Carrier Class, High Density VoP

Packet-based voice has quickly evolved to enable a single network to deliver a new generation of business and consumer voice and data services. Carriers have announced that new network build out will be based on packet technology that supports a converged network that carries both voice and data.

Packet-based voice provides several advantages over circuit switched voice. Bandwidth required for a voice call can be significantly reduced through Voice Activity Detection (VAD) techniques and use of low bit rate CODECs. VAD removes silence, which accounts for as much as 40% of the voice information that is transmitted. Low bit rate CODECs reduce the amount of bandwidth for a voice call from 64 Kbps to as little as 8 Kbps.

	IP telephony has grown up and is now becoming part of the mainstream telecoms scene. All of the world's top telcos are exploiting IP telephony in some way, although few are reacting to the full breadth of its impact. Also, hundreds of new players are active Internet telephony service providers (ITSPs), and are generating revenues in what has almost become a new industry. Exploiting Market Opportunities, An Ovum Report—December 2000

Based on open standards, a packet-based voice and data infrastructure allows faster time to market for new services and enhancements with third party developers offering products to service providers. Capital investment for packet-based platforms is significantly less than circuit-switched equivalent. The savings in operational costs have been estimated at 40-50% by carriers based on the consolidation of platforms and network management applications.

In a few short years packet-based voice has evolved from a technology demonstration to an integral part of next generation networks and services. The accelerated pace at which this technology has evolved can be attributed to some key factors:

• Voice over Packet (VoP)—which includes all packetized voice such as VoIP, voice over frame relay (VoFR), voice over ATM, and voice over DSL, plus combinations—technology feasibility
• DSP-based technology with low power consumption and scalable channel density
• Enabling transport technologies, e.g. DSL, Cable
• Interoperability and standards.

Early VoP applications were targeted at providing lower cost long distance services. One early application was transporting long distance voice traffic between certain cities that traditionally experienced a high volume of traffic, for example between Hong Kong and Vancouver. A second early application was carrying voice traffic between corporate facilities on their existing data network, significantly reducing the voice traffic expenses.

	The VoP market is expected to grow from 7.7 billion minutes in the year 2000 to 570 billion minutes by 2005. Probe Research

It is clear that a wide range of consumer and business services will be available utilizing packet-based voice as part of bundled voice and data service offerings. VoP gateways are being implemented for a number of applications ranging from small two-port Integrated Access Devices (IADs) for the consumer and telecommuter to very large carrier class gateways used as Class 5 switch replacements.

Many technical barriers have been overcome to bring VoP technology to its current state. Large scale VoP networks have become increasingly feasible as DSP-based technology has been refined to provide lower power and higher densities with manufacturers meeting the challenge of interoperability. Silicon and software technology for VoP has evolved to support hundreds of channels and will soon support OC-3 channel density on a single chip (greater than 2000 channels).

Meeting Service Provider Requirements with Solution Density

Service providers have a great deal to gain through VoP implementations. They envision new revenue opportunities in leveraging the open architecture of packet-based services to quickly bring to market new and enhanced services while reducing operating costs through the deployment of packet-based networks. Service providers, however, will not sacrifice the tradition of toll quality voice for the benefit of deploying a single converged infrastructure.

TI has developed the solution density concept to help service providers and platform developers more clearly understand and implement high density VoP. VoP solutions cannot be evaluated merely for the number of channels on a chip. From a system engineering perspective, a solution must be evaluated on how the combination of system elements deliver a complete solution with lowest power and smallest area without compromising voice quality and features. Solution density then, refers to the optimization of channel density, power, architecture, integration of system functions, and I/O with the required software-based features for the targeted application (i.e. high-density carrier class gateways).

The solution density concept provides a framework to evaluate solutions based on the integration of all major system elements and features by considering factors beyond just the number of channels and area for any single component. Areas that are important to service providers as they move to implement packet-based voice networks include:

• Quality and Reliability
  Standards for VoP quality and network reliability are the same as for traditional telephone networks. Customers expect service quality to be consistent with traditional telephone networks. Packet-based voice is transparent to the customer. High density VoP implementations are often discussed in terms of carrier class or toll quality, which are universally associated with the high quality voice services. These terms set the expectation for quality for service providers and, more importantly, their customers.

  Features such as tone processing, packet playout, voice activity detection (with comfort noise generation), and echo cancellation are key in meeting quality expectations. These features require a robust and in-service-hardened implementation to be considered toll quality. The absence of any of these features will result in less than toll quality voice service.

• Scalability
  The ability to scale VoP networks to large volumes of traffic is critical to service providers justifying deployment of a packet-based infrastructure. Scalability requires VoP gateways to support very high volumes of traffic without degrading voice quality. This places increased pressure on gateway vendors to support thousands of voice channels on a single platform driving the need for a system level evaluation of density.

  In most cases density will be limited by the power requirements for the total system. Service providers must maintain power and cooling within industry guidelines (i.e. NEBS specifies a maximum of 1275 W for a 23-inch bay with forced air cooling). For platform developers this important design criterion can only be met with a solution that optimizes the design for power and area while maintaining carrier class features. Therefore, the most important performance specification is not channels-per-chip but power-per-channel. It is possible that a service provider will run out of power-budget before shelf space in a bay. This would result in under utilization of central office space driving higher operational costs. Power-per-channel is the key metric.

• Flexibility
  Flexibility includes the ability to add new services and to react to standards evolution. VoP gateway platforms based on solutions that support features beyond PCM voice (features such as low bit rate CODECs and fax relay) enable service providers to add services without the disruption and expense of replacing equipment. These software-based features allow platform developers to distinguish their gateways form the competition. From a solution density perspective, architectural considerations such as sizing of DSP resources and memory will determine a solution's level of flexibility.

Table 1:  A summary of the key criteria for high density solutions from a solution density perspective.

High Density VoP Architecture

High-density VoP architectures are driven by several critical elements:

• Power per channel of the solution expressed in milli-Watts (mW) per channel
• Cost per channel of the solution, which includes silicon/hardware, software, and intellectual property licensing costs
• Channel Density of the solution expressed in channels per square inch
• System Partitioning including packet aggregation and routing
• Software Features that define the functionality of the product
• Network Management Capabilities to address high availability and accountability.

Cost, power, and area must be evaluated on a total system basis and must be a function of the features and capabilities supported.

Figure 1:  An example of a high-density VoP reference design consisting of an Alarm Monitor and Control (M&C) Module, Call Processing Modules, PSTN Interface Modules, Packet Interface Modules, VoP/Universal Port (UP) Modules, and a Backplane Interface.

The Alarm Monitor and Control (M&C) Module performs the overall network management for the equipment. This includes configuration on a per channel basis, status and statistics collection, call record reporting, and alarm processing. The Call Processing Modules perform call establishment and call tear down for the system and performs interworking functions between the PSTN and packet network. Depending on the application and location of the equipment, signaling that may be performed includes:

• PSTN Telephony Signaling:
  • SS7
  • ISDN
  • TR08
  • TR303
• VoP Network Signaling
  • H.323
  • MGCP
  • Megaco
  • SIP
  • ATM Broadband Local Emulation Services (BLES).

Depending on the architecture, call processing may be centralized or distributed with the VoP modules performing lower levels of the signaling protocols.

The PSTN Interface Modules provide the interface to the Public Switched Telephone Network. Traditionally, PSTN interfaces for VoP consisted of T1 (24 channels) and E1 (30 channels). High Density VoP systems being designed today typically have multiple DS3 (28 T1s or 672 channels) and even multiple OC-3 (2016 channels) PSTN interfaces as manufacturers offer equipment capable of handling in excess of 100,000 voice channels in a single rack of equipment.

The Packet Interface Modules provide the interface to the packet switch network. The two most prevalent networks are ATM and IP. Depending on the application, VoP equipment may be ATM-centric, IP-centric or support a hybrid of both ATM and IP voice. In many cases, it is important for the equipment to support both Voice over ATM (VoATM) and Voice over IP (VoIP) on a per call basis to provide interworking between the ATM and the IP worlds. Packet interfaces include OC-n (OC-3, OC-12, etc.) optical interfaces for ATM and Packet Over SONET (PoS) as well as multiple 100BaseT and Gigabit Ethernet interfaces.

The Switch Fabric Module performs the routing of cells/packets through the system. Line cards fill out the appropriate header information that is used by the switch fabric to direct cells/packets to the appropriate line card/external interface.

Figure 2:  The VoP high density module

The VoP Modules consist of a "farm" of DSPs that perform the actual conversion of the voice streams between the PSTN and packet worlds. In the PSTN to packet network direction, the VoP Modules receive 64 Kbps data streams from the PSTN Interface Modules and output packets or cells to the Packet Interface Modules. Similarly, in the packet network to PSTN direction, the VoP Modules receive packets or cells from the Packet Interface Modules and output 64 Kbps streams to the PSTN Interface Modules. The DSPs are controlled by a host processor that is responsible for configuration and software download of the DSPs as well as assisting in call establishment and termination and other network management functions.

In order to concentrate a large number of VoP channels, aggregation logic is required. This logic:

• Aggregates packet streams from multiple DSPs to the Backplane/Packet Network Interface
• Routes incoming packets from the Backplane/Packet Network Interface to the appropriate DSP
• Provides a standard interface to the Backplane/Packet Network Interface
• Filters network management and call setup/teardown information to a Host processor.

There are many different backplane interfaces that are used in systems such as these. Most typical are PCI and cell bus variants as well as Packet over Sonet (PoS). TDM samples from the PSTN can be relayed over an H.110 TDM bus or the PCM samples can be encapsulated in ATM cells to be sent over the same cell bus that is used for packet traffic.

Figure 3:  The VoP aggregation logic

Software is a critical ingredient of high quality VoP systems. There are many features that must be implemented for Carrier Class systems. The most important software features are:

• Echo Cancellation
• Voice Compression
• Packet Play-out Software
• Tone Processing
• Fax and Modem support
• Packetization
• Signaling Support
• Network Management.

Figure 4:  The architecture for high density VoP software.

High density VoP software should be designed to minimize delay and maximize scalability. This includes:

• Efficient and adaptive algorithms
• Header encapsulation in VoP device, (RTP, AAL1, AAL2) on a per channel basis
• Low-latency implementation
• Features to off-load host processor to drive overall channel density.

Solutions based on the concept of solution density that offer high density, low power and provide full-featured performance will enable a new generation of high density carrier class gateway platforms. This new generation of platforms will enable service providers to deploy a common, packet-based infrastructure for voice and data services enabling a new generation of services for businesses and the consumer.