Introduction to QoS

1. What is QoS?

To understand Quality of Service (QoS), imagine you’re driving on a busy highway. The road is full of cars, and everything is moving very slowly.

Visual representation of Quality of Service (QoS) with heavy traffic on a highway where cars move aside to prioritize an emergency vehicle.
Figure 1 – Highway Analogy Without QoS

Now, picture an ambulance appearing. Even though traffic is heavy, all the cars move aside to let the ambulance pass because its journey is urgent and must be prioritized.

Quality of Service illustrated by an ambulance prioritized through traffic, showing how urgent data is given preference in a congested network.
Figure 2 – Ambulance Prioritization Analogy

In a network, the situation is similar. Different types of traffic—such as emails, voice calls, and video streams—are sent at the same time.

Network traffic without Quality of Service showing equal treatment of emails, voice calls, and video packets, leading to delays in critical data delivery.
Figure 3 – Network Topology Without QoS

Without QoS, every data packet is treated equally, much like cars obeying a strict “first in, first out” rule. This means even critical information can end up waiting its turn.

Illustration of Quality of Service prioritizing voice traffic over regular data in a network, ensuring high-priority packets reach their destination first.
Figure 4 – Network Topology With QoS Prioritization

However, with QoS, you can prioritize important traffic. Just as the ambulance gets the right of way, QoS ensures that high-priority data (like voice packets from an IP phone) is delivered first, keeping calls clear and smooth.

2. Network Congestion

Before we explore QoS further, let’s look at the challenge it addresses: Network Congestion.

Network congestion occurs when a network device receives more traffic than it can send out because the outgoing interface’s capacity is exceeded.

Imagine three Gigabit interfaces, each running at 1 Gbps, all sending data towards a destination via interface g0/3. Since g0/3 can handle only 1 Gbps, it becomes overwhelmed.

Diagram showing network congestion on router R1 where multiple 1 Gbps interfaces overload a single 1 Gbps output link, causing packet buffering and drops.
Figure 5 – Network Congestion and Buffer Overflow

In this situation, the router (R1) will temporarily store the extra packets in a buffer (a kind of temporary memory) until it can process them. But if the congestion continues and the buffer fills up, new packets may be dropped, leading to packet loss.

Understanding congestion is essential because it highlights the need for QoS: to make sure that even when the network is busy, important data gets through without unnecessary delay.

3. QoS Metric

Now, let’s look at some key metrics used to evaluate Quality of Service in a network.

Bandwidth

Bandwidth is the maximum amount of data a network interface can transmit in a given time period, typically measured in bits per second (bps). For example, a Gigabit Ethernet interface operates at 1000 Mbps, meaning it can theoretically handle 1,000,000,000 bits per second.

Illustration of QoS bandwidth showing a Gigabit Ethernet interface capable of 1000 Mbps, representing the data capacity of a network link.
Figure 6 – QoS Metric: Bandwidth

💡 Think of bandwidth as the width of a highway, the wider it is, the more traffic (or data) it can accommodate.

Delay

Also known as latency, delay is the time it takes for a packet to travel from its source to its destination. High delay can disrupt real-time applications like VoIP or video conferencing.

Diagram illustrating QoS delay with a 16ms latency between two routers, showing packet travel time from source to destination in a network.
Figure 7 – QoS Metric: Delay (Latency)

In our example, a packet takes 16 ms (milliseconds) to reach its destination !

Jitter

Jitter refers to the variation in delay between packets arriving at their destination. Ideally, packets should arrive at consistent intervals for smooth data flow. When there’s too much variation, packets may arrive out of order, affecting real-time services like streaming video or voice.

The diagram below shows an ideal scenario with packets arriving at regular intervals.

Diagram illustrating QoS jitter with equal 1ms inter-packet delays, showing a stable and consistent packet flow between two routers without jitter.
Figure 8 – QoS Metric: No Jitter

In real-world networks, various factors can cause delays to vary, as shown in the diagram below.

QoS jitter example showing variable inter-packet delays of 1ms, 1ms, and 2ms between packets, illustrating inconsistent delivery timing that affects real-time network performance.
Figure 9 – QoS Metric: Jitter Example

Packet Loss

Packet loss occurs when packets are dropped during transmission and never reach their destination. This usually happens during network congestion when a device’s buffer is full.

For instance, if a client sends three packets to a server and one packet is lost, the service quality is affected.

QoS packet loss diagram showing a client sending 3 packets and a server receiving only 2, illustrating how dropped packets during transmission degrade service quality.
Figure 10 – Packet Loss Example

Packet loss can result from hardware issues or simply because too much data is sent through a device at once.

Next Steps: Exploring Traffic Types

In the next lesson, we will explore various traffic types including Voice, Video and Data. Understanding these traffic categories is crucial for applying QoS effectively, as it helps you decide which data needs the highest priority when congestion occurs.