Data communication sends information as a sequence of distinct states, usually on and off, rather than as a continuous waveform, so a receiver only has to decide which state it saw and a relay can re-form a damaged pulse into a clean one. The price is overhead: framing each 8-bit byte with a start and a stop bit gives 80% efficiency, and at 9,600 baud that is about 960 bytes per second. Errors are caught by adding check bits.
Part of Telecommunications
Sending messages as discrete symbols over telegraph and telephone lines: why digital signalling resists noise, how bytes are framed, and what error checking costs.
A Receiver That Chooses Between Two States Tolerates Noise That Would Ruin A Waveform
A voice on a wire is a continuous waveform; every distortion and every bit of noise stays in it, and an amplifier increases both (Amplification). A telegraph key sends only current or no current. If the pulse reaching a relay is weak and rounded, the relay still closes a fresh local circuit and sends a clean pulse on, which is exactly what Repeater Stations describes. The decision at each hop is only: was there a pulse or not?
That is the strength of digital communication: as long as the receiver can still tell the states apart, noise does not accumulate along the route. Morse code was the first widely used digital code, written as dots, dashes and spaces whose length is a ratio of the dot (Morse Code System). Later machine codes use a fixed number of bits per character, as in Teleprinter Systems.
Framing Marks Where Each Character Starts When There Is No Shared Clock
If sender and receiver have no common clock, each character needs markers. Framing gives start-stop framing: 2 overhead bits for every 8 data bits, so efficiency is 8 / 10 = 80%. Serial Communication works out the speed at 9,600 baud: one 8N1 byte is 10 bit periods.
- Bit period at 9,600 baud: 1 / 9,600 = 104 µs
- One byte: 10 × 104 µs = 1.04 ms
- Throughput: 1 / 1.04 ms = about 960 bytes per second, capped whatever the line quality
At 300 baud, a speed the networking articles also mention, the same arithmetic gives 300 / 10 = 30 bytes per second, so a 1,000-byte message takes about 33 s. Speed trades against distance: Networking puts a basic serial link at about 100 m at 9,600 baud and past 500 m at 300 baud.
Check Bits Catch What The Line Corrupts
Noise still flips bits sometimes. The corpus lists the standard tools in order of cost:
- Parity bit. One extra bit per byte, 1 / 8 = 12.5% overhead, catches any single-bit error (Parity Bits).
- Checksum. 4 to 32 extra bytes per block, catches accidental corruption but not tampering (Checksums).
- CRC-32. Catches every single-bit error and every burst under 32 bits (Error Detection).
- Error-correcting codes repair rather than just detect, at higher overhead (Error Detection and Correction).
Line noise from a poor contact or a nearby storm lasts many bit periods, so errors tend to arrive in bursts. That is why a single parity bit alone is weak on such lines and why a CRC is the better check. Detection then needs an answer: Acknowledgments covers the receiver telling the sender whether a block arrived.
Digital Does Not Remove The Line’s Limits
A telegraph pulse is a DC on-off signal. A telephone connection runs through transformers and coupling capacitors, which block DC (the ringer capacitor in the carrier article is there for exactly that), so only the 300 to 3,400 Hz voice band can be relied on. Sending bits over a telephone connection therefore means converting them to tones, which is the work of Modem Concepts.
What You Need
- A code, either Morse or a fixed-length code
- A framing rule and an agreed bit rate at both ends
- A check method matched to how noisy the line is
- An acknowledgment rule so lost blocks are resent
- A means to key or read the line: a relay and key for telegraph, a serial interface for machines
Common Mistakes
- Ignoring framing overhead. 960 bytes per second is the ceiling at 9,600 baud, not the plan.
- Using one parity bit on a burst-noise line. Use a CRC.
- Choosing a speed the line cannot hold. Cut the bit rate for a long run.
- Counting on digital to fix a wrecked signal. It tolerates noise only until the two states can no longer be told apart.
What’s Next
- Learn next: Teleprinter Systems, the first machines that sent text as bits
- Combine with: Modem Concepts, putting bits through a voice channel