The world largely uses Signaling System 7 (SS7) for
communications, not for IP telephony. And although there
is consensus that IP telephony is the future of voice
communications, SS7 still dominates. Therefore, getting
the best out of IP telephony means understanding SS7,
the capabilities it can bring you, and how to go about
What Is SS7?
Chances are, unless you are involved in public network
telecommunications, you have never heard of SS7.
However, you have probably used it. SS7 is what runs
public wireless and wireline networks. It is the
communications network for the communications network,
both a suite of protocols as well as an architecture.
When you pick up the telephone in your home, or one
connected to your company's private branch exchange
(PBX), it connects to your local telephone exchange. The
exchange then routes your call using SS7 protocols over
SS7 links. In seconds, it can send your call around the
world through all kinds of equipment. If the person you
are calling has moved to a different state in the U.S.,
SS7 will find out where. If the recipient is out and has
voice-mail, SS7 will route the call to the mailbox,
wherever it may be.
On top of that, when you dial a mobile phone or send
a text message, the SS7 network finds out where in the
world that mobile phone is and routes the call there,
making sure the appropriate charges are logged. It can
also reroute your call when you roam into the domain of
another switching center, or cut you off if you run out
of money on your prepaid phone.
In short, the intelligence in the public network is
SS7 Versus ISDN
To be useful, IP telephony networks need to be connected
to the public network. Today corporate telephony systems
usually connect using ISDN PRI interfaces -- so why not
just use that to connect to the public network? For
small systems, ISDN is very cost effective. But once you
start to scale up, SS7 becomes more economical. A single
SS7 link can handle the messaging traffic for thousands
of voice circuits. However, the most compelling reason
for using SS7 is not cost, but the extra features and
capabilities it brings.
Fundamentally, ISDN just makes calls. The
network-side equivalent of ISDN is the SS7 protocol ISUP,
which makes calls much faster and more efficiently than
ISDN. ISUP also gives you more information about the
call and routes it more efficiently.
And ISUP is only one part of the SS7 suite. SS7 also
includes protocols for data traffic through the network,
such as querying databases or more complicated
interactions between different nodes. The underlying
transport and management are also much more
sophisticated, with redundancy, fault reporting, and
correcting as more integral parts of the design.
The basic difference? With ISDN, you're just a user.
With SS7, you're the network.
What's The Catch?
There is always a price to pay for sophisticated
capabilities -- and this is true of SS7.
First, SS7 is very complex, with many different
national variants. Operation is often covered by
legislation -- which is, after all, part of the core
infrastructure of most countries. But there are methods
and tools you can use to overcome these problems (which
we will discuss later).
The second problem is that once you are a part of a
network, the network expects you to be properly
functioning 24/7. Firing garbage into an ISDN
application only affects you. That is the idea behind an
access protocol like ISDN. Firing garbage into an SS7
network gets the attention of the operator, and you'll
lose the connection. Therefore, when you start playing
with SS7, you need to get it right.
I'm In IP Telephony; Why Do I Need SS7?
The installed base for SS7 is massive -- more than $1
trillion. IP telephony deployment and revenues are
microscopic by comparison. IP-based services need to
become parasites on the SS7 network, leveraging that
access instead of competing on an installed base.
Consider an example. Today, you can get a British
Internet service provider (ISP) service that tells you
when a call is incoming while you are using your line
for connecting to the Internet. You can then choose
whether to send the call to voice-mail, hang up and take
the call, or even take the call over IP through your
modem. This is a good example of IP telephony services
sneaking in by leveraging existing SS7 technology. It is
good for the ISP, since people use its service without
worrying about missing calls. It also gives the ISP's
customers a taste of IP telephony. It also allows the
ISP to terminate the incoming call, tapping into another
source of revenue.
How about a service that lets you consolidate your
different phones and voice mail? For someone to contact
you, they need to know where you are and which number to
use: Office phone, home phone, or cell phone. Callers
can also end up leaving voice mail in three different
places, all of which you need to check periodically. The
answer to this problem could come from IP service logic
without the calls actually leaving the SS7 network.
SIP developers have promised a unified environment
that unhooks addresses from physical equipment, letting
users process calls more intelligently. However, today's
SIP service logic needs access to the SS7 network to
reach the market. Without SS7 connectivity, a SIP
service is a novelty. With SS7 connectivity, service
platforms can intercept and reroute normal calls. They
can monitor when your mobile phone is switched on and
where it is, even controlling the response the network
sends to the caller. Voice mailboxes can be amalgamated,
with the presence of a message communicated over e-mail,
mobile short messaging, or any medium that is most
The technologies now available can provide amazing
integration and new services, but they need to be
deployed as an integral part of the SS7 network -- not
simply as customers.
Getting Into SS7
The advantages of understanding and using SS7 are
becoming clear. And you can enter the world of SS7 at
several levels, using protocol interworking, service
interfaces, protocol interfaces, or abstraction layers,
or by implementing an SS7 stack. Each level provides a
different trade-off of flexibility and control versus
simplicity and speed.
The simplest method is with an SS7 protocol
interworking device. This lets you use an SS7 connection
just by configuring the unit. It is ideal for taking
advantage of the economics of SS7 for large systems.
However, the amount of control and number of extra
features you get are limited by the lowest common
denominator of SS7 and the other protocol it is
interworking to (for example, SIP, H.323, or ISDN).
Some of these interworking devices also provide an
application programming interface (API) or other
interface for simple services. This gives you more
control and flexibility without forcing you to become an
SS7 expert. Similar to interworking devices are service
interfaces like those provided by the JAIN and PARLAY
groups. These give you a service-oriented API so that,
again, you have a high-level abstraction between you and
the complications of the SS7 network.
Getting a good level of flexibility and control
requires visibility down to the SS7 message level. Until
recently, this would be done using a proprietary API
from a specific vendor. Now the JAIN group provides
protocol interfaces, as well as their service
interfaces, to the three key SS7 protocols: ISUP, MAP,
and INAP. ISUP is the SS7 call control protocol; MAP is
the wireless protocol for database lookups, roaming and,
short messaging; and INAP is the intelligent networking
protocol used for advanced services in the SS7 network.
There is also a JAIN API for TCAP, the SS7 protocol used
by MAP and INAP, which can give a higher degree of
This combination of protocol interfaces gives you the
ability to create most kinds of SS7/IP service. The
underlying protocols all have main variants and national
sub-variants, but the JAIN APIs shield you from that
extra complexity. This allows you to deploy a system
almost anywhere without extra development.
The JAIN protocols are for use in a Java environment,
and they don't end at SS7. JAIN also includes an
interface for SIP, allowing you to develop in a single
environment, with the same type of interface, for all
protocols. It also solves the problem of vendor
dependence, since the JAIN API is an open standard. The
only problem with JAIN is that not all the work is
complete. You may be forced to use a vendor-specific API
to implement it today.
The final option is to implement an SS7 stack. This
is usually done by buying-in a hardware design and
protocol source code. This option gives you the ultimate
flexibility, but the advantages over an API are minimal.
The investment for this approach is massive, so it is
unlikely to give you the best relative return on your
Choosing SS7 Hardware and Software
SS7 links are usually presented on a timeslot/circuit in
an E-1, T-1, or J-1 connection or on a serial interface.
Most SS7 hardware supports these interfaces. They are
likely to have more of a certain type, depending on
which regional market the designer had in mind.
The number of major and minor protocol variants
supported by the SS7 stack in use is important, since it
limits where you can deploy. The main variants are U.S.
(ANSI), International Telecommunications Union (ITU --
which is the basis for European ETSI ISUP), and
Japanese. Most other variants are based on one of these.
Protocol interworkers are a chassis-based system,
usually with a TCP/IP interface used for management and
the service API (if there is one). These systems may
already have approvals from network operators for SS7
interconnects, so this route is often the quickest to
If you can use a standard API like JAIN, you will
enjoy a measure of vendor independence, since you can
instantly use any stack or hardware certified as JAIN-compliant.
The choice, then, is whether to integrate a board into
your system or to connect using an independent chassis.
For smaller systems, the board is the obvious choice.
However, chassis systems often provide a convenient
IP-based API. SS7 systems like this can allow you to
consolidate your expensive SS7 links in one place,
sending and receiving the messages from many other
chassis in a large IP network.
Another alternative is to use a system based on the
new SIGTRAN standard. SIGTRAN is an Internet Engineering
Task Force (IETF) working group that has produced a
number of protocols for sending SS7 messages over IP.
The higher-layer SS7 protocols then run on your system,
along with an adaptation layer. This adaptation layer
communicates over the new IP protocol SCTP (a part of
the SIGTRAN suite) with a signaling gateway, allowing
messages to be sent on the SS7 network.
The best choice for your system depends on several
factors and may also vary with time. You can quickly get
connected using a protocol interworker, then integrate
SS7 to gain better access as your expertise grows.
Systems may vary in size for different deployments,
or expand as demand for a service grows. Therefore, the
scalability a vendor provides is also important. The
vendor needs to demonstrate a roadmap that meets your
ongoing needs, including support for new standards and
methods as they become available.
The Future Of SS7
The days of SS7 are numbered, but not over. SS7
technology is becoming higher density, more powerful,
and less expensive. It is also becoming significantly
easier to integrate and use. This, combined with the
huge installed base, gives SS7 a profitable -- and not
insignificant -- future.
It is also worth keeping in mind that SS7 is an
architecture as well as a technology. New technologies
are allowing the architecture to exist in software. For
example, the SIGTRAN suite allows an entire SS7 network
to be run over IP, with SS7 providing many features not
currently available on any other technology.
The pivotal moment will not be when the last SS7 link
is taken down, but when SS7 is no longer dominant, which
could happen suddenly. Currently, European and Asian
mobile operators are committed to deploying packet-based
systems. Fixed-line operators are rolling out large data
networks capable of handling the voice traffic.
The last mile will eventually bring high-bandwidth to
most users and make the IP phone something the man in
the street has heard of. At this point, SS7 could
suddenly find itself playing second fiddle to IP
telephony protocols like SIP. When that happens, being
in IP telephony won't mean you need to understand SS7 to
be successful. But remember, as long as SS7 has more
users than IP telephony, you need it more than it needs
David Evans is product manager, Telecommunications
and Embedded Group, Intel
Corporation. The Intel Telecommunication and
Embedded Group develops advanced communications
technologies and products that merge data and voice
technologies into a single network. Products from the
Telecommunications and Embedded Group are available to
customers at multiple levels of integration: silicon,
software, boards, platforms, systems, and appliances.
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