What is QoS?

The problem QoS solves

Picture a road with a single lane. A big truck (a cloud backup, an update download, a video call) enters the road, and small cars get stuck behind it — the voice packets of the calls. Voice doesn't need much room: about 100 kilobits per second per call. But it needs to arrive on time, one packet every 20 milliseconds. A packet that arrives late is thrown away, and it sounds like a break in the audio.

Without QoS, the router handles all the packets in the order they arrived: first come, first served. That is fair, but not smart. QoS is the "fast lane": the router and the switch recognize the voice packets and pass them ahead of everything else, even when the pipe is full.

It is important to understand what QoS does not do. It doesn't increase internet speed, and it doesn't fix weak Wi-Fi or a damaged cable. It only decides who goes first when there is congestion. If there is no congestion, it has nothing to do. That is why it is most useful in offices with many users on one connection — exactly where jitter shows up.

How it works behind the scenes

QoS works in three stages: identify, mark and prioritize.

1. Identify — the equipment needs to know which packet is voice. You can identify by device (everything that comes out of the phones), by network (everything in the phones' VLAN), by ports, or by the addresses of the PBX servers.
2. Mark — a "sticker" is attached to the packet that says what its priority is. On the internal network the 802.1p standard is used (priority 0 to 7 inside the switch). At the internet level a field called DSCP is used, and for voice the common marking is EF (Expedited Forwarding). Most IP phones can mark their own packets.
3. Prioritize — when there is congestion, the router and the switch keep several internal "queues." The voice queue goes out first, and only when it is empty do they move on to the queues of the rest of the traffic.

There is another important tool called traffic shaping. The router limits itself to slightly less than the real speed of the connection — for example, 90 percent. That sounds odd, but it is what lets it control the queue: if it sends faster than the line can handle, the queue forms at your internet provider, and there you have no control at all.

Where QoS works, and where it doesn't

Here is a point that surprises many people: QoS works only on the segments you control.

• Inside the office — in the managed switch and in the router. Here QoS works very well.
• On the way out to the internet — the router decides what goes out first. This is the most important place, because upload is usually the bottleneck.
• On the way in from the internet — here the control is partial. The packets have already crossed the line before the router saw them. Traffic shaping in the download direction helps, but isn't perfect.
• On the internet itself — in most cases internet providers don't honor your markings. Between the office and the server, the packet is just another packet.

The practical conclusion: QoS protects you from the congestion you create — the employee uploading videos, the computer backing up at night, the cameras. That is exactly what causes most quality problems in offices. It doesn't solve congestion at the provider, and for that you choose a stable connection.

Who needs QoS and who doesn't

Not every office needs this. A business with three phones on a fiber connection with 200 Mbps upload will probably feel no difference. The general guidelines:

The clearest sign that you need QoS: calls are excellent in the morning and evening, and sound bad during the busy working hours, or the moment someone starts uploading files. This is almost always internal congestion.

QoS and VLAN: a natural combination

The simplest way to identify voice traffic is to separate it in advance. When all the phones sit in their own VLAN, the switch doesn't have to guess: everything coming from this network is voice, and gets priority. This is also why the specifications of IP phones always list the two settings together — the VLAN number and the priority level.

In a small office, a single rule in the router, based on the addresses of the PBX servers, can be enough. In a large office, combining separation and prioritization gives an arrangement that is easy to maintain and diagnose over the years.

Common mistakes

• Turning on QoS without setting the line speed. A router that thinks it has 1000 Mbps on a 100 Mbps line won't build a queue, and prioritization won't work. You need to enter the real speed, or slightly less.
• Relying on "game mode" or "automatic QoS." Many home routers offer a button like this. Sometimes it helps, and sometimes it prioritizes other traffic entirely. It is better to have a clear rule: phones first.
• Giving priority to everything. When all the traffic is "important," nothing is important. Keep high priority for voice only, and at most for video calls too.
• Forgetting the switch. A well-configured router won't help if, inside the office, the cheap switch drops packets under load.
• Thinking QoS fixes Wi-Fi. If the phone is on a weak wireless connection, no prioritization will save it. A network cable first.

A practical tip: after you set it up, test it. Start a call, and at the same time upload a large file from one of the computers. If the call stays clean — the setup works.

A worked example: an office with 20 Mbps upload at nine in the morning

Let's take a made-up law office: ten phones, eight computers, a connection with 20 Mbps upload. At nine in the morning six handsets are picked up (0.6 Mbps), and in that same minute the sync software on one of the computers starts uploading a folder of scans of 4 GB. The software does what software does: it takes all 20 Mbps.

Without QoS. The router receives packets faster than the line can send them out, and keeps them in a queue. The queue fills with packets of the scans, and a voice packet that arrives waits behind them. Instead of arriving every 20 milliseconds, the voice packets arrive after 80, after 150, after 40. The jitter buffer in the phone on the other side can't handle the jumps and throws some of them away. The result, in the numbers you see in the call statistics: jitter of 60–100 milliseconds, loss of 2–4 percent, and a customer who says "I can't hear you well." The call stays bad until the upload finishes — almost half an hour.

With QoS. The router was set to 18 Mbps (90 percent of the line), with a priority queue for packets marked EF. The sync is still running, but every voice packet that arrives bypasses the queue and goes out first. The scans get 17.4 Mbps instead of 20 — no one notices — and the voice gets its 0.6 Mbps on time. Jitter: under 10 milliseconds. Loss: zero. The customer doesn't know that 4 GB is being uploaded from the office at this moment.

What we learned: the problem was not the 20 Mbps. Even 100 Mbps would have filled up, because sync software takes whatever there is. The problem was that there was no order in the queue, and QoS is exactly that order.

How to set it up in practice: the markings, and seven steps

The markings you will meet on the setup screens repeat themselves on every piece of equipment. The table saves you searching:

1. Measure the line. A speed test over a cable, during working hours. Write down the real upload.
2. Enter 85–90 percent of it in the router as the upload speed. This is the step most people skip, and without it the queue forms at the provider and not at your end.
3. Make sure the phones mark their packets. On most IP phones, the EF marking for voice is the default; check in the settings file or in the menu.
4. The switch "trusts" the marking. A managed switch has a setting that says whether to honor the markings arriving from the devices. Turn it on at the phone ports.
5. A rule in the router: traffic with DSCP EF, or everything coming from the voice VLAN, goes to the highest priority queue. Another rule, lower, for signaling.
6. Don't touch the rest. There is no need to rank browsing, email and printers. One queue for everyone.
7. Test. An active call, and at the same time upload a large file. Look at the call statistics on the phone: if the jitter stays in single digits, you're done.

And Wi-Fi? It has prioritization too, in a standard called WMM: the wireless device gives packets marked as voice a faster turn over the air. This helps a mobile app, but doesn't fix a weak signal — and for a desk phone a cable is still better.

QoS vs. the neighboring solutions: what solves what

When calls sound bad, you are offered four different things, and sometimes all of them together. The table helps you choose by the problem, not by what the salesperson happens to know:

The most common combination in a mid-size office is VLAN + QoS: the separation identifies voice without guessing, and the prioritization passes it first. A separate line for the phones is a "heavy" solution reserved for call centers, and then it also serves as a backup for the computers' line. And a plan upgrade — only after you have checked that the problem really is quantity and not order.