What is Latency?
Also known as: Round Trip Time, RTT
Latency (or round-trip time, RTT) is the time required for a packet to travel from a source to a destination and back, measured in milliseconds, and is a critical metric in network performance.
Latency, often measured as round-trip time (RTT), is the total delay that a data packet experiences as it travels from a sender to a receiver and back. It includes propagation delay (time for signal to traverse the medium), transmission delay (time to push bits onto the link), processing delay at routers, and queuing delay where packets wait in buffers. In practice, latency is dominated by physical distance and the speed of light in the medium, with additional overhead from network hops.
For example, a packet traveling over a fiber-optic cable between New York and Los Angeles (roughly 4000 km) incurs a minimum propagation delay of about 20 ms due to light speed in glass (about 200,000 km/s). Real-world RTT often exceeds this due to intermediate routing, congestion, and device processing. Latency directly affects application performance: high latency degrades interactive applications like web browsing, video conferencing, and online gaming, while throughput-bound transfers are less sensitive.
Latency is distinct from bandwidth: a high-bandwidth link can still have high latency if the physical path is long or congested. In networking, low latency is achieved through proximity (e.g., edge computing), quality of service (QoS) policies, and protocols optimized for minimal delays (e.g., UDP over TCP).
Key facts
- Latency is measured in milliseconds (ms) for typical internet connections.
- It includes propagation, transmission, processing, and queuing delays.
- Lower latency is critical for real-time applications like VoIP and gaming.
- Distance and medium (fiber, copper, satellite) significantly impact latency.
- RTT is the common operational measure of latency in TCP/IP networks.
How it works in practice
Related terms
References
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