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Video conferencing (VC) allows people located at two or more remote locations to see, hear and interact with each other in real time using computer and communication technology. Each video-conferencing system consists of a number of audio and video sources connected to a CODEC (Coder DECoder) which is the heart of any VC system. The video sources could be cameras and PC. The remote video is usually displayed on a Monitor/TV or projection system. The audio of the remote speakers are available from CODEC through audio amplifier and speaker system.
The local and remote systems (CODEC) can be connected either through Internet or ISDN (Integrated Services Digital Network) line and accordingly two technologies are involved in setting up a call i.e. H.320 for ISDN and H.323 for IP (Internet Protocol). The calls can be between two parties i.e. point-to-point or more than two i.e. multiparty.
Besides the CODEC and all associated audio-video components and networking connection, other essential centralized components of any VC system are Gatekeeper, Gateway and MCU (Multipoint Conferencing Unit). All VC systems in a network are assigned with a unique IP or ISDN No. and they are registered with the gatekeeper. As it is not always possible to remember the IP address of each VC points, they are assigned some aliasing numbers. The gatekeeper takes care of the function of address translation (alias no. to its corresponding IP) of all registered VC systems as well as bandwidth management.
For a multiparty conference involving more than two parties, the MCU which is a central multimedia server, acts like a virtual meeting room. Each MCU has the limitation of number simultaneous calls that it can handle. However, the concurrent call handling capacity can be increased by cascading multiple MCUs.
As most of the CODECs have the provision of both IP and ISDN connectivity, so there can be calls from IP system to any ISDN system or vice-versa. As both use different protocols (H.320 or H.323) so a third component called Gateway does the necessary functionality to allow call between the two dissimilar protocols.
The availability of sufficient bandwidth is the main constraint factor for the quality of communication. The volume of data generated per second being very high cannot be transmitted over the limited bandwidth. So the data is compressed before transmission and decompressed before reproduction at the remote end in the CODEC depending on the available bandwidth. Different compression techniques are used to compress the audio and video like H.264/MPEG-4 AVC for video and G722/G722.1 for audio.
Besides high resolution cameras and monitors at each end, we need at least 512 Kbps for IP based system and at least 384 Kbps for good quality video conferencing. Most of the current day VC systems support a bandwidth of about 4 Mbps for IP and 2 Mbps for ISDN call. With 8 Mbps leased line connectivity between Kharagpur and two of its extension campuses at Kolkata and Bhubaneswar, we operate three concurrent VC sessions each operating with 2 Mbps bandwidth.
The cameras used are usually high definition (HD) PTZ (Pan/Tilt/Zoom) type having number of preset positions which can be recalled instantaneously. This allows focusing on to a speaker quickly at the touch of a button, thereby minimizing the time to focus on to a student in a classroom environment. A HD TV/ Monitor or projector system is used for viewing the remote site. A picture-in-Picture (PIP) and Picture-and- Picture (PAP) feature allows viewing both local and remote sites simultaneously. The monitor screen layout in a multiparty conference can be adjusted locally, setting either fixed size of all participating sites or bigger
window for the active speaker.
All the components connected with the CODEC, like camera, monitor, projector, microphone, speaker system can be operated independently using separate remote controls provided with each. As it is cumbersome to have so many remote controls at the disposal of operator for proper operation, Crestron control is employed than can control all devices remotely from a single source using either touch panel or laptop. The Crestron processor is interfaced with all devices, whose operations are desired to be controlled remotely through IR (infrared) probes and it is programmed in such a way that the operator can control them easily using user friendly GUI (Graphics User Interface) of the touch panel or laptop.
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Center For Educational Technology. IIT Kharagpur