transmission time

Transmission time is the elapsed period between the moment the first bit of a message enters a communication channel and the moment the last bit exits it. For a message of L bits traveling over a link with data rate R bits per second, transmission time equals L divided by R. This is a fixed property of the link and the payload; it does not include propagation delay (the time for the signal to physically traverse the medium) or processing delays at nodes.

The distinction matters because total latency in a network includes transmission time plus propagation delay plus queuing and processing times at each hop. A 1000-bit message on a 10 Mbps Ethernet link takes 0.1 milliseconds to transmit, but the signal may take several milliseconds to propagate across the fiber, and then face additional delays waiting in a queue at the next router. Confusing these terms leads to badly tuned systems and missed deadlines in real-time applications.

Practical implications

In circuit switching, transmission time is often negligible compared to setup time; in packet switching, it directly competes with other messages for use of the channel. On high-speed links (10 Gbps and above), transmission time for small packets becomes so short that propagation delay dominates the round-trip latency, a fact critical to financial trading networks and distributed control systems where geographic distance becomes the constraint.

The term applies equally to serial protocols (RS-232, CAN bus), wireless links, and optical fiber. A 64-byte frame on a 1 Mbps serial line takes 512 milliseconds; the same frame on a 1 Gbps Ethernet takes 0.512 microseconds. This scaling directly affects buffer sizing, timeout values, and whether acknowledgment-based flow control becomes the bottleneck in the system.