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Lesson 1
CCS7 Overview
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Objectives
Explain the functions of the CCS7 network
Describe the CCS7 protocol model
Identify the nodes of the CCS7 network
Explain how nodes communicate with one another in the CCS7 network
Identify the CCS7 signal units
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GSM CCS7 Overview
The Universal Mobile Telecommunications system (UMTS) Public Land Mobile Network (PLMN) is composed of the following main components:
Digital Multiplexing System Mobile-services Switching Center (DMS-MSC)
Gateway Digital Multiplexing System Mobile-services Switching Center (GMSC)
Digital Multiplexing System Home Location Register (DMS-HLR)
Visitor Location Register (VLR)
UMTS Terrestrial Radio Access Network (UTRAN)
These components work with the Public Switching Telephone Network (PSTN) to process User Equipment-related calls.
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Functional Architecture
Core Network
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Topics
The information in this lesson is divided into the following sections:
signaling
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Signaling
The evolution of communications technology has created an increasing demand for new and enhanced network services.
To meet these and other signaling demands, networks use two distinct types of signaling:
Per Trunk signaling
Common Channel signaling
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Characteristics of C7
Per Trunk signaling
Common Channel signaling
Large message repertoire
Signaling cannot occur during speech phase
Signaling for multiple voice circuits simultaneously
Signaling on a per trunk basis
Error detection and correction
Security - all messaging has multiple paths over which to travel
Capability to launch query and response messages to databases
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UMTS CCS7 Protocol
CCS7 is a message-based signaling protocol that separates the interconnection and exchange of information that occurs between signaling points in a network into layers.
CCS7 protocol is defined by CCITT and ANSI
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CCS7 Protocol (continued)
Transaction
Capabilities
Application
Network
Link
Physical
BSSMAP
DTAP
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CCS7 Nodes
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CCS7 Nodes (Continued)
Service Switching Point
Generates query messages to the application databases located at Service Control Points.
SSPs are also capable of trunk signaling.
A DMS-MSC is an SSP.
Signaling Transfer Point
Is the only node that can transfer messages.
The DMS-MSC is capable of acting as a STP in addition to its function as an SSP.
Nortel provide standalone STP known as DMS-STP.
SSP
STP
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CCS7 Nodes (continued)
Service Control Point supports transaction services subsystems. An SCP:
Accepts a query for information from a subsystem in the CCS7 network.
Retrieves the requested information from one of its subsystems.
Sends a response message to the originator of the request.
A DMS-HLR is an SCP. The IN service Center is another example of an SCP.
SCP
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CCS7 Nodes (continued)
Integrated Node:
Integrates the function of more than one CCS7 function within a single node.
Are NORTEL specific products.
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CCS7 Protocol Architecture
BSSMAP
DTAP
MTP
Network
Link
Physical
MAP
TCAP
User
Part
SCCP
BSSMAP
DTAP
MTP
Network
Link
Physical
MAP
TCAP
User
Part
SCCP
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Network Addressing
Network addressing is a collection of functions and conventions that CCS7 uses to route messages accurately through the network.
Each node in the CCS7 network has a unique address, and network addressing routes every message to its intended address.
There are three kinds of network addressing:
point code routing
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ANSI Point Codes
The ANSI point code is a three-part number defined as follows:
Network Identifier
Cluster Identifier
identifies the cluster, within a network, to which a particular node belongs (0-255)
Network Cluster Member
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CCITT Point Codes
The CCITT point code will be one of the types:
basic
international.
The CCITT basic point code is composed of one number.
The range for this number is 0-16,383.
The CCITT international point code is composed of three numbers:
Zone
identifies the world zone to which a node belongs (0-7)
Area
identifies area, within a network, to which a particular node belongs (0-255)
Point
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Subsystem Routing
A user of the SCCP protocol is called a subsystem.
Subsystems are application processes used for transaction services. HLRs and VLRs are examples of subsystems.
Subsystems which reside at a CCS7 node are identified and addressed with a SubSystem Number (SSN).
SSCP uses the SSN to deliver the message to the particular subsystem at the node.
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Global Title Routing
Messages do not always contain all the MTP level CCS7addressing information (point code or subsystem number) needed to route them to their ultimate destination.
Another method of routing is to use a specified digit string and translate it at subsequent nodes into the CCS7 address.
These dialed digits are called a Global Title (GT).
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CCS7 Message Categories
Signaling messages are transferred over signaling links by means of signal units. Three signal units exist and are as follows:
Link Status Signal Units (LSSU)
Message Signal Units (MSU)
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SS7 Signal Units
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Message Type Length Indicator Value(s)
The value of the LI (Length Indicator) field determines the signal unit type:
LI Value
3.63
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Service Indicator Values
1
4
7
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CCS7 Tables
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Signaling Link Definition
A signaling link:
Is used to pass signaling messages.
For example: a TS dedicated to CCS7 on a PCM30.
One link
PC=100
PC=200
Node A
Node B
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Linkset Definition
A linkset is a set of links connecting two nodes
Link 2
Link 3
Link 1
PC=100
PC=200
Linkset AB:
OPC=100
FEPC=200
Node A
Node B
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Linkset
A linkset is:
Is a group of links directly connecting one node to another.
Consists of one to 16 links. (A linkset of C-links is a special case; it consists of up to eight links.)
All links within a linkset share the same attributes.
For example, they are all A-links or all D-links. Multiple link linksets are deployed for redundancy and traffic distribution.
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Routeset Definition
Linkset AB:
OPC=100
FEPC=200
Linkset CB
PC=100
PC=200
PC=300
Routeset AB:
Linkset AB
Linkset AC
OPC=100
DPC=200
Linkset BA
Linkset BC
Linkset CA
Routeset BA:
Linkset BA
Linkset BC
Linkset AC:
OPC=100
FEPC=300
A routeset:
Is made of linksets.
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Modes of Operation
CCS7 signaling modes are the relationship between the signaling and voice components of a call.
CCS7 networks typically operate in one of the following two modes of operation:
Associated mode
Quasi-associated mode
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Associated Mode
LKSET AB
The routeset is made of only one linkset.
DMS-MSC
A
DMS-MSC
B
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Quasi Associated Mode
DMS-MSC
A
DMS-MSC
B
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CCS7 Signaling Types
Connection-oriented signaling
Connectionless signaling
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Connection-oriented Signaling
Voice trunks
Signaling link
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Connectionless Signaling
signaling link
signaling link
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Signaling Link Types
STP pair A
STP pair B
an E-Link is impossible.
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Supported Linkset Types
SCP
CSP06 and beyond
A, E, F on SSP component with SOC option TEL00010 (Multiple Network Address for CCS7) activated.
A, B, C, D, E on STP component with SOC option TEL00010 (Multiple Network Address for CCS7) activated.
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CCS7 Signaling
Signaling link
Voice trunk
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CCS7 Signaling Interfaces
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CCS7 Protocol and Interfaces
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Operation and Maintenance Subsystem
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PTS vs CCS
Single trunk carries
signaling and voice
Trunks dedicated to
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CCS7 Protocol (continued)
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Format of Signal Unit (LSSU)
Flag
Check
Status
Field
LI
F
I
B
FSN
B
I
B
BSN
Flag
LSSU
Flag
Check
Signal
Information
Field
LI
F
I
B
FSN
B
I
B
BSN
Flag
SIO
MSU
Flag
Check
LI
F
I
B
FSN
B
I
B
BSN
Flag
FISU