- The Evolution of Data Transmission Technologies
- Contemporary Bandwidth Requirements
This chapter is from the book
This chapter is from the book
This chapter is from the book
1.2 -Contemporary Bandwidth Requirements
For decades, only alphanumeric information was stored on computers. Since the early 1990s, however, a rapid drop in the cost of mass storage media and high-speed processors has opened the door to widespread use of multimedia applications in computer systems. Today, information is increasingly stored in the form of image, video, or audio files, which take up many times the amount of space required by text files. A parallel development in the field of data communications has been the shift from text-oriented user interfaces, such as FTP or Telnet, to more advanced multimedia communications infrastructures, including the World Wide Web, video conferencing, IP telephony, and multimedia email, to name just a few examples. This has meant an increase both in the amounts of data transmitted and in the sensitivity of that data to transmission delay. To meet today's requirements, networks must have extensive bandwidth available, and use the latest transport mechanisms. Figure 1.2 shows the real-time processing capabilities required by various applications, as well as the volume of data transmitted. Classic applications such as file transfer, backups, and LAN-to-LAN connections have the lowest real-time requirements. At the high end of the scale, with regard to both the real-time capabilities required and data volume, are supercomputer networks and virtual reality applications. The four main types of user data—text, image, audio, and video files—and the demands that each of these place on networks are discussed individually below.
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Figure 1.2 Network services: Bandwidth and real-time processing requirements.
1.2.1 Voice Communication
When voice signals are restricted to the 4 kHz frequency bandwidth used in telephony, a data rate of 64 kbit/s is required for digital transmission. This is corroborated by Nyquist's equation, which holds that a sampling rate of 8 kHz is required to digitize all the information in a 4 kHz voice channel. Every sampled value is coded in 8 bits for a resolution of 256 signal levels. This yields a bit stream of 8000 x 8 bits per second, or 64 kbit/s. In North America this bandwidth is often reduced to 56 kbit/s, as 4 kbit/s is used to carry signaling; this data rate reduction is known as "bit stealing." If compression mechanisms are used, however, the same information can be transmitted over a fraction of this bandwidth.
1.2.2 Alphanumeric Data Communication
A typical text-based application uses a 40-line screen page at 80 characters per line. As a rule, each character is coded in 8 bits, so that a screen page equals 25.6 kilobits of data. Thus it takes 2.6 seconds to transmit the content of one screen page over a 9.6 kbit/s line. Transmission of an entire page, however, is only occasionally necessary—when a new dialog mask is displayed, for example. When only screen input and output are updated, the average data speed required drops significantly, so that a rate of 2.4 to 4.8 kbit/s can be entirely adequate for interactive alphanumerical applications. Again, bandwidth requirements can be significantly reduced by using data compression.
1.2.3 LAN-to-LAN Communication
With the integration of WANs into enterprise data communication infrastructures, LAN-to-LAN communication has gained steadily in importance ("enterprise" networks are networks confined to handling the private data of a corporation or other large organization where data may have to be moved between sites often separated by long distances). The bandwidth required for a connection between two LANs can vary widely, depending both on the network load within individual segments and on intersegment traffic levels. Measurements performed on LAN-to-LAN connections have shown that network levels at peak use times can be as much as 25 times higher than during low-use periods. In most cases, however, less than 10% of LAN data traffic actually travels over LAN-to-LAN links (see Figure 1.3). Depending on the LAN topology used (100/1000 Mbit/s Ethernet, Token Ring, or FDDI), bandwidths between 1 and 10 Mbit/s are usually sufficient.
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Figure 1.3 Profile of LAN-to-LAN traffic.
1.2.4 Bandwidth Requirements for Video Applications
The bandwidth required by today's video applications ranges from 10 Mbit/s for video to 900 Mbit/s for uncompressed, broadcast-quality high-definition television, or HDTV. Again, the volume of data actually transmitted can vary greatly depending on the optimization and compression techniques used.
Graphic Data Communication
A single screen page contains around 1 million pixels. On a color monitor, each pixel is defined by 24 bits. Thus, sending a single screen page in color entails transmitting some 24 megabits. If the data is not compressed, it takes 6 minutes to render the image over a 64 kbit/s line, or 0.15 seconds over a 155 Mbit/s line (See Figure 1.4 and Table 1.1).
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Figure 1.4 Image rendering and bandwidth.
Video Data Communication
Of the four kinds of data examined here, video sequences place the heaviest load on network infrastructures. Thanks to the advanced data compression techniques developed over the past few years, however, data speed requirements for transmitting video data have dropped significantly. The first international standard for transmission of compressed video data, MPEG-1, was introduced in 1992. MPEG-1 processes 25 images per second at a resolution of 352 x 288 pixels. The MPEG-1 compression ratio is 26:1, resulting in a bit stream of 1.15 Mbit/s. As for audio communication, however, the bandwidth required for video conferencing increases with the number of conference participants.
At the end of 1993, the MPEG-2 standard was introduced for broadcast-quality video transmission. MPEG-2 processes 25 video images per second at a resolution of 720 x 576 pixels, which yields a level of quality equal to that of broadcast television. Transmission of MPEG-2 data requires over 4 Mbit/s, however. The high-resolution HDTV format (1000–1200 screen lines) requires 30 Mbit/s—after compression! Table 1.3 shows the average bandwidth requirements for various video applications. Depending on the screen content, however, the amount of bandwidth required can vary significantly. Video sequences that contain only slow or very little movement generate far less data than fast-changing images (Figure 1.5).
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Figure 1.5 Profile of data traffic generated by moving images.
Table 1.1 Bandwidth Requirements for Image DataTable 1.2 Comparison of Bandwidths
| Information Service | Bandwidth | Typical Application |
| Image transmission | <1 Mbit/s | Monochrome images |
| 1Ð10 Mbit/s | Color images | |
| 10Ð100 Mbit/s | High-resolution color images |
Table 1.2 Comparison of Bandwidths
| User Interface | Resolution | Bandwidth (1 channel, half duplex) | Bandwidth (Video Conference 4 users) |
|
Video (MPEG-1 compressed |
352 x 288 |
1.15 Mbit/s |
13.8 Mbit/s |
| Video (MPEG-2 compressed) | 720 x 576 | 4 Mbit/s | 48 Mbit/s |
| Video (MPEG-3 compressed) (HDTV) | 1920 x 1080 | 20 Mbit/s | 240 Mbit/s |
| Video (MPEG-4 compressed) (Videophone) | 176 x 144 | 0.064 Mbit/s | 0.768 Mbit/s |
| ASCII-based interfaces | 40 x 80 characters | 0.0096Ð0.0144 Mbit/s | -------- |
| Graphical user interfaces in LAN environments | 800 x 600 | * Peak traffic: up to 4 Mbit/s Average traffic: 5Ð50 kbit/s | |
| * Loading of office applications such as Word or Excel; duration of traffic peak: app. 5 seconds | |||
To deploy multimedia applications to new types of networks, including those employing relatively low bit rates such as wireless telephony and wireless LAN networks, the MPEG-4 standard was developed. It became an international standard in the year 2000 and for the first time represents the various aural, visual, and audiovisual components of multimedia applications as separate units, called media objects. The data streams associated with these media objects can be multiplexed and transported over network channels providing QoS (Quality of Service) appropriate for the nature of the specific media object. This allows, for example, the transportation of the same MPEG-4 file as video and audio objects across high bandwidth networks, as still image and audio objects across low bandwidth networks.
Table 1.3 Bandwidth Requirements for Video Conferencing
| Service | Bandwidth | Application |
| Video/Multimedia conferences | <1 Mbit/s | Talking heads |
| 1Ð10 Mbit/s | Small screen, high quality | |
| 10Ð100 Mbit/s | Large screen, high quality |
Table 1.4 Transfer Delay and Audio Transmission Quality
| Delay per Direction | Impact on Communication |
| > 600 ms | No communication possible. |
| 600 ms | No continous communication possible. |
| 250 ms | Delay perceptibleÑ communciation style needs to be adapted. |
| 100 ms | No delay perceptible if listener occurs via the network only, and not in parallel direct. |
| 50 ms | No delay perceptible. |
1.2.5 End-to-End Delay
Another important communication parameter for multimedia applications is the end-to-end delay. ITU studies have shown that, for transmission of lower-quality conversational voice services, the maximum acceptable network delay is 150 ms. For real-time graphic visualization systems, the upper limit is 30 ms.
In general, for high-quality multimedia services the delay should not exceed 100 ms in WANs, or 30 ms in LANs. In LAN workgroups, propagation delay should be limited to 10 ms to allow for additional latency in intersegment components such as routers and bridges.
As discussed in the following chapters, ATM is a technology that can provide both the high bandwidth and the real-time processing capabilities demanded by current multimedia applications.