SDH-based transmission Cell-based transmission Synchronous pulse frame( 125 us) Frame overhead Frame payload ATM onous digital hierarchy ATM FIGURE 72.2 Transmission principles for B-ISDN Class A Class B Class C Class D Timing relation between source and destination Not required Bitrate Constant variable Connection oriented Connection Connection mode FIGURE 72.3 AAL service classification The AAL protocols are subdivided into two parts. The lower part performs, at the sending side, the segmen- tation of long messages into the cell payload and, at the receiving side, reassembly into long messages. The upper part is service dependent and provides the AAL service to the higher layer B-ISDN Signaling For signaling in B-ISDN, existing protocols and infrastructure will be reused as much as possible. Figure 72.4 shows the protocol stacks for UNI and NNI. The upper part concerns signaling applications and the lower part signaling transfer. For the introduction of simple switched services in B-ISDN, at UNI and NNI, existing signaling application protocols will be reused. The 64-kbit/s ISDN-specific information elements will be removed and new B-ISDN specific information elements will be added. Right from the beginning these protocols will provide means that llow smooth migration toward future applications, which will include highly sophisticated features like mul timedia services [Huber et al., 1992]. This approach guarantees compatibility for future protocol versions. the NNi the existing signaling system no. 7( SS7)can be reused(see right part of the NNI protocol stack in Fig. 72.4). SS7 is a powerful and widespread network that will continue to be applied for rather a long period until ATM penetration has been reached. For the middle term, however, a fully ATM-based network will be available which also carries signaling messages(see left part of the NNI protocol stack in Fig. 72. 4). ATM-based signaling at the NNI needs a suitable AAL which provides the services of the existing message transfer part level 2 the UNI, right from the beginning, all kinds of traffic(including signaling) is carried within cells. An AAL for signaling at the UNI is also required. This aal has to provide the services of the existing layer 2 UNI e 2000 by CRC Press LLC© 2000 by CRC Press LLC The AAL protocols are subdivided into two parts. The lower part performs, at the sending side, the segmen￾tation of long messages into the cell payload and, at the receiving side, reassembly into long messages. The upper part is service dependent and provides the AAL service to the higher layer. B-ISDN Signaling For signaling in B-ISDN, existing protocols and infrastructure will be reused as much as possible. Figure 72.4 shows the protocol stacks for UNI and NNI. The upper part concerns signaling applications and the lower part signaling transfer. For the introduction of simple switched services in B-ISDN, at UNI and NNI, existing signaling application protocols will be reused. The 64-kbit/s ISDN-specific information elements will be removed and new B-ISDN￾specific information elements will be added. Right from the beginning these protocols will provide means that allow smooth migration toward future applications, which will include highly sophisticated features like mul￾timedia services [Huber et al., 1992]. This approach guarantees compatibility for future protocol versions. At the NNI the existing signaling system no. 7 (SS7) can be reused (see right part of the NNI protocol stack in Fig. 72.4). SS7 is a powerful and widespread network that will continue to be applied for rather a long period until ATM penetration has been reached. For the middle term, however, a fully ATM-based network will be available which also carries signaling messages (see left part of the NNI protocol stack in Fig. 72.4). ATM-based signaling at the NNI needs a suitable AAL which provides the services of the existing message transfer part level 2. At the UNI, right from the beginning, all kinds of traffic (including signaling) is carried within cells. An AAL for signaling at the UNI is also required. This AAL has to provide the services of the existing layer 2 UNI FIGURE 72.2 Transmission principles for B-ISDN. FIGURE 72.3 AAL service classification