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Basics of Wireless Networking Basics of Wireless Networking

Neil TangNeil Tang1/21/20091/21/2009

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OutlineOutline

Wireless Communication Basics

Wireless Networks

Interference Model

IEEE 802.11 MAC Protocols

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Wireless CommunicationsWireless Communications

There is no physical link in wireless networks. Signals are transmitted on a certain frequency, propagate in the space and are captured by the receiver tuned to the same frequency.

Wireless communication is normally broadcast communication, i.e., all nodes within the transmission range of a particular node can receive the transmitted packets.

Transmissions in a common neighborhood will interfere with each other. If the Signal-to-Interference-Noise-Ratio (SINR) in the receiver is large enough, a packet can be correctly decoded.

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Wireless NetworksWireless Networks

Single-hop wireless networks: cellular network, wireless LAN.

Multihop wireless networks: mobile ad hoc network, wireless mesh network, wireless sensor network.

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Interference ModelInterference Model

Primary Interference:

A B C

A B C

A B C

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Interference ModelInterference Model

Protocol Model (for the fixed transmission power case):

A B

C D

a

b

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Interference ModelInterference Model

Physical Model (for the variable transmission power case):

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MAC Protocol – 802.11MAC Protocol – 802.11

Basic architecture: Infrastructure mode and ad hoc mode

MAC protocol: CSMA/CA (MACA)

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Basic ArchitectureBasic Architecture

AP

Infrastructure mode

Ad hoc mode

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Hidden Terminal ProblemHidden Terminal Problem

Transmission Range

A B C

Node A and C are hidden terminals.

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Exposed Terminal ProblemExposed Terminal Problem

Transmission Range

A B CD

Node A and B are exposed terminals.

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802.11802.11

Distributed Coordination Function (DCF) : Simple protocol with no centralized control.

Point Coordination Function (PCF): APs control medium access and provide collision-free communications. Bandwidth (timeslots) will be carefully allocated to nodes which has signed up for transmission.

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DCFDCF

Basic Idea: CSMA/CA (MACA)

Request-To-Send (RTS) and Clear-To-Send (CTS) are used to reserve space and time for transmission.

Both physical and virtual carrier sensing are used to determine the time for transmission. Network Allocation Vector (NAV) is used for virtual carrier sensing.

For each transmitted data frame, an ACK frame will be sent back to from the sender. Stop-and-wait protocol are used to provide reliability.

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DCFDCF