Chapter 11: Wireless LANs

Business Data Communications, 6e

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Wireless LAN Applications

• LAN extension• Cross-building interconnect• Nomadic access• Ad hoc networks

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LAN extension

• Originally targeted to reduce cost of wiring, but new buildings now have sufficient wiring in place

• Still useful in buildings where wiring is problematic– buildings with large open areas, – historical buildings with insufficient twisted pair – small offices wired LANs are not economical

• Typically, a wireless LAN will be linked into a wired LAN on the same premises

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Single-Cell Wireless LAN

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Multiple-Cell Wireless LAN

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Cross-building interconnect

• Connect LANs in nearby buildings, be they wired or wireless LANs

• Point-to-point wireless link is used between two buildings (e.g. two microwave or infrared transmitter/receiver units can be placed on the rooftops of two buildings within the line of sight of each other)

• Devices are typically bridges or routers.

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Nomadic Access

• Provides a wireless link between a LAN hub and a mobile data terminal (e.g. laptop computer)

• Examples– Enable an employee returning from a trip to transfer

data from a personal portable computer to a server in the office.

– Access in an extended environment such as a campus or a business operating out of a cluster of buildings.

– In both of these cases, users may wish access to the servers on a wired LAN from various locations

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Ad Hoc Networks

• A peer-to-peer network (no centralized server) set up temporarily to meet some immediate need

• For example, a group of employees, each with a laptop or palmtop computer, may convene in a conference room for a business or classroom meeting. The employees link their computers in a temporary network just for the duration of the meeting

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Wireless LAN Requirements

• Efficient throughput• Support for multiple nodes• Connection to backbone LAN• Broad service area (~ 100-300m)• Allows for reduced power consumption while not using

the network (e.g. sleep mode)• Transmission robustness and security• Co-located network operation• License-free operation• Handoff/roaming• Dynamic and automated addition, deletion, and relocation

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Wireless LAN Technology

• Spread spectrum LANs– In most cases, these LANs operate in the ISM

(Industrial, Scientific, and Medical) bands so that no FCC licensing is required for their use in the U.S.

• OFDM LANs– For higher speeds; this is known as orthogonal

frequency division multiplexing and is superior to spread spectrum.

• Infrared (IR) LANs– Individual cells are limited to a single room, because

infrared light does not penetrate opaque walls

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IEEE 802.11 Architecture

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IEEE 802.11 ServicesService Provider Used to Support

Association Distribution system MSDU delivery

Authentication Station LAN access and security

Deauthentication Station LAN access and security

Disassociation Distribution system MSDU delivery

Distribution Distribution system MSDU delivery

Integration Distribution system MSDU delivery

MSDU delivery Station MSDU delivery

Privacy Station LAN access and security

Reassociation Distribution system MSDU delivery

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IEEE 802.11Medium Access Control

• Reliable Data Delivery– Basic data transfer mechanism involves an

exchange of two or four frames (data, ACK, and optional CTS/RTS)

• Access Control– DFWMAC (distributed foundation wireless

MAC)

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IEEE 802.11Protocol Architecture

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IEEE 802.11 Physical Layer

• 802.11 (1997)– MAC layer and three physical layer specifications;

two 2.4-GHz band, one infrared, all operating at 1 and 2 Mbps

• IEEE 802.11a (1999)– operates in the 5-GHz band at up to 54 Mbps

• IEEE 802.11b (1999)– operates in the 2.4-Ghz band at 5.5 and 11 Mbps.

• IEEE 802.g (2002) - operates in the 2.4-Ghz band and 54 Mbps

• IEEE 802.n– operates in the 2.4-Ghz band and hundreds of Mbps

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Original 802.11Physical Media Definitions

• Direct-sequence spread spectrum (DSSS) operating in the 2.4 GHz ISM band, at data rates of 1 Mbps and 2 Mbps

• Frequency-hopping spread spectrum (FHSS) operating in the 2.4 GHz ISM band, at data rates of 1 Mbps and 2 Mbps

• Infrared at 1 Mbps and 2 Mbps operating at a wavelength between 850 and 950 nm

• All of the original 802.11 products were of limited utility because of the low data rates

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IEEE 802.11b

• Extension of the IEEE 802.11 DSSS scheme, providing data rates of 5.5 and 11 Mbps (higher data rate is achieved with more complex modulation)

• Apple Computer was first, with AirPort wireless networking, followed by other vendors

• Wireless Ethernet Compatibility Alliance created to certify interoperability for 802.11b products

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Problems with 802.11 and 802.11b

• Original 802.11 and 802.11b may interfere with other systems that operate in the 2.4-GHz band– Bluetooth– HomeRF– other devices--including baby monitors and

garage door openers

• Limited data rate results in limited appeal

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Higher-Speed 802.11 Options

• 802.11a– Uses 5-GHz band. – Uses orthogonal frequency division multiplexing

(OFDM) rather than spread spectrum – Possible data rates are 6, 9, 12, 18, 24, 36, 48, and 54

Mbps• 802.11g

– Higher-speed extension to IEEE 802.11b. – Combines physical layer encoding techniques used in

802.11a and 802.11b to provide service at a variety of data rates

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Higher-Speed 802.11 Options

• 802.11n– Uses both the 2.4 GHz and 5-GHz band. – Improves data transmission and effective throughput– Uses Multiple Input Multiple Output (MIMO) antenna

architecture– Uses channel bonding , allowing for 2X as many

subchannels, doubling the transmission rate– Uses orthogonal frequency division multiplexing

(OFDM) rather than spread spectrum – Aggregates multiple MAC frames into a single block

for transmission

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IEEE 802.11 Security Considerations

• Workstations are not physically connected to the network

• Privacy concerns since any station in range can receive data

• Wi_Fi Protected Access (WPA) used to address these concerns