| |
SS7 (from RB thread, courtesy InternetTelephony.com)
How SS7 works
SS7: The tie that binds
next-generation services
DANIEL KOZIN
SS7 has been in existence since 1984 and has become nothing less than the central nervous system of the worldwide
telecom network. Computer-telephony integration (CTI) and the emergence of next-generation services means that more
kinds of equipment than ever will have to interface to SS7. Next-generation systems must include SS7 interfaces for the
systems to provide a complete spectrum of services.
The Telecom Act of 1966 pried open Pandora?s Box, forcing competition on the unwilling telecommunications giants.
Previously, SS7 had been a virtual secret employed mainly by the telecommunications giants. Now CTI has directed
considerable attention toward it.
Worldwide coverage
The main point is clear and simple: For users of next-generation services to communicate with anywhere in the world that they
could call with an ordinary phone, then SS7 will figure in the picture somewhere.
SS7 provides the two essential services needed for telephony in all its forms:
Call setup, including redirecting phone calls from a dialed number to a different target number.
Data queries, including translating a 1-800 toll-free number into a local phone number, or determining the local carrier used for
a called number (local number portability).
Voice over IP
To succeed with users, voice over IP (VoIP) must operate just like basic telephone service always has. A caller will dial into
the VoIP gateway using a special number. The caller will then be prompted to enter a destination number. Once it is entered,
the IP "cloud" has to find the nearest VoIP gateway--of which there are many--to the destination phone. When the destination
phone is picked up, then the user?s voice signals are digitized and assembled into packets and the packets are transmitted
via the IP cloud through the VoIP gateways.
Most would assume that the IP cloud is the Internet. It is not. To get the reliability and quality of service users expect from a
telephony system, communications companies are constructing their own managed IP networks.
So far, this scheme looks pretty good. But what happens when a user tries to call a destination such as Europe, where the
communications company has no IP network usable for voice? For these calls, the VoIP traffic will have to be carried at least
part of the way by the conventional telephone system.
To pass the IP phone call to the public switched telephone network (PSTN), there needs to be a connection from the IP
network to the SS7 network. Figure 1 shows the method being proposed today to merge VoIP networks with the traditional
PSTN. With this architecture, service providers can offer worldwide coverage to their subscribers using the PSTN for
destinations that are not covered by their IP networks.
Under this architecture, the VoIP network is connected directly to the SS7 network and can be accessed through one-stage
dialing (dialing with a single number prefix like 1+) instead of requiring the caller to dial a POP number before the destination
number. Notice that the VoIP gateways have split into two different gateways, the signaling gateway handling only the call
setup, and the media gateways handling only the data transmission related to the real-time, bidirectional voice conversations.
Voice over DSL
DSL should soon to be carrying large amounts of traffic between phone companies? central offices and customer premises,
virtually replacing T-1 lines. Because DSL uses the phone companies? ordinary copper wire for communication, carrying voice
traffic simultaneously with data traffic is imperative.
At the customer premises, both the data network and conventional phones connect to a integrated access device (IAD),
transferring both voice and data over the DSL signal (Figure 2). At the CO end, the DSL gateway extracts the voice traffic
from the data traffic and sends it over the conventional telephone system--again using SS7. Thus, the DSL gateway can be a
replacement for a standard Class 5 switch (or co-exist as a "peer"), which means it must have an SS7 interface.
This scenario becomes a very attractive service to companies that want to compete with the local telephone operating
company. By transferring telephone traffic over the DSL data path, the company can capture the local as well as long-distance
phone traffic and provide bundled data service as to differentiate itself from competitors.
Voice over Cable
Although the actual medium for sending and receiving data in voice over cable is very different from voice over DSL, the
principle is the same. At the customer premises, is a set-top box that has a phone, data, and TV connections. At the cable
company?s office, voice packets are split off from the stream of data packets and sent out over the conventional phone
network--SS7 (Figure 3).
The business dynamics are similar for voice over DSL because cable companies can now begin to provide a complete
bundle of services that include local and long-distance phone calls, data and TV.
Practical Matters
Including SS7 functionality in a product sold worldwide will be difficult. Even though a basic specification for SS7 exists and
deregulation has forced the various PTTs to make their specifications public, each country has developed its own SS7
variation. Designing and certifying SS7 equipment is a lengthy and exacting exercise because of stringent regulations.
Therefore, equipment manufacturers should buy off-the-shelf SS7 components rather than trying to develop them in-house.
Daniel Kozin is the Director of Business Development for Brooktrout, Inc. Previously, he was Director of Engineering and was
responsible for starting the IP Telephony product line at Brooktrout Technology. He can be reached at
dkozin@brooktrout.com.
Visit the Brooktrout website.
RETURN TO TOP
--------------------------------------------------------------------------------
How SS7 works
Signaling System 7 (SS7) is the global standard for telecommunications. It is an International Telecommunication Union (ITU)
standard. The standard covers procedures and protocols that network elements in the public switched telephone network
(PSTN) use to exchange data packets over a digital network.
The purpose of exchanging data is to enable wireless and wireline call setup, routing and control as well as voice
communications. The ITU definition is an umbrella standard that allows for variants such as the American National Standards
Institute (ANSI) and Telcordia standards used in North America and the European Telecommunications Standard Institute
(ETSI) standard used in Europe.
SS7 performs a host of functions including:
The fundamental operations of an ordinary phone call: call setup, management and call teardown.
Toll and toll-free wireline services.
Advanced services such as call forwarding, caller ID and multiparty calls.
Wireless services including wireless roaming.
Subscriber authentication.
Local number portability (LNP).
SS7 separates voice channels from signaling. SS7 data packets go over 56 or 64 kb/s bidirectional channels dubbed
signaling links. Signaling occurs on dedicated channels that are different from the voice/data channels. This provides faster
call-setup times and more efficient use of voice circuits. Out-of-band signaling is also needed to support Intelligent Network
(IN) services.
Signaling points
SS7 has three kinds of switches or "signaling points":
SSP--Service switching point.
SCP--Service control point.
STP--Signal transfer point.
SSPs originate, terminate or tandem calls. An SSP sends signaling messages to other SSPs to setup, manage and release
voice circuits. An SSP may also send a query message to an SCP, which is a centralized database, to figure out how to route
a call such as a toll-free call. Network traffic between signaling points may be routed via a packet switch called an STP.
SS7 Protocol Stack
The hardware and software functions of the SS7 protocol stack are divided into levels. These levels map loosely to the Open
Systems Interconnect (OSI) seven-layer model defined by the International Standards Organization (ISO).
Message Transfer Part
The Message Transfer Part (MTP) of the SS7 protocol stack is divided into three levels. The lowest level, MTP Level 1, is
equivalent to the OSI Physical Layer. MTP Level 2 ensures accurate end-to-end transmission of a message across a
signaling link. MTP Level 2 is equivalent to the OSI Data Link Layer. MTP Level 3 provides message routing between
signaling points in the SS7 network. MTP Level 3 is equivalent to the OSI Network Layer.
ISDN User Part (ISUP)
The ISDN User Part (ISUP) defines the protocol used to setup, manage and release trunk circuits that carry voice and data
between calling parties. ISUP is used for both ISDN and non-ISDN calls. In some parts of the world such as China and Brazil,
the Telephone User Part (TUP) affects basic call setup and teardown. TUP handles analog circuits.
Transaction Capabilities Applications Part
TCAP supports information exchange between signaling points using the SCCP connectionless service for intelligent
networks (IN). An SSP uses TCAP to query an SCP to determine the routing number(s) associated with a dialed toll-free
number. The SCP uses TCAP to return a response containing the routing number(s) back to the SSP. Calling card calls are
also validated using TCAP. When a mobile subscriber roams into a mobile switching center (MSC) area, the integrated
visitor location register requests service profile information from the subscriber?s home location register (HLR) using mobile
application part (MAP) information carried within TCAP messages.
Signaling Connection Control Part
SCCP provides connectionless and connection-oriented network services above MTP Level 3. SCCP allow messages to be
addressed to specific applications (called subsystems). SCCP is used as the transport layer for TCAP-based services such
as toll-free calls, calling card, local number portability, wireless roaming and personal communications services (PCS). |
|
