Communication Methods

Serial transmission sends one bit at a time over a single wire.

Parallel transmission sends multiple bits simultaneously, each on its own wire. An 8-bit parallel bus sends a full byte per cycle.

Why serial dominates over long distances:

	Serial	Parallel
Wires	1	$n$ (one per bit)
Bits per cycle	1	$n$
Skew	No skew	Skew possible (bits arrive at different times)
Distance	Long-distance reliable	Short-distance only
Cost	Cheap	More wires, more cost
Examples	USB, Ethernet, SATA	Internal memory buses, parallel printer ports (legacy)

Skew: in parallel transmission, each wire has slightly different electrical properties. Bits sent simultaneously may arrive at the receiver at different times — the further the distance, the worse the skew, causing data corruption. Serial transmission avoids this entirely.

USB uses serial transmission despite its name ("Universal Serial Bus") and is faster than the older parallel printer ports it replaced.

Both methods address the same question: how does the receiver know when each bit begins and ends?

Synchronous transmission

• Sender and receiver share a common clock signal
• Data is sent as a continuous stream timed to the clock
• No need for start/stop bits — the clock defines boundaries
• Used for: high-speed bulk transfers (e.g. USB in synchronous mode, network links)
• Requires clock synchronisation between devices

Asynchronous transmission

• No shared clock between sender and receiver
• Each data unit (typically a byte) is framed by a start bit (signals transmission is beginning) and a stop bit (signals end)
• The receiver re-synchronises its own clock to the start bit edge for each byte
• Flexible timing — sender can transmit whenever ready
• Slightly less efficient (overhead of start/stop bits)
• Used for: keyboard input, serial ports (RS-232), UART

Comparison:

	Synchronous	Asynchronous
Clock	Shared	Not shared
Framing	Clock defines bit boundaries	Start/stop bits frame each byte
Efficiency	Higher (no framing bits)	Lower (extra bits per byte)
Use	Continuous bulk data	Bursty, irregular data

Baud rate is the number of signal changes (symbols) per second. It measures how frequently the transmission medium changes state.

Bit rate is the number of bits transmitted per second.

If each symbol represents a single bit (binary signalling): bit rate = baud rate.

If each symbol represents more than one bit (e.g. using 4 voltage levels to encode 2 bits per symbol): bit rate = baud rate × bits per symbol.

Example:

• Baud rate = 1000 symbols/second
• 2 bits per symbol (4 possible signal levels)
• Bit rate = 1000 × 2 = 2000 bits/second = 2 kbps

Key rule: bit rate ≥ baud rate, with equality only when each symbol encodes exactly one bit.

Bandwidth is the range of frequencies a communication channel can carry. A wider frequency range allows more signal changes per second, supporting a higher maximum bit rate. Bandwidth is measured in Hertz (Hz) for the physical channel or in bits per second (bps) for digital throughput.

Latency is the time delay between a signal being sent and being received. Latency is distinct from bandwidth:

• High bandwidth with high latency: can send lots of data but each packet takes time to arrive
• Low latency with low bandwidth: fast response but limited throughput

Latency arises from: propagation delay (signal travel time), queuing at routers, processing delay at each hop.

Protocol is an agreed set of rules that governs communication between two or more devices. A protocol specifies:

• The format and structure of messages
• How connections are established and terminated
• Error detection and correction procedures
• Addressing conventions

Without a protocol, two devices cannot reliably communicate even if they share the same physical medium.

Example 1 — calculating bit rate from baud rate:

A channel has a baud rate of 2000 symbols/second. Each symbol encodes 3 bits. What is the bit rate?

$\text{Bit rate}=2000\times 3=6000\text{ bps}=6\text{ kbps}$

Example 2 — time to transmit a file:

A 4 MB file is sent over a 2 Mbps (megabits per second) connection. How long does transmission take?

$4\text{ MB}=4\times 8=32\text{ Mb (megabits)}$ $\text{Time}=\frac{32}{2}=16\text{ seconds}$

Example 3 — comparing two channels:

Channel	Baud rate	Bits per symbol	Bit rate
A	10 000	1	10 000 bps
B	5 000	4	20 000 bps

Channel B has a lower baud rate but a higher bit rate — it uses more efficient encoding.

Key conversions:

• 1 byte = 8 bits
• 1 kbps = 1000 bits/second (networking uses SI, not binary prefixes)
• Transmission time = file size (bits) ÷ bit rate (bps)

Note: latency is not included in transmission time calculations above. A high-latency link adds delay before and after data transfer but does not change the raw bit rate.

1. Confusing baud rate and bit rate

Baud rate counts signal changes per second; bit rate counts bits per second. They are equal only when each signal change represents exactly one bit. Modern modems use multi-level signalling so bit rate > baud rate.

2. Stating parallel transmission is faster than serial

Parallel transmission sends more bits per cycle but suffers from skew and is limited to short distances. Modern serial standards (USB 3.x, PCIe, Thunderbolt) achieve far higher bit rates than legacy parallel systems by running at much higher frequencies without skew problems.

3. Confusing synchronous and asynchronous with other meanings

In networking, synchronous/asynchronous describe clock sharing and start/stop bit usage — not the programming concept of asynchronous function calls. The networking meaning is distinct.

4. Omitting both start AND stop bits for asynchronous

Asynchronous transmission uses a start bit (signals beginning) AND a stop bit (signals end) per byte. Mentioning only one loses marks.