Internet Protocol (IP)/Intelligent Network (IN) Integration Definition The convergence of the public switched telephone network (PSTN) and Internet protocol (IP) data networks promises exciting opportunities for local and longdistance wireline and wireless carriers, Internet service providers (ISPs), equipment manufacturers, and value-added resellers (VARs). An important step in the fulfillment of this promise is the extension of intelligent network (IN) capabilities to and from IP networks. Telephony IN services such as 800 toll-free service, local number portability (LNP), and calling-card services are now being complemented by a corresponding set of IP based call management and intelligent routing services. The signaling system 7 (SS7) protocol is the only means by which these applications can achieve interoperability. This integration of IN and IP technologies represents a significant breakthrough in the ongoing convergence of voice and data networks. Overview This tutorial contrasts several alternative architectures for an IP/IN integrated network, considering the advantages and disadvantages of each. An introduction to the relevant industry standards work is also presented. Topics 1. The Intelligent Network 2. Voice over IP 3. Small IP Telephony Switches with SS7 4. Large IP Telephony Switches with SS7 5. Distributed IP Telephony Switch with SS7 6. The Distributed IP Telephony Network 7. The Future of Internet Telephony in the PSTN Self-Test Correct Answers 1/18
Glossary 1. The Intelligent Network The PSTN is an intelligent network throughout much of the world. In practical terms, this means that the network has the capacity to utilize real-time database interactions to control the routing of telephone calls. Many of the services that modern telephone users expect rely upon this capability, including the following: 800 toll-free calling services Unlike traditional telephone numbers that explicitly identify a telephone, an 800 telephone number is simply an identification number. To route an 800 call, the network must first locate the appropriate database (which depends on the carrier) and then look up the terminating phone number to be used. wireless roaming When wireless users roam from their home territories, the network uses a complex series of signaling messages to enable incoming and outgoing phone calls. The subscribers' home databases are queried to determine what rights exist for roaming service in new areas. Temporary records are created in a visitor database, which is used to complete subsequent phone calls. calling cards When a calling card phone call is placed, a database is accessed to confirm the validity of the calling card and to enable proper billing to occur. local number portability LNP is a feature that allows phone users to hange local carriers while maintaining the same phone number. Traditionally, phone calls have been routed to switches based on their exchange codes. The ramification of LNP is that this exchange code to switch mapping is no longer valid. Once LNP has been introduced for a given exchange, every phone call made to any phone number within that exchange code will require a database query to determine the switch to which the call should be routed. In many markets, including the United States and Canada, LNP has been mandated by regulatory agencies. In the PSTN, IN services are controlled by SS7 protocol. The SS7 protocol is a layered protocol, with functionality isolated in specific software layers. The ISDN user part (ISUP) layer is used for setting up and tearing down phone calls. The transaction control application part (TCAP) layer is used for exchanging arbitrary information, including database queries and responses. Thus, the ability to support IN services in the PSTN requires both ISUP and TCAP support. 2/18
A simple diagram of the network elements involved is shown in Figure 1. Signal service points (SSPs) are telephone switches that initiate and terminate SS7 messages. The signal transfer points (STPs) are devices that route the SS7 messages within the network. The service control points (SCPs) are database servers. Figure 1. IN Architecture For more information on the network architecture and the SS7 protocol itself, please refer to the tutorials offered on the MicroLegend home page at http://www.microlegend.com/aboutss7.htm. The PSTN is characterized by its reliability and guaranteed quality of service (QoS). The voice and data traffic is carried on switched circuits or dedicated connections, the entire bandwidth of which is available to the called and calling parties throughout the duration of the call. As the network becomes busier, each user will experience busy signals more frequently but will enjoy consistently high performance whenever they are connected. This guaranteed bandwidth per connection is one of the fundamental aspects that differentiates the PSTN from IP networks. The signaling architecture in the PSTN is extremely robust. Note that each SS7 link in Figure 1 is duplicated, as are the STP and SCP network elements. The SS7 protocol includes comprehensive procedures to handle link and node failures, providing alternate routes and message retransmission to ensure that messages reach their desired destinations. 3/18
2. Voice over Internet Protocol While the PSTN evolved into an IN, data networks evolved in tandem. With the creation of the World Wide Web (WWW), the Internet exploded into a global network of research, business, and personal users. As a result, IP has become the de facto standard for data networking. Unlike the circuit-switched architecture of the PSTN, IP networks are data packet networks. The computers on the network are all interconnected by persistent connections, the bandwidth of which is shared by all active users. As the network gets busier, each user will remain connected but will experience performance degradation. Although PSTN and IP networks are fundamentally different in terms of routing and performance, it is possible for the networks to be connected, exchanging voice and data traffic. Figure 2 depicts the first generation of technology to achieve this integration. Figure 2. PSTN and IP Network Integration If a subscriber connected to an SSP places a direct call through the PSTN to another SSP in a distant area, the voice traffic is routed through an intermediary toll SSP. There can be many intermediary switches involved in routing a call from end to end. SS7 signaling is used to reserve a voice trunk from each switch to the next and to send messages from end to end to set up the phone call. The SS7 signals themselves are routed through some number of STPs. In Figure 2, a simple example is shown with a single toll SSP and a single pair of STPs. In this 4/18
example, the phone call is a long-distance call, and the calling party will be billed appropriately. Voice over Internet protocol (VoIP) technology offers an alternative route. A local call is placed to the IP telephony switch that digitizes and compresses the voice traffic into data packets and then sends these packets across the IP network. The SSPs on both ends of the call communicate with their respective IP telephony switches using ISDN connections, which include both the voice and the signaling information. Calls can be initiated or terminated on either the PSTN or the IP network. On the IP networks, users send and receive voice using an IP telephony terminal, which is a multimedia computer equipped with telephony software. On an IP network, VoIP devices communicate using either a proprietary protocol or one of the emerging standards such as H.323 or MGCP. Because of the lack of fully defined standards, the first generation of VoIP products required that gateways from the same vendor exist on both sides of the IP network. By the middle of 1998, the first example of multivendor interoperability (based on H.323) was demonstrated. For more information about the ongoing standards development, please refer to Topic 7. The signaling used by the first generation of VoIP product is limited in its functionality. Basic call setup and tear down is possible, but IN services cannot be accessed. These services require the ability to use the TCAP layer of the SS7 stack to query databases that exist in the PSTN. 3. Small IP Telephony Switches with SS7 Recognizing the importance of the SS7 protocol, some IP telephony switch manufacturers began to announce plans to support SS7. The first of such products were small IP telephony switches (fewer than 1000 trunks supported) with an SS7 stack configured as an end node (SSP). A network topology showing IN and IP integration based on these types of switches is shown in Figure 3. 5/18
Figure 3. Small IP Telephony Switches with SS7 This architecture is conceptually simple. Each IP telephony switch is connected directly to the STP pair using SS7 links, enabling the support of IN services. Because the IP telephony switches are so small, the result is a highly distributed IP telephony network, which limits the local trunk network congestion for the SSPs. Also, the distributed architecture is inherently more reliable, without any nodes that could potentially cause a network-wide failure. However, there are several critical problems with this topology. The first difficulty with the architecture shown in Figure 3 is that it is extremely expensive. SS7 links consist of dedicated, guaranteed bandwidth, full-duplex signaling links. Each SS7 link is subject to a number of monthly operational costs, including physical link charges, connectivity charges, and per-message charges. Each SS7 link has the capacity to control a much larger number of trunks than these small IP telephony switches support. The SS7 links are inefficiently utilized, and the overall network SS7 costs are excessive. Another issue with this network configuration is that each of the small IP telephony switches must have a distinct exchange code to support traditional SS7 routing. Users are forced to dial different access codes when moving among the different switches. Perhaps the most significant shortcoming of the SS7 enabled small IP telephony switch is the requirement of a large number of point codes. Every node in an SS7 network requires a unique identifier, called a point code. There is a limit on the number of point codes that are available. This is particularly true for networks that use the International Telecommunications Union Telecommunications (ITU) T version of SS7, which is more severely constrained than the American National Standards Institute (ANSI) version. Because it is simply not possible to assign an arbitrarily large number of point codes in an SS7 6/18
network, it is impractical to adopt an architecture such as that depicted in Figure 3, in which a large number of small switches each have distinct SS7 identities. 4. Large IP Telephony Switches with SS7 The next logical step for the VoIP equipment manufacturers is to simply build bigger IP telephony switches with greater than 1000 trunks. This architecture, shown in Figure 4, solves most of the problems related to small IP telephony switches. Figure 4. Large IP Telephony Switch with SS7 Cost efficiency is improved as more trunks are controlled with each SS7 link. An Internet service provider (ISP) can provide VoIP services to a large number of subscribers using a single exchange and a single phone number. Fewer point codes are required, as fewer IP telephony switches are required. However, this network configuration has one particularly severe disadvantage. As users place Internet telephone calls from various switches, they are all routed to the IP telephony switch through its local switch. This architecture ties up the trunks between the switches, causing congestion in the local loop. The phone network trunks are expensive resources, with dedicated bandwidth. Traffic that is bound for an IP network, with its more efficient shared bandwidth, should be transferred onto the IP network as soon as possible, rather than being carried a substantial distance over the PSTN first. Another problem with the architecture shown in Figure 4 is that network survivability is low. All of the IP network traffic flows through a single switch. If that switch becomes inactive for any reason, the entire VoIP service is lost. 7/18
5. Distributed IP Telephony Switch with SS7 A distributed IP telephony switch offers a good compromise between the large and the small IP telephony switches. Figure 5 depicts a distributed IP telephony switch. This switch includes all of the functions of the previous examples, but the functional modules are logically separated. The processes can run in the same central office (CO), on the same machine, or distributed around an IP network on multiple machines. The computer platforms on which the processes run can range from small PCs to large servers deployed in simplex, redundant, highavailability and fault-tolerant configurations. Figure 5. Distributed IP Telephony Switch Like the network shown in Figure 3, this topology uses multiple, relatively small trunk groups. These can be deployed physically near to the SSPs, minimizing the transport of the information on the relatively costly PSTN trunks. By offloading this traffic from the PSTN quickly, such a configuration also avoids the network congestion problems that exist with large IP telephony switches. While media gateways are distributed around the network, the SS7/IP gateway is centralized, connected to the SS7 network in a convenient location near an existing STP. Despite the distributed nature of this configuration, the entire collection of modules appears as a single switch to the SS7 network. It has a single point code, which is used to manage all of the media gateways. This is more cost-efficient than the small switch configuration, and it avoids the network congestion caused by the larger configuration. 8/18
With the utilization of database applications in the distributed IP telephony switch, SS7 queries can be initiated by an SSP to request the most efficient media gateway to which to route the call. In this way, IN functionality can be supported on both the PSTN and the IP network. The SS7/IP gateway communicates with the media gateways and the other modules using the IP network (see Figure 6). If each of the modules understands a consistent data packet format, then the necessary signaling information can be exchanged. If the SS7/IP gateway is sufficiently sophisticated, the other modules in the switch can be duplicated to provide increased reliability. If any module becomes unavailable for any reason, the SS7/IP gateway can route the traffic to the remaining redundant module until the problem can be addressed. The only module within the distributed IP telephony switch shown in Figure 5 that is not duplicated for reliability is the SS7/IP gateway. The SS7 network will route all messages intended for a switch's point code to a single device. Figure 6. Interprocess Communication in a Distributed IP Telephony Switch 6. The Distributed IP Telephony Network The ideal communication signaling integration between the IP network and the PSTN takes the distributed IP telephony switch one step further. The SS7/IP 9/18
gateway supports multiple distributed IP telephony switches. Rather than requiring that each and every IP telephony switch provide native SS7 support, this architecture, which is shown in Figure 7, provides well-defined signaling interface points between the two networks. Figure 7. Distributed IP Telephony Network Architecture As was the case in each of the previous examples, the SS7/IP gateway connects to an STP pair on the PSTN. In this configuration, however, the SS7/IP gateway itself is deployed as a mated pair, eliminating the single point of failure for the distributed IP telephony switch. The two SS7/IP gateways share a single point code and appear to the SS7 network as a single switch. SS7 gateways are typically deployed in different geographic locations to increase overall network survivability. The two devices in the mated pair use a high-bandwidth connection to maintain synchronization at all times. On the PSTN, the SS7 protocol includes mechanisms for the rerouting and retransmission of message traffic in the event of the failure of either SS7/IP gateway. On the IP network, the SS7 gateway extends this same level of reliability. Primary and alternate routes for messages are supported, as is load sharing between redundant modules. The centralized network-management module uses protocols such as signaling network management protocol (SNMP) and common object request broker architecture (CORBA) to manage the various processes around the network, including the SS7/IP gateway. 7. Future of Internet Telephony in the PSTN The communications network of the future includes both traditional PSTN circuit-switched technology and IP data-packet technology. The convergence of 10/18
these two different network architectures creates capabilities that neither network can support on its own. The inevitable progress of this convergence depends upon many factors, but two are particularly significant: protocol standardization and internetwork billing policies. If the convergence shown in Figure 7 is to achieve its full potential, the industry must agree on standard protocols to be used on the IP network. These protocols must cover all aspects of communications, including voice compression, multimedia transmission, and signaling. The adoption of standards will enable equipment from multiple vendors to coexist and interoperate on the network. Several different standards are currently under development by standards bodies and industry consortiums to address these issues (see Table 1). Table 1. Standard Protocols Standard References H.323 ITU T, Recommendation H.323, "Visual Telephone Systems and Equipment for Local Area Networks that Provide a Nonguaranteed Quality of Service" ITU T, Recommendation H.225, "Call Signaling Protocols and Media Stream Packetization for Packet-Based Multimedia Communications Systems" ITU T, Recommendation H.245, "Line Transmission of Nontelephone Signals" Media Gateway Control Protocol (MGCP) Session Interface Protocol (SIP) MGCP Specification from Telcordia Techologies www.cs.columbia.edu/~hgs/sip/sip.html A single standard is unlikely to be settled for some time. In the meantime, vendors are extending and adapting the emerging candidates or relying on proprietary protocols. Telephone network operators, ISPs, and regulatory agencies have been discussing the issues related to Internet telephony billing for some time now. The issues remain unsolved and contentious. The telephone networks are tightly regulated, with tariffs and billing policies designed to foster competition. The Internet, in contrast, has grown in an ad hoc and largely unregulated fashion. There are many non-technical questions that must be addressed as these networks converge, and the policies that are adopted will have a profound effect on the future of IP telephony. 11/18
Self-Test 1. The SS7 protocol is required to enable telephone calls between the PSTN and IP networks. 2. The SS7 protocol enables an Internet telephony switch to support which of the following? a. toll-free calling b. calling-card calling c. local number portability d. all of the above 3. Each Internet telephony media gateway needs its own dedicated SS7 link. 4. Supporting a large Internet telephony switch with over 1,000 trunks can cause. a. inefficient use of the SS7 signaling links b. excessive delays across the IP network c. network congestion in the local loop d. signal degradation in the PSTN 5. The media gateway controller, billing, and database application modules in a distributed IP telephony switch run on which of the following? a. the same machine b. different machines in the same CO c. different machines distributed around the network d. any of the above 12/18
6. A mated pair of SS7/IP gateways can support which of the following? a. multiple media gateways and media gateway controllers b. load sharing among redundant IP telephony modules c. primary and alternate routes d. all of the above 7. IP networks provide guaranteed bandwidth for each phone call. 8. The voice/data and signaling packets on IP networks have all been standardized to enable interoperability between multiple IP telephony vendors. 9. A single SS7 link can support more than 1,000 trunks. 10. An SS7/IP gateway can support multiple distributed IP telephony switches with a single point code. 11. IP telephony switches can support calls from which of the following? a. the PSTN to the IP network b. the PSTN to the PSTN via the IP network c. the IP network to the PSTN d. the IP network to the IP network e. all of the above 13/18
Correct Answers 1. The SS7 protocol is required to enable telephone calls between the PSTN and IP networks. See Topic 1. 2. The SS7 protocol enables an Internet telephony switch to support which of the following? a. toll-free calling b. calling-card calling c. local number portability d. all of the above See Topic 1. 3. Each Internet telephony media gateway needs its own dedicated SS7 link. See Topic 5. 4. Supporting a large Internet telephony switch with over 1,000 trunks can cause. a. inefficient use of the SS7 signaling links b. excessive delays across the IP network c. network congestion in the local loop d. signal degradation in the PSTN See Topic 4. 14/18
5. The media gateway controller, billing, and database application modules in a distributed IP telephony switch run on which of the following? a. the same machine b. different machines in the same CO c. different machines distributed around the network d. any of the above See Topic 5. 6. A mated pair of SS7/IP gateways can support which of the following? a. multiple media gateways and media gateway controllers b. load sharing among redundant IP telephony modules c. primary and alternate routes d. all of the above See Topic 6. 7. IP networks provide guaranteed bandwidth for each phone call. See Topic 1. 8. The voice/data and signaling packets on IP networks have all been standardized to enable interoperability between multiple IP telephony vendors. See Topic 1. 9. A single SS7 link can support more than 1,000 trunks. 15/18
See Topic 3. 10. An SS7/IP gateway can support multiple distributed IP telephony switches with a single point code. See Topic 6. 11. IP telephony switches can support calls from which of the following? a. the PSTN to the IP network b. the PSTN to the PSTN via the IP network c. the IP network to the PSTN d. the IP network to the IP network e. all of the above See Topic 2. Glossary ANSI American National Standards Institute CO central office Class-4 switch a telephone switch that supports trunks to other switches, but does not support direct lines to telephones Class-5 switch a telephone switch that supports both trunks to other switches and direct lines to telephones H.323 a specification defining how voice, video, and data traffic will be transported on the Internet IEC International Engineering Consortium 16/18
IP Internet protocol IN intelligent network; a communications network with the capability of accessing information stored in one or more databases to control the routing of information ISDN integrated services digital network ISUP integrated services digital network user part; a layer of the SS7 protocol; ISUP messages are connection-oriented messages used to set up and tear down telephone calls; ISUP defines a handshaking protocol that initiates the phone call, reserves a path for the voice or data between the originating and destination devices, and ultimately releases the call; note that despite the name of this part of the SS7 stack, ISUP messages are not limited to ISDN calls IPDC Internet protocol device control ITU International Telecommunications Union ITU T International Telecommunications Union, Telecommunications Standardization Sector LNP local number portability PSTN public switched telephone network SCP service control point SGCP simple gateway control protocol SIP session initiation protocol SS7 signaling system 7 17/18
SSP service switching point STP signal transfer point TCAP transaction capabilities application part; a layer of the SS7 protocol; transaction capability application part messages are used to support non circuit-related, connectionless information exchange; among other things, TCAP messages are used to send queries to databases (such as toll-free [freephone] databases) and to return the database response VoIP voice over Internet protocol 18/18