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Chapter 9
Common Channel Signaling No. 7
(CCS7)
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1 Common Channel Signaling
• In CCS, a Common Signaling Link (SL) carries
Signaling Messages for a number of trunks.
• CCS was developed for the third-generation
SPC exchanges that were introduced in the
1960s such as 4ESS, 5ESS, and DMS-100.
• First-generation CCS was introduced in the
1970s.
• Signaling System No. 7 (SS7) also known
CCS7 specified by ITU-T in 1980s.
• It is widely in service in the International and
National Networks.
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Fully disassociated channel signaling
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2 Signaling System No. 7
• SS7 is a CCS system that defines
the Procedures and Protocol by
which Network Elements (Signaling
Points) in the PSTN Exchange
Information over a Digital Signaling
Network to facilitate Wired and
Wireless (Cellular) Call Setup,
Routing and Control.
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• The SS7 Network and Protocol are used for:
1. Basic call setup, management, and tear down.
2. Wireless services such as wireless roaming
and mobile subscriber authentication.
3. Toll free (800/888) and toll (900) wireline
service.
4. Enhanced call features such as call forwarding,
calling party name/number display, and threeway calling.
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•
In an SS7 Network, Control Messages
(Packets) are routed through the network
to perform:
1. Call management (setup, maintenance,
and termination) and,
2. Network management functions.
• Therefore, the CCS7 network is a packetswitched network, that controls circuitswitched network.
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2.1 Signaling Network
• Telecommunication networks that
employ CCS require, in addition to the
network of Trunks and Exchanges, a
Signaling Network.
• This network consists of Signaling
Points (SPs), interconnected by
Signaling Links (SLs).
• SP: is an entity in the network to which
CCS links are attached.
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SS7 sample network
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• For example, an exchange that serves CCS
Trunk Groups has CCS links and is
therefore a SP.
• Likewise, a Network Database that is
accessed via CCS links is a SP.
• SL: is a bidirectional 56 kbps or 64 kbps
transport facility for CCS messages between
two SPs.
• A Signaling Relation exists between any
pair of SPs that need to communicate by
CCS.
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• For example, when two exchanges, say, A
and B, are interconnected by a group of
CCS trunks, there is a signaling relation
between these SPs.
• Signaling Route: is a predetermined path
for the CCS Messages of a particular
relation.
• Usually, there is a signaling route set,
consisting of several routes, for each
signaling relation.
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3 Signaling Points
• An SS7 Network is comprised of Network
Elements (NEs) connected together using
Signaling Links (SLs).
• Such a NE that is capable of handling SS7
Control Messages is called a SP.
• All SPs in an SS7 Network are identified
by a unique address known as a point
code that defines the Signaling Relation.
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•
In SS7 Messaging, both the origination
point code and the destination point
code are provided.
• There are three different basic types of
Network Elements:
1. Service Switching Point (SSP).
2. Signal Transfer Point (STP).
3. Service Control Point (SCP).
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SSP
STP
STP
SSP
STP
STP
Service Switching
Point
Signal Transfer
Point
Service Control
Point
Signaling Network Elements
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3.1 Service Switching Point
(SSP)
• SSP is the Brain of the SS7 Network and
is located in the Switch.
• The SSP is the Local Exchange in the
Telephone Network.
• An SSP can be:
1. The combination of a Voice Switch and
an SS7 Switch.
2. Or an Adjunct Computer connected to a
Voice Switch.
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• The SSP must convert signaling from
the Voice Switch into SS7 Signaling
Messages, which then can be sent to
other Switches in the Network.
• The Switch typically will send
messages related to its voice circuits
to the switches with a direct voice
trunk connection to it.
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• In the case of database access, the
SSP will send database queries
through the SS7 Network to Computer
Systems located centrally to the
network (or regionally).
• The SSP function uses the information
provided by the calling party (such as
dialed digits) to determine how to
connect a call.
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• A Routing Table in the Switch itself
will identify which Trunk Circuit to
use to connect the call and at which
Switch this Trunk terminates.
• An SS7 Message must be sent to
this adjacent Switch requesting a
Circuit Connection on the
specified Trunk.
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• This SS7 Message contain:
1. The calling and called telephone
numbers,
2. The Information about the type of
decoding used in the voice transmission,
3. Type of the call.
• The adjacent Switch Grants
Permission to connect this Trunk by
sending back an Acknowledgment to
the Originating Switch.
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• Using the Called Party Information in the
Setup Message, the adjacent Switch
then can determine how to connect the
call to its final destination.
• The same process is followed using a
Setup Message to any adjacent
Switches and Circuits connecting those
Switches.
• The entire call may require several
connections between several switches.
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• The SSP function in each Switch,
manages these connections but really
has no knowledge of the status of
remote connections (nonadjacent
connections).
• The SSP only has visibility of its own
connections and does not maintain the
status of all the connections needed to
connect and maintain a call.
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3.2 Signal Transfer Point (STP)
• All SS7 Packets travel from one SSP to
another through the services of an STP.
• The STP serves as a Router in the SS7
Network.
• To maintain redundancy and diversity in
the network, STPs are always deployed in
pairs.
• Should one STP node fail, the other node
assumes all the traffic.
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• Both STPs in a pair process traffic using
Load sharing.
• An STP does not originate messages.
• The STP only processes the Transport
Layers (Message Transfer Part (MTP) or
TCP/IP-based protocols).
• The STP Routes SS7 Messages as received
from the various SSPs throughout the
Network to their appropriate destinations.
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• The STP can be an Adjunct to a Voice
Switch or a Standalone Packet Switch.
• Many Tandem Switches provide both
Switching Functions and STP Functions
(through the use of an adjunct computer).
• Although several manufacturers provide
STP equipment, very few provide a
Standalone STP.
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• Routing to Databases is another
important STP function.
• For example, the SCP may support
calling cards, 800 routing, and
calling-name display.
• Each of these services in the SCP
must have a unique address.
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• The STP uses a function called Global
Title Translation (GTT) to determine
which Database will receive the queries
generated by SSPs.
• By using an STP and GTT, the SSPs
only need to know the address of the
STP.
• The SSP sends a Database Query to
the local STP with the destination
address
of
the
STP.3
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• The STP looks at the dialed digits in the
Signaling Connection Control Part (SCCP)
portion of the Message Signal Unit (MSU)
(or global title digits as they are often called)
and determines the address of the Database
through its own Translation Tables.
• The GTT consists of the Subsystem
Number (Address) of the Database and the
Point Code of the SCP that interfaces with
the Database.
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3.3 Service Control Point (SCP)
•
The SCP serves as an interface to
telephone company databases.
• These databases provide:
1. The storage of information about
subscribers’ services,
2. The routing of special service numbers
(such as 800 and 900 numbers), callingcard validation, and even Advanced
Intelligent Network (AIN) services
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• The SCP is actually a function of the computer
used as a front end to the database application
itself.
• The SCP does not necessarily have to be a
standalone computer system.
• Some new SCP database applications are
being implemented in STPs, providing an
integrated solution.
• In all cases, the address of the SCP is a Point
Code, whereas the address of the database is
a subsystem number.
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• Entities within the network route to SCPs
using the SCCP protocol.
• The SCP function does not necessarily store
all the data, but it is the interface to the
mainframe or minicomputer system that is
used for the actual database.
• These computer systems usually are linked
to the SCP through X.25 or IP links.
• The SCP communicates with the database
application through the use of primitives.
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• A primitive is an interface that provides
access from one level of the protocol to
another level.
• In the case of the database, the database
is considered an application entity, and
TCAP is the protocol used to access and
interface with this application entity.
• The type of database depends on the
network.
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•
The databases used most commonly within
either of these networks are as follows:
1. Call Management Services Database
(CMSDB): It includes certain network
management functions used to prevent
congestion on the network. It also provides
billing information.
2. Local Number Portability (LNP): It enable
subscribers to change telephone companies
without having to change their telephone
numbers.
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3. Line Information Database (LIDB):
provides information regarding
subscribers, such as calling-card
service, third-party billing instructions.
The user’s personal identification
number (PIN) is stored in this
database for comparison when a user
places a call.
4. Calling Name (CNAM): This database
provides the name of the calling party.
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5. Home Location Register (HLR): is
found in wireless networks and is
used to store information regarding
a wireless subscriber. Billing and
feature information is stored in this
database along with location
information.
6. Visitor Location Register (VLR): is
used for determining the location of
a wireless subscriber in real time.
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• Each database is given a
unique address called a
subsystem number .
• The subsystem number is used
to route queries from SSPs
through the SS7 network to the
actual database entity.
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