CISSE Advanced Level
Networking and Telecommunications cheat sheet
Every chapter's key points, formulas and definitions on one page, for the last week before the exam. Each heading links to the chapter that explains it.
Introduction to network and telecommunication systems
- 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.
Key terms
- Network
- An interconnected set of devices able to exchange data, ranging from two directly connected computers to the global internet.
- Communication system
- The complete arrangement of hardware, software and protocols that gets a message from a source to a destination, of which a data network is the transport layer.
- Simplex
- Data flows in one direction only, sender to receiver, never the reverse, e.g. a broadcast radio signal.
- Half-duplex
- Data flows in both directions, but only one direction at a time, e.g. a walkie-talkie.
- Full-duplex
- Data flows in both directions simultaneously, e.g. an ordinary telephone call.
- Bandwidth
- The range of frequencies a medium can carry (analogue), or the maximum data rate it can support (digital), commonly measured in Hz or bits per second respectively.
- Throughput
- The actual achieved data rate in practice, which is always less than or equal to the theoretical bandwidth once overhead and contention are accounted for.
- Latency
- The time delay between a signal being sent and being received.
- Jitter
- The variation in latency from one packet to the next, particularly damaging to real-time voice and video, which expects a steady, predictable arrival rate.
- Attenuation
- The weakening of a signal's strength as it travels along a medium, which is why long transmission runs need amplification or regeneration.
- Thermal noise
- Random noise from the thermal agitation of electrons in a conductor, present in every electronic system and unavoidable in principle, only reducible.
- Intermodulation noise
- Produced when signals of different frequencies share the same medium and interact, generating spurious signals at sum and difference frequencies.
- Crosstalk
- Unwanted coupling between adjacent transmission paths, a signal on one cable or channel bleeding into another running alongside it.
- Impulse noise
- Irregular, high-amplitude spikes from external sources (lightning, electrical switching), brief but capable of corrupting a burst of data outright.
The rest of the sheet covers
- Fourier theory and communication signals
- Communication systems
- Modulation methods for analogue data
- Encoding and modulation methods for digital data
- Multiplexers, multiplexing and access techniques
- Network access
- Fibre-optic cabling
- Circuit and packet switching
- Asynchronous Transfer Mode (ATM) networks
- Wireless technologies
- Voice over Internet Protocol (VoIP)
- Worked examples
- Exam technique and revision
- Glossary