July 1999
IP Telephony Building Blocks
BY LAURENCE J. FROMM
The pace of change in the Internet industries is increasing. In an environment of rapid
change, the winners are the companies that not only have the best products or services,
but those that can most rapidly modify and improve their offerings. Speed is critical.
For a product company, then, fundamental architectural and design decisions
often made in the early under-funded, over-hurried days of a company have long-term
implications on the extensibility of the product family, and hence affect the ongoing
competitiveness of the company.
IP telephony gateway vendors, for example, need to consider their view of the future in
designing their products.
VIEWS OF THE WORLD
One view of the future is that IP telephony gateways are relatively simple,
fixed-function devices. They convert telephony traffic to and from IP traffic, making a
limited set of signal processing and protocol translations in the process. The best way to
add value in this view of the world is to minimize cost and maximize performance.
Adherents to this view handcraft software to minimize memory space for program code and
data, and reduce procedure calls between software interfaces. They employ simple
schedulers that efficiently dispatch a series of related tasks. They build embedded
devices based on private architectures.
THE NEED FOR SPEED
Another view of the future is that IP telephony gateways sit on the edge of a new
kind of network that enables new kinds of services. The gateways may be used to perform
unknown and unknowable signal processing and protocol translations for some future
services. Design for extensibility becomes another key criteria, along with cost and
performance. Adherents to this view are more likely to define well architected interfaces
between software modules; to use higher level programming languages more often; and to use
real-time operating systems, enabling multiple tasks to execute within the same subsystem.
They use industry standards at the interfaces to enable incorporation of third-party
technology.
Gateways may be initially deployed doing a limited set of signal processing
translations. Those built for extensibility can add capabilities to enable new services.
The gateways may be extended, for example, to provision network-based interactive voice
response (IVR), or for waveform capture for natural language speech recognition. Basic
designs can be extended for video signal processing, or for multiparty, multimedia
conferences.
DESIGN CONSIDERATIONS
Regardless of world view, gateway vendors consider a range of design choices in
building gateways. Some of these include:
DSP Processor Architectures
DSP processor performance is characterized by various factors, including arithmetic format
(fixed or floating point), instruction set type (CISC or RISC), clock speed, pipelining,
memory organization, and peripherals.
There are two basic generations of DSP processors.
The current generation represented by the TI C5000 and Motorola 563xx families
of DSPs are based on programmable filter architectures. These processors have a
data path architecture targeted at DSP multiply accumulate operations. They incorporate a
multiplier, an arithmetic logic unit, shifter, registers, and other specialized units.
The next generation of DSPs is based on RISC technology, with load/store architectures
with multiple function units accessing large register files. Most, like the TI C6x family,
are VLIW (Very Long Instruction Word)-based. Their orthogonal instruction set allows for
optimized compiler implementations. The advantage of this architecture is that the
processor can run at a much higher clock speed for increased performance.
Other Design Choices
Besides the DSP type and specific DSP, other DSP system design choices exist:
Memory organization: Design factors include the amount of internal memory, and whether it
can be used as cache; whether to deployed local or shared external memory; and memory type
(SRAM, DRAM, SDRAM).
I/O access: Multifunctional serial ports or hardware DMA are choices.
Run-time scheduling: Monolithic runtime environments are simple and limited. A program
polls for data at fixed points, executes a fixed algorithm, and outputs data at the rate
required. Priority stack schedulers are more powerful. These schedules support multiple
contexts (or threads) with multiple priorities, but are limited to a single context stack
and a single active thread per priority. Task-based real-time operating systems are the
most general schedulers. RTOS support multiple contexts with multiple priorities, each
with a completely separate context including a private stack.
THE BEST CHOICE
Comparing DSP processors solely on core functions like FFT or filtering can be
misleading. The DSP system architecture needs to be factored into the decision process.
The best performance benchmarking is done by comparing performance on the set of functions
needed for a particular service or application. For example, an application that requires
DSP processors to execute limited functions from internal memory is best matched with a
processor and architecture with low power and footprint and adequate internal memory. A
software-driven architecture, however, that supports multiple functions executing on a
single processor for multiple channels, requires a powerful processor with high throughput
memory and I/O interfaces, and excellent high-level language support.
If the new services enabled by converged networks require constant new capabilities
from the edge devices, vendors that have a compatible world view win. If evolution of
gateways, conversely, is characterized by bigger, faster, denser versions, then vendors
with that world view win. Let the battle begin. c
Laurence J. Fromm is vice president, new business development for Dialogic
Corporation. Dialogic is a leading manufacturer of high-performance, standards-based
computer telephony components. Dialogic products are used in fax, data, voice recognition,
speech synthesis, and call center management CT applications. The company is headquartered
in Parsippany, New Jersey, with regional headquarters in Tokyo and Brussels, and sales
offices worldwide. For more information, visit the Dialogic Web site at www.dialogic.com. |