1 As we enter the 21st century, we are experiencing a telecommunications revolution. From a technological perspective, the distinction between voice information and other kinds of data is blurring as circuit-switched and packet-switched technologies converge. From a market perspective, the level of competition for telecommunications business is escalating rapidly as telecommunications network operating companies, wireless communication companies, data access providers, cable companies, and even utilities set their sights on each other s traditional markets. From a subscriber s perspective, there is an endlessly shifting set of options for local and long-distance telephone service, wireless communications, always-on Internet hookups, as well as satellite and cable television packages in different combinations and permutations. Additionally, from all sides, there is the promise of brand new services and capabilities that will simplify and enrich our lives. To realize the promise of this telecommunications revolution, one of the fundamental requirements is the establishment of a global Voice-over-IP (VoIP) communications network. While there are commercially successful VoIP networks operating today, they are isolated and limited in scope. They are essentially islands of VoIP that intercommunicate through the Public ed Telephone Network (PSTN). The vast majority of subscribers are still PSTN subscribers with either PSTN telephones or PBX connectivity. As the technology behind VoIP networks matures and stabilizes, more of these VoIP islands will be created and many will grow larger. Ultimately, these islands will become directly interconnected to form a global VoIP network, which will be referred to as the Public Internet Telephone Network (PITN). Most subscribers will eventually access the PITN directly using IP-based telephone sets or IP-PBX connectivity. Ideally, the PITN will combine the best network features of the PSTN and the Internet. Like the PSTN, the PITN must handle real-time data in an efficient and reliable manner. Like the PSTN, the PITN will therefore utilize a three-tiered architecture in which an end-to-end signaling infrastructure provides real-time control and management for both an underlying transport layer and a higher-level advanced services layer. Like the Internet, the PITN will be based on industry-standard interfaces, non-proprietary systems, and open architectures. In addition, the PITN will achieve global reliability through the wide deployment of systems from many different vendors and the sheer magnitude of the connections between them. Current vs. next-generation VoIP networks Figure 1 shows the current PSTN architecture. Local Exchange Carriers (LECs) that run networks composed of some number of Class 5 switches, such as Signal Transfer Points (s) and Service Control Points (SCPs), support the subscribers. Interexchange Carriers (IXCs) that use Class 4 switches, also known as toll switches, interconnect these local loops. This combination of local loops and IXCs literally span the world to create a global PSTN. In contrast, the current VoIP architecture is shown in Figure 2. IP Local Exchange Carriers (IP LECs) offer VoIP services to their PSTN IXC PSTN LEC PSTN LEC SCP Class 4 Class 4 SCP Class 5 Signaling Media Figure 1. Today s PSTN networks Class 5
2 GW GW PSTN IP IXC PSTN IP/IN GW Signaling Media GW IP/IN IP LEC IP LEC Figure 2. Today s VoIP network architecture over IP, essentially uniting the islands into a global PITN. As in the previous diagram, the gateways provide interconnection with the PSTN. The architecture has been complemented by the addition of a pure IP switch. In an H.323 network, this device would be a gatekeeper, while in an SIP network, it would be a proxy server. In this article, these switches will be referred to by the generic term IP signaling switch. Unlike the gateways that support both IP and PSTN protocols, the IP signaling switch operates entirely within the IP domain. Its purpose is to support IP telephones and to create the real-time signaling infrastructure that is required for a global IP telephony network. The IP signaling switches connect the gateways, IP telephones, and IP-hosted Intelligent Network service platforms (IP/IN) within each IP telephony network and between networks as well. Figure 3. A gateway subscribers with connectivity to the PSTN through some number of gateways. A gateway can be one or more physical devices that inter-work the media and signaling traffic between the PSTN and IP telephony networks. As shown in Figure 3, the logical functions that these gateways provide are: Signaling gateways, which inter-work the signaling traffic Media gateways, which inter-work the media traffic Media gateway controllers, which manage the media gateways and provide call control Another class of VoIP carrier shown in Figure 2 is the IP Interexchange Carrier (IP IXC) that offers VoIP service to other carriers. Unlike heavily regulated PSTN carriers, VoIP carriers can offer both LEC and IXC functionality in their networks. Today s VoIP networks generally are interconnected only through the PSTN. As the vast majority of telephone subscribers are on the PSTN, connectivity with the PSTN is the primary concern for VoIP carriers at the outset. However, as the IP telephony networks grow larger and more sophisticated, for reasons of cost and efficiency, direct connection between them over IP will become increasingly important. Figure 4 shows the future of IP telephony networks. In this picture, the IP telephony networks have been interconnected directly The SCP in Figure 1 is the device that hosts Intelligent Network (IN) services for the PSTN. These services are controlled by the PSTN s signaling infrastructure, which is based on Signaling System 7 (SS7). This IN architecture has evolved over the last 30 years and it is extremely reliable. The services offered by the IN network include 800 toll-free service, calling card services, Virtual Private Networks (VPN), caller name display, and Local Number Portability (LNP). While the PSTN has been successful at offering reliable voice communications, it has been much less successful at offering a development environment that fosters innovation. The PSTN service creation environments (SCEs) are proprietary and inflexible, demanding tremendous resources to make even minor improvements. In contrast, the PITN SCEs (shown as IP/IN servers in Figures 2 and 4) will take advantage of the basic tenets of IP network architecture. They will be based on open architectures and industry standard interfaces. The PITN will therefore have the ability to support rapid service innovations in an open, multi-vendor environment. The stark contrast in development environments will mean that the PSTN will continue to offer existing voicecentric services, while new converged voice, audio, and video services will be developed on the PITN. The IP signaling switch The IP signaling switch provides the PITN with both intra-carrier and inter-carrier IP signaling, thereby eliminating the VoIP network dependence on PSTN SS7 signaling as the call signaling bridge. IP signaling provides the means for VoIP networks
3 networks to be interconnected and yet isolated from each other when needed. With this architecture, a failure in any given network need not place other networks in jeopardy. In addition, with multi-vendor (or multi-service provider) networks, there is a to interact, to establish hierarchy, to exchange signaling data, and to provide an infrastructure to deliver advanced multimedia services. As the center of IP signaling, the IP signaling switch addresses the following four fundamental requirements of the PITN: 1. Mediated signaling relationships to enable scalability of VoIP networks to a global network 2. Dynamic telephony route discovery and updating 3. Global telephony address resolution 4. Dynamic policy-based routing to provide flexibility and choice Mediated Internetworking Figure 5 shows that the PITN will consist of multiple IP LECs interconnected either directly or through one or more IP IXCs. Within each of these networks, carriers may create different zones for ease of administration. On a global scale, there will be a large number of these VoIP networks, some of which will be divided into a large number of zones. Interconnecting these networks requires scalability and hierarchy. Hierarchy and scalability are the result of mediated peering relationships between network control elements. The notion of internetworking hierarchical control has long been utilized in telephone networks. In the PSTN, SS7 manages the connection of traffic from the sending user to the receiving user. Through mediated internetworking control, SS7 allows multiple What is the Public Internet Telephone Network (PITN)? The PITN is an IP-based, real-time global communications network. The PITN is based on a scalable architecture that creates hierarchy and allows multiple carriers to operate in a peering relationship. The PITN is compatible with the PSTN: It receives or delivers telephone calls from the PSTN. It allows IP telephones access to PSTN services. The PITN is a platform for the delivery of voice, audio, and video services independent of the PSTN. Figure 4. The PITN
4 desire to isolate each provider s internal network architecture, security, and control from a neighboring vendor s network. An excellent example of mediated internetworking control relationships exists in the combination of numerous data IP networks that form the Internet. In an IP network, intra-enterprise, peer-topeer mediation is accomplished via Internal Gateway Protocols (IGPs). IGPs, such as RIP and OSPF, flood the routing information between network elements in adjacent autonomous systems. Inter-enterprise peer-to-peer and peer-to-superior mediated relationships are controlled by the Exterior Gateway Protocols (EGPs), such as BGP. These protocols enable multiple autonomous systems to share limited border information. The use of IGPs and EGPs has allowed autonomous systems to be joined and scaled on a global basis to create the Internet. In order for VoIP networks to scale globally, call control data must be shared in a mediated relationship between networks and between zones within a network. This function requires the establishment of mediated control relationships analogous to the IP internetworking relationships. Like the PSTN and the data IP networks, the PITN has three distinct mediated control relationships. The type of information shared across the boundary and the network positioning will determine the nature of the relationship. Three different logical arrangements of networks are shown in Figure 5. In each case, the network zones depicted might be in the same carrier s network or in different carriers networks. When two network zones are operating in a peer-to-superior hierarchical network configuration, the information they exchange can be limited to border information. There is no need for the superior device to know all subordinate zones network configurations. When two zones are operating at equivalent network positions, the information they share depends on who owns and operates the two zones. If they are both within the network of a single provider, then it might be in the best interest of the network to share all information equally across the network. This setup is not applicable between two similar networks operated by two different companies, e.g., competing IP LECs. Inter-working of different carriers networks is typically limited to the minimum information exchange required to route and make calls. As was noted earlier, mediated inter-working in the data IP world is accomplished using IGPs and EGPs. Mediated interworking between zones in the PITN will be accomplished using standard protocols. One such protocol is the Telephony Routing over IP (TRIP) protocol from the International Engineering Task Force (IETF). Like the IGPs described above, TRIP enables the flooding of routing information between peers within a given domain. TRIP also supports EGP functionality, propagating selected routing information between peers in adjacent domains, or between a peer and a superior node. As shown in Figure 6, the IP signaling switch uses TRIP to share routing information with peers, both within the same zone and in adjacent zones. The figure illustrates another essential feature of TRIP route aggregation. The best routes from all peers are aggregated intelligently for efficiency before being propagated to peers in external zones. Dynamic telephony route discovery and updating The ability for two devices to exchange information in a mediated relationship is a fundamental requirement of network growth. However, the information they exchange will change over time. Zones will be added; new networks will be created; end users will come and go; and network elements will be taken off-line. Manual provisioning of these changes into network control devices is expensive and inhibits rapid network growth. Dynamic telephony route discovery and updating allows the network to adapt, change, and grow rapidly without the need for constant manual intervention. Not only does TRIP allow two control devices to exchange call routing information in a mediated relationship, but network administrators can also use it to provide dynamic synchronization and updates of network changes to connected control devices. This capability enables each change in routing information to be derived, broadcast, and dynamically synchronized throughout the entire PITN. The concept of dynamic network provisioning is equally powerful for the subscriber registration process. When the location of a user changes or when a user is added or removed, this call reaching information must be entered into the network and broadcast between elements. Again, when this is done dynamically, the required network management resources are greatly reduced. Once the network has the ability to dynamically broadcast and synchronize information, this function can be used not only for call destination routing but to broadcast other network Figure 5. Logical arrangement of mediated networks
5 control parameters both inside a carrier network and between multiple carriers. Global telephony address resolution The addressing schemes in the PSTN and IP networks are fundamentally different. In the PSTN, telephone numbers that are defined by the ITU E.164 standard represent access lines. Telephone numbers identify a port on a switch rather than an individual telephone. Multiple telephones can be connected to the same access line and they will respond to the same telephone number. In contrast, each device on an IP network is identified by a unique IP address, either permanently or dynamically assigned. These numeric addresses can be cumbersome to work with directly, so most IP routing is done using textual aliases called domain names. Domain names are commonly used in URLs and addresses. The domain names are mapped to IP addresses by a large distributed database called the Domain Name Server (DNS). The PITN exists to provide real-time telephony services over IP networks. Therefore, it must be able to support both PSTN and IP addressing schemes. IP addresses, URLs, domain names, or E.164 numbers can reach PITN subscribers. Either the PSTN or the PITN can originate or terminate calls. In any case, the address must ultimately be translated into a routable IP address for any IP portions of the call. The IETF defines a protocol called E.164 Number Mapping (ENUM), which uses the proven DNS mechanism to map E.164 telephone numbers to IP addresses. Policy-based routing As independent VoIP networks become interconnected, organizations need the freedom to implement packet forwarding and routing according to their defined policies. In a global network, there are multiple paths between individual endpoints. Where administrative issues dictate, one path may be preferred over another. This preference may be variable based on the time of day, network usage, inbound carrier, quality of service, or a variety of other factors. Policy-based routing introduces the concept of choice into the network infrastructure. From the end user s perspective, this concept will provide the option to choose between competing backbone and inter-exchange carriers. Like today s customer choice between competing long-distance providers, this decision may be based on price, provider preference, quality, protocol, or services offered. From the network provider perspective, policy-based routing will provide the ability to direct traffic through different parts of the network for reasons of cost, performance, or convenience. Network administrators will be able to implement routing policy based on parameters such as: Time of day Network usage Inbound carrier Figure 6. Mediated Internetworking using TRIP
6 Outbound carrier Quality of service Protocol Cost Network compatibility Through the use of TRIP s dynamic updates of peering policy, policies can be defined and propagated inside a carrier s network or between carriers networks based on the desire of the network providers. These policies can range from simple static operations to complex, rule-driven operations. Establishing the global PITN Various carriers around the world have begun to create commercial IP telephony networks. However, these networks are generally isolated from each other, interconnecting only through the existing PSTN. As the IP telephony networks grow and expand, they will need to interconnect directly over IP. This development will establish a global PITN and can occur only with the creation of a global IP Telephony signaling infrastructure. The basic building block of this signaling infrastructure is the IP signaling switch. With its support for SIP, H.323, TRIP, and ENUM, the IP signaling switch provides the fundamental signaling capabilities required by global IP telephony networks: Mediated signaling relationships Dynamic telephony route discovery and updating Global telephony address resolution Dynamic policy-based routing These signaling capabilities will enable VoIP service providers to scale up their networks to support very large numbers of users with widely distributed network resources and services. Furthermore, as these disparate and disjointed networks grow larger, the IP telephony signaling infrastructure will enable operators to interconnect their networks on a global scale to create the PITN. When that setup has been accomplished, the telecommunications revolution will have profoundly changed the way that we communicate with each other. Our turn-of-the-century telecommunications services will appear decidedly quaint by comparison. Michael Gaines has more than 15 years of experience in the fields of telecommunications and information management. After obtaining a degree in electrical engineering, Mr. Gaines held a variety of technical and customer-facing positions at Mitel Corporation, Fulcrum Technologies, MicroLegend Telecom Systems, and Performance Technologies. As marketing director for SS8 Networks, Mr. Gaines is focused on product marketing for IP telephony signaling and services products. For more information, contact Mike Gaines, marketing director, at , ext SS8 Networks (www.ss8.com) is a leading developer of Internet signaling and services solutions that enable service providers to deliver high-performance data, telephony, and video services over the Internet. The company s carrier-class products enable next-generation network operators to create and deploy innovative new services that transcend the PSTN. SS8 Networks has headquarters in San Jose, California, with R & D in Ottawa, Canada.