3. Broad-band ISDN

3.1 Outline of Broad-band ISDN

The broad-band ISDN (B-ISDN) being standardized at CCITT can be understood through such keywords as the asynchronous transfer mode (ATM), the fibre-to-the-home (FTTH), and a multimedia network covering data up through a high-definition TV (HDTV).

As shown in figure 7, the transmitted data stream is divided into sequences of a small amount of data called a cell. Each cell has a 5 octets header and a 48 octets information field. At the transmission line, cells are packed without any gap, as shown in figure 7. Various identifiers in the header specify a cell and contain sufficient information to deliver a cell. When a high bitrate is necessary, more cells are captured. In other words, network customers can define the capacity of a channel at their own will. This is called a virtual channel. Customers can take as many virtual channels as desired in their subscribers' lines.

B-ISDN is widely believed to be the telecommunication infrastructure for the twenty-first century. On the basis of B-ISDN, NTT announced the twenty-first century telecommunication service vision, by the name Vl&P, an acronym for visual, intelligent, and personal communications services.

Figure 7 Asynchronous Transfer Mode (ATM)

Table 2 HDTV digital transmission

Usage	Bitrate
Studio grade	624M b/s
Program distribution grade	100-150M b/s
High quality entertainment	40-50M b/s
Application specific	10-20M b/s

3.2 Multimedia Service Example

3.2.1 Visual Telephone

Visual telephone is believed to be one of the major terminals for the twenty-first century. Its picture quality will be much better than the CIF-based picture in the NISDN environment.

3.2.2. HDTV

Depending on the application of high-definition TV (HDTV), necessary bitrates will become versatile, as shown in table 2. FTTH in B-ISDN will make a wide range of HDTV applications easy.

3.2.3 Personal Multimedia Teleconferencing

A desktop workstation (WS) used in the office will become multimedia. Office workers will be able to do many kinds of work at their WSs. Teleconferencing is an example. In order that office workers with their own WSs will be able to participate in meetings from their desks, multipoint capability is essential. The need for participants in a meeting to see one another makes multi-motion-picture-windowing important. It may be desirable to distinguish who is speaking.

Figure 8 shows a typical screen image of such a multimedia teleconferencing system developed by NTT. From the WS, a participant can send text, scanned image, and telewriting information; his own voice and image; and stored video in colour. The system has audio-window capability. The voice of a person whose picture window is at the left comes from the left, and the voice at the right from the right. Another feature is private conversation capability. Any two persons can talk confidentially during the meeting; this private conversation voice comes from the extreme right.

3.2.4 Network-casting

Network-casting is a notion that can be positioned somewhere between broadcasting and conventional telecommunication. Broadcast media such as radio or TV are called mass media, by which information is distributed to many people, e.g. several thousands of people. Conventional telecommunication media such as telephone and facsimile can distribute information to only a small number of people, e.g. less than 30 people in a certain amount of time, and thus may be called "mini-media." In relation to these, network-casting might be called a midi-medium.

Figure 8 Typical picture in the personal multimedia teleconferencing

A baseball game in a local stadium may be network-cast to people in a big city through B-ISDN. In an ATM switch, an incoming cell could be duplicated to branch into two or more outgoing cells, as shown in the upper part of figure 8. Then information from the visual source could be distributed to many points. The other method is to branch by using local nodes, as shown in the lower part of figure 9. These two methods could be combined for the economical realization of network-casting.

Different from broadcasting, which is dependent on radio waves, network-casting goes through optical fibre cables, whose capacity is unlimited. TV programmes that are represented by terms such as personalized TV and interactive TV can be distributed through B-ISDN. This is one reason for characterizing network-casting as a midi-medium.

Figure 9 Network-casting through B-ISDN