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March 1999

SS7 - The Intelligent Choice for Telephony Services


The early cost driver for Internet telephony is arbitrage versus telephone taxes. But as arbitrage opportunities diminish, engineering economics take over as the primary cost driver. Full-duplex, real-time audio communication can be provisioned more cheaply over data networking equipment than over circuit-switched phone equipment. But for IP backbones to complement, and ultimately replace traditional telephony equipment, they must seamlessly integrate with today's public telephone network. To do this, IP backbones must support Signaling System 7 (SS7).

Development of SS7 began in the 1970s to provide a reliable and open mechanism to control telephone calls in which call control is separated from the media stream. Technically, SS7 is characterized by three elements:

Reliable packet network. SS7 networks share some characteristics with IP networks. Instead of IP addresses, however, SS7 networks have point codes (much more limited than IP addresses, with only 16 bits of addressing). Also, SS7 networks are designed to deliver highly reliable, low-latency signaling. The network goal is for more than 99.999 percent availability with less than 100 ms average end-to-end signaling delay.

Comprehensive call control. SS7 provides switch-to-switch signaling via two protocols, SS7 Telephone User Part (TUP) and ISDN User Part (ISUP). SS7 is only a network protocol, and there is no such thing as an SS7 phone. SS7 call control provides faster call setup through the network than the robbed-bit signaling protocols. Also, SS7 ISUP enables implementation of services such as caller identification and call forwarding.

Basic transaction protocol. SS7 is the basis for Intelligent Network services like 800, CNAM(Calling Name service), and virtual private network, and for wireless network services such as roaming. These are implemented via a data protocol referred to as Transactional Capabilities Application Part (TCAP). By some estimates, intelligent network services account for more than 50 percent of service provider revenue.

Financially, it is important to know that SS7 is the most universally deployed telecommunications protocol. More than 99 percent of the public network switches in the United States support SS7. Obviously, there is significant capital investment in SS7 technology worldwide. It is also the most widely supported telecommunications protocol, and is understood by probably more than 100,000 public network maintenance, engineering, and support personnel around the world. As such, there is significant human investment in SS7 technology.

Service providers like Qwest (www.qwest.com) and Level 3 (www.level3.com) are deploying data networks as a carrier's carrier. Essentially, they provide the transport and switching for long-distance service providers. Long-distance carriers connect to other interexchange carriers and local exchange providers through SS7 ISUP. To preserve end-to-end features such as caller ID and call forwarding, the next-gen carriers transparently transport SS7 ISUP through the network. Since they have no native H.323 Internet telephony clients in the network, H.323 is not required.

Among the competing protocols for integrating Internet telephony with the PSTN (see Industry Insight in the November issue of INTERNET TELEPHONY´┐Ż), the Media Gateway Control Protocol (MGCP) -- a combination of the Internet Protocol Device Control (IPDC) and the Simple Gateway Control Protocol (SGCP) -- has emerged as the clear leader. The MGCP is based on the concept of an end-to-end transport of ISUP. An element in the MGCP architecture, called the signaling gateway, uses MGCP signaling based on SS7 ISUP messages to control the switch connections on the media gateways.

Initial Internet telephony services like Internet call waiting and Internet telephony connections are now being deployed by forward-looking ISPs to support simultaneous use of a single line for both data and voice.

This service is a boon for consumers and ISPs alike, and provides a good example of the interrelation between SS7 and Internet telephony. An ISP provides on-screen notification of incoming calls and, optionally, the ability to answer the call over IP while the subscriber is logged onto the Internet. Deploying this service requires enabling a local exchange switch feature called "Busy -- Don't Answer." With this service enabled, the ISP can receive an indication of the incoming call by the local exchange carrier through another line. The trouble is, if the subscriber is already using the Busy -- Don't Answer feature for another service like voice mail or call forwarding, the ISP cannot enable the Internet call waiting service.

The intelligent network, enabled by SS7, then comes to the rescue. Competitive Local Exchange Carriers (CLECs) are defining an intelligent network service that determines whether you are logged into the ISP. If you are, it will optionally forward your incoming calls to the ISP. When you are not logged into the ISP, the incoming calls can go to your voice mail as they normally would. All this is enabled through SS7 interworking.

Emerging CLECs are interested in providing basic dial tone as part of their standard service. In addition to the need to interwork with the local exchange carrier and interexchange carrier through SS7 ISUP for incoming and outgoing calls, implementing basic dial tone requires a basic switch model and intelligent network services like 800.

Several standards bodies, including the European Telecommunication Standards Institute (ETSI) Project TIPHON (Telephone and Internet Protocol Harmonization Over Networks), the Internet Engineering Task Force (IETF), and others still unannounced, will all have a hand in defining the next-generation network architecture. (Information on ETSI/TIPHON can be found on the Web at www.etsi.org/tiphon. Information about the IETF can be found at www.ietf.org.) Still to be decided is what role SS7 plays in this next-generation network beyond interworking with the legacy network.

As the debate continues, there is still no clear answer. This issue will certainly have the emerging carriers on the side of a pure IP protocol and the incumbent carriers on the side of leveraging their existing SS7 network. Who will win? Stay tuned.

Laurence J. Fromm is vice president, new business development for Dialogic Corporation. Dialogic is a leading manufacturer of high-performance, standards-based computer telephony components. Dialogic products are used in fax, data, voice recognition, speech synthesis, and call center management CT applications. The company is headquartered in Parsippany, New Jersey, with regional headquarters in Tokyo and Brussels, and sales offices worldwide. For more information, visit the Dialogic Web site at www.dialogic.com.

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