The idea
Cellular and WiFi both need ground equipment nearby. Satellites reach places that have none, and Bluetooth reaches devices that have no infrastructure at all. Both ends of that range appear in the last slides of Lecture 11.
How it works
Satellite applications
The lecture lists three. Sensing covers weather, land use and human activity, at about a metre per pixel. Broadcast covers direct-to-home TV, video and radio for consumers, content to many cable head ends for business, and GPS, which relies on one send reaching many users. Internet covers connectivity to unserved regions, an alternative to wired Internet, low latency over long distances and access in airplanes.
How it works
GEO and LEO
| GEO | LEO | |
|---|---|---|
| Meaning | Geosynchronous | Low Earth Orbit |
| Motion relative to ground | Stationary | Moves, 27,000 km/hour |
| Altitude | 35,768 km above Earth | 550 to 1200 km above Earth |
| Round-trip time | About 800 msec | About 30 msec in practice |
| Coverage per satellite | Wide area | Smaller area |
| In line of sight of a ground station | Not stated on the slide | 5 to 15 minutes |
Aside
The slide prints the GEO altitude as 35,768 km. The commonly quoted figure is 35,786 km, so the digits look transposed. The page keeps the lecture’s value, as the exam marks the lecture.
A LEO network is a constellation of many satellites that together cover a wide area, with inter-satellite links (ISL). The lecture’s component figure shows the satellite channel as broadcast (multicast) down and unicast up, with ground stations, mobile user terminals, airborne and maritime vehicles and rural or remote communications, and gives a satellite’s coverage area on the ground as its footprint. It names the Starlink dish “Dishy McFlatface”.
How it works
Link or network?
The lecture asks whether satellite is a link or a network.
- Links in the sky. A single satellite hop between base stations. This is also called the bent pipe architecture.
- Network in the sky. Multiple satellite hops between base stations, with routing among satellites.
The lecture’s table of LEO constellations has columns for satellites deployed in 2025, satellites planned and whether ISL is planned. Its rows merged on extraction, so only Starlink can be read reliably, with 7,239 deployed, 12,000 planned and ISL planned. The other constellations named are Eutelsat OneWeb, Kuiper (Amazon) and Telesat, and their numbers are not reproduced here.
How it works
Starlink
All figures are as dated in the lecture.
- April 2025, about 7,239 operational LEO satellites. Planned 12K, requested 30K.
- First launches in 2019.
- Three low-Earth-orbit shells at 525, 530 and 535 km.
- Satellites weigh from 500 to 2700 lbs.
- Internet service, with 4M subscribers in September 2024. Residential price in the US is $120 per month.
- Service in Ukraine, where terrestrial networks were damaged.
- In 2024, direct-to-smartphone tests, which would use cellular spectrum from SpaceX’s US mobile partner, T-Mobile.
How it works
Bluetooth personal area network
Bluetooth is the lecture’s example of a wireless ad hoc network, a network with no infrastructure such as an AP or base station to connect to. Devices must find each other and organise themselves into a network from scratch. That means linking devices at the link layer with channel allocation, address assignment, service discovery, application-to-application communication over the Bluetooth network and security. The lecture’s conclusion is that an entirely new architecture and protocol stack, different from the Internet’s, is needed.
A Bluetooth network is a piconet.
- Coverage is generally under 10 m in diameter.
- No more than 8 devices per piconet. The device that formed the network has the central role and up to 7 others have the peripheral role.
- Communication is only between the central and a peripheral. There is no direct peripheral-to-peripheral communication.
- Each device has a unique 48-bit address.
- It operates in the unlicensed ISM band at 2.4 GHz, specifically 2.4 to 2.5 GHz.
| Classic Bluetooth | BLE | |
|---|---|---|
| Bandwidth per channel | 1 MHz | 2 MHz |
| Channels | 79 | 40, made up of 3 advertising and 37 data |
| Maximum rate | 2.1 Mbit network maximum | Not stated on the slide |
A Bluetooth channel is TDM with a slot length of 625 microseconds. Channel access is by polling. The central device advertises and grants channel access to peripherals in its network. Transmissions use frequency hopping, which the lecture calls a form of spread spectrum. The slide prints the slot length as 625 sec, with the microsecond sign lost in extraction.
Where marks get lost
Bluetooth is not an 802.11 ad hoc network
802.11 ad hoc mode is a BSS without an AP. Bluetooth is also described as ad hoc, but it is a separate architecture with its own protocol stack, its own central and peripheral roles and polling instead of CSMA/CA.
In the exam
- GEO against LEO. Altitude, RTT, motion, coverage. Quote the lecture’s values.
- Bent pipe is links in the sky. A network in the sky has several hops and routing among satellites.
- Starlink figures with their dates. 7,239 in April 2025, 12K planned, 30K requested, first launches 2019, shells at 525, 530 and 535 km, 4M subscribers in September 2024.
- Piconet. Under 10 m, no more than 8 devices, one central and up to 7 peripherals, only central to peripheral, 48-bit address, 2.4 GHz ISM.
- Classic against BLE channels. 79 channels of 1 MHz, against 40 channels of 2 MHz with 3 advertising and 37 data.
Check yourself
- GEO is stationary at about 800 msec RTT. LEO moves and is about 30 msec in practice.
- A bent pipe is one satellite hop. A network in the sky routes among satellites using ISL.
- Starlink had about 7,239 operational satellites in April 2025.
- A piconet is one central and up to seven peripherals, and the central polls them.