Voice packet design and voice gateway for client equipment

    The attractiveness of Voice-over-Packet (VoP, Voice-over-Packet) services and equipment is mainly reflected in three aspects: cost saving (due to high transmission efficiency), simplicity (voice and data are transmitted on one network) and value-added services ( far superior to circuit-switched networks). Traditional voice calls are connected to the Public Switched Telephone Network (PSTN) through dial-up, forming a dedicated communication "transparent channel". During the call, both parties occupy 64Kb/s bandwidth. This method is very reliable and the call quality is almost impeccable, but it will cause a waste of capacity when only one person is speaking and between calls. The packet-switched network takes full advantage of the high efficiency of data transmission and can save several times the bandwidth compared to the circuit-switched network.

    Because of this, the VoP market is showing a trend of rapid expansion. Its global turnover will rise from US$2 billion in 2000 to US$88 billion in 2004, with a comprehensive annual growth rate of 102%. These numbers include VoIP, VoFR, VoATM, and value-added IP services such as IP fax.

    However, like other emerging markets, it is difficult to accurately determine where the VoP market will evolve and which standards will emerge as mainstream technologies. This makes it difficult to decide which technologies should be used in implementing VoP. For example, the echo cancellation algorithm of the G.168 network and several ITUG.72x encoding and decoding specifications can be improved using various configurations of processing resources. Current solutions mostly combine microprocessors with existing digital signal processors (DSPs). However, these solutions have been unable to meet the increasingly higher requirements due to the continuous increase in channel density.

    In order to improve the running performance of various algorithms, using a configurable DSP chipset is an ideal solution. Compared with general-purpose DSP chips, configurable customized DSP can improve performance indicators several times. In addition, the channel cost is greatly reduced, the area occupied by the backplane is greatly reduced, energy consumption is reduced, and expansion and reduction can be carried out on the entire network.

Application Segmentation

    The VoP market is divided into three different segments. The first is the home market, bringing VoP into the home and small home office market, with two to four voice lines on a digital subscriber line (DSL) or cable modem. DSL and cable service providers can now add several phone lines and high-speed data channels to a twisted pair or existing cable television cable. If the same service is provided through a traditional network, multiple copper wires need to be used, and fees are paid monthly per wire. Value-added services such as DSL voice and virtual private networks are opening up markets for DSL and cable TV companies and increasing profits.

    The second is for the large market of back-office systems, giving corporate users the ability to integrate telephone networks with wide-area Internet architectures. This type of customer premises equipment (CPE) enables companies to significantly reduce costs because analog phone lines and DS-0 (digital transmission system level 0) channels are significantly reduced.

    The third is the rapidly emerging market for “gateway” products for central offices. It can handle large numbers of phone lines. The current application system is still in its infancy. It mainly transfers multiple telephone lines and back-office products from different households to DSL or Tl lines, and selects routes for voice data entering and leaving the traditional PSTN through Category 5 switches. Switches also select transmission routes for data entering and exiting the Internet infrastructure.

CPE designs

    the above-mentioned emerging markets and proposes a series of design issues. For CPE, the focus is on system-on-a-chip (SoC) integration and flexibility. Such a platform must be cost-effective and support a wide variety of network and voice processing software. For a typical VoP CPE design, the following components are prioritized:

    Network interface. CPE design requires LAN and WAN networks to have connectivity. Almost all LAN interfaces are specified as Ethernet 10/100Mbit interfaces. WAN interfaces are difficult to restrict and must, to a large extent, support a range of broadband modems. In the same CPE equipment series, the reason why there are many CPE SoC manufacturers providing Ethernet, Utopia, PCI, USB and SSI is to cover the various physical interfaces for broadband data transmission. 

    Digital telephone interface. The requirement for "seamless" connection with various analog and digital telephone components necessitates the provision of a synchronous time division multiplexing (TDM) interface and supporting time slot switching equipment. Second, the level of system integration and target cost require this interface to have flimsy connections to analog telephony components, such as subscriber line interface cards and digital T1/E1 frame interfaces. 

    Network processor. The CPE is required to support a variety of network control software and dedicated gateway device signaling interfaces, which requires the addition of embedded processor resources in addition to the general routing equipment for voice and data packets. These resources must also be positioned appropriately to add value-added capabilities to the converged network in the future, such as network address translation, encryption, and intelligent call processing. One of the most difficult issues in adopting VoP technology is provisioning and configuring the CPE infrastructure from the central office. CPE processing resources, especially network signaling software, play a decisive role in the economical utilization of VoP infrastructure by local exchange operators. 

    Digital signal processing. CPE is required to support various standards-based voice processing, which requires the use of high-performance and flexible DSP. It would be advantageous to pair such a platform with specialized hardware enhancements that accelerate voice processing and organize voice flows into and out of the WAN. It is also advantageous to minimally combine the DSP with the network processor to handle the packaging and unpacking of voice. One of the most challenging issues with DSP platforms is how to configure the "real" transmission process for any codec on any channel. The DSP must work with the network processor to seamlessly implement the real-time call establishment functions required to manage WAN bandwidth. 

    System support components. The CPE design requires supporting components such as Universal Asynchronous Receiver Transmitter, SPI, II2C and external memory interface controller. These components are already used in embedded designs, and their high level of integration helps reduce the design cost of CPE equipment. 

Gateway Equipment Design

    For gateway equipment, the most important thing is channel density and system-level integration. The typical design of a VoP gateway considers the following factors:

    Packet network interface. Gateway designs are typically accompanied by ATM or IP switches and routing equipment. This allows the gateway's packet interface to be dedicated to managing voice and voice control information. There is also a trend currently: using multiple physical packet interfaces to provide a level of redundancy.

    Voice network interface. The gateway design should also provide a digital trunking interface for legacy Category 5 switching equipment. VoDSL and VoCable gateways utilize this interface to provide PSTN services. Both types of gateways use the GR-303 standard signaling protocol for call establishment and control. This is the basis for providing Category 5 PSTN old-style ordinary telephone services over a broadband packet network.

    Voice packet interface. One of the challenges in gateway design is the flow of voice packets within the system. The DSP resources within the gateway must have flexible interfaces to offload the system's control processor from voice packet routing. This interface is often used in conjunction with the system's direct memory access (DMA) control and packet routing functions to rationalize voice flows in the system.

Digital telephone switching interface. The digital telephony interface differs from the CPE design in that it is not seamlessly integrated with external components as envisioned, but is integrated with high-density distributed switching fabric within the gateway. Since the DSP of a designated gateway can handle hundreds of voice channels, it must be easy to switch voice channels to specific DSP resources within the system. One of the distributed switching standards commonly used in gateway design is the H.110 CT bus used by Compact PCI. H.110 provides up to 4,096 DS-O time slots. This number results, to some extent, in the considerable amount of DSP resources that must be installed on a single gateway rack.

    Digital signal processing. The DSP resources in gateway design need to maximize the gateway's processing capabilities for a certain voice processing standard. Because DSP resources are in a distributed switching environment, the key is to properly configure the DSP resources to maximize the channel density of the gateway, not that each DSP supports all voice processing capabilities. In the current VoDSL and VoCable markets, the focus is on the two codecs G.711 (64Kb/s) and G.726 (32Kb/s) and the corresponding G.168 echo canceller. More traditional VoIP and high-density application systems are implementing several codecs such as G.723.1 (5.3/6.3kb/s), G.728 (16kb/s) and G.729 with G.168 echo cancellation. .

    Another important issue with gateway DSP resources is how much of the voice packet processing is done by the DSP. You should know that the processing volume involved in the packaging and unpacking of voice within the gateway is quite large.

    If all such operations are handed over to the control processor, the channel density of the gateway will inevitably decrease. An effective approach is to offload the VoP header information necessary to constitute the voice packet flow during call establishment, thereby eliminating the load on the host or control processor during call activation.

    Ideally, the control processor only needs to interact with designated voice streams on the DSP during call setup and teardown phases.

    Gateway VoP designs are particularly susceptible to poor system partitioning. From the above description of gateway design components, it can be inferred that the correct allocation of operations to hardware, software, and gateway interfaces is related to the success or failure of the design. The real-time data and voice flows within the gateway, under the load of calls, have the potential to saturate certain design components, especially the network control processor. Care must be taken when designing the DMA control and routing functions to minimize inter-processor communication when handling the activation of a voice call.

Configuration is Key

    As the VoP market expands, CPE and central office gateway application system designers have a variety of supply and configuration requirements. For this reason, designers need a variety of configurable techniques.

    Designers use configurable DSP chips to customize algorithms for applications to improve performance, reduce costs, and reduce power consumption. In addition, it also makes the entire network easy to expand and reduce. There is no doubt that the prospect of DSP chips with configurable resources will become more and more attractive compared with the traditional microprocessor plus spot DSP approach.