Building bridges with SS7 technology.

See how the past, present and future of telecommunications will converge.

Creating a converged multiservice network, seamlessly integratng existing services with new technologies, can be complex and challenging. Implementation of new technologies over the public switched telephone network (PSTN) offers new revenue-generating services, reduces costs and protects investment in existing infrastructure. The converged network has to maintain the characteristics and services of the circuit switched network (CSN) in terms of reliability, quality of service, interoperability and manageability.

Available signaling gateway technology is now available to transparently bridge the interface between the PSTN network and the IP network, making the converged network dilemma a problemof the past. For the next-generation PSTN gateway to be effective, it has to recognize the signaling requirements of voice and data. It needs to be able to understand, translate, process and respond to voice and data signals in a ubiquitous environment. To achieve that, it has to be able to function as an end point (the recipient of the signals for processing) and a signaling transfer point (the processor and translator of signals.)

In order to bridge the interface between the PSTN and IP networks, carriers are turning to open network signaling gateways (SGs). These gateways enable signaling messages to flow transparently between the two domains. On the IP side, they use the emerging protocols defined by the Sigtran (signaling translator) work group within the Internet Engineering Task Force. These protocols are optimized for adapting signaling system 7 (SS7) signaling to the IP network. To enable a smooth transition, all variants of integrated services user part (ISUP) protocols are supported.

TWO NETWORK CONFIGURATIONS

In protocol interworking between message transfer part Layer 3 (MTP3) on the SS7 side and MTP3 user adaptation (M3UA) on the IP side, typical applications are data offload and long-distance bypass. In this example, the node in the IP network would be a media gateway controller (MGC) with ISUP traffic being used. All types of MTP3 user protocols are supported, however, including telephone user part (TUP) and signaling connection control part (SCCP), in addition to ISUP. The M3UA protocol defines the messages and procedures for delivering MTP3 user data between the IP-based node and the SG. M3UA uses the services of the stream control transmission protocol (SCTP) for transporting its messages.

There are two distinct types of network configurations that are possible with this use case--one in which the SG acts as a signaling end point (SEP) and one in which the SG acts as a signal transfer point (STP). In the "SG as SEP" scenario, the SS7 network views the SG as an end point with a single point code. All of the elements in the IP network that use this SG "share" this point code. The SG uses configurable routing criteria, such as the circuit identification code (CIC) for routing traffic to the appropriate IP node. The SG may perform circuit management functions on behalf of the IP nodes to enable reliable behavior in the SS7 network.

In the "SG as STP" scenario, the SS7 network views the SG as a signal transfer point. The SG has a unique point code. Point codes are also allocated for nodes in the IP network. This configuration has the advantage of offering network redundancy. While both SGs are available, they can share the network load. In the event that one SG fails, traffic can be routed via the alternative SG.

Another example involves protocol interworking between SCCP on the SS7 side and SUA on the IP side. It allows nodes in the IP network access to transmission control application part (TCAP) databases in the CSN domain. Typical applications are local number portability and home location register (HLR) queries. One advantage of using SCCP user adaptation (SUA) instead of M3UA in this case is that the IP-based client does not need to include an SCCP layer. The SUA layer provides the SCCP upper interface to the user (e.g., TCAP), but the SCCP routing data is centralized in the SG.

In an STP replacement example, the use of an IP backbone network with a mesh of signaling gateways, is an alterative to traditional SS7 STPs and an SS7 backbone network. For a customer who needs an IP network anyway, use of that network for SS7 signaling traffic can be more cost-effective than setting up a parallel network dedicated to SS7.

The SS7 SEPs can be any type of SS7 end node (e.g., service switching point or SSP, HLR, mobile switching center or MSC). The type of traffic can be ISUP, TUP or SCCP. To eliminate network single point of failures, all SS7 end nodes should include links to at least two signaling gateways. An SG always appears as an STP toward the SEPs. One special feature offered is "point code preservation," which allows SGs at different sites to share the same SS7 point code and thus reduce the need for the allocation of new point codes.

TWO ROUTING OPTIONS

Toward the IP network, SG makes use of the SCTP and M3UA protocols. This usage of the M3UA protocol (between peer signaling gateways) is not explicitly described in the M3UA protocol specification. The M3UA specification concentrates on the interworking between an IP-based node, such as a media gateway controller and an SG. Nonetheless, the M3UA protocol works for this scenario and permits IP-enabled end nodes that conform to M3UA/SCTP protocol to interoperate in the network.

There are two routing options with this use case--routing based on destination point code and routing based on global title. In the simplest case, the SG will route traffic that it receives from its SS7 interface, based on network indicator and destination point code. The SCCP layer will not be used in this case. SCTP associations are used for passing traffic between any pair of SGs. The associations are also used to pass management-related messages, such as ones related to destination availability.

MOVING TOWARD CONVERGENCE

To take a step in the direction of convergence, the seamless migration from the PSTN circuit-switched network to an IP-based telephony network has to start with products that recognize the different aspects of signaling in the current PSTN network and form the basis of the next-generation signaling network. By introducing the complexity of signaling into a single building block, the massive overhaul of hardware is eliminated by retaining the current investment and adjusting for the transition.

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Granstrom, Niermann and Yong are with Ericsson, Richardson, TX.