Internet telephony is still an emerging network technology. As with any new
technology, there is often misunderstanding and confusion. In many cases,
the new terminology, the new acronyms, and the new concepts are difficult to
tie together. This article will attempt to alleviate some of this confusion
by introducing and explaining the basic elements that make up an Internet
telephony gateway. The Internet telephony gateway is at the heart of the RAS
(remote access server) Internet telephony system. It can be viewed in terms
of its hardware, which are the physical components that are put together to
form the gateway. It can also be viewed in terms of its software or
firmware, which are the program modules that work together to form the
virtual gateway. We will look at both the hardware and software aspects of
the gateway in two separate articles. The first will focus on the hardware
elements of a gateway.
WHAT'S IN A GATEWAY?
The basic hardware elements of a gateway include the interface to the
multichannel serial line that comes from the Telco, the core digital signal
processor (DSP) performing the voice processing, and the interface to the
network. At the front end of the gateway there is the interface to the
PSTN's multichannel digital serial lines. At the back end of the gateway is
the interface to a LAN (local-area network), WAN (wide-area network), or the
Internet.
What exactly happens when the signal leaves a user's home? It all starts
with an analog signal generated by the handset's mouthpiece or microphone.
This analog signal, along with analog signals from other users, travels down
the local loop wires to the local central office (CO). The CO takes these
calls, digitizes, and combines them into a single multichannel digital
serial signal. These multichannel serial signals travel on trunk lines that
connect one CO to another CO. The trunk lines will connect the CO of the
caller to the CO of the call/end user. At the end user's CO the multichannel
line gets broken up into a number of individual lines that are converted
back to analog signals that travel over similar local loop wires to the end
user's destination.
The first thing the gateway needs to do is interface to the PSTN's
multichannel serial trunk lines. The type of multichannel serial trunk line
connecting to the gateway is a concern because of the need for accessing the
highest number of channels, or calls, from the PSTN. The gateway's interface
must be able handle these high-capacity trunk lines. Ultimately, a design
should offer as many calls as possible, enabling the highest number of calls
per area, or highest density. The ability of a gateway to handle a large
number of calls is directly related to the performance and flexibility of
the DSP that is used as the core processing device in the gateway.
THE DSP SUBSYSTEM
What gives the gateway its performance and characteristics is the DSP
chosen to implement the gateway. Typically the DSP includes the core
processing units, memory, and I/O (input/output) peripherals. The import
peripherals for a gateway application are high-capacity serial ports, and a
high-bandwidth parallel port or DMA port.
A DMA port is a host processor that loads programs/firmware into the
DSP's memory via the parallel/DMA port. It is the firmware that ultimately
gives the gateway its personality. That is, whether it acts as a modem port,
a fax port, or a voice port. Via the DSP's DMA port, the host processor also
handles all the network data flow into and out of the DSP. The PSTN's serial
data flows into and out of the DSP via the DSP's serial port.
The DSP is the processing engine that performs all the voice processing
functions. These functions include voice compressing and expanding, also
referred to as the voice codec, echo cancellation, data packetization, error
correction, and in some cases process the higher-level network protocols.
Similar, but generally more sophisticated, processing is also performed in
the implementation of a modem or fax port.
The importance of the hardware with respect to flexibility lies within
the application programming interface (API). The API provides instruction on
how to access and control the many voice processing functions implemented in
the DSP firmware. The flexibility of the API affects both the designer and
the end user. For example, the API can allow for direct access to the
various data buffers within DSP memory. This enables the real-time
examination and tracking of the performance of many different processing
components of the DSP's firmware. This is especially useful in detecting and
correcting problems with the local loop connection between the central
office and the end-user. Modem flexibility is another issue that ties
directly back to the DSP subsystem that allows for
"any-protocol-on-any-port" where there is a legacy V.32, V.34, or
V.90 connection that needs to be accommodated within the same box.
Voice coders generally have a variety of different hardware and software
requirements. They depend on the speed and memory of the DSP to function at
maximum performance level. When it comes time for the service provider to
upgrade a system to add additional voice coders, to increase performance
and/or increase port density in the gateway, if the gateway was not
implemented with adequate hardware there may be serious limitations in the
quality of service of the product.
For instance, if a service provider chooses a basic voice coder to start,
knowing that they will eventually need to add or upgrade to higher voice
quality, then the hardware needs to be chosen such that these additional
voice coders can be easily added to the gateway in the future. The system
host processor coordinates all of the activity of the box and the DSP. The
host needs to be able to keep up with bandwidth and capacity. If a DSP has
more capacity than the host, it could put a strain on the host processor.
QUALITY & CAPACITY ISSUES
Choosing a gateway that supports high voice quality depends on the
gateway's ability to accommodate a variety of codecs. The more codecs a
system can accommodate, the better off the system will be and the more
likely it will be compatible with the other end and be able to offer a full
range of voice quality services. Another quality issue is packet management.
This occurs when the Internet does not provide perfect data transfer and the
packets are not continuous for real-time processing. All gateways have
different packet management functions and while some gateways may feature
high processing performance, the data management functionality may be low.
Examining the entire gateway with respect to overall functionality, not just
performance, is key. These management functions occur in the DSP along with
other functions such as jitter buffer management. There are other functions
that occur on the DSP as well, these include the voice codec
implementations, echo cancellation, and other related functions.
Voice capacity starts with the DSP. The DSP provides support for the
number of channels that a system can provide. The size, power, and
integration of the DSP, the box size and host system all determine the
output and total capacity of a system. The majority of the performance of
the box lies within the DSP. Power is also a concern. A typical gateway
consumes several hundred milliwatts per channel, while there are DSPs that
consume substantially less, as low as 35-40 mW per channel. If you have
one-tenth the power, you can integrate ten times the amount of channels onto
a card. The integration of the DSP is also important. The DSP's integration
level -- including performance, memory, and number of features -- will
enable more channels without extra hardware to be contained on the DSP. In
addition, the smaller the physical size, the more DSPs can fit on a single
board, so the size of the DSP should be an important consideration when
choosing a gateway.
CONCLUSION
An evaluation of the total solution, including taking into consideration all
of the elements of the gateway needed to offer the highest level of capacity
and flexibility possible, is extremely important for choosing the right
hardware for implementing an Internet telephony gateway. These elements are
not independent of one another. The best example is the integration of the
DSP. If highly integrated with a lot of memory and functions, bundled with a
full suite of voice and modem software, the DSP can provide a total solution
that will enable the highest levels of performance and functionality. The
importance of software and its role within the gateway with respect to the
hardware choices will be explained in next month's article. Software options
and configurations for system capacity and interoperability, as well as
voice quality will also be looked at in more detail next month.
Scot Robertson is the product line manager for Remote Access Products
at Analog Devices, Inc., a leading manufacturer of precision
high-performance integrated circuits used in analog and digital signal
processing applications. Scot can be reached at [email protected].
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