What is attenuation?
It is the loss of intensity as signal moves through a medium. In copper cables, you’ll have electrical signals that will attenuate as they move through the cable. If it’s a fiber connection, the same thing happens with light.
What are decibels?
(dB) It is the signal strength ratio measurements.
What are dB loss symptoms?
- No connectivity(no signal)
- Intermittent connectivity(just enough signal to sync the link)
- Poor performance(signal too weak, CRC errors, data corruption)
- Test each connection(test distance and signal loss)
What is latency?
It’s a delay between the request and the response(waiting time)
When and how do we test the latency?
When the response time is longer than normal, we examine the response times at every step along the way(it may require multiple measurement tools like packet captures.
What is a jitter?
It is the time between frames(excessive jitter can cause you to miss information, “choppy” voice calls).
How can we troubleshoot excessive jitter?
- confirm available bandwidth(nothing works if the tube is clogged)
- Make sure the infrastructure is working as expected(check queues in switches and routers for dropped frames)
- Apply QoS(Quality of service by prioritize real-time communication services, switch, router, firewall, etc)
What is XT and how do we measure it?
(XT:crosstalk)It’s when signal on one circuit affects another circuit(leak of information from one wire to another). We measure it with a TDR.
- Near End Crosstalk(NEXT) at the transmitting end.
- Far End Crosstalk(FEXT) away from the transmitter.
How do we troubleshoot XT?
- It’s almost always a wiring issue(check your crimp)
- Maintain your twists(twist helps to avoid crosstalk)
- Category 6A increases cable diameter(increased distance between pairs.
- Test and certify your installation(Solve problem before they are problems)
What is EMI? and how can we avoid it?
It’s Electromagnetic interference and we can avoid it with:
- Cable handling(no twisting, watch your bend radius, not using staples and watching the cable ties)
- EMI and interference with copper cables(avoid power cords, electrical systems, etc)
- Test after installation
What are opens and shorts?
A short circuit happens when connections are touching(or wires inside of a cable or connection)
An open circuit is a break in the connection.
How can we troubleshoot opens and shorts?
- Replace the cable with the short or open.
2. Advanced troubleshooting with a TDR(Time Domain Reflectometer)
What are pin-outs and how do we troubleshoot them?
It’s very easy to switch cables around and have the incorrect pin-outs on these wires and can cause pin-out. For fixing the problem:
- Have a good cable mapping device
- Get a good cable person
What kind of problem can using the wrong kind of cable cause? how can we confirm the type pf cable we have?
Excessive physical errors, CRC errors(check your layer 1 first)
We can confirm it by:
1. check the outside of the cable
2. Confirm the cable specifications with a TDR.
How do we troubleshoot interfaces?
- Interface errors may indicate bad cable or hardware problem
- Verify configurations(speed, duplex, VLAN, etc.
When does the transceiver mismatch happen?
Transceivers can mismatch if any of the following don’t meet or we can have signal loss, dropped frames or missing frames:
- Transceivers have to match the fiber
- A transceiver needs to match the wavelength
- Use the correct transceivers and optical fiber
When does TX/RX reverse occur?
It happens with wiring mistake. They are easy to find with a wire map but some interfaces will automatically correct(using Auto-MDIX)
How can we troubleshoot TX/RX reversal?
- No connectivity(Auto-MDIX might connect
2. Locate the reversal location(often at a punch down)
What are bottlenecks?
A bottleneck is a point of congestion in a production system that occurs when workloads arrive too quickly for the production process to handle.
How do we resolve the network bottleneck?
Check on I/O bus, CPU speed, storage access speed, network throughput, etc. We must monitor all of the, to find the slowest one but there is never just one performance metric.
When do we know we have an interface configuration problem?
- Auto vs. Manual configuration
- Light status(no light, no connection)
- Speed(must be identical on both sides)
- Duplex(if mismatched, speed will suffer)
What do we have a VLAN mismatch? and can we check for it?
- Switch is configured with the incorrect VLAN
- Link light is on but no surfing
We check the switch configuration for VLAN configuration
Whats duplex/speed mismatch happen?
Incorrect speed happens when we have less than expected throughput. Incorrect duplex happens when duplex is not the same on both sides and can cause significant slowdowns.
