December 2002

BY RAVI RAVISHANKAR

Signaling is the command and control mechanism in today�s telecommunication network. Some form of signaling is required at virtually every step in the call process -- from the desktop phone to the corporate PBX, from the PBX to the local central office (CO), and from the local CO to the destination CO. In addition to basic call control, signaling delivers the network intelligence that enables advanced services like caller ID, calls to be routed to toll-free numbers, and mobile customers to roam outside their home network.

As voice and data networks converge, signaling requirements will become increasingly complex. The network will have to evolve to support multimedia sessions as well as a variety of new access modes, services, and players. A new class of signaling architecture is required that not only delivers maximum capacity and flexibility but also reduces operational costs. And, it will have to continue to deliver at least the same capabilities and features as the PSTN�s signaling network. Enter IP signaling.
 

WHAT IS IP SIGNALING?

IP means different thing to different people. In the context of this column, IP does not refer to the Internet, the world�s largest computer network. Rather, IP is the Internet protocol, one of the most important protocols on which the Internet is based.

IP signaling protocols currently being developed in the industry can be classified into three broad categories:

� IP signaling that adapts legacy-signaling architectures such as SS7 and ISDN PRI to the IP network. A large number of network nodes exist, which are based on legacy signaling protocols such as SS7. It is not realistic to assume that there will be a wholesale replacement of those elements. However, by using SS7 at the call control and application level and replacing the lower layer transport network with IP, tremendous efficiencies can be gained without sacrificing interoperability with legacy switches and applications. MTP3-User Adaptation protocol (M3UA), for example, allows existing SS7 entities to use SS7 call control protocols over an IP transport network.

� IP signaling developed by traditional telecommunication organizations such as the International Telecommunications Union (ITU). These protocols carry the telco heritage. They are designed as entire, unified systems and deliver about the same robustness and interoperability as found in today�s PSTN. Examples in this category include the H.323 protocol suite for voice over IP and Bearer Independent Call Control (BICC) for inter-softswitch signaling.

� IP signaling developed by Internet organizations. Groups such as the Internet Engineering Task Force (IETF) are active in the development of these protocols, which are built on the open-architecture model of the Internet. Session Initiation Protocol (SIP), VoiceXML, Hypertext Transfer Protcol (HTTP), and Lightweight Directory Access Protocol (LDAP) are some examples.

THE NEED FOR SPEED

The adoption of IP signaling is being driven by a number of factors including:

� Operational cost savings;

� Architectural limitations of SS7;

� Voice and data convergence; and

� Faster deployment of new services.

IP�s efficient bandwidth utilization can mean significant savings for carriers. Time division multiplexed (TDM) links in the voice network must be engineered to the �worst case� scenario; that is, they must be engineered to handle the peak traffic load. The SS7 protocol specifies that TDM links can be engineered for a maximum, normal operational occupancy of 40 percent. In the SS7 network, the maximum linkset size is 16, which translates to a maximum bandwidth of 358K (Max bandwidth = 16 X 56K [per link] X 0.4 = 358K). And, the reality is that most TDM links actually handle an average load of only 20-30 percent occupancy. The result is that 70-80 percent of the bandwidth is not utilized, which can create an expensive, over-engineered network with under-utilized bandwidth. IP provides dynamically scalable bandwidth assignment that delivers capacity on demand and can cost up to 75 percent less than the equivalent dedicated circuit bandwidth.

The TDM bandwidth limitation also impacts the processing power of other network elements including application servers, home location registers (HLRs), and service control points (SCPs). While those elements may have the capacity to handle large volumes of traffic, their processing power can be throttled back by the SS7 constraints on the TDM links that interconnect them to other network elements. To increase the overall network throughput, carriers must purchase additional HLRs or SCPs even though the capacity on the existing elements is not being fully utilized. Additionally, every new element added to the network requires a point-to-point link as well as revisions to routing tables, which drives up capital and operational costs. IP�s scalable bandwidth allows carriers to accommodate fluctuations in traffic, simplify the network architecture, and run databases at their peak capacity.

In addition to the savings realized from efficient bandwidth and higher resource utilization, carriers can grow their networks more economically with IP technology. They can virtualize a large number of servers, which appear to the SS7 network as a single entity with a single point code, operating under the control of a signaling gateway. When a new server needs to be added to the network, it can just be connected to the LAN, and, with some provisioning at the gateway, it�s ready to go. Growth can be accommodated with minimal impact to the rest of the network, which translates to lower cost for operators.

IP bandwidth is scalable and well suited to applications that generate bursty, seasonal traffic such as short message service (SMS). SMS service is used to provide a variety of data applications such as e-mail and Web information retrieval. Carriers typically see spikes in the SMS traffic during holiday season. Planning and growing an SS7 network to accommodate this kind of traffic can be complex and expensive due to TDM�s inherent static bandwidth allocation. An economical and efficient solution for this problem is to offload the SMS traffic from the SS7 network and transport over an IP signaling network.

IP signaling enables faster deployment of new services. IP is rich in intellectual capital -- a large number of people understand the technology and can create software to support new services. IP networks, built on a distributed model, are open by nature and facilitate third-party application development.

COME TOGETHER

The convergence of voice and data is also driving the adoption IP signaling. It is becoming apparent that the new converged network will be multi-protocol in nature. A variety of protocols will be required to support the myriad functions of a multimedia network. Megaco and MGCP likely will be used to separate call control from the bearer path. RTP and RTCP will support real-time media exchange, and BICC and SIP will establish multimedia sessions. SIP will enable network-to-network and end-user-to-network connections. And, adaptation protocols such as M3UA and M2PA will be required to interface to the existing SS7 network. All of these protocols are IP-based, so it makes sense to carry them over a common IP signaling transport network that allows horizontal integration of IP signaling protocols

REQUIREMENTS

The signaling network forms the backbone for all services. It must meet or exceed the quality of service (QoS) that exists in today�s signaling network. Signal packet delay and signal loss can have significant impact on QoS and can result in post-dial delays and lost calls. Planned operational events such as software upgrades and unplanned events such as natural disasters cannot impact service delivery. Practices employed in today�s telecommunication signaling network such as site diversity and software prove-in prior to full implementation must be migrated to the new signaling network to ensure that there is zero downtime. The ability to handle multiple protocols is essential to the success of the new signaling infrastructure. The network will not only have to interoperate with the existing SS7 network but must support a variety of new protocols as well.

CONCLUSION

Multiple media, a variety of access technologies, service growth, and an increasing number of competitors are driving the need for a distinct signaling architecture in the next-generation network. While the signaling network has not yet been fully defined, it is apparent that its core will be access independent and will blend of IP and SS7 technologies. It will facilitate network interconnection and application deployment. As network requirements become more complex, signaling intelligence will be viewed and managed as an information system, which will be utilized for network planning, fraud control, billing, service assurance, and other business applications.

Ravi Ravishankar is director, Advanced Technology Planning, at Tekelec. His focus is on defining signaling solutions and products for the next-generation packet telephony and 3G wireless networks. Tekelec is a leading developer of telecommunications signaling infrastructure, softswitches, testing and diagnostic solutions, and service applications. Please visit their Web site at www.tekelec.com.

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