Enterprise networks handle a wild mix of traffic—video calls, file transfers, cloud apps, and more—all competing for the same resources. Quality of Service (QoS) is the mechanism that brings order to this chaos, ensuring that critical applications get the performance they need.
This blog breaks down the core concepts of QoS across both wired and wireless networks, explaining queuing, marking, policing, and shaping, and how policies drive traffic prioritization.
Why QoS Matters in Enterprise Networks
Without QoS, all traffic is treated equally—first come, first served. In peak hours, time-sensitive traffic like voice and video suffers, resulting in jitter, delay, or drops. QoS ensures that high-priority applications consistently perform well, even under congestion.
Key Concepts
1. Classification and Marking
Classification identifies traffic types (e.g., voice, video, web), while marking tags packets so network devices can treat them accordingly.
- Wired Networks: Use Layer 2 CoS (Class of Service) or Layer 3 DSCP (Differentiated Services Code Point) markings.
- Wireless Networks: Mapping occurs between DSCP and wireless QoS profiles (WMM Access Categories).
Example:
- DSCP EF (Expedited Forwarding, value 46) is often used for voice.
- DSCP AF41 is suitable for video conferencing.
2. Queuing
When traffic exceeds interface capacity, packets are placed in queues. Queuing mechanisms determine which packets are sent first.
- FIFO (First-In, First-Out): No prioritization.
- CBWFQ (Class-Based Weighted Fair Queuing): Allocates bandwidth per traffic class.
- LLQ (Low Latency Queuing): Adds a strict-priority queue for delay-sensitive traffic like VoIP.
3. Policing and Shaping
Policing drops or re-marks excess traffic instantly; shaping buffers and sends it at a regulated rate.
- Policing: Common on inbound traffic, ensuring users/applications don’t exceed allowed rates.
- Shaping: Used outbound to smooth bursty traffic, often paired with queuing.
4. Trust Boundaries
Define where markings are accepted or rewritten. For example, in a wireless deployment, trust is usually given to the AP if it’s known to enforce QoS settings accurately.
QoS in Wireless Networks
Wireless adds extra complexity due to shared medium and variable transmission conditions.
- WMM (Wi-Fi Multimedia) defines 4 Access Categories:
- Voice (AC_VO)
- Video (AC_VI)
- Best Effort (AC_BE)
- Background (AC_BK)
APs translate DSCP into WMM categories, ensuring consistent QoS treatment end-to-end.
Important: Congestion can occur on both the wired uplink and the wireless RF. QoS must be applied at both points.
Policy-Driven QoS
QoS policies use class maps, policy maps, and service policies in Cisco IOS.
- Class Map: Matches traffic types.
- Policy Map: Assigns QoS actions like bandwidth or marking.
- Service Policy: Applies the policy to an interface.
Config Insight: Simple LLQ for Voice Traffic
class-map match-any VOICE
match ip dscp ef
policy-map QOS_POLICY
class VOICE
priority 1000
class class-default
fair-queue
interface GigabitEthernet0/1
service-policy output QOS_POLICY