4.4 QoS Fundamentals
Key Takeaways
- QoS manages bandwidth, delay, jitter, and packet loss to prioritize critical traffic.
- DSCP (Differentiated Services Code Point) uses 6 bits for 64 possible values in the IP header.
- Voice traffic requires <150ms one-way delay, <30ms jitter, and <1% packet loss.
- Per-Hop Behavior (PHB) defines how a router treats packets based on their DSCP marking.
- The three PHB categories: EF (Expedited Forwarding for voice), AF (Assured Forwarding for data), BE (Best Effort default).
QoS Fundamentals
Quality of Service (QoS) is a set of technologies that manage network traffic to ensure critical applications receive the bandwidth, low latency, and reliability they need.
Why QoS Matters
Without QoS, all traffic is treated equally (best-effort). When congestion occurs, packets are dropped indiscriminately. This causes problems for time-sensitive applications like voice and video.
QoS Parameters
| Parameter | Definition | Impact |
|---|---|---|
| Bandwidth | Available capacity on a link | Determines throughput |
| Delay (latency) | Time for a packet to travel from source to destination | Critical for voice/video |
| Jitter | Variation in delay between packets | Causes choppy voice/video |
| Packet loss | Percentage of packets that don't arrive | Causes gaps in audio/video |
Traffic Requirements
| Traffic Type | Bandwidth | Delay | Jitter | Loss |
|---|---|---|---|---|
| Voice (VoIP) | Low (30-100 Kbps per call) | < 150ms one-way | < 30ms | < 1% |
| Video conferencing | High (384 Kbps - 20 Mbps) | < 150ms | < 30ms | < 1% |
| Streaming video | High | < 4-5 seconds | Moderate | < 2% |
| Web browsing | Variable | < 2-4 seconds | N/A | Low |
| File transfer | High | Tolerant | N/A | 0% |
| Low | Tolerant | N/A | 0% |
QoS Mechanisms
Classification and Marking
Traffic is classified (identified) and marked with a QoS value that follows the packet through the network.
Layer 2 marking: CoS (Class of Service)
- 3-bit field in the 802.1Q tag (values 0-7)
- Only works on trunk links (not on access ports)
Layer 3 marking: DSCP (Differentiated Services Code Point)
- 6-bit field in the IP header (values 0-63)
- Works end-to-end across Layer 3 networks
- Preferred marking method for the CCNA
DSCP Values and Per-Hop Behaviors
| PHB | DSCP Value | Decimal | Use Case |
|---|---|---|---|
| EF (Expedited Forwarding) | 101110 | 46 | Voice (low latency, low jitter) |
| AF41 | 100010 | 34 | Video conferencing |
| AF31 | 011010 | 26 | Mission-critical data |
| AF21 | 010010 | 18 | Transactional data |
| AF11 | 001010 | 10 | Bulk data |
| CS0/BE (Best Effort) | 000000 | 0 | Default (no priority) |
Assured Forwarding (AF) Classes
AF uses two components: Class (1-4, higher = more important) and Drop precedence (1-3, higher = more likely to be dropped during congestion).
| Low Drop (1) | Medium Drop (2) | High Drop (3) | |
|---|---|---|---|
| Class 4 | AF41 (34) | AF42 (36) | AF43 (38) |
| Class 3 | AF31 (26) | AF32 (28) | AF33 (30) |
| Class 2 | AF21 (18) | AF22 (20) | AF23 (22) |
| Class 1 | AF11 (10) | AF12 (12) | AF13 (14) |
Queuing
After marking, routers use queuing to prioritize traffic:
- LLQ (Low Latency Queuing): Strict priority queue for voice (EF traffic)
- CBWFQ (Class-Based Weighted Fair Queuing): Bandwidth guarantees for different classes
- WFQ (Weighted Fair Queuing): Proportional bandwidth sharing
Trust Boundaries
The trust boundary is the point in the network where QoS markings are trusted:
- Traffic from IP phones is typically trusted at the access switch
- Traffic from PCs is not trusted (could be manipulated)
- The access layer switch is typically the trust boundary
On the Exam: The CCNA focuses on QoS concepts, not detailed configuration. Know DSCP marking, the PHB categories (EF, AF, BE), voice traffic requirements, and the trust boundary concept. Be able to identify which DSCP value is appropriate for voice traffic (EF/46).
Which DSCP Per-Hop Behavior is used for voice traffic?
What is the maximum acceptable one-way delay for voice traffic (VoIP)?