Domain 5 of 5 · Chapter 4 of 5

Performance Issues

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Included in this chapter:

  • Bandwidth vs throughput, and where congestion comes from
  • Latency, jitter, and packet loss: the real-time metrics
  • Quality of Service: marking and queuing under congestion
  • Wireless interference and exam-pattern recognition

Performance symptom to fault to measurement to fix

SymptomPerformance faultHow to measurePrimary fix
Link near 100% utilization, rising delay and lossCongestion / bandwidth saturationiperf throughput vs rated bandwidth; utilization graphAdd bandwidth or a second path; apply QoS
High round-trip time but link not fullLatency (propagation / queuing)ping round-trip time; traceroute per hopShorten path or reduce queuing; QoS for priority flows
Choppy or robotic voice and videoJitter (latency variation)ping variation; VoIP / call-quality monitorJitter buffer on receiver; QoS marking (DSCP)
Dropped packets, stuttering callsPacket losscontinuous ping (ping -t) loss percentageQoS or bandwidth if congestion; replace hardware if physical
Wi-Fi slows in crowded areasCo-channel interferenceWi-Fi analyzer (channel utilization, RSSI)Channel plan 1/6/11; move to 5 GHz or 6 GHz
Wireless noisy near equipmentAdjacent-channel / non-Wi-Fi interferenceWi-Fi analyzer (SNR, noise floor)Change channel, shield or remove the source

Decision tree

Wired or wireless?measure the symptom firstWired: link near 100%?utilization + iperfWireless: RSSI + SNRWi-Fi analyzerwiredwirelessAdd bandwidth or QoScongestion / saturationNot full: which metric?latency / jitter / lossyes, fullnoShorten path / QoShigh latency (ping)Jitter buffer + QoSchoppy voiceQoS/bandwidth or replace HWpacket loss: congestion vs physicallatencyjitterlossPlan 1, 6, 11 / 5-6 GHzco-channel interferenceChange channel / remove sourcenon-Wi-Fi noise, low SNRco-channelnoiseQoS reorders the queue; it never adds bandwidthuse it to manage scarcity, not manufacture capacity
Cheat sheet

Cheat sheet

  • Bandwidth is the rated capacity; throughput is what you actually measure
  • Congestion is demand exceeding capacity, and it raises delay and loss together
  • Tail drop is what a full interface buffer does to overflow traffic
  • QoS reorders the queue under congestion; it never creates bandwidth
  • QoS marks traffic with DSCP, a 6-bit value in the IP header's DS field
  • Latency is total delay: propagation plus serialization plus queuing
  • Jitter is the variation in latency, and it is what breaks VoIP
  • A jitter buffer smooths arrival timing, not packet loss
  • Packet loss has several causes, so confirm the cause before the fix
  • Measure packet loss with a continuous ping, not a single probe
  • Match the tool to the metric: ping for latency and loss, iperf for throughput
  • Co-channel interference is access points sharing one channel via CSMA/CA backoff
  • Adjacent-channel interference corrupts frames and lowers SNR
  • On 2.4 GHz, only channels 1, 6, and 11 are non-overlapping
  • 5 GHz and 6 GHz offer more non-overlapping channels for crowded areas
  • Non-Wi-Fi sources raise the noise floor in the 2.4 GHz band
  • RSSI measures signal strength; SNR measures signal above noise
  • Mark voice and video with QoS so they survive a saturating bulk flow
  • TCP retransmissions and duplicate ACKs in a capture point to packet loss
  • NetFlow identifies top talkers and bandwidth-hogging applications
  • SNMP polling at intervals builds utilization and error baselines
  • An SNMP trap is an unsolicited push for immediate event alerting
  • ifInDiscards and ifOutDiscards count no-error drops from buffer congestion

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References

  1. Using the Extended ping and Extended traceroute Commands
  2. RFC 2474: Definition of the Differentiated Services Field (DS Field) in the IPv4 and IPv6 Headers Whitepaper
  3. Channel Planning Best Practices