Why Your 100 Gb LAN Still Needs QoS: The Hidden Traffic Jams in High-Speed Networks
Facts, targets, and a simple plan for leadership and engineers
Modern enterprise network architecture requiring Quality of Service management across high-speed interconnections
🎯 Executive Summary
High throughput does not remove queues. Short bursts, mixed link speeds, shared Wi‑Fi airtime, and converged workloads still add delay and jitter. QoS protects time‑sensitive traffic during those moments.
The Highway Analogy
Picture a ten lane highway. The core runs fast. Access uplinks act like toll booths. Older 100 Mb endpoints behave like scooters in fast lanes. Wi‑Fi acts like a side road where users share airtime. Without QoS, a board meeting call stalls while a backup job or a 4K camera stream grabs airtime.
⚠️ Business Risk in One Minute
Unplanned downtime is expensive. Recent ITIC surveys report over 90 percent of mid size and large firms estimate one hour of downtime above 300,000 USD. Many report one million USD or more.
What Engineers Face
1. Bandwidth Does Not Equal Low Delay
Microbursts create short queues that average graphs miss. Fan in leads to incast where many senders overload a single receiver. Active Queue Management with ECN reduces queuing delay and loss.
2. Mixed Link Speeds Remain
Access ports often run at 1 Gb. Printers, sensors, and legacy clients still run at 100 Mb in many offices. Oversubscription and bursts drive drops on uplinks.
3. Wi‑Fi is Shared Airtime
EDCA and WMM prioritize voice, video, best effort, and background. Map DSCP values to access categories.
4. Cabling Facts That Matter
Category 6A supports 10GBASE‑T to 100 meters. 25GBASE‑T and 40GBASE‑T use Category 8 for up to 30 meters. Most high speed access links use DAC or fiber.
5. Targets for Real Time Traffic
Keep one way latency under 150 ms for interactive voice. Keep jitter near 10 ms and under 30 ms to 50 ms worst case. Keep loss near 1 percent or lower.
| Metric | Target | Maximum Acceptable | Impact if Exceeded |
|---|---|---|---|
| One-way Latency | < 150 ms | 200 ms | Noticeable delay in conversation |
| Jitter | ~10 ms | 30-50 ms | Audio quality degradation |
| Packet Loss | < 1% | 3% | Audio dropouts, video pixelation |
💡 Design That Works
- Classify and mark at the edge with DSCP
- Map to service classes per RFC 4594. Use EF for voice. Use AF4x for video. Allocate bandwidth for AF queues
- Use strict priority for EF. Keep the queue small
- Enable AQM and ECN where supported
- On Wi‑Fi, enable WMM. Map DSCP to AC_VO, AC_VI, AC_BE, and AC_BK
- Monitor per queue delay and drops during change windows
Validation Under Load
- Run tests while backups and OS updates run
- Track one way latency, jitter, and loss for voice and video
- Review per queue utilization and drops
- Adjust shaping, scheduling, and markings with evidence
🎯 Quick Wins for Leadership
- Require QoS on executive spaces and boardrooms
- Ask for monthly QoS reports with latency, jitter, loss, and queue drops
- Fund Cat 6A for new copper runs. Use fiber or DAC for 25 Gb and above except for short Cat 8 server links
Implementation Appendices
Appendix A: Suggested DiffServ Map
- Voice media: EF, DSCP 46
- Video: AF41 to AF43
- Call signaling: CS3
- Best effort: CS0
- Background: CS1
Appendix B: Wi‑Fi DSCP to WMM Map
- EF maps to AC_VO
- AF4x maps to AC_VI
- BE maps to AC_BE
- CS1 maps to AC_BK
Appendix C: Test Checklist
- Define service classes and DSCP values
- Trust boundaries on access ports
- Edge marking for voice and video
- Map DSCP to switch queues
- Enable strict priority for EF
- Enable AQM or ECN where supported
- Wi‑Fi WMM on all SSIDs
- DSCP to WMM mapping verified
- Test during backup and patch windows
- Track one way latency, jitter, and loss
- Monthly review of per queue drops
- Quarterly review of DSCP markings
- Run MOS tests for voice and video
References
- ITIC 2024 Hourly Cost of Downtime survey. https://itic-corp.com/itic-2024-hourly-cost-of-downtime-report/
- ITIC 2024 report PDF summary. https://www.calyptix.com/wp-content/uploads/Hourly-Cost-of-Downtime-ITIC.pdf
- Arista. Microbursts, Jitter and Buffers. https://www.arista.com/assets/data/pdf/TechBulletins/AristaMicrobursts.pdf
- H. Wu et al. ICTCP: Incast Congestion Control for TCP. https://conferences.sigcomm.org/co-next/2010/CoNEXT_papers/13-Wu.pdf
- RFC 7567. IETF Recommendations Regarding AQM. https://www.rfc-editor.org/info/rfc7567
- RFC 3168. Explicit Congestion Notification. https://www.rfc-editor.org/info/rfc3168
- Wi‑Fi Alliance WMM overview and history. https://docs.broadcom.com/doc/12358327
- IEEE 802.11 QoS tutorial on EDCA. https://ieee802.org/1/files/public/docs2008/avb-gs-802-11-qos-tutorial-1108.pdf
- Cisco, Wireless QoS design notes for WMM and EDCA. https://www.cisco.com/en/US/docs/solutions/Enterprise/Mobility/emob30dg/WANQoS.html
- IEEE 802.3an 10GBASE‑T and Cat 6A to 100 m. https://en.wikipedia.org/wiki/10_Gigabit_Ethernet
- IEEE 802.3bq 25GBASE‑T and 40GBASE‑T over Category 8 to 30 m. https://standards.ieee.org/ieee/802.3bq/6227/
- Fluke Networks. Category 8 fact sheet. https://www.flukenetworks.com/knowledge-base/applicationstandards-articles-copper/category-8-cabling-fact-sheet
- ITU‑T G.114 one way transmission time. https://www.itu.int/rec/T-REC-G.114-200305-I
- Cisco voice design, jitter target near 10 ms and not more than 50 ms. https://documentation.meraki.com/Architectures_and_Best_Practices/Cisco_Meraki_Best_Practice_Design/Best_Practice_Design_-_MR_Wireless/Wireless_VoIP_QoS_Best_Practices
- Cisco QoS for VoIP. Loss near 1 percent, one way delay 150 ms. https://www.cisco.com/c/en/us/support/docs/voice/voice-quality/20371-troubleshoot-qos-voice.html
- RFC 4594. DiffServ service class guidelines. https://www.rfc-editor.org/info/rfc4594