The report of the death of the public switched telephone network (PSTN) has been greatly exaggerated.
A mere 25 million of the more than 450 billion domestic minutes in 1998 were from packet voice. Further, intelligent network (IN) services were, until recently, unavailable to users of voice over packet services. The PSTN infrastructure and the signaling protocols that support it are far from moribund.
Many existing and emerging service providers recognize the significant capital, facilities, support systems and personnel savings that utilization of a data, rather than circuit-switched, infrastructure can provide. Asynchronous transfer mode (ATM) and transfer control protocol/Internet protocol (TCP/IP)-based networks are springing up that will be used not just to provide traditional data services such as frame relay and Internet access, but also to provide the traditional telephony services enterprises and consumers have come to expect and rely on. With distributed call control, centralized signaling and billing as well as integration with existing PSTN protocols and services, these networks will deliver innovative, cost-effective voice and data service solutions into the 21st century. Today's service providers can make this transition easier by effectively leveraging signaling system 7 (SS7).
When 2nd Century Communications Inc., Tampa, Fla., designed its network infrastructure, it reviewed a number of possible architectures, considering the potential capital, facilities and support costs. It decided the best and most cost-effective method of delivering the voice and data services small and medium-sized businesses needed and could afford was via an ATM network. Their challenge was finding a means to seamlessly integrate ATM call control with regional Bell operating company (RBOC) and interexchange carrier (IXC) SS7-based networks and deliver core SS7-based voice services without building a traditional circuit-switched network.
The company determined a signaling and service gateway between their ATM network and the SS7 networks offered the most cost-effective and manageable solution to its problem (see "ATM in the Local Loop to PSTN Interworking via SS7" diagram, below). 2nd Century uses the gateway to provide competitive and cost advantages. The gateway allows it to deliver integrated services from common hardware, provisioning and billing platforms. "Our goal is to deliver services, previously accessible only to large businesses, to the small and medium-sized business segment by integrating voice, data, video and Internet access onto a unified network," says John Prisco, president and CEO of 2nd Century. "We have identified a significant opportunity, and by deploying an SS7 to ATM gateway, we can provide next-generation services and applications that are unmatched by other carriers."
Graph: ATM in the Local Loop to PSTN Interworking via SS7
Clearly, ATM in the local loop provides valuable services previously unattainable for small to medium-sized businesses and some compelling economic advantages for competitive local exchanges (CLECs). In this type of deployment, SS7 provides two significant benefits. First, it provides CLECs the ability to deliver critical features such as calling party name (CNAM), local number portability (LNP) and 800 number lookup, additional to features that may reside closer to the customer premises. Second, SS7 allows the CLEC to deploy new packet switching technologies and to trunk with the PSTN.
Leveraging ATM for Voice
Carriers interested in providing wholesale long distance services must be particularly selective when designing their network architecture. Due to the projected growth of voice and data traffic, many carriers providing backbone services must build their networks to support a variety of traffic types as well as to meet the increasing demand for cost effective service. Accordingly, some have, or are considering, implementing dense wavelength-division multiplexing (DWDM) to maximize capacity within the fiber infrastructure.
Graph: Worldwide SS7 Shipments
In addition, some long-haul carriers are planning to use their ATM network to reduce, and ultimately eliminate, the need to purchase costly circuit switches for voice tandeming. ATM voice tandeming will allow them to redeploy existing circuit switches to new or expanding central offices (COs) and offer voice transport at a highly competitive rate to their customer base. Critical to these new tandem deployments is the ability to interwork with the PSTN. Between the ATM backbone and the ATM multiservice switching gateway, the industry standard Q.2931 protocol can provide the signaling (see "SS7 Bridges the Gap Between ATM Backbones and the PSTN" diagram, below). However, between the ATM switching gateway and the PSTN, SS7 is required to bridge the gap. This can be accomplished via the SS7 gateway, which would interface with the switching gateway and SS7 signaling transfer points (STPs) via A-links.
Graph: SS7 Bridges the Gap Between ATM Backbones and the PSTN
The U.S. Department of Commerce estimates there are as many as 28,000 switches in the telecommunications network of the United States. So it is unlikely to expect a strategy of complete replacement anytime soon. As a result, carriers deploying the ATM tandem switch solution require close integration of SS7 STP and service control point (SCP) gateway, routing and database interface functionality with that of the ATM network. Without SCP functionality in particular, access to critical LNP, call management services database (CMSDB), line information database (LIDB), business services database (BSDB), home location register (HLR) and visitor location register (VLR) information would be unavailable. Further, critical billing data would be inaccessible.
Is Integration for Everyone?
Many early adopters of voice over packet services delivered only delivered transmission facilities and could not deliver the enhanced voice services we have come to expect from the PSTN and its SS7-based infrastructure.
With the emergence of packet-SS7 network service integration, the market opportunity for arbitrage, centric service providers may well be at an end. As voice over-packet services offer traditional voice network functionality--Centrex, call forwarding, caller identification, etc.--arbitraged, transmission services may prove less appealing to businesses and consumers, espec-ially as emerging service providers begin to offer robust, packet-based voice services.
"I've never been interested in arbitrage," states Deb Mielke, principal of Treillage Network Strategies, McKinney, Texas. "I believe that only when packet-based voice services offer comparable, 'IN-like' voice capabilities, will they threaten traditional PSTN-based services. Customers will not go 'back to the future.' They have a right to expect that voice service capabilities will not diminish as service providers migrate voice to packet infrastructures."
With service providers of all stripes announcing their intentions to offer voice over packet services in the near future, the need to integrate with the billions of dollars of existing PSTN infrastructure will take on increasing importance. Additionally, the rate of replacement of PSTN equipment in existing service provider networks is hampered by regulatory and philosophical influences. Ultimately, savvy service providers intent upon limiting or eliminating their investment in circuit-switched architectures should ensure that packet network infrastructure plans include the ability to deliver packet over voice services with the functionality available in the PSTN today. This functionality is delivered in large part by SS7 and the advanced intelligent network (AIN). Accordingly, carriers must develop the architectures that can point-to-point transmission. Even those offering PC-to-phone, phone-to-PC or phone-to-phone over a packet network only seamlessly integrate with, and take advantage of, the database access functionality and service features in the SS7 network. Tight integration will allow them to not only complete calls of all types, but also provision and bill for services delivered across divergent technologies.
Circuit-switched networks will be usurped by ATM and TCP/IP-based infrastructures, but the robust services supported by the SS7 network will continue to set the standard for delivery of robust voice services. Successful service providers will embrace the opportunity afforded by ATM and IP to incorporate traditional voice services while implementing technologies that include SS7 protocols.
Bing Yang is chief technology officer with Convergent Networks Inc., Tewksbury, Mass. He can be reached at (978) 640-1970.
