October 2000

SCTP For Intelligent Call Control 

BY RICHARD HAYTER

The SS7 network incorporates robust and reliable mechanisms including support of error checking mechanisms, link changeover techniques under failure conditions, and management messages, which constantly monitor and report on the health of signaling network elements. Most importantly, SS7 introduced the concept of out-of-band signaling, which provided the ability to signal without occupying a dedicated speech circuit. SS7 is message based, allowing the support of a vast range of services including access to remote databases for service support. Signaling messages can be efficiently packed and transported in this dedicated data network far more efficiently than an in-band signaling solution.

There is no question that SS7 is at the heart of the modern, circuit-switched telecommunications network. The Intelligent Network was made possible through the ubiquitous deployment of SS7. Today, the SS7 network performs its task quietly and with extremely high reliability. However, the next-generation networks emerging using the latest in packet technology will have to coexist with the circuit networks for many years. These new, packet-based networks promise to deliver Internet-style innovation with the flexibility to plug and play new services and remove barriers to entry for new entrepreneurial service providers. The challenge now faced by incumbent telecommunication service providers and this new breed of entrepreneurial service providers alike, is how to interface the existing SS7 network with the emerging next-generation, packet-based networks. How can providers achieve call control between the legacy and the new networks with no loss of reliability or service?

TRADITIONAL SS7 FUNCTIONALITY
The existing SS7 network has many attributes, which must be maintained or emulated in the new networks. This represents a formidable task as evidenced by the amount of effort now being put into standards body activities such as the International Engineering Task Force (IETF) and the SIGTRAN (Signaling Transport) working group. Both today's service-conscious subscribers and the network operators themselves insist on carrier-class reliability over packet networks. Carrier-class is defined by the characteristics of today's SS7 circuit-switched network: Redundancy, no single point of failure, fault tolerance, and 99.999 percent reliability. The fact of the matter is that many of today's packet networks, such as LANs and WANs, were not designed with these characteristics in mind.

Emulating the reliability of the SS7 network in new packet networks presents several engineering challenges. Chief among these is maintaining the overall reliability of the network by providing path diversity. A second requirement is the timely and sequenced delivery of messages according to defined rules, which are dependent on the type of information being transferred. Network-level timers running in today's SS7 networks monitor message delivery performance and may generate alarms, clearing down circuits if they expire as a result of delays. This functionality must extend to next-generation networks for reliability reasons.

EXTENDING INTELLIGENCE TO IP NETWORKS
Next-generation protocols will be critical in guaranteeing reliable call control services. The recent adoption by the IETF of the stream control transmission protocol (SCTP) as a proposed standard is a positive step in addressing some of the shortcomings of transmission control protocol (TCP) over IP-based implementations. Previously, these limitations were often worked around by creating somewhat more complicated upper-level protocols. For example, ITU Q.2111 and TALI are readily available protocol alternatives that, when paired with TCP/IP, can deliver reliable signaling transport over packet networks. While these protocols are not directly replaced by SCTP, some messages or timers will become redundant when these protocols evolve to support both SCTP/IP and TCP/IP as their lower layer.

SCTP was developed to run over IP and provide an alternative to TCP in some applications, including PSTN signaling. SCTP raises packet transport technology to a level of reliability and fault tolerance comparable to signaling transport in today's voice network. SCTP offers the following services to its users:

• Acknowledged error-free, non-duplicated transfer of user data;
• Data segmentation to conform to discovered path maximum transmission unit size;
• Sequenced delivery of user messages within multiple streams, with an option for order-of-arrival delivery of individual user messages;
• Optional multiplexing of user messages into SCTP datagrams; and
• Network-level fault tolerance through support of multi-homing at either or both ends of an association.

The adoption by the IETF and the industry of SCTP is a significant advance in providing a purpose-built protocol for the reliable delivery of signaling messages. However, much work remains to be done. Defining standards are needed in many areas, such as the many user adaptation layers, where an early version of TALI (Transport Adaptation Layer Interface) has already been deployed. SIGTRAN is also engaged in defining standards in this area, e.g., M2UA, M3UA, and others.

Judging by the increasing level of interest from all corners of the industry, it is clear that early success in interfacing the legacy SS7 networks to the new packet networks will be key to rapid deployment of new technology. Let us ensure that there is a smooth transition from TDM to packet networks and that we continue to improve the signaling control mechanisms and call control in our new networks by dedicating ourselves to what can be improved.

Richard (Dick) Hayter is the assistant vice-president, marketing for the Network Systems Division of Tekelec. Tekelec, a leading supplier of signaling and control systems, develops innovative network switching and diagnostic solutions enabling the convergence of traditional and converged wireline, wireless, and IP voice and data communications networks. The company also provides products and solutions for call centers and other telecommunications markets. 

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