downed power lines or electrical failures. Power spike
A power spike is a short burst of excessive power, usually lasting less than 1/60 of a second. Unprotected electronic equipment is vulnerable to this sudden, potentially massive, increase in voltage. Power spikes are similar in nature to power surges, which can last as long as several seconds. Brownouts
Brownouts are in-line power reductions of ten percent or more. They are usually caused by utility company problems or a sudden drain of electricity from a particular part of the power grid. Dirty power
Dirty power is caused by electrical circuits experiencing transients and noise. Transients are brief high-speed electrical fluctuations caused by lightning or improper grounding. Noise is electromagnetic or radio frequency interference in the power signal caused by disruption from the external power grid or by feedback from local mechanical devices such as printers and copiers.
Content 3.1 Characteristics of Physical Layer Failure Problems 3.1.5 Noncritical characteristics – console messages With a physical layer failure problem, sometimes a problem is discovered when a device shows console messages indicating that an interface is not functioning. For instance, the show interfaces command for a failing interface may show one of the following: An unattached Ethernet LAN interface can be spoofed to an interface is up, line protocol is up state by issuing the no keepalive command. Therefore, this command should not be used in a live network as a substitute for testing at the physical layer. Figure shows possible problems that can cause the line status or the line protocol to be down. In a Frame Relay environment, troubleshooting is not so simple. For example, if the circuit goes down at one side of the non-broadcast network, but the connection to the local loop on the other side is functional and configured on a physical interface, then the interface for that device will still show as administratively up and line protocol up. The connection of the remote router to the local switch is still up, even though the circuit with its partner is down. If the connection is configured with subinterfaces on both ends, then a loss of connectivity on either side will cause both subinterfaces to go down.
Content 3.2 Characteristics of Physical Layer Optimization Problems 3.2.1 Performance lower than baseline If there is a problem with sub-optimal operation at the physical layer, the network will be operational, but performance will be consistently or intermittently lower than the level specified in the baseline. If performance is unsatisfactory all the time, then the problem is probably related to a poor configuration, inadequate capacity somewhere, or some other systemic problem. If performance varies and is not always unsatisfactory, then the problem is probably related to an error condition or is being affected by traffic from other sources: A physical layer optimization problem occurs when the physical properties of the connection are substandard, causing data to be transferred at a rate that is constantly less than the rate of data flow established in the baseline. A number of factors can be involved in decreasing the rate at which data is transmitted across media. Major causes of networks performing below baseline are:
Content 3.2 Characteristics of Physical Layer Optimization Problems 3.2.2 Exceeding cable design limits, poor quality cabling and connections Attenuation
A common issue of exceeding the design limits of a media type is the attenuation of the bit-stream transmitted along the media. Attenuation depends on the media over which the traffic is being transmitted. Attenuation may occur to such an extent that the receiving device cannot always successfully distinguish the component bits of the stream from each other. This ends in a garbled transmission and results in a request from the receiving device for retransmission of the missed traffic by the sender. Each network media is rated for a specific distance. Category 5 media has a maximum rated cabling distance of 100m. Beyond that distance, the signal must be regenerated or it may degenerate and be unreadable by the receiving end. To get an accurate measurement of the length of a cable, use a cable tester. A far more common source of attenuation is a very poor connection resulting from a loose cable or dirty or oxidized contacts. One bad patch cable can easily cause an entire link to fail. Another source of this fault is the wrong Category cable used, such as Category 3 cable used for a link being tested to Category 5e limits. Return Loss
Return loss is a measure of all reflections that are caused by the impedance mismatches at all locations along the link. It indicates how well the characteristic impedance of the cable matches its rated impedance over a range of frequencies. The characteristic impedance of links tends to vary from higher values at low frequencies to lower values at the higher frequencies. Return loss is expressed in decibels. The termination resistance at both ends of the link must be equal to the characteristic impedance of the link to avoid reflections. A good match between characteristic impedance and termination resistance in the end equipment provides for a good transfer of power to and from the link and minimizes reflections. Return loss results vary significantly with frequency. One small source of return loss is variations in the value of the characteristic impedance along the cable. This may be due to slight untwisting or separation of wires in the pairs, or due to variations in the metal of the wire and the uniformity of the insulation. Another source of return loss is reflections from inside the installed link, mainly from connectors. Mismatches predominantly occur at locations where connectors are present. The main impact of return loss is not on loss of signal strength but rather the introduction of signal jitter, the variation in delay over time from point-to-point. Jitter is of special concern for time sensitive traffic such as VoIP, excessive jitter seriously degrades call quality. Signal reflections truly cause loss of signal strength but generally this loss, due to return loss, does not create a significant problem. Return loss is of particular concern in the implementation of Gigabit Ethernet.
Content 3.2 Characteristics of Physical Layer Optimization Problems 3.2.3 Noise Local Electromagnetic Interference (EMI) is commonly known as noise. There are four types of noise that are most significant to data networks: The default threshold level for the detection and registration of impulse noise is 270 mV (determined by 10BASE-T specification in the IEEE 802.3i standard). For high-speed network applications such as 1000BASE-T (Gigabit Ethernet), the recommended threshold value for impulse noise detection is 30 or 40 mV. Random Noise may be generated by many sources ranging from wireless communications such as FM radio stations, police