The idea
Lecture 11 opens with a fact: the number of wireless phone subscribers now exceeds the number of wired phone subscribers. Users expect Internet access anytime, anywhere and on any device.
Everything earlier in the unit assumed a cable. A cable keeps a signal inside a known path. A radio signal spreads in all directions, weakens as it goes, bounces off things and is shared with whoever else is using the spectrum. This topic names the pieces of a wireless network and the ways the radio medium makes the job harder. The next topics cover how 802.11, cellular, satellite and Bluetooth deal with those problems.
How it works
Elements of a wireless network
The lecture breaks a wireless network into four elements.
- Wireless hosts. The end-system devices that run applications, such as a laptop or a smartphone.
- Base station. It sends and receives to and from each wireless host associated with it, and it coordinates the transmissions of multiple hosts. Cell towers and access points are base stations.
- Wireless links. The connection between a host and a base station. The lecture names its key characteristics as coverage and rate.
- Network infrastructure. The larger network a wireless host may want to reach.
The slide that draws these four together is a figure, and it extracted to nothing. The text above is all the lecture’s wording that survives.
How it works
Wireless versus wired links
The lecture lists four problems that a wireless link has and a wire largely does not.
- Signal attenuation. An electromagnetic signal weakens with distance. The lecture calls this path loss.
- Interference from other sources. Wireless signals are broadcast, so any other transmitter on the same spectrum can disturb reception.
- Multipath propagation. Reflection delivers more than one copy of the signal. This may cause fading and inter-symbol interference (ISI).
- Errors. The lecture says errors are a more serious issue on wireless links than on wired ones.
These four problems are the reason the rest of the module looks different from the wired modules. CSMA/CD cannot work, so 802.11 uses collision avoidance. Signal strength changes with position, so 802.11 adapts its transmission rate. Hosts move, so cellular networks need handover.
How it works
Challenges the lecture raises
Alongside the demand for access anytime and anywhere, the introduction lists four challenges: mobility management, QoS management, channel conditions and security. Mobility management returns in the cellular topic as handover. Channel conditions return as rate adaptation in the 802.11 channels topic.
How it works
The families of wireless network
The lecture sorts wireless networks into four groups.
| Family | What the lecture lists |
|---|---|
| Wireless LAN | IEEE 802.11 (a, b, g, n, h, ax), ad hoc and wireless mesh |
| Cellular | 1G AMPS. 2G D-AMPS, GSM, IS-95 (CDMA-ONE). 2.5G GPRS. 2.75G EDGE. 3G UMTS (WCDMA) and CDMA2000. 3.5G HSPA. 3.75G HSPA+. 4G LTE, LTE-A and some versions of WiMAX. 5G NR |
| Satellite | Iridium, Globalstar |
| Other | Sensor networks (BAN), PAN (Bluetooth, ANT), LP-WAN for IoT |
The lecture describes the WLAN as one of the most important access network technologies in the Internet today. It gives high speeds with limited mobility, in places such as the home, the office and the airport. Its low cost and its use of unlicensed spectrum keep the user’s cost down.
Where marks get lost
Do not file Bluetooth under WLAN
Bluetooth and ANT sit under PAN, in the group the lecture labels Other. They are not 802.11 standards, and the lecture later says Bluetooth needs an entirely new architecture and protocol stack, different from the Internet’s.
In the exam
- Four elements, each with its job. Base station is the one that does two things, communicating with its associated hosts and coordinating their transmissions.
- Four wireless link problems, named as the lecture names them: signal attenuation (path loss), interference, multipath propagation (fading, ISI), errors.
- Link characteristics are coverage and rate.
- Examples of base stations are cell towers and access points.
- The lecture’s four challenges are mobility management, QoS management, channel conditions and security.
Check yourself
- A wireless network has hosts, a base station, wireless links and the wider network infrastructure.
- The base station both serves its hosts and coordinates their transmissions.
- Path loss, interference, multipath propagation and errors make wireless harder than wired.
- The lecture’s families are WLAN, cellular, satellite and a group of other networks that includes PAN and LP-WAN.