September 1999
WIRELESS IMPLEMENTATION: Which Boards Work Best?
BY EMRE �NDER
Today's wireless networks run on proprietary hardware that is costly to maintain and
difficult to upgrade. Over the years the wireless protocols have emerged as standards,
although some of the older networks run proprietary protocols and place gateways to access
different networks. When implementing wireless networks, it is important to note that
building solutions that are both scalable and upgradeable will protect your investment of
years to come. Selecting products that interoperate with each other and offer upgrade
paths to newer protocols and line interfaces helps prepare the network for the third
generation (called 3G) of protocols.
Consider all the types of components that are used to make up a wireless network. The
BTS (Base Transceiver Station) is used to convert the RF radio signal to a digital signal.
Multiple BTSs connect to BSCs (Base Station Controllers), which allow the user to travel
between antennas and automatically switch between sites. Finally, you have an MSC (Mobile
Switching Center) designed to connect the mobile user to the public network or switch
between carriers.
CompactPCI AND WIRELESS
CompactPCI platforms raise the bar for telephony applications, offering higher
density solutions for different media applications. Using DSPs as a general purpose
hardware component has made applications like wireless networks, voice conferencing, and
Internet fax more scalable. Now the CompactPCI platform makes this infrastructure possible
in a form factor that is both affordable and flexible.
Although originally conceived for embedded computing applications, CompactPCI’s
potential in wireless applications is immediately clear. It offers the advantage of a true
industrial strength circuit board in a form factor that is rackable and stackable. Unlike
PC cards, cPCI cards are configured in rackmount chassis and are stackable in a “real
chassis” — just like real telephony modules. The cards are hot swappable. Cables
can break out from either the front or the back, and the modules are easily upgraded.
Thanks to powerful DSPs and 4 or 8 T1/E1 spans on one board, you can now do it all
— handle wireless connections and compress the voice calls from ADPCM to A-Law or
mu-Law. Products today offer enough DSP resources from as little as 200 MIPS up to 1800
MIPS all on a single card. This can include the WAN interfaces or a separate DSP resource
card. Not only can that lower the cost of raw switching by an order of magnitude — it
can lower the cost of adding high-value applications to those existing switches
But such potential riches raise an important design issue: Can a plethora of new
DSP-enabled functions be brought on board without adding development complexity, impeding
run-time performance, and ultimately increasing costs? They can, if packaged in modules
that scale as easily at the application level as they do at the board level.
SELECTING THE RIGHT BOARD
What makes this architectural challenge even more exciting is that resources are
potentially as varied and complex as each individual application and network. Functions
and combinations of functions are implemented as single DSP resources. Typically, these
are DSP chips that apply standard algorithms for analog-to-digital, digital-to-analog, and
digital-to-digital conversion, allowing wireless phone calls to switch from the standard
wired phone to the wireless phones.
Over the years, these algorithms have been perfected and become more robust so that
developers need to use fewer of them (or can keep going back to the same ones). While this
simplifies the programming challenge, it still leaves many different functions with
different real time, inter-process, and host communication requirements for the developer
to meld together.
In selecting the right board for wireless DSP resource cards, the programmer must
select the desired functions and the desired software architecture. This will prepare them
to properly partition the application’s services they are building within and between
the various telephony modules and the host CPU — and then to write the program.
In other words, for most CT applications there is usually a best way to designate DSP
resources, allocate functions, and partition the system. Not only should the deployment
platform be standardized before development begins, it should be application-appropriate.
This eliminates the complexity of designing a DSP resource card interface and writing
complex DSP algorithms.
WIRELESS NETWORK PROTOCOLS
Today the protocols that are used for compressing the voice calls — like
Adaptive Differential Pulse Code Modulation (ADPCM) — must meet industry standards.
To access legacy networks, the uncompressed voice calls must be converted to either A-Law
or mu-Law depending on your regional/international requirements. These conversions are
done using the DSPs.
On another note, the network interface to the public network is either ISDN or T1/E1
signaling. The wireless network side will run a network specific protocol like ISDN (i.e.,
Q931/LAPD), and in Europe and Asia the GSM protocol is used.
Look to vendors who are committed to these protocols and will work with you to add
newer and higher density solutions.
Emre �nder is vice president of marketing, Brooktrout Technology, Data
Technologies Division. Brooktrout Technology, a Brooktrout Company, provides enabling
technologies for customers to deliver voice, fax, and data solutions for the electronic
communications market. For more information, please visit their Web site at www.brooktrout.com |
