link. Crosstalk originating nearest the transmission source is usually the loudest and has the greatest amplitude or leaks into the measured pair. To minimize cross talk, ensure that the cable, at each termination point, is left untwisted for no more than the allowed 13 mm (0.5 inch). Crosstalk becomes correspondingly worse as more cable is untwisted. The cross talk occurring outside the link is actually cross talk from other adjacent cables, or noise from a variety of sources. Noise sources include nearby electric cables and devices, usually with high current loads. These may include large electric motors, elevators, photocopiers, coffee makers, fans, heaters, welders, compressors, and so on. Another less obvious source is radiated emissions from transmitters. This would include TV, radio, microwave, cell phone towers, hand-held radios, and anything else that includes a transmitter more powerful than a cell phone.
Content 3.2 Characteristics of Physical Layer Optimization Problems 3.2.4 Collisions Collision domain problems affect the local medium, and disrupt communications to Layer 2 or Layer 3 infrastructure devices, local servers or services. They typically result from the following problems: Collisions are normally a more significant problem on shared media than on switch ports. Average collision counts on shared media should generally be below 5 percent, though that number is conservative. Be sure that judgments are based on the average and not a peak or spike in collisions. If the average utilization is high (sustained peaks in excess of 60 percent for shared media, and in excess of 80-90 percent for switched links) and collision counts are acceptable (average is below 5 percent for shared media, and below 1 percent for switched links), then the network may simply be saturated. There may be too many stations transmitting within this collision domain, or the network architecture may need optimizing for shorter distances between distant stations. Physical layer or Layer 1 network devices act to increase collision domains, effectively increasing the scope of a potential problem from bad to worse. Layer 1 devices such as repeaters and hubs act to regenerate signals and therefore increase the maximum distance over which a network can operate. Hubs additionally act to increase the collision domain. They contain from four to forty-eight ports for station connectivity. A hub will regenerate any signal received and resend it out all active ports. Be very careful when troubleshooting collision problems because the obvious answer is usually wrong. The addresses found in collision fragments belong to stations that transmitted legally. Stations that sent enough of the current frame to have a source address in a collision fragment usually started transmitting first, though that depends in part on the monitoring point within the collision domain. Most of the stations that collide with those legally transmitted frames are also operating legally. They did not “hear” anything on the wire, so they began to transmit. If there is a station that has gone “deaf” and is stepping on other transmissions because it does not hear them, it will probably never be discovered because its frame collides with another transmission and its data is always corrupted. Troubleshoot the presence of too many collisions, but don’t examine the fragments closely. Using the corrupted data from collision fragments will just cause frustration. Late Collisions
A late collision is counted when a collision is detected by a device after it has sent the 512th bit of its frame. No more than a few late collisions should ever occur in any environment. If a device is incrementing a collision counter, further investigation is needed as a significant problem is occurring. If the number of late collisions is occurring at a steady rate, performance degradation may be noted.
Any of the following conditions may be causing late collisions: Note: Collisions and late collisions should never occur on any router, switch, or NIC port operating a full-duplex. Some Cisco Catalyst switches, such as the Catalyst 6000/6500, will disable ports on excessive late collisions, even when operating a half-duplex. The old attribution that a late collision is literally the result of a too-long cable is challenged by calculating the delay introduced by the cable only. While delay introduced by the cable is a factor, it is much more likely that other factors such as cable impedance mismatches, signal attenuation along the cable, too many repeaters, and marginal interfaces result in situations where a shortened cable eliminates late collisions.
Content 3.2 Characteristics of Physical Layer Optimization Problems 3.2.5 Other data transmission issues Short Frames
The most likely cause of a short frame is a faulty card, or an improperly configured or corrupt NIC driver file. Jabber
Jabber, is often defined as the condition in which a network device continually transmits random, meaningless data onto the network. IEEE 802.3 defines a jabber as a data packet whose length exceeds the standard. These packets are called long frames. Cyclically lock the port out, then check later to see if it is ok. The standard says that after the jabber timer expires (20,000 to 50,000 bit times) then the hub should close the port for awhile before reopening the port to see if the attached device has stopped transmitting. If jabber is again detected the port may be closed for another cycle. This may continue forever. In general jabbers are not a common occurrence. The most likely causes of jabber are a faulty NIC and/or faulty or corrupt NIC driver files, bad cabling, or grounding problems. Ghosts
Ghosts are easily created by a variety of causes on coaxial Ethernet. They may also be caused by something as simple as installing a second crossover cable between two hubs on half duplex 10BASE-T. The parallel path sometimes causes very strange symptoms. The hub will do one of the following two things: The error level often fluctuates between very little and the available bandwidth for no apparent reason. In addition to ghosts, this second crossover cable fault