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Converged Networks
October 2000

Scot Robertson  

Elements Of An Internet Telephony Gateway

BY SCOT ROBERTSON


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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