A centralized topology is defined simply as a switch network design that requires all calls, regardless of the origination and destination endpoints, to be connected through the same TDM bus or switch matrix or the center stage switch complex.
Any PBX system with a switch network system comprised of a single TDM bus or switch matrix is classified into the centralized switch network design category because all calls are handled through the same “centralized” switch network element. Most small PBX systems have a switch network design based on a single TDM bus because the traffic requirements for equipped systems with fewer than 100 ports (stations and trunks) can easily be supported without multiple TDM bus requirements and/or a center stage switch complex. A single TDM bus design may also be used by PBXs with larger port capacity limits, if the TDM bus bandwidth is sufficient to support the port traffic requirements. For example, the Avaya Definity’s switch network design is based on a 32-Mbps TDM bus that can easily support the very small port (20 to 40 stations) traffic requirements of the Definity One model and the larger port (40 to 400 stations) traffic requirements of the Definity ProLogix model.
Many intermediate/large PBX system models have centralized switch network designs because a center stage switch complex handles all call connections regardless of the originating and destination call endpoints. It is easy to see the necessity of using a center stage switch complex to support switch connections between port interface circuit cards housed in different multiple carrier port cabinets because a local TDM bus is not configurable across common cabinets, and the system installation may include many port cabinets. The system switch network architecture is easier to design and program if all calls are connected with a centralized switch network element because the same call processing steps are followed for each and every type of call. It is more difficult to see the necessity of using a center stage switch complex to support switch connections between port interface circuit cards housed on the same port carrier shelf or even between ports on the same port interface circuit card, but a centralized switch network design dictates the same switch network connection protocol (center stage switch complex connection) regardless of originating and destination port interface circuit proximity.
The Nortel Networks Meridian 1 Option 81C, Siemens Hicom 300H, and Fujitsu F9600 XL models are examples of centralized switch network designs. Each of these systems can be installed with multiple port cabinet stacks, with several port carriers per cabinet stack. Each system uses a center stage switch complex to support connections between each port carrier’s local switching network (single or multiple local TDM buses), even if the two connected telephones are supported by the same port circuit interface card and connect to the same local TDM bus that connects to the same the Highway bus that connects to the center stage switch complex. It may appear a waste of switch network resources (talk slots, switch connections) to use the center stage switch complex for a call of this type, but that is the way the system is designed and programmed. Figure 1 illustrates the call communications path for a Meridian 1 Option 81C between port interface circuits in different cabinet stacks and between port circuit interface circuits on the same port circuit interface card. The call connection protocol is similar, if not identical, for the Siemens and Fujitsu systems.
A centralized switch network design offers no customer benefits, but it can be problematic because a large number of potential switch network elements (local TDM buses, Highway buses, switch network interface/buffer, TSI, center stage switch elements) are required to complete any and all calls. This can affect switch network reliability levels because the probability of switch network element failure or error affecting a call connection is increased. For example, center stage switch complex failures or errors affect all system port connections in the PBX system.
A major disadvantage of the centralized switch network design is when a customer needs to install a remote port cabinet option to support multiple location communications with a single PBX system. A remote port cabinet option requires a digital communications path between it and the main PBX system location. Most remote port cabinet installations are supported with digital T1-carrier trunk services. If the PBX switch network topology is centralized, all calls made or received by station users housed in the remote cabinet must be connected through the center stage switch complex at the main location, even if calls (intercom or trunk) are local to the remote port cabinet. A T1-carrier circuit, with a limited number communications channels, must be used for every remote cabinet call to access the center stage switch complex. Most remote PBX cabinet options require two T1-carrier channels per call connection, thus limiting the number of active simultaneous conversations at the remote location. This may force the customer to install additional T-1 carrier circuits to support the port traffic requirements at the remote location, but there are limits on how many T1-carrier interface circuit interfaces can be supported by the remote port cabinet. The limitations of the centralized switch network design may force a customer to install multiple remote port cabinets at the remote location or a standalone PBX system.
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