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New Trends in Mobile Computing Architecture

Antnio Miguel Cruz

Dep. Informtica, Universidade do Minho4710 - 057 Braga, Portugal

antonio.rosadodacruz@edinfor.edp.pt

Abstract. The increasing nomadicity of our society places new challenges to the hardware

design of mobile computers. This communication introduces the subject and identifies the main

requirements of mobile stations, namely portability, mobility and wireless communications. It

then focuses on the architectural issues of these mobile computers with a brief overview on the

main features of Digital Signal Processors for control signalling in mobile digital networks.

1 Introduction

Research in wireless networking technology has allowed portable computers to be

equipped with wireless interfaces, enabling networked communication to take place while

mobile. This lead to an enormous increase of productivity to those professionals that need

to be constantly moving while requiring connection to the companys server. An example is

the expert from an assurance company that connects to his companys database server,

from his notebook computer, using his mobile phone (GSM enabled handset). This makes

possible for that expert to give an answer to his client, accepting or not the garages budget.

This type of nomad teleworking has been called mobile computing, which should not be

confused with portable computing. In mobile computing and networking, applications are

not interrupted when the user changes the computers point of attachment to the network.

Instead, all the needed reconnection occurs automatically and without human intervention

[1].

Mobile Computing is a new paradigm of computing where mobile computers have

access to a wireless network, independently of their current location. The required tasks to

adapt to the environment conditions can be included into the operating system, such as in

Coda [2], or both the operating system and the application, such as in Odissey application-

aware adaptation [3].

This communication will address mobile computers as computing devices that connect

to a server or to the Internet from anywhere in the world, provided that its location iscovered by a similar digital radio network. These devices may be notebooks, handheld

computers (PDA), or systems embedded in automobiles with integrated wireless data

communications, or mobile phones with computing capabilities (PDA/cell phone

combination) such as the Nokia Communicator; or they may also include wearable

computers, such as the Matchbox PC [4].

The requirements for mobile computing can be included into three main groups [5]:

Portability. With desktop PCs, the main development trend has been the

execution performance, while keeping the size. For mobile computing are needed

machines that are small, lightweight and have battery autonomy, so that they can

be truly mobile. Thus, there is aportability requirementthat imposes constraints in

the use of space, power, cabling and heat dissipation.

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Mobility. The ability to change locations while connected to a network places a

mobility requirement, in the sense that data must be valid, independently of the

point of access to the network. This also requires that the access to location

dependent information should be sensitive to the network access point of the

mobile computer. A mobile computer also needs a mechanism to know which

server to use, depending on its location.

Wireless communication. In wireless communication the surrounding

environment interacts with the signal, blocking its path or introducing distortion

and noise. Wireless connections have lower quality than wired connections: lower

bandwidth, higher error rates and more frequent unwanted network disconnection.

The next sections discuss the three classes of requirements for mobile computing. In

section 5, an overview on Digital Signal Processors (DSP), that are used to control

signalling in digital networks, will be given.

2 Portability requirements

Two types of computers for mobility can be considered:

Computers that have the same functionality as desktop computers, such as

notebooks;

Computers that gave up some functionality, so that they can be shrunk to fit in a

pocket, such as handheld computers or PDA/cell phone combinations.

In the near future, the wireless data market will continue to be dominated by

notebooks [6], because notebook users need more connectivity than handheld users do, as

they run more powerful programs and more communication demanding applications, such

as mobile client/server applications.

Mobile computers differ from desktop computers in some aspects, such as size, weight

and power autonomy. The technological trends to meet this constraints are [7],[8]:

At the hardware level:

embedding code within the system, allowing it to slow down, suspend or

shutdown part or all of the system platform; advances in microelectronics;

increasing code density;

higher peripheral integration.

At the hardware/software level:

let the operating system (OS) to have control over the power management.

2.1 Embedding code within the system to manage power consumption

This technology was first introduced by Intel with its processor 386SL. The SL technology,

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still present in todays Pentiums, consists in embedding code within the BIOS of the CPU,

allowing it to slow down, suspend or shutdown part or all of the system platform, including

the CPU itself, enabling an effective power management in order to extend battery life,

thus increasing computers autonomy [7]. The Intel Mobile Pentium, for notebooks, and

the Motorola Dragon Ball (MC68328), for handheld computers, are examples of

processors that have these features.

2.2 Advances in Microelectronics

Reducing space between components within the integrated circuit, as permitted by the 0.25

micron CMOS process technology, or the more recent 0.18 micron process technology,

minimises the die area of silicon needed to produce a chip and increases the number of

chips per silicon wafer. This directly reduces production costs, and allows the processor to

be smaller and faster.

Reducing space between components within the circuit also allows a voltage reduction

inside the CPU. Reducing voltage inside the CPU, consists of powering the CPU core with

a lower voltage than the motherboard. CPUs working this way are called split coreprocessors. The first implementation of the split core CPU technology was the Pentium.

The core of the chip operated at 2.9 V while interfacing to the motherboard at 3.3 V [7].

This enabled a CPU power saving of 23%. The 0.25 micron technology made possible an

even larger voltages reduction, enabling the manufacturing of split core processors

running at 1.8 V while interfacing to other components at 2.5 V.

The 400 MHz version of Pentium III is available with both the 0.18 micron and the

0.25 micron technology, having a core voltage of 1,5 V in both cases. The former

dissipates 7.5 W of power, while the last dissipates 9.2 W of power, a difference of about

20%.

2.3 Increasing Code Density

Code density is inversely proportional to memory size. And reducing memory size, reduces

both the overall size of the system and the use of power to maintain memory data. Due to

their fixed instruction length, applications on RISC architectures have longer code than on

CISC processors [8]. Nevertheless, embedded systems that require some performance

improvement techniques, as is the case of handheld computers, normally prefer RISC

processors, because fixed-instruction length enables the use of techniques like pipelining.

The way some handheld designers found to increase code density using RISC

architectures was to reduce the instructions length. Another way is to enlarge the

instruction set of the CPU having one specialised instruction to do what several standardinstructions could do. This not only increases code density but also improves the system

performance, allowing faster DSP, multimedia, etc..

In the field of mobile computing, this is only a constraint to handheld computers, as

they require an even higher portability. Notebooks usually do not care about code density.

2.4 Peripheral Integration

Integrating the CPU and peripheral controllers within the same chip reduces the overall

system size and cost, while contributing to save power in handheld and embedded systems

[8]. Processors that have peripheral integration, used for handheld computers, include the

NEC VR4111 and the Intel StrongARM SA-1100. These processors include logic fordisplay and keyboard controlling, among other integrated functions. They also have a DSP

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functional unit , which is an important feature for mobile computing.

2.5 Allowing the OS to have control over the power management

Power management as a functional unit within the CPU has some limitations. It only

allows shutdowns after a certain period of inactivity of the peripherals or the CPU, and theOS is never informed of the parts of the system that have been shutdown. The first step

towards informing the OS of the parts of the system that were not available was made by

Microsoft and Intel, introducing the APM (Advanced Power Management) enabling

communication between the OS and the power management code embedded into the

BIOS. Now the OS knows what the BIOS is going to do, but it remained a power

management based on a time out concept.

In 1997, Intel, Toshiba and Microsoft introduced the Advanced Configuration and

Power Interface (ACPI). The ACPI allows for collection of information about power

consumption from all over the system, giving that information to the OS, which has

complete device activity control, enabling it to provide power to the devices only when

they need it [7]. Support for ACPI has to be provided both by the processor and the OS.Pentium based notebooks running Windows 95 (or higher) have support for ACPI.

3 Mobility requirements

Mobility places many demands on a system. The network address of a mobile computer

(mobile station) changes dynamically, as it moves. Its current network access point may

determine system configuration, such as current servers location, if the nearest server shall

be used. Another problem is accessing location dependent data, like the possible ways to

go to the City Hall, or which movies are in the Town Cinemas. In this section, the threemain issues introduced by mobility are discussed.

3.1 Address Migration

In the Internet Protocol (IP), moving to a new location implies acquiring a new IP name.

To communicate with a mobile computer, messages must be sent to its most recent address.

The basic mechanisms that can be used to determine the current address of a mobile

computer are:

Selective broadcast. A message is sent to a set of cells near the cell corresponding to

the last known address of the mobile station, asking it to report its current location. Central services. In this method, the current address for each mobile computer is

maintained in a logically centralised database. Each time a mobile computer

changes its address it sends a message to update the database. This database may be

physically distributed or replicated in the network nodes to improve response time.

Home bases. This method is similar to the previous one. But, here only one server

knows the current address of a mobile computer.

Forwarding pointers. Each time a mobile computer changes its address, a copy of

(pointer to) the new address is kept at the old location. Each message is propagated

through the chain of pointers until it reaches the mobile computer. This method

requires a process at the old location to receive and forward messages.

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3.2 Location Migration

While taking an effort to determine which server is near to it, for a given service, a mobile

computer may keep moving making the path to that server grow. Because the physical

distance between two points may not be proportional to the network distance, moving themobile computer a few meters may result in a much longer path in the network. And,

longer paths mean greater risks of disconnection or data losses.

3.3 Accessing Location Dependent Data

In traditional desktop computers, information that depends on location is configured

statically, by human intervention, such as the local name server, printers, etc.. A

requirement for mobile computing is to provide mechanisms to obtain configuration data

for the current location.

4 Wireless Communications requirements

The need for network access together with the need for mobility leads mobile computing to

the use of wireless communication. Wireless network access is more susceptible of

unwanted disconnection, low bandwidth availability and highly variable network

conditions.

Due to mobility, wireless connections may be lost or degraded, and due to the cellular

nature of mobile telecommunications networks, such as GSM1or UMTS, if there are too

many mobile users within a cell, network capacity may overload. The main constraints

addressed by wireless communications are:

Network disconnection

Low bandwidth and bandwidth variability

Security risks

There are several ways to deal with these constraints.

4.1 Network Disconnection

Mobile computers run great risks of getting disconnected from the network. To overrun

this problem, mobile computing has to either try to prevent a disconnection or try to deal

autonomously with disconnection. Trying to prevent a disconnection usually compromises

portability. Thus, the best way to deal with network disconnection is to treat network

access in an adaptive manner, enabling disconnected operation. Such adaptation to the

network conditions may either be performed by the OS or by both the OS and the

applications. If disconnection is handled transparently by the OS, mobile applications may

be developed without disconnection concerns, just like traditional client/server applications

1 Global System for Mobile Communications (GSM) is a digital cellular radio networkoperating in most countries world-wide. It operates in three different frequency ranges:

900 MHz (most common in Europe and the world), 1800 MHz (rapidly increasing inEurope), and 1900 MHz (the only frequency used in USA and Canada for GSM). A goodoverview of GSM network architecture can be found in [10].

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over wired networks. One way the OS has to deal with short term disconnection is working

asynchronously [5]. As opposed to synchronous remote procedure calls, where the client

stops waiting for the servers reply, in asynchronous operation a client sends multiple

requests before asking for an acknowledgement. Prefetching and lazy write-back of data in

the server, also improves toleration to short term disconnection. These techniques, enable

the masking of some network failures.Another way to have the OS dealing with total disconnection, permitting disconnected

operation, is to cache a set of entire files in the mobile computer based on the users

profile, such as in the Coda file system [2]. When the network reconnects, the cache is

automatically synchronised with the repository server.

Having the OS providing primitives so that the applications can deal with network

conditions and other mobile environment conditions, such as low bandwidth, is the aim of

the Odissey API [3].

Naturally, not all network disconnections can be dealt transparently to the users point

of view. In those cases, the user should be informed of the operations that cannot be

performed during disconnection.

4.2 Low Bandwidth and Bandwidth variability

Wireless networks deliver lower bandwidth than wired networks. Radio transmission

limitations, in terms of bandwidth and cost, do not allow GSM users to communicate with

other users at rates higher than 9.6 Kbps. With UMTS2 it will be possible to communicate

at rates higher than 11 Mbps. It will always be lower than wired networks bandwidth.

With ISDN, users can communicate at rates higher than 2 Gbps.

To improve network capacity there can be installed more wireless cells, either by

overlapping cells on different wavelengths or by reducing transmission ranges in order to

put more cells in a given area.

Certain software techniques, such as compression, can also be used to deal with low

bandwidth.

Wireless Networking also suffers from a greater variation in network bandwidth than

wired networking. Applications can deal with this bandwidth variability in the following

ways:

assuming low bandwidth connections, and not using available bandwidth when

it is available;

adapting to the currently available resources, providing a variable level of

quality to the user.

4.3 Security Risks

Because it is easier to connect to a wireless link than to a wired one, the security of

wireless communication can be of greater concern than the security of wired

communication. Secure communication over insecure channels is accomplished by

encryption [5]. This can either be done with software or, more efficiently, with hardware.

2 Todays cellular networks, such as GSM (Global System for Mobile Communications)are mainly circuit-switched, having connections depending on circuit availability. With

packet-switched networks, such as UMTS (Universal Mobile TelecommunicationsSystem) it is possible to implement Virtual Circuits, having virtual connections alwaysavailable to any other end point in the network.

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5 Digital Signal Processing

Control signalling aims at managing the establishment, maintenance, and termination of

signal circuits. Control signalling is used either between the subscriber and the network or

between functional entities within the network. GSM, just like ISDN, uses the signalling

system number 7 (SS7), which is an open-ended common-channel signalling standard that

can be used over digital circuit-switched networks.

There are specialised processors for signal processing, called Digital Signal Processors

(DSPs). The main difference between a general purpose processor and a DSP processor is

that a DSP has features designed to support high-performance, repetitive, numerically

intensive tasks, which include [9]:

Single-cycle multiply-accumulate capability. Some high performance DSPs have

two multipliers that do two multiply-accumulate operations per instruction cycle.

Specialised addressing modes, for example pre- and post-modification of address

pointers.

On-chip memory and peripherals controllers. DSPs generally feature multiple-

access memory architectures that enable DSPs to complete several accesses to

memory in a single instruction cycle.

Specialised execution control. DSPs often provide a loop instruction that allows a

loop to be repeated without having to spend any instruction cycles for updating and

testing the loop counter or for jumping to the top of the loop.

Several operations encoded in a single instruction. DSPs have irregular instruction

sets, usually allowing several operations to be encoded in the same instruction. For

example, a processor that uses 32-bit instructions may encode two additions, two

multiplications and four 16-bit data moves into a single instruction. In general,

DSPs instruction sets allow a data move to be performed in parallel with an

arithmetic operation.

6 Conclusions and Comments

This communication is an overview of mobile computing architectural needs and the latest

trends in fulfilling those needs. It has been stated the difference between portable

computing and mobile computing, and the major features of mobile computing were

identified.

Today, the number of voice communications is, yet, greater than data communications.

In fixed networking, the break-even is expected to happen in about one or two years. Inmobile networking it will take some years to have a break-even, though. Communication

requirements of mobile computing will evolve as mobile applications get more complex,

and wireless communications technology evolves.

Due to the limitations of handheld systems, the wireless data market will be dominated

by notebooks [6] for a few more years. Nevertheless, with current microelectronics

technologies it is possible to integrate into the same chip a general-purpose processor withan instruction set extension for digital signal processing, encryption, and other functional

units of a mobile station, allowing the production of notebooks and handheld systems

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