Chapter 11:
Wireless LANs
Business Data Communications, 6e
Wireless LAN Applications
•
•
•
•
LAN extension
Cross-building interconnect
Nomadic access
Ad hoc networks
2
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
3
Single-Cell Wireless LAN
4
Multiple-Cell Wireless LAN
5
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.
6
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
7
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
8
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
9
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
10
IEEE 802.11 Architecture
11
IEEE 802.11 Services
Service
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
12
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)
13
IEEE 802.11
Protocol Architecture
14
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
15
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
16
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
17
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
18
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
19
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
20
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
21