McGraw Hill－Signaling System Number 7(5th edition)

Chapter

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Signaling System #7

Signaling System 7 (SS7) has become one of the most important assets within any

carrier’s network. Already deemed important for interconnecting calls from one net-

work to the next, SS7 also has become a network rich in user data.

SS7 is really a control protocol, used to provide instructions to the various elements

within a telephony network. These instructions may be how to route a call through the

network, what features a caller has subscribed to, or in the case of number portability,

(which, of course, there is), carriers would find a rich resource for identifying the users

of their network.

The data can be used for determining new marketing campaigns, the success of new

feature offerings, and much more. In fact, I often refer to these data as the three W’s:

who is using the network, when they are using the network, and why they are using

the network. These data are crucial to the success of any business to ensure that it is

meeting the needs of its customers.

Many carriers are just now realizing the benefits of mining the data traversing the

SS7 network and are using these data to maintain revenue assurance in all aspects of

the business. SS7 has even become an important revenue source for carriers who have

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In addition to changes in the transport layer, the industry already has begun to

evolve the network itself to a new architecture designed to support not just voice but

all forms of media, including video and messaging. Much as the Intelligent Network (IN)

was designed to support the delivery of voice services under the control of SS7, the IP

Multimedia Subsystem (IMS) has been developed to support the delivery of all services,

Why is signaling needed? To understand the answer to this question, you must first

understand the mechanics of a telephone call. When a subscriber picks up a telephone

receiver, an electric signal is sent over a wire to a telephone switch. The telephone

switch detects the electric current on this wire and interprets this “signal” as a request

for a dial tone.

But let’s say the subscriber wants to transmit data over this same line using packet

switching rather than an analog modem. The information sent to the telephone switch

has to define the transmission as digital data and not voice before the switch can de-

termine how to handle the call. This is only one portion of the call.

regarding the type of transmission, how the transmission is to be routed (call destina-

tion), and what the contents of the transmission are (audio, video, data, and so on).

If there is to be special handling or routing for a call, the telephone switches involved

in routing the call must be able to obtain these instructions. Rather than store routing

instructions for every single telephone number in the world within each and every tele-

phone switch, each network is responsible for its own network database. The telephone

switches then need the capability to connect and communicate with these databases to

obtain the special instructions.

This is a high-level view of what signaling networks really do. They enable tele-

phone switches (and now packet switches) to communicate directly with one another

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Ever since the beginning of the telephone, signaling has been an integral part of

telephone communications. The first telephone devices depended on the receiving party

standing next to the receiver. Early telephones did not have ringers like today’s tele-

phones and used crude speakers to project the caller’s voice into the room. If the person

being called was not within close proximity of the speaker, he or she would have no

immediate impact, this method became important when the first telephone exchange

was created. Lifting the receiver and allowing a dc current to flow through the phone

and back through the return of the circuit would light a lamp on the operator’s switch-

board. This signaled the operator when someone needed to place a telephone call and

often was accompanied by a buzzer.

Once the operator answered the switchboard, information was exchanged with the

operator. The person calling was identified so that a record of the call could be made

(for billing if it was a long-distance call), and the person being called was identified

so that the operator knew how to connect the call. This is not much different from

telephone signaling today, although signaling has evolved over the decades to include

significantly more information than these early methods could.

Consider the typical long-distance telephone call today. When a caller dials the area

that could be represented. They also were limited to audible tones because they used

the same circuit for both signaling and voice. The tones sometimes would interfere with

the call in progress, and sometimes the voice transmission itself would be interpreted

as part of the signaling and release the call.

Another problem with early signaling methods was the fact that the circuit used for

the call would be busy from the time the caller started dialing until the call was com-

pleted. Since the signaling was sent through the same circuit as the voice transmission,

it was necessary to connect the facility end to end even if there was no voice transmis-

sion. For example, if the number being called was busy, the facility still would be con-

nected end to end so that a busy tone could be sent through the circuit to the caller.

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a way to consolidate their facilities, making more efficient use of what they had. In ad-

dition, they needed a service that would vastly improve their network’s capability and

support the many new services being demanded by subscribers.

Europe had already begun the process of digitizing the network in the early 1960s.

One of the first steps was to remove signaling from the voice network and place the

The concept is simple. Rather than use voice trunks for signaling and voice trans-

mission, they are used only when a connection is established and voice (or data) trans-

mission takes place. For instance, when a call is placed to a distant party using con-

ventional signaling, the signaling for that call begins from the time the caller lifts the

receiver and goes off-hook until the caller goes back on-hook. After the end office has

received the dialed digits, an outgoing trunk to the destination end office is seized,

based on a routing-table entry and the digits dialed.

The voice circuit remains connected and in use by the telephone switch, even if the

distant party never answers the call, until the calling party hangs up. Meanwhile, other

circuits are being tied up in a similar fashion. By removing the signaling from the voice

network and placing it on a network of its own, the voice circuit remains available for a

longer period of time. This means that the availability of voice circuits is higher, and the

the audible tone. The same is true for intercept recordings.

With SS7, the caller’s local office could provide these tones and recordings at the

command of the distant office. This implementation would offer even more efficiencies

than the present method used throughout North America (voice circuits are two-way

transmission circuits, with a transmit and receive, so that tones and recordings are

sent in one direction).

SS7 does provide much faster connections and teardowns than conventional signal-

ing methods. Even if voice circuits do get connected, with the speed of the signaling

network, circuits can be disconnected and quickly connected again for a new call. This

is especially true of long-distance calls, where many segments of circuits are bridged

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can carry the signaling information for thousands of trunks and maintain thousands

of telephone calls. When TCP/IP is used as the transport for signaling, this number

increases significantly.

With SS7, the amount of information that can be provided is virtually unlimited. This

opens the potential for many more uses of the signaling data. Signaling takes place in two

even answered. Interoffice signaling now includes information obtained from regional

databases pertaining to the type of service a subscriber may have or billing informa-

tion. In fact, the first use of SS7 in the United States was not for call setup and tear-

down but for accessing remote databases. The opposite is true of Europe and other

international communities, where C7 is used for call setup and teardown, but the con-

cept of centralized databases for custom call routing is still new. In the 1980s, the U.S.

telephone companies offered a new service called Wide Area Telephone Service (WATS),

which used a common 800 area code regardless of the destination of the call. This posed

a problem for telephone-switching equipment, which uses the area code to determine

how to route a call through the Public Switched Telephone Network (PSTN).

To overcome this problem, a second number is assigned to every 800 number. This

second number is used by the switching equipment to actually route the call through

for the 800 number. The switching equipment then can route the call using conven-

tional signaling methods.

SS7 provides the data communications link between switching equipment and tele-

phone company databases. Shortly after 800 number implementation, the SS7 network

was expanded to provide other services, including call setup and teardown. Still, the

database-access capability has proven to be the biggest advantage behind SS7 and is

used widely today to provide routing and billing information for all telephone services,

including 800 numbers, 900 numbers, 911 services, custom calling features, caller iden-

tification, and many new services still being defined.

800 numbers at one time belonged to one service provider. If subscribers wanted to

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