Network Troubleshooting · Topic 4 of 6
Troubleshooting Performance Issues
Diagnose slow networks by measuring congestion, bottlenecks, latency, jitter and packet loss, and fix wireless interference, coverage and roaming faults.
- 4 min
- Bandwidth is capacity. Throughput is the rate actually achieved.
- Latency is delay. Jitter is variation in delay. Packet loss is dropped packets.
- Voice and video are most sensitive to latency, jitter and loss.
What it is
A network that works but works badly is a performance problem. Users report slow pages, choppy calls or file copies that take forever.
To fix it you must measure the right thing. The exam asks you to tell congestion, latency, jitter and packet loss apart, and to find the cause on wired and wireless links.
How it works
Key performance terms
| Term | Meaning |
|---|---|
| Bandwidth | The maximum capacity of a link |
| Throughput | The data rate actually achieved, which is lower than bandwidth |
| Latency | The delay for a packet to travel from source to destination |
| Jitter | Variation in latency from packet to packet |
| Packet loss | The percentage of packets that never arrive |
Real time traffic such as voice and video is very sensitive to latency, jitter and loss. File transfers tolerate delay but need throughput.
Congestion and bottlenecks
Congestion occurs when more traffic arrives than a link can carry, so queues build up and packets are delayed or dropped. Contention is competition among devices for the same shared resource, such as a wireless channel.
A bottleneck is the slowest point on a path that limits the end to end speed, for example a 100 Mbps link in an otherwise gigabit path or an overloaded firewall. Find it by checking utilization along the path, from the user to the server.
| Cause | Typical fix |
|---|---|
| Link utilization near the limit | Add bandwidth, use link aggregation or reduce unnecessary traffic |
| Bulk traffic hurting voice or video | Apply Quality of Service (QoS) to prioritize sensitive traffic |
| Slow or overloaded device | Check central processing unit (CPU) and memory, upgrade or rebalance |
| Duplex mismatch or bad cable | Correct the setting or replace the cable |
| Broadcast storm or loop | Find and remove the loop, and enforce spanning tree |
Latency, jitter and loss
High latency can come from long distances, satellite links, overloaded devices or slow Domain Name System (DNS) lookups. Jitter appears when queues vary in length, and it causes broken audio. Packet loss comes from congestion, bad cables, or interference, and it forces the Transmission Control Protocol (TCP) to retransmit, which slows transfers.
Use ping to see delay and loss, and traceroute to find where delay appears along the path. A repeating test over time, such as continuous ping, shows jitter and intermittent loss.
A sudden jump in delay at one hop points to that device or the link before it.
Wireless performance problems
| Problem | Cause and fix |
|---|---|
| Interference | Other Wi-Fi networks, microwaves, Bluetooth or cordless phones. Change channels or move to 5 or 6 gigahertz (GHz) |
| Signal degradation | Distance, walls or metal reduce signal strength. Move the access point closer or add one |
| Insufficient coverage | Dead spots, found with a site survey and fixed by adding or repositioning access points |
| Client disassociation | Clients drop off because of weak signal, authentication timeouts or an overloaded access point |
| Roaming misconfiguration | Different Service Set Identifiers (SSIDs) or security settings on neighboring access points, or poor overlap, so clients stick to a distant one |
| Too many clients | One access point serves too many devices, so use more access points or better band use |
A wireless site survey and a spectrum analyzer show signal strength and noise. A low signal strength reading, or a poor signal to noise ratio, explains slow speeds.
Method
Compare current results to the baseline, change one thing at a time and re-test. Without a baseline you cannot tell whether a number is unusual.
What to remember for the exam
- Bandwidth is capacity. Throughput is the rate actually achieved.
- Latency is delay. Jitter is variation in delay. Packet loss is dropped packets.
- Voice and video are most sensitive to latency, jitter and loss.
- Congestion means more traffic than capacity. A bottleneck is the slowest point on the path.
- Use QoS to protect voice and video traffic during congestion.
- Traceroute shows where delay appears. Ping shows delay and loss.
- Wireless issues include interference, weak signal, poor coverage, roaming mistakes and too many clients.
- Compare against a baseline and change one thing at a time.
If only calls are choppy while file transfers are fine, think jitter and QoS. If everything is slow at the same time every day, think congestion.
Common mistakes
Mixing up bandwidth and throughput. A fast link can still deliver slow throughput.
Adding bandwidth without finding the bottleneck. The slow point may be a device, not the link.
Ignoring wireless noise. Strong signal with high noise still performs badly.
Confusing latency and jitter. Constant high delay is latency, while changing delay is jitter.
Testing once. Intermittent problems require repeated or continuous tests.
A speed test shows throughput from your device to one test server at one time. It does not prove the whole network is healthy.
Check yourself
Try these three.
What is the difference between latency and jitter?
Latency is the delay of packets, and jitter is the variation in that delay.
Which tool helps find the hop where delay increases?
Traceroute (tracert on Windows).
A conference room has poor Wi-Fi when many people join. What are likely causes?
Too many clients on one access point, channel contention or interference.
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