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Date: 2015-10-07; view: 437.

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DIGITAL VIDEO

Scan the text and match the headings (a – d) with the paragraphs (1 – 4).

Text A

Reading

a) Video Reliability

b) Video Quality

c)Implications for Network Infrastructure

d)Compression Techniques

With digital video we are able to take two of our senses, sight and sound, convert the analog signals, and combine them in the digital realm. By converting our analog world into the digital realm, we can more easily manipulate sight and sound.

Our vision is inherently analog based. To convert that analog world to a digital one, we need a device to sample analog signals and convert them into the digital domain. This is done using a Charged Coupled Device¹ (CCD). A CCD performs sampling and outputs digital information. Once an image is captured, raw video is converted to more efficient formats that can be manipulated, transported, and stored. In order for businesses to take advantage of the benefits of digital video and to make digital video applications more affordable to implement, numerous compression techniques have been developed. Video compression methodologies take the original, raw video data and shrink it using methods that can either restore the video back to its original state called lossless compression when uncompressed or to a close approximation to the original called lossy compression.

Historically, video transmission (synchronous video with audio) was accomplished using traditional analog communication techniques over a coaxial cable physical infrastructure. For both residential and business environments this typically means supporting overlay networks. In addition to the added costs associated with designing, implementing, and maintaining separate networks, video networks have became increasingly complex as the size of the video network increases. Once the video signal (synchronous video with audio) has been digitized, transporting this signal over a communications network based on standard networking technologies like ATM and Ethernet/IP becomes much easier and cost effective for most businesses and even residential video service delivery. However, due to the latency requirements for video signals, the supporting network infrastructure must exhibit several key characteristics discussed below.

One of the most difficult tasks is to determine the level of video quality that is adequate and required for a certain need. Needs range from video conferencing in a business environment to video surveillance in a public safety environment, to broadcasting for entertainment purposes. The trade-off surrounds quality versus cost, and it centers on the level of video quality necessary to achieve the desired level of realism from the video transmission. These decisions cover the range of digital video solution components discussed above. Cameras, viewing devices, compression methods, and appropriate network infrastructure must be designed and selected to ensure that your video will meet users expectations. Regarding the network infrastructure segment of the overall digital video solution, latency is the main issue. Latency in networking is the amount of time it takes a packet to travel from source to destination. Together, latency and bandwidth define the speed and capacity of a network. In order to address the latency requirements of high-quality digital video transmission, a network based on ATM provides the best solution on the market today. ATM is an International Telecommunications Union – Telecommunication Standardization Sector² (ITU-T) standard for cell relay. Cells are the basic unit of transferring data in an ATM network. It is fixed sized and contains destination information and payload. Since the cells are of fixed size, exacting algorithms have been invented to ensure that when data is sent, it will arrive at the appropriate destination intact, with minimal jitter. This trait is better known as Quality of Service³ (QoS). Today, ATM is the accepted standard technology for video networking. While there are many IP video solutions available on the market today, only ATM can provide the necessary features to support video transmission flawlessly. Since video is very loss and jitter sensitive, ATM QoS guarantees video transmissions will arrive at the destination address intact.

Another main attribute necessary for digital video transmission is network reliability. Network reliability affects latency. In the event of a network outage due to an equipment failure or physical layer problem, the network supporting digital video transmission must be robust enough to identify the outage point and re-route traffic so no interruption in service is noticed by the users. Digital video networks based on ATM technology have an inherent advantage over Ethernet-based networks due to a robust, hierarchical routing protocol called Private Network to Network Interface⁴ (PNNI). Network infrastructures leveraging PNNI technology have the proven ability to re-route traffic within 50ms of an outage. This more than supports the latency requirements of digital video. Conversely, IP/Ethernet networks based on RIP⁵ and OSPF⁶ routing protocols can take up to 30 seconds to re-route traffic due to an outage. This delay will result in unacceptable video service interruptions. Finally, ATM technology has been widely accepted as the network technology of choice in every telecommunication service providers network.

Comments:

¹CCD (Charged Coupled device) – прибор с зарядовой связью, ПЗС