Chapter 2

Mobile Computing Architecture and Technology

Brisk advances in computer hardware and wireless network technologies have led

to the development of mobile computing [W92] [W93]. The expanding

technology of cellular communication, wireless LAN, and the satellite services

make it possible for mobile users to access information. In this ubiquitous wireless

environment of the future, large number of users equipped with the battery

powered palmtops or portable computer can access information anywhere and at

anytime.

In this chapter, we present the various existing wireless communication

technologies that support the mobile computing environment. In Section 2.1, we

briefly describe the network technology and the different variations of

communication system. In Section 2.2, we present the battery technology, in

which selective tuning is resorted to conserve the battery power, such that the

mobile unit have the capacity to run for a longer period. Section 2.3 addresses the

networking architecture, which supports the wireless mobile computing, wireless

information services are also provided in the same Section. Various practical

communication issues are introduced in Section 2.4. Finally, we conclude the

chapter with Section 2.5.

2.1 Network Technology

The wireless communication systems began with the launch of cordless and

cellular telephones. Cordless telephones were initially designed for very limited

range and cellular telephones were largely intended to extend the telephone

service to users in their vehicles. Each cellular phone of this initial generation of

wireless communication systems has a dedicated support (base) station. This first

generation communication system uses analog frequency modulation for speech

transmission.

System configuration of a cellular network consists of fixed information

network extended with wireless network elements. These elements comprise,

wireless terminals, base stations, and switches. The entire geographical area is

partitioned into cells. Each cell is covered by a base station, which is attached to a

fixed network and provides a wireless communication link between the mobile

users and the rest of the network. The next generation of wireless communication

systems will provide digital speech transmission and increase the range of

applications to data transfer, and video conferencing. These wireless units have

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sophisticated capabilities for transferring information to and from the network.

This generation of wireless communication systems was proposed to extend the

services offered by the first generation system to a business environment. These

cellular network, which are in the process of being deployed, are digital networks

having radio links to connect mobile users to their base stations.

Recently, American Federal Communication Commission (FCC) allocated

bandwidth in the 2 GHz range for the third and last generation wireless

information systems, more commonly referred to as Personal Communication

Services (PCS) or Personal Communication Network (PCN). This proposes to

provide ubiquitous communication (voice, data, and multimedia) coverage,

facilitating people to call each other and exchange data irrespective of their

location. These systems are anticipated to include wireless access such as paging

and wireless local area networks, in addition to amalgamation of the previous two

generations of cellular and cordless services. Satellite communication is also an

integral part of PCSIPCN. It is speculated that ubiquitous communications will be

provided by pes in a hybrid fashion. Heavily populated areas will be covered by

cheap based stations of small radius (picocells). Whereas areas with less density

will be covered by base stations of larger radius, and remote areas, and highways

with satellites that will provide the bridge between these different islands of

population density. pes involve two types of mobility: terminal and personal

mobility [MJ94]. Terminal mobility allows a terminal to be identified by a unique

terminal identifier independently of its point of attachment to the network.

Personal mobility allows PCS users to make and receive calls independently of

both their network point of attachment and the specific PCS terminal.

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Wireless infonnation services can be broadly classified into two mam

categories: Land Based and Satellite Based.

• \Vireless LAN (Local Area Networks) and wireless WAN (Wide Area

Networks) services, and packet over circuit technology are Land Based

wireless infonnation services.

• Geosynchronous satellites and Low Earth Orbit (LEO) satellites are

Satellite Based infonnation services.

The land based wireless infonnation services are very popular and are being used

globally. These services are described as follows:

o Wireless LANs provide wireless communication in a building in which

each cell has only one channel covering entire available bandwidth.

Typically its bandwidth is 1 to 2 Mbps. NCR's wavelan, Motorola's Altair,

Infra-Red (IR) based LAN and Telesystem's ARLAN are the examples of

Wireless LAN. These systems operate in the 902-928 Mhz Industrial,

Scientific, and Medical (ISM) band.

o \Vireless WAN schemes include technologies like the Trunk Radio. This

Trunk Radio technology supports both voice and packet data and is useful

for the trucks, police and emergency vehicles. Trunk Radio has a

bandwidth of up to 56 Kilobits per second (Kbps).

o Cellular Digital Packet Data (CDPD) is an example of packet over circuit

technology. In CDPD packetized data is transmitted on unused cellular

voice channels. In this, voice has higher priority over data transmission.

Data can be transmitted on a channel only if there is no voice call, which is

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using that channel. V oice transmission can preempt data transmission. In

such cases, another channel must be found for the data transmission.

Geosynchronous Satellites are synchronized with the rotation of earth and at an

altitude of approximately 36,000 KM above the equator. These satellites are

suitable for paging and other broadcast-only services. Motorola's proposal for a

satellite based service; called the Iridium project, consist of 66 satellites in a low­

earth-orbit. Qualcomm's Globalstar uses 48 satellites orbiting the earth. TRW's

Odyssey also has 12 satellites that are orbiting the earth. All the above have the

same goal of providing information services (mostly voice) around the world.

2.2 Battery Technology

Energy supply is a major bottleneck for mobile wireless computers. Battery is the

energy source of a portable computer. The portability of a computer is determined

largely by the weight of its power source. In the case of palmtops, batteries

constitute the largest weight. Also longer battery life is a most desired feature of

the mobile users. The lifetime of a battery is expected to grow only 20% over the

next 10 years [SCB92]. This constraint of limited available power is expected to

derive all solutions for mobile computing on palmtops. Thus, energy efficiency is

a necessary feature both at the level of hardware and software.

To illustrate the constraint of limited available power, consider a palmtop

with a CD-ROM and display. An AA cell is rated to provide 800 mAIHr at 1.2 V

(0.96 WIHr). Assume that the power source of a palmtop with a CD-ROM and a

display is 10 AA cells. The constant power dissipation in a CD-ROM (for disk

spinning itself) is about 1 W, and the power dissipation for display is around 2.5

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W. The assumed power source will last for only 2.7 Hours. Thus, to enhance the

battery life, the CD-ROM and the display may have to be powered off for most of

the time.

The power management enhances battery life, allowing users to work

longer without having to plug into wall power or replace battery. Apart from CD­

ROM and display, the CPU and the wireless receiver of the palmtops also

consume power. There is an increasing pressure on hardware vendors to come up

with power efficient processors. One such processor is the Hobbit Chip from

AT&T that operates in two modes: full operation mode called the active mode,

and the power-conserving mode called the doze mode. This chip utilizes 250 mW

in an active mode and the consumption in doze mode is 50,uW . The ratio of

power consumption in active mode to doze mode is 5000. Due to this, there is also

a growing pressure on software vendors to incorporate energy management

features. The CPU consumes more power than some receivers do, especially, it

has to be active to examine all incoming buckets. This is true, if on the average,

only a new data buckets are of interest to the units. Therefore, it will be beneficial

if the CPU can slip into the doze mode most of the time and come into the active

mode only when the data of interest arrives on the broadcast channel. This

requires the ability of selective tuning.

Transmitting and receiving consume power as well. In practice, the power

required for transmitting increases as a fourth power of the distance between the

transmitter and receiver [L89]. However. the number of other aspects like the

terrain, landscape, the height and the kind of trees, foliage, rain etc., play an

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important role in determining the power required by the client in transmitting to

the server. This constitutes a major drain of the power at the unit.

Power efficient solutions are important due to the following reasons:

o Selective tuning makes it possible to run the same applications for a larger

period with smaller batteries. Smaller batteries are significant from the

portability point of view since palmtops can be more compact and weigh

less. This is principally true in palmtops, since batteries constitute the

largest source of weight.

o The power management enhances longevity of battery, allowing clients to

work longer without having to plug into wall power or replace battery.

With the same batteries, a client can run for a very long time without the

problem of changing the batteries often. This avoids frequent recharging

and results in considerable monetary saving. Recharging can be

cumbersome especially if the user is on the move or does not have any

source to plug into the wall power. With power efficient solutions batteries

may have to be recharged only every few days, rather than every few

hours. The frequent memory effect (it occurs when certain batteries are

charged without being fully discharged first. This results, in the batteries,

not being able to be recharged fully, ever again) problem that is prevalent

in most rechargeable batteries can be avoided.

2.3 System Architecture

A model view of the global architecture to support mobile wireless computing is

sho\\'TI in Figure 2.1. The geographical area covered by a base station is called a

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cell. Each cell is managed by a base station called the Mobile Support Station

(MSS) (a fixed host) [IB94]. The MSS is a server (transmitter) as well as the

WIreless LAN cell ... -.....

WIreless Radio cell .. _ ................... ..

" .

· · · · · , , \ 1 Mbps .: , . , . . ,

" . WIreless LAN cell

.'

. , ,

, ,

WIreless Radio cell

MU Mobile Unit (whether dumb terminal or walkstations),

MSS Mobile Support Station (equipped with wireless interface), and

Fixed Hosts (without wireless interface).

Figure 2.1: Wireless Computing Architecture

source of all data. The MSS can periodically broadcast data on the wireless

network and the clients (units) can 'filter' the required data items. The MSS can

also be the server in a client-server interaction (on-demand mode). In addition, the

MSSs also provide commonly used application software, so that a mobile user can

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download the software from the closest MSS and run it on the palmtop. The

wireless cells typically have restricted bandwidth. Due to constraints in the

hardware, the wireless cells continue to have limited bandwidth [AK93]. These

cells may vary in terms of their wireless networks like satellite channel, radio

channel, infra-red link etc. Wireless information services of the future are

characterized by their geographical scope. These services are classified as follows:

• Picoservices: Small picocells may have a very small range covering only

relatively few clients. That is, these services will be very local and picocell

will cover a few hundred meters in diameter. Examples of picoservices are

as follows:

•

o Local parking availability of lot.

o Layout of the building.

o Empty seats in a stadium/restaurant/concert halVtheatre.

o Activities in Schools and colleges.

o Items in a store, and shopping mall advertising and local schemes (a

user in particular mall receive advertisements about sales and events

which are local with respect to his location).

Microservices: The geographical area of Microservices (microcells) is

provided in a range of few miles in diameter. Some of the Microservices

include:

o Train and bus station information.

o Local sports and cultural information.

o local traffic information or airline information.

o Local telephone directories information.

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• Macroservices: These types of services could be provided in an area

extended to tens of miles. These services are crucial on a regional basis

(macrocells). Following are the examples of macroservices:

o Regional maps.

o weather information.

o local news services are all instances of macroservices.

• Wide Area Services: The geographical area covered by these services is

very large and these can be offered on a national basis. The examples of

these services are as:

o Stock market information and national commodity prices.

o general political, financial and sports news are all examples of the

wide area services.

Due to the types of data services made available to clients and the cell size,

broadcasting is an efficient data dissemination mechanism in microcells.

Broadcasting may not be efficient in picocells or macrocells in view of the fact

that in picocells the number of clients may not justify broadcasting and clients

may not change macrocells over and over again. In the architecture, the MSSs

(wireless information servers) are equipped with wireless transmitters capable

of reaching thousands of users residing in the microcells. Users equipped with

small battery powered palmtops, with wireless connections can move freely

between cells. The palmtop, which is similar to the Motorola's Embarc

receiver, but equipped with a power efficient chip like the hobbit chip, will

pick up information delivered by MSS.

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2.4 An Outline of Communication Issues

In a wireless broadcasting environment. Imielinski et al. [IVB97] addressed the

number of practical communication issues. In this Section, we discuss these issues

in brief.

o Setup Time: The process of tuning out of the broadcast channel or tuning

back in is referred to as setup process. The time taken by CPU for shifting

from doze mode to the active mode or vice -versa is known as set up time.

It also refers to the time required to switch the receiver on (or oft). In all

the algorithms proposed for broadcasting, we assume that the set up time is

negligible. In reality, each tuning out (dozing oft) and tuning in (waking

up) does not occur right away, but takes small amount of time. Typically,

the set up time is negligible compared to the time it takes to broadcast a

bucket [R92] and hence it can be disregarded and ignored.

o Reliability Issues: Due to disconnections, handoffs and communication

noises, the error rate in wireless transmissions are much higher than the

error rates on fixed networks. Thus. maintaining the reliability of

broadcasting in wireless networks is an important issue. Initially, we first

assume that no additional steps are taken to increase the reliability of

broadcasting. Observe that broadcasting is ultimately reliable due to its

periodic nature; the client can always wait for the next broadcast in case

the replicate does not exist.

o Synchronization: For selective tuning, clients have to be guaranteed that

data will appear on the broadcast channel at a pre specified time, i.e.,

clients and the server (which broadcast the data) have to be synchronized.

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This calls for the network to be detenninistic. This is reasonable

assumption for exclusively used channels such as Motorola's Satellite

Networks (Embarc) where infonnation is assured to come at a specific

time because there is only one server and the clients only listen.

Synchronization is not difficult, especially with the current quartz

technology. In order to take care of small discrepancy in the clock

synchronization, the clients may tune in, epsilon (bucket) ahead of time

(the required bucket is expected to arrive on the broadcast channel)

[IVB97] [V94]. That is, the tuning time starts epsilon prior to the expected

timestamp of broadcast and continues until a bucket received by the client.

Epsilon varies from client to client depending upon the accuracy of its

clock.

In general, synchronization can be easily accomplished if the MSS

has a higher access priority than any client. This is not true for the

Ethernet, where the broadcasting traffic has to be interleaved with the on­

demand request and it is very complex to adequately predict the time of

transmission. For such channels, the methods presented in this thesis have

to be modified.

2.5 Conclusion:

In this chapter, we presented the vanous existing wireless communication

technologies that support the mobile computing environment. We also delve with

the network technology and the different variations of communication system.

The importance of battery technology, where selective tuning is resorted to

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conserve the battery power, such that the mobile unit has the capacity to function

for a longer time, is also briefly discussed. Finally, the networking architecture,

which supports the wireless mobile computing along with the wireless information

services in different regions in terms of geographical areas are discussed.

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