Local Switching Network Design: TDM Buses and Highway Buses

The local switching network in a PBX system can support several basic functions:

1. Port interface circuit card access and egress into the circuit switched network

2. Direct switched connections between port interface circuit cards

3. Switch connections into the center stage switching complex

The primary function of the local switching network is to provide the local communication path for calls between system ports. Small PBX systems without a center stage switching complex depend on the local switching network for all communication paths between station and trunk ports. Much of the communications traffic in many intermediate or large PBX systems is carried exclusively over the local switching network without connections across the center stage switching complex, if the design topology is dispersed or distributed (see next section). When switched connections between endpoints must be made across the center stage switching complex, it is the local switching network that handles most of the call’s transmission requirements.

A PBX system’s local switching network design may be comprised of the following elements:

1. Local TDM buses

2. Highway TDM buses

3. Switch network interfaces/buffers

4. Time slot interchangers

A traditional PBX switch network local TDM bus is an unbalanced, low characteristic impedance transmission line that directly supports the traffic requirements of port circuit interface cards without intermediary TDM buses. The ends of the TDM bus are usually terminated to ground, with a separate resistor for each bit. Port interface circuit cards typically connect to the TDM bus through a customized bus driver device. A bus driver is a switchable constant current source so that, in the high “output” state during transmission, there is no bus loading to cause reflections.

A Highway TDM bus consolidates traffic from multiple lower bandwidth local TDM buses to facilitate switch network connections between local TDM buses and provide a communications path to the central stage switching complex when needed to connect the originating and destination call endpoints across different local TDM buses and Highway buses.

Although all circuit switched PBXs depend on local TDM buses for transporting communications signals to and from port interface circuit cards, the local switching network design usually differs from one system to another. The local TDM bus in a PBX system may support a few port interface circuit cards, a full port carrier shelf, or an entire port cabinet. The number of port interfaces a TDM bus can adequately support is based on its bandwidth. A limited bandwidth TDM bus that supports 32 time slots may be used to support only a few low-density port circuit cards, whereas a high bandwidth TDM bus that supports 512 time slots can easily support the traffic requirements of a high-density port carrier shelf or a moderate density port cabinet. A few examples illustrate the differences in local TDM bus design:

1. A Fujitsu F9600 16-port card slot Line Trunk Unit (LTU) carrier is supported by eight 2.048 Mbps TDM buses (32 time/talk slots per bus); each local TDM bus supports a maximum of two port interface circuit cards (the number of ports across the two cards must be equal to or less than 32).

2. The switch network architecture of the Avaya Definity PBX family is based on a 32-Mbps TDM bus (512 time slots, 483 talk slots) that can be configured to support a single port carrier shelf or a five-carrier shelf cabinet. Each Definity G3si/r port carrier shelf supports 20 port interface card slots. The 512 time slot TDM bus can support very high traffic requirements for a single port carrier or moderate traffic requirements across a multiple carrier cabinet if traffic engineering guidelines are used.

3. A Nortel Meridian 1 Option 81C Intelligent Peripheral Equipment Module (IPEM), single port carrier cabinet with 16 port interface card slots, can be configured with one, two, or four Superloops (128 time slots, 120 talk slots per Superloop). A Superloop is the Nortel Networks name for its Meridian 1 local TDM bus. The IPEM port carrier shelf can have access and egress to 120, 240, or 480 talk slots; the number of configured Superloops depends on the traffic capacity requirements of the local ports. Basic traffic requirements can usually be supported by a single Superloop, but nonblocking switch network access requirements may dictate four Superloops per IPEM. A single Superloop can also be configured to support two IPEM stackable cabinets, with a total of 32 port card slots (a maximum of 768 voice ports), if there are very low traffic requirements.

The Fujitsu example illustrates a PBX system with multiple local TDM buses per port carrier shelf, with each local TDM bus supporting only two port cards. The Avaya example illustrates a PBX system with a local TDM bus designed for and capable of supporting a multiple port carrier cabinet capable of housing dozens of port cards and hundreds of ports. The Nortel example illustrates a flexible local TDM bus design that can support low, medium, or high traffic requirements per port carrier shelf by provisioning the appropriate number of local TDM buses.

Even though the Fujitsu, Avaya, and Nortel PBXs use local TDM buses to provide a communications path for port interface circuit cards, the bandwidth of the TDM buses and the number of TDM buses per carrier shelf or cabinet varies among the three systems. There is no standard for local TDM bus bandwidth and provisioning in a PBX system. The concept is the same, but the implementations differ.

In the Fujitsu example, the backplane of the port circuit cards connects directly to the local TDM bus. The Definity port carrier backplane also provides a direct connection to the local TDM bus. In the Nortel example, a Superloop bus supports the communication transmission needs of the port interface circuits in an IPEM cabinet, but there is no direct link between the cards and the Superloop. An interface card is used as a buffer to link the carrier shelf backplane to the electrical transmission wire operating as the Superloop TDM bus. The switch net- work buffer function is embedded on the IPEM Controller Card, which also provides local processing functions to the port carrier shelf.

Switch network interfaces/buffers are used to consolidate communications signals from multiple port interface circuit cards for access to and egress from the local TDM bus. These specialized interface cards may be dual function interfaces because several PBX switch network interface/buffer cards also have an on-board microprocessor controller used for localized processing functions.

The Siemens Hicom 300H has an interface card similar to the Nortel Meridian 1 to support both switching and processing functions as the local port carrier level. The Siemens Line Trunk Unit Controller (LTUC) card provides a link between the main system processor and the port interface circuit card microcontrollers and also serves as a buffer interface between the high-speed 32-Mbps Highway transmission bus (512 time slots) and two segmented TDM buses (256 time slots per TDM bus, 128 time slots per segment) that connect directly to the port circuit interface cards. The Hicom 300H LTUC functions like a TSI because it is multiplexing several moderate bandwidth TDM buses onto a higher bandwidth TDM bus.

From high-level diagrams, it appears that the Nortel and Siemens switch network designs are very similar, but major differences exist. The Meridian 1 Superloop functions as a local TDM bus but requires a buffer interface to link to the port interface carrier; the Hicom 300H has four TDM bus segments directly connected to the LTU port circuit interface cards and requires the LTUC, functioning as a TSI, to link to the Highway bus. The Meridian 1 Superloop and Hicom 300H Highway Bus provide a communications to the central stage switching complex of their respective PBX systems, but the design structures are not identical.

The NEC NEAX2400 IPX also uses a TSI to multiplex local TDM buses onto a higher bandwidth Highway bus. A 384 time slot local TDM bus supports each NEAX2400 PIM single carrier shelf cabinet. Up to four PIMs can be stacked together, and the individual local TDM buses communicate over a common 1,536 time slot Highway bus. A TSI links each PIM’s local TDM bus to the Highway bus. Multiple Highway buses across cabinet stacks communicate over a higher bandwidth Highway bus. In the largest NEAX2400 configuration, a Super Highway bus links Highway buses across the entire switch network complex. The broadband Super Highway bus functions as a center stage control complex but only is used for switched connections between local Highway buses. Most system traffic is localized at the PIM cabinet level and uses the Highway and Super Highway buses infrequently if the system is proper- ly engineered. The Highway bus design of the NEAX2400 is not nonblocking for a worse case traffic situation but is essentially nonblocking for most customer requirements.

Highway buses typically operate at very high transmission rates because they are required to provide the communications path across many local TDM buses or between many local TDM buses and the center stage switch complex. The terminology used to describe a PBX switch network system Highway bus varies from system to system, but the function is essentially the same. Avaya calls the optical fiber cable link used to connect Definity Port Network cabinets an Archangel Expansion Link (AEL), and Ericsson calls its MD-110 LIM cabinet communications links FeatureLinks (formerly PCM links), but the two perform the same primary function: linking port cabinets together directly or through a center stage switch complex. The Definity AEL is always a fixed high-bandwidth optical fiber link and provides nonblocking access to the center stage switch complex for each port network cabinet TDM bus. The Ericsson FeatureLink operates at only 2 Mbps and can support only 32 time slots (30 talk slots). Based on customer traffic requirements, up to four FeatureLinks can be equipped per LIM, for a total bandwidth of 8 Mbps, to support a maximum of 120 talk slots. The limited number of talk slots supported by the MD-110’s Highway bus would seem to cause switch network access problems, but analysis of customer traffic patterns (inbound trunk calls, outbound trunk calls, intercom calls) indicates that the four FeatureLink capacity is more than sufficient for most customer configurations.

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 3-5).

Figure 3-5: 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.

Time Division Multiplexing | Fundamentals of PBX Circuit Switching

The core design element of a traditional digital PBX is the local transmission bus that connects to a port circuit card. Many port circuit cards may share a common local transmission bus, and a PBX system may have many local buses dedicated to designated port circuit cards housed in different port carrier shelves and/or cabinets. Port circuit cards are used to connect peripheral equipment devices, such as telephones and telephone company trunk circuits, to the internal circuit switched network, where the local transmission bus is the point of entry and exit. Voice signals transmitted from the port circuit card onto the transmission bus are in digital format. The transmission and coding standard used by all current circuit switched PBX systems is known as Time Division Multiplexing/Pulse Code Modulation (TDM/PCM). To fully understand the workings of the PBX circuit switched network, it is necessary to define the basic terminology (Figure 1).

Figure 1: TDM/PCM.

Multiplexing is the sharing of a common transmission line (bus) for transport of multiple communications signals. A communications transmission bus is a collection of transmission lines used to transport communications signals between endpoints. TDM is a type of multiplexing that combines multiple digital transmission streams by assigning each stream a different time slot in a set of time slots. TDM repeatedly transmits a fixed sequence of time slots over a single transmission bus. In a PBX system, the transmission bus is usually referred to as the TDM bus.

A PBX TDM bus is used to transport digitized voice signals that originate as continuous (analog format) sinusoidal waveform signals. Digital sampling of a continuous audio signal is a technique used to represent the analog waveform in digital bit format. The sampling technique that has become the accepted standard for circuit switched communications is PCM

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.