# Wireless Networks: Overview

### Two challenges
- Wireless
 - wireless link
- Mobility
 - change of the point attachment to the network
  
### Technology
- Cellular
- Wireless LAN/PAN (local/personal area network)
- GPS
- Satellite Based GPS
- Home networking
 - home router
- Ad hoc networks
 - mobile network with laptop
- Sensor network
 - IoT sensors
- Bluetooth
- IoT
- VLC (visual light communications)
- ...

### Wireless Network Types
- Wireless LAN: Local
 - IEEE 802.11 a/b/g/n/ac...etc.
- Wireless PAN: Personal; home or vehicle
 - IEEE 802.15 bluetooth
- Wireless BAN: Body; wearable devices
 - IEEE 802.15.6 belongs to bluetooth
- Wireless MAN: Metropolitan
 - IEEE 802.16; using WiMAX, 3G, 4G

## Architecture
- Network infrastructure; Backbone; Core network
- Base station; Access point
 - provide channels
 - to communicate with another base station, it needs switching
 - **relay**: deal with packets between hosts
- Wireless link
 - used to connect mobile to the base station
 - also used as backbone link
 - multiple access protocols for link access
- Wireless hosts
 - stationary or mobile

## Wireless Communication Link
![image.png](https://cdn.hashnode.com/res/hashnode/image/upload/v1662440369877/Ksuzsh_b-.png align="left")
- Higher data rate -> shorter distance
- WiFi: only 10-30m

### Characteristics
- Attenuation; signal strength decreases as it propagates through matter (path loss)
- Interference; frequencies shared by other devices; hard to decode
- Multipath propagation; radio signal reflects off objects or ground; receive signal in different path; arriving at destination at slightly different times



## Infrastructure vs Ad-Hoc
- Infrastructure mode
 - we have base station.
 - handoff: MS move to another base station
- Ad hoc mode
 - no base stations.
 - nodes can only transmit to others in their coverage
 - nodes organize themselves into a network.

### Taxonomy
![image.png](https://cdn.hashnode.com/res/hashnode/image/upload/v1662441257530/_T_4TF6e4.png align="left")
- Infrastructure
 - single hop: host <-> base station
 - multiple hop: host -> base station -> infra -> base station -> host



# Multiplexing
## FDMA; Frequency Division Multiple Access
![image.png](https://cdn.hashnode.com/res/hashnode/image/upload/v1662441683381/iyGP9tWVq.png align="left")
- frequency -> smaller channels
- 1 user per channel
- Bandwidth structure

## TDMA; Time Division Multiple Access
![image.png](https://cdn.hashnode.com/res/hashnode/image/upload/v1662441789741/__Lw_UzG1.png align="left")
- users can use the whole frequency
- 1 user per time division
- Frame structure

## CDMA; Code Division Multiple Access
![image.png](https://cdn.hashnode.com/res/hashnode/image/upload/v1662441942190/vFr2vnmHL.png align="left")
- all users using same frequency and time slot
- but using different code

## OFDM; Orthogonal Frequency Division Multiplexing
- conventional
![image.png](https://cdn.hashnode.com/res/hashnode/image/upload/v1662442082668/a4r6lmNVa.png align="left")
 - gap; guard band
- orthogonal
![image.png](https://cdn.hashnode.com/res/hashnode/image/upload/v1662442107392/vq7M5Fu5F.png align="left")
 - no gap
 - using less bandwidth

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## Cellular Network Infrastructure
### Cell
![image.png](https://cdn.hashnode.com/res/hashnode/image/upload/v1662442204826/jqCQP0Wal.png align="left")
- hexagons

### MS, BS, BSC, MSC and PSTN
- MS; mobile station
- BS; base station
- BSC; base station controller
- MSC; mobile switching center
- PSTN; public switching telephone network

### Control and Traffic Channels
- Control channel: request signal, smaller data
- Traffic channel: bigger data

### Call Setup
- BS & MS

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## WiFi; Wireless Fidelity
- packet-based (<-> circuit based)

### WiMAX; IEEE 802.16
- Lost to LTE

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## Satellite Systems
- cover very large area

### GPS; Global Positioning Systems

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