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Chapter 11:
Wireless LANs
Business Data Communications, 5e
Wireless LAN Applications
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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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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
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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
• Infrared (IR) LANs
– Individual cells are limited to a single room, because
infrared light does not penetrate opaque walls
• 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.
• Narrowband microwave
– Do not use spread spectrum. Some of these products
operate at frequencies that require FCC licensing,
while others use one of the unlicensed ISM bands
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IEEE 802.11 Architecture
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IEEE 802.11 Services
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Association
Reassociation
Disassociation
Authentication
Privacy
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IEEE 802.11
Medium 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 802.11
Protocol 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)
– extends IEEE 802.11b to higher data rates
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Original 802.11
Physical 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 2.4GHz 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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Bluetooth
• Always-on, short-range radio hookup that resides
on a microchip
• Low-power short-range wireless standard for a
wide range of devices
• Uses 2.4-GHz band (available globally for
unlicensed low-power uses)
• Two Bluetooth devices within 10 m of each other
can share up to 720 kbps of capacity
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Examples of
Bluetooth Capability
• Make calls from a wireless headset connected remotely to
a cell phone
• Eliminate cables linking computers to printers,
keyboards, and the mouse
• Hook up MP3 players wirelessly to other machines to
download music
• Set up home networks to remotely monitor air
conditioning, appliances, and Internet surfing
• Call home from a remote location to turn appliances on
and off, set the alarm, and monitor activity.
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Bluetooth Applications
• Up to eight devices can communicate in a
small network called a piconet; ten of these
can coexist in the same coverage range of
the Bluetooth radio
• Three general application areas
– Data and voice access points
– Cable replacement
– Ad hoc networking
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Bluetooth Standards
• Core Specifications
– Describes layers of the protocol architecture, from
radio interface to link control
• Profile Specifications
– Discusses the use of the technology defined in the core
specifications to implement a particular usage model
– General access profile specifies how the baseband
architecture should be used between devices that
implement one or multiple profiles
– Other profiles fall into one of two categories: cable
replacement or wireless audio
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Bluetooth Protocol Architecture
• Core Protocols
• Cable Replacement Protocol (RFCOMM)
– presents a virtual serial port that is designed to make
replacement of cable technologies as transparent as
possible
• Telephony Control Protocol (TCS BIN)
– a bit-oriented protocol that defines the call control
signaling for the establishment of speech and data
calls between Bluetooth devices
• Adopted Protocols
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Bluetooth Core Protocols
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Radio
Baseband
Link manager protocol (LMP)
Logical link control and adaptation
protocol (L2CAP)
• Service discovery protocol (SDP)
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Bluetooth Adopted Protocols
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PPP
TCP/UDP/IP
OBEX
WAE/WAP
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Bluetooth
High-Priority Usage Models
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File Transfer
Internet Bridge
LAN Access
Synchronization
Three-in-one Phone
Headset
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Piconets and Scatternets
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