4.9.1.1–4.9.1.2 - Transmission, bit rate and bandwidth

Serial and parallel transmission

Transmission moves encoded data between devices over a communication medium such as copper, optical fibre or radio.

For an eight-bit value, an eight-channel parallel connection can carry one bit on each channel during a transfer. A serial connection carries the eight bits successively. The words describe the organisation of the bits, not whether the data can travel in both directions. A serial link can have separate send and receive channels.

Parallel transmission appears faster if each channel has the same signalling rate as the serial channel. However, separate conductors do not behave identically. Over longer cables, bits sent together may arrive at slightly different times: skew. The receiver must still decide which arriving bits belong to the same group. Signals on neighbouring conductors may also interfere through crosstalk.

Serial transmission avoids the need to align a group of bits arriving on separate data channels. It needs fewer conductors and simpler, cheaper cables and connectors, and is less affected by crosstalk between parallel data lines. This makes it useful over longer distances and permits high signalling rates. It does not mean every serial connection is faster than every parallel connection: compare the actual rates and implementation.

For example, a short internal parallel connection can transfer groups efficiently, while an external connection to a distant device benefits from fewer wires and no inter-channel bit skew. The appropriate method depends on distance, interference, cost and required throughput.

Synchronous and asynchronous timing

The receiver needs to know when to sample a signal so that it can recover the intended bits.

In synchronous transmission, sender and receiver maintain synchronised timing while a block or stream is transferred. Timing may be provided by a clock signal or recovered from the encoded signal; a separate clock wire is not essential. Once synchronised, the receiver samples at agreed bit intervals. Blocks can contain framing and other control information, but there is no need to add a start and stop bit to every character.

In asynchronous transmission, the sender can start a character independently after an idle period. Sender and receiver still agree a bit rate and a character format in advance. They do not dispense with timing: each start bit establishes the timing reference for that character, and the receiver's clock samples the remaining bits at the agreed intervals.

A common illustrative character frame is one start bit, eight data bits and one stop bit. The start bit is a transition away from the idle state that tells the receiver a character is beginning. The stop bit returns to the agreed idle state, marks the character boundary and provides a framing check before another character starts. The receiver can resynchronise at the next start bit. Optional parity and extra stop bits depend on the agreed protocol; they are not universal requirements.

In the illustrative frame, ten transmitted bits carry eight data bits. At a line bit rate of $9600\,\mathrm{bit\,s^{-1}}$, continuous frames therefore carry $9600\times8/10=7680\,\mathrm{data\ bits\,s^{-1}}$. Idle gaps reduce the average useful rate further. Start and stop bits are overhead, not extra bits in the character's value.

Synchronous transmission suits sustained transfers because timing is maintained across many data units and per-character overhead is reduced. Asynchronous framing suits intermittent characters and simpler exchanges because a character can begin after a gap. Both methods require an agreed way to identify boundaries; neither guarantees error-free delivery.

Baud, bit rate, bandwidth and latency

A connection has several different measurements because signalling speed, useful information and delay describe different properties.

Baud is often described as the rate of signal changes. More precisely it is the symbol rate: consecutive identical symbols need not produce a visible transition every time. A symbol can represent one bit or several bits, depending on the signalling scheme. With four distinguishable symbols, each can encode two bits: 00, 01, 10 or 11.

A link sending $2400$ symbols per second with four choices per symbol carries $2400\times2=4800\,\mathrm{bit\,s^{-1}}$. Its bit rate exceeds its baud rate. If each symbol encodes one bit, the numerical rates are equal. Neither rate automatically equals the useful application-data rate, because transmitted bits may include headers, framing or error-control information.

For a fixed encoding and comparable channel conditions, the available bit rate is directly proportional to bandwidth. Doubling the available frequency range permits twice the bit rate under those assumptions. Greater bandwidth allows the channel to accommodate more rapid signalling. Encoding efficiency and noise also matter, so this proportionality does not mean that a value in Hz is numerically the same as a value in bit/s. In everyday broadband advertising, “bandwidth” often means a data-rate capacity; distinguish that usage from the frequency range in this specification.

Latency includes propagation time and may also include processing and queuing. Sending an entire message additionally takes transmission time: a $12\,000$-bit message on a $6000\,\mathrm{bit\,s^{-1}}$ link takes $2$ seconds just to place its bits on the link. A high-bit-rate satellite connection can transfer large files efficiently yet have noticeable propagation delay. Increasing bit rate reduces transmission time; it does not remove the time a signal takes to travel.

Protocols and shared rules

Devices can interpret each other's signals only when they agree the rules of the exchange.

Rules can specify the format of a message, the meaning of its fields, how a destination is identified, how timing or message boundaries work, how errors are detected, and what response or acknowledgement is expected. A protocol may govern one layer of an exchange rather than every aspect at once.

For example, two devices that agree 9600 bit/s but disagree about eight versus seven data bits may still misinterpret characters. Agreeing a rate alone is insufficient. Likewise, a receiver that recognises data but does not know the acknowledgement rule can leave the sender waiting unnecessarily.

Think of a request followed by a reply: the sender must format a request the receiver understands, and the receiver must send the expected kind of reply. The agreed behaviour permits independently built devices to communicate. An implementation can fail to follow those rules; having a named protocol is not a guarantee of compatibility or security in every configuration.