7.2 Physical Layer Problems Flashcards
A large proportion of networking problems are related to physical components or problems with the physical layer. Physical problems are concerned mainly with the hardware aspects of computers and networking devices, and the cables that interconnect them. Physical problems do not include the logical (software) configuration of devices.
Remember, the physical layer (Layer 1) deals with the physical connectivity of the network devices. Some of the more common Layer 1 problems include the following:
Device power turned off
Device power unplugged
Loose network cable connection
Incorrect cable type
Faulty network cable
Faulty wireless access point
To troubleshoot at Layer 1, first check that all devices have the proper power supplied, and that the devices are turned on. This may seem to be an obvious solution, but many times the person reporting the problem may overlook a device that is within the network path from source to destination. Ensure there are no errors showing on any LEDs that display the connectivity status. If on-site, visually inspect all network cabling and reconnect cables to ensure a proper connection. If the problem is with wireless, verify that the wireless access point is operational and that wireless settings are configured correctly.
The Sense of Sight
Vision is used to detect problems such as improperly connected or poorly constructed cables:
Cables which are not connected
Cables connected to the wrong port
Loose cable connections
Damaged cables and connectors
Use of the wrong type of cable
Vision also allows us to view the condition and function of various network devices with LEDs.
The Sense of Smell and Taste
Smell can alert troubleshooters to components which are overheating. The smell of burning insulation or components is very distinct and is a sure sign that something is seriously wrong.
The sense of taste is directly related to the sense of smell because both use the same receptors. You may also taste the acridness of something burning.
The sense of Touch
Troubleshooters can use touch to feel for overheated components as well as to detect mechanical problems with devices such as cooling fans. These devices usually create a small vibration in the component that can be detected using touch. The absence of this vibration or the presence of excessive amounts of vibration can indicate that the cooling fan has failed or is about to do so.
The sense of Hearing
Hearing is used to detect major problems such as electrical issues and the proper operation of cooling fans and disk drives. All devices have characteristic sounds and any change from the normal sounds usually indicate a problem of some sort.
Wireless Router LEDs
Most devices will have activity LEDs, which are often called link lights. A normal condition is for these LEDs to flash indicating that traffic is flowing through the port. A solid green light typically indicates that a device is plugged into the port, but no traffic is flowing. No light typically indicates one or more of the following:
Nothing is plugged into the port.
There is an issue with the wired or wireless connection.
A device or port has failed.
There is a cabling issue.
The wireless router is improperly configured, for example, a port was administratively shut down.
The wireless router has a hardware fault.
The device does not have power.
Cabling Problems
If the wired client is unable to connect to the wireless router, one of the first things to check is the physical connectivity and cabling. Cabling is the central nervous system of wired networks and one of the most common issues when experiencing inactivity.
There are several issues to watch for when cabling:
Be sure to use the correct type of cable. Two types of UTP cables are commonly encountered in networking: straight-through cables and crossover cables. Using the wrong type of cable may prevent connectivity.
Improper cable termination is one of the main problems encountered in networks. To avoid this, cables should be terminated according to standards. Terminate cables via the T568A or the T568B termination standard. Avoid untwisting too much of the wire pairs during termination. Crimp connectors on the cable jacket to provide strain relief.
Maximum cable run lengths exist based on characteristics of the different cables. Exceeding these run lengths can have a serious negative impact on network performance.
If connectivity is a problem, verify that the correct ports are being used between the networking devices.
Protect cables and connectors from physical damage. Support cables to prevent strain on connectors and run cable through areas that will not be in the way.
