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Feature Article
June 2003

Increasing Value Using Standards-Based IP Telephony Solutions


Designers of embedded systems have many choices to make when choosing a platform for their embedded design. Choosing an off-the-shelf solution is good for both time to market and compatibility with third-party content. This article details the advantages of using standards-based platforms based on the more popular embedded system platforms in the industry. In addition, an application in the IP telephony segment is examined.

A key decision that a designer must make is whether to design a proprietary system in house or to purchase a more general solution from a third party. Every company has core competencies that differentiate it in the marketplace. Designers must focus on where this core competency lies and leverage that in the development of their overall solutions. This allows a company to dedicate resources to their core business where their added value is. A company who buys products in the embedded system marketplace is generally one whose core competency or value proposition lies somewhere else. Even though a telecom company knows how to design and manufacturer computer boards, it doesn�t make sense for them to do so if what they need is available on the open market. Their value-add is in the software and in the integration of the pieces that make up the end solution. These companies benefit from not having to re-invent the wheel. They can have their engineers concentrate on their market niche and focus on gaining market share through time to market and functionality enhancements that can be realized through the use of commodity computing components. There are situations where a company must design their own platforms. For example, a telecom company may not be able to procure a wide-band digital modem off-the-shelf or at least not one that can give them a competitive advantage in their marketplace. Similarly an imaging company may not be able to buy a signal amplifier board or sequencer board due to the unique nature of the application. In these cases it makes perfect sense to design and manufacture the product in house.

Systems designed to industry standards can meet the needs of many market segments. This is achievable due to the ecosystem of compatible products that are created based upon the common specifications. Interoperability with third-party content, time to market, and reduced cost are all benefits that are achieved through the use of industry standards. Bodies such as The PCI Industrial Computer Manufacturers Group (PICMG), Institute of Electrical and Electronics Engineers (IEEE), Internet Engineering Task Force (IETF), and the International Telecommunications Union (ITU) govern these standards. These organizations are made up of representatives from various manufacturers in the industry who work together to create standards based on the input of not one company, but of the industry as a whole. This ensures that standards represent a general list of requirements that apply to the market and not just one company. It also ensures that manufacturers that make products based on these standards will interoperate on some level. This opens up the options available to a designer who chooses an open platform rather than designing a closed proprietary one.

A highly scalable VoIP media gateway can be put together using off-the-shelf hardware based on industry standard components. These platforms are largely based upon PICMG 2.x standards. CompactPCI (PICMG 2.0) is based on the PCI specification. It uses industry standard mechanical components and high-performance connector technologies. Since it is based upon the PCI spec, the vast amount of low-cost PCI components can be utilized. The goal is to provide an optimized system intended for rugged applications. In a media gateway system the CompactPCI bus is used for internal control of system elements rather than for bearer traffic. Bearer traffic is typically routed on the computer telephony bus (PICMG 2.5). The goal of this specification is to extend the capabilities and utility of the CompactPCI based system architecture to support the application needs of the computer telephony industry. It essentially adds the H.110 TDM bus by a standard implementation. This allows interoperability between CompactPCI system vendors and computer telephony board vendors thereby allowing an ecosystem of compatible products to develop in the market. Smaller media gateway systems utilize the H.110 bus by interfacing line cards to it and switching voice traffic internally within the chassis.

Using PICMG 2.5 for bearer traffic allows for 2,048 simultaneous full duplex voice calls per chassis. Applications that require higher call density utilize the Packet Switching Backplane (PICMG 2.16) to carry the bearer traffic. This specification defines an IEEE 802.3�2000, Ethernet packet switching backplane in addition to the existing suite of PICMG 2.x specifications. High availability is also provided through the use of redundant interconnections and switching components. Use of IEEE 802.3-2000 provides an industry standard framework for network communication within the chassis. Larger gateway systems can be created utilizing the backplane IP network to switch voice and data calls between boards within a chassis. The number of simultaneous voice/data streams scales by the number of slots in the system and by the call capacity for each board.

Required Interfaces
A VoIP media gateway typically has three types of interfaces to the outside world. The media gateway controller (MGC) interface provides a data path between the call agent located on the MGC, sometimes called a softswitch, and the media gateway located in the data path. The media gateway controller uses Megaco/H.248 to set up endpoint connections in the media gateway. This provides an industry standard protocol connection between the gateway and controller. A circuit switch interface is also required to communicate directly with the PSTN. Typically T1/E1, T3/E3 interfaces must be provided. Since a media gateway provides a bridge between the TDM streams and packet-based data, a packet network interface is required. The packet network interface is typically 100/1000Mbit Ethernet.

System Implementation
A scalable system that encompasses all of these interfaces can be built using off-the-shelf hardware and software. There are a number of vendors that provide scalable PICMG 2.x systems that can accommodate a variety of different telephony resource boards. These systems include TDM and packet switching interconnects embedded into the backplane for simple interconnection between payload cards. In addition to the chassis infrastructure, switch cards are also available to provide a redundant data path between the packet network elements and the payload cards. For media gateway applications, the conversion between circuit switched voice and data to packet switched voice and data is accomplished on payload cards called packet voice resource boards (PVRB). These boards contain DSP resources that provide transcoding functionality. In addition a packet processor and control path processor are also provided to interface to the packet network via the 2.16 backplane interconnections (Figure 3). These types of boards are also capable of taking TDM data in on the H.110 bus embedded in the backplane. This interface could be used for TDM switching between PVRBs, voice channel tandem applications, or for taking TDM streams in from H.110 line interface cards located elsewhere in the system. Scalable call capacity is achieved through the use of multiple boards in the system.

Media gateways utilize a number of different processor technologies to provide the overall solution. DSP resources are required for transcoding, network processors are required for packetization, and general purpose processors for control. Having the correct software framework to make each part work together is absolutely essential in order to focus the software effort on the top-level application rather that the integration of each of these components.

The utilization of standards-based, off-the-shelf systems, boards, and software is vital in accelerating the development of media gateway solutions. This allows a company to focus on the value added parts of the solution rather than on the development of the commodity parts. Years ago, companies were forced to develop proprietary systems solutions. Today the situation has changed dramatically. There are many hardware, software, and system vendors who make this equipment and products are extremely flexible. Companies that leverage off-the-shelf technology are able to focus on where they add the most value and outsource products outside of their core competency. This enables them to work more efficiently and ultimately to be more profitable.

Mike Werning is principal staff engineer at Motorola Computer Group, a leader in the innovation of intelligent building blocks for standards-based embedded computing. These building blocks include open-architecture hardware, rich software, and application-ready platforms that enable equipment manufacturers to quickly and cost-effectively embed leading-edge functionality into their next-generation systems. More information can be found at www.motorola.com/computer.

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