软交换技术

1 Introduction

Recent technology advances continue to drive the shift towards converged IP networks. Networking hardware and software are simultaneously getting better, faster, and cheaper. Packet-based technology has finally matched the circuit-based in terms of reliability. The explosive growth of IP networks provides ample bandwidth to accommodate voice calls. We have witnessed in the last few years a steady transition from a circuit-switched to a packet-based infrastructure among the service providers and switching vendors worldwide.

Economic forces are behind the shift away from traditional circuit-based switching and towards packet-based switching which takes voice signals, digitizes them, breaks them into packets, and sends the packets over a standard IP based data network, or is commonly referred as to VoIP. Compared to the traditional PSTN networks, IP networks offer several distinct advantages. An IP network can carry calls with less bandwidth, integrate voice with data and video, and deliver new telecommunications services with ease. It uses the low cost Internet infrastructure, rather than the traditional PSTN proprietary circuit networks. Finally, IP networks allow telecommunications service providers to introduce new services less expensively than using proprietary methods, often by just adding a new server and integrating it with the existing network.

An IP network is especially well suited to handle the convergence of voice, data, fax, and video. There is already more data traffic than voice in the United States and British, and data is growing over ten times faster in many other countries. Some analysts predict that more than 30％ of voice traffic worldwide will be over IP networks by 2004. In addition to driving Internet growth, consumers are demanding more and more converged services that combine voice, data and video services to manage their increasingly complex telecommunications needs and new solutions are required to satisfy these. One such solution is the Softswitch technologies. Softswitch technologies are a collection of products and services to enable telecommunications services to run on the data network, notably, the Internet. It can integrate voice, data, and video, and handle protocol translations between separate networks such as wireline, wireless and cable systems. Softswitch technologies enable cost-effective VoIP implementations and give service providers the flexibility to add value while adhering to legacy switching standards like SS7. Although the majority deployment of Softswitch solutions today is to replace the traditional PSTN Class 4 and/or Class 5 switches, the key of these solutions is their ability to effectively integrate the circuit and packet networks and to deliver converged service.

This article gives a brief introduction on the emerging Softswitch technologies that refer to a set of technologies that perform the functions of switching and converged service. In the following Section we shall provide an overview on the Softswitch architecture and discuss some of the major architecture considerations in the design of ZTE Softswitch solution.

2 Softswitch Technologies Overview

Softswitch technologies refer to a set of technologies that perform the functions of (a) switching??establishing connections for end-to-end communications, and (b) converged service??providing combined service of voice, data, fax, video, and new services that could be offered in the future via Softswitch open API. Softswitch solutions can coexist with traditional circuit-based switch and provide similar services using packet-based switch technology. The end user devices may include regular POTS phones, IP phones, Multimedia Computers, Beepers, Pagers, Video Conferencing Terminals, etc. Softswitch technologies may extend beyond IP networks to other forms of wired, wireless or broadband networks.

Softswitch technologies as implemented by ZTE logically break into 4 functional planes as seen in figure one. Components that reside in each of these functional planes will be briefly described here. Note that Softswitch technologies usually refer to a set of technologies accomplished by network components from several functional planes, i.e., the Softswitch component in the Control Plane, the Media Gateway, Signaling Gateway and Media Server components from the Access Plane and Application Servers from the Service Plane. A softswitch alone usually refers to the Softswitch component in the Control Plane.

Figure 1 Softswitch Architecture

The Media Gateway Controller is the key component in the Control Plane. In fact, the most popular word used in the industry for this component is the term Softswitch. It mainly performs the call control function providing connection and call control, translations and routing, media gateway management, bandwidth management, signaling, provisioning, security, and call detail record generation.

The switch plane consists of standard IP routers and/or ATM switches. Unlike the traditional PSTN networks where telephone switches use their switching fabric to perform the function of switching, the new Softswitch based network uses the network itself as the switching fabric.

The Access Plane consists of Signaling Gateway, Media Gateway, and Media Server. A Signaling Gateway provides conversion between the SS7 signaling network (SS7 links) and the packet network, including protocols such as ISUP and TCAP. It is a key unit within the Softswitch architecture. A Media Gateway connects to the telephone switch side and deals with the payload voice media data. It provides conversion between circuit-switched resources (lines, trunks) and the packet network (IP, ATM), including voice compression, fax relay, echo cancellation, and digit detection. In the near future the video media data may also apply. A Media Server provides for specialized media resources, such as IVR, conferencing, facsimile, announcements, speech recognition, and handles the bearer interface to media gateways. An announcement server can be a type of media server.

Application Servers reside on the Service Plane and are another kind of key components that provide for the execution and management of enhanced services and billing, and handle the signaling interface to softswitches. It also provides Application Programming Interface (API) for creating and deploying services. Examples of enhanced services are Internet Call Waiting, Multiparty Conferencing, etc.

The major architecture considerations in the design of ZTE Softswitch solution are the following aspects:

• Performance. It should achieve high Busy Hour Call Attempt (BHCA) for a given CPU type and speed.
  
• Scalability. The architecture must be flexible enough so that it can grow to support many millions of BHCA as well as potentially many millions of subscribers.
  
• Redundancy. The state persistence and database replication are some of the main architectural considerations. The architecture is so designed that there is no single point of failure in the system.
  
• Failure Recovery. A highly effective failover scheme must be built in such that the system is able to recover from any type of failures quickly enough to achieve the high availability comparable to those (99.999％) currently seen with the PSTN switches. At the very least stable calls must be recoverable in failure.
  
• Interoperability. To achieve interoperability with media gateways and softswitches made from other vendors, multiple device protocols and their variants are implemented.
  
• Service Extensibility. The ability of rapidly creating and providing innovative new services is one of the strength of the architecture. Realizing the importance of time to market of new services to the service providers today, the system supports open standard service creation API such as JAIN/PARLAY which are best suited for creating new generation convergence services. Services built on these standard API are not constrained by Softswitch solution vendors.
  

3 Key Components of the Softswitch Technologies

In this section we shall elaborate on those key components that are collectively known as the Softswitch technologies. Descriptions of the major functions provided by these components are given here.

3.1 Softswitch(Media Gateway Controller)

The Softswitch component, sometimes called a media gateway controller or a call agent, is responsible for bridging the traditional wireline and wireless, SS7 signaling and IP networks together.This “bridging" function involves establishing phone calls and managing voice and data traffic (media) throughout the various networks. The Softswitch is the controlling element and the call control engine of the Softswitch solution architecture.

The major functions of a Softswitch are as follows:

• Perform the call control function
  
• Control media gateways using industrial standard media gateway control protocols such as MGCP and Megaco/H.248
  
• Support voice call establishment protocols such as H.323 and SIP
  
• Support Class features such as Class 4 features and/or Class 5 features
  
• Provide Class of service/quality of service control
  
• Interface with SS7 network via SIGTRAN (SS7 over IP)
  
• Support various forms of routing and numbering plan
  
  • -E.164 numbering plan
    
  • -digit analysis overlap and/or channel associated signaling
    
  • -digit translation support for IP, FR and ATM
    
• Call Detail Records (CDR) for billing
  
• Provisioning
  
  • -for media gateway controlled:
    
    • DS0 channel assignments
      
    • assignment and real time configuration of DSP resources
      
    • media transmission (voice coding, compression and packetization)
      
  • -for SS7 signaling gateway:
    
    • linkset configuration
      
    • point code/routing configuration
      
    • SS7 variants such as ANSI or ITU
      
    • timers
      
• Optionally support Gatekeeper functionality
  
• Optionally support bandwidth management control
  

A Softswitch typically runs on commercially available servers such as SUN Netra workstations. A few vendors including ZTE have chosen to have it run on more fault tolerant machines than commercial workstations to increase the reliability and shorten the time required for the component failure recovery.

3.2 Media Gateway

The Media Gateway provides a way to transport the media (voice/data/fax/video) between the ATM/IP packet network and the legacy PSTN network. In the PSTN network, toll quality voice payload is riding in a DS0. For the purpose of transporting in the packet network, samples of this toll quality voice are compressed and packetized. Typically Digital Signal Processors perform the functions such as analog to digital conversion (Mu-Law, A-Law), voice-audio code compression (G.723.1, G.729a, G.711, etc.), echo cancellation, presence of voice detection, silence detection and compression, comfort noise generation, out of band DTMF signal transport, etc. The DSPs control the playout of voice into the IP network.

　　Some of the key functional requirements of a media gateway are as follows:

• Transmission of voice-data (media) using transmission protocol RTP
  
• DSP resource and TDM timeslot allocation controlled by the Media Gateway as a result of MGCP/H.248 messages
  
• Manage DSP resources to provide voice and packet services
  
• Support for legacy protocols such as Loop-Start, Ground-Start, E＆M, CAS, Q.SIG and TR-303
  
• Support of TI clear channel configuration for transferring voice traffic payload in SS7 networks
  
• Manage TDM resources and link

3.3 Signaling Gateway

A Signaling Gateway (SG) bridges the SS7 network with the PSTN and ATM/IP networks for the Softswitch and establishes the protocol, timing and message framing requirement of the SS7 network and the functional equivalents in the ATM/IP network. It makes the Softswitch look like another SSP in the SS7 network. The SG only handles the SS7 signaling, while the media gateway will handle the voice circuits established by means of the SS7 signaling. If SIGTRAN is used as the protocol between the SG and Softswitch, then only MTP1, and MTP2 and SIGTRAN will reside on the SG. MTP3 and the above will reside on the Softswitch. SIGTRAN defines a suite of protocols and user adaptation layers for transporting signaling information over IP-based networks. It uses SCTP for reliable signaling transport, streaming, congestion avoidance and control, and M3UA for the transfer of SS7 MTP3 user part messages for ISUP, SCCP, and TUP. If another protocol such as SIP is used between the SG and Softswitch, then the entire SS7 protocol will reside on the SG.

The major functional requirements of SG are:

• Provide physical connectivity to the SS7 network via T1/E1/V.35 physical connection
  
• Transport SS7 information between SG and Softswitch via packet network

3.4 Application Server

An Application Server is an application level server that provides for the execution and management of advanced services. It typically has a signaling interface to a call control function entity such as Softswitch. It also provides APIs for creating and deploying services.

Advanced services provided by Application Servers generally include several services such as:

• Virtual Call Center
  
• Unified Messaging
  
• IP centrex
  
• Internet Call Waiting
  
• Pre-paid Calling cards
• Calling Card applications
  

3.5 Media Server

A Media Server provides for specialized media resources (IVR, conferencing, facsimile, announcements, speech recognition), and handles the bearer interface to media gateways. An announcement server can be a type of media server. Media Server is not strictly required as part of the media gateway functions. However, in many implementations the media server functionality is also built into the media gateways. MGCP is now often used as the controlling protocol for media servers.

Typically, media servers provide the following functions:

• Pre-recorded announcements, IVR, record/playback
  
• n-way station conference
  
• DTMF/MF tone detection/generation
  
• Fax support
  

4 Softswitch TechnologiesAre the Heart to the Next Generation Network

The following combination of market and technical forces contribute to place Softswitch technologies at the heart of the next generation network infrastructure.

4.1 The market place and acceptance of Softswitch technologies

Any company that provides telecommunications services, whether voice, Internet Access, wireless, or a bundle of these, can use Softswitch technologies to provide advanced functions to its customer base. Long distance and local phone companies can use Softswitch technologies to replace Class 4 and Class 5 telephone switches, add capacity, increase services offered, or boost capabilities to handle multimedia service. Internet service providers and Internet telephony service providers can broaden the range of services they offer beyond basic Internet connectivity. Emerging next generation carriers who sell bandwidth to others can use the private fiber optic networks they built to carry converged data, including voice, fax, and video, over their own lines with the help of Softswitch technologies. Softswitch technologies provide service providers the capability to seamlessly link to the PSTN to reach new customers not served by their fiber optic backbones. We believe that all companies that deploy Softswitch technologies will gain the benefits of cost reduction, service differentiation, and leveraging the PSTN while migrating to IP-based networks.

The market acceptance of Softswitch technologies has been overwhelming. Many service providers in North America and Europe have started or planed to deploy Softswitch technologies. For example, Global Crossing Ltd., which is building and operating the world´s most extensive global IP-based fiber optic network, announced on September 27  2000 that it had lit up its VoIP backbone in its domestic production network using Softswitch technologies. The company completed the first phase of its U.S. VoIP network by the end of 2000, allowing it to originate and terminate calls on its own core network in the US. Global Crossing plans to transfer its entire voice traffic from the circuit-switched network to the packet-based network by 2002.

4.2 Cost reduction by Softswitch technologies

Softswitch technologies allow telecommunication companies to reduce the costs of handling traffic, especially the exploding volumes of data that their networks must carry. It is known that IP transport and switching cost less than circuit switching. Softswitches in PSTN networks can off load dial-up data calls and intercept fax data and route them to lower cost IP networks. They can also be used to shift high-priced international long distance calls to less expensive data networks.

Service providers can use Softswitch technologies to replace existing Class 4 and/or Class 5 telephone switches more cost effectively. New service providers have lower barriers to entry since they no longer need to purchase an expensive Class 4 or Class 5 phone switch. They can also purchase only the capacity they need, starting with as few as several hundred ports or customers, and add more as they expand their customer base.

The Softswitch is easier to manage and operate than the traditional circuit-based switch. As the control function is performed in the software based softswitches, the upgrade and configuration changes are much more manageable than the traditional PSTN switches. All these can help service providers bring down the maintenance cost of their network dramatically.

4.3 Softswitch technologies improve service delivery

The key to the rapid acceptance of Softswitch technologies in the telecommunications industry is their ability to deliver new service. In the new architecture, services are programmed through open APIs and this is one of the crucial elements in enhanced service delivery. Basic services that already exist in today´s Class switches are typically done in softswitches while enhanced services that may be far too complex for traditional Class switches are offered by separate application servers. Different services may be provided by separate application servers and this architecture allows carriers to maintain a robust infrastructure for completing calls independent of the additional features they provide to individual customers.

By adding new services either as an upgrade to the software based softswitches, or as a replacement of an application server, service providers can deploy new services more quickly and at lower cost than with traditional methods. New value-added offerings or service bundles represent an excellent way for companies to differentiate themselves from their competition.

Examples of basic services include call waiting and forwarding, call blocking, 800 numbers, and caller ID. Instances of enhanced services are such as Unified Communications, IP Conferencing, Local Number Portability, and IP Centrex. Many of the existing Softswitch implementations support customer self-configuration which means that individual customers can go to a specific web site provided by the service provider to self-configure the services .

4.4 Softswitch technologies are the key to the seamless migration to IP Networks

The migration of the traditional circuit-based PSTN networks to all IP networks will take years to complete and until then, PSTN and IP networks will coexist. The Softswitch technologies offer interoperability across these two types of networks as they transfer from circuit to packet based switching and are the key to facilitate carriers to seamlessly migrate their networks to all IP networks.

The distributed architecture of Softswitch technologies is well suited to allow the phone networks to continue to evolve by allowing service providers to add capacity or services when and where they are needed. By implementing various protocol adaptors, softswitches can leverage existing resources such as directory and network SCP databases currently available in PSTN network, allowing new services to interoperate with legacy databases. By simply adding new device control protocol adaptors in the softswitches, service providers can easily incorporated new types of devices into the networks and expand their networks and services.

4.5 Open architecture and interoperability of Softswitch technologies

One of the most important aspects of the Softswitch technologies is that they break apart the linkages between the software and the intelligence, and the hardware, transport and switching layers. Unlike traditional PSTN switches, where there were few manufacturers, and modifying an existing switch was difficult and expensive, many vendors compete to provide hardware, software, and application components for Softswitch based solutions. Not only does this keep pricing competitive, it also allows service providers or phone companies to select the best solution for their specific needs, mixing and matching pieces from different vendors and combining them using standard open interfaces.

Softswitch solutions from different vendors have begun to demonstrate interoperability recently. The International Softswitch Consortium (ISC) was formed in 1999 to promote the interoperability of Softswitch components from different vendors and several interoperability test laboratories have been established that are open to Softswitch vendors. ISC currently has more than 200 members.

4.6 Standards related to Softswitch technologies

There have been major efforts going on in ITU-T and IETF to standardize the protocols and architecture in Softswitch technologies. Most early players in the VoIP market have focused primarily on H.323 for voice and video call setup. Recently, packet voice protocols, such as MGCP and Megaco/H.248 for media gateway control, SIP for session initiation and management, SIP-T and BICC for Softswitch peer-to-peer communications, SIGTRAN for SS7 Over IP, have emerged. These protocols allow seamless traffic exchange within or between service provider networks and improve the interoperability among components from different vendors.

Note that several of the protocols, such as H.323, SIP and MGCP/H.248, can work interchangeably. For example, a softswitch using the H.323 protocol can talk to and pass messages to a SIP-based softswitch or a MGCP based softswitch. This interoperability among protocols adds to the Softswitch flexibility and ease of deployment. Standard based software components of Softswitch solutions can be distributed across hardware elements but function as a single unit, and standard based media gateways can be easily integrated with legacy systems via SS7 interface, bringing new services to an existing subscriber base. Service providers can exploit Softswitch technologies modular architecture and expand their systems to accommodate new market needs.

[摘要] 文章简要介绍了软交换技术，并在概括软交换体系结构的基础上，进一步阐述了软交换技术的关键组成部分。从市场和技术两方面论证了软交换技术将会成为下一代网络基础设施的核心。

[关键词] 软交换技术 基于IP的语音 下一代网络 增强型业务 会聚型业务

[Abstract] This article gives a brief introduction on Softswitch technologies and provides an overview on the Softswitch architecture. Key components of Softswitch technologies are further elaborated. Market and technical aspects that contribute to place Softswitch technologies at the heart of the next generation network infrastructure are examined.

[Keywords] Softswitch technologies VoIP Next generation network Enhanced service Converged service