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SS7 Connectivity:The Foundation
of Telephony
Signaling System 7 A R T I C L E
Daniel Teichman
and Donald Reaves
December 1997
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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 functionscall 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 SS7infrastructure 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.
1
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2
SS7 Network Architecture
Most SS7 networks in North America use a quasi-associated architecture. Signaling betweenendpoints 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 andtransaction 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 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)
Physical trunking
SS7 signaling
(TCAP & ISUP)
SS7 signaling
(TCAP)
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3
Importance of SS7 Signaling
A quasi-associated architecture has certain inherited attributes because of the design of theSS7 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 ofthe 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 stringentreliability 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, andextensive 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.10 Minutes/Year
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Figure 1.
5
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.
10 minutes/year
A-links
A-links
C-links
SPSP
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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6
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 inan 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.
Figure 3. Cable Telephony Network Infrastructure
Local Exchange
Carriers
Long Distance
Carriers
Toll calls
800 calls
Local callsWireless calls
800 callsHybrid fiber coax (HFC)
distribution plant
SCPSCP
STP
STP
SSP
Head end
withtelephony
interface
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7
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.
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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.
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.
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.
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.
a.
ISUPTCAP
SCCP
NETWORK
LINK
PHYSICALMTP
2
1
3
4567
SS7 Protocol Model
Figure 4. SS7 Protocol Model
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The ISUPcorresponds 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.
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18004 NORTELwww.nortel.com
For more information, please contact your
local Nortel account representative or call1-800-4 NORTEL (1-800-466-7835).
Northern TelecomP.O. Box 13010Research Triangle Park, NC 27709
1998 Northern Telecom Inc.Published by Northern Telecom50001.25/11-97Printed in USA January 1998
Nortel, the Nortel Globemark are trademarks of NorthernTelecom Limited.
Information subject to change. Northern Telecom reserves
the right to make changes, without notice, in equipmentdesign as engineering or manufacturing methods warrant.Product capabilities and availability dates described in thisdocument pertain solely to Northern Telecoms marketingactivities in the United States and Canada.
World Wide Web URL:http://www.nortel.com
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