Durchsatz

The Gap Between Capacity and Reality

BandwidthThe maximum data transfer rate of a network link, typically measured in bits per second (Mbps, Gbps). Bandwidth represents capacity, not actual speed; real-world transfer rates depend on latency, congestion, and protocol overhead. defines the maximum theoretical data rate a link can carry. Throughput is the actual rate at which useful data moves from sender to receiver over a measured interval. The gap between the two is caused by protocol overhead, retransmissions due to Packet LossThe percentage of data packets that fail to reach their destination, typically caused by network congestion, faulty hardware, or wireless interference. Even 1-2% packet loss can noticeably degrade voice and video quality., LatencyThe time delay for a data packet to travel from source to destination, typically measured in milliseconds (ms). Lower latency is critical for real-time applications like video calls, gaming, and financial trading.-induced TCP window limitations, and application-level bottlenecks. On a high-latency satellite link, a 100 Mbps BandwidthThe maximum data transfer rate of a network link, typically measured in bits per second (Mbps, Gbps). Bandwidth represents capacity, not actual speed; real-world transfer rates depend on latency, congestion, and protocol overhead. connection may achieve only 10 Mbps of TCP throughput if the congestion window fills before ACKs return — the bandwidth-delay product problem.

TCP Window and Congestion Control

TCP flow control uses a receive window to limit how much unacknowledged data can be in flight. The maximum throughput achievable is window_size divided by RTT. A 64 KB window over a 200 ms RTT satellite path yields only 2.56 Mbps regardless of raw bandwidth. Modern TCP stacks use window scaling to negotiate windows up to 1 GB, and congestion algorithms like BBR probe for available bandwidth without relying on loss as a signal.

Measuring Throughput Accurately

Internet Speed Test performs a multi-stream download and upload test designed to saturate the connection and report sustained throughput, not peak burst rate. For more controlled measurement between two specific hosts, iperf3 sends TCPTransmission Control Protocol. A reliable, connection-oriented transport protocol that guarantees ordered, error-checked delivery of data through three-way handshakes, acknowledgments, and retransmission. The foundation of HTTP, SSH, and most internet services. or UDPUser Datagram Protocol. A connectionless transport protocol that sends datagrams without establishing a connection or guaranteeing delivery. Faster than TCP, it is preferred for real-time applications like DNS queries, VoIP, gaming, and streaming. streams and reports achieved throughput, jitter, and Packet LossThe percentage of data packets that fail to reach their destination, typically caused by network congestion, faulty hardware, or wireless interference. Even 1-2% packet loss can noticeably degrade voice and video quality. simultaneously. Comparing speed test results to ISPInternet Service Provider. A company that provides internet access to consumers and businesses, assigning public IP addresses and routing traffic to the wider internet. Examples include Comcast, AT&T, and SK Broadband.-advertised BandwidthThe maximum data transfer rate of a network link, typically measured in bits per second (Mbps, Gbps). Bandwidth represents capacity, not actual speed; real-world transfer rates depend on latency, congestion, and protocol overhead. at different times of day reveals whether congestion or throttling is reducing throughput during peak periods.