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TABLE OF CONTENTS
APPENDICES |
GlossaryConnectivity Connectivity is the way in which videoconferencing stations share audio, video, and file data. (Is this really the definition? Are the references throughout text correct, then? Connectivity vs. connection--terms must be consistently and clearly used.) An overview of connectivity can be broken down into the signal and the components that carry and manipulate it, how the whole videoconferencing system functions, how the videoconferencing system is controlled, and the basic setup of the videoconferencing system. Signal The signal is either an electric current or an electromagnetic field, whose fluctuations can be used to encode information. The carrier signal for a videoconference contains the encoded information used to produce audio, video, and data displays in the videoconference classroom. The example of the telegraph, in the sidebar, exemplifies an extremely simple use of a carrier signal. The need to send complex information has led to innovations that allow more than the simple "on" or "off" information sent on the carrier signal described in the telegraph example. However, the principles of sending information over long distances, and of coding and decoding, remain the same as for the simple telegraph example. Telegraph The telegraph uses DC current as a carrier signal. The sender presses a key to turn on the current, and releases the key to turn off the current; information is encoded in terms of whether the current is on or off at a given time. Decoding is accomplished by a device that responds to whether the current is on or off. Thus, when the sending key is depressed, the circuit is completed and an electromagnet causes the "clicker" to make a sound; when the send key is released, the electromagnet is turned off and the "clicker" is reset. The decoding device interprets the on/off signal as sound or no sound, and the tele-graph operator decodes the series of sounds--dots and dashes--as alphanumeric characters. Analog Signal An analog signal is generated as an electronic response to something in the real world. For example, a microphone in a telephone converts sound waves from the human voice to an electrical analog signal. This analog signal is then carried over telephone lines to the receiving telephone where it is converted back into sound waves. Transmitting an analog signal to the receiving site allows more chance for "noise" (error) to get into the signal than transmitting a digital signal. Digital Signal A digital signal is created by converting analog information into a binary format--into a series of ones and zeroes which describe the analog signal. Computers and compact disk players process digital signals. Digital signals are less susceptible to "noise" than analog signals. Digital signals may be "compressed" (see "Codec" below), thus reducing the amount of information that must be sent to have a usable signal at the other end. Codec (coder/decoder or compression/decompression) The codec converts analog data to digital form, and digital data to analog form. A codec that converts analog data to digital form may also "compress" data, by eliminating redundant information, which reduces the amount of data that needs to be sent for the videoconference; another codec that converts digital data to analog form may "decompress" such data. This reduces the bandwidth needs of a videoconferencing system (see "bandwidth" below) and thereby speeds up transmission. Digital Decompression Decompression is the process by which compressed information is reconstructed to share the information that was sent. The point of digital decompression is to reproduce an analog signal that will, in turn, produce sound that sounds like what went into the microphone, or video on the monitor that fairly represents the video collected by the camera at the other end. Bandwidth Bandwidth is a measure of the range of frequency that a signal occupies. It is directly proportional to the amount of data that can be transmitted. A larger bandwidth means that more data can be sent. Bandwidth is important in videoconferencing for determining the quality of the video that can be sent and displayed reasonably--inadequate bandwidth means unsatisfactory video transmission. Because bandwidth costs money, schools and institutions try not to buy more than they anticipate needing. This consideration has been a major factor in the development of technologies for digitizing and compressing signals. Modulation Modulation technology is used to take advantage of the characteristics of carrier signals (optical or electronic) to send additional information. It uses markers that recognize changes in things like voltage, waveform phase, frequency, and binary encoding (digitizing). Better modulation technology results in more efficient use of the carrier signal and ultimately provides better quality video and audio produc-tion. As modulation technology becomes more complex, it becomes more difficult for different systems to communicate because of the different ways of coding and then decoding the carrier signals. That is a reason why there is a movement to establish standards that will allow communication between different brands of telecommunication systems. Multiplexing Multiplexing is technology that permits better use of the carrier signal bandwidth by sending more than one modulated signal within a bandwidth. This means that more than one simple signal can be transmitted as part of a single complex signal, and that these simple signals can be separated at the receiving end. Transmission Medium The transmission medium is the means by which carrier signals are moved between end points e.g., videoconferencing stations. This includes a particular technology (which can include hardware and/or software) and a physical medium to transmit the signals. A brief comparison of some better-known ones is shown below (adapted from Whatis.com http://www.whatis.com/thespeed.htm). This is a simplified display of a few common transmission media. Fully interactive videoconferencing requires at least 384 Kbps of bandwidth. Carrier Technology Speed Physical Medium Application ISDN 1.544 Mbps T-1 Medium and large enterprise access Satellite 400 Kbps Air Faster home and small enterprise access. Cable modem 512 Kbps to 52 Mbps Coaxial Cable Home, business, school access Ethernet 10 Mbps 10-base-T Most popular for local area networks (LAN)
Bridge Bridges and routers connect the participants in a videoconference to one another. The distinction be-tween a bridge and a router has to do with whether or not addresses and the physical path to that address is set or not. A more detailed account of bridges and routers can be found at Connected: An Internet Encyclopedia, at http://www.freesoft.org/CIE/index.htm. The System A videoconferencing system is a group of components that work together to support communication by sharing video, audio, and data files with distant sites. The diagram below identifies some of those components and how they interrelate. The different color lines show where the signals are generated and the arrows show the direction that information flows. The components can be grouped into viewers (monitors and speakers), senders (cameras, microphones, whiteboards, computers), controllers (key-board, tablet, or remote), processors (codec and bridge/router), and carriers (wires, microwave in air, satellite signals, fiberoptic cable). Router (see Bridge)
Interrelationships of videoconferencing components
Senders Senders generate analog signals that are digitized, compressed, and sent to distant videoconferencing sites. These might include any or all of the following.
Viewers When the compressed and digitized sender signals are received at the receiving site, they are decoded and displayed through the receiving site viewers--usually monitors and speakers. Lead teachers should use a monitor to observe what the lead site is sending to receiving sites. This is particularly important while presenting. Lead teachers should also have a screen, separate from the videoconferencing monitors, for any computer they use, in order to get computer displays ready without having to manipulate the views.
Processors
Physical Signal Carriers Physical signal carriers are the wires, fiberoptic cable, and air that carry satellite and microwave signals. If physical signal carriers fail, lead and receiving sites cannot communicate. Troubleshooting Problems with connectivity generally relate to a broken link between controllers, senders, or viewers. Troubleshooting should begin with the simple things. Is the device plugged in? Has the connection come loose? If it appears that these are not the problem or it is obvious that the problem lies with the communication between codecs, then the system should be rebooted by shutting it down and starting it again. If this does not reestablish connectivity, technicians should be called. Control Control consists of interface devices, called controllers, and how they are deployed to facilitate presentation, called the basic setup. Controllers Controllers are interface devices that interact with software and allow you to control the videoconference. The keyboard, tablet and pen, and remote control are the most common interfaces. They all allow lead teachers to control such things as camera angle and zoom, microphone volume, putting a picture in a picture, which video feed will be sent to receiving sites, which receiving sites will be connected through a bridge or router, and transfer of files. Controllers Controllers are interface devices that interact with software and allow you to control the videoconfer-ence. The keyboard, tablet and pen, and remote control are the most common interfaces. They all allow lead teachers to control such things as camera angle and zoom, microphone volume, putting a picture in a picture, which video feed will be sent to receiving sites, which receiving sites will be connected through a bridge or router, and transfer of files.
Basic Setup The basic setup of equipment and the layout of the lead site is an important part of the control that lead teachers have over their presentations. These are some issues that lead teachers should consider.
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