Source mc mcse Certification Resources

Crosstalk
Symptoms: Slow network performance and/or an excess of dropped or unintelligible packets. In telephony applications, users hear pieces of voice or conversations from a separate line.

Causes: Generally crosstalk occurs when two cables run in parallel and the signal of one cable interferes with the other. Crosstalk can also be caused by crossed or crushed wire pairs in twisted pair cabling.

Resolution: the use of twisted pair cabling or digital signal can reduce the effects of crosstalk. Maintaining proper distance between cables can also help.
Near-End Crosstalk
Symptoms: Signal loss or interference

Causes: Near-end crosstalk is crosstalk that occurs closer along the cable to the transmitting end. Often occurs in or near the terminating connector.

Resolution: Test with cable tester from both ends of the cable and correct any crossed or crushed wires. Verify that the cable is terminated properly and that the twists in the pairs of wires are maintained.
Attenuation
Symptoms: Slow response from the network.

Causes: Attenuation is the degradation of signal strength.

Resolution: Use shorter cable runs, add more access points, and/or add repeaters and signal boosters to the cable path. Or, evaluate the environment for interference. The interference you would look for would depend on the spectrum used.
Collisions
Symptoms: High latency, reduced network performance, and intermittent connectivity issues.

Causes: Collisions are a natural part of Ethernet networking as nodes attempt to access shred resources.

Resolution: Depends on the network. For example, replacing a hub with a switch will often solve the problem.
Shorts
Symptoms: Electrical shorts—complete loss of signal.

Causes: Two nodes of an electrical circuit that are meant to be at different voltages create a low- resistance connection causing a short circuit.

Resolution: Use a TDR to detect and locate shorts. Replace cables and connectors with known working ones.

Open Impedance Mismatch
Symptoms: Also known as echo, the tell-tale sign of open mismatch is an echo on either the talker or listener end of the connection.

Causes: The mismatching of electrical resistance.

Resolution: Use a TDR to detect impedance. Collect and review data,interpret the symptoms, and determine the root cause in order to correct the cause.

Interference
Symptoms: Crackling, humming, and static are all signs of interference. Additionally, low throughput, network degradation, and poor voice quality are also symptoms of interference.

Causes: RFI can be caused by a number of devices including cordless phones, Blue-Tooth devices, cameras, paging systems, unauthorized access points, and clients in ad-hoc mode.

Resolution: Remove or avoid environmental interferences as much as possible. This may entail simply turning off competing devices. Ensure there is adequate LAN coverage. To resolve problems proactively, test areas prior to deployment using tools such as spectrum analyzers.

Port Speed
Symptoms: No or low speed connectivity between devices.

Causes: Ports are configured to operate at different speeds and are therefore incompatible with each other.

Resolution: Verify that equipment is compatible and operating at the highest compatible speeds. For example, if a switch is running at 100 Mbs, but a computer’s NIC card runs at10 Mbs, the computer will run at the slower speed (10 Mbs). Replace the card with one that runs at 100 Mbs and throughput will be increased to the higher level (or at least higher levels since there are variables such as network congestion, etc.)

Port Duplex Mismatch
Symptoms: Late collisions, alignment errors, and FCS errors are present during testing.

Causes: Mismatches are generally caused by configuration errors. These occur when the switch port and a device are configured to use a different duplex setting or when both ends are set to auto-negotiate the setting. Resolution: Verify that the switch port and the device are configured to use the same duplex setting. This may entail having to upgrade one of the devices.

Incorrect VLAN
Symptoms: No connectivity between devices.

Causes: Devices are configured to use different VLAN’s

Resolution: Reconfigure devices to use the same VLAN.

Incorrect IP Address
Symptoms: No connectivity between devices.

Causes: Either the source or destination device has an incorrect IP address.

Resolution: Use the ping command to determine if there is connectivity between devices. Resolution will depend on the problem. If a network is running a rouge DHCP server, for example, two computers could have leased the same IP address. Check TCP/IP configuration information using ipconfig /all on Window machines and ifconfig on Linux/UNIX/Apple machines. In that case troubleshoot DHCP (it may be off line, etc.). It could be the case that a static IP address was entered incorrectly. Check IP addresses; empty the arp cache on both computers.

Wrong Gateway
Symptoms: No connectivity between devices.

Causes: The IP address of the gateway is incorrect for the specified route.

Resolution: Change the IP address of the gateway to the correct address.

Wrong DNS
Symptoms: No connectivity between devices.

Causes: A device is configured to use the wrong DNS server.

Resolution: Open the network properties on a Windows machine. Open TCP/IP properties and check the IP address of the DNS server listed for the client. Put in the correct IP address. Test for connectivity.

Wrong Subnet Mask
Symptoms: No connectivity between devices.

Causes: Either the source or destination device has an incorrect subnet mask.

Resolution: Use the ping command to determine if there is connectivity between devices. Check the IP address on both devices. Change the incorrect subnet mask to a correct subnet mask. Test for connectivity.

Issues that should be ID’d but Escalated

• Switching Loop: Need spanning tree protocol to ensure loop free topologies.
• Routing Loop: Packets are routed in a circle continuously.
• Route Problems: Packets don’t reach their intended destination. This could be caused by a number of things: configuration problems, convergence (in which you have to wait for the discovery process to complete), or a broken segment (a router is down, etc.).
• Proxy arp: If mis-configured, DoS attacks can occur.
• Broadcast Storms: The network becomes overwhelmed by constant broadcast traffic.

Wireless Connectivity Issues

Interference
Symptoms: Low throughput, network degradation, dropped packets, intermittent connectivity, and poor voice quality are all symptoms caused by interference.

Causes: RFI can be caused by cordless phones, Bluetooth devices, cameras, paging systems, unauthorized access points, metal building framing, and clients in ad-hoc mode.

Resolution: Remove or avoid environmental interferences as much as possible.

Incorrect Encryption
Symptoms: For wireless, if encryption levels between two devices (access point and client) do not match, connection is impossible. Similarly, if different encryption keys are used between to devices they can’t negotiate the key information for verification and decryption in order to initiate communication.

Causes: Improper configuration.

Resolution: Ensure that security settings match between and among devices.

Congested Channel
Symptoms: Very slow speeds.

Causes: Interference from neighboring wireless network; congested network channel.

Resolution: Many wireless routers are set to auto configure the wireless channel. Try logging into the router and manually change the channel the wireless router is operating on.

Incorrect Frequency
Symptoms: No connectivity.

Causes: In wireless, devices must operate on the same frequency. A device for a 802.11a frequency can’t communicate with one designed for 802.11b.

Resolution: Deploy devices that operate on the same frequency.

ESSID Mismatch
Symptoms: No connectivity between devices.

Causes: Devices are configured to use different ESSIDs.

Resolution: Set the devices to use the same SSID. Ensure that the wireless client and the access point are the same. Note: SSIDs are case sensitive.

Standard Mismatch
Symptoms: No connectivity between devices.

Causes: Devices are configured to use different standards such as 802.11a/b/g/n.

Resolution: Devices have to be chosen to work together. 802.11a, for example, is incompatible with 802.11b/g because the first operates at 5 GHz and the second at 2.4 GHz. O a 802.11g router could be set only for “g” mode and you are trying to connect with a 802.11b wireless card. Change the mode on the router.

Distance
Symptoms: Slow connection and low throughput.

Causes: The distance between two points may be to blame for this connectivity issue. The longer the distance between the two points the prominent the problem may become. Issues that can occur between the two points include latency, packet loss, retransmission, or transient traffic.

Resolution: I f the issue is with cabling, do not exceed distance limitations. If the issue is with wireless, you may need to increase coverage. Use a spectrum analyzer to determine coverage and signal strength.

Bounce
Symptoms: No or low connectivity between devices.

Causes: Signal from device bounces off obstructions and is not received buy the receiving device.

Resolution: If possible, move one device or the other to avoid obstructions. Monitor performance and check for interference.

Incorrect Antenna Placement
Symptoms: No or low signal and connectivity.

Causes: The position of the access point’s antenna can negatively affect overall performance.

Resolution: Change the position of the antenna and monitor device performance.

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