The latency of a network connection represents the amount of time required for data to travel between the sender and receiver. While all computer networks possess some inherent amount of latency, the amount varies and can suddenly increase for various reasons. People perceive these unexpected time delays as lag.

The Speed of Light On a Computer Network

No network traffic can travel faster than the speed of light.

On a home or local area network, the distance between devices is so small that light speed does not matter, but for Internet connections, it becomes a factor. Under perfect conditions, light requires roughly 5 ms to travel 1,000 miles (about 1,600 kilometers). Furthermore, most long-distance Internet traffic travels over cables, which cannot carry signals as fast as light due to a principle of physics called refraction. Data over a fiber optic cable, for example, requires at least 7.5 ms to travel 1,000 miles.

Typical Internet Connection Latencies

Besides the limits of physics, additional network latency is caused when traffic is routed through Internet servers and other backbone devices. The typical latency of an Internet connection also varies depending on its type. The study Measuring Broadband America - February 2013 reported these typical Internet connection latencies for common forms of U.S. broadband service:

• fiber optic: 18 ms
• cable Internet: 26 ms
• DSL: 44 ms
• satellite Internet: 638 ms

Causes of Lag on Internet Connections

The latencies of Internet connections fluctuate small amounts from one minute to the next, but the additional lag from even small increases becomes noticeable when surfing the Web or running online applications. The following are common sources of Internet lag:
Internet traffic load: Spikes in Internet utilization during peak usage times of day often cause lag. The nature of this lag varies by service provider and a person's geographic location. Unfortunately, other than moving locations or changing Internet service, an individual user cannot avoid this kind of lag.

Online application load: Multiplayer online games, Web sites, and other client-server network applications utilize shared Internet servers. If these servers become overloaded with activity, the clients experience lag.

Weather and other wireless interference: Satellite, fixed wireless broadband and other wireless Internet connections are particularly susceptible to signal interference from rain. Wireless interference causes network data to be corrupted in transit, causing lag from re-transmission delays.

Lag switches: Some people who play online games install a device called a lag switch on their local network. A lag switch is specially designed to intercept network signals and introduce significant delays into the flow of data back to other gamers connected to a live session. A person can do little to solve this kind of lag problem other than avoiding playing with those who use lag switches; fortunately, they are relatively uncommon.

Causes of Lag on Home Networks

Sources of network lag also exist inside a home network as follows:
Overloaded router or modem: Any network router will eventually bog down if too many active clients are using it at the same time. Network contention among multiple clients means that they are sometimes waiting for each other's requests to be processed, causing lag. A person can replace their router with a more powerful model, or add another router to the network, to help alleviate this problem. Similarly, network contention occurs on a residence's modem and connection to the Internet provider if saturated with traffic: Depending on the speed of their Internet link, homeowners should avoid too many simultaneous Internet downloads and online sessions to minimize this lag.

Overloaded client device: PCs and other client devices also becomes a source of network lag if unable to process network data quickly enough. While modern computers are sufficient powerful in most situations, they can slow down significantly if too many applications are running simultaneously. Even running applications that do not generate network traffic can introduce lag; for example, a misbehaving program can consume 100% of the available CPU utilization on a device that delays the computer from processing network traffic for other applications.

Malware: A network worm hijacks a computer and its network interface, which can cause it to perform sluggishly, similar to being overloaded. Running antivirus software on network devices helps to detect these worms.

Use of wireless: Enthusiast online gamers often prefer to run their devices over wired Ethernet instead of Wi-Fi because home Ethernet supports lower latencies. While the savings is typically only a few milliseconds in practice, wired connections also avoid the risk of wireless interference that results in significant lag if it occurs.

How Much Lag Is Too Much?

The impact of lag depends on what a person is doing on the network and, to some degree, the level of network performance they have grown accustomed to. Users of satellite Internet, expect very long latencies and tend not to notice temporary lag of an additional 50 or 100 ms. Dedicated online gamers, on the other hand, strongly prefer their network connection to run with less than 50 ms of latency and will quickly notice any lag above that level. In general, online applications perform best when network latency stays below 100 ms and any additional lag will be noticeable to users.