QoS (Quality of Service)

What is QoS?

Quality of Service (QoS) refers to mechanisms that control how network traffic is handled to ensure critical applications receive the bandwidth, latency, and reliability they require. QoS is essential in environments where multiple applications compete for limited network resources.

QoS is commonly implemented on routers, switches, firewalls, and WAN devices.

Why QoS matters

QoS is important because it:

• Ensures predictable performance for critical applications
• Reduces latency and jitter for real-time traffic
• Prevents bandwidth starvation
• Improves user experience for voice and video
• Supports service-level objectives (SLOs) and SLAs

Without QoS, time-sensitive traffic can suffer during congestion.

Common QoS use cases

QoS is typically used for:

• VoIP and video conferencing
• Business-critical applications
• Remote desktop and VDI
• ERP and transactional systems
• Control and management traffic

Best-effort traffic (e.g., bulk downloads) is usually deprioritized.

How QoS works (simplified)

QoS generally involves four steps:

1. Classification - identify traffic types
2. Marking - tag packets (DSCP, CoS)
3. Queuing - place traffic into priority queues
4. Scheduling & policing - control bandwidth and order of delivery

These steps determine how packets are treated during congestion.

Key QoS metrics

QoS focuses on managing:

• Latency - delay before data arrives
• Jitter - variation in latency
• Packet loss - dropped packets
• Bandwidth - available throughput

Real-time applications are especially sensitive to latency and jitter.

QoS techniques

Common QoS techniques include:

• Priority queuing
• Weighted fair queuing
• Traffic shaping
• Traffic policing
• Rate limiting
• Bandwidth reservation

Different techniques are combined depending on network design.

QoS in WAN and MPLS

In MPLS networks, QoS is a major advantage:

• Traffic classes are defined end-to-end
• Carriers enforce QoS using SLAs
• Voice and video receive guaranteed treatment

In internet-based WANs, QoS is typically best-effort and local only.

QoS and SD-WAN

Modern SD-WAN platforms enhance QoS by:

• Being application-aware
• Dynamically steering traffic based on performance
• Applying policies across multiple links
• Combining QoS with real-time monitoring

SD-WAN often replaces static QoS with adaptive policies.

QoS limitations

QoS cannot:

• Create bandwidth where none exists
• Fix poorly designed applications
• Guarantee performance across uncontrolled networks (e.g., public internet)
• Replace proper capacity planning

QoS is a management tool, not a magic fix.

Common misconceptions

• "QoS makes networks faster"
• "QoS is only for voice traffic"
• "QoS works end-to-end on the internet"
• "QoS is unnecessary with high bandwidth"