July 2004

Developer's Toolkit: Residential VoIP Devices at the Processor Level

BY FRED ZIMMERMAN & DEBBIE GREENSTREET

Residential Voice over IP (VoIP) services in the U.S. represent a significant validation that 2004 is emerging as the year of VoIP. As original equipment manufacturers (OEMs) compete to win service provider tender, original design manufacturers (ODMs) are searching for the optimal solution: a balance of product performance and cost. While there are a variety of second- and third-generation VoIP chipsets on the market, the selection of the right feature set and performance level is not always apparent. The residential VoIP gateway devices evolving for the consumer market are available in a variety of flavors and subsequently dictate different interfaces, processor speeds, and channel density requirements.

SERVICE PROVIDER STRATEGIES
Until recently, the majority of residential VoIP services have been deployed in the Asia-Pacific region, leading with YahooBB in Japan. Over the last few months, however, there has been a significant expansion of residential services offerings. In the U.S., Vonage, a relatively new, independent service provider has launched Session Initiated Protocol (SIP)-based residential voice service, while AT&T has announced its residential VoIP service in several markets and plans to increase coverage significantly in the next 12 months. These services typically start with a series of regional trials designed to validate the network infrastructure stability, service performance, and user satisfaction levels. Early tenders issued by service providers usually address a more simple set of functionality, while after trials are executed, requirements levied on products are more complex. For example, several service providers have started trials based on the Media Gateway Control Protocol (MGCP) and basic call services. As deployments evolve, SIP is expected to be the network protocol of choice with supplementary call services such as call forwarding, call transfer, etc. being added to the various service providers feature suite. The challenge for VoIP product developers is implementing configurations that meet the requirements of the trials while supporting an architecture that allows migration to future features.

PRODUCT CONFIGURATIONS
Early residential gateway product configurations consisted of a relatively simple architecture, typically referred to as a voice gateway terminal adapter (TA). The TA usually has a DSP for voice processing, a telephony interface consisting of a SLIC/CODEC pair (for two telephony user ports), a RISC for telephony and network protocol, management software processing, and a single Ethernet port for connectivity to the home broadband modem. This configuration of a residential VoIP gateway continues to be deployed today. In other cases, the telephony interface, along with the DSP and associated memory is co-located on the broadband modem motherboard itself, offering a voice enabled modem in a single box.

A common residential VoIP architecture that has emerged as a popular configuration is an expansion of the TA model. This CPE gateway includes two Ethernet ports; one for the WAN connection to the broadband modem, and another as a LAN connection to either a standalone, personal home computer or to a home router device. This requires the VoIP gateway to support additional functionality to pass through the computer data as well as the voice data. A variation of this is to essentially include the router box in the voice gateway chassis as well, resulting in a four-port LAN (for PC connections) and one-port WAN set of Ethernet interfaces.

It is important to understand what level of functionality is expected for residential broadband in the specific VoIP gateway. With that understanding, the appropriate processor architecture can then be selected.

VoIP FUNCTIONAL ELEMENTS
First and foremost, it is important to understand the functional level blocks required for the VoIP gateway processing. POTS Interfaces
To connect an analog phone to the voice gateway, a Foreign Exchange Station (FXS) is needed. The FXS interface typically consists of a codec (encoder, decoder) and a Subscriber Line Integrated Circuit (SLIC). The codec provides analog to digital and digital to analog conversion. In some applications, for outside calling using the PSTN, a Foreign Exchange Office (FXO) connection is needed. The SLIC device emulates a PSTN network�s voltage levels. It needs to detect, on-hook, off-hook, and generate ringing voltages which can range to 120V. The main function is to combine the analog signal with the PSTN voltages.

Telephony Processing
The telephony signal processing portion of the solution addressing the media stream is typically implemented in software, usually on a DSP processor. They can, however, be implemented in a RISC processor. This is advised only when the additional processing requirements for the typical RISC functions are minimal. Care must be taken to size the overall megahertz required to execute all functions of the VoIP gateway.

Voice activity detection (VAD) and related silence suppression, whether incorporated in the codec or as an external software function, should also be supported as a configurable (enable/disable) feature in VoIP designs. This includes a comfort noise generation function that provides low-level background noise in order to create the perception that the connection is still maintained. Voice encoding and decoding functions are most often necessary for the reduction of network bandwidth utilization. The vocoders chosen can have significant impact on the required DSP horsepower and related memory requirements. Robust echo cancellation is another key element to acceptable quality in a VoIP solution.

Packet and Telephony Network Signaling
Translation of telephony signals to packets is only a part of the VoIP gateway solution. Telephony signals, such as on-hook, off-hook functions are executed and processed very differently than packet network signals or protocols such as SIP. There is a comprehensive amount of processing that is typically executed on a RISC processor, which translates the telephony signals/protocols to the packet protocols and vice versa.

Supplementary Services and Device Provisioning
VoIP gateways used in residential applications require the support of functions typically available in phone services today. This includes features such as call waiting, call forward, visual message waiting indicator, and call transfer. Software is required to interpret these commands and to execute the function through the gateway to the telephone.

As a remote device in the service provider network, the residential gateway must have the capability of being configured either on premise or remotely. This impacts the overall software program design, as well as FLASH and SDRAM requirements.

Other Functions
Depending on the residential gateway configuration requirements, additional functionality such as data routing and a variety of security features may also be required. Such features can have a significant impact on the processor speed requirements and, of course, real-time operating systems, IP protocol stacks, and Ethernet drivers.

PROCESSOR ARCHITECTURE
An important aspect in designing a residential VoIP system is the selection of the processor. The processor handles multiple tasks and must be efficient in moving data through the processor and between various modules and peripherals. An efficient processor architecture has a hierarchical internal bus structure with similar speed modules connected to similar frequency peripherals. This separation and isolation of similar performance components offers efficient utilization of bandwidth by preventing low throughput components (e.g., the inter-integrated circuit standard, I2C) from inhibiting the performance of higher throughput components (e.g., Ethernet).

Since VoIP systems involve the movement of data (LAN to WAN, LAN to voice, etc.), an efficient architecture for moving data between various modules is important. A distributed DMA architecture is designed to move data; each component is able to move data independent of the processor via its own DMA engine. This architecture enables each component to move data on its own without loading the processor with mundane data movement functions and concentrate on what processors are designed for � processing. This distributed DMA architecture uses an interrupt programming model. With multiple streams of data moving around the processor, the bus architecture has to support multiple simultaneous data streams with access arbitration between all its peripherals. This allows the LAN port to transmit data from external memory while the DSP is receiving voice packet data from the WAN port. At the same time, the RISC CPU can download data into the DSP program space. The bus arbitration scheme should be fair so that all masters have access to slave peripherals without excessive buffering.

Another aspect of selecting a processor is to have a consistent platform � one with a consistent memory map, internal bus architecture, interrupt scheme, peripheral set, peripheral programming model, timer, etc. This enables software portability, a quick development cycle and re-use across applications. With a consistent bus structure, integrating or removing peripherals is lower risk because the risk is in the new modules rather than the system.

THE END SYSTEM
The voice processing, network and telephony signaling, POTS interface, and Ethernet interface are the bare-bones, minimal functions required for residential VoIP gateways. While low cost is paramount, it is important to select components, which achieve optimal quality and performance. It is essential to understand the types of supplementary services and provisioning functions to ensure a complete product. The regions where the product will be shipped and the programmability requirements will dictate the type, and ultimately cost of, the POTS interface. In addition, the residential gateway under design may require more advanced features such as data routing functionality and security features. Care in product development should be taken to ensure the proper processing power and appropriate architecture is present to support such requirements.

Debbie Greenstreet is Product Management Director and Fred Zimmerman is Executive Director, Customer Premise Solutions in the Voice over Packet Business Unit at Texas Instruments. Texas Instruments is a leader in digital signal processing and analog technologies, the semiconductor engines of the Internet age. More information can be found online at www.ti.com.

 

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