An Alternative Approach to CallKonnection Control in Broadband Switching Systems

WAI SUMLAI is with AT&T Bell Laboratories. ignaling protocols a re used in switching systems to support realtime procedures for the establishment and release of on-demand calls/connections. For broadband switching systems based on Asynchronous Transfer Mode (ATM), the ITU-T has defined Broadband ISDN User Part (B-ISUP) [ 11 as the network-node (network-to-network) interface signaling protocol. B-ISUP is based on the ISDN User Part of the Signaling System No. 7 (SS7) protocol [2]. Currently, B-ISUP is defined to only support point-to-point ATM connections. Since ATM switches are expected to provide multicasting, signaling protocols must evolve to support connections with multiple endpoints. The B-ISUP signaling protocol and associated call processing software in broadband switching systems use a hop-by-hop connection establishment algorithm. This does not lend itself to easy extension for multiple endpoints. Negative responses from some of the endpoints cause the protocol state machines in the switches to become fairly complex. We observe that a client-server approach for signaling communication would greatly simplify the handling of multiple endpoints, where one server coordinates the responses from the endpoints. Secondly, unlike in circuit switches, where for on-demand connection establishment the switch software simply performs next-hop routing and configures its switch fabric for each connection, connection establishment in ATM networks is significantly more complex due to the statistical multiplexing feature of ATM. Early work on congestion control in high-speed ATM networks indicates that Connection Admission Control (CAC) procedures should be employed at the switches to implement preventive congestion control ra ther than reactive control [3]. This requires more complex processing in ATM switches during connection establishment. Using a hop-by-hop sequent ia l approach with the increased CAC functions in each switch will significantly increase the end-to-end connection setup delay. Based on these two observations, we describe a new approach to connection establishment. The approach allows for the easy handling of multiparty connections and also improves endto-end connection setup delay by parallelizing certain operations that occur during connection establishment. We use a client-server based signaling protocol, the Transactions Capabilities Application Part (TCAP) of SS7. Existing SS7 implementations in switches contain TCAP software, used for switch-to-service control point communication, for services such as 800-number. The B-ISUP signaling protocol supports an additional functionality of service/call control. It continues to support Intelligent Network (IN)based services [4), as its predecessor, ISUP. For example, to support the IN-based virtual private network service, B-ISUP signaling messages carry service control parameters, such as Closed User Group Information. These parameters are used as triggers in the Basic Call State Model (BCSM) maintained in each switch for IN-based networks. Thus, broadband switching systems implemented with the B-ISUP standard interface must include call processing software to maintain the BCSM to trigger service logic processing for IN-based services. Since ATM was developed for networks to carry integrated traffic (i.e., isochronous voice and video traffic and bursty data traffic) broadband switching systems must be developed to work equally well in networks that specialize in one or the other type of traffic. We observe that while many of the IN-based services may be useful to even data-based LANs, the initial usage of