How to Set Up Quality of Service (QoS) for VoIP

Voice over IP has transformed business communications across the United Kingdom. From small accountancy practices in Bristol to large financial services firms in the City of London, organisations of every size have moved away from traditional analogue phone lines and embraced internet-based voice communications. The benefits are substantial — lower costs, greater flexibility, advanced features, and seamless integration with other business applications.

However, VoIP comes with a fundamental challenge that traditional phone lines never had: it shares your internet connection with everything else. When a staff member downloads a large file, streams a training video, or backs up data to the cloud, that activity competes with voice traffic for bandwidth. Without proper management, the result is poor call quality — choppy audio, delays, echoes, and dropped calls that frustrate both your team and your customers.

Quality of Service, or QoS, is the solution. QoS is a set of network technologies and configurations that prioritise certain types of traffic over others. When properly configured, QoS ensures that voice traffic always gets the bandwidth and low latency it needs, regardless of what else is happening on your network. This guide explains how QoS works, why it matters for VoIP, and how to set it up correctly for your UK business.

Understanding Why VoIP Needs Special Treatment

To understand why QoS matters, you need to understand how VoIP differs from other types of network traffic. When you browse a website, your browser requests data from a server. If a packet of data is delayed or arrives out of order, the browser simply waits for it to arrive and reassembles the page. You might notice a slight delay in loading, but the end result is a complete, accurate web page.

Voice traffic cannot tolerate this kind of delay. A phone conversation happens in real time, and even small delays — as little as 200 milliseconds — are noticeable and disruptive. If packets arrive out of order or are lost entirely, the result is not a slightly delayed conversation but a broken one, with missing words, robotic sounds, and awkward pauses that make business communication impossible.

Three key metrics define VoIP call quality: latency (the time it takes for a voice packet to travel from sender to receiver), jitter (the variation in latency between packets), and packet loss (the percentage of voice packets that never arrive). QoS addresses all three by giving voice traffic preferential treatment on your network.

How QoS Works: The Technical Fundamentals

QoS works by classifying network traffic into different categories and then applying rules that determine how each category is treated. Think of it as creating a fast lane on a motorway — certain vehicles (in this case, voice packets) get priority access, while other traffic uses the remaining lanes.

Traffic Classification and Marking

The first step in QoS is identifying which traffic is voice traffic and which is not. This is done through a process called classification and marking. Voice packets are tagged with a special marker — typically a Differentiated Services Code Point (DSCP) value — that tells every network device along the path to treat this traffic with priority. The standard DSCP value for voice traffic is EF (Expedited Forwarding), which has a decimal value of 46. Call signalling traffic, which sets up and tears down calls, is typically marked with CS3 (decimal 24).

Queuing and Scheduling

Once traffic is classified and marked, network devices use queuing algorithms to determine the order in which packets are processed. The most common approach for VoIP is to create a strict priority queue for voice traffic and a weighted fair queue for everything else. This means voice packets are always processed first, and other traffic shares the remaining bandwidth fairly. The key is to limit the priority queue to a reasonable percentage of the total bandwidth — typically 20 to 30 percent — to prevent voice traffic from monopolising the entire connection.

Traffic Shaping and Policing

Traffic shaping smooths out bursty traffic to prevent congestion, while policing drops or re-marks packets that exceed defined limits. Together, these mechanisms ensure that no single type of traffic can overwhelm the network. For VoIP, this means that even during periods of heavy data usage, voice traffic is protected and consistently delivered within acceptable quality parameters.

Step-by-Step QoS Configuration for VoIP

Setting up QoS for VoIP involves configuring multiple devices on your network. The exact commands and settings vary depending on your equipment, but the principles are consistent across all platforms.

Step 1: Assess Your Bandwidth

Before configuring anything, you need to understand your bandwidth. Determine your internet connection speed (both download and upload), and calculate how much bandwidth your VoIP system requires. Each concurrent call using the G.711 codec requires approximately 100 Kbps in each direction. If you have 20 staff members and typically see 10 simultaneous calls, you need at least 1 Mbps of dedicated upload and download bandwidth for voice alone. Many UK businesses on standard fibre connections have asymmetric speeds — for example, 80 Mbps download but only 20 Mbps upload. It is the upload speed that typically creates the bottleneck for VoIP, so pay particular attention to this figure.

Step 2: Configure Your Router or Firewall

Your edge router or firewall is the most important point for QoS configuration because it controls the connection between your internal network and the internet. Configure it to classify voice traffic by identifying SIP signalling packets (typically on ports 5060 and 5061) and RTP media packets (typically on a defined port range such as 10000 to 20000). Mark these packets with the appropriate DSCP values and apply a strict priority queue for the voice traffic.

Step 3: Configure Your Switches

If you use managed switches — which you should in any business environment — configure them to trust and preserve DSCP markings. Many switches default to stripping DSCP markings, which means your carefully configured QoS policies at the router are ignored on the internal network. Enable QoS on each switch port that connects to a VoIP phone or a network segment carrying voice traffic.

Step 4: Configure Your Wireless Access Points

If any of your VoIP devices connect wirelessly — which is increasingly common with softphones on laptops and mobile devices — your wireless access points must also support QoS. Wi-Fi uses a standard called WMM (Wi-Fi Multimedia) that provides traffic prioritisation over the air. Ensure WMM is enabled and that voice traffic is mapped to the highest priority access category. Most enterprise wireless systems, including Cisco Meraki and Ubiquiti UniFi, support this out of the box but may need specific configuration to work optimally with your VoIP system.

VoIP VLANs: Separating Voice from Data

One of the most effective techniques for improving VoIP quality is to separate voice traffic from data traffic using Virtual LANs (VLANs). By placing all VoIP devices on a dedicated VLAN, you create a logical separation that makes it easier to apply QoS policies and prevents data traffic from interfering with voice communications.

Most enterprise VoIP phones support VLAN tagging, which means a single network cable can carry both voice and data traffic to a desk — the phone connects to the voice VLAN, and a computer connected to the phone's pass-through port connects to the data VLAN. This is the standard configuration in UK offices and is supported by all major VoIP phone manufacturers.

Testing and Monitoring Your QoS Configuration

Configuring QoS is not a set-and-forget exercise. You need to test your configuration and monitor it ongoing to ensure it continues to deliver the expected results. Start by conducting test calls under various load conditions — make calls while simultaneously running large file transfers, cloud backups, and video streams. Listen for any degradation in voice quality and check the call quality metrics provided by your VoIP system.

Most modern VoIP platforms provide a Mean Opinion Score (MOS) for each call, which rates quality on a scale of 1 to 5. A MOS of 4.0 or above indicates good quality, while anything below 3.5 suggests a problem that needs investigation. Monitor these scores over time and investigate any drops, as they may indicate that your QoS configuration needs adjustment or that your bandwidth has become insufficient for your current usage patterns.

Network monitoring tools such as PRTG, SolarWinds, or even the built-in monitoring capabilities of platforms like Cisco Meraki can provide real-time visibility into your QoS performance. Set up alerts for key metrics — latency above 100ms, jitter above 20ms, or packet loss above 0.5% — so you can address issues before they affect call quality.

Common QoS Mistakes and How to Avoid Them

Even experienced network administrators sometimes make mistakes with QoS. Here are the most common pitfalls and how to avoid them.

The first is over-prioritising. If you allocate too much bandwidth to the priority queue, you starve other traffic and create a different kind of problem. Never allocate more than 33% of your total bandwidth to the strict priority queue. The second mistake is forgetting about upload bandwidth. Most UK broadband connections have much less upload capacity than download, and VoIP requires bandwidth in both directions. Always configure QoS based on your upload speed, not your download speed.

The third common mistake is not configuring QoS end-to-end. If your router has QoS but your switches do not trust DSCP markings, the QoS is only effective for the last hop to the internet. Every device in the path needs to participate. Finally, many organisations configure QoS once and never revisit it. As your organisation grows, adds more users, and adopts new applications, your QoS configuration needs to evolve with it. Schedule a quarterly review of your QoS settings and voice quality metrics to ensure they remain optimal.