INTRODUCTION
Mobile computing can be
defined as computing environment over physical mobility. The user of a mobile
computing environment is able to access data, information or other logical
objects from any device in any network while on the move. Mobile computing
system allows a user to perform a task from anywhere using computing device in
public (the Web), corporate (business information) and personal information
spaces. While on the move, the preferred device is a mobile device, while back
at home or in the office the device could be a desktop computer. To make the
mobile computing environment ubiquitous, it is necessary that the communication
bearer is spread over both wired and wireless media. Be it for the mobile
workforce, holidaymakers, enterprises or rural population, the access to
information and virtual objects through mobile computing are absolutely
necessary for optimal use of resource and increased productivity.
The mobile computing
environment should provide the functions — User mobility, network mobility,
bearer mobility, device mobility, session mobility, service mobility and
finally host mobility.
Mobile computing
environment is made up of three main components: a computer, a network and
co-ordination software that ties them together. In mobile networking, computing
activities are not disrupted when the user changes the computer’s point of
attachment to the internet. Instead, all the needed reconnection occurs
automatically and non-interactively.
NETWORK COMPONENTS
Router
A
Router is a device that
transfers data from one network to another in an intelligent way. It has the
task of forwarding data packets to their destination by the most efficient
route.
In
order to do this, it holds a table in memory that contains a list of all the
networks it is connected to, along with the latest information on how busy each
path in the network is, at that moment. This is called the 'routing table'.
When
a data packet arrives, the router does the following:-
o
Reads the data packet's destination
address
o
Looks up all the paths it has
available to get to that address.
o
Checks on how busy each path is at
the moment
o
Sends the packet along the least
congested (fastest) path
o
Exchange Protocol information across
networks
o
Filter traffic - useful for
preventing hacker attacks for example
Routers
operate at the network level of the OSI model.
Repeater
All signals fade as they travel from
one place to another. Each type of network cable has a maximum useable length.
Beyond that length, the signal becomes too weak to be useful.
Computers
on a real network can easily be more than 200 meters apart. Therefore the
network cable is split up into segments. Each segment is less than the maximum
length allowed. Joining the segments together is a device known as a
'Repeater'.
A repeater boosts the signal back to its correct level.
Bridges
A Bridge joins two
networks together so as far as data packets are concerned it looks like one
large network.
A bridge is not as capable as a Router - but it
is less expensive. Both networks have to be using the same protocol.
Hub
To
allow the Star and Tree network topologies to work properly, each computer must
be able to send data packets to any other computer on the network.
The network 'Hub' allows computers to share data
packets within a network.
Each
computer will be connected to a single 'port' on the hub. So in an 8 port hub,
one is able to connect up to eight computers together.
Switches
A network cable can only have one
data packet in it at any instant. So if two or more computers want to place a
data packet on to the network at exactly the same time, then a 'data collision'
will take place.
The
network protocol is set up to deal with this but it can handle only a few
devices. A switch is used to solve this problem.
A
switch has a number of ports and it stores the addresses of all devices that
are directly or indirectly connected to it on each port. As a data packet comes
into the switch, its destination address is examined and a direct
connection is made between the two machines
Here
AE and BD need to communicate. As such, links are established only between them
by the switch. This also prevents collision.
Gateway
A gateway
converts the data passing between dissimilar networks so that each side can
communicate with each other i.e. converts data into the correct network
protocol.
The gateway is a mixture of hardware components
and software. This is unlike a standard 'Bridge' which simply joins two
networks together that share the same protocol.
Filters
For
keeping the network secure, filtering is essential.
A filter can be set up so that it accepts data
packets coming from that particular laptop. Other filtering rules would block
unwanted packets trying to come in. A filter can also prevent data packets from
leaving the company network.
A filter is an essential component of a
'Firewall'.
Modem
A modem converts the digital data from the
computer into a continuous analog wave form that the telephone system is
designed to deal with (Modulation). The reason for this is that the
telephone system was originally designed for the human voice i.e. continuous
signals. The modem also converts the analog signal from the telephone
network back into digital data that the computer can understand (Demodulation).
Network
Card
Network
cards are required in every machine connected to the network. They allow
the signal from the network to be transmitted to the machine – this could be
via a fixed cable, infra red or radio waves.
The vision of wireless communications supporting information exchange
between people or devices is the communications frontier of the next few
decades, and much of it already exists in some form.
This wireless communication allows multimedia communication from
anywhere in the world using a small handheld device or laptop. Wireless
networks connect palmtop, laptop, and desktop computers anywhere within an
office building or campus, as well as from the corner cafe.
In the home these networks enable a new class of intelligent electronic
devices that can interact with each other and with the Internet in addition to
providing connectivity between computers, phones and security/monitoring
systems. Such smart homes can also help the elderly and disabled with assisted
living, patient monitoring, and emergency response.
Video teleconferencing can take place between buildings that are blocks
or continents apart, and these conferences can include travelers as well.
Wireless video enable remote classrooms, remote training facilities, and remote
hospitals anywhere in the world.
Wireless sensors have an enormous range of both commercial and military
applications.
Commercial applications include monitoring of fire hazards, hazardous
waste sites, stress and strain in buildings and bridges, carbon dioxide
movement and the spread of chemicals and gasses at a disaster site. These
wireless sensors self-configure into a network to process and interpret sensor
measurements and then convey this information to a centralized control
location.
Military applications include identification and tracking of enemy
targets, detection of chemical and biological attacks, support of unmanned
robotic vehicles, and counter-terrorism.
HISTORY
The first wireless networks were developed in the
Pre-industrial age. These systems transmitted information over line-of-sight
distances using smoke signals, torch signaling, flashing mirrors, signaling
flares, or semaphore flags. Observation stations were built on hilltops and
along the roads to relay these messages over large distances.
These early communication networks were replaced
first by the telegraph network (invented by Samuel Morse in 1838) and later
replaced by telephone.
In 1895, a few decades, after the telephone was
invented, Marconi demonstrated the first radio transmission from the Isle of
Wight to a tugboat 18 miles away, and radio communication was born. Radio
technology advanced rapidly to enable transmissions over larger distances with
better quality, less power, and smaller, cheaper devices, thereby enabling
public and private radio communications television, and wireless networking.
Early radio systems transmitted analog signals. Today most radio systems
transmit digital signals composed of binary bits, where the bits are obtained
directly from a data signal or by digitizing an analog signal. A digital radio
can transmit a continuous bit stream or it can group the bits into packets. The
latter type of radio is called a packet radio and is characterized by heavy
transmissions: the radio is idle except when it transmits a packet.
The first network based on packet radio, ALOHANET, was developed at the
University of Hawaii in 1971. This network enabled computer sites at seven
campuses spread out over four islands to communicate with a central computer on
Oahu via radio transmission. The network architecture used a star topology with
the central computer at its hub. Any two computers could establish a
bi-directional communications link between them by going through the central
hub. ALOHANET incorporated the first set of protocols for channel access and
routing in packet radio systems, and many of the underlying principles in these
protocols are still in use today.
The U.S. military tried to combine packet data and broadcast radio
inherent to ALOHANET. Throughout the 1970’s and early 1980’s the Defense
Advanced Research Projects Agency (DARPA) invested significant resources to
develop networks using packet radios for tactical communications in the
battlefield but the resulting networks fell far short of expectations in terms
of speed and performance. These networks continue to be developed for military
use. Packet radio networks also found commercial application in supporting
wide-area wireless data services.
The introduction of wired Ethernet technology in the 1970’s steered many
commercial companies away from radio-based networking. Ethernet’s 10 Mbps data
rate far exceeded anything available using radio, and companies did not mind
running cables within and between their facilities to take advantage of these
high rates.
The current generation of wireless LANs, based on the family of IEEE
802.11 standards, has better performance, although the data rates are still
relatively low and the coverage area is still small. Wired Ethernets today offer
data rates of 100 Mbps, and the performance gap between wired and wireless LANs
is likely to increase over time without additional spectrum allocation. Despite
the big data rate differences, wireless LANs are becoming the preferred
Internet access method in many homes, offices, and campus environments due to their convenience and freedom from wires.
By far the most successful application of wireless networking has been
the cellular telephone system. The roots of this system began in 1915, when
wireless voice transmission between New York and San Francisco was first
established. In 1946 public mobile telephone service was introduced in 25
cities across the United States. The inefficient use of the radio spectrum
coupled with the state of radio technology at that time severely limited the
system capacity. A solution to this capacity problem emerged during the 50’s
and 60’s when researchers at AT&T Bell Laboratories developed the cellular
concept. The first analog cellular system deployed in Chicago in 1983 was
already saturated by 1984, at which point the FCC increased the cellular
spectral allocation from 40 MHz to 50 MHz.
The second generation of cellular systems, first deployed in the early
1990’s, were based on digital communications. The shift from analog to digital
was driven by its higher capacity and the improved cost, speed, and power
efficiency of digital hardware. While second generation cellular systems
initially provided mainly voice services, these systems gradually evolved to
support data services such as email, Internet access, and short messaging. The
most compelling feature of these systems is their ubiquitous worldwide
coverage, especially in remote areas or third-world countries with no landline
or cellular system infrastructure.
A wireless network consists of
several components that support communications using radio or light waves
propagating through an air medium. Some of these elements overlap with those of
wired networks, but special consideration is necessary for all of these
components when deploying a wireless network.
Users
·
A user can be anything that directly utilizes
the wireless network. One of the most common types of user is a person. For
example, a business traveler accessing the Internet from a public wireless LAN
at an airport is a user.
·
The user initiates and terminates use of a
wireless network. Typically, a user operates a computer device, which often performs a
variety of application-specific functions in addition to offering an interface
to the wireless network.
·
Users of wireless networks tend to be mobile,
constantly moving throughout a facility, campus, or city. Mobility is one of
the most prominent benefits of deploying a wireless network.
Computer
Devices
·
Many types of computer devices, sometimes
referred to as clients, operate on a wireless network. Some computer devices
might be specifically designed for users, whereas some computer devices are end
systems. In generally, any computer device might communicate with any other computer
device on the same wireless network.
NICs
·
The network interface card provides the
interface between the computer device and the wireless network infrastructure.
·
Wireless
network standards define how a wireless NIC operates. For example, a wireless
LAN NIC might implement the IEEE 802.11b standard.
·
Wireless
NICs also comply with a specific form factor, which defines the physical and
electrical bus interface that enables the card to communicate with the computer
device.
Air
Medium
·
Air
provides a medium for the propagation of wireless communications signals, which
is the heart of wireless networking. Air is the conduct by which information
flows between computer devices and the wireless infrastructure.
·
Wireless
information signals also travel through the air, but they have special
properties that enable propagation over relatively long distances. Wireless
information signals cannot be heard by humans, so it's possible to amplify the
signals to a higher level without disturbing human ears.
·
The
quality of transmission, however, depends on obstructions in the air that
either lessen or scatter the strength and range of the signals. Rain, snow,
smog, and smoke are examples of elements that impair propagation of wireless
communications signals.
Wireless
Network Infrastructures
The infrastructure of a wireless network
interconnects wireless users and end systems. The infrastructure might consist
of base stations, access controllers, application connectivity software, and a
distribution system. These components enhance wireless communications and
fulfill important functions necessary for specific applications.
Base
Stations
·
The base station is a common infrastructure
component that interfaces the wireless communications signals traveling through
the air medium to a wired network—often referred to as a distribution system.
·
Therefore, a base station enables users to
access a wide range of network services, such as web browsing, e-mail access,
and database applications. A base station often contains a wireless NIC that
implements the same technology in operation by the user's wireless NIC.
·
Base stations go by different names, depending
on their purpose. An access point, for instance, represents a generic base
station for a wireless LAN.
·
Residential
gateways and routers are more advanced forms of base stations that enable
additional network functions. The gateway might have functions, such as access
control and application connectivity, that better serve distributed, public
networks.
·
On
the other hand, a router would enable operation of multiple computers on a
single broadband connection. A base station might support point-to-point or
point-to-multipoint communications.
Access
Controllers
·
In the absence of adequate security, quality of
service (QoS), and roaming mechanisms in wireless network standards, companies
offer access-control solutions to strengthen wireless systems.
·
The key component to these solutions is an
access controller, which is typically hardware that resides on the wired
portion of the network between the access points and the protected side of the
network. Access controllers provide centralized intelligence behind the access
points to regulate traffic between the open wireless network and important
resources.
·
Access controllers apply to a wide range of
applications. In a public wireless LAN, for example, an access controller
regulates access to the Internet by authenticating and authorizing users based
on a subscription plan.
·
Access controllers often provide port-based
access control, allowing administrators to configure access to specific
applications on a per-user basis.
·
The use of an access controller reduces the need
for smart access points, which are relatively expensive. These access
controllers have many advantages: Lower Costs, Open Connectivity and
Centralized Support. Access controllers provide following features:
Authentication, Encryption, Subnet Roaming, Bandwidth Management.
Application
Connectivity Software
·
Web surfing and e-mail generally perform well
over wireless networks. All it takes is a browser and e-mail software on the client
device. Users might lose a wireless connection from time to time, but
the protocols in use for these relatively simple applications are resilient
under most conditions.
·
Beyond these simple applications, however,
special application connectivity software is necessary as an interface between
a user's computer device and the end system hosting the application's software
or database.
Ã
Wireless Middleware
·
Wireless middleware software provides
intermediate communications between user computer devices and the application
software or database located on a server.
·
The middleware—which runs on a dedicated
computer (middleware gateway) attached to the wired network—processes the
packets that pass between the user computer devices and the servers.
·
The middleware software primarily offers
efficient and reliable communications over the wireless network while
maintaining appropriate connections to application software and databases on
the server through the more reliable wired network. Sometimes this is referred
to as session persistence.
Distribution
System
·
A wireless network is seldom entirely free of
wires. The distribution system, which often includes wiring, is generally
necessary to tie together the access points, access controllers, and servers.
In most cases, the common Ethernet comprises the distribution system.
·
The IEEE 802.3 standard is the basis for
Ethernet and specifies the use of the carrier sense multiple access (CSMA)
protocol to provide access to a shared medium, such as twisted-pair wiring,
coaxial cable, and optical fiber.
WPAN describes an application of wireless technology
that is intended to address usage scenarios that are inherently personal in
nature. WPANs are short-range networks that use Bluetooth technology.
They are commonly used to interconnect compatible
devices near a central location, such as a desk. A WPAN has a typical range of
about 30 feet. The emphasis is on instant connectivity between devices that
manage personal data or which facilitate data sharing between small groups of
individuals.
An example might be synchronizing data between a PDA
and a desktop computer or spontaneous sharing of a document between two or more
individuals. Wireless communication adds
value for these types of usage models by reducing complexity (i.e. eliminates
the need for cables).
For example, both Bluetooth radio and invisible
Infrared light provides a WPAN for interconnecting a headset to a laptop.
ZigBee also supports WPAN applications.
WLAN
WLAN on the other hand is more focused on
organizational connectivity not unlike wire based LAN connections.
The intent of WLAN technologies is to provide
members of workgroups access to corporate network resources be it shared data,
shared applications or e-mail but do so in way that does not inhibit a user’s
mobility.
WLAN are wireless networks that use radio waves. The
backbone network usually uses cables, with one or more wireless access points
connecting the wireless users to the wired network.
The range of a WLAN can be anywhere from a single
room to an entire campus. The emphasis is on a permanence of the wireless
connection within a defined region like an office building or campus.
WMAN
Wireless
Metropolitan Area Networks are a type of wireless network that connects several
Wireless LANs. WiMAX is a type of Wireless MAN and is described by the IEEE
802.16 standard
WWAN
Whereas WLAN addresses connectivity
within a defined region, WWAN addresses the need to stay connected while
traveling outside this boundary. Wireless wide area networks are wireless
networks that typically cover large areas, such as between neighboring towns
and cities, or city and suburb.
WWANs are created through the use of
mobile phone signals typically provided and maintained by specific mobile phone
(cellular) service providers. These networks can be used to connect branch
offices of business or as a public internet access system.
The wireless connections between
access points are usually point to point microwave links using parabolic
dishes. A typical system contains base station gateways, access points and
wireless bridging relays. Other configurations are mesh systems where each
access point acts as a relay also.
