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6.4.1. Bandwidth and Latency Problems
| Issue | Description | Common Cause |
|---|---|---|
| Congestion | Too much traffic for capacity | Oversubscription, burst traffic |
| Bottleneck | Single point limiting throughput | Slow link, overloaded device |
| High latency | Excessive delay | Distance, congestion, processing |
| Jitter | Latency variation | Congestion, uneven queuing |
| Packet loss | Data not delivered | Congestion, errors |
QoS (Quality of Service): Classifies and prioritizes latency-sensitive traffic (voice, video) ahead of bulk data when queues fill—for example, marking voice as DSCP EF. If VoIP is choppy while data applications are fine, check that voice is being marked and prioritized end to end.
Slow to some destinations only: If speed tests are normal but certain sites are slow, the problem is on the path to those destinations (routing or ISP peering), not local bandwidth—run traceroute or mtr to a slow site to see where latency climbs.
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Written byAlvin Varughese
Founder•20 professional certifications