September 1998

BY KIERAN TAYLOR

If there is a Holy Grail of the networking industry, it must be the integration of voice and data communications. While most efforts in the CTI space have centered on developments on the customer premise, similar integration is needed in service provider wide area networks. As a result, telcos are beginning to confront the growing problem of maintaining disparate voice and data networks on the backbone in much the same way IT managers are tackling similar issues at the customer premise.

The good news is that this integration does not depend on some new untried technology. Instead, the solution for achieving voice and data integration in the PSTN (Public Switched Telephone Network) may be found in existing infrastructure. One of the leading choices for many service providers is to leverage their existing Signaling System 7 (SS7) infrastructure to work with traditional remote access concentrators. The resulting architecture gives service providers the ability to logically combine voice and data networks that today remain separate.

SIGNALING SEMANTICS
In the most basic terms, signaling is the exchange of information between communications components in order to establish or maintain a connection. Traditionally, the use of signaling in telecommunications networks has been for tasks as basic as providing a dial-tone or a busy signal. Over the past decade, however, signaling has evolved to support a much more complex flow of call-related information across public switched telephone networks.

Today’s Public Switched Telephone Network consists of two logically separate networks that transport either user information or control information. The switched/Time Division Multi-plexed (TDM) network provides the physical infrastructure for voice and data communications and comprises telephony switching machines, transmission network facilities, and operational support systems.

The second network, the SS7 network, is the PSTN’s primary means for controlling circuit-switched connections and value-added telephone services such as calling card validation, calling number ID, and intelligent network services. SS7 is, essentially, an out-of-band data communications network that is laid over the carrier’s switching network. Rather than burden the communications network with signaling, out-of-band SS7 creates a separate path for signaling, thereby significantly increasing network efficiency. Working in parallel with the voice network, SS7 provides the information necessary for a switch to know which other switches to involve in the set-up of a telephone call. Prior to 1976, all signals were sent over the same path as the voice or data communication, resulting in inefficient use of telecommunications resources. SS7 provides carriers with an out-of-band option that is interoperable, thanks to ratification of an SS7 standard by the International Telecommunication Union.

DATA CONGESTION ON THE PSTN
The exploding demand for Internet access has created significant challenges for traditional providers of telephony services who are struggling to manage the growth of data traffic on their voice networks. Users connecting to the Internet or to corporate networks have been using dial-up modem connections over the PSTN as their primary means of connecting. This method has become highly problematic, as modem calls last far longer, on average, than voice calls. This increased "call time" has resulted in congestion on the PSTN, which at times has actually forced telecom service providers to deny service to lifeline telephony and emergency 911 services.

As data traffic pushes central office voice switches beyond their intended capacity, carriers must keep switch performance above certain levels in order to provide critical emergency services. Consequently, service providers often find themselves upgrading voice switches to accommodate data applications for which they were not designed. This can be costly and inconvenient, when one considers that the data traffic flow from dial-up modem calls is a low-margin business for carriers when compared to voice transport.

Currently, dial-up modem traffic is terminated on modems or modem pools contained within a remote access concentrator. Remote access concentrators support a large number of subscribers and accept digital lines such as T1/E1 or ISDN PRI (primary rate interface) connections. ISDN PRI lines, where available, are a viable alternative to T1 lines because they provide a higher quality connection with greater manageability. Today, PRI lines are generally accepted as the best way to manage a modem pool, although they are also the most costly.

In addition to its basic call control functionality, the SS7 network is equipped with the ability to provide for intelligent network (IN) applications such as advanced call routing features, conferencing, and third party billing. Using SS7 to integrate voice and data networks will allow service providers to offer new services that span the Internet and the PSTN. Ultimately, it will also allow them to recoup some of the revenues previously lost on data services by generating new revenues from data traffic services.

VOICE AND DATA INTEGRATION
Network service providers are in need of a solution that will keep data traffic off the voice network, while providing data users with the level of service associated with voice. Several methods exist that enable service providers to direct data traffic to "bypass" the PSTN. The deployment of SS7-enabled remote access concentrators is one of these methods.

Nearly all remote access concentrators (RACs) utilize the Q.931 signaling found in the D channel of ISDN PRI links to terminate incoming modem calls. One of the most straightforward ways to integrate voice and data networks is to translate between SS7 and Q.931 signaling. Because the RAC must integrate seamlessly into the existing SS7 network, it must be outfitted with an enhanced Q.931 interface and use an SS7 gateway to provide a translation function between SS7 and Q.931 protocol. This gateway performs SS7 signaling with ingress PSTN switches and non-SS7 signaling to the RAC.

Bringing the RAC into the SS7 network enables the RAC to be deployed to offer intelligent IP services. Whereas modem banks today are simply used to terminate data calls unintelligently, SS7 gateways empower RACs, allowing them to communicate with IP. Each port on a RAC is now capable of intelligently directing data traffic to the appropriate resource by leveraging SS7 signaling. The voice switch can now dynamically route a user to the appropriate processor on the RAC, thereby enabling service providers to offer a broad range of new services.

SS7 is critical to call setup since a RAC cannot cut through an SS7 connection on its own. The RAC relies on signaling from the SS7 gateway to tell it which circuit a call is coming in on, and to complete the connection. The RAC and gateway communicate to determine if a modem is available. If not, the call is released and a cause of "user busy" is indicated. This approach provides a rerouting of data calls onto a public switched data network and frees up the PSTN for voice communications, thereby solving the congestion problem.

SAVING WITH SS7
In addition to freeing up the PSTN lines, another immediate strength of this SS7-integrated approach is that it provides high-quality signaling at the lower cost of T1 lines. According to the Aberdeen Group, Inc., this strategy offers payback within five to eight months for even small service providers. Substantial operational cost advantages rapidly offset the capital cost of an SS7 signaling strategy.

The Aberdeen Group directly compares the relative costs of doing business over T1 versus PRI lines: At $500/month, 4 T1s would cost a total of $2,000. The same service with PRI at $900/month would cost $3,600. For a single POP servicing 1,000 users, the PRI approach costs $1,600/month more than the T1 strategy. If the ISP had 10,000 customers distributed evenly across 10 similar POPs, and you added in the monthly cost of an SS7 A-link connection ($2,500), the PRI signaling approach would cost an additional $13,500/month. At a $13,500/month cost savings, a service provider can repay the up-front costs of an SS7 gateway server and software in five to eight months.

SERVICE PROVIDER OPPORTUNITIES
Longer term, the SS7-integrated approach enables an ISP to manage the call control of modem connections as if they were regular telephone calls. This provides the ISP with an excellent opportunity to offer traditional enhanced services as well as IP telephony and other voice and data converged services that span the Internet and the PSTN. For example, it is not currently possible for an IP telephony call to carry caller ID information from the PSTN to an IP network, or vice versa. An SS7-integrated strategy would be an effective way to transport this information between the networks.

SS7 has traditionally been a source of high-margin revenues for carriers. By bridging the SS7 and data environments, carriers have additional high-margin service opportunities. In the near future, enhanced SS7-integrated services will include: call waiting and connection for individuals connected by a modem; enhanced billing features supporting complex rating plans; IP telephony services; and pre-switch bypass and management of telephony services. As additional existing services are combined with an SS7-integrated platform, the opportunities for these enhanced services multiply.

Projections that a significant fraction of voice traffic will utilize IP networks in the future suggest that the ability for PSTN users and VoIP users to locate and talk to each other and access similar services will become essential. Therefore, remote access concentrators containing both SS7 and VoIP functionality will provide carriers with an edge in launching new competitive voice offerings. Service providers will need to integrate PSTN SS7 signaling with directory and service information in IP networks to support number translation, routing, and calling card services for calls transiting from PSTN to IP and vice versa. This will enable transparency of routing and service to users on PSTN and VoIP networks.

CONCLUSION
As data and voice networks converge, it is increasingly apparent that the circuit-switched networks built for voice communications are being adversely affected by the tremendous growth in dial data access. Rather than create a new class of equipment to divert data traffic from the PSTN, it is more prudent to develop strategies that leverage existing features found in the voice network today. The voice network offers maturity and advanced features that cannot be found in today’s data networks.

Despite their differences, voice and data networks do not have to remain disparate from a management and control standpoint. With the right equipment and planning, network service providers can leverage their existing voice infrastructure while augmenting it with existing data access equipment.

Signaling System 7 is deployed worldwide and is the technology that enables service providers to construct networks in which voice and data communications complement one another. Service providers that integrate SS7 in conjunction with next generation data platforms will be well-equipped to serve their current and future customers, and to introduce new services in a new millennium that will be known for its increased competition.

Kieran Taylor is a product marketing manager at Bay Networks, and former broadband consultant at TeleChoice, Inc. Bay Networks is a leader in the worldwide networking market, providing a complete line of products that serve corporate enterprises, service providers, and telecommunications carriers. The company offers frame and ATM switches, routers, shared media, remote and Internet access solutions, IP services, and network management applications, all integrated by Bay Networks’ Adaptive Networking strategy. With headquarters in Santa Clara, California, Bay Networks markets its products and services around the world, providing 7x24 support coverage. For more information, contact the company at 800-8-BAYNET, or visit their Web site at www.baynetworks.com.