Latency vs. lag: what’s the difference?

Latency is the amount of time it takes for a single packet to traverse a network.  It is normally expressed as a round-trip-time (RTT) which is the amount of time it takes a packet to go from A to Z and back again. A “ping” (from submarine sonar) is a common tool used to measure latency.

There are three primary sources of latency in a network:

  • Time-of-flight: the time is takes a packet to physically traverse the cables in the network.
  • Buffers: Temporary storage areas in routers and switches used to absorb bursts of data between links of different speeds. This is a variable number based on utilization and router/switch design but is usually measured in hundreds of milliseconds for loaded networks.
  • Play-out-time: The amount of time it takes to get a packet onto a link.  It is equal to packet size/link speed.

On a loaded network, it is buffer latency that dominates.  On an unloaded network, time-of-flight dominates.

Lag is the Quality of Experience (QoE) impact on different network issues, resulting from latency.  It is the delay experienced by a user when they press a button or drag a window and have to wait for something to happen.  Latency causes lag but so does packet loss.

When a user experiences lag, it manifests itself as: slow mouse movement, characters being delayed from appearing on the screen after typing or poor user experience with the interface. An example is when a user clicks on an icon but nothing happens so they click again and then skip two screens past where they want to go. Also, a user dragging on a window with the mouse with no immediate movement resulting.

When you fix packet loss you reduce lag.

IPQ from LiveQoS helps to reduce latency by reducing the retransmissions required.

The result to the end user will be an improved quality of experience that can be measured in the reduction of the lag that is seen.

View our latest video, which demonstrates virtual desktop lag.

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