Wireless LANs - Data Communications - Lecture Slides, Slides of Data Communication Systems and Computer Networks

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Chapter 17 Wireless LANs

Overview

• A wireless LAN uses wireless transmission

medium

• Used to have high prices, low data rates,

occupational safety concerns, and licensing

requirements

• Problems have been addressed

• Popularity of wireless LANs has grown rapidly

Single Cell Wireless LAN

Configuration

Multi-Cell Wireless LAN

Configuration

Applications - Nomadic Access

• Link between LAN hub and mobile data terminal

—Laptop or notepad computer

—Enable employee returning from trip to transfer data

from portable computer to server

• Also useful in extended environment such as

campus or cluster of buildings

—Users move around with portable computers

—May wish access to servers on wired LAN

Infrastructure Wireless LAN

Add Hoc LAN

Wireless LAN Requirements

• Same as any LAN — High capacity, short distances, full connectivity, broadcast capability
• Throughput: efficient use wireless medium
• Number of nodes:Hundreds of nodes across multiple cells
• Connection to backbone LAN: Use control modules to connect to both types of LANs
• Service area: 100 to 300 m
• Low power consumption:Need long battery life on mobile stations — Mustn't require nodes to monitor access points or frequent handshakes
• Transmission robustness and security:Interference prone and easily eavesdropped
• Collocated network operation:Two or more wireless LANs in same area
• License-free operation
• Handoff/roaming: Move from one cell to another
• Dynamic configuration: Addition, deletion, and relocation of end systems without disruption to users Docsity.com

Infrared LANs

Strengths and Weaknesses

• Spectrum virtually unlimited — Infrared spectrum is unregulated worldwide — Extremely high data rates
• Infrared shares some properties of visible light — Diffusely reflected by light-colored objects - Use ceiling reflection to cover entire room — Does not penetrate walls or other opaque objects - More easily secured against eavesdropping than microwave - Separate installation in every room without interference
• Inexpensive and simple — Uses intensity modulation, so receivers need to detect only amplitude
• Background radiation — Sunlight, indoor lighting — Noise, requiring higher power and limiting range — Power limited by concerns of eye safety and power consumptionDocsity.com

Infrared LANs

Transmission Techniques

• Directed-beam IR — Point-to-point links — Range depends on power and focusing - Can be kilometers - Used for building interconnect within line of sight — Indoor use to set up token ring LAN — IR transceivers positioned so that data circulate in ring
• Omnidirectional — Single base station within line of sight of all other stations - Typically, mounted on ceiling — Acts as a multiport repeater — Other transceivers use directional beam aimed at ceiling unit
• Diffused configuration — Transmitters are focused and aimed at diffusely reflecting ceiling

Spread Spectrum LANs

Peer-to-Peer Configuration

• No hub

• MAC algorithm such as CSMA used to control

access

• Ad hoc LANs

Spread Spectrum LANs

Transmission Issues

• Licensing regulations differ from one country to another
• USA FCC authorized two unlicensed applications within

the ISM band:

— Spread spectrum - up to 1 watt — Very low power systems- up to 0.5 watts — 902 - 928 MHz (915-MHz band) — 2.4 - 2.4835 GHz (2.4-GHz band) — 5.725 - 5.825 GHz (5.8-GHz band) — 2.4 GHz also in Europe and Japan — Higher frequency means higher potential bandwidth

• Interference — Devices at around 900 MHz, including cordless telephones, wireless microphones, and amateur radio — Fewer devices at 2.4 GHz; microwave oven — Little competition at 5.8 GHz - Higher frequency band, more expensive equipment Docsity.com

Licensed Narrowband RF

• Microwave frequencies usable for voice, data, and video licensed within specific geographic areas to avoid interference — Radium 28 km — Can contain five licenses — Each covering two frequencies — Motorola holds 600 licenses (1200 frequencies) in the 18-GHz range — Cover all metropolitan areas with populations of 30,000 or more in USA
• Use of cell configuration
• Adjacent cells use nonoverlapping frequency bands
• Motorola controls frequency band — Can assure nearby independent LANs do not interfere
• All transmissions are encrypted
• Licensed narrowband LAN guarantees interference-free communication
• License holder has legal right tointerference-free data channel

Unlicensed Narrowband RF

• 1995, RadioLAN introduced narrowband wireless LAN

using unlicensed ISM spectrum

— Used for narrowband transmission at low power

• 0.5 watts or less — Operates at 10 Mbps — 5.8-GHz band — 50 m in semiopen office and 100 m in open office
• Peer-to-peer configuration
• Elects one node as dynamic master — Based on location, interference, and signal strength
• Master can change automatically as conditions change
• Includes dynamic relay function
• Stations can act as repeater to move data between

stations that are out of range of each other