BandwidthThe basic principle of multiple-use (Section 2.6) says that for distance education, we will use the hardware infrastructure, software, and other technology produced for the suite of NII applications including business, health care, and entertainment.
End-user access to the Web can be based on 28.8 kilobits/second or similar speed modems using POTS (Plain Old Telephone Service) connections, which in practical terms, limits us to displaying audio, text and graphics. Applications involving quality video delivery require ISDN access (128 kilobits/second) or ATM service (scales to Gigabit speeds, individual user typically requires a maximum of a few megabits/second).
Full screen NTSC video display requires about 1.5 megabits/second of bandwidth assuming MPEG1 compression. By comparison, a quad speed CDROM provides transfer rates of 0.6 megabits/second. We note that ISDN provides 10% of the performance needed for full motion video, but for educational applications we need perhaps only quarter screen displays with 15 frames per second (typical educational clip is one to two minutes). Improved compression schemes such as wavelets are becoming available but are not standardized and built into PCs as is MPEG.
The bandwidth required for digital video transmission (1.5 megabits/second) also supports the following applications relevant to distance education:
Role of caching Caching content for use in distance education applications has important performance and quality of content implications. Education is typically carried out in a classroom setting, therefore a group of students browsing related content will naturally provide efficient use of cached resources. In a school experiment conducted by NASA Langley, 95% of the material accessed by students was found in cache, allowing a 28.8 kilobaud connection purchased by a school to look like a dedicated T1 line [Fox:95m].
Caching is also important to content quality. Pre-selecting material for later use in class can be used to ensure students browse relevant and appropriate material, and maintain efficiency in teacher preparation. Many schools limit Internet access due to the perception of easily accessible inappropriate material. Datamining the World Wide Web for resources relevant to K-12 classroom adds great value to Web content. KidsWeb [Kids:95a] was initially developed to support an educational program at NPAC for eighth graders in the Syracuse Area [YSP:95a], and is NPAC's most popular Internet access point.
As described in Section 2.2 and Figure 7, caching is also needed to support a hierarchical Web server/InfoVISION server scenario where content in high demand is replicated and stored close to end-user populations for bandwidth efficiency, and migrated to centralized archives for storage efficiency when demand lowers.
In future distance education experiments, we intend to distribute a CD-ROM to each student. Lessons will be delivered synchronously (all students are on the same page) by transmitting the same relative URL to each client. This approach uses modern (Java) collaboration technology such as Habanero [NCSA:97b] or Tango [TANGO:97b] to fetch data from the local CD-ROM, to display the same page on each client, for example a 100 kilobyte image. This effectively saves a factor of 1,000 in needed bandwidth and allows simple 28.8 kilobaud lines to support a graphically rich curricula.