bit interval

The bit interval is the duration of time during which a single bit occupies the transmission line or medium. In serial communication, this is the fundamental unit of timing, measured in seconds. A modem transmitting at 9600 baud sends one bit every 104 microseconds (1 / 9600 hertz). The bit interval determines the maximum rate at which information can flow and sets the tempo for all synchronisation between sender and receiver.

The bit interval depends entirely on the agreed transmission rate, called the baud rate or bit rate. These terms are technically distinct: baud measures signal changes per second, while bits per second (bps) counts information bits. In many systems, especially older serial protocols, one baud equals one bit, so a 115200 bps connection has a bit interval of approximately 8.7 microseconds. However, the terms are often used interchangeably in practice, and confusion arises when modulation schemes pack multiple bits into each signal change.

Timing and synchronisation

The bit interval is the foundation of clock recovery in digital receivers. When data arrives asynchronously, the receiver must sample the incoming signal at precisely the midpoint of each bit interval to read its value correctly. Sampling too early or late introduces errors. Phase-locked loops and timing circuits in the receiver measure incoming transitions and adjust their sampling clock to align with the transmitter's rhythm. Even a small drift accumulates quickly: a receiver clock that runs 1 percent fast will be misaligned after receiving fewer than 100 bits.

In synchronous systems, the transmitter sends a separate clock signal alongside the data, one pulse per bit interval, eliminating the need for clock recovery. Asynchronous systems (like standard RS-232 serial ports) rely on agreed bit rates and start/stop bits to resynchronise at the beginning of each character. The receiver's local clock must be stable and accurate enough to sample correctly across multiple bit intervals without drifting beyond acceptable limits, typically within 5 percent of the nominal rate.

When transmission fails or produces errors, checking the bit interval is a first step. Baud rate mismatches are common when devices are configured differently: a transmitter sending at 9600 bps connected to a receiver listening at 19200 bps will see garbage because the receiver's bit interval is half as long. Temperature-induced frequency drift in oscillators can also cause slow slippage. In noisy channels, intersymbol interference may cause the rising or falling edge of one bit to spill into the next bit interval, especially at high rates, degrading the signal-to-noise margin.