2. The narrow-band ISDN

2.1 User-Network Interface

The present ISDN is generally referred to as "narrow-band ISDN" (NISDN) to distinguish it from B-ISDN. N-ISDN has two kinds of user-network interface (UNI): the basic-rate interface (BRI) and the primary-rate interface (PRI). BRI has two 64Kb/s channels, called B. and one D channel, and thus it is frequently called the 2B+D interface. PRI has the bitrate 1.536Mb/s, and can be divided into B (64Kb/s), Ho (384Kb/s), and D (64Kb/s). Typical division is 23B+D, and thus it is frequently called the 23B+D interface. However, when high bitrate is necessary, the whole bitrate 1.536Mb/s can be used as one channel, and it is called Hii channel.

The D channel is mainly used for control between user terminals and the network. All information is "packetized" in D channel. A customer can use D channel as a packet communication channel; in this case, control information and customer information are packet-multiplexed in D channel. The packet switching service is also provided through B channel. In this case, the whole bitrate (64Kb/s) in B channel can be assigned to customer packets.

In the long run, ISDN offers switched 64Kb/s service, including telephone, switched 384Kb/s service, switched 1.536Mb/s service, and switched packet service, through both B and D channels. Furthermore, ISDN has various features suitable to multimedia communication.

2.2 Multimedia Communication Features

One of the biggest differences between ISDN and the telephone network is that ISDN is receptive to various communication media, while the telephone network is strongly biased to the telephone service. ISDN is equipped with universal communication protocols according to the standard OSI reference model. The layer 1 interface of ISDN consists of BRI and PRI. The layer 2 and layer 3 protocols go through D channel. The layer 2 protocol mainly establishes the data link between a terminal and the network. The layer 3 protocol is very important to multimedia communication.

Among various commands in the layer 3 protocol, bearer capability, low layer compatibility, and high-layer compatibility commands are closely related to multimedia communication. Using the bearer-capability command, a terminal requests to the network the kind of channel that is necessary; namely, the bitrate, B. Ho, or Hii, and speech, audio, or unrestricted 64Kb/s in the case of B. Low-layer compatibility is used to select end-to-end transfer capability. High-layer compatibility is used for match-making of terminals at both ends; namely, high-layer compatibility specifies the kind of terminals: telephone, facsimile, telex, teletex, MHS, etc.

In many cases, various kinds of terminals, e.g. telephone, facsimile, and videotex, are connected to a BRI. When a call is originated by a Group 4 facsimile, the transfer bitrate is specified as unrestricted 64Kb/s and the kind of terminal as a facsimile using the high-layer compatibility. At the receiving side, terminals different from a facsimile do not respond, and thus only a facsimile gives a response.

Thus, various kinds of terminals connected to the same BRI can communicate independently with the same kind of terminals at the receiving end. This feature is very valuable to multimedia communication.

2.3 ISDN Application to Video Telephone and Video Conferencing

Figure 1 shows the standard model of video telephone/video conferencing system. For standardization of video codec, a common interface format (CIF) for TV signals was defined to enable the interconnection of the 525/60 (North American, Japanese) and 625/50 (European) TV systems, as shown in figure 2. The standard codec encodes TV signals with CIF. The interchange between a national TV standard and the CIF can be done freely in each country. For low-cost video codec, a quarter CIF (QCIF) was also defined.

Figure 1 Standard video telephone/video conferencing system

Figure 2 International connection through the Common Interface Format (CIF)

Table 1 shows typical usages of ISDN channels to video telephone/video conferencing services. The use of B or 2B is likely suitable to video telephone, and Ho or Hii to video conferencing. The combination of QCIF, 48Kb/s video, and 16Kb/s audio may facilitate offering low-cost video telephone service.

Figure 3 shows a one-board video codec developed by NTT. This codec can be used with B and 2B in table 1. Its functions cover NTSC/CIF conversion, video codec, and transmission codec. A 16Kb/s audio codec can be mounted as a child board on this board.

Figure 4 shows a codec for 384Kb/s video. Four newly developed DSPs are used in the former and eight in the latter.

2.4 Colour Picture Transmission and Colour Facsimile

The standard colour picture codec, based on the JPEG standard, was developed as shown in figure 5. This codec has an interface to the VME bus and can encode a picture having maximally 8192 pixels in one direction. By assigning bitrates suitable to brightness and colour difference signals, the data compression rate can come down to 1/20 without noticeable degradation. An optical disk memory and the JPEG codec board are attached to a UNIX workstation to constitute a colour picture filing system, as shown in figure 6. A colour copier attached to the workstation can be used as an I/O terminal. Colour pictures are transmitted through ISDN. A JPEG codec is useful to save transmission time and storage capacity.

Table 1 Bitrates for video telephone/video conferencing

Channel	Division	Coding
H11	B(audio) + 23B(video)	audio: SB-ADPCM PCM
H0	B(audio) + 5B(video)	video: hybrid coding
	B(audio) + B(video)
2B	B(audio) + 112k(video)	audio: LD-CELP
B	16k(audio) + 48k(video)	video: hybrid coding

Figure 3 64K/128Kbls video codec (28 cm x 28 cm)

The JPEG standard will stimulate the standardization of colour facsimile. Important points of colour facsimile may be:

- Interworking with existing facsimiles, particularly with Group 4 facsimile;

- the same resolution as Group 4 facsimile in the black and white portion;

- application of JPEG for the full colour portion;

- distinction of the black and white and the full colour portions in the scanning process; and

- application of the standard open document architecture to distinguish the black and white portion from the full colour portion.

These should be guidelines for standardizing colour facsimile.

Figure 4 384Kb/s video codec

Figure 5 JPEG codec

Figure 6 Still colour picture filing and transmission