Multiple TDM Bus Design | Fundamentals of PBX Circuit Switching

Most PBX systems have multiple TDM buses supporting the communications needs of the system ports. The individual TDM bus segments can be linked through a variety of methods based on the topology of the switching network design (see section below). Two user stations connected to port interface circuit cards housed in different port equipment cabinet frames would each be supported by different local TDM buses. A PBX switched network TDM bus may support a few port circuit cards (perhaps half a port carrier shelf), an entire port carrier shelf, or even multiple port carrier shelves within the same cabinet frame. Stackable single carrier cabinet designs sharing a common backplane for process- ing and switching functions also may be supported by a single TDM bus, but it is more than likely that port circuit interfaces housed in different cabinets will not share the same TDM bus. Based on these TDM bus segment scenarios, it is possible that a call between two user stations housed on the same port carrier shelf would require four talk slots per call and two station users on different port carrier shelves in the same equipment cabinet frame would require only two talk slots. The number of required talk slots will depend on the switch network design.

Whenever two communicating ports do not share a common TDM bus, the PBX processing system will assign the originating port a talk slot on its local TDM bus (the TDM bus directly connected to its port interface card) and a talk slot on the TDM bus that is local to the destination port. The destination port will likewise be assigned two talk slots: one on its local TDM bus and another on the originating port’s local TDM bus. This scenario requires at least four available talk slots divided between two TDM buses. Additional communications channels may be required to link the TDM buses, and the PBX processing system makes the necessary assignments per call. Multiparty conference calls across multiple TDM buses will require one talk slot per party per TDM bus used to complete the call connection. For example, a three-party conference call among three internal station users, each supported by a different TDM bus, may require nine talk slots: three talk slots per party (one per TDM bus) × three parties = nine talk slots (Figure 1).

Figure 1: Call across multiple TDM buses.

To minimize the number of inter-TDM bus connection requirements and increase the traffic handling capability of the PBX system, it is often recommended that groups of station and trunk circuit ports share a common TDM bus, instead of dedicating different TDM buses to different station or trunk interface port circuits. Station user groups with high intercom traffic requirements also should share common switch networking facilities to minimize inter-TDM bus connection requirements.

In some instances there will not be talk slots available on a local TDM bus when a station call is initiated. Voice-based communications systems traditionally have been designed to support more system ports than available local TDM bus talk slots. In a typical PBX system environment, it is rare that every station or trunk port will be active simultaneously, but there may be a blocked call if there are more provisioned station/trunk ports than total local TDM bus talk slots. PBX systems are designed with traffic engineering calculations to minimize blocked call attempts. Call blocking situations have a low probability of occurring if the system is correctly traffic engineered, but they may occur if there are more potentially active ports than available talk slots required to provide the circuit switched connection between the ports