Integrating Computer Telephony Systems with CompactPCI Technology.

Developers of large-scale, open Computer Telephony (CT) systems have long faced a challenge when choosing which computing platform to build. While VME offers superior maintainability and ruggedness, it suffers from a limited range of component suppliers--resulting in higher prices. The PC is much less expensive and offers a wide range of sources. However, historically, the telecom community has viewed the PC as a less reliable and maintainable platform.

The popular solution has been to meet the reliability challenges with arrays of small, redundant PC-based systems. This solution can be cost effective, but is an inefficient use of space. A different solution, the CompactPCI (cPCI) standard, now offers an ideal compromise. cPCI marries the rugged, maintainable form factor of VME with the excellent price/performance offered by PCI.

The cPCI Standard

The cPCI standard and specifications were collaboratively developed. The PCI Industrial Computer Manufacturers Group (PICMG), a consortium of more than 450 companies with a common goal of adapting PCI technology for use in industrial and telecommunications computing applications, maintains them.

Standard PC platforms present a challenge to developers trying to minimize overall system downtime. Consider what happens if one of the CT boards in one of the PCI or ISA expansion slots of a PC fails. The entire PC must be powered down. The PC must be pulled out from the rack. The cover must be removed. The mezzanine TBM bus (H.l00 or SCbus) ribbon cable must be removed. Care must be taken not to drop any of the screws into the PC. Then, the whole process must be reversed.

System developers usually resort to swapping out the entire PC assembly with an identical replacement and servicing the failed unit off-line. Furthermore, the PC's horizontal motherboard/backplane design creates a barrier to the efficient vertical flow cooling arrangement used by rack systems.

cPCI addresses these maintenance issues by using the VME's standard Eurocard format. The Eurocard (See Fig) uses 160-by-233mm cards (called the 6U format) vertically mounted into a vertical backplane through a front opening. The nearly square board shape is optimal for printed circuit board layout, allowing vendors to pack on more features and power. Metal faceplates eliminate the need for covers while reducing electromagnetic emissions. The chassis is split front/rear to accommodate separate I/O modules (rear) from the computing modules (front).

The Eurocard format uses high-density, gas-tight connectors that are less susceptible to shock, vibration, and corrosion than standard PCI card-edge connectors. The backplane connectors are organized into logical groups that enable 32-bit PCI devices and to accommodate a smaller, 3U-format (100mm by 160mm) card. The larger 6U boards have access to additional connectors that are specified for telephony use.

More Powerful Bus

cPCI improves on VME by using the more modern and powerful 64-bit PCI bus, capable of bandwidths up to 132MB/sec. The PCI architecture is inherently processor independent. Thus, cPCI integrators will enjoy a wide choice of processors, operating systems, and application software.

Since the PCI bus is being widely used by enterprise and consumer PCs, the silicon devices used to implement the interface are highly integrated, widely available, and produced in volume. Designers of cPCI products will enjoy the benefits of easy design and cost effective implementation. Predictions are that a cPCI chassis will command no more than a 15 percent price premium over a comparable PCI platform.

Through careful design and impedance control, the cPCI bus supports up to eight slots per bus--twice as many as a PCI system. This reduces the need for performance-robbing bridge chips. A popular configuration is two side-by-side, 8-slot PCI buses in a single 19-inchwide, rack-mountable chassis. The two buses can either be bridged together or operate as two independent systems. Typically, the CPU occupies one slot, leaving seven open slots for peripherals and CT boards.

State-of-the art CT boards can support up to four T-1 digital network interface trunks on a single-slot cPCI card. With up to 14 slots available, the bandwidth of nearly two DS-3 interfaces can be serviced in a single chassis. A system that once required three racks of PCs may now be reduced to perhaps a single rack, resulting in a lower footprint--an important consideration for deployment in the floor-space-limited public network Central Office (CO).

The latest revision of the cPCI specification, PICMIG 2.5, goes even further by incorporating the new H.110 Computer Telephony Bus (CT Bus) directly into the backplane. The CT Bus is an isochronous Time-Division Multiplex (TDM) bus ideal for carrying live telecommunications traffic across multiple CT boards. The CT Bus can carry and switch up to 4,096 DSOs (2,048 conversations). In contrast, the CT Bus implementation for the PCI platform (H.100) uses a ribbon cable, making service more difficult.

The PICMIG 2.5 specification for CT applications details backplane connectoring for telephony I/O (tip and ring signals) and telephony power (such as battery and ring signaling). The variations of telephony interfaces found around the world result in the need for many different connectors and magnetic components. With cPCI's separate rear I/O configuration, board vendors can simplify their product inventories by addressing these differences on a series of modules that are physically separate from the more flexible--and expensive--integrated electronics.

Building a large CT system would not be practical, unless it were designed redundantly so that a single common failure would not bring down the whole system. To facilitate serviceability, PICMG has a specification that provides for hot-swapping cPCI boards using a staged-pin sequence. This will enable a single failed card to be quickly replaced without tools and without disturbing the rest of the system. With separate rear I/O modules, boards can be replaced without the need to remove connectors. Visible front panel indicator lights can be viewed to easily determine an individual board's status.

Removing a board can be triggered by a switch on the handle of front panel of the card, or at the console using remote maintenance software such as SNMP. Either way, the software is notified in advance of the pending removal of the card. After the software reacts, it indicates to the technician by either a front panel indicator light or at the console that the board is ready to be removed.

The system software can automatically detect a board inserted into the system. The cPCI specification supports both geographical and slot-sensitive addressing. cPCI uses the native PCI mechanism for identifying the board type, using pre-assigned OEM codes.

As the table (System Bus Comparison) illustrates, cPCI combines the reliability and maintainability of VME with the performance and wide availability of PCI. By adding the integrated CT Bus, cPCI has now become the ideal platform for the next generation of open, standards-based telecommunications equipment.

Martin Zary is a product manager at Dialogic Corp. (Parsippany, NJ).

                           System Bus Comparison
    Features               VME                 cPCI
  Address Space            4GB                  4GB
   (Physical)
Addressing Method      Geographic        Slot-sensitive or
                                            geographic
                    ANSI/VITA 6-1994
   TDM Bus and    2,048 time slots std.       H. 110
    Bandwidth     4,096 time slots ext.  4,096 time slots
 CPUs in System         Multiple            1 (master)
  Hot Swap/Live       VITA 1.4-199x       PICMG hot swap
    Insertion
 Cabling Options      SCbus and I/O     Rear I/O transition
                     ribbon cables,          modules,
                        ANSI/VITA         H.110 embedded
                       backplanes           backplanes
   System Bus        40 to 80 MB/sec        132 MB/sec
    Bandwidth
    Features             PCI                ISA
  Address Space          4GB             1 or 16MB
   (Physical)
Addressing Method   Slot-sensitive     Not supported
                        H.100              SCbus
   TDM Bus and     Up to 4,096 time   Up to 2,048 time
    Bandwidth           slots              slots
 CPUs in System       1 (master)             1
  Hot Swap/Live    PCISIG hot plug     Not supported
    Insertion
 Cabling Options     Bracket I/O,       Bracket I/O,
                  H.l00 ribbon cable SCbus ribbon cable
   System Bus         132 MB/sec          8 MB/sec
    Bandwidth