Content Overview The physical layer
transmits bits from one computer to another and regulates the
transmission of a stream of bits over the physical medium. This
layer defines how the cable or antenna is attached to the
network adapter and what transmission technique is used to send
data over the medium. Repeaters, hubs, multiplexers, and
network interface cards (NICs) operate at the physical layer. A
NIC is primarily a Layer 2 device, but also operates at the
physical layer of the OSI model. A NIC performs Media Access
Control (MAC) sub-layer functions, but it also encodes bits and
sends them out on the medium. Repeaters, hubs, and multiplexers
have no interest in higher layer function and simply regenerate
and propagate received bit-streams to outgoing ports. Problems
that occur at Layer 1 are distinctly different from problems
that occur at higher layers. The physical layer is the only
layer with physically tangible properties such as wires, cards,
and antennas. In this lesson, identifiable characteristics and
commands will be used to isolate problems at the physical
layer. Failures and suboptimal conditions at the physical layer
do not merely inconvenience users. Networks that experience
these kinds of conditions usually come to a grinding halt.
Since the upper layers of the OSI model depend on the physical
layer in order to function, a network technician must have the
ability to effectively isolate and correct problems at this
layer. Upon completing this module, the students will be able
to: - Identify the characteristics of a physical layer
failure problem
- Identify the characteristics of a
physical layer optimization problem
- Identify
end-system commands and applications that gather physical layer
component information
- Identify the Cisco commands and
applications that gather physical layer component information
- Recognize possible causes of common physical layer
problems
- Isolate a problem at the physical layer
Content 3.1 Characteristics
of Physical Layer Failure Problems 3.1.1
Critical characteristics – connectivity Anytime that there
is a physical layer failure, a loss of connectivity will be
experienced. A user or a group of users affected by the problem
will usually call the help desk complaining that everything is
down. Less astute users may only complain that a specific
service or application is not working. If a technician can log
in to one of the affected devices and start gathering
information, it will be evident that no component above the
physical layer is operating. Unlike network failures, all cable
failures are approached in approximately the same manner,
whether the link is newly installed or has failed during
operation. There are many instances where a poor quality link
has been in service, but due to the operating environment and
influences it has stopped working. These influences include
visible damage to the cable, new electrical noise sources near
the cable, or accidental movement of the cables. Most networks
have converted from coaxial cable to Category 5 or Category 5e
UTP links. However, there is still a surprising amount of
coaxial cable in legacy network segments. Fiber-optic cable is
approaching a price point where it will rival Category 6 UTP in
overall cost. The overall cost of a fiber-optic cable
installation includes materials, labor, and network adapters.
All of these factors have become less expensive in recent
years. There are different installation and maintenance issues
for each cable type. Lab Activity Lab Exercise:
Applying a Logical Layered Model to a Physical Network After
completing this lab, the student will be able to understand the
data flow in the classroom network based on the logical network
model.
Content 3.1 Characteristics of Physical
Layer Failure Problems 3.1.2 Critical
characteristics – upper layer component operation With the
exception of Layer 1, each layer of the OSI model depends on
the layer below it for communication to work. In the event of a
Layer 1 connectivity problem, no component above the physical
layer will be operational. For example, name resolutions, ARP
requests, and DHCP requests all rely on Layer 1 for
connectivity. An example of upper layer protocol failure would
be a corporate user attempting to access a file server from a
workstation. Depending on the location of the fault, the
following types of communication components may fail:
- All pings to external devices would timeout.
- The
user would not be able to telnet into any other device.
- The user would not be able to access network drives.
- E-mail messages would not be sent or received and there may
be an undeliverable message to this effect when attempting to
send.
- “Page cannot be displayed” messages occur when
attempting to gain intranet or Internet access.
Content 3.1 Characteristics of Physical
Layer Failure Problems 3.1.3 Noncritical
characteristics – equipment indicators The Light Emitting
Diodes (LEDs) on a device can give feedback for diagnosing the
operational status of the device. When there is a physical
problem with equipment, the LEDs of the failing device are
usually off, flashing, or a different color than usual. Most
LED link lights are now software controlled, so they are no
longer reliable as a sole indicator of connectivity. If the
link light is illuminated, it may or may not mean that a valid
link is present. If the port is faulty, it may be possible to
disconnect the cable and still have the link light illuminated.
If the link light is off however, then it is still a fairly
good indication that no link is present. For devices using
multispeed interfaces such as 10/100 and 10/100/1000 NICs, it
is not uncommon to see link lights at one end of a failing
link, but not at the other. If a 10BASE-T device is connected
to a switch that is ‘hard-set’ or only capable of 100BASE-TX,
the link LED on the 10BASE-T device will often show that the
link is active, but the network card will fail to communicate.
The 100BASE-TX switch will not show link, and the 100BASE-TX
Ethernet segment is unaffected. If a 100BASE-TX only station is
connected to a shared media 10BASE-T hub, then it will not show
a link. The workstation will cause somewhere between 33 percent
and 100 percent collisions on the 10BASE-T collision domain.
The 10BASE-T hub will show the link, and if it has status LEDs
it will usually also show constant utilization and/or
collisions with those status LEDs. To troubleshoot mismatched
speeds, a more thorough examination than just looking at the
link state LED is needed. If a cable fault is the cause of link
failure, an auto-negotiating 10/100 device may successfully
negotiate a link to 100BASE-TX, but it may not be able to
subsequently establish and maintain signaling synchronization.
This results in the link light blinking on and off as the
Ethernet chipset repeatedly negotiates a connection speed, then
fails to link. If the cable fault is not severe, then the link
may come up and operate very poorly for a short time before the
link fails and is again renegotiated. If the affected port is a
parallel uplink between switches, this fault can cause
spanning-tree problems and loss of logical connectivity across
a broadcast domain.
Content 3.1
Characteristics of Physical Layer Failure Problems
3.1.4 Noncritical characteristics – power failures
A major category of physical layer problems which may be
encountered are power related. If power is not supplied to each
network device within specific tolerances, then either a loss
of service or damage to the unit may occur. Either the device
will not have sufficient power to function, or it may be
damaged, if the power fluctuates above the design
specifications for that device. These fluctuations could
include the following: Power failure
A power failure
is characterized by a loss of power. Incoming power is subject
to blackouts, which are complete power outages, often caused by