What Is Packet Loss, and Why Does the Sound Cut Out?

What is a "packet"

Everything that travels over the internet is broken into small pieces called packets. In a call over the network (VoIP), the voice is cut into pieces of usually 20 milliseconds each. That means every second of speech is sent as about fifty small packets, in each direction.

Each packet sets out on its own, passes through several routers and reaches the other side, where the phone stitches them back into a stream of sound. When everything is working, the packets arrive on time and in the right order, and the ear never notices that the voice was taken apart.

Packet loss is a situation where some of the packets simply don't arrive. They are dropped along the way — because of congestion, interference, a damaged cable or equipment that can't keep up.

Why it is so noticeable in a call

When you download a file or open a website, a lost packet is simply sent again. It takes another moment, but in the end the file arrives complete. That is why packet loss is almost never felt during ordinary browsing.

In a call there is no time to send again. A voice packet that arrives half a second late is already useless — the conversation has moved on. So the phone gives up on it, and in its place you hear silence, a metallic sound or a "jump" in the speech.

This explains a familiar situation: "The internet works great, websites open, but calls keep cutting out." Browsing hides the problem, and the call exposes it.

How much packet loss is a lot

No network is perfect, and a lost packet here and there is normal. The question is how many, and above all — whether the loss is spread out or comes in bursts. A commonly accepted rule of thumb for calls:

Bursty loss is worse than spread-out loss at the same percentage. If ten packets in a row are lost, that is a gap of a fifth of a second — enough for a whole word to disappear. Phones know how to "fill in" a single missing packet, but not a long run.

How to test for packet loss

The simple way is a continuous ping. In Windows, type ping -t followed by an address on the command line, let it run for a few minutes and stop it with Ctrl+C. The summary shows a Lost line with the number of lost packets and their percentage.

To find out where the loss happens, test in two steps: ping the office router, and ping a server on the internet. If there is loss already toward the router, the problem is inside the office — Wi-Fi, a cable, a switch. If toward the router there is none and toward the internet there is loss, the problem is in the line or with the internet provider.

There are also tools that combine ping and tracert and show the loss percentage at each hop along the way (for example pathping in Windows). Note: loss at a single hop in the middle that doesn't continue to the following hops is usually not a real problem — that router is simply prioritizing other traffic over test packets.

One more tip: write down the results. The time, from which computer, toward which address, and what the percentage was. After three or four days a pattern almost always shows up — a fixed hour, a certain room or a certain device — and that pattern leads to the cause much faster than any guess. An internet provider also treats a complaint differently when it comes with an organized table.

Common causes

• A congested line — when the upload or download is completely full, the router has to drop packets. Cloud backups, streaming cameras or a heavy download are the usual suspects.
• Wi-Fi — interference, distance, walls and a load of devices. A wireless network is the most common source of loss inside the office.
• A damaged cable or a loose connector — a network cable crushed under a chair or a broken connector can cause intermittent loss, which is very hard to track down.
• Weak equipment — a home-grade router or switch that can't handle the number of devices.
• A problem at the internet provider — a noisy line, faulty equipment in the neighborhood, or congestion on the provider's network at peak hours.

Among the causes there is also one that is hard to guess: a double connection. When a network cable is plugged into the same switch at both ends, or two switches are connected to each other with two cables, a "loop" forms in the network. The packets circle around in it again and again, the whole network slows down, and calls cut out throughout the building. Managed switches know how to detect this situation and block it.

Packet loss, jitter and latency — three siblings

Three measurements determine the quality of a call over the network, and they are related to one another:

In practice they show up together. High jitter causes loss, because a packet that arrives too late is dropped by the phone as if it never arrived. And a congested line raises all three at once.

What the phone does to hide loss

Modern phones and PBXs don't just suffer from loss — they also try to hide it. They have a jitter buffer: the phone collects a few packets before playing them, so a packet that is slightly late still makes it into place.

When a packet is missing entirely, the phone guesses what was in it based on what came before and after it, and "smears" the sound over the gap (Packet Loss Concealment). With a single packet this works well, and the ear doesn't notice.

But all of this has a limit. A jitter buffer that is too large adds delay, and guessing a long run of packets sounds artificial. That is why packet loss can't be solved only in the phone settings — it has to be treated at the source.

A field example: the yeshiva with the Wi-Fi

In a small yeshiva (a made-up name: "Tiferes Shmuel"), wireless phones were installed that connected to the building's Wi-Fi network. In the secretary's office the calls were fine, but in the rebbeim's room at the end of the hallway the voice cut out every few seconds.

A continuous ping from the far room showed 4% lost packets toward the router itself — meaning the problem was inside the building. Two stone walls and a beverage refrigerator along the way weakened the signal. The solution was to run a network cable to the room and connect the phone by cable. The loss dropped to zero.

How to handle it

1. Find where the loss is: ping the router and ping the internet, from a computer connected by cable.
2. If the problem is inside — replace Wi-Fi with a cable, replace suspect cables, check the switch.
3. If the line is full — turn on QoS in the router, move backups to the night, separate the cameras.
4. If the problem is in the line itself — contact the internet provider with the test results in hand: time, percentage and hop.
5. In an office where calls are critical — consider a backup line.

Loss in one direction only

A common and confusing situation: I hear the customer perfectly, and he hears me choppy. The reason is that the voice flows in two separate directions, and each direction takes its own path. My voice goes out through the office's upload, and the customer's voice comes in through the download.

In most internet plans the upload is much smaller than the download, so it chokes first. A cloud backup, a camera that streams, or someone sending a large file by email fills the upload, and it is my voice packets that get dropped.

That is why, when checking a complaint, it is important to ask who hears badly — us or the other side. The answer points right away in the right direction.

Packet loss at home and on the road

People who work from home and representatives who talk from a softphone on a laptop are more exposed to packet loss: a home network with children watching videos, Wi-Fi shared across the whole house, and sometimes a connection through a mobile phone.

On a cellular connection, the loss depends heavily on location and time. Inside a building, in an elevator or while traveling, it can rise sharply within seconds. Someone who works from home regularly will gain a lot from one simple network cable from the router to the desk.

For a kosher phone or a phone without internet there is a different solution: a SIM extension, where the call goes through the regular cellular network and not as packets over the internet.

What doesn't help

• Increasing the download plan — if the problem is in the Wi-Fi or the cable, a faster line won't change anything.
• Restarting again and again — restarting the router clears congestion for a moment, but the problem will come back.
• A cheap Wi-Fi booster — a booster (repeater) doubles the wireless path and sometimes increases the loss. An access point connected by cable is better.
• One short test — intermittent loss won't show up in a ten-second test. Test over time and during busy hours.