Wireless LAN Technology

Wireless LAN
Class Index
Wireless LAN’s
 Wireless
LAN applications
 Nomadic Access and Ad Hoc Networking
 Wireless LAN requirements
 Wireless LAN Technology
Infrared LAN’s
 Strengths
& Weaknesses
Class Index
 Transmission
Spread Spectrum LAN’s
 Configuration
 Transmission
Narrowband Microwave LAN’s
 Licensed
Narrowband RF
 Unlicensed Narrowband RF
Wireless LANs
Wireless LANs are an indispensable adjunct to
traditional wired LANs,
They satisfy requirements for:
 relocation,
 ad hoc networking, and
 coverage of locations difficult to wire.
Wireless LAN applications
Early wireless LAN products, were
marketed as substitutes for traditional
wired LANs
 In a number of environments, there is a
role for the wireless LAN as an alternative
to a wired LAN.
Wireless LAN applications
Buildings with large open areas (manufacturing
plants, stock exchange trading floors, warehouses).
Historical buildings with insufficient twisted pair and
where drilling holes for new wiring is prohibited.
Small offices where installation and maintenance of
wired LANs are not economical.
Wireless LAN applications
In all of these cases, a wireless LAN
provides an effective and more attractive
In most of these cases, an organization
will also have a wired LAN to support
servers and some stationary workstations.
Typical Wireless LAN Configuration
– Single Cell Wireless LAN
There is a backbone
wired LAN, such as
Ethernet, that
supports servers,
workstations and one
or more bridges or
routers to link with
other networks.
There is a control
module (CM) that
acts as an interface
to a wireless LAN
Typical Wireless LAN Configuration
– Single Cell Wireless LAN
Some of the end
systems are
standalone devices,
such as a
workstation or a
Hubs or other user
modules (UMs) that
control a number of
stations off a wired
LAN may also be
part of the wireless
LAN configuration.
Typical Wireless LAN Configuration
– Multiple Cell Wireless LAN
Nomadic Access and Ad Hoc
Nomadic access provides a wireless link
between a LAN hub and a mobile data terminal
equipped with an antenna.
Users may move around with their portable
computers and may wish access to the servers
on a wired LAN from various locations.
An ad hoc network is a peer-to-peer network
(not centralized server) set up temporarily to
need some immediate need.
Nomadic Access and Ad Hoc
Nomadic Access
Ad Hoc Network
Wireless LAN requirements
Wireless LAN must meet the same sort of
requirements typical of any LAN, including:
 High
 Ability to cover short distances.
 Full connectivity among attached stations.
 Broadcast capacity
Wireless LAN specific requirements
Throughput: The MAC protocol should
make as efficient use as possible of the
wireless medium to maximize capacity.
 Number of nodes: Wireless LANs may
need to support hundreds of nodes across
multiple cells.
Wireless LAN specific requirements
Connection to backbone LAN: In most cases,
interconnection with stations on a wired backbone LAN
is required.
Service area: A typical coverage area for a wireless
LAN has a diameter of 100 to 300 m.
Battery power consumption: Mobile workers use
battery-powered workstations that need to have a long
battery life when used with wireless adapters.
Typical wireless LAN implementations have features to reduce
power consumption while not using the network, such as a sleep
Wireless LAN specific requirements
Transmission robustness and security:
Unless properly designed, a wireless LAN
may be interference prone and easily
eavesdropped. The design of a wireless
LAN must permit reliable transmission
even in a noisy environment and should
provide some level of security from
Wireless LAN specific requirements
Collocated network operations: As wireless
LANs become more popular, it is quite likely for
two or more wireless LANs to operate in the
same area or in some area where interference
between the LANs is possible.
License-free operation: Users would prefer to
buy an operate wireless LAN products without
having to secure a license for the frequency
band used by the LAN.
Wireless LAN specific requirements
Handoff/roaming: The MAC protocol used in
the wireless LAN should enable mobile stations
to move from one cell to another.
Dynamic configurations: The MAC addressing
and network management aspects of the LAN
should permit dynamic and automated addition,
deletion and relocation of end systems without
disruption to other users.
Wireless LAN – Mobile Data
Network – Wired LAN comparison
Kiviat Graphs provides a pictorial means of comparing
systems among multiple variables.
Wireless LAN Technology
Wireless LANs are generally categorized
according to the transmission technique that is
All current wireless LAN products fall into one of
the following categories:
 Infrared
 Spread spectrum LANs
 Narrowband microwave LANs
Wireless LAN Technologies – Key
Spread Spectrum
Direct Beam
Direct Sequence
Data Rate
1 to 4
1 to 10
1 to 3
2 to 20
10 to 20
Range (m)
15 to 60
30 to 100
30 to 250
10 to 40
With LOS
Stationary / Mobile
Token Ring,
25 mW
: 800 to 900 nm
(8000 to 9000 Å)
902 to 928 MHZ
2.4 to 2.4835 GHZ
5.725 to 5.85 GHz
[1] CSMA: Carrier Sense Multiple Access. Check IEEE 802.11 Class 11
Yes unless ISM
902 to 928 MHz
5.2 to 5.775 GHz
18.825 to 19.205 GHz
Infrared LANs – Strengths
The two competing transmission media for
wireless LANs are microwave radio, and
infrared (IR).
Infrared LANs – Strengths
IR offer a number of significant advantages over
the microwave radio approaches:
 The
spectrum of IR is virtually unlimited, which
presents the possibility of achieving extremely high
data rates.
 The IR spectrum is unregulated worldwide, which is
not true of some portions of the microwave spectrum.
 IR shares some properties of visible light that make it
attractive for certain types of LAN configurations.
Infrared LANs – Strengths
IR light is diffusely reflected by light-coloured
objects; thus it is possible to use ceiling
reflections to achieve coverage of an entire
IR light does not penetrate walls or other opaque
objects. This has two advantages:
 IR
communications can be more easily secured
against eavesdropping than microwave
 A separate IR installation can be operated in every
room in buildings without interference, enabling the
construction of very large IR LANs.
Infrared LANs – Strengths
Equipment is relatively inexpensive and
simple. (IR data transmissions usually use
intensity modulation).
Infrared LANs - Weaknesses
Many indoor environments experience rather
intense IR background radiation, from sunlight
and indoor lighting.
This ambient radiation appears as noise in an IR
receiver, requiring the use of transmitters of
higher power than would otherwise be required
and also limiting the range.
However, increases in transmitter power are
limited by concerns of eye safety and excessive
power consumption.
IR – Transmission Techniques
There are three alternative transmission
techniques commonly used for IR data
 Direct
beam IR
 Omnidirectional
 Diffused
IR – Transmission Techniques
Directed-beam IR can be used to create pointto-point links.
In this mode, the range depends on the emitted
power and on the degree of focusing.
A focused IR data link can have a range of
An IR link can be used for cross-building
interconnect between bridges or routers located
in buildings within a LOS of each other
Token Ring LAN using Point-toPoint IR link
IR – Transmission Techniques
An omnidirectional configuration involves a
single BS that is within LOS of all other stations
on the LAN.
Typically, this station is mounted on the ceiling.
And acts as a multi-port repeater.
The ceiling transmitter broadcasts an
omnidirectional signal that can be received by all
of the other IR transceivers in the area.
These other transceivers transmit a directional
beam aimed at the ceiling base unit.
Omnidirectional IR Transmission
using LOS
IR – Transmission Techniques
In a diffused network, all of the IR
transmitters are focused and aimed at a
point and a diffusely reflecting ceiling.
IR radiation striking the ceiling is
reradiated omni-directionally and picked
up by all of the receivers in the area.
Omnidirectional IR Transmission Diffuse Networks
Spread Spectrum LANs
A SS wireless LAN makes use of a
multiple cell arrangement.
Adjacent cells make use of different centre
frequencies within the same band to avoid
Spread Spectrum LANs
Within a given cell, the topology can be either
hub or peer to peer.
In the former, the hub is typically mounted on the
ceiling and connected to a backbone wired LAN
to provide connectivity to stations attached to the
wired LAN and to stations that are part of
wireless LANs in other cells.
A peer to peer topology is one in which there is
no hub. A MAC algorithm such as CSMA is used
to control access. This topology is appropriate
for ad hoc LANs.
Transmission Issues in Wireless
A desirable, though not necessary, characteristic
of a wireless LAN is that it be usable without
having to go through a licensing procedure.
The licensing regulations differ from one country
to another, which complicates this objective.
Narrowband Microwave LANs
The term “narrowband microwave” refers to the
use of a microwave RF band for signal
transmission, with a relatively narrow bandwidth
(just wide enough to accommodate the signal).
Until recently, all narrowband microwave LAN
products have used a licensed microwave band.
More recently, at least one vendor has produced
a LAN product in the ISM band.
Licensed Narrowband RF
Microwave radio frequencies usable for voice,
data and video transmissions are licensed and
coordinated within specific geographic areas to
avoid potential interference between systems.
A narrowband scheme typically makes use of
the cell configuration.
Adjacent cells use non-overlapping frequency
bands within the overall band.
Licensed Narrowband RF
Licensed narrowband LAN guarantees
interference-free communication.
Licensed spectrum gives the license holder a
legal right to an interference free data
communications channel.
Users of an ISM-band LAN are at risk of
interference disrupting their communications, for
which they may not have a legal remedy.
Unlicensed Narrowband RF
In 1995, “RadioLAN” became the first vendor to
introduce a narrowband wirelss LAN using the
unlicensed ISM spectrum.
This spectrum can be used for narrowband
transmission at low power (<0.5 W).
The product operates at 10 Mbps in the 5.8 GHz
band and has a range of 50 m in a semi-open
office and 100 m in an open office.
Unlicensed Narrowband RF
The product makes use of a peer-to-peer
configuration with an interesting feature:
 As
a substitute for a stationary hub, it automatically
elects one node as the Dynamic Master, based on
parameters such as location, interference, and signal
 The identity of the master can change automatically
as conditions change.
The LAN also includes a dynamic relay
functions, which allows each station to act as a
repeater to move data between stations that are
out of range of each other.

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