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DESIGNING A DSP-BASED RAS SERVER

Part 2: Implementation
by Shawn Arnold

Start ý Network I/F ý Telco I/F ý Sources and PDF

In Part 2 of this series, I will discuss the examination of the key issues in the implementation of the RAS server hardware. More specifically, I'll focus on a DSP as the device that will implement a universal RAS port.

Iýll begin by examining why a DSP is an appropriate choice for the RAS port solution and the important issues surrounding the connection of the DSP to the Telco system I/F and Network I/F. Also, I will briefly examine other areas of the RAS server design.

THE DSP SOLUTION

As I noted in Part 1, each of the required modem, fax, and voice port services are computational intensive data processing services. Because the data processing is so numerically intense, DSPs become a possible candidate solution for the implementation of the port. Now, letýs examine how well a DSP meets the critical design criteria.

Recall what the most critical design criteria are:

ý specifications: size and power consumption
ý features: flexible (re-programmable) and complete solution (total solution)

Because most RAS servers contain several RAS ports (as little as 24 to as many as 100), it is important that the RAS port solution be small. Today's DSPs meet this critical size criteria. The actual physical size of the package is the most obvious aspect because it is the most apparent. Most DSPs are available in small TQFP packages. Some are available in the even smaller BGA type packaging. Some manufacturers go as far as to provide multiple DSPs in a single MCM package.

The less apparent but much more significant aspect of the DSP's size is what is actually integrated onto the DSP silicon, or chip. Several important system features are integrated onto today's DSP chips. These features include:

ý internal SRAM
ý integrated serial port
ý integrated parallel port

Without integrated SRAM, the single most costly part of a RAS server would be its RAS port external SRAM. The cost comes in the form of PCB real estate necessary to mount external memory, as well as the actual cost of the SRAM chips, components, and devices themselves.

DSPs with internal SRAM provide great savings in the total cost of a RAS server. Although DSPs with internal SRAM are more expensive than a DSPs without, the cost of the integrated SRAM DSP is less than the total cost of the sum of its individual component parts. However, the biggest cost savings comes in the reduced size of the RAS port, or the increased port density.

Similar to memory integration, the integration of I/O sub-systems, such as serial ports and parallel ports, has also contributed to the reduced size and cost of the RAS server. Most all of today's general-purpose DSPs come equipped with a variety of integrated I/O sub-systems. Specific to the RAS port application, the integration of serial ports and parallel ports have contributed to the reduced size of the overall RAS port application. Serial ports provide for direct I/O connections to the trunk-line transceiver within the Telco I/F block, and parallel ports provide direct I/O connections to the host controller within the Network I/F block.

With today's trend in transistor geometry shrinkage, DSP power consumption has dropped dramatically. Other application areas, such as hand-held telephone sets, have driven DSP manufacturers to design the lowest power general-purpose DSPs. RAS port applications are benefiting from this overall reduction in power consumption specifications.

By nature, DSPs are programmable. Therefore, in the RAS port application, they are universal. Firmware can be developed to implement any of the three services—modem, fax, or voice—required by today's RAS servers.

As RAS servers evolve, there will be new demands placed on the port block of the RAS server. The programmability of DSPs allows the design to be flexible and robust and enables the port to be easily adaptable to the ever-changing modem, fax, and voice standards.

The most attractive feature is that many companies offer complete RAS port solutions. Not only do they provide a small, low-power DSP, they also offer the firmware that turns the DSP into a modem, fax, or voice port. Even if the DSP manufacturer does not offer the firmware directly, chances are a third-party vender will offer firmware for the specific DSP platform chosen to implement the port. RAS server manufacturers can drastically reduce their design cycle time by purchasing the technology, rather than investing hundreds of engineering hours to acquire and implement the technology on their own.

Because DSPs are the desired solution for today's RAS ports, the issue becomes how to best use the abundance of features offered by today's DSP in designing a RAS server. From a hardware perspective, I will focus on how best to integrate the DSP into the RAS server as a RAS port. The two major issues are:

ý connection to the Network I/F host controller: DSP parallel port and DSP control signals
ý connection to the Telco I/F trunk line: DSP serial port

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