Challenges of Mobile Computing Forman Zahorjan 1994 Wireless

Challenges of Mobile Computing [Forman & Zahorjan, 1994] • Wireless Communication – – – Disconnection Low Bandwidth High bandwidth variability Heterogenous network Security

• Mobility – Address migration – Location-dependent information – Migrating locality • Portability – – Power consumption (see Table 1) Form factor Small user interface and storage capacity Risks to data and physical damage

Xerox PARCTAB A first attempt at a thin handheld client for ubiquitous computing – office environment

Ubiquitous Computing Philosophy • Demand less of our attention than current computers • Advantages of an intelligently orchestrated and highly connected system. • Context-aware features – – User’s current location Identities of user and nearby people Identities and status of nearby resources Physical parameters (time, temperature, …)

PARCTAB Design Goals • Small, light and aesthetically pleasing • Reliable wireless connectivity • Tracking mechanism to know its location (resolution of a room) • Run on batteries for atleast one day without recharging • Allow casual interaction (even with one hand) • Display graphics & text, touch-screen • Reasonable cost of hardware and network

Hardware (1995!) • 12 MHz, 8 -bit Intel 87 C 524 Microcontroller (with low power modes) • 128 KB memory • 2. 4 in*1. 8 in LCD (128*64 monochrome pixels) • Buttons + Touch-screen • 215 g in weight (battery is 70 g) • Consumes 27 m. A at 5 V (normal) and 30 u. A (low power mode) – needs only around 1 charge per week (around 400 minutes)

Communication • Space and power constraints – Infrared (IR) • 9600/19200 baud • IR signals are contained by walls • One transceiver per room (cell) • Transceiver is connected to RS-232 port of a workstation in the room (which is on a LAN)

Transceiver • Coverage around 20 ft radius • Performs transmission, receiving, coding, decoding, buffering, protocol checks. • Transmission: 2 dozen IR emitters placed at 15 degree intervals • Reception: 2 detectors provide 360 degree coverage

Transmission Control 1 1 Type Len. 4 Dest. 4 Src. 3 -247 Payload 2 CS • Time division multiplexing of medium • CSMA protocol to provide access to IR channel with exponential backoff when busy. • Ack needed bit in Type forcing explicit acknowledgements

User Interface • Buttons + Touch-screen • Keyboard entry + Unistrokes • Display can only show 8 lines of 21 characters • Elision and incremental searches

Software Architecture Tab 1 Agent RS 232 Tab 1 IR Gateway Tab 2 Agent Tab 4 Tab 3 Shell Locate Shell Gateway IR Video Tab 3 Agent Vote Calendar Tab 2 IR Tab 4 Agent Gateway Transceiver Ethernet Shell Mail

• Tabs resemble dumb terminals • Execute simple local functions in response to remote procedure calls (RPCs) • There is one tab agent for each tab that maintains current info. (eg location) • Request from application comes to agent, which forwards to appropriate gateway, which then sends it to transceiver on RS 232 to be broadcast using IR • Events from tab are sent to the appropriate agent in the reverse direction

• Simple tab functions (displaytext, displaybitmap, generatetones, wakeup, etc) • Generates beacon events every 30 secs for location • Gateway uses a name service to locate tab agent • Appends a return address and location id to the agent • A centralized location service keeps track of each tab’s location that is updated by tab agent

Example Applications • Information access – Weather forecast, dictionary, thesaurus, file browser, WWW, Diary (of day’s activities) • Communication – Mail, locator, pager • Media applications • Computer supported collaboration – Group pointing, voting, … • Remote Control

The Info. Pad Mutimedia Terminal • For wireless information access and display of multimedia data • Again a very thin client – does only wireless communication and I/O processing • It is like a switch between the backend and the I/O devices

Info. Pad Hardware Wireless Network Interface (FPGA) Speech Codec Video Decomp Processor ARM 60 10 MHz 512 K RAM, 128 K ROM Display Keyboard, Pointer Other I/O

Power breakdown (9. 6 Watts)