Glossary

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ADSL (Asymmetric Digital Subscriber Line) - Modems attached to twisted pair copper wiring that transmit from 1.5 Mbps to 9 Mbps downstream (to the subscriber) and from 16 kbps to 800 kbps upstream, depending on line distance. NOTE: webcam technology requires higher upstream speeds than downstream so ADSL lines with lower-end (16Kbps to 256Kbps upstream speeds) will not typically yield the best results.

ActiveX - A loosely defined set of technologies developed by Microsoft. ActiveX is an outgrowth of two other Microsoft technologies called OLE (Object Linking and Embedding) and COM (Component Object Model). As a moniker, ActiveX can be very confusing because it applies to a whole set of COM-based technologies. Most people, however, think only of ActiveX controls, which represent a specific way of implementing ActiveX technologies.

Applet - An applet is a small program written in Java. The EarthCam Applet is a program that looks at a specific image URL and checks if the image has updated. If so, it loads and displays it seamlessly.

Bandwidth - The measure of how quickly you can move information from one point to another. If you are using a 14.4 kbps modem, you have at least six times the bandwidth of somebody using a 2400 bps modem. It's similar to roadways - a four-lane highway can carry more traffic than a two-lane highway.

Cable Modem - A modem designed to operate over cable TV lines. Because the coaxial cable used by cable TV provides much greater bandwidth than telephone lines, a cable modem can be used to achieve extremely fast access to the Internet. Cable bandwidth is a very fast and typically reliable choice for Webcam systems if the upstream speeds are not significantly restricted.

Client - A software program that is used to contact and obtain data from a Server software program on another computer, often across a great distance. Each Client program is designed to work with one or more specific kind of Server program and each Server requires a specific kind of Client. A Web Browser is a specific kind of Client.

Digitize - Converting NTSC or other analog video formats into a digital format that can be stored and viewed on a computer DSL (Digital Subscriber Line) - A method for moving data over regular phone lines. A DSL circuit is much faster than a regular phone connection. The wires coming into the subscriber's premises are the same (copper) wires used for regular phone service.

FTP (File Transfer Protocol) - FTP is the underlying protocol on the Internet for copying files between sites. Whenever a user clicks on a Web link that delivers a file to their machine (for example: <A HREF="FTP://bible.acu.edu/Technology/ctt/netmap.zip">), it is actually FTP which transports the file, even though the user is using a Web browser at the time. There are many "FTP sites" on the Internet, where a system administrator has made files available for "anonymous FTP." That is, users can connect with an FTP tool and download files without having to register on the system where the FTP site is located. This can make large amounts of material easily available on the Internet, and is, in fact, one of the biggest reasons why the Internet is such an incredibly useful tool. The kinds of files made available for anonymous FTP can be nearly anything, from the text of Bunyan's Pilgrim's Progress to specification sheets for out-of-production hard drives. For example, an FTP site run by a software company may have demo versions of the software for sale, device drivers, patch files, help files, and so on.

GIF (Graphic Interchange Format) - A common format for image files, especially suitable for images containing large areas of the same color. GIF format files of simple images are often smaller than the same file would be if stored in JPEG format, but GIF format does not store photographic images as well as JPEG.

Host - Any computer on a network that is a repository for services available to other computers on the network. It is common to have one host machine provide several services, such as WWW, TELNET, and FTP.

HTML (HyperText Markup Language) - The coding language used to create Hypertext documents for use on the World Wide Web. HTML looks a lot like old-fashioned typesetting code, where you surround a block of text with codes that indicate how it should appear. Additionally, in HTML you can specify that a block of text, or a word, be linked to another file on the Internet. HTML files are viewed using a World Wide Web Client Program, such as Netscape or Mosaic.

ISDN (Integrated Services Digital Network) - A way to move more data over existing regular phone lines. ISDN is available throughout the USA and, in most markets, is priced very comparably to standard analog phone circuits. It can provide speeds of roughly 128,000 bits-per-second over regular phone lines. In practice, most people will be limited to 56,000 or 64,000 bits-per-second.

ISP (Internet Service Provider) - An institution that provides access to the Internet in some form, usually for money.

Java - Java is a network-oriented programming language invented by Sun Microsystems that is specifically designed for writing programs that can be safely downloaded to your computer through the Internet and immediately run without fear of viruses or other harm to your computer or files. Using small Java programs (called "Applets"), Web pages can include functions such as Webcam viewers.

JPEG (Joint Photographic Experts Group) - JPEG is most commonly mentioned as a format for image files. JPEG format is preferred to the GIF format for photographic images as opposed to line art or simple logo art. Webcams typically produce JPEG images.

Megapixel Camera - Megapixel Cameras are IP (or Network) based cameras built around Megapixel technology. These cameras produce large, hi-resolution images. The quality of a Megapixel image is usually very high and can be used for print applications. This type of system is more suited for archival applications than for live, real-time viewing.

MJPEG - MJPEG stands for "Motion JPEG" and is a JPEG-based codec. MJPEG is identical to JPEG except that the MJPEG codecs have translators built-in to support the different capture cards. MJPEG is not the same as MPEG, although the names are confusingly similar. The primary difference is that MPEG provides temporal compression, while MJPEG only provides spatial compression. MJPEG codecs are often used as storage formats for large files that need to be archived with good quality. It is a lossy codec, but at 100% quality, the image degradation is minimal. All the JPEG codecs require significant amounts of CPU power and are not well suited for video playback. Large image and/or high frame rate movies usually don't play smoothly.

MPEG (Moving Picture Experts Group) - Pronounced m-peg, a working group of ISO. The term also refers to the family of digital video compression standards and file formats developed by the group. MPEG generally produces better-quality video than competing formats, such as Video for Windows, Indeo and QuickTime. MPEG files can be decoded by special hardware or by software.

MPEG-1 - The most common implementations of the MPEG-1 standard provide a video resolution of 352x240 at 30 fps. This produces video quality slightly below the quality of conventional VCR videos.

MPEG-2 - MPEG-2 offers resolutions of 720x480 and 1280x720 at 60 fps, with full CD-quality audio. This is sufficient for all the major TV standards, including NTSC and even HDTV. MPEG-2 is used by DVD-ROMs and can compress a 2-hour video into a few gigabytes. While decompressing an MPEG-2 data stream requires only modest computing power, encoding video in MPEG-2 format requires significantly more processing power.

MPEG-4 - MPEG-4 is a graphics and video compression algorithm standard that is based on MPEG-1, MPEG-2 and Apple QuickTime technology. Wavelet-based MPEG-4 files are smaller than JPEG or QuickTime files, so they are designed to transmit video and images over a narrower bandwidth and can mix video with text, graphics and 2-D and 3-D animation layers. MPEG-4 was standardized in October 1998 in the ISO/IEC document 14496.

NTSC (National Television Standards Committee) - A video standard established by the United States (RCA/NBC) and adopted by numerous other countries. This is a 525-line video with 3.58-MHz chroma subcarrier and 60 cycles per second. Frames are displayed at 30 fps.

PAL (Phase Alternative Line System) - The European TV standard based upon 50 cycles per second electrical system and 625 lines per frame and 25 fps. (NTSC, the North American standard is based on 30 frames per second; French use SECAM).

Plug-In - A software program that enhances a larger program. Common examples are plug-ins for web browsers that would allow a Webcam to be viewed over a network. A specific browser plug-in is usually required to view streaming media like Windows Media. The idea behind plug-ins is that a small piece of software is loaded into memory by the larger program adding a new feature allowing users to only install the few plug-ins that they need out of a much larger pool of possibilities.

POTS (Plain Old Telephone Service) - The only name recognized around the world for basic analog telephone service. POTS takes the lowest 4kHz of bandwidth on twisted pair wiring. Any service sharing a line with POTS (like DSL) must either use frequencies above POTS or convert POTS to digital and interleave with other data signals.

Router - A hardware device that routes data from a local area network (LAN) to another network connection. A router is connected to at least two networks, commonly two LANs or WANs or a LAN and its ISP's network. Routers are located at gateways - the places where two or more networks connect.

SDSL (Symmetric Digital Subscriber Line) - DSL line with identical upstream and downstream speeds. Typically better suited for webcam applications than ADSL.

Server - A computer or a software package that provides a specific kind of service to client software running on other computers. The term can refer to a particular piece of software, such as a Web Server, or to the machine on which the software is running (e.g. our mail server is down today, that's why e-mail isn't being sent out). A single server machine could have several different server software packages running on it, thus providing many different services to clients on the network.

Streaming Audio/Video - Technology that allows you to play audio and/or video while it is still downloading.

T-1 - A leased-line connection capable of carrying data at 1,544,000 bits-per-second. At maximum theoretical capacity, a T-1 line could move a megabyte in less than 10 seconds.


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