What Is QoS?

Quality of Service (QoS) is a collection of networking techniques used to classify, prioritize, queue, schedule, and sometimes police or shape traffic.

The goal is to provide better and more predictable network performance for applications that are sensitive to delay, variation in delay, or packet loss.

```

Traffic
|
v
+-------------------+
| Traffic Classifier|
+---------+---------+
|
v
+-------------------+
| Priority / Queue  |
+---------+---------+
|
v
+-------------------+
| Network Interface |
+---------+---------+
|
v
Network 

Why Do We Need QoS?

Imagine a company network where employees are simultaneously:

• Downloading large files
• Making VoIP calls
• Joining video conferences
• Using cloud applications
• Browsing websites

If the network becomes congested, all traffic competes for limited bandwidth. Without appropriate policies, delay-sensitive applications can experience poor performance.

Traffic Priority

QoS can classify traffic into different classes and assign different treatment to those classes.

```

High Priority
|
+-- Voice
|
+-- Critical real-time traffic
|
Medium Priority
|
+-- Video conferencing
|
+-- Business applications
|
Normal / Lower Priority
|
+-- Web browsing
|
+-- Large file transfers 

The exact policy depends on the organization. QoS does not simply mean "make everything faster"; it manages competition for network resources.

Voice Traffic and QoS

Voice traffic is highly sensitive to delay, jitter, and packet loss.

A large file download can usually tolerate additional delay. A real-time voice conversation cannot tolerate the same amount of variation without affecting the user experience.

```

Without QoS:

File Download --------+
Video ----------------+----> Congested Link
Voice ----------------+

With QoS:

Voice --------------------> Priority Queue
Video --------------------> Managed Queue
File Download ------------> Best-Effort Queue 

Video Traffic

Video conferencing and interactive video can require significant bandwidth and can be sensitive to delay, jitter, and packet loss.

QoS policies can classify video traffic and give it appropriate queueing or bandwidth treatment.

Latency

Latency is the time required for data to travel from one point to another.

High latency can make applications feel slow and can negatively affect interactive voice and video communication.

Jitter

Jitter refers to variation in packet delay.

For example, if voice packets arrive after 20 ms, 22 ms, 21 ms, 80 ms, and 25 ms, the large variation can create problems for real-time communication.

Bandwidth

Bandwidth represents the capacity available to carry data over a network link, commonly measured in bits per second.

Common values include:

• 100 Mbps
• 1 Gbps
• 10 Gbps
• 40 Gbps
• 100 Gbps

QoS cannot create additional physical bandwidth. Instead, it helps determine how available resources are managed during congestion.

QoS Techniques

QoS Classification Example

            Network Traffic
                   |
         +---------+---------+
         |         |         |
       Voice     Video     Data
         |         |         |
      Priority   High      Normal
       Queue     Queue     Queue
         |         |         |
         +---------+---------+
                   |
                Router
                   |
                 WAN
    

QoS and DSCP

DSCP (Differentiated Services Code Point) is a field used in the IP header to mark packets for differentiated handling.

Network devices can use these markings when implementing QoS policies across a network.

Practical Cisco QoS Concept

Cisco IOS platforms provide multiple QoS mechanisms. A common approach is to use Modular QoS CLI (MQC) to define classes and policies.

```

class-map match-any VOICE
match dscp ef

policy-map WAN-QOS
class VOICE
priority

interface GigabitEthernet0/0
service-policy output WAN-QOS 

The exact commands and available QoS features vary by Cisco platform and IOS/IOS XE release, so configurations should be validated against the target device.

Real-World Example

Consider a company with a 100 Mbps WAN connection.

During peak hours, employees download large files while the company's VoIP system and video meetings are active.

A QoS policy can classify real-time traffic and provide it with appropriate priority or bandwidth treatment during congestion.

Does QoS Increase Internet Speed?

No. QoS does not increase the physical capacity of a network link.

Instead, QoS controls how traffic competes for existing resources. It is most useful when congestion causes important applications to suffer.

Quick Quiz

1. What does QoS stand for?
2. Why is voice traffic sensitive to network conditions?
3. What is jitter?
4. Does QoS increase the physical bandwidth of a link?
5. What is DSCP used for?

Key Takeaways

• QoS manages traffic during network congestion.
• Voice and interactive video are sensitive to delay and jitter.
• Latency is packet delivery delay.
• Jitter is variation in packet delay.
• Bandwidth represents link capacity.
• QoS uses classification, marking, queuing, and scheduling.
• QoS improves traffic management but does not create bandwidth.