Quality of Service (QOS)

• QOS is about managing network resources to provide predictable service levels - it’s traffic prioritization, not traffic acceleration
• Works by classifying, marking, queuing, and shaping traffic flows based on business requirements
• Essential for time-sensitive applications like VoIP, video conferencing, and real-time data
• Operates on the principle that not all traffic is created equal - mission-critical apps get priority over web browsing

QOS Models

• Best Effort: No QOS guarantees (default Internet model)
• Integrated Services (IntServ): Per-flow resource reservation using RSVP
• Differentiated Services (DiffServ): Class-based treatment using DSCP markings (most common in enterprise)

Traffic Classification & Marking

• Classification identifies traffic types (by protocol, port, source/destination)
• Marking tags packets with priority values for downstream devices to honor
• Layer 2 marking: 802.1p CoS (Class of Service) field in VLAN header (3 bits = 8 priority levels)
• Layer 3 marking: DSCP (Differentiated Services Code Point) in IP ToS field (6 bits = 64 possible values)

QOS Mechanisms

• Queuing: Multiple queues with different scheduling algorithms

  • FIFO: First In, First Out (no prioritization)
  • PQ: Priority Queuing (strict priority - higher queues starve lower ones)
  • WFQ: Weighted Fair Queuing (automatic flow-based fairness)
  • CBWFQ: Class-Based WFQ (manual class configuration with bandwidth guarantees)
  • LLQ: Low Latency Queuing (CBWFQ + priority queue for voice)

• Traffic Shaping: Smooths traffic bursts by buffering excess packets

• Traffic Policing: Drops or remarks traffic exceeding configured rates (more aggressive than shaping)

QOS Trust Boundaries

• Trust boundary defines where you believe QOS markings are accurate
• Typically set at access layer switches or IP phones (not end devices)
• Untrusted devices get their markings overwritten to prevent abuse
• For example: Trust IP phone markings but not PC markings on same switch port

Vocabulary

DSCP: Differentiated Services Code Point - 6-bit field in IP header for traffic marking CoS: Class of Service - 3-bit 802.1p field in Ethernet frame for Layer 2 marking
Expedited Forwarding (EF): DSCP marking (46) reserved for voice traffic Assured Forwarding (AF): DSCP class providing bandwidth guarantees with drop precedence Committed Information Rate (CIR): Guaranteed bandwidth rate for traffic shaping/policing Jitter: Variation in packet delay (critical for real-time applications) Latency: One-way packet delay (voice requires <150ms end-to-end)

Notes

• Voice traffic is the most demanding - requires <150ms latency, <30ms jitter, <1% packet loss
• QOS is only effective during network congestion - it can’t fix insufficient bandwidth
• AutoQOS feature automatically configures common QOS policies (use auto qos voip on switch ports)
• Remember the 80/20 rule: Reserve 80% bandwidth for data classes, 20% for voice/video
• QOS markings are preserved across trunk links but may need remarking at Layer 2/3 boundaries
• Always implement QOS end-to-end - one weak link breaks the entire chain
• In government/military networks, QOS often maps to security classifications (SECRET traffic gets priority over UNCLASSIFIED)