Unifying Multivendor Networks

In order to unify networks, carriers will have to integrate a new SS7 signaling messaging architecture.

The convergence of networks is fast approaching: voice over IP (VoIP), fax over IP and other internetworking systems already are moving into place. The Internet continues to grow at break-neck speed, forcing carriers to rethink connectivity issues and the public switched telephone network (PSTN). Fortunately, current solutions tend to be small-scale or customized solutions in an Enterprise environment where all of the information is carried over the same interface, such as ISDN or the Internet. Although these solutions work, they have yet to realize their potential in a fully unified network environment. To achieve a fully integrated or unified network that takes advantage of the capabilities of IP-based networks, a new telephony architecture is required. To accomplish this involves the integration of the SS7 IN model and the adoption of the SS7 signaling messaging infrastructure. This is necessary to integrate IP-based networks, hence the need for a new variant on IP-based signaling protocols.

Challenges
The PSTN carries the traditional land-based, hard-wired constant bandwidth of information flow between users. A statically designed circuit-switched network, it was optimized for voice traffic and voice calling patterns. However, today this same network must carry fax and data traffic on its way to the IP-based network world.

One of the more wasteful characteristics of the circuit-switched network is that when a connection is made, bandwidth is occupied even when there is no information being passed. On the other hand, voice-quality circuits aren't really necessary to move data. In the PSTN network, elements tend to be centralized and highly engineered. Therefore, a significant degree of coordination is required to add or modify network elements.

Even before the onslaught of fax and data traffic, SS7 was developed to improve the throughput of the efficiency and throughput of the PSTN. In this new structure, SS7 separated the traditional signaling paradigm into a separate control-messaging overlay, comprising out-of-band links. These were used to deliver all of the control information related to setting up, billing and terminating a connection. Although SS7 links contained only data packets, filler was used to ensure the reliability of constant bandwidth-dedicated connections. The key advantage of this approach ensured that any network disturbance could be detected quickly and routed around, enabling reliable and rapid message delivery. This approach reduced call-setup time and optimized trunk capacities. The SS7 also enabled the storage of information making possible a range of new subscriber services known as advanced IN services. Because SS7 crosses many international boundaries, it also has to support the protocol variants observed by these countries.

Like SS7, the IP also is packet-oriented, but differs in that IP networks are configured dynamically; they are self-healing and self-learning and not centrally managed. In an IP network, data is broken into manageable pieces called packets, each with the address of the sender and recipient. Packets may have different routes and may arrive at their destination in random sequence. In some instances, a few IP packets might not arrive at all. Higher-level protocols such as transmission control protocol (TCP) are used to guarantee information delivery in IP networks. The IP network is based upon a distributed open architecture, rather than a closed centralized model. Although this affords the opportunity to grow the network with little coordination beyond address convention, it does open the system to some peril.

Unifying the PSTN-IP Networks
Assuming the right combination of network interfaces with the ability to direct and control these interactions, there could be a convergence of the PSTN, SS7 and IP technologies. To achieve a seamless internetworking of the PSTN, SS7 and a multipurpose IP-based communications network, a new set of network-control elements must be defined.

To support the development of such an IP model for network elements, which will interface the PSTN, SS7 and IP networks, vendors such as Nortel and other industry members proposed a reference model to the Internet Engineering Task Force (IETF) in mid-1998. The model comprises three basic functional elements: a signaling gateway, a media gateway and a media controller. These three elements would provide the basis for unification, supporting VoIP — voice, fax, video and a full set of digital services over IP networks. Other optional elements to this basic architecture include application and database servers that can be added to support the modular addition of enhanced services. This approach allows for the separate development of new services independent of the basic functional service elements.

Media Gateway
This function enables various media such as voice, fax, video and modem data to be packetized and transported over either an IP network or a circuit-switched network. These data packets would move over the network without any loss or degradation of services or quality. This is accomplished through various coding, compression, echo-cancellation schemes and decoding schemes. Essentially, the media gateway function provides a bi-directional interface between an IP and a circuit-switched network. Media gateways interface between the end-user application or with other gateway units. Media gateway units also would support a number of physical interfaces to the PSTN such as high-speed time division multiplexing trunk interfaces.

Media Controller
The media controller provides the instruction sets to make decisions regarding the media gateway and flow-related information. The controller would provide the instructions for interconnecting IP elements so that information exchange could take place. The media controller also would implement much of the call-control functions found in the switching elements in the PSTN. The media controller would provide the parameters associated with bandwidth allocation and quality-of-service characteristics. The media controller also would serve to instruct the media gateway on setting up, handling and terminating individual media flows.

The signaling gateway is another key in this new unified networking environment. This gateway unit supports bi-directional interfaces between the SS7 network and various call-control elements in an IP network. It must implement highly reliable SS7 messaging that obeys all of the rules of the SS7 network.

The signaling gateway also enables the media controller to perform its designated administrative functions. It repackages the information contained in various high-level SS7 message protocols such as: ISDN User Part (ISUP) and Transaction Capabilities Application Part (TCAP) into formats that can be understood by IP elements.

The signaling gateway also must contain the programming that enables it to recognize all of the variants of the SS7 protocol used around the world. In addition, the signaling gateway also must serve as a security measure in that it filters out inappropriate traffic.

Internetworking Ahead
The basic functions of media gateway, media controller and signaling gateway can be provided as separate functional elements or integrated depending upon the needs of the targeted end user. However, with well-defined standardized protocols, product designers would be free to combine or separate functions into the most appropriate product mix.

Internetworking products are emerging. Most of these tend to integrate multiple functions together into one or two products. However, unlike the PSTN in which reliability is controlled through centralized elements, the IP model relies upon cooperating, multiple instances of independent servers. This approach provides more flexibility in building system implementations, because controllers can be deployed on an as-needed basis. Reliability can be achieved with potentially less equipment.

A New SS7 Element: IPS7
To reach the goal of unification, there is the need to develop a next-generation SS7 element that will provide the functionality of a signaling gateway while acting as a fully transparent component on the SS7 network.

IPS7 is a newly proposed open-architecture definition that encompasses all SS7-related signaling protocols and functions to communicate SS7 information in IP networks. IPS7 is a non-proprietary architecture aimed at providing a set of common standards that support multivendor PSTN-IP network environments. The IPS7 provides for VoIP by fully integrating more than 15 new protocols and functions. The IPS7 open architecture supports bi-directional signaling between the two disparate networks. This enables operators to assemble VoIP networks that take advantage of the full PSTN SS7-based IN capabilities while protecting the SS7 network against IP irregularities.

A list of VoIP protocols has been submitted to the signaling transport working group of the IETF. These protocols, when programmed as part of a signal gateway, would enable the gateway to bridge VoIP calls into an SS7 network. Similarly, calls into an IP network could be bridged into the IP network using the reverse protocols.

Issues Remain
Issues remain to be resolved by the working committees. For example, an approach must be developed to support variants to some of the signaling protocols to include Q.931, SS7 ISUP, SS7 TCAP and application protocols such as INAP and GSM MAP. There also is the requirement to develop a standard for signaling performance requirements on an IP network. In addition, reliability issues such as failover need to be addressed as well as gain the support of the media gateway control, another IETF working group, which is working on standards for VoIP gateway and device control.

Although there is much to be accomplished in the way of coordination within the IETF and its working groups, some customized solutions have been evolving for some time.

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Working Solutions

A number of SS7 bridge solutions have been around for some time. These were usually solutions that served to bridge applications into the SS7 world. For example, DGM&S for years had a software product that supported SS7 bridging and as part of the software contained APIs. These application interface modules allowed the implementer to develop its own interfaces between an application server and a switch interfaced into the SS7 networking environment.

Summa Four, a special applications switch manufacturer, has supported its own SS7 bridge into an SS7 networking environment. Its systems have been an important part of many of the voice-messaging systems used by service providers around the world.

It's not surprising to see Nortel announce its own signal server. This system is a compact version of a broadband signal transfer point (STP) with scaleable configuration. The system can be configured with four link system nodes with expansion up to 16 system nodes. Additional shelves can be added to bring the system up to 10 shelves.

A GUI interface provides for a simplified provisioning process. The system is designed to integrate other related applications such as number portability on the same platform.

Another SIGTRAN member, MicroLegend, also has submitted draft specifications for transporting SS7 signaling information over the IP networks. This specification identifies performance requirements for SS7 signaling transport to IP-based application processes residing on one or more hosts. This draft defines the concept for a signaling gateway, which acts like an intelligent bridge between the SS7 and IP networks. A message set is defined in the draft that allows remote devices to take advantage of the flexibility offered by a signaling gateway deployed as a SS7 end node or as an STP. MicroLegend is a Canadian vendor of the versatile signal point, a family of SS7 platforms that have been used as custom STPs in GSM networks as well as custom-programmed signaling gateways and basic SS7/IP gateways.

Llana (llana@bellatlantic.com) is a Vermont Studies Group consultant.