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: - interface is down, line
protocol is down (FastEthernet)
- initialized,
down state (Token Ring)
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: - Unstable routing due to a marginal port
or link somewhere beyond the broadcast domain, possibly the
result of a bad cable
- Excessive traffic across a low
speed LAN or WAN link, possibly causing traffic to be discarded
or buffer capacity to be exceeded
- Overloaded server
or service
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: - Exceeding the
design limits of the media in terms of cabling distance or of
network devices
- Large collision domains in shared
media networks such as CSMA/CD Ethernet
- Electromagnetic Interference (EMI) effects
- Faulty
media or hardware
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:
- Impulse noise - voltage fluctuations or current spikes
induced on the cabling
- Random (white) noise
distributed over the frequency spectrum
- Alien cross
talk
- Near End Cross Talk (NEXT)
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