Voice over IP (VoIP) has become one of the fastest growing applications
for both wireline and wireless telecommunications equipment
manufacturers. Not only does VoIP afford end-users affordable access to
voice calls, but it allows media gateway designers the ability to
engineer equipment that serves many different purposes " a situation
that typically would provide a cost benefit for both.
In the case of VoIP, however, the
opposite is true. As VoIP gains greater and greater acceptance, profit
margins for equipment manufacturers are minimized based on the
competitive nature of the application and the high cost of designing IP
PBX and multiservice access nodes (MSANs)
at many different channel densities. Engineering costs are rising and
product prices are declining.
Although many companies have designed proprietary hardware/software
media gateway solutions, they must make compromises in order to offer
cost-competitive products. As a result, most VoIP functionalities are
best offered today using DSP technology, but not without a new set of
challenges.
As media gateway or IP PBX designers tackle the
daunting task of bringing a cost-effective product to market in a
time-sensitive, ever-changing environment, the high-tech dilemma
recurs: performance vs. functionality. The main problem with DSP
technology is finding the perfect balance of performance and
interfaces.
For example, designers often find that only the very high-end DSPs
on the market can support their most sophisticated
applications/interfaces, including SGMII Ethernet or RapidIO.
So, when it comes time to interconnect these DSPs in larger systems,
a field programmable gate array (FPGA) is often used to glue devices
together and perform functions such as back-pressure on packet
transmission or packet classification. This is where programmable logic
has found its way into hardware design.
Application portability is one of the key values of DSPs using
programmable logic. The same underlying media processing technology can
achieve performance results in application areas as diverse as:
- Large-scale media gateways
- IP PBX
- HMP media servers off-loading
- MSAN / Access equipment
Even though the basic feature set may be the same for all VoIP
applications, requiring the same list of codecs and voice quality
features, the interfaces required for each may be quite different.
For example, a PCI card designed for host-based media processing
(HMP) off-loading would require a PCI Express interface, whereas a
MSAN may require a lightweight interface to an Ethernet PHY and to T1/E1
framers, as well as a lot of glue logic to the various
components on the board.
One of the beauties of DSPs for the designer is the inherent ability
to make the solution unique " not an off-the-shelf, one-trick pony.
This technology flexibility allows designers to explore more
cost-effective ways of doing things, but they can often find that they
are limited by what their DSPs support.
This is where programmable logic " FPGA " provides the added
enhancement. For example, in an IP PBX, the USB, rather than Ethernet,
is the ideal interconnect for low-cost systems.
Not only is it hot-pluggable by design, a USB hub device is less
expensive and less complex than an Ethernet switch, which means that
the base cost for a system is lower. In addition, as a system grows "
particularly a blade-based system -- the incremental cost is lower,
because for each processing blade added to the system, very simple,
low-cost RISC processors are available with USB interfaces.
Sadly, most DSPs provided to the VoIP gateway market do not offer
USB support, leaving designers with only the high cost product model.
Happily FPGA vendors provide IP cores for such an interface, allowing
for newer, lower-cost innovation.
In a perfect world, a media gateway designer would be able to get
exactly the interfaces needed, without the cost and side effects of
buying a fully-featured DSP. But today, most high-end DSPs have
multiple high-speed interfaces " many of which are not needed " and
host a very large package with a very high pin count.
These large packages are dense ball grid arrays (BGAs) that are
expensive and require a printed circuit board (PCB) with a high number
of layers, which can all contribute to design delays.
Although some manufacturers have tried combining an FPGA with a DSP
to achieve design nirvana -- particularly in cases where the DSP
interfaces are judged inadequate or customization was required -- this
solution can be overkill. It adds unnecessary cost and board real
estate, only to correct short comings in the DSP.
