At present, the market offers a variety of video terminals, ranging from high-performance devices for professional settings to budget-friendly options for everyday consumers. These terminals cater to diverse needs, ensuring accessibility across different sectors.
When it comes to video conference transmission, there are primarily two types of network environments in use. One is circuit-switched networks like ISDN, DDN, and PSTN, while the other comprises packet-switched networks such as ATM, IP, and Frame Relay.
For most individual users and small businesses, ISDN lines offer an economical and practical choice. ISDN leverages the public telephone network to provide end-to-end digital channels for various telephony services, including voice and non-voice. ISDN lines can transmit conference television at speeds from 384kb/s to 2Mb/s. Given that ISDN charges based on the utilized B channel (64 kb/s), domestic communication via a single B channel matches the standard telephone communication rate.
Users with limited traffic and shorter communication times find ISDN usage more cost-effective than leasing DDN lines or frame relay circuits. It boasts high-speed connectivity, low initial investment, and ease of integration, fulfilling basic video conferencing requirements. Nevertheless, the nationwide ISDN infrastructure is still evolving, with reliability and stability lagging behind dedicated lines.
Large enterprises and institutions often require higher video quality, making DDN private lines a preferred option. DDN provides superior transmission quality, minimal network latency, high circuit reliability, robust network security, and straightforward network setup. Despite these benefits, DDN's semi-permanent connections—where the link remains active regardless of data transmission—result in higher costs. Thus, DDN is unsuitable for short-term communication but excels in point-to-point and multipoint-to-point setups. For instance, connecting multiple branch offices to a corporate headquarters via a multi-site DDN network is feasible. However, if these branches frequently interact, point-to-point connections could lead to excessive costs.
PSTN (Public Switched Telephone Network) serves as an accessible alternative for individual users keen on testing video conferencing. PSTN is widespread, offering broad coverage, ease of use, affordability, and straightforward networking. Yet, its maximum speed of 56 kbps, poor line quality, and low transmission rates limit its potential for video conferencing. Software-based solutions can achieve a 4f/s image rate, but the subpar image quality significantly impacts meeting quality, making it less popular.
ATM (Asynchronous Transfer Mode) networks provide another viable option for those seeking excellent video quality. ATM combines circuit-switching and packet-switching advantages, enabling broadband signal transmission of any speed and supporting voice, data, images, and videos. Its key feature is Quality of Service (QoS) assurance, making it ideal for organizations requiring high-quality lines. Despite high equipment costs and the necessity of an ATM network for access, its reliability and superior image quality are commendable.
The simplest and most affordable way to access video conferencing is through IP networks. IP-based networks rely on packet-switching, which lacks guaranteed orderliness and fixed delays, affecting real-time communication quality. To enhance service quality, protocols like UDP/IP, RTP, RTCP, and RSVP are employed. Video conferencing systems leveraging broadband IP networks like ADSL and FTTB+LAN deliver impressive results. Ubiquitous IP networks are convenient and cost-effective, though their “best-effort†delivery model results in inferior video conferencing compared to dedicated lines like ISDN and DDN. Still, their cost-effectiveness attracts many users, particularly small and medium enterprises with ample bandwidth.
Satellite networks excel in mid- to long-distance video conferencing, covering remote areas like mountains and seas. They offer high transmission rates, strong security, and flexibility in venue setup and relocation. Multinational corporations find satellite networks appealing, as they support multi-channel video, document, and data transmission. However, satellite conferencing is costly, with satellite channel rental fees and ground station construction costs reaching around $200,000. Additionally, satellite communication suffers from significant delays, making it unsuitable for high-real-time-demand systems.
Frame relay access is another professional option, primarily for data services. Frame relay networks are efficient, offer high throughput, and maintain low communication delays. Access rates range from 64 kbit/s to 2 Mbit/s, sometimes reaching 34 Mbit/s. While expensive, these networks provide excellent video quality.
When purchasing video terminals, users should consider audio and video effects, application needs, and environmental factors. Hardware products, featuring dedicated DSP processors, outperform software-based counterparts in video/audio decoding. Hardware terminals also support multiple network environments beyond just IP, ensuring broader compatibility.
Environmental factors influence product suitability. Government offices prioritize audio/video quality and stability. Educational institutions focus on interactive real-time capabilities and multimedia content transmission. Enterprises emphasize product interoperability and network interface richness.
Blindly chasing trends can lead to unnecessary expenses. Users should follow these guidelines: specify application purposes, ensure compatibility with existing equipment, and assess after-sales service. High-quality products supported by reliable technical assistance ensure long-term value.
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