Switch Network Redundancy | PBX Switch Network Issues

Switch network redundancy is a design criterion that minimizes switch network downtime for one station user, a few station users, or all station users in the system. The term redundancy is often confused with the term duplication. A switch network design incorporated with duplicated elements is said to be redundant, but a redundant switch network may not necessarily have any duplicated elements that might prevent downtime for some or all station users. Duplication is the highest form of redundancy, but it is not the only type of redundancy, particularly in PBX switch network architectures, as we will shortly see.

If a customer wants a redundant switch network design that is based on duplication of critical design elements, the checklist of duplicated elements may include any or all of the following:

1. Center stage switch complex

2. Local TDM buses

3. Highway buses

4. Switch network interface (including embedded TSI)

5. Intercabinet cabling

Duplication of the center stage switch complex is a vital redundant switch network requirement in a centralized design topology because all calls are connected through this switch network element. Center stage switch complex errors or failure affect every call in the PBX system. Center stage switch problems may be slightly less important in a dispersed design topology, but it would still be highly desirable to have duplication of the switch network element because it is needed to con- nect all calls between local switch networks. A few PBX systems have a fully duplicated center stage switch complex as a standard design feature, such as the Nortel Networks SL-100, a modified version of the supplier’s DMS-100 central office switching system. It is more common that the duplicated center stage switch complex is available as an option, although some intermediate/large PBX systems do not offer it as a standard or optional design element. The Siemens Hicom 300H is available in two models: the large line size Model 80 has an optional duplicated center stage switch complex and the smaller Model 30 does not offer it as standard or optional.

Loss of the local TDM bus will negatively affect the communications capabilities of all ports to which it connects. Redundancy of the local TDM bus can vary between different PBX systems based on the definition of redundancy. The Fujitsu F9600 XL has a fully duplicated local switching network design, including duplication of the local TDM buses. The Avaya Definity PBXs have a redundant local TDM bus design: the local 32-Mbps TDM bus (512 time slots) supporting all of the communications needs within a Port Network cabinet is based on two independent TDM buses, each with a 16-Mbps bandwidth (256 time slots), but operating as a single TDM bus from the viewpoint of the port interface circuit cards. If one of the two 16-Mbps TDM buses fails, all system ports can still connect to the remaining TDM bus. The Siemens Hicom 300H offers a similar redundant design concept for its TDM bus architecture. Two 8-Mbps TDM buses support eight port interface circuit card slots (one half of an LTU carrier shelf), each operating independently and accessible by any of the eight port interface cards. In these Avaya and Siemens models, loss of one TDM bus will place a heavier traffic load on the remaining TDM bus and may increase the number of blocked call attempts due to the reduced number of available time slots. The major difference between the two designs, however, is that a Definity 32 Mbps TDM bus can support a five-carrier shelf cabinet with several hundred stations and associated trunk circuits, and the Siemens 16-Mbps TDM bus design supports only eight port card slots (nominally 192 ports). Failure of a Definity TDM bus segment will have greater traffic handling consequences than failure of a Siemens TDM bus segment. Siemens offers switch network redundancy at a more local level than does Avaya.

Nortel Networks has claimed that the multiple Superloop design in its intermediate/large Meridian 1 models is a form of redundancy because loss of a single Superloop affects only a limited number of the ports in a cabinet stack. The term limited, however, can be misleading because a Superloop can support up to 32 port card slots, and each port card slot can support 24 digital telephones. If strategic system ports, such as attendant consoles or trunk circuits, are affected by loss of a Superloop, the redundancy level of the design might not be acceptable.

Highway buses may be fully duplicated, or loss of a TDM bus segment comprising the Highway bus may not affect the remaining bus segments (although traffic handling capacity will be reduced). Highway buses are used for connections between local TDM buses and to provide communications paths to the center stage switch complex. Loss of a Highway bus can be significant in a centralized switch network design.

Switch network interfaces are printed circuit boards connecting local switch networks to each other or the center stage switch complex. It is an electronic switch network design element that can fail and affect communications traffic between port cabinets. A duplicated switch network interface typically links the local switching network element, such as a TDM bus, to a high-speed fiber optic cable communications link. Duplicated center stage switch complex designs usually have duplicated switch network interfaces and duplicated cabling links for intercabinet communications connections.