A Teacher's Guide to Distance Learning
A major concern for distance learning is the connectivity and transmission speed between the teaching site and the students. Some distance learning technologies use analog transmissions and some use digital. Traditional distance learning techniques (such as telephone and videotape) are analog (represented by a continuous waveform). Newer technologies (such as the computer and desktop videoconferences) are digital (represented by binary codes of zeros and ones).
The trend is to move toward primarily digital systems. The problem is that digital files (especially audio and video) are huge, and they require "pipes" or cables with tremendous capacity to transmit quickly and effectively. The transmission capacity of a cable or a technology is referred to as the bandwidth. The greater the bandwidth, the greater the amount of digital information that can be transmitted per second.
Access to the Internet through a standard modem that transmits at 28,000 bits per second (28.8Kbps) can be excruciatingly slow -- causing jerky movies, disjointed sounds, and long wait times. There are several options available now or in the near future that will help to expand the bandwidth and increase the speed of information transfer. These options include ISDN lines, T1 lines, ADSL modems, cable modems, and satellite delivery.
Standard Modems
The "standard" speed for modems is currently between 28.8 Kbps and 56 Kbps. Those speeds can provide effective communications via e-mail and Web sites that do not have extensive graphics. Advantages of standard modems include low cost and compatibility with standard telephone lines.Although the bandwidth and speed of modems continues to improve, they are far too slow for most video applications. In addition, two modems of different speeds will communicate at the slower of the rates. For example, if you have a 56 Kbps modem, but your Internet Service Provider has 28.8 Kbps modems, you will only be able to communicate at 28.8 Kbps. Other factors, such as the amount of congestion on the Internet, also affects the transmission rate.
ISDN
ISDN stands for Integrated Services Digital Network. It is a totally digital system designed to transmit information faster than standard modems. ISDN is often used for desktop videoconferencing or Internet access. A single ISDN line with two channels can transmit data at 128 Kbps (about five times faster than a regular modem). ISDN telephone lines use interface devices (called ISDN terminal adapters or ISDN modems) to connect to computers (see Figure 3).
Figure 3. ISDN connections.
ISDN has great potential for distance learning because it can use the copper telephone wire system that is currently in place. To implement ISDN on a large scale, however, telephone companies need to upgrade their switching equipment, and homes and schools need to upgrade their telephones and computer interfaces.
At present, ISDN availability and costs vary dramatically. In some areas, ISDN lines are available for nearly the same cost as standard voice lines, but, in other areas, they are either very expensive or unavailable. When checking on the price of an ISDN connection, be aware that some systems require a connection fee, a monthly fee, and a charge per minute.
T1 and T3 Lines
A standard T1 line (also referred to as DS1) allows digital information to be transmitted at 1,544 Kbps (1.544 Mbps). This transmission speed is almost 54 times faster than a 28.8 Kbps modem. Because T1 lines can be quite expensive to lease, many schools lease a "fractional" T1 line through which they have access to a portion of the bandwidth.T3 lines (also referred to as DS3) are even faster than T1 lines. The T3 lines can transmit data at 44.736 Mbps. This is roughly equivalent to 29 simultaneous T1 lines. T3 lines are extremely expensive, though. In most cases, T3 lines are used to connect parts of the Internet backbone or to connect supercomputers at government and research sites. Both T1 and T3 lines can support video, audio, and data transmissions.
ADSL Modems
ADSL stands for Asymmetric Digital Subscriber Line. ADSL modems can transmit data to users at up to 9 Mbps. The return rate (back to the ISP or Internet) is not quite as fast -- only 640 Kbps. In most cases, the difference in the transfer rates is acceptable for Internet access. We are most likely to receive large files from the Internet (such as graphics and video) that require the faster rates. On the other hand, we generally do not send back as much data to the Internet (perhaps an e-mail message or a click on a hyperlink). Therefore, the slower rate on the return segment is not detrimental.A major advantage of ADSL technology is that it uses standard, copper telephone lines; however, the telephone lines in many areas need to be upgraded to allow the rapid transmission of data. Several companies, including Bell Atlantic, Pacific Bell, and GTE are promoting ADSL and plan to charge approximately $100 per month (Feeley, 1997). An ADSL modem is required as well as an Ethernet card for your computer.
Cable Modems
In some areas, cable companies are offering Internet access through the same cable that delivers television signals to our homes. If your area has been configured for this service, you can connect a cable line to a network card on your computer.The main advantage of cable modems is the bandwidth. Cable modems can bring data to your computer at roughly 400 times faster than a regular modem (Salvador, 1996). If you have a 10 Mbps network card in a computer, you may be able to receive information at that speed. As illustrated in the table below, cable modems offer one of the fastest technologies available for Internet access (Feeley, 1997).
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Figure 4. Comparison of sample transfer rates.
Even though cable modems are faster than most other technologies, they are not the most expensive. The relative expenses for monthly use of the various technologies are illustrated in the table below.
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Figure 5. Comparison of approximate monthly fees. (Feeley, 1997)
Disadvantages of cable modems are that you must have a computer with a network card and you must purchase a cable modem (see Figure 6). In addition, the transfer rate may be slowed if too many people in your neighborhood all connect to the Internet at the same time. Although this technology is new and the standards for cable modems are not firmly established, cable modems offer great potential for high-speed access to the Internet for schools and homes.
Figure 6. Cable modem in a home.
Satellite Delivery
It is also possible to receive information from the Internet via satellite. Satellite access is relatively fast, does not require the installation of telephone or data lines, and is not adversely affected by the number of users.Satellite delivery, however, is usually one-way; you cannot send information back up to the satellite (not on a school budget, anyway). In most cases, a telephone line is used to send information back to the Internet or service provider, and the satellite is used to receive information (see Figure 7). This configuration works well in most cases, because the information you send back is generally very small (a mouse click or an e-mail message); whereas, the information you receive can be quite large (video files, Web pages, etc.).
Figure 7. Connecting to the Internet via satellite.
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Produced by the Florida Center for Instructional Technology,
College of Education, University of South Florida © 1998, 1999.