Mobile Computing

``Supporting the WWW in Wireless Communications Through Mobile Agents'' Hadjiefthymiades, Matthaiou and Merakos, Mobile Networks and Applications, August 2002.

Build on proxy approach. See Figure 1. Base stations cache content for mobile hosts.

Issues:

1. How to handle caches when clients move? Figure 2 shows that base station cache contents move.
2. How much of cache contents to move? Full or partial? See Fig 2.
3. Relates to the problem of path prediction algorithm (PPA).
4. Also the issue of merging cache contents.

Implement wireless architecture with IBM Aglets (Java mobile agents) framework.

Show that basic framework can be built with these mobile agents. Details less important.

Issues with Mobile Devices

• Connectivity. Getting better with 3G networks.
• Physical limitations of devices - CPU, memory, display, input devices, power supply.
• high percentage of sensitive (personal) information.
• intent -- not to access all content, but to answer specific questions.

Solutions

• content delivery

  • delta encoding
  • compression
  • image transcoding (distillation)
• base proxy server does work for the mobile device (DNS lookups, prefetch objects)
• reuse cached object for use with ``back'' button
• WebViews: macros of web navigations from a wired connection, then apply the macros on a mobile device.
• content summarization--either by original content provider or by proxies using heuristics

Caching not as effective when wireless bandwidth is poor because of discrepancy between wired and wireless rates.

Wireless Application Protocol

WAP-enabled phone usability study in 2000 said 70% of 20 trial users would not buy the technology. Jacob Nielsen: WAP ``Wrong Approach to Portability''. But using slower technology at the time.

http://www.wapforum.org. Works with IETF and W3C.

WAP 1.0 1998. WAP 2.0, January 2002.

• support for IP, TCP, TLS and HTTP. ``interoperable optimizations''
• WAP application environment (WAE) or ``WAP browser'', XHTML Mobile Profile.

WAE supports style sheets (CSS).

WAP Programming Model

See Fig 1 and 2.

Generally a ``pull'' model based on user requests.

WAP 2.0 does not require a WAP gateway (as WAP 1.0 did), but can use a WAP proxy for enhanced features--location, privacy and presence based services.

One example is push-based services where proxy pushes content to mobile devices.

WAP Protocol Stack

WAP 1.0, many specific protocols (see Fig 3).

• Wireless Session Protocol (WSP)
• Wireless Transport Protocol (WTP)
• ...

WAP 2.0 for wireless networks supporting IP (see Fig 4):

• Wireless Profiled HTTP (WP-HTTP). Message body compression, establishment of secure tunnels.
• Transport Layer Security (TLS) - wireless profile of TLS.
• Wireless TCP (WP-TCP) - connection-oriented services.

Other Features

• WAP Push - a proxy can push content to devices. Client polling is expensive. Could use timed invalidations so wireless devices know when to wake up.
• User Agent Profile - based on Composite Capabilities/Preference Profiles (CC/PC) work of W3C.
• Data Synchronization
• Pictogram - consistent use of images