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SS7 Connectivity: The Foundation of Telephony Daniel Teichman and Donald Reaves December 1997

SS7 Connectivity: The Foundation of Telephony Signaling System 7 (SS7) is the foundation of telephony infrastructures worldwide. SS7 is an agreed-to set of standards for how telephone switches and networks communicate with each other. For telcos, SS7 provides several vital functions—call setup efficiency, deployment of network-wide services, service availability, and rapid service creation. These attributes are both cost-effective and revenue-generating. Each attribute also applies to a new entrant in the telephony business. It is also important to recognize how SS7 allows telcos to implement regulatory changes that open up the industry to local competition. For example, the Telecommunications Act of 1996 requires LNP (local number portability) to ensure fair local competition. Without an SS7 infrastructure and SS7 interconnection between network providers, LNP cannot be implemented to any meaningful degree. By examining how traditional telcos have implemented SS7 networks and how SS7 networks have evolved, we can understand the specific value of the SS7 network. Furthermore, by projecting the demands a rapidly changing industry will have on the SS7 infrastructure, we can see how an SS7 network is an integral key to the success of a new telephony service provider.

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SS7 Network Architecture Most SS7 networks in North America use a quasi-associated architecture. Signaling between endpoints is based on a common network (e.g., signal transfer points for efficient transfer of signaling messages) rather than being directly routed between each node. See Figure 1. Other SS7 networks SS7 signaling (TCAP & ISUP)

STP

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SCP POTS SS7 signaling (TCAP) Physical trunking

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Figure 1. Typical SS7 (QuasiAssociated) Network Architecture

SSP

ISDN

ADSI - Analog Display Services Interface ISDN - Integrated Services Digital Network ISUP - ISDN User Part (signaling for voice/data call connections) POTS - Plain Old Telephone Service SCP - service control point (database of network or service information) SSP - service switching point (central office switches) STP - signal transfer point (SS7 message switch) TCAP - Transaction Capabilities Application Part (signaling for database queries)

As Figure 1 shows, the signaling path between service switching points (SSPs) is independent of the physical connection. STPs (signal transfer points) are always deployed as mated pairs for redundancy and load-sharing efficiencies. SCPs (service control points) are repositories for network or service intelligence and can be deployed as mated pairs sharing the traffic load or in an active/ backup configuration. The two primary uses of the SS7 network are call setup and transaction messaging, such as database queries. Because SS7 is a network signaling protocol, the information SS7 signaling carries is used to work with a variety of access signaling methods, such as Integrated Services Digital Network (ISDN) and Analog Display Services Interface (ADSI).

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Importance of SS7 Signaling A quasi-associated architecture has certain inherited attributes because of the design of the SS7 protocol (see Appendix). Three key attributes are efficiency, service enabling, and network reliability.

Efficiency Because SS7 uses an overlay network of separate high-speed “out-of-band” links operating at 56 or 64 kbps, it may reduce network provider expenses for call setup procedures. When SS7 is used instead of in-band signaling, trunks are reserved (rather than seized) until the network is assured of completing a call. Through this procedure, call setup savings come from two sources: shorter information transfer time and the ability to fall back to the originating end of the call to provide call treatment (e.g., busy signal to end user). This method frees up trunk facilities to carry optimal traffic. More network capacity is available, and network efficiency is increased.

Service Enabling The SS7 protocol carries critical information that enables residential and business services to work harmoniously across the network. Both residential services (e.g., automatic callback and calling number delivery) and business services [e.g., network message service and network automatic call distribution (NACD)] depend on SS7 to work beyond the limits of a single switch. SS7 also provides the ability to develop services that store information at a centralized database. Two examples of this ability are 800 number service (where an 800 number is mapped to a real directory number for call routing) and calling card validation. When services are expanded beyond the boundary of a single switch, service value is greatly enhanced. The SS7 network provides intelligent service information throughout the network. It is within this scope that advanced access signaling methods, such as ISDN1 and ADSI, become important. ISDN access and SS7 network signaling provide nationwide (and potentially international), end-to-end, digital common channel signaling for data and voice connections. ADSI provides the ability to transfer network/service intelligence to and from analog-based display terminals (phones with small display screens). The combination of access signaling and network signaling (SS7) enables network operators to maximize service revenues and end users to maximize service usage.

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Network Reliability The SS7 protocol, developed to carry user information, also carries extensive network management messages. This attribute handles abnormal network conditions and meets stringent reliability requirements. Because of these characteristics, the SS7 protocol ensures service availability to the end user. As an illustration of SS7 reliability, when an STP receives an incoming message, it performs message discrimination. It determines the message destination node and the application or user of the message information. For example, the STP might distinguish between a TCAP (Transactional Capabilities Applications Part) query message destined for an SCP and an ISUP (ISDN User Part) release message destined for an originating switch to tear down an established voice call. After it determines the destination address of the next signaling point, network management procedures check the available state of the node and its primary route. Assuming that no faults are detected within its routing database, the message is transferred to its primary route. If the primary route is unavailable, the message follows secondary routes. The North American requirement for availability between any two directly connected switches of a quasi-associated network architecture is 10 minutes (measured in downtime/year). Figure 2 shows the reference model for the Message Transfer Part (MTP) network downtime objective. MTP is used for the reference model because MTP Level 3 (see Appendix) is responsible for the routing of data between nodes in the SS7 network. This objective is accomplished by providing mated pairs of STPs, diverse paths between signaling points, and extensive network management capabilities.

Three Industry Changes Affecting SS7 Networks As each telco expands its services, the value of its SS7 network may dramatically increase. In fact, this trend applies to the entire telecommunications industry. As new service providers enter the market and as all service providers add new innovative network-based services, the common factor will be the SS7 infrastructure and connectivity.

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10 Minutes/Year

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User interface segment 3 min/yr

Network access segment 2 min/yr

Backbone network segment Negligible

STP Network access segment 2 min/yr

Network access STP processor A-link interfaces A-link transport

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User interfaces Common failure Modes/signaling

Illustration adapted from ANSI T1.111-1988

First, consider the introduction of LNP, a critical factor that will promote the successful entrance of new service providers into the world of telephony. LNP lets subscribers keep their telephone numbers when they change service providers. LNP specifications have been developed using SS7, common number portability databases, and requirements to carry certain information in the SS7 protocol. While it will take time to introduce and fully deploy, LNP has become regulatory table stakes and is likely to influence the SS7 network decisions of all telephone service providers. Second, given the proliferation of service providers (local wireline, local wireless, and long distance), how can the increasing complexity of SS7 interconnection be handled? SS7 standards and industry agreements help manage this complexity2. For example, two specific STP capabilities are global title translation (GTT) and gateway screening. When GTT is centralized at an STP, the amount of data each switch or network must retain locally is simplified. Gateway screening is essential to ensure security and integrity between interconnected networks. Parallel to standards organizations, industry forums (e.g., the Network Interconnection Interoperability Forum) address issues, such as link diversity guidelines and the procedures for problem resolution between interconnected networks. 5

Third, service expansion is an immediate factor. Most local service providers have implemented or plan to implement some form of Advanced Intelligent Network (AIN) capabilities. Instead of placing the intelligence to deliver key features in each switch, AIN places it in an SCP or in an intelligent peripheral (IP). Software triggers in individual SSPs (switches) momentarily interrupt call processing and send a query to an SCP for instructions on how to process features for individual calls. AIN also enables a standardized service creation environment (SCE). This allows any vendor, including the service provider, to develop software for the SCP. Service providers can quickly create (or have other specialized companies create) custom features and load them into the SCP. Any SSP in the network can then access and use these features.

Example of SS7 Investment for Cable Telephony As a result of the value SS7 may bring to a service provider, successful entrants clearly must have an SS7 network. Figure 3 shows a possible network for providing cable telephony. As shown, SS7 will be used to interconnect to other local and long-distance service providers as well as to access SCPs for network and service information.

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Toll calls 800 calls

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Figure 3. Cable Telephony Network Infrastructure 6

Head end with telephony interface

Hybrid fiber coax (HFC) distribution plant

Each service provider must decide whether to own or lease its SS7 network. Leasing involves purchasing network capacity from another network provider, while owning means building your own SS7 network. The lease versus own decision is complex and will be dictated by the tradeoffs encountered with ownership, network control, deployment costs, timing, and degree of desired service flexibility. With a flexible SS7 infrastructure, a new entrant will have the potential to maximize network investment quickly by making available relevant features and services which fill unanswered market needs. New entrants will gain the ability to be more competitive which will, in turn, benefit end users. With a robust and reliable SS7 infrastructure, both service availability and service assurance can be given to end users. Finally, careful planning of an SS7 infrastructure will make the uncertainty of ongoing industry evolution more manageable.

Endnotes 1 Two Integrated Services Digital Network (ISDN) user-to-network interfaces have been standardized: Basic Rate Interface (BRI) supports single terminals or small groups of terminals and Primary Rate Interface (PRI) gives PBXs access to the SS7 network. 2 The Alliance for Telecommunications Industry Solutions (ATIS) oversees the activities of both American National Standards Institute (ANSI) and Network Interconnection Interoperability Forum (NIIF). Both organizations address issues of national scope with respect to SS7.

Authors Daniel Teichman is Manager, Business Development Strategic Planning for the Nortel Signaling Solutions Group. Donald Reaves is Account Marketing Manager for CLECs and IXCs for the Nortel Signaling Solutions Group.

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Appendix: SS7 Protocol Figure 4 shows the structure of the SS7 protocol. It is based on the Open Systems Interconnection (OSI) reference model in which functions are partitioned within the seven independently standardized layers and the well-defined interfaces between adjacent layers.

MTP Level 2 (Data Link) ensures reliable exchange of information between two signaling points by error control, flow control, and other link control activities. Errors in transmission can be detected and recovered, thereby masking deficiencies in the transmission quality.

SS7 Protocol Model TCAP

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The physical, electrical, and functional characteristics of the signaling link are defined within MTP Level 1. MTP Level 1 relays the bit streams of data containing call control information between two endpoints over a physical medium such as a 56 kbps or 64 kbps clear-channel link.

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Figure 4. SS7 Protocol Model MTP Level 3 (Network) is the workhorse of the SS7 protocol. It transparently transfers data by routing and relaying data between end users. This includes procedures for connection and connectionless addressing, message discrimination and routing, network management, multiplexing several logical links onto a single link, and congestion control. Connectionless addressing is performed by the Signaling Connection Control Part (SCCP). SCCP is responsible for determining the network address supporting a connectionless-based service (e.g., 800 number services or calling card validation), relaying the translated address to the MTP, and for network management related to connectionless services. OSI layers 4, 5, and 6 are the Transport, Session, and Presentation layers. They optimize resources based on the type of communication and application.

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The ISUP corresponds to these three layers. ISUP uses fixed messaging procedures for setting up, coordinating, and taking down voice/data trunk calls. ISUP also transports data about the signaling user (such as calling and called party number) in the ISUP message parameters. OSI layer 7 (Application) specifies the nature of the communication required to satisfy the user's needs such as ISUP (call setup) and TCAP for database queries. End-user applications reside within this layer.

For more information, please contact your local Nortel account representative or call 1-800-4 NORTEL (1-800-466-7835). Northern Telecom P.O. Box 13010 Research Triangle Park, NC 27709 © 1998 Northern Telecom Inc. Published by Northern Telecom 50001.25/11-97 Printed in USA January 1998 Nortel, the Nortel Globemark are trademarks of Northern Telecom Limited. Information subject to change. Northern Telecom reserves the right to make changes, without notice, in equipment design as engineering or manufacturing methods warrant. Product capabilities and availability dates described in this document pertain solely to Northern TelecomÕs marketing activities in the United States and Canada. World Wide Web URL: http://www.nortel.com

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