PBX Circuit Switching Design

PBX circuit switched network designs differ between each manufacturer’s product portfolio and even among models within a portfolio. Although there are differences in the individual PBX system switch network designs, the main functional elements are the same. All port circuit interface cards transmit and receive communications signals via a directly connected TDM bus, but the time and talk slot capacities are likely to differ between systems. A very small or small PBX system switching network design may consist of a single TDM bus backplane connected to every port interface circuit card, but a larger PBX system with more than one TDM bus must be designed to provide connections between the TDM bus segments. The TDM bus connections may be direct connections or center stage switch connections. The center stage switching system complex may be based on a space switch matrix design using circuit switched connections or a broadband TDM bus interconnecting lower bandwidth TDM buses. Two of the leading suppliers of PBX systems, Avaya and Alcatel, also offer customers of their very large PBX system models a center stage ATM switching option that can also support switched LAN data communications applications.

Center Stage Switch Complex
The primary function of the center stage switching complex is to provide connections between the local TDM buses, which support port carrier interface transmission requirements across the internal switching network. Complex center stage switching systems may be used in PBX systems designed for 100 user stations, although the smaller systems typically have a single TDM bus design or multiple TDM buses with direct link connections between each bus. A center stage switching complex may consist of a single large switching network or interconnected switching networks.

A very small PBX system usually does not require a center stage switching complex because the entire switching network might consist of a single TDM bus. Individual TDM bus switch network designs require a TDM bus with sufficient bandwidth (talk slots) to support the typical communications needs of a fully configured system at maximum port capacity. Most small PBX systems based on a single TDM bus design can provide nonblocking access to the switch network at maximum port capacity levels. If the TDM bus has fewer talk slots than station and trunk ports, the switch network can still support the communications traffic requirements, if properly engineered.

There are a few small and intermediate PBX systems that have multiple TDM buses but no center stage switching complex. For example, an Avaya Definity G3si can support up to 2,400 stations and 400 trunks using three-port equipment cabinets, each with a dedicated TDM bus, but does not use a center stage switching complex to connect the TDM buses. PBX system designs like the Definity G3si use direct cabling connections between each TDM bus for intercabinet connections between ports. This type of design can support a limited number of TDM buses without a center stage switching complex, but more TDM buses require more direct connections between each bus. When the system design includes more than three TDM buses, the switch network connection requirements may become unwieldly and often very costly. During the 1980s the Rolm CBX II 9000 supported up to 15 port equipment nodes that required dedicated fiber optic cabling connections to link each cabinet’s TDM bus switching network because it lacked a center stage switching complex. A fully configured system required 105 direct link connections (fiber cabling, fiber interface cards), resulting in a very costly alternative to a center stage switching complex. Every new nodal addition to the system required new fiber optic connections to every existing cabinet node. The advantage of a center stage switching com- plex in an intermediate/large PBX system design is to simplify switch network connections between endpoints.

There are several center stage switch designs typically used in digital circuit switched PBXs:

Broadband (very large bandwidth) TDM bus

Single-stage switch matrix

Multistage switch matrix

Broadband TDM Bus
Most local TDM buses have limited bandwidths capable of supporting between 32 and 512 time slots. A TDM bus functioning as a center stage switching complex capable of supporting switch connections between many local buses must have a transmission bandwidth equal to or greater than the total bandwidth of the local TDM buses it supports for nonblocking access. For example, a single TDM bus with a bandwidth of 128 Mbps (2,048 time slots) can support switch connections for sixteen 8-Mbps TDM buses or four 32-Mbps TDM buses.

The center stage TDM bus must also support a sufficient number of physical link connections to support all local TDM buses. If the bandwidth of the center stage TDM bus is not sufficient to support switched connections for every local TDM bus time slot, there is a probability of blocking between the local TDM bus and the center stage TDM bus. The number of local TDM bus connections is always limited to ensure nonblocking access to the center stage TDM bus.

Local TDM buses typically interface to the center stage TDM bus through a switch network element known as a Time Slot Interchanger (TSI). The TSI is a switching device embedded on the physical interface circuit card that supports the physical local/center stage bus connection. The primary function of a TSI is to provide time slot connections between two TDM buses with different bandwidths. The simplest definition of a TSI is a portal between the local TDM bus and the center stage bus.

If a single broadband TDM bus cannot support nonblocking connections for all of the installed and configured local TDM buses, it may be necessary to install additional center stage TDM buses. A center stage switching complex based on multiple high bandwidth TDM buses requires connections between each center stage bus, in addition to switched connections to the local buses. Switched connections between any two local TDM buses in the PBX system may require transmission across two center stage buses, which are linked together, because each center stage TDM bus has dedicated connections to a select number of local TDM buses. The bandwidth connections between the high-speed center stage TDM buses must be sufficient to support the port-to-port traffic needs of the local TDM buses. For this reason, system designers use very high-speed optical fiber connections to ensure the switched network traffic requirements.

Single-Stage Circuit Switch Matrix
The most popular center stage switching design is a single-stage circuit switch matrix. A single-stage circuit switch matrix is based on a physical crosspoint switched network matrix design, which supports connections between the originating and destination local TDM buses. A single-stage circuit switch matrix may consist of one or more discrete switch network matrix chips. Most small/intermediate PBX systems use this type of design because of the limited number of local TDM buses needed to support port circuit interface requirements.

The core element of a crosspoint switching matrix is a microelectronic switch matrix chip set. The switch matrix chip sets currently used in PBXs typically support between 512 and 2,048 nonblocking I/O channels. A 1K switch matrix supports 1,024 channels; a 2K switch matrix supports 2,048 channels. Each channel supports a single TDM bus time slot. Larger switch network matrices can be designed with multiple switch matrix chips networked together in an array.

Based on the size of the switch network matrix and the channel capacity of a single chip set, a center stage switching complex may require one or more printed circuit boards with embedded switch matrix chip sets. The number of chips increases exponentially as the channel (time slot) requirements double. For example, if a single 1K switch chip can support 1,024 I/O communications, four interconnected 1K switch chips are required to support 2,048 I/O channels. Doubling the number of channels to 4,096 will require 16 interconnected 1K switch chip sets. Large single-stage switching networks use a square switching matrix array, for example, a 2 × 2 array (four discrete switch matrix chip sets) or a 4 × 4 array (16 discrete switch matrix chip sets).

A 1K switch matrix can support any number of TDM buses with a total channel (time slot) capacity of 1,024, for example, eight 128 time slot TDM buses or four 256 time slot TDM buses. The total bandwidth (time slots) of the networked TDM buses cannot be greater than the switch network capacity of the center stage switch matrix. The physical connection interfaces for the TDM buses are usually embedded on the switching network board, but this is not always the case. The intermediate/large Nortel Networks Meridian 1 models require an intermediary circuit board, known as a Superloop Card, to provide the switch connection between the local TDM buses (Superloops) and the center stage 1K group switch matrix.

Multistage Circuit Switch Matrix
A single-stage circuit switch matrix design is not feasible for the center stage switching system complex of a large or very large PBX system because such a system would have a system traffic requirement for as many as 20,000 time slots. A very large array of switch matrix chip sets would lead to design complications and require several switch network array printed circuit boards. The better switch matrix design solution for a large or very large PBX system is a multistage design. The most common multistage switch network design type is a three-stage network design known as a Time-Space-Time (T-S-T) switch network. A T-S-T switch network connects three layers of switches in a matrix array that is not square (Figure 1).


Figure 1: TDM bus connections: center stage space switch matrix.

In a T-S-T switching network design, each switch network layer consists of the same number of switch matrix chips. The first switch network layer connects the originating local TDM buses to the second switch network layer; the third switch network layer connects to the second switch network layer and the destination local TDM buses. In this design, the second network switch layer is used to connect the first and third layers only, with no direct connection to the local TDM buses. The term Time-Space-Time was derived from the fact that the first and third switch network layers connect to TDM buses, and the second switch network layer functions solely as a crosspoint space connection switch for the two outer layers.

In a T-S-T switch network configuration, each TDM bus channel entering the first switch network layer has access to each outbound switch connection to the second switch network layer. In turn, each outbound switch connection in the second switch network layer has access to each switch connection in the third switch network layer. Each switch matrix in the first and second layers is connected according to the same pattern.

The T-S-T switch network is contained on a combination of printed circuit boards. Multiple first and third layer switch matrix chip sets may be packaged on a single board, although the usual design is a single switch matrix per board to simplify connections between the local TDM buses and the second switch network layer. Multiple second layer switch matrices are usually packaged on a single board. The total number of boards required for the center stage switching complex will depend on the number of I/O TDM channels configured in the installed system. An 8K switch network will require fewer boards than a 16K switch network.

ATM Center Stage
During the early 1990s, it was believed that traditional circuit switched voice networks would someday be replaced by ATM switch networks. Several PBX manufacturers worked to develop a PBX switch network based on ATM switching and transmission standards. An ATM switching network can provide the same high quality of service as traditional circuit switched networks can for real-time voice communications; it also offers the additional advantage of very high switching and transmission rates. Lucent Technology’s enterprise communications system division (now Avaya) and Alcatel developed, announced, marketed, and shipped ATM center stage switching options for its largest PBX models. Implementing the ATM center stage switching option requires a stand-alone ATM switching system equipped with customized interface cards to connect to the PBX processing and switch network subsystems. A gateway interface card is used to link the local TDM buses to the ATM switching complex for intercabinet communications. The gateway interface card converts communications signals from time-based PCM format to ATM packet format.

Shipments of the option have been negligible since its introduction for two important reasons: few customers have installed ATM-based LANs, opting instead to upgrade their IP-based Ethernet LAN infrastructure, and the cost to install the PBX option is greater than the cost of a traditional TDM/PCM center stage switching complex. In addition to the cost of the ATM switching system, there is the cost of high-priced interface cards used to convert TDM/PCM communications signals to ATM format for connecting the local TDM buses to the center stage switch complex. Nortel Networks tested an ATM-based version of its Meridian 1, but canceled development in the late 1990s after determining that the cost to upgrade a customer’s installed system was too high.

The Avaya Definity ECS and Alcatel OmniPCX 4400 ATM-based offerings are still being marketed, but too few customers have shown enough interest to make it a viable center stage switching option for the future. Growing market demand for IP-based PBX systems appears to have stunted development of the ATM center stage switching option.