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BEYOND WIRELESS
Posted Date: 22 Mar 2008 Resource Type: Articles/Knowledge Sharing Category: Computer & Technology
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Posted By: arunkumar Member Level: Gold
Rating:  Points: 5
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The need for wireless communications
arose when people started using
more than one electronic device to
store personal data, such as contacts, photos,
appointments, etc. All this data was
stored all over the place: in laptops, PDAs,
mobile phones, and even in wrist watches.
Using wires for communication beats the
very purpose of mobile devices. So, the first
wireless standard to gain acceptance in the
market was infra-red. Then Bluetooth
arrived, and it is slowly but steadily entering
the market. These standards, however,
had low data transfer rates; so a new standard
named WiFi was born. It supports
high data rates and is used primarily in laptops
for network access.
Infra-red
Infra-red is similar to normal light and is
just another electromagnetic wave. It is
called infra-red because the frequency and
wavelength takes it below the red end of
the spectrum, making it invisible to
humans. Infra-red is the simplest and one
of the most prevailing wireless communication
standards. It uses an infra-red beam
to transfer data between devices. Remote
controls for various devices—televisions,
CD players, air conditioners, etc.—all use
this technology. IrDA (Infra-red Data Association)
is an organisation that maintains
the standard for communication via Infrared,
and there are currently more than 100
member companies and many more
devices that have an IrDA port.
Like any other standard, IrDA is a
cable replacement standard; it creates a
virtual wire using an infra-red beam to
transfer data. It is a half-duplex, point to
point protocol—only one device can
transmit data at a time. Also, the devices
need to have a clear line of sight between
their ports to function—the IrDA ports of
the communicating devices must face
each other without obstructions. The two
devices that take part in this communication
are called primary and secondarylook for IrDA-enabled devices through a
process called discovery. Once it finds a
secondary device, two random 32-bit
addresses are generated for each device—
even if the same address is generated for
both devices, there are mechanisms to
resolve them. Once the addresses are
assigned, a connection is established and
the devices figure out each other’s capabilities:
supported data rates, data size,
turnaround time, etc. After these rituals,
the data transfer commences at the highest
supported speed. A ‘permission to
transmit token’ is used, which ensures
only one device talks at a time. Only the
device possessing the token can transmit
data. Though more than two devices can
communicate with each other, only one
can talk.
There is no fixed speed for IrDA
devices, but all devices need to be capable
of at least 9.6 Kbps, as this is the speed at
which initial discovery and negotiation
takes place. Currently, the maximum supported
speed is 4 Mbps, but a 16 Mbps standard
is in the pipeline. IrDA devices can be
up to a metre apart, but work best within a
range of 5 cm to 60 cm. As IrDA communication
happens only by conscious effort
of a user, the range is short and security
isn’t considered an issue—there isn’t any in
the standard.
Bluetooth
Bluetooth is designed to be a personal area
network, where participating entities are
mobile and require sporadic communication
with others. It’s omni directional; i.e.,
it doesn’t have the line of sight limitation
like infra-red does. Ericsson started the
wcork on Bluetooth, and being a Denmarkbased
company, they named it after the
Danish king Harold Bluetooth. Later, a
Bluetooth special interest group, comprising
companies such as
Nokia and Ericsson, was
formed to oversee the
development and standardisation
of the technology.
Bluetooth operates in
the 2.4 GHz area of the
spectrum, and provides a
range of 10 metres. It
offers transfer speeds of
around 720 Kbps. When
Bluetooth devices are connected to each
other, they form a network called piconet.
Each piconet can have a master, and up to
seven slaves. Communication takes place
only between the master and the slave. If
the slaves need to communicate with each
other, they can’t route via the master—they
have to form a separate piconet. A device
can participate in more than one piconet at
any given time—it can be a slave in one
piconet, and a master in another. Such
networks are called scatternets, where different
piconets are connected using a common
device.hopping. The devices frequently change
the frequency at which they operate—as
often as 1,600 times a second. This prevents
interference with other devices that
may be operating at the same frequency.
When a piconet is formed, the master
informs all the slaves of the frequency hopping
sequence, and the slaves follow these
instructions.
To conserve power, devices can go
into three modes when they aren’t actively
involved in a piconet. In increasing
order of power consumption, they can be
in park mode, hold mode and sniff mode.
All the slave devices in a piconet are
assigned an active member address
(AM_ADDR). When a slave enters the park
mode, it will give up its AM_ADDR and
get a park address, PM_ADDR. The parked
devices will listen to the master at regular
intervals, allowing them to conserve
power, and will yet be able to rejoin a
network when required. When a device
enters hold mode, it is assigned a hold
timer and no data transfer takes place
until the timer ends. In sniff mode, the
device retains its AM_ADDR, but reduces
its role in the piconet, thus conserving
power. This mode is flexible, and parameters
such as sniffing interval can be programmed.
One of Bluetooth’s advantages is that
it can handle both data (asynchronous)
and voice (synchronous), which others
such as infra-red can’t. One must understand
that the requirements of a voice and
data channel are very different. A data
channel needs to be strictly error free
while it can tolerate timing errors, for a
voice channel it is exactly the opposite.
Both synchronous and asynchronous
channels can be handled simultaneously.
This has lead to products such as Bluetooth
headsets for mobile phones; these
headsets allow users to talk via the cordless
headsets while the mobile phone
resides in their pockets.
Every Bluetooth device has a 48-bit
BDA (Bluetooth Device Address) burned
into its ROM. This address can’t be easily
changed by the user. Similarly, each device
can be given a user-friendly name such as
‘Sachin’s mobile’. These names will make
it easy for the user to identify a device in
a piconet. A Bluetooth device can be set to
periodically scan for other devices in its
vicinity, or users can perform manual
scans. After a scan is performed, a list of all
Bluetooth devices detected is shown. Each
device in the list will have the name
assigned by its respective owner, and the
user can select a device and communicate
with it. A Bluetooth
device will show up in a scan only
when it is in discoverable mode.
Discoverable mode should be
turned off to prevent bluejacking—
sending anonymous messages to
users via Bluetooth.
WiFi
Bluetooth is a convenient way of communication
for a personal network, but it is
not suitable as an LAN replacement, or for
perennial data transfer. But the growing
popularity of laptops necessitated the need
for a wireless standard that would allow
people to walk around with their laptops,
and still be connected to a network. WiFi,
an abbreviation for Wireless Fidelity, is a
wireless standard that promises mobility
while offering data rates comparable to
those of a wired LAN. WiFi is a collection
of standards ratified by the IEEE (Institute
of Electrical and Electronics Engineers). In
1997, IEEE approved the 802.11 standard,
which laid down the specifications for
wireless LAN. Subsequently, revisions were
made to this standard and these resulted in
three other standards, namely 802.11a,
802.11b and 802.11g. The entire family of
802.11 standards are collectively called
WiFi. The letters appended to the standard
To accommodate interference from
other signals, Bluetooth uses frequency
devices. Primary devices are the ones that indicate the task group that proposed the
modifications For example, task group ‘b’
put forth the 802.11b standard.
A wireless network can work in two
modes; namely ad-hoc and base station. In
an ad-hoc (also called peer to peer) network,
several computers equipped with
wireless hardware come together to form a
network. All the computers communicate
with each other directly. Unless one of the
computers participating in the ad-hoc network
is connected to a wired network, no
system can gain access to a wired network.
Three friends meeting in a park and sharing
files between their wireless laptops is
an example of an ad-hoc network.
The other type of network involves the
use of an access point (also called base station).
The access point acts as the central
point of the network, and is the link
between the wired and wireless networks.
An access point can be a dedicated hardware
system, or just software running on a
system. In airports and coffee shops, mainly
in the US and Europe, access points are
provided so that customers can walk into
the shops and start using the Internet from
their laptops. It may not be possible to cover
an entire airport, or an office, with a single
access point. In such cases, more than one
access point is required. Users can take their
devices and roam across different access
points. All access points have a SSID (Service
Set ID) assigned to them, as do all the
clients connected to them. When an access
point is installed, the default SSID must be
changed to increase security, and to prevent
users from connecting to some other spot.
The name WiFi was first given to the
802.11b standard. Approved in 1999,
802.11b offered extensions to the original
802.11 that improved the highest data
rate from 2 Mbps to 11 Mbps. It operated
in the 2.4 GHz frequency range, like as the
original and has a range of about 300 feet.
Keep in mind that range and speed are
interrelated—as the wireless clients move
further away from each other, the speed
is bound to deteriorate.
802.11a is a standard that operates at a
frequency of 5 GHz, and hence it is incompatible
with 802.11b. So, a laptop with an
802.11b card can’t connect to an access
point running on 802.11a. Also, the range
of 802.11a is lower than that of 802.11b;
hence, more access points are needed to
cover a large area. On the brighter side, it
offers a much higher throughput of 54
Mbps. Interference is a problem with wireless
networks. Bluetooth devices and cordless
phones are some sources of this interference.
802.11a is less susceptible to interference
when compared to other standards
such as 802.11b.
To solve the limitation of ‘b’ and at the
same time provide the speed of ‘a’, the
802.11g was introduced. 802.11g offers
speeds of 54 Mbps, while maintaining compatibility
with 802.11b networks, so a laptop
with a 802.11g card will be able to use
a 802.11b access point.
WiFi has a basic level of security provided
at the physical level, called WEP
(Wired Equivalent Privacy). But vulnerabilities
in WEP have already been discovered,
and it can’t be used as the only security
mechanism. However, it is recommended
that WEP be turned on, as it provides
a basic level of security. Additional
levels of security can be provided at the
software level in the form of firewalls. The
level of security offered by WEP depends
on the number of bits used for the encryption
key; almost all WiFi products come
with a 40-bit encryption key. A 104-bit
encryption key is also available, and it is
recommended that the largest available key
should be used.
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