Time Slot Access and Segmentation | PBX Switch Network Issues

Some switch network designs are based on universal port access to the local switching network; that is all ports in a carrier shelf or cabinet can use the full bandwidth capacity of the local TDM bus. For example, any port interface circuit housed in a Definity PPN cabinet can be assigned any talk slot on the local 32-Mbps TDM bus regardless of its port circuit card slot location. A five-carrier shelf PPN cabinet has 100 port card slots, and the local TDM bus supports every port interface circuit card in the cabinet. The Definity TDM bus is said to be universally accessible to all PPN port circuit terminations. The 512 time slot (483 talk slots) TDM bus supports the traffic needs of hundreds of system ports in the cabinet.

A switch network design is said to be segmented if the local switching network is based on segmented TDM buses supporting a single port carrier shelf or cabinet. For example, the Siemens Hicom 300H LTU carrier connects to the center stage switch complex via a 32-Mbps Highway bus. The local switching network consists of two 16-Mbps segmented local TDM buses, with each local TDM bus supporting different port card slots. Although the total TDM bandwidth at the LTU carrier shelf level is 32 Mbps, the 512 time slots are divided equally between both halves of the shelf (eight port card slots per half). If the segmented TDM bus supporting port card slots 1 to 8 fails, available time slots on the second operational TDM bus are not accessible to port circuit interfaces on the port card housed in slots 1 to 8. The LTU carrier shelf is said to be based on a segmented TDM bus design. This is the downside of a segmented TDM bus design when compared with a universally accessible design. The upside is that the Siemens system can be traffic engineered to a greater degree. The local TDM bus supports only eight port card slots, a fraction of the number the Definity local TDM is required to support (Figure 1).

Figure 1: Segmented bus design.

The segmented TDM bus design of the F9600 was described earlier. Each port carrier shelf is supported by a 16-Mbps Highway bus that segments into eight 2-Mbps local TDM buses, with each bus supporting two port card slots. Minimizing the number of port card slots supported by a TDM bus is not always a good design objective because there may be less flexibility when configuring the port circuit cards. The F9600 backplane used to access the local TDM bus can support only 32 connections, which is also the maximum number of total port circuit terminations allowed for the two adjacent port cards. If 16 port circuit cards are installed, there is no problem, but problems may occur when a higherdensity digital trunk card is installed. A 24-port T1-carrier interface card will limit the flexibility in configuring the adjacent port card slot that shares the 32 connections to the 2-Mbps local TDM bus (32 time/talk slots). Only an eight-port card can be housed in the second port card slot if the configuration rules are followed. If a 16-port card was installed and only eight telephones were installed, thereby limiting the number of configured ports to 32 (24 + 8), the system configuration guidelines would still prohibit such an installation. The Fujitsu system was programmed for nonblocking switch network access only, and more ports than fixed TDM bus connections/time slots are not allowed. The TDM bus segmentation design can limit port configuration flexibility, if the number of backplane connections per TDM bus is relatively small.