Реферат: Wireless connectivity
Реферат: Wireless connectivity
Wireless Connectivity
Content
Abstract
Introduction
Operation
Wireless Application Design
Caching
Prefetching
Data
reduction
Mobile
WWW Browsers
Active documents
Dynamic
URLs
Data over Cellular links
Radio-based wireless
connectivity
IEEE 802.11 protocol
Interworking Units
for wireless connectivity
Internet Mobile Host Protocol
IP
Tunneling
Abstract
Traditional
networking technologies offer tremendous capabilities from an office or home
via the Web. But, limitations to networking through the use of wired-based
systems exist because you cannot utilize these network services unless you are
physically connected to the system. As mobile computing becomes more prevalent,
systems and applications must deal with scarcity of resources such as
bandwidth. Mobile devices and wireless workstations should handle some of the
work that has been traditionally carried-out by the network through techniques
such as document partitioning. Dynamic documents can also be used to cache and
prefetch documents while the network connection is not being utilized fully.
Meanwhile,
the need for higher speed wireless connections is growing with multimedia rich
contents on the World Wide Web (WWW). The IEEE 802.11 protocol and the Medium
Access Control part of the protocol (DFWMAC) will allow wireless networks to
operate at high data rates (1 to 20 Mbps). Furthermore, the 802.11 only effects
the bottom two layers of the OSI's seven layered architecture; hence, through
an access point (Router), wireless packets are routed to the Web.
Introduction
Wireless
LANs will provide the first layer of connectivity between mobile users and the
global information infrastructure. Wireless devices such as Personal Digital
Assistants (PDAs) and Notebooks will be an extension of the Web. The user
should not know nor care whether the information travels over a wire or a radio
frequency. Depending on the power of the transmitters and the sensitivity of
the receivers, wireless devices may become the first truly universal form of
virtual LAN. By mixing the wireless Networks with other wireless communication
technologies such as cellular and satellite, the user can have full
connectivity at all times and more importantly everywhere on the globe.
Wireless
connectivity to the web can also be achieved through the use of existing
cellular telephone links. Using Spread Spectrum Technologies (SST) such as
time-division multiple access (TDMA), code-division multiple access (CDMA) and
extended time-division multiple access (ETDMA) has allowed the cellular links
to carry more information and as a result better suited for data transmission.
Although the overhead in cellular data transmission is somewhat high, but data
reduction techniques, and caching is used to reduce network latency.
With
the Introduction of PDA, people began to see the natural progression of
Wireless technology into these devices. However, the current state of these
devices has obvious limitations. Computational power, storage, communication
bandwidth, display size and power consumption are just a few of these
limitations. Nevertheless, presently such devices are running Web browser, mail
clients and etc.Presently there is a variety of pen based computer systems like
palmtops, notebooks and different versions of what John Sculley, Apple's vice
president in 1992 introduced as a PDA.
Personal
communication is the primary motivation for wireless connectivity, but in addition,
wireless users need access to on-line information in real time. There are three
reasons why users need to be connected to the Web. First, it is often
difficult, if not impossible, to determine the data of interest ahead of time
and download it to the hand-held device. Second, even then, space limitations
may prevent caching of all data. Finally, some data changes dynamically with
time such as weather forecast, or stock market activities. (Watson, 1994)
The
current application environment is ill-suited for the wireless Web, the wired
web squanders bandwidth through unusable information on the client's side. In
the wired world these inefficiencies amount to only milliseconds, but as the
bandwidth is reduced over wireless links, milliseconds can add up to seconds
and perhaps time-outs by the underlying protocols such as TCP/IP. As a result
various groups have proposed new HyperText Mark-up Language (HTML) or new
protocols such as HTTP+. But these avenues of solutions are rigid and the need
for standardization is greater than a temporary increase in throughput for a
particular scheme.
Operation
Similar
to any transmission system, a wireless system needs a transmitter, a receiver
and a transmission medium. In a wireless system, the transmission medium is air
rather than the cables used by conventional wired systems. The use of air as a
transmission medium utilizes two major spectra: infrared and radio frequency.
The
key difference between the use of infrared and radio frequency is the support
of roaming. Infrared is a line-of-sight technology. There has to be a direct
line of sight or at least a surface to bounce the waves from the transmitter to
the receiver. On the other hand, radio frequency systems can penetrate through
objects such as walls and doors in most office buildings; hence their
popularity in present wireless systems. FCC rules allow only small sections of
the electromagnetic spectrum (figure below) to be used for wireless data
networks; thus techniques are needed to avoid interference from other devices
that share the space or perhaps multiple stations using the same frequency.
A
technique developed by the military in the 1970s to help secure transmissions
offers a way around this problem. This technique is called Spread Spectrum
Technology (SST). It involves spreading transmissions across a range of
frequencies, rather than transmitting on one frequency all the time.
One
approach known as Frequency-Hopping Spread Spectrum (FHSS) involves dividing a
range of the radio spectrum into individual channels, each on a specific
frequency. A transmitter can hop from one channel to the next and if the
receiver is aware of the hopping pattern of the transmitter, it can follow the
pattern and receive the information. The second method of spread spectrum is
Direct Sequence Spread Spectrum (DSSS). The source data to be transmitted is first
exclusive ORed with a pseudorandom binary sequence. The bits making up the sequence
are random but the same sequence is made much larger than the source data rate.
When this data is modulated and transmitted it occupies a wider frequency band
than the original source data bandwidth. This would make the signal appear as
noise to any other devices using the same frequency spectrum. All the members
of this wireless system know the binary sequence being used .(Halsall, 1996) Thus,
all receivers first search for the known preamble sequence, once it has been
recognized, the receivers start to interpret the bit stream.
FCC
rules for DSSS transmission requires 10 or more redundant data bits to be added
to each signal. This limits the maximum throughput of DSSS transmitters to
approximately 2 Mbps when using the 902-MHz band, and approximately 8 Mbps in
the 2.4-GHz band.
Wireless Application Design
Designing
a web application for a wireless node is different from designing a web application
for a workstation. Bandwidth is a precious resource in the wireless domain and
it must be utilized in the most efficient fashion. Research focuses on
streamlining applications to make the best use of the available bandwidth.
These options include using dynamic documents which use the resources of the
mobile node itself to generate parts of a document or partitioning the application
between a client and the server.
Dynamic
documents can address the variable resources requirement of mobile computers
accessing the Web. Dynamic documents are programs executed by programs such as
Web browsers in order to generate the actual information displayed to the user.
Execution of a dynamic document cause the client to perform any number of
actions in order to generate a final presentation to the user. Dynamic
documents are flexible enough to address many mobile computing resource
constraints. Documents can be customized at the client depending on available
resources. (Kaashoek, 1994)
Application
partitioning can also be used over a wireless link for more effective use of
the wireless link. Much like a client/sever system, applications and their
functionality can be divided into different parts. the boundaries between how
much of the application should be run on the client side vs. the server side
can be determined dynamically and based upon the availability of the bandwidth.
The data and their functions are packaged into hyperobjects. The purpose of
hyperobjects is to expose a certain level of application structure and semantics
to the system in a uniform and manageable way. The system will use this
hyperobject structure, along with observations of access patterns to make
informed decisions. (Watson, 1995)
Partitioning
documents are combined with several other well-known techniques to increase the
effectiveness of wireless clients such as browsers.
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