Distributed and Dispersed Switch Network Designs

A distributed topology is defined simply as a switch network design comprised of multiple, independent local switching networks that are connected with direct communications links instead of a center stage switch complex. Each local switching network operates independently of the others and supports all of the communications needs of the local port interface circuits it connects to. Communications between user ports housed in different cabinets require a direct communications path between each cabinet’s local switch network. There is no center stage switch complex (standard in centralized switch network designs with multiple local switch networks) in a PBX based on a distributed switch network design, which is a potential cost benefit to the customer. Another benefit of a distributed switch network design as opposed to a centralized design is its flexibility in supporting multiple location customer requirements. Without a center stage switch complex, the communications links between remote locations and the main customer site are minimized because most station user traffic is local to the cabinet’s switch network. Only intercabinet traffic requires communications link resources.

Figure 1: Distributed switching network topology.

A distributed switch network design is usually limited to PBX systems with a minimal number of local switching networks supporting two or three port cabinets. Once the number of local switching networks exceeds three, it usually becomes a cumbersome, and expensive, process to upgrade the system because of the necessity of having direct communications links between each cabinet, unless a cabinet can be used as a tandem switching node within the distributed cabinet configuration. The two most popular PBXs based on a distributed switch network design are the Avaya Definity G3si and the Alcatel OmniPCX 4400. A Definity G3si can be installed with up to three port network cabinets (a PPN control cabinet and two EPN expansion port cabinets). Each cabinet has a local switching network based on a 32-Mbps TDM bus and can be equipped with expansion interface circuit boards to connect to an EAL (see above) for intercabinet communications. There is no center stage switch complex, and each port network cabinet TDM bus functions independently.

The Alcatel OmniPCX 4400 is an example of a PBX with a distributed switch network design that can support more than three cabinets, making it the exception that proves the rule. As part of its Alcatel Crystal Technology (ACT) system architecture, a single OmniPCX 4400 system can support up to 19 discrete cabinet clusters (control cabinet and expansion cabinets); each cabinet cluster has a local TDM bus (420 two-way channels) and can be linked to other cabinet clusters over a variety of communications paths based on PCM, ATM, or IP communications standards. A single interface board in the cluster’s control cabinet can support up to 28 communications links. The bandwidth of each PCM link is 8 Mbps; the ATM link can operate at transmission rates of up to 622 Mbps. Direct links between any two cabinets can be established, or a control cabinet can function as a tandem switching node to link two or more distributed control cabinets. The availability of very high-speed communications links between cabinet clusters can minimize the number of physical transmission circuits supporting intercabinet cluster communications requirements, and the use of hop-through connections through a tandem switch node allows Alcatel to design large and very large system configurations without a center stage switch complex. Alcatel markets a multiple system version of the OmniPCX 4400, capable of supporting a maximum of 50,000 stations, and can design the network to handle communications traffic between all cabinet clusters across all systems without a center stage switch complex.

The third type of switch network design is dispersed topology. A dispersed switch network combines the design attributes of a distributed design (functionally independent local switch networks) and centralized design (center stage switch complex connecting local switch networks). A dispersed switch network design is comprised of local switch networks that support all of the local communications requirements of its connected port interface circuits and a center stage switch complex that is used only to provide switched connections between local switch networks for calls between ports connected to different local switch networks. For example, a call between two ports in the same cabinet sharing a common switch network would be connected by using only the resources of the cabinet’s local switch network, such as a local TDM bus. If a call were placed between ports in different cabinets, the call would be connected through the center stage switch complex, and access to the center stage switch complex would be via the local switching networks.

Figure 2: Dispersed switching network topology.

For example, the Avaya Definity G3r, a larger version of the Definity G3si, can support up to 40 port network cabinets. The G3r EPN expansion port cabinets are identical to the G3si cabinets; each is designed with a local switching network capable of handling all local communications requirements—calls exclusively between ports (stations and/or trunks) in the same cabinet. Calls between ports in different EPN cabinets are handled through a center stage switching complex in the PPN cabinet (common control cabinet). The Ericsson MD-110 is another example of a dispersed switch network design; communications between LIM cabinets are handled through a centralized group switch network complex, but local communications traffic remains within the LIM. The NEC NEAX2400 IPX also can be considered a dispersed switch network design because communications between ports in the same PIM cabinet (a single carrier shelf cabinet) are supported exclusively on the local TDM bus; intercabinet and intermodule group communications are supported over a hierarchy of Highway buses.