Networks Essay, Research Paper
Survey of Wireless Computing
School Of Computer Science
Florida International University
Abstract
Wireless technology can provide many benefits to computing including faster response to queries,
reduced time spent on paperwork, increased online time for users, just-in-time and real time control,
tighter communications between clients and hosts. Wireless Computing is governed by two general
forces: Technology, which provides a set of basic building blocks and User Applications, which
determine a set of operations that must be carried out efficiently on demand. This paper summarizes
technological changes that are underway and describes their impact on wireless computing development
and implementation. It also describes the applications that influence the development and
implementation of wireless computing and shows what current systems offer.
1 Introduction
Wireless computing is the topic of much conversation today. The concept has been around for some
time now but has been mainly utilizing communication protocols that exist for voice based
communication. It is not intended to replace wired data communication but instead to be utilized in
areas that it would be otherwise impossible to communicate using wires. Only recently has the industry
been taking steps to formulate a standard that is more suitable to data transmission. Some the problems
to be overcome are:
(1) Data Integrity – relatively error free transmission,
(2) Speed – as close as possible to the speed of current wired networks,
(3) Protection – making sure that the data now airborne is encoded and cannot be tapped by
unwelcome receivers,
(4) Compatibility – ensuring that the many protocols that sure to be created subscribe to a standard
to allow inter-operability,
(5) Environmentally safe – strengths of electromagnetic radiation must be kept within normal levels.
In our study of the theories and implementation concerns of wireless computing, we found that it is
being treated in an object oriented fashion. Scientists and development crews, including the IEEE, are
doing their best to implement wireless connectivity without changing the existing computer hardware.
As a result, a lot of focus is on using existing computer hardware and software to convert data to a
format compatible with the new hardware which will be added to the computer using ports or PCMCIA
connections that already exist. This means that wireless communication will be transparent to the user
if and when wireless computing is utilized on a wide scale.
Wireless computing applications covers three broad areas of computing today. Replacement of normal
wired LAN’s need to retain the speed and reliability found in wired LAN’s. Creation of semipermanent
LAN’s for quick and easy setup without the need for running wires. This would be necessary for events
such as earthquakes. The last category is that of mobile computing. With advent of PCMCIA cards,
notebook computers are being substituted for regular desktop machines with complete connectivity of
the desktop machine. However, you lose the connectivity when out of the office unless you have a
wireless means of communicating.
On the compatibility issue, the ability to mix wireless brands on a single network is not likely to come
soon. The IEEE Standards Committee is working on a wireless LAN standard — 802.11, which is an
extension of the Ethernet protocol. Because the field of wireless communication is so broad, the IEEE
was not able to set a standard by the time private researchers were ready to test their theories hoping to
set the standard for others to follow.
2 Methods
There are a few methods of wireless communication being theorized and tested.
(1) Radio: This is the method that makes use of standard radio waves in the 902 MHz to 928 MHz
frequency range. Although these frequencies are well used, methods have been developed to
ensure data integrity. Spread spectrum transmission of data is a method where the transmitter
will send information simultaneously out over many frequencies in the range increasing the
change that all data will eventually reach the receiver. Frequency hopping is an additional
measure that also enables data security. The 26 MHz range of frequencies is further divided in
to channels. The transmitter then sends out data hopping from one channel to the next in a
certain pattern known to the receiver. Within each channel, spread spectrum transmission can
be used to maintain interference avoidance. Some of this transmission manipulation can be
avoided by transmitting at a frequency that is less used. Some developers have tried
transmitting in the gigahertz range. The disadvantages here are: 1) Higher frequencies mean
shorter wavelengths and shorter wavelengths do not penetrate solid objects like walls and floors;
2) The same transmission strength employed by lower wavelength transmitters yields a shorter
range at higher frequencies. This means that transmission strength will need to be boosted
something hard to accomplish using portable tools and potentially dangerous to humans; 3)
Transmission frequencies of 3 GHz and higher are licensed by the Federal Communications
Commission. Developers in the range have the additional hassle of obtaining a license every
time an installation is done.
(2) Laser: Laser-based communication is the fastest way to communicate without wires.
Information travels at the speed of light. The drawbacks however far outweigh the speed
advantage and prevent this method from becoming the standard. The major drawback is that
communication is restricted to line of sight. Also, very thick fog or blizzard conditions will
diffuse the laser beam and causing interference and reducing data integrity.
(3) Infrared: This method is similar to Laser. High speed communications are easy to achieve
using this method. However, it suffers from the same problems that plague laser
communications. It requires line of sight transmission and can be disrupted by strong ambient
light. Infrared wireless computing exists more commonly in the form of peripheral connections
in a small area.
(4) Cellular connections although expensive to use now is the area of much development by private
companies. Cellular computing can be likened to the current wire-based internet network. Data
is packaged in to units, size of the unit is dependent on the actual hardware, and is sent to the
nearest participating cell. That cell then forwards the packet to the next cell and so forth until
the packet reaches its destination.
(5) Microwave: This method of communication has been utilized for quite some time now.
However this method has makes little provision for data aware transmission. It used extensively
in Europe where wired transmission of any type including voice is poor. For data transmission,
a lot of technology is utilized in packaging the data into a form that is compatible to voice
communication. On the receiving end, the process is reversed. The advantage of this method
however is that communication can be accomplished using existing satellite connections making
worldwide connectivity possible.
3 Standards
The IEEE 802.11 committee has voted to create a minimum requirement for wireless computing
connections. In their consideration:
(1) Use the frequencies 2.4 to 2.5 GHz. This is in the low end of the high frequency spectrum and
is currently not licensed by the FCC.
(2) Use spread spectrum technology. Compared to the current bandwidth 26 MHz, 902 MHz to
928 MHz, the range 2.4 to 2.5 GHz yields a bandwidth of 100 MHZ. Spread spectrum
transmission now gives 385% percent increase in data reliability.
(3) Many more sub-channels can be formed in a bandwidth of 100 MHZ. This increases the
capability of frequency hopping which in turn yields greater data security.
(4) Utilize Gaussian Frequency Shift-Keying. Frequency shift-keying is a form of frequency
modulation in which binary signaling is accomplished by using two frequencies separated by
some Df Hz. The frequency duration is small compared with the carrier frequency, fc. A signal
received at frequency fc, would represent a digital low and signals received at frequency fc + Df,
would represent a digital high. Note that this does not interfere with spread spectrum or
frequency hopping capabilities since those function on frequencies separated by 1 MHz or more.
As part of setting a wireless standard some modifications of the standard set by the IEEE 802.3
committee have been adopted. The most significant of these is the modification to the carrier sense
multiple access / collision detection, or CSMA/CD, protocol used in wired networks today. This is a
method whereby any machine at any time, wishing to send a message on the net, will first send a token
out to ensure that a carrier exists (network ready). After establishing this, the message will be sent.
Because any machine may send at any time, collisions of information will occur. If any machine detects
a collision, it will send out a jamming signal to all the others. All machines will then wait on a random
interval timer after which they will try to send again.
For wireless networks however, since a machine is not in constant communication with the rest of the
LAN, detecting a collision and notifying all other machines on the net is impossible. A modification
in the way of the collision handling had to be made. A method known as collision avoidance is
employed to create the
CSMA/CA standard. In a
collision avoidance strategy, the
net estimates the average time
of collisions and send a
jamming signal at that time. A
wireless transceiver will not
only sense a carrier but will also
listen out for the jamming
signal. When all is clear it then send its message. This collision avoidance method has two drawbacks:
1) It cannot completely filter all collisions since it operates on estimated times of collisions; 2) and if
it did, it slows the network significantly by sending jamming signals whether or not a collision actually
occurs.
4 Physical Layer
Much of the focus of wireless computing development is centered on the physical and media access
control layers of a system. It is on this level of the LAN protocol of which wireless products like
modems and transceivers
On the physical layer issue, the 802.11 is focusing on the one proposed by Apple Computer
Corporation.
The Apple physical-layer protocol appears the most robust of any considered to date in 802.11. Apple’s
system is a full-duplex, slow frequency-hopping protocol. By using a frequency-hop spread-spectrum
radio, the system fits with the spread-spectrum methods of virtually all 802.11 specifications.
Apple splits the data-transport protocol into two layers:
— The RF Adoption Layer is similar in some respects to cell-based data protocols, such as
Asynchronous Transfer Mode and IEEE 802.6 Switched Multimegabit Data Services; like ATM and
802.6, the RF Adoption Layer includes segmentation/reassembly functions and Protocol Data Unit
generation functions, and it also includes Forward Error Correction (FEC) generation and verification
functions which substantially increase packet integrity in wireless environments but adds FEC overhead.
— The RF Hopping Protocol Physical Layer consists of a transmission convergence sublayer including
header generation, RF framing, and RF hopping protocol functions and the physical- medium-dependent
sublayer, in which the actual characteristics of the RF channel are handled.
In the RF Adoption Layer, a Protocol Data Unit is split into three segments, and two error-correcting
data units are added. The RF Hopping segments, and two error-correcting data units are added. The RF
Hopping Physical Layer builds special Burst Protocol Data Units out of the data and FEC units and uses
carrier-sense methods borrowed from Ethernet to determine whether an RF Hop Group is clear for
transmission. Each hop group consists of five separate radio channels. The controller scans hop groups
via state-machine operation with four states: scan, receive, carrier-sense, and transmit. In early tests at
Apple, the hop system showed 80-microsecond hop times, 57-microsecond clock recovery, and a
5-microsecond lapse between the time an empty channel is sensed and transmission begins. Since each
cluster of wireless LANs can use different hop groups, multiple LANs could operate in the same area
without interference. One concern is whether the overhead for error correction for each packet, which
can be as much as 50% is too high to give the proposal a chance.
The safety of those operating new equipment now plays a larger role in determining the direction of
technological growth now more that ever. Factors under consideration are the effect of infrared and
strong electromagnetic radiation that would pervade the workplace on the workers. This limits the
strength of and communication device that would be used in accomplishing transmission.
For the Personal Computer. The adapters have a small attached antenna through which they send and
receive network traffic as radio signals. Some wireless products are small boxes that attach to your PC’s
parallel port. In either case, the signals may travel from PC to PC, forming a wireless peer-to-peer
network, or they may travel to a network server equipped with both wireless and standard Ethernet
adapters, providing notebook users a portable connection to the corporate network. In either case,
wireless LANs can either replace or extend wired networks.
Standards are lacking. Wireless networking is still a technology looking for a standard, which is why
very few wireless products can work with one another. Each vendor uses a different protocol, radio
frequency, or signaling technology. If wired networks still operated like wireless, you would have to
use the same brand of network interface card throughout your network. Right now you are, for the most
part, tied to whichever brand of wireless LAN you pick. Most of the products in this comparison listed
their wireless protocol as Ethernet carrier sense multiple access/collision avoidance (CSMA/CA), a
variation of standard Ethernet. Unfortunately, each vendor has put its own spin on CSMA/CA, which
means even their protocols are incompatible.
5 Wireless services
As technology progresses toward smaller, lighter, faster, lower power hardware components, more
computers will become more and more mobile. For space concerns this paper will exclude any further
discussion of the hardware developments toward mobility except for devices directly related to wireless
connectivity such as modems.
A wireless computer is not connected via a wireline and thus has mobility and convenience. A wireless
LAN provides the convenience of eliminating the wires, yet is not necessarily mobile.
(What is mobility?)
Mobility is a characteristic where the wireless computer may connect, loose the physical
communication (possibly due to interference) and reconnect (possibly to another sub-network) and retain its virtual connections and continue to operate its applications. The
network protocols will be discussed later.
(Then, what is portable?)
Portable is defined that the wireless computer may connect, loose the connection and
then re-connect, as well. However, the mobile unit will have to restart if it is
reconnected to another sub-network, requiring that running processes be shut-down and
windows closed.
Mobility may be limited by the wireless service subscribed. Four basic service zones are described:
Global/National service zone: Ubiquitous radio coverage throughout a region, country or the
entire globe, low user densities, and minimal bandwidth
requirements. Typically satellite systems.
Mobile service zone: Radio coverage in urban, suburban and populated rural areas,
medium to high user densities, low to medium bandwidth
requirements (tens of Kbps), and high vehicular speed. Cellular
(AMPS) system is a good example.
Local/micro service zone: Radio coverage in densely populated urban areas, shopping
malls, and transportation centers. High enduser densities,
medium bandwidth requirements, hand-held portable terminals,
low-speed mobility.
Indoor/pica service zone: in-building radio coverage, low to high user densities, medium
to high bandwidth requirements (Mbps), very low mobility.
Prior to the cellular phone network, base station radio covering a single cell geographic area with a fixed
number of channels was the only service available.
The cellular phone service divides the service area into cells and assigns a subset of the available
channels to any given cell. This way the channels can be reused and interference from neighboring cells
is reduced. The system tracks the active mobile unit, delivers calls, and maintains connections as units
move between cells (Hand-off: a realtime transfer of a call between radio channels in different cells).
This system is called Advanced Mobile Phone Service (AMPS). Current cellular systems use analog
FM technology. However, implementation of digital radio technology is being deployed now. These
systems utilize Time Division Multiple Access (TDMA) or Code Division Multiple Access (CDMA)
to increase throughput up to ten times the previous analog system. Additionally, end users will access
a wider range of telecommunications as the implementation of integrated services digital network
(ISDN) principles are utilized. Personal Communication Services, similar to the current cellular system,
will soon be available from the larger telecommunication services, but with reduced price and wider
availability.
Wireless
Advantages
Limitations
Traditional Cellular
no restrictions on length
or type of data
transmission
national coverage
bill by minute
potential line
interruptions,
congestions in urban
areas
limited throughput
CDPD
enhanced technology
for data over cellular
bill by message size
integrated voice and
data
packet switching error
correction techniques
lack of applications
development
not fully developed
Dedicated packet switched
mobile networks
integrated applications
and communications
no call setup time
inherent reliability and
security of packet
switching
coverage not full
nationwide
limited packet size
require specialized
modems
data only
Specialized mobile radio
voice and data
vehicle based
limited coverage
Satellite-enabled networks
geographic reach
expensive equipment
and service costs
The application of the wireless computing system determines the type of wireless medium system to be
employed. Circuit switched or packet switched, both are available through wireless technology and
provide connectivity. Circuit switched systems provide a continuous connection established to the
destination by the switching system. The most popular examples are the wireline public switched
telephone network (PSTN) and cellular telephones systems. This method of communication can be
relatively expensive. If the phone systems offers voice grade bandwidth, then a standard modem can
provide speed of 14.4 Kbps (at the time of this writing). However, if a digital line is provided then
higher communication rates can be achieved with more specialized equipment.
Packet switched systems provide a delivery system of information packets. The packet contains the data
and an address to the destination. Packet switching is far less expensive than circuit switching.
Examples would be RAM, ARDIS, and Internet networks. Packet radio networks have been the target
of many studies since the military has a vested interest in the communication medium. Concerns such
as reliability, throughput optimization and re-routing of packets have been recent topics.
Packet Switched
Systems
ARDIS
RAM Mobile
Data
Circuit Cellular
CDPD
Network
Capacity
1,300 base
stations in
approx. 325
metro service
area (MSA)
840 base stations
in 210 MSAs
8,000 cell sites
in 734 metro
areas
potentially entire
cellular network
Coverage (cities
and towns)
10,700
6,300
NA
NA
Transmission
speed
4.8Kbps.
19.2Kbps
upgrade in major
metro areas
8Kbps
38.4Kbps to
56Kbps
19.2Kbps
Message
capacity
256 bytes
512 bytes
NA
114 bytes
National
roaming
completed by
mid Sept 94
yes
no
yes
In-building
coverage
yes
top 20 MSAs by
June 1993
limited
limited
Cellular Digital Packet Data technology (CDPD)
utilizes the space between the voice segments on
cellular (AMPS) network channels and inserts a
data packet. The user pays only for the packet
sent as opposed to a cellular circuit switched
connection. CDPD cellular communications
systems such as the Ubiquity 1000 from PCSI,
offer packet burst rate of 19.2 Kbps with full
duplex. This CDPD modem offers the option to
use circuit switched cellular, wireline PSTN and
voice support. However, in a large urban area
with thousands of stations using any packet
switching service at current speeds, delay may be
unacceptable.
Satellite can be used as long distance links within
wireless networks. Three major projects have
been proposed. The Teledesic system, composed
of 840 low orbit satellites, was proposed by Bill
Gates (Microsoft) and Craig McCaw (McCaw
Cellular). Second, the Pentagon, solicited a
system, using 1,000 smaller satellites, from TRW and Martin Marietta. Both the Teledesic and the
Pentagon systems cost around $9 billion. The third system, called Iridium, from Motorola, will use 66
satellites to offer mobile phone service all over the globe. This project will begin this year and the rest
in place by 1996.
6 Software
Software concerns in a wireless computing environment can be broken into two areas, system and
application.
7 System Software
Network operating systems must be able to handle the uniqueness of a wireless computer. Advanced
operating systems utilizing distributed technology must be adapted to the specific communication
media. The advancement of technology has provided that even mobile computer systems the size of
notebooks are capable of internetworking as a host in global networks. Mobile host protocols
compatible with TPC/IP have been developed to allow continuous network connectivity where ever the
host may be. Due to the unpredictable nature of wireless connections, even operating systems may have
to be written to provide support services for mobile network. The WIN*OS, a micro kernel for a
wireless-compatible operating system, was developed to “support concurrent and composable objects
and coordinated communication among groups of objects through a process of agreements.”
8 Application Software
Application software concerns in the wireless computing environment vary depending on the type of
application and wireless medium used. For example, E-mail software must know how to communicate
with the packet switched network as compared to the traditional cellular network. Software developer
kits (SDK) and application programmers interfaces (API) are usually available by the service provider.
Remote access software allows the remote user to connect to a host workstation to view the screen and
control the keyboard as if the user was there. The data does not have to be communicated to the remote
user and thus allows processing locally. Carbon copy and PC anywhere are among the programs which
provide remote access for microcomputers. High baud rate is needed especially when a graphical user
interface (GUI) is used.
9 Wireless Local Area Networks (WLAN)
WLAN offers the same features as a wireline LAN but without the wires. Coverage can range from a
room to a building to a “campus” (wide-spread, multi-building). Both stationary desktop systems and
mobile notebook computers can connect using specialized wireless LAN adapter cards. Another
configuration allows wireless additions to current networks. Wireless Hubs have been developed which
bridge the wireless units into the wireline network.
As mentioned before, during the recent natural
disasters in California, the Federal Emergency
Management Agency (FEMA) set up field offices
with WLAN very quickly. Here is a great example
of how WLAN can be used: An ETHERNET
connection over a radio link provided data from a
low-power PC in a buoy to a PC on a ship. The
system provided a megabyte/sec data rate for four
days while guaranteeing error-free delivery of
data. Even more incredible is the MBARI
acoustic LAN. Since under water, radio waves
travel only a few feet but sound waves can travel
for miles, the acoustic LAN uses the better carrier
of wireless data signals. The acoustic LAN has
two 5Kbps data channels and two slow-speed
command channels. The LAN is used to
communicate with tilt meters and buoys.
Personal Data Assistants (PDA) are the new
handheld computers which also have wireless options. Using a pen-based GUI operating system, the
applications are accessed from local storage. Fax, data and voice can be transferred to and from the
PDA via cellular phone system. The AT&T EO can run a program called Gnosis which when also
loaded on a remote server host will allow the user to search for documents and have them downloaded
in minutes including graphics.
Even though all these nifty devices such as radio
modems and PDAs are developed and marketed,
a recent study of mobile professionals shows that
currently relatively few spend time far from their
desks. In fact, only 13 percent of mobile users
spend time outside their metro area and just 1
percent outside the country. As the technology
becomes more common place, more users will
find themselves moving further out of their wired
areas and into the wireless field.
10 Security
Security becomes essential in wireless
computing. Especially since the data is
broadcast to the receiving unit. International
Standards Organization (ISO) has published
security services which provide for secure data
and computer systems on standard wireline
networks. However, these must be modified to
meet the needs of mobile users and systems. Data encryption and Two possible solutions include
exchanging security information between a small number of entities, or even more complex involving
an information center.
Infrared offers the least problem of security due fact that stations must be in the line-of-sight and the
limited area of coverage, usually one room. Spread spectrum RF transmissions spread the data over a
range of frequencies making interception extremely difficult. Also, low power limits the coverage area,
although the signal will penetrate walls. Cellular phone networks offer no security of their own. Even
though listening to these transmissions has been made unlawful, the signals can be overheard by a radio
scanner. Data encryption is left up to the connecting unit. Packet radio offers inherent data security by
scrambling the data packets.
Clipper chip will replace the digital encryption statndard (DES). The Clipper chip boasts to be 16
million times stronger with 80-bits as compared to the old DES, which has a 56-bit binary key. This
chip will be used in many communication products, especially wireless. The Department of Justice and
AT&T will be installing them in their telephone products. The controversy about these chips stems from
the fact that they are programmed with a back door. The government can, with a court order, access
the chip and monitor the communication.
11 Conclusion
In the relatively short time of the Information Revolution, the world has seen several technologies, first
introduced as “convenient”, become “essential” the basic structure of the modern lifestyle. The
automobile, telephone, and the refrigerator are easy examples to cite. The wireless revolution will
transform another “convenience” to a necessity. “Emerging wireless systems will provide the technology
to allow people and machines to communicate anytime, anywhere, using voice, [video,] data and
messaging services through telecommunications.” The wireless revolution began with the introduction
of the cellular phone networks. This coupled along with the reduction in size of the microcomputer and
an increase in the applicable technologies.
After surveying the many aspects of wireless computing, several areas stand-out and appearently require
further research and development. Among those are mobile internetworking protocols, which would
allow a mobile host to connect to any part of the network. Mobile “aware” ope