Timothy M. Logan
Associate Director, Information Technology Center
Instructional Technology and Distance Education
Joseph R. Radke
Virtual Teaching Center
Center for Occupational Research and Development
Videoconferencing is the transmission of video and audio between two
sites in relatively real time. This allows interaction between participants at
both sites similar to that which would be possible if they were in the same
room. Videoconferencing has been used in business for years. Now it is
beginning to make its way into education.
This guide is an introduction to the technology of videoconferencing and
its uses in education. It will begin by discussing the different components
required for videoconferencing. This includes not only the videoconferencing
equipment and networking technology but also the international standards
that make them all work together. The guide will then look at some
hypothetical situations and explain how videoconferencing is applied in each.
We hope you find this guide useful in developing a technology plan to
incorporate the many opportunities afforded by the rapidly changing
technology now becoming available to the educational community.
2 Videoconferencing Technology: An Educator’s Guide
2 Overview of Videoconferencing
The technology involved in videoconferencing ranges from simple
cameras and microphones to sophisticated and powerful networking
equipment. The technology seems to be constantly changing and improving.
It doesn’t have to be hard to understand though. The following sections will
explain, as simply as possible, some of the equipment, cables, and
telecommunications techniques used in videoconferencing.
4 Videoconferencing Technology: An Educator’s Guide
Today there are several ways to get data, including videoconferencing
signals, from the sender to the receiver. In evaluating these different
technologies, certain factors are important. Bandwidth, the amount of data
that can be transmitted in a given amount of time, is a major consideration.
This will be the main determinant of audio and video quality. Bandwidth is
usually measured in bits per second. A bit is the basic unit of digital data. A
thousand bits is known as a kilobit (kb), and a million bits is a megabit
Certain technologies are limited to private point-to-point connections
while others are public-switched services. Public-switched services can
connect with any other end-point that has the same type of service. Unless
you have an unlimited budget, cost will also be a concern. Finally, the
service must be available in your area.
Plain Old Telephone Service (POTS)
The most basic telecommunication technology available today is plain
old telephone service, known as POTS. This is the system you use every
time you pick up the telephone. The maximum bandwidth available through
POTS is 33.6 kilobits per second (kbps). As you will see, this is slow
compared with other technologies. The big advantages of POTS are that it
is widely available—nearly every home and business has at least one
telephone line—and it is inexpensive.
A T1 is a digital trunk phone line, capable of transmitting at the rate of
1.544 megabits per second (Mbps). It carries 24 channels of data at 64 kbps
per channel. Each channel is referred to as DS-0. There are different
formats for transmitting data on a T1 circuit. Alternate Mark Inversion
(AMI) and Binary 8 Zero Suppression (B8ZS) refer to the electrical
characteristics of signal as it is put on the line. D4 and Extended Superframe
(ESF) concern the size of blocks (frames) of data transmitted. B8ZS and
ESF are recommended for videoconferencing applications. T1 connections
are dedicated point-to-point connections. Since the connection is always
active, you pay a flat monthly fee and get unlimited usage of the line. A T1
is also very reliable. T3 is a related but higher-speed technology. It has a
bandwidth of over 45 Mbps, the equivalent capacity of 28 T1s.
Overview of Videoconferencing Technology 5
Integrated Services Digital Network (ISDN)
ISDN is a fully digital switched service available from local and long
distance telephone companies. It is similar to POTS in that you are assigned
a telephone number or numbers. Since it is fully digital, higher bandwidths
are available. ISDN comes in two types:
1. Basic Rate Interface (BRI) ISDN
BRI ISDN is made up of two channels of 64 kbps, called B channels,
and one 16-kbps channel, called a D channel. Because of this
configuration, BRI ISDN is sometimes call 2B+D. The D channel is
used by telephone companies for signaling. The customer has access
to the two B channels for a total accessible bandwidth of 128 kbps.
BRI lines can be combined to achieve higher bandwidths. This
process is called inverse multiplexing or IMUXing. Each B channel is
usually assigned a separate telephone number. Each B channel is also
assigned a service profile identification number, known as an SPID.
The service profile to which the SPID refers contains information on
the capability of the channel. Channels can be provisioned for voice or
data. For videoconferencing, both B channels should be provisioned
for data. When configuring ISDN BRI equipment it is often necessary
to know your SPIDs as well as the type of switch the telephone
company is using and the ISDN standard the switch uses. 5ESS and
DMS250 are common switch types. Most ISDN telephone company
switches conform to the National ISDN 1 (NI1) or National ISDN 2
(NI2) standard, although some are custom configurations. The
availability of ISDN is not as great as POTS but is constantly
increasing. Usage charges per B channel are similar to regular toll
2. Primary Rate Interface (PRI) ISDN
PRI ISDN has a bandwidth of 1.536 kbps. It consists of 23
B channels of 64 kbps and one D channel also of 64 kbps. ESF
framing and B8ZS line coding are used on PRI circuits. PRIs are
relatively expensive to install. Usage charges are based on the number
of B channels used and the duration and distance of a call.
6 Videoconferencing Technology: An Educator’s Guide
Cables connect the parts of videoconferencing network systems. They
carry signals, as either electricity or light, from one component to the next.
Different types of cable have different capabilities and costs and thus
different applications. You may encounter the following types of cables.
As the name implies, twisted-pair cables are made up of coated copper
wires twisted around one another. The twists help reduce interference that
can develop when wires run beside each other. Twisted-pair cable comes in
two varieties: shielded twisted pair (STP) and unshielded twisted pair
STP has an extra covering that shields it from external electrical
interference. This may be necessary when running the cable close to
machinery or devices that create a lot of electrical noise.
UTP is rated into five categories. Category 3 (cat 3) cable or above is
adequate for most telephone applications, including ISDN. Category 5 (cat
5) cable is required for some higher-speed networking. Because of its low
price, ease of installation, and flexibility use, cat 5 cable is one the most
popular networking cables in use today.
Coax cable is also made of copper wire. It has a single strand surrounded
by insulation and a grounded shield. Because of the extra shielding, coax is
less susceptible to interference than UTP cable. It has a higher bandwidth
capacity and can run longer distances. It is, however, harder to install and
more expensive. Coax is used on some digital networks. It is also used to
carry analog audio and video signals.
Overview of Videoconferencing Technology 7
Unlike other cables, fiber-optic cables do not carry electricity. They carry
light. Fiber-optic cables are constructed from flexible glass or plastic. They
can carry data faster and farther than any copper cable. The main drawbacks
of fiber-optic cable are the cost and the difficulty of installation. Currently,
the main application of fiber-optic cable is as a high-speed backbone for
A V.35 cable consists of up to 34 wires. It can transmit data at as much
as 2 Mbps. V.35 cables are limited to 100 feet in length before data loss
starts to occur. Therefore, they are not used for general networking
installations. In videoconferencing, they are used to connect the
videoconferencing hardware to networking equipment. V.35 cables use
M-34 connectors. These connectors have 34 gold-plated pins and built-in
screws for secure connection.
8 Videoconferencing Technology: An Educator’s Guide
RS-366 cable, like V.35 cable, is used primarily to connect to pieces of
equipment directly. It is much slower and less expensive than V.35. RS-366
is used primarily for transmitting dialing information, such as ISDN
telephone numbers, to networking equipment. DB-25 connectors, like those
found on most parallel printer cables, are used on RS-366 cable. The cable
has a length limit of 100 feet.
So far, we have covered how data are transmitted over
telecommunication networks. We looked at the features of various
networking technologies and the capacity and application of several cable
types. Let us now turn our attention to how our images and words are
captured and converted into the data that race along these networks.
Overview of Videoconferencing Technology 9
Codec stands for coder/decoder. This is the heart of the
videoconferencing system. A codec takes the incoming audio and video
signals from the cameras and microphones and converts (codes) them to a
digital format. During the coding, the data are also compressed. A regular
television signal contains 90 million bits of information per second. This
information must be transmitted across lines with a bandwidth of as little as
128 thousand bits per second. That is a ratio of over 700:1 for video alone.
The codec must select which pieces of information are important and code
them in an efficient manner. It also takes the incoming data signal and
decodes it back into audio and video signals. Codecs can be a complex
series of electronics in a custom chassis or as simple as a card installed
inside a computer. They can even be just software and use components
already installed in a multimedia computer. In general, the more complex
the codec, the more expensive and higher quality.
The basic function of a camera is to capture video information and send
it to the codec. Simple cameras used in some desktop videoconferencing do
little more than perform this task. More sophisticated remote-control pan-
tilt-zoom (PTZ) cameras offer the ability to rotate side to side (pan) or up
and down (tilt) and change focal length (zoom). These cameras may also be
able to store preset locations that can be recalled with the push of a button.
Some camera systems have the capability to track the instructor. The
tracking system can be an infrared device worn by the instructor or a video-
pattern-recognition system incorporated into the codec. Others have
sophisticated audio camera control systems that use multiple sensors to
locate the speaker in the room and then instruct the camera to move to that
location. The use of 3 CCD or digital cameras is not necessary in
videoconferencing. Since much of the video information is discarded by the
codec, these types of high-quality, expensive cameras do little to improve
picture quality. They may even make the job of the codec harder by
providing more data to be compressed.
A document camera is another type of video input used in
videoconferencing. A document camera has a lens mounted on an arm over
a small flat platform. Paper documents, transparencies, or 3-D objects can
be placed on the platform and transmitted to the codec.
10 Videoconferencing Technology: An Educator’s Guide
Monitors are video output devices. They range from computer monitors,
used mainly for desktop videoconferencing, to large-screen TVs and
projectors. Television monitors in North America and Japan follow the
NTSC standard. Other countries use PAL or SECAM standards. Monitors
should be large enough to be easily seen by all participants. It is frequently
necessary to add more monitors in the classroom for student viewing. A
small, indecipherable video image will soon lose the attention of the
students participating in the videoconference, so special attention must be
paid to providing the students with a clear view of monitors located close
enough for comfortable viewing. In addition, the instructor needs the ability
to see both the far-end and near-end views while presenting.
Lighting in the room should provide sufficient illumination for the
cameras to do a good job of picking up and transmitting the video from
each classroom. Fluorescent lighting with even coverage in the room often
suffices, but additional lights may be needed to illuminate the instructor.
Diffuse or angled lighting helps to eliminate harsh shadows under the eyes
and chin. Outside windows should be covered with blinds or drapes to
eliminate sunlight during a videoconference. Video cameras are not able to
adapt well to the differences in brightness between artificial lighting and
Good-quality audio reproduction is extremely important in the design of
a distance-education facility. Good video with bad audio is nearly useless;
good audio with bad video is less than ideal, but still functional in an
instructional setting. Good audio systems incorporate two elements: pickup
Pickup is accomplished by microphones. Tabletop microphones allow
students to ask questions. These microphones are frequently the push-to-
talk variety. Microphones of this type are muted until a button is pushed to
activate them. This allows microphones to be placed throughout the room
without picking up unwanted noise of pages turning, students’ side
conversation, or pencils tapping. While ceiling-mounted microphones seem
to offer a technically simple solution for student questions and responses,
many vendors and consultants strongly recommend that tabletop
microphones be used instead to ensure that ambient noise and extraneous
noises do not interfere with the educational experience.
Instructor microphones are usually either podium mounted, worn (called
lavalier or lapel microphones), or carried (hand held). A wireless lavalier
Overview of Videoconferencing Technology 11
microphone allows the instructor to retain a natural teaching style while
providing clarity for effective instruction and meaningful student interaction.
Reproduction is accomplished with speakers. Proper placement of
speakers in each classroom allows clear transmission of the spoken
instructional material and questions, ensuring that the interaction between
student and teacher is not interrupted by problems in an inadequate audio
system. With the significant expenses associated with other aspects of
video-based instruction, the proper design and installation of audio systems
are often overlooked. It is imperative that significant attention be paid to the
classroom audio systems that are part of a distance-education facility
Inverse Multiplexer (IMUX)
An IMUX takes the signal produced by the codec and breaks it into
pieces for transmission over separate lines. For example, a
384-kilobit-per-second signal would be broken into three 128-kbps signals
for transmission over ISDN BRI lines (3 × 128 = 384). The IMUX also
takes the incoming three signals and reassembles them into one before the
signal enters the codec.
Wide Area Network (WAN) Access Switch
In businesses and schools, there isn’t a separate telephone line for each
phone in the building. Usually, there is a PBX system that allocates a limited
number of incoming lines to extensions on an as-needed basis. A WAN
access switch performs the same type of function. By using a switch, you
eliminate the need to run lines to every videoconferencing unit. The switch
will allocate available lines to whatever unit is making a call. IMUXing is
often performed by the switch as well. In this case, the switch assigns
multiple lines to each call. Some switches are slotted chassis. This means
that cards can be inserted into them to customize their configuration. As
your videoconferencing needs grow, additional cards can be added.
Channel Service Unit/Data Service Unit (CSU/DSU)
A CSU/DSU is required at each end of a T1. It is responsible for the line
coding and framing of the signal. It also performs diagnostic functions to
help troubleshoot problems on the circuit.
12 Videoconferencing Technology: An Educator’s Guide
The demarc is not a piece of equipment but a place. It is the location at
which the telephone company terminates its lines in your building. It marks
the end of the telephone company’s responsibility.
ISDN BRI lines come into your building on two copper wires. This two-
wire interface is called an U interface. Most videoconferencing and
networking equipment require four wires, known as an S/T interface. An
NT1 device converts the U interface to a usable S/T interface. By
terminating the network, NT1s also provide the telephone company the
ability to perform testing on your line. If an NT1 is turned off for an
extended time, the telephone company may assume there is trouble on the
line and turn the line off. Some ISDN cards and networking equipment have
NT1s built in, eliminating the need for an external one.
Multipoint Control Unit (MCU)
When three or more sites want to conference together at the same time,
an MCU is required. All sites connect to the MCU, and it connects the sites
to one another. During multipoint calls, all sites must connect at the same
speed and use the same algorithms. There are a number of options for
determining which site’s video image and audio appear at the other
participating sites. The MCU can manage the conference so that the site
speaking appears on all screens. The MCU can also rotate among the
participating sites, showing each site in turn for a selectable amount of time.
Some MCU systems allow one location to be designated as the chair site,
controlling which site appears on the participants’ screens. Other MCU
options include the ability to divide the screen into four parts, displaying up
to four separate locations simultaneously at all sites. This option is called
continuous presence or, more informally, “Hollywood Squares.”
Overview of Videoconferencing Technology 13
Remote Distance Modules (RDM)
V.35 and RS-366 cables are used to connect CSU/DSUs, IMUXes,
switches, and other networking components to codecs. These cables have a
distance limit of only 100 feet. What happens when you want to place the
codec more than 100 feet from this equipment? The answer is that you buy
a set of remote distance modules (RDM). An RDM can convert the V.35
and RS-366 signal to a format that can be carried over cat 5 UTP or fiber-
optic cable. An RDM on the other end of the cable, as much as over a mile
away, converts the signal back.
All the equipment in the world will do you no good if it can’t
communicate. A standard is a common language that different
manufacturers’ equipment speaks, allowing it to communicate with others.
Imagine having a telephone and being able to call only someone with the
same make and model of telephone. That was the world of
videoconferencing before standards. The International Telecommunications
Union (ITU) is the United Nations body responsible for setting the
standards for videoconferencing. Each standard is given a letter and a
14 Videoconferencing Technology: An Educator’s Guide
H.320 is actually a group of standards for ISDN and T1
videoconferencing. It includes a standard for compressing audio, called
G.711, a standard for compressing video, called H.261, and a standard for
transmitting these signals, called H.221. By using these standards, any
H.320 system should be able to connect to any other H.320 system. All
H.320 systems are not created equal, however. To meet the standard,
systems are required only to have a frame rate of 7.5 frames per second
(fps). Regular television pictures and better videoconferencing system run at
30 fps, producing a much higher-quality image. G.711 also provides
telephone-quality audio. Other optional audio standards that provide
differing sound quality. G.722 requires more bandwidth but provides
excellent audio. G.728, on the other hand, requires only 16 kbps of
bandwidth and provides less than ideal quality.
H.323 and H.324.
H.323 specifies how videoconferencing is done over the Internet and
local area networks. H.324 concerns videoconferencing over analog
telephone lines. Neither of these methods provides sufficient quality for
T.120 is a group of emerging standards. Instead of dealing with
videoconferencing, T.120 is concerned with data and graphics conferencing.
It address such topics as application sharing, file transfer, and whiteboarding
among multiple sites. With T.120, participants at several different locations
can work on a document or receive a graphical presentation from a single
site. This standard can work alone or with an H.320 videoconferencing
system that is T.120 compatible.
Now that we know all the components that go into a videoconferencing
network, let’s take a look at how this technology is applied. Below are
sample applications for videoconferencing in education. In each case, the
appropriate level of technology was selected for the desired outcome.
Although the solutions presented here are complete and workable, they are
not the only solutions. Each situation you encounter will be different as will
the required approach.
16 Videoconferencing Technology: An Educator’s Guide
Simple single-classroom setup
Ms. X wanted to set up a small system in her classroom to allow students
to take virtual field trips and have guest speakers she would not normally be
able to get into the classroom. Since she already had a multimedia-equipped
computer, she ordered an H.320-compatible desktop videoconferencing
package. When it arrived, she installed the videoconferencing card in an
available expansion slot in her computer and plugged the included camera,
microphone, and speaker into the card.
In the meantime, the technology director for the school ordered a single
BRI ISDN line to be installed at the school’s demarc. From the demarc, he
ran cat 5 UTP cable to an NT1. Through a patch panel, the director
connected the NTI to Ms. X’s classroom. He had the telephone company
provision both B channels for data. Ms. X received the SPIDs, switch type,
and ISDN standard from the technology director. She plugged a cat 5 cable
from the wall jack to the videoconferencing card in the computer and
installed the software. She then dialed a test number provided by her software
vendor and confirmed that her system was working. Ms. X now has a 15-fps
desktop videoconferencing system in her classroom. She can connect with
any other H.320-compatible system with an ISDN connection any place in the
world. She ordered a little more expensive system and received a remote-
control camera and the ability to add an extra video source.
Ms. X now brings speakers in from around the world. Among other uses,
Ms. X’s students have watched a session of the state legislature and later
questioned their representative on the proceedings.
Several people had come to Mr. Y, information technology director for
the school district, requesting videoconferencing services. Mrs. Z had been
approached by several rural schools to share her calculus course. Miss Q, the
guidance counselor, wanted students to be able to take a CAD course at the
technical school across town without busing them back and forth. Finally,
Principal H wanted to get more training for her teachers without spending a
lot of money on travel and substitute teachers.
Mr. Y purchased a group videoconferencing system. It is H.320
compatible and has T.120 capabilities. Three BRIs were ordered, provisioned
for data on all B channels, and installed by the telephone company. Mr. Y
also purchased an IMUX with built-in NT1s. He programmed the IMUX
with the SPIDs and directory numbers of the ISDN lines and the switch
information. The IMUX was mounted directly in the videoconferencing
system’s cart. An existing computer lab was converted for distant learning. A
simple audio system was installed, and extra lighting was added. The 3 BRIs
were connected with cat 5 cable through the patch panel to the room.
Now, Mrs. Z teaches her course to three rural schools that cannot offer
the course. Students take the CAD course for college credit before they
graduate from high school. They use the T.120 capabilities to share their
work with the instructor on the other side of town. Teachers receive regular
professional development sessions without leaving school.
18 Videoconferencing Technology: An Educator’s Guide
A university has made a major commitment to videoconferencing. The
business department both delivers and receives distance education courses. In
addition, they offer virtual job interviews for graduating students. The
medical school has a partnership with a local hospital to research
telemedicine. Finally, the dean wants a desktop system to conference with
other administrators in the state system.
At the center of this complex system is a slotted WAN access switch. This
switch is outfitted with cards for T1 and PRI coming into the switch from the
telephone company. It also needs V.35 cards for the two group systems and a
BRI card for the desktop system. A PRI with B8ZS line coding ESF framing
is installed to connect to the public telephone network. A T1, configured the
same way, is installed between the switch and a similar switch at the hospital.
Two sets of RDMs are installed, one between the switch and the medical
school’s telemedicine laboratory. The other RDM is installed between the
switch and the business school’s distance-learning classroom. A cat 5
connection is made between the switch and the dean’s office. Although this is
not a BRI provided by the telephone company, the switch makes it appear as
such to the desktop videoconferencing system.
The T1 provides the medical school with high-quality, low-delay audio
and video required for telemedicine. The medical school can also call out
using the PRI to reach other H.320-compatible sites. The business school has
the flexibility to offer distant education courses to sites around the world and
bring prospective employers on campus electronically. The dean can attend
important meetings without leaving his office.
District or regional network
The university is selected to be a hub for a videoconferencing network of
area schools, libraries, and hospitals. Since multipoint conferencing is a major
part of the project, an MCU is included in the plan. Additional cards are
bought for the switch to accommodate a connection to the MCU and T1s.
T1s are installed from the switch to an IMUX at each site. Each site sets up a
local system similar to those described above.
Any site on the network can make a point-to-point call to any other site.
Three or more sites can participate in a multipoint call. Each site now also
has access to the PRI installed at the university. All these connections are
managed through the switch at the central site.
20 Videoconferencing Technology: An Educator’s Guide
The development of videoconferencing technology has opened countless
opportunities for educators. The ability exists today to bring the world right
into the classroom in an effective and affordable way. The future will be only
brighter. Videoconferencing systems are getting better and less expensive.
New network technologies promise to bring higher-bandwidth connections to
more places around the world. These capabilities, if used correctly, can open
whole new horizons for teachers, administrators, and students.
22 Videoconferencing Technology: An Educator’s Guide