Background

The ISDN is intended to be a worldwide public telecommunication network to replace existing public telecommunication networks and deliver a wide variety of services. The ISDN is defined by the standardization of user interfaces and will be implemented as a set of digital switches and paths supporting a broad range of traffic types (e.g., data, voice, image, video, etc.) and providing value-added processing services. It has been generally accepted and implemented all over the world. ATM is the transfer method of the future B-ISDN, which is expected to support a wide range of services including data communications. As B-ISDN deployment will be gradual and ISDN provides a cheap and efficient access interface, it is envisaged that ATM will not replace ISDN immediately, and that both of them will coexist to access the B-ISDN.

In this project, we identify the scenarios for internetworking between ISDN and ATM based B-ISDN. Further, we develop a distributed ISDN/B-ISDN server and demonstrate its functionality through an application example running on NYNET.

Approach

ATM based B-ISDN supports both connectionless and connection-oriented communications, while ISDN is mainly circuit switching and connection-oriented, respectively. To provide the service of ISDN over ATM B-ISDN, we first need to decide how to use ATM virtual channels to establish the connection-oriented communications for ISDN. In this project, we investigate a distributed server approach for establishing services between ISDN and B-ISDN. There are several techniques that can be used to design such a server: 1) Centralized ISDN Server, 2) Complete Connection ISDN Server, and 3) Distributed ISDN Server.

In Centralized ISDN server, all ISDN connections to the NYNET ATM are assigned ports to the ATM, and are referred to as ISDN nodes. In this approach, an ISDN node is chosen to be the centralized ISDN server, such that all ISDN communication services are first transmitted to the server and then forwarded to the desired destination nodes. In this project, we are not going to consider this approach because it suffers from a potential problem that the system near the server will become a major performance bottleneck due to both congested traffic and the processing time needed for forwarding the data. Moreover, due to the nature of data forwarding through the server, the performance overhead for communications might become a serious concern.

In the Complete connections Server approach, a permanent ATM virtual channel is established between any two ISDN/FRS nodes such that a complete graph between ISDN nodes using the NYNET ATM as a backbone is constructed. This approach achieves better performance at the expense of ATM virtual channels. Totally, N(N-1)/2 ATM virtual channels will be needed. Due to the large number of virtual channels occupied by the ISDN connections, the performance of the "regular" ATM in supporting B-ISDN will be significantly degraded. As a result, we will not consider this approach in this project. Another weakness of the above two approaches is due to the fact that the effect of underlying ATM switches topology, congestion information, and/or user application natures are not taken into account.

To overcome all the weaknesses mentioned earlier, we propose to investigate a distributed server approach, which in a dynamic way partitions the duty of a centralized server to a number of distributed/local servers, such that the traffic congestion and performance bottleneck at distributed servers will no longer be a serious problem. In this approach, each distributed server will forward the data communications from local ISDN nodes. A complete (or partially complete) graph of ATM virtual channels will be established only between those distributed servers. Clearly, it overcomes the problems associated with the earlier approaches.

To reduce the complexity and the processing time of the local server, we will investigate techniques to implement the server functions. One approach to implement these functions can be based on using two devices: Gateway and Routing. The gateway handles the network layer processing needed to do fragmentation, routing, etc; it gets its routing information from the routing device. The routing device stores the the static information about the network topology, links and switches status, etc. The routing device provides the gateway with the proper routing information needed to process the packets arriving at the gateway. The dynamic status of the network can be discovered by the gateway device and later used to update the knowledge of the routing device.

An important task of this approach is to develop methodologies to determine the number of distributed servers, the sites of the distributed ISDN servers, and the boundaries of the regions managed by the distributed servers. The criteria for selecting the distributed servers need to include the topology of ATM switches, traffic congestion information, and the natures of "heavy" B-ISDN multi media application users of the ATM. The reason to take into account of the nature of of B-ISDN application users is to allow us to roughly estimate and predict the future application requirements of those users over the ATM. Of course, our goal is to establish the ATM permanent virtual channels to adequately support the ISDN, while affecting the performance of the regular functions of the ATM based B-ISDN to the minimum.