The Challenges and Requirements of Mobile Computing

Pradeep Loganathan

Information Systems Engineering (ISE II)

Department of Computing

Imperial College of Science, Technology and Medicine

"Mobile Computing constitutes a new paradigm of computing which is expected to revolutionise the way computers are used."

_{George H. Forman}

1 Introduction

Advancements in technology have enabled portable computers to be equipped with wireless interfaces, allowing mobile networked communication. This combination of mobility and networked communications will give rise to a range of new applications and services. However, the challenges involved in establishing this goal prove to be non-trivial. This article identifies and briefly discusses some of the issues of mobile computing.

2 The Constraints of Wireless Communication

Generally wireless computers have less resources relative to stationary (wired) computers, this is because wireless computers are required to be smaller, lighter and consume less power than stationary computers. Wireless communication is more difficult to implement than wired communication because of the interaction of the surrounding environment with the message signal. Problems caused by the environment include blocked signal paths, echoes and noise. Hence wireless connections are more error prone, have much lower bandwidths, and have frequent spurious disconnections when compared to wired connections. These factors can increase communication latencies due to error control checks, retransmissions, time-out delays and brief disconnections.

2.1 Disconnections

Since wireless communication is so susceptible to disconnection, it is of great concern when designing successful mobile computers. Resources can be allocated to handle disconnections more elegantly, or to try and prevent those disconnections from happening. In environments with frequent disconnections it is better for the mobile computer to act as a stand-alone unit rather than a mobile terminal (i.e. splitting the application and the user interface across the network). For wide-area networks, round-trip RPC delays will tend to be expensive in terms of wasted processor clock cycles, hence round-trip latencies and brief disconnections can be made less expensive by operating asynchronously. The advantages of using synchronous systems include a substantially simpler design, implementation and debugging. Whereas the asynchronous model will yield considerably higher performance because the receiver does not block waiting for the requested data. Caching techniques could be used to enhance the performance of weakly-connected and disconnected operation, but preserving cache coherence under weak connectivity can be expensive. The Coda file system solves this problem by maintaining cache coherence at multiple levels of granularity and by the use of callbacks. In the Coda solution, fast cache validation is performed by comparing version stamps maintained by the clients and servers, then preserving the validity through callbacks. This approach provides a trade-off between precision of invalidation for speed of validation.

2.2 Low Bandwidth and Bandwidth Variability

Wireless networks deliver lower bandwidth than wired networks, hence mobile computing designs need to be very concerned about bandwidth consumption. The deliverable bandwidth per user depends on the number of users sharing a cell. The network's capacity can be measured by its bandwidth per cubic meter. This value can be improved in two ways:

Maintain multiple cells at different frequencies. This technique, although more flexible, is limited by the range of frequencies of the electromagnetic spectrum available for public consumption.
Limiting transmission ranges so that more cells can fit in a given area. This preferred approach is simpler, reduces power requirements, and may decrease corruption of the signal. It is also known that transceivers covering less area can achieve higher bandwidths.

Techniques such as compression, logging (making large requests out of several short ones), prefetching (guessing which files will be needed soon), and write-back caching can help cope with low bandwidth. System performance can be further enhanced by scheduling communications intelligently.

Mobile computing designs must cope with much greater variations in network bandwidth than traditional designs. A good design would be able to adapt to the currently available resources, providing the user with a variable level of quality. As a mobile element leaves the range of one network transceiver it switches to another, and there may also be places where they can access multiple transceivers on different frequencies. Concurrent use of a wired network and a wireless network maybe possible too. There may be a need to change access protocols, for example when switching from cellular coverage to satellite coverage.

2.3 Security Concerns

The security of wireless communication is more easily compromised than wired communication, especially for wide area networks. This is further complicated if users are allowed to cross security domains, for example in a large firm where sensitive data accessible to only authorised personnel and other data that is freely accessible to anybody with a mobile computer. Secure communication over such networks is accomplished by encrypting the sensitive data usually by using an encryption key, only parties knowing the key will be given access to the data.

3 Mobility

The volatility of some information is increased with the ability to change locations while connected to the network. Certain data that may have been considered static for stationary computing now becomes dynamic for mobile computing.

As mobile computers change location, they will use different network access points, or 'addresses'. To communicate with a mobile computer, its latest address must be known. Several techniques may be used to determine the current network address of a mobile unit.

Selective Broadcast: If a mobile computer is known to be in a set of cells, then a message could be 'broadcasted' to these known cells asking the required mobile unit to reply with its current network address.
Central Services: A logically centralised database contains the current addresses of all mobile units. Whenever a mobile computer changes its address, it sends a message to update the database.
Home Bases: This is essentially the limiting case of distributing a central service, i.e. only a single server knows the current location of a mobile computer.
Forwarding Pointers: This method places a copy of the new address at the old location. Each message is forwarded along the chain of pointers leading to the mobile computer. This requires an active entity at the old address to receive and forward messages.

Traditional computers do not move, hence location dependent information such as the local name server, available printers etc. can be configured statically. A mechanism is needed for mobile computers to obtain configuration data appropriate to the present location. There maybe a need to obtain information on other mobile devices, and this may cause a breach of security. Privacy will need to be maintained in some cases where location information of a user maybe misused.

4 Portability

The design of a hand-held mobile computer should strive for properties such as small size, light weight, durable, and long battery life. Minimising power consumption can improve portability and thus lengthen the life of a charge.

Power consumption is proportional to CV²F, where C is the capacitance, V is the voltage swing, and F is the clock frequency. Hence reducing any of those factors will in turn reduce the power consumed. New portable systems include Advanced Power Management systems that power down components of the computer when idle. Applications too can help conserve energy by reducing on their appetite for computation and communications.

Storage space on a mobile computer is limited by the size and power requirements. Since disks consume more power than RAM chips they are less popular and tend to be a liability. Some solutions to storage problems are compression, code sharing, remote access storage, and interpreting script languages instead of executing compiled code.

4.1 Small User Interface

For smaller and more portable devices current windowing techniques are inadequate. It is impractical to have several windows open at the same time on a small screen even at high resolutions. Due to a shortage of surface area of portable computers, it may be feasible to trade buttons for some other form of input. The forms that may be feasible are hand writing, voice and gesture recognition. Most PDAs implement hand writing recognition, and are typically 96-98% accurate when trained. Common pointing devices such as a pen or a mouse are used with mobile computers because of their ease of use while mobile, their versatility and their ability to substitute the keyboard.

5 Conclusion

Wireless communication brings challenging new problems. Mobility makes information dynamic. Portability entails limited resources available on board to handle the variable mobile computing environment. The challenge for mobile computer designers is to incorporate wireless communication, mobility, and portability to adapt the system designs that have worked well for traditional computing.

References:

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M. Satyanarayan, Mobile Computing: Where's the Tofu?

School of Computer Science, Carnegie Mellon University, 1996.

G.H.Forman, J.Zahorjan, The Challenges of Mobile Computing,

Computer Science & Engineering, University of Washington, 1994.