Ch14. Wireless LAN

Objective

• Introduction
• First 
  • Wireless LAN vs. Wired
  • Wireless Networks
  • Characteristics of the wireless networks
  • How to access controlled in these types of networks
• Second
  • Wireless LAN defined by the IEEE 802.11 Project.
  • Define the architecture of this type of LAN and describes the MAC sub-layer
• Third
  • Bluetooth technology as a PAN(personal area network)
  • Addressing mechanism, Packet format
  • Different layers used in this protocol are also briefly described and compared with the ones in the other wired and wireless LAN

 

Introduction

 

- Wireless LANs can found on college campauses, office building ETC...

 

First 

• Ad Hoc Network
  • There was no infrastructure -> Organized by nodes
  • 네트워크를 구성 및 유지하기 위해 기지국, AP등과 같은 기본 네트워크 장비가 필요하지 않다.
• Connection of a Wired LAN and a Wireless LAN to Other Networks

• Characteristics
  • Attenuation
    • Signal spreads in all directions
  • Interference
    • The receiver can receive not only the signal of the intended sender, but also the signals of other senders
  • Multipath
    • Signal can be reflected on obstacles such as walls, objects etc.
  • Error
    • SNR(Signal-to-Noise Ratio)

 

• Access Control
  • Wireless LAN is access control 
    • How a wireless host can get access to the shared medium(air)
    • CSMA/CD algorithm does not work in wireless LANs for three reasons
      • Wireless hosts do not have enough power to send and receive at the same time
      • The hidden station problem prevents collision detection
      • The distance between stations can be great

Hidden station problem

 

Second

• IEEE 802.11 Project
  • IEEE has defined the specifications for a wireless LAN
  • That is covers the physical and data-link layers
  • It is sometimes called wireless Ethernet
  • Wireless LAN that is certified by the Wi-Fi Alliance
• The standard defines two kinds of services
  • Basic service set(BSS)
  • Extended service set(ESS)
• BSS
  • Consists of fixed / mobile stations and access point(AP)
  • Ad hoc Network : network without AP
  • Infrastructure network BSS : network includes AP

BSS

• ESS
  • Consists of two or more BSSs with APs
  • Distribution System : connect APs of BSS

 

• MAC Sub-layer
  • IEEE 802.11 defines two MAC sublayers
    • Distributed Coordination Function(DCF)
      • Selecetive approach of infra network
      • Centralized polling used by AP
    • Point Coordinaion Function(PCF)
      • Ccarrier-Sense Multiple Access
      • CSMA/CA used by station

- RTS : source -> Des로 data송신이 가능한지 물어보는 단계

- CTS : RTS에 대한 응답(Yes/No)

- Data : If CTS answer is 'Yes' -> send data

- ACK : Response of the Data

- NAV : CTS에서 데이터 통신이 얼마 만큼의 시간동안 전달이 될지에 대한 정보가 담겨있기에, 이를 이용해 NAV결정

- Size : RTS<CTS

 

Frame Format

Subfields in FC field

• Exposed Station Problem

Exposed Station Problem

- RTS, CTS both are waiting for transmission

- You can see the above figure, There is exists B, A, C, D

- A and C are communication So, it is caused 'Explosed Problem'

- What is 'Explosed Problem'? well, C can send to D becasue this area is free, but C erroneously refrains from sending because of received RTS

- 전송거리 제약 -> 무선에서 신호를 보내는 세기가 약해서 한 번에 통신을 못 하게 되는 경우가 발생한다. 이 때문에, 여러번 거쳐가면서 전달한다.

- Mobile Ad-hoc Net -> Sensor Network

- A에서 B까지 통신을 하려고 하는데 서로 거리가 너무 멀 경우, 무선에서는 신호가 갈 수 있는 범위가 한정 돼 있으므로 범위 내 center에 대해서 도착지까지 도달할 수 있을 때까지 계속 신호를 보낸다.

 

• OFDM vs. OFDMA
  • OFDMA은 Wi-Fi6에서 중요한 기술로 대두된다.

Compare OFDM and OFDMA(via ASUS)

채널을 세분하면 작은 패킷을 동시에 여러 장치로 병렬 전송할 수 있다.

도착한 패킷은 전송되기 시작하며 다른 패킷을 기다릴 필요가 없고, 다운링크 OFDMA에서 라우터는 서로 다른 부반송파 그룹을 사용하여 서로 다른 클라이언트에 패킷을 보낼 수 있다.  (ASUS의 설명)

 

• Variable Data signal
  
  • MIMO
    • Multi Input Multi Output
  • Beam-Forming
    • 특정 방향으로 에너지를 더 많이 받아서 신호를 전파한다.
  • HARQ
    • 원래 받은 데이터와 원래 데이터에서의 발생한 에러와 합쳐보면, original Data를 만들어 낼 수 있다.

 

• Wi-Fi6 & Wi-Fi7 Comparison
  • QAM Comparison

 

 

Third

• Bluetooth
  • 짧은 거리에서 무선랜 기술로 운영되는 방식
  • A Bluetooth LAN is an ad hoc network(somtimes called gadgets, find each other and make a network called a piconet)
• Basic Spec

• Piconet
  • Master가 Hoppin patern을 사용하면 그에 따라 slave들도 해당 패턴에 맞춘다. -> piconet형성

 

• Scatternet
  • Scatternet (Connected Piconet between Piconet)
  • Pattern Change(Frequency Change) : Based on ID -> Created Pattern

 

 

• Bluetooth 4.0 Bluetooth Low Energy(BLE)
  • What is Bluetooth Low Energy? NEW, open, short range radio technology
  • Basic Attributes of BLE 
• 40 physical channels are used
• These channels was selected to avoid channel interference with Wi-Fi communications

1, 6, 11 와 같이띄엄띄엄 범위가 있는 이유 : 신호 대역 간 간섭을 방지하기 위해

 

• Overview of communication
  • Scanning & Advertising -> Parsing -> Connection -> Data Transmission
  • "Data" Transmission without connection
  • Communication range à 50~80m in real environment

Example of BLE Topology Architecture