SS7 Connectivity:The Foundation of Telephony

Signaling System 7A R T I C L E

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, LNPcannot 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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2

SS7 Network Ar chitecture

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.

Figure 1.Typical SS7 (Quasi-Associated) Network Architecture

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).

SSP SSP

ADSI

POTS

ISDN

SCP

STPSTP

Other SS7 networks

ADSI - Analog Display Services InterfaceISDN - Integrated Services Digital NetworkISUP - ISDN UserPart (signaling for voice/data call connections)POTS - Plain Old Telephone ServiceSCP- service control point (database of network or service information)SSP- service switching point (central office switches)STP- signal transferpoint (SS7 message switch)TCAP - Transaction Capabilities Application Part (signaling for database queries)

Physical trunking

SS7 signaling(TCAP & ISUP)

SS7 signaling(TCAP)

3

Impor tance 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.

EfficiencyBecause 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 EnablingThe 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.

4

Network ReliabilityThe 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 STPreceives 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 STPmight distinguish between a TCAP

(Transactional Capabilities Applications Part) query message destined for an SCPand 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. MTPis used for the reference model because MTPLevel 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.

Thr ee Industry Changes Af fecting 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.10 Minutes/Year

Figure 1.

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First, consider the introduction of LNP, a critical factor that will promote the successful

entrance of new service providers into the world of telephony. LNPlets subscribers keep their

telephone numbers when they change service providers. LNPspecifications 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, LNPhas

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

STPcapabilities are global title translation (GTT) and gateway screening. When GTTis

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.

10 minutes/year

A-links

A-links

C-li

nks

SP SP

STPSTP

STP STP

B/D-links

B/D-links

Userinterfacesegment3 min/yr

BackbonenetworksegmentNegligible

Userinterfacesegment3 min/yr

Networkaccesssegment2 min/yr

Networkaccesssegment2 min/yr

Backbone networkSTP interfaceB/D-linksC-linksTransport facilities

Network accessSTP processorA-link interfacesA-link transport

User interfacesCommon failureModes/signaling

Illustration adapted from ANSI T1.111-1988

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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 SCPor in

an intelligent peripheral (IP). Software triggers in individual SSPs (switches) momentarily

interrupt call processing and send a query to an SCPfor 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 SSPin the network can then access and use these features.

Example of SS7 Investment forCable 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.

Figure 3. Cable Telephony Network Infrastructure

Local ExchangeCarriers

Long Distance Carriers

Toll calls800 calls

Local callsWir eless calls800 calls Hybrid fiber coax (HFC)

distribution plant

SCPSCP

STP

STP

SSPHead endwith telephonyinterface

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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 trade-

offs 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.

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 standardizedlayers and the well-defined interfaces between adjacent layers.

The physical, electrical, and functional characteristics of the signaling link aredefined within MTPLevel 1. MTPLevel 1 relays the bit streams of datacontaining call control informationbetween two endpoints over a physicalmedium such as a 56 kbps or 64 kbpsclear-channel link.

MTP Level 2 (Data Link) ensuresreliable exchange of informationbetween two signaling points by errorcontrol, flow control, and other linkcontrol activities. Errors in transmission can be detected andrecovered, thereby maskingdeficiencies in the transmission quality.

MTP Level 3 (Network) is the workhorse ofthe SS7 protocol. It transparently transfers data by routing and relaying data between end users. Thisincludes procedures for connection and connectionless addressing, message discrimination androuting, network management, multiplexing several logical links onto a single link, and congestioncontrol.

Connectionless addressing is performed by the Signaling Connection Control Part (SCCP). SCCPisresponsible for determining the network address supporting a connectionless-based service (e.g., 800number services or calling card validation), relaying the translated address to the MTP, and fornetwork management related to connectionless services.

OSIlayers 4, 5, and 6 are the Transport, Session, and Presentation layers. They optimize resourcesbased on the type of communication and application.

a.

ISUPTCAP

SCCP

NETWORK

LINK

PHYSICALMT

P

2

1

3

4567

SS7 Protocol Model

Figure 4. SS7 Protocol Model

The ISUPcorresponds to these three layers. ISUPuses fixed messaging procedures for setting up,coordinating, and taking down voice/data trunk calls. ISUPalso transports data about the signalinguser (such as calling and called party number) in the ISUPmessage parameters.

OSIlayer 7 (Application) specifies the nature of the communication required to satisfy the user'sneeds such as ISUP(call setup) and TCAPfor database queries. End-user applications reside withinthis layer.

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© 1998 Northern Telecom Inc.Published by Northern Telecom50001.25/11-97Printed in USA January 1998

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