---
# System prepended metadata

title: Chapter7 - Wireless and Mobile Networks
tags: [Internet]

---

---
tags: Internet
---

# Chapter7 - Wireless and Mobile Networks

## 7.1 Introduction

![](https://i.imgur.com/2MzMdr7.png)

- Elements of a wireless network
    - Wireless hosts
    - Wireless link
    ![](https://i.imgur.com/7gl5LyB.png)
        - Range 
            - Indoor
            - Outdoor
        - Data rate
    - Base station
        - Cell Tower
        - Access Point (AP)
        - **Infrastructure** mode
            - 以一個 AP (access point) 作為所有節點通訊的中心。
        - **Ad hoc** networks
            - 兩個節點的裝置均有無線網路的功能即可連線 （對等）。
            - ==[Infrasctructure vs ad hoc](https://ithelp.ithome.com.tw/articles/10208110)==
        - Handoff or handover
            - 從一個基站覆蓋區移動到另一個基站覆蓋區時，切換連接到更大或相關的網路。
            - soft handoff / handover: 在不中斷與原基站的連通的情況下與新的基站連通，再與原基站斷開連接 （**先通後斷**）。
    - Network infrastructure

:::info
補充:
bps = bit per second
- K = $2^{10}$
- M = $2^{20}$
- G = $2^{30}$
- T = $2^{40}$
:::

- Single hop infrastructure-based
    - 802.11
    - 4G LTE
- Single hop infrastructure-less
    - Bluetooth
- Multiple hops infrastructured-based
    - Wireless **mesh** networks
- Multiple hops infrastructure-less
    - MANET (mobile ad hoc networks)
    - VANET (vehicular ad hoc networks)

## 7.2 Wireless Link and Network Characteristics

- ==Difference between a wired link and a wireless link==
    - **Decreased signal strength** (wireless)
    - **Interference** from other sources (wireless)
        - 2.4 GHz 為免執照頻段
    - Multipath propagation
        - 部分電磁波從物體或地板反射時，傳送端到接收端的路徑長度會改變，使收到的訊號變得模糊。

---

- **SNR** (signal-to-noise ratio)
- **BER** (bit error rate)

![](https://i.imgur.com/uyzp8Qz.png)
==考分別固定 modulation scheme、BER、SNR 的情況==

- For a given modulation scheme
    - **The higher the SNR, the lower the BER**.
- For a given SNR
    - A modulation technique with **higher bit transmission rate** will have a **higher BER**.
- Dynamic selection of the physical layer modulation technique
    - Can be used to adapt the modulation technique to channel conditions.
- **Hidden terminal** problem
    - Obstacle （障礙物）
    - Fading （特指快速衰減）
![](https://i.imgur.com/kGXheLM.png)

---

- **CDMA** (code division multiple access)
    - Code (orthogonal) (code length = $m$)
    - Encoding
    - Decoding
    ![](https://i.imgur.com/aruQ3qC.png)
    ![](https://i.imgur.com/UHxubAs.png)
- TDMA (time division multiple access)
- FDMA (frequency division multiple access)


## 7.3 WiFi: 802.11 Wireless LANs

![](https://i.imgur.com/ZpMNwEJ.png)

### 7.3.1 The 802.11 Architecture
- BSS (Basic Service Set)
    - BS (Base Station)
     - AP (Access Point)
- 802.11b 2.4GHz-2.485Ghz spectrum
    - 11 channels
    - Associate (Figure 7.9)
        - Passive scanning
        - Active scanning
![](https://i.imgur.com/o9Jkv4b.png)

### 7.3.2 The 802.11 MAC Protocol
- Avoid collision
- CSMA/CA
    - 偵測訊號衰減 （CSMA/CD 沒有）

![](https://i.imgur.com/RLTa0kI.png)

#### Dealing with Hidden Terminals: RTS and CTS
- Reserve 
    - RTS (requset-to-send)
    - CTS (clear-to-send)
![](https://i.imgur.com/XgzygGb.png)

![](https://i.imgur.com/ZDpqDvg.png)

### The IEEE 802.11 Frame

![](https://i.imgur.com/3O57cgu.png)
![](https://i.imgur.com/jzjDNO1.png)