Part A: Signals and systems
Introduction to network and telecommunication systems
Key takeaways
- Every telecommunication system reduces to five blocks: source, transmitter, transmission medium or channel, receiver and destination.
- Networks are classified by scale: a PAN covers one person's own devices, a LAN a building or campus, a MAN a city and a WAN a country, a continent or the globe.
- Simplex sends one way only, half-duplex both ways but one direction at a time, and full-duplex both ways at once.
- Bandwidth is a medium's theoretical capacity; throughput is the rate actually achieved, always less than or equal to the bandwidth once overhead and contention are counted.
- Noise (thermal, intermodulation, crosstalk and impulse) degrades the receiver's ability to recover the signal, seen as a higher bit error rate in a digital system.
This opening chapter fixes the vocabulary the rest of the paper assumes: what a network and a telecommunication system actually are, how a message travels from sender to receiver in the abstract, and the impairments (noise) that any real transmission must contend with.
Overview of networks and communication system
Telecommunication model
Every telecommunication system, however complex, reduces to the same five functional blocks, the model this whole paper's technical chapters elaborate one block at a time.
- Source: originates the message (a person speaking, a sensor reading, a file)
- Transmitter: converts the message into a signal suitable for the transmission medium, the job of the chapters on modulation and encoding
- Transmission medium/channel: carries the signal (copper cable, fibre-optic cabling, or the air itself for wireless media), and is where noise is introduced
- Receiver: converts the received signal back into a form the destination can use
- Destination: the intended recipient of the message
Types of networks
Networks classified by geographic scale
| Type | Typical scale |
|---|---|
| Personal Area Network (PAN) | A single person's own devices, a few metres |
| Local Area Network (LAN) | A single building or campus |
| Metropolitan Area Network (MAN) | A city |
| Wide Area Network (WAN) | A country, continent, or the whole globe |
The chapter on wireless technologies revisits this same scale classification for wireless networks (WPAN, WLAN, WMAN), applying exactly this hierarchy to a medium without physical cabling.
Modes of data transmission
A second, independent axis classifies transmission by how bits within a single message are sent: serial (one bit at a time, over a single line, the standard for long-distance transmission since it needs only one channel) versus parallel (multiple bits simultaneously over multiple lines, faster over short distances but impractical over long ones due to signal skew between the lines).
Telecommunications information type
- Voice: real-time, latency-sensitive analogue or digitised audio
- Data: discrete digital information (files, records), generally more tolerant of delay than voice or video
- Video: high-bandwidth, latency-sensitive moving images, the most demanding of the three on a network's capacity
- Text/fax: low-bandwidth textual or image-based information
Each information type carries different network requirements, chiefly bandwidth (how much capacity it needs) and latency tolerance (how much delay it can absorb before quality degrades noticeably), the two dimensions that drive most of the design decisions in the chapters that follow (VoIP, in the closing chapter, is precisely voice information carried over a network originally designed for data's more relaxed latency tolerance).
Media performance terminologies
Noise, effects and types of noise
Noise is any unwanted signal that corrupts the intended signal, and no real transmission medium is entirely free of it, which is precisely why the Shannon theorem expresses a channel's maximum capacity as a function of both its bandwidth and its signal-to-noise ratio, not bandwidth alone.
The effect of noise is, in every case, the same: it degrades the receiver's ability to correctly recover the original signal, seen as an elevated bit error rate in a digital system or audible distortion in an analogue one. Managing it, through shielding, filtering, error detection and correction, and appropriate signal strength, is a thread running through nearly every later chapter of this paper.
Exam tip
The five-block telecommunication model (source, transmitter, medium, receiver, destination) is the frame the whole paper hangs on, when a later chapter introduces a new technique (modulation, multiplexing, switching), be able to say which block of this model it belongs to.
Self-check
(a) State the five blocks of the telecommunication model, in order. (5 marks)
(b) Distinguish simplex, half-duplex and full-duplex transmission, giving one example of each. (6 marks)
(c) Distinguish bandwidth from throughput. (2 marks)
(d) Explain why jitter is particularly damaging to voice and video traffic specifically. (3 marks)
(e) State two types of noise and briefly describe each. (4 marks)
Show worked answer
(a) Source; transmitter; transmission medium/channel; receiver; destination.
(b) Simplex: one direction only, e.g. broadcast radio. Half-duplex: both directions, but only one at a time, e.g. a walkie-talkie. Full-duplex: both directions simultaneously, e.g. an ordinary telephone call.
(c) Bandwidth is the theoretical maximum capacity of a medium; throughput is the actual achieved data rate in practice, always less than or equal to bandwidth once overhead and contention are accounted for.
(d) Voice and video are real-time traffic that expects a steady, predictable arrival rate to reconstruct a smooth signal; jitter, variation in latency from packet to packet, causes uneven arrival that produces audible or visible glitches, even where average latency and bandwidth are both otherwise acceptable.
(e) Any two of: thermal noise (random noise from electron thermal agitation, always present); intermodulation noise (spurious signals from interacting frequencies sharing a medium); crosstalk (unwanted coupling from an adjacent transmission path); impulse noise (brief high-amplitude spikes from external sources).