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.