Unit 2: Wireless transmission and reference models
Computer Networks notes · PTU syllabus (BSIT404/BSBC603)
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Unit summary
Signals also travel through air and space, and networks follow layered reference models. This unit covers microwave, infrared, laser, radio and satellite transmission, Bluetooth, the frequency spectrum, circuit, message and packet switching, the OSI and TCP/IP reference models, and their comparison.
After this unit you can
- Compare wireless transmission methods
- Explain Bluetooth and the frequency spectrum
- Compare circuit, message and packet switching
- Explain and compare the OSI and TCP/IP models
PTU syllabus topics
- Microwave/infrared/laser/radio/satellite transmission
- Bluetooth
- frequency spectrum
- circuit/message/packet switching
- OSI reference model
- TCP/IP reference model
- comparison of OSI and TCP/IP
Top
Application, presentation, session
Application
Middle
Transport
Transport (TCP, UDP)
Routing
Network
Internet (IP)
Bottom
Data link, physical
Network access
Topic 1
The frequency spectrum
- 1Radio waves
3 kHz–1 GHz — AM, FM, TV
- 2Microwaves
1–300 GHz — mobile, Wi-Fi, satellite
- 3Infrared
300 GHz–400 THz — remotes
- 4Visible light
Lasers, optical fibre
- 5Ultraviolet, X-rays, gamma rays
Not used for data networks
- Bands: governments license the spectrum; ISM bands (2.4 GHz, 5 GHz) are unlicensed and used by Wi-Fi and Bluetooth. Higher frequency carries more data but travels shorter distances and is blocked more easily.
Topic 2
Wireless transmission
Radio
Omnidirectional; passes through walls; long range
AM/FM, Wi-Fi, mobile
Microwave
Line of sight; dish antennas on towers about 50 km apart
Long-distance telephone, cellular backhaul
Infrared
Short range; cannot pass walls; no licence
TV remotes, short links
Laser (free-space optics)
Narrow light beam; high bandwidth; affected by fog
Building-to-building links
Satellite
Microwave relay through satellites (GEO, MEO, LEO)
TV broadcast, GPS, remote internet
- Satellite orbits: GEO at about 36,000 km (fixed position, high delay); LEO at a few hundred to about 2,000 km (low delay; constellations such as Starlink).
Topic 3
Bluetooth
- Bluetooth (IEEE 802.15.1): short-range wireless (about 10 m, up to 100 m for class 1) in the 2.4 GHz band using frequency hopping.
- Piconet: one master and up to seven active slaves; scatternet: several connected piconets.
- Uses: earphones, keyboards, file sharing, smartwatches; Bluetooth Low Energy for wearables and IoT.
Topic 4
Circuit, message and packet switching
Circuit switching
A dedicated path is set up before communication and held throughout
Traditional telephone; guaranteed bandwidth, wasted when idle
Message switching
Whole message stored at each node and forwarded (store-and-forward)
Telegram, early email; no dedicated path but long delays
Packet switching
Data split into packets, each routed independently and reassembled
The internet; efficient sharing, variable delay
- Packet switching approaches: datagram (each packet routed independently, e.g., IP) and virtual circuit (a logical path set up first, e.g., MPLS, ATM).
Topic 5
The OSI reference model
The OSI (Open Systems Interconnection) model, from ISO, divides communication into seven layers.
| Layer | Function | Example |
|---|---|---|
| 7 Application | Network services to users | HTTP, FTP, SMTP, DNS |
| 6 Presentation | Translation, encryption, compression | JPEG, SSL/TLS, ASCII |
| 5 Session | Establishes, manages and ends sessions | NetBIOS, RPC |
| 4 Transport | End-to-end delivery, segmentation, error and flow control | TCP, UDP |
| 3 Network | Logical addressing and routing | IP, ICMP, routers |
| 2 Data link | Framing, MAC addressing, error detection | Ethernet, switches |
| 1 Physical | Transmission of raw bits | Cables, hubs, signals |
Exam tip
Mnemonic (top to bottom): "All People Seem To Need Data Processing".
Topic 6
The TCP/IP reference model and comparison with OSI
The TCP/IP model is the practical model used by the internet. It has four layers: Application (OSI layers 5–7), Transport, Internet (network) and Network Access (data link + physical).
Layers
7
4
Developed by
ISO, as a reference model
US Department of Defense, from real protocols
Approach
Model first, protocols later
Protocols first, model describes them
Use
Teaching and reference
Used on the internet
Transport
Connection-oriented and connectionless
TCP and UDP
Key terms
- Spectrum
- Range of electromagnetic frequencies
- Line of sight
- Transmission requiring a clear straight path
- Piconet
- Bluetooth network of one master and up to seven slaves
- Packet switching
- Sending data as independently routed packets
- Reference model
- Layered framework describing network functions
Quick revision
- Radio, microwave, infrared, light; ISM bands.
- Microwave line of sight; satellites GEO and LEO; laser links.
- Bluetooth, piconet, scatternet.
- Circuit, message, packet switching; datagram vs virtual circuit.
- OSI seven layers; TCP/IP four layers; comparison.
Important exam questions
Practice questions written to the PTU exam pattern for this unit's syllabus: short answers (Section A style) and long answers (Sections B and C style).
Short-answer questions
- Q1.What is an ISM band?
- Q2.Why does microwave need line of sight?
- Q3.What is a piconet?
- Q4.Distinguish circuit switching and packet switching.
- Q5.Name the seven OSI layers.
- Q6.State two differences between OSI and TCP/IP.
Long-answer questions
- Q1.Explain wireless transmission methods.
- Q2.Explain Bluetooth and the frequency spectrum.
- Q3.Compare circuit, message and packet switching.
- Q4.Explain the OSI model and compare it with TCP/IP.
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