What functions must take place for a message to travel from a source to a destination across a communication system?
Describe the communication framework as the functions performed when transmitting and receiving data, including message creation, organisation, control, addressing, transmission, synchronisation, error detection and decoding
A focused answer to the HSC Information Processes and Technology dot point on the communication framework. The functions performed from source to destination, the role of protocol levels, and the traps markers look for.
Reviewed by: AI editorial process; not yet individually human-reviewed
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What this dot point is asking
NESA wants you to describe the communication framework: the ordered set of functions that must happen for data to travel from a source to a destination and be understood. This is the detailed view of the transmitting and receiving information process, and it sits alongside the protocols and handshaking dot point by showing where each protocol level does its work.
The answer
Source, transmitter, medium, receiver, destination
A communication system is often described by five parts: the source where the message originates, the transmitter that prepares and sends it, the transmission medium that carries the signal, the receiver that takes the signal in, and the destination where the message is used. The framework describes the functions that happen across these parts.
The functions at the source
- Message creation
- The data to send is generated, for example an email typed by a user or a record produced by a program.
- Organising the message
- The data is arranged into a form suitable for sending, typically broken into packets, each a manageable unit that can be routed and checked independently.
- Control and addressing
- Information is added so the message can be delivered and managed: the destination and source addresses, sequence numbers so packets can be reassembled in order, and control data that governs the exchange.
- Conversion for transmission
- The organised, addressed data is turned into a signal the medium can carry, for example modulated onto a carrier or encoded as light pulses or radio waves.
The functions across the medium
- Transmission
- The signal travels across the medium (cable, fibre or wireless) from transmitter toward receiver.
- Synchronisation
- Sender and receiver must agree on timing so the receiver samples the signal at the right moments and knows where each unit of data begins and ends. Without synchronisation the bits cannot be read correctly.
- Error detection
- The receiver checks whether the data arrived intact using methods such as parity bits, checksums or cyclic redundancy checks, and where the protocol allows, requests retransmission of corrupted data.
The functions at the destination
Reception and decoding. The receiver takes in the signal, converts it back to data, strips the control and addressing information, reassembles packets in sequence, and decodes the message into a form the destination can use.
Presentation. The message is delivered to the destination application and ultimately displayed to the user, completing the journey.
How the framework relates to protocols
Each function in the framework is governed by a protocol, and protocols are grouped into levels, commonly application, communication control and addressing, and transmission. The framework is the what (the functions that must happen); the protocols are the agreed rules that make each function work between different devices. Handshaking, covered in the protocols dot point, is the function that establishes the connection before the bulk of the data flows.
Exam-style practice questions
Practice questions written in the style of NESA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
2019 HSC1 marksIn which level of the communication system framework does the physical transfer of data between nodes occur? A. Addressing. B. Application. C. Communication. D. Transmission.Show worked answer →
The answer is D, Transmission.
The transmission level (the lowest level of the framework) is where the actual physical movement of data signals between nodes happens - data is sent across the medium (cable, fibre or wireless) as electrical, light or radio signals.
The application level is where the user interacts and the message is created, the addressing level identifies source and destination so data is routed correctly, and "communication" is not a standalone level here. The physical transfer of bits between nodes is the transmission level, option D.
2020 HSC2 marksTwo data bytes were transmitted, with the last bit of each byte the parity bit. Why was there no error detected in the transmission? (The sent and received bytes differed but parity checking did not flag an error.)Show worked answer →
For 2 marks explain the limitation of a single parity bit.
Parity checking adds one bit so that the total number of 1s in each byte is even (even parity) or odd (odd parity). The receiver recalculates the parity and flags an error only if it does not match.
No error was detected because the corruption changed an even number of bits in the byte. When two bits flip, the count of 1s changes by an even amount, so the parity stays the same and the check still passes - the error goes undetected.
So a single parity bit can only reliably detect an odd number of bit errors; an even number of errors slips through. Markers reward identifying that an even number of bits changed, leaving parity unchanged.
2020 HSC2 marksHow could checksum be used to detect that there was a transmission error in this situation? (Two transmitted data bytes were corrupted but parity did not detect the error.)Show worked answer →
For 2 marks explain the checksum method and how it catches the error parity missed.
How it works. Before sending, the sender adds the values of the data bytes (or runs a calculation over them) to produce a checksum value, which is transmitted with the data. The receiver performs the same calculation on the received bytes and compares its result with the transmitted checksum.
Why it detects the error. Because the corruption changed the bit values, the receiver's recalculated total no longer matches the sent checksum, so a mismatch flags a transmission error - even where a single parity bit (which only counts bits per byte) missed it.
Markers reward describing the calculate-send-recalculate-compare process and that a mismatch indicates corruption.
2019 HSC2 marksA government is developing an electronic driver licence system. Outline how cyclic redundancy check (CRC) can be used in this system.Show worked answer →
For 2 marks outline what CRC does and how it helps this system.
CRC is an error-detection method. The sender treats the data as a number and divides it by a fixed divisor, then sends the remainder (the CRC value) along with the data. The receiver performs the same division on the received data; if its remainder matches the transmitted CRC the data is accepted, and if not the data is rejected and resent.
In the driver licence system, CRC checks that licence data transmitted between drivers' devices and the central system has not been corrupted during transmission, so that displayed licence details are accurate and trustworthy. Markers reward identifying CRC as error detection plus its role in verifying transmitted licence data.