Airdisks and airRAID (expanded extract)

Abstract

A new generation of low-cost, low-power, and portable personal computer systems is emerging; sometimes these are referred to as palmtops or Personal Digital Assistants (PDAs). One of their key features is that they utilize wireless communication media, thus freeing the user from the constraints of wired or tethered communication. In fact, the wireless medium becomes a critical component of the I/O subsystem, allowing communication with fixed servers and other users. In particular, the broadcast nature of the wireless medium can be exploited to efficiently transmit information required by a large number of PDA users (e.g. stock quotes, sports updates, etc.), with software on the PDA being used to filter the information and present only the information of interest to the PDA user.We introduce a simple model, called the airdisk, for modeling the access of data transmitted periodically over wireless media as being analogous to the access of data from a standard magnetic disk. We consider several issues related to airdisks, such as their mean rotational latency under certain assumptions. The problem of scheduling the order in which data items are broadcast is analogous to that of determining how data should be laid out on the disk. Two problems of laying out data so as to minimize read time, given information about which data items are of most interest to the clients, are defined; both are shown to be NP-complete. We discuss ways in which the information about which items are of interest to clients can be obtained. Finally we consider how to increase the performance and storage capacity of airdisks, using the magnetic disk analogy as a guide. We suggest using multiple-track airdisks or borrowing the idea of Redundant Arrays of Inexpensive Disks (RAID) which is used for magnetic disks; for the wireless data broadcast environment we call the latter approach airRAID.