Packet loss happens when data packets sent across a network fail to reach their destination. Data travels in small units called packets, and a lost packet may be dropped by a congested router, corrupted by a faulty cable or interference, or discarded because it arrived too late to be used. It is usually expressed as a percentage of packets sent. Small amounts can be enough to make voice calls break up or slow down applications.
At a glance
- Packet loss is the share of packets that never arrive, or arrive too late or damaged to be used.
- Real-time traffic suffers most: lost voice and video packets show up as gaps, clipped words or frozen frames.
- File transfers and web traffic resend lost packets, so loss shows up as slowness rather than missing data.
- Congestion, bad cabling, Wi-Fi interference and faulty equipment are common causes.
- Loss is often intermittent, so measurement over time and at busy hours matters more than a single test.
What problem it solves
Packet loss is a symptom to diagnose and reduce. It explains many complaints that do not look like network problems at first: a voice that cuts out mid-sentence, a cloud application that is slow although the link is not full, a VPN that disconnects. Understanding it helps a buyer separate capacity problems, which more bandwidth fixes, from quality problems, which it often does not.
It also gives buyers a concrete metric to hold providers to. Business connectivity services often include a loss target for the provider’s own network, and SD-WAN and monitoring tools can show where and when loss occurs.
How it works
Every packet passes through a series of links and devices. Loss happens when one of them cannot or does not pass the packet on.
Congestion. When more traffic arrives at a link than it can carry, packets queue in a buffer. When the buffer fills, new packets are dropped. This is the most common cause and tends to follow busy periods.
Physical problems. Damaged cables, dirty fiber connectors, failing optics and electrical interference corrupt packets, which are then discarded.
Wireless conditions. Wi-Fi and cellular links lose packets to interference, weak signal and contention; radio protocols retry, but some loss and delay remain.
Equipment and configuration. Overloaded firewalls or routers, mismatched port settings and policy limits can all drop traffic.
Late arrival. For real-time traffic, a packet that arrives after the receiving app’s jitter buffer has moved on is useless, so high jitter shows up as effective loss.
How applications react. Protocols such as TCP detect loss and resend the data, slowing down as they do, so loss combined with high latency can cut throughput well below the link’s capacity. Voice and video generally do not wait for resends; they conceal small gaps and otherwise play through them.
Reducing it. Fixing physical faults, relieving congested links, applying Quality of Service (QoS) on links you control and using SD-WAN to steer traffic away from a lossy path all help. Some SD-WAN products can also duplicate or add error correction to critical traffic, depending on the vendor. Our SD-WAN page covers how multi-link designs handle degraded circuits.
When it matters for buyers
- Rolling out voice, video or contact center. These are the first applications to show loss.
- Persistent “slow network” complaints. If the link is not near capacity, look for loss before upgrading bandwidth.
- Evaluating a provider or SLA. Dedicated internet access and private network services often state a loss target; best-effort broadband usually does not.
- Choosing SD-WAN. Ask how the product measures and reacts to loss on each link.
- After an equipment or cabling change. New loss after work on site often points to a physical or configuration fault.
Questions to ask vendors
- Does your SLA include a packet loss target? Between which points is it measured, and over what period?
- Can we see loss, latency and jitter for each circuit or site over time?
- How quickly does your equipment detect loss on a link and move traffic, and what threshold triggers that?
- Do you offer packet duplication or error correction for voice and video, and what does it cost in bandwidth?
- When we report loss, how do you determine whether it is in our network, your access circuit or beyond your network?
How it differs from jitter
Packet loss means packets never arrive, or arrive unusable; jitter means they arrive with uneven timing. The two are closely related in real-time traffic, because packets delayed past the receiver’s buffer are thrown away and count as lost. Both are often caused by congestion, but loss can also come from physical faults that create no jitter at all. Monitoring tools usually report them together, with latency, so you can see which one is behind a quality problem.