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Transcript of SS7 Nortel
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.
1
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.
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. 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
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 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
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.
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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