1. Common Channel Signaling No. 7 (CCS7)
Chapter 9
Common Channel Signaling No. 7 (CCS7)
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2. 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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3. Continue… Fully disassociated channel signaling 19/04/2013
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4. 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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5. Continue… The SS7 Network and Protocol are used for:
Basic call setup, management, and tear down.
Wireless services such as wireless roaming and mobile subscriber authentication.
Toll free (800/888) and toll (900) wireline service.
Enhanced call features such as call forwarding, calling party name/number display, and three-way calling.
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6. Continue…
In an SS7 Network, Control Messages (Packets) are routed through the network to perform:
Call management (setup, maintenance, and termination) and,
Network management functions.
Therefore, the CCS7 network is a packet-switched network, that is been controlled by circuit-switched network.
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7. 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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8. Continue…
SS7 sample network
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9. Continue…
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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10. Continue…
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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11. 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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12. Continue…
In SS7 Messaging, both the origination point code and the destination point code are provided.
There are three different basic types of Network Elements (Signaling Points):
Service Switching Point (SSP).
Signal Transfer Point (STP).
Service Control Point (SCP).
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13. Service Switching Point
Continue…
SSP
Service Switching Point
STP
Signal Transfer Point
Service Control Point
Signaling Network Elements
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14. 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:
The combination of a Voice Switch and an SS7 Switch.
Or an Adjunct Computer connected to a Voice Switch.
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15. Continue…
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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16. Continue…
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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17. Continue…
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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18. Continue… This SS7 Message contain:
The calling and called telephone numbers,
The Information about the type of decoding used in the voice transmission,
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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19. Continue…
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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20. Continue…
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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21. 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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22. Continue… 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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23. Continue…
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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24. Continue… 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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25. Continue…
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.
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26. Continue…
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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27. 3.3 Service Control Point (SCP)
The SCP serves as an Interface to Telephone Company Databases.
These Databases provide:
The storage of information about subscribers’ services.
The routing of special service numbers such as 800 and 900 numbers, and calling-card validation.
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28. Continue…
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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29. Continue…
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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30. Continue…
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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31. Continue…
The Databases used most commonly within either of these networks are as follows:
Call Management Services Database (CMSDB):
It includes certain network management functions used to prevent congestion on the network.
It also provides billing information.
Local Number Portability (LNP):
It enable subscribers to change telephone companies without having to change their telephone numbers.
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32. Continue… Line Information Database (LIDB):
provides information regarding subscribers, such as calling-card service, third-party billing instructions.
Calling Name (CNAM):
The user’s Personal Identification Number (PIN) is stored in this Database for comparison when a user places a call.
This Database provides the name of the calling party.
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33. Continue…
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.
Visitor Location Register (VLR): is used for determining the location of a wireless subscriber in real time.
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34. Continue…
A Visitor Location Register (VLR) is a Database that contains information about the Subscribers Roaming within a Mobile Switching Center’s (MSC) Location Area.
The primary role of the VLR is to minimize the number of queries that MSCs have to make to the Home Location Register (HLR), which holds permanent data regarding the cellular network’s subscribers.
Ideally, there should be only one visitor location register per MSC, but it is also possible for a single VLR to serve multiple MSCs.
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35. Continue…
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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36. 4 Basic Signaling Architecture
Figure on the next slide shows a small example of how basic Network Elements (NE) of an SS7 Network are deployed to form two Interconnected Networks.
The following points should be noted:
STPs W and X, perform identical functions, they are redundant, and referred to as mated pairs of STPs. Similarly are Y and Z.
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37. Continue…
SS7 sample network
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38. Continue…
Each SSP has two links, one to each STP of a mated pair. All SS7 Signaling to the rest of the world is sent out over these links. Messages sent over either link will be treated equivalently.
The STPs of a mated pair are joined by a link or set of links.
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39. Continue…
Two mated pairs of STPs are interconnected by four links which are referred to as quad.
SCPs are usually (not always) deployed in pairs which are referred to as mated pair of SCPs. They are not directly joined by a pair of links.
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40. 4 Signaling Links
All signaling points are interconnected via signaling links.
These links are 56 or 64-kbps.
Links are Bidirectional, using transmit and receive pairs for simultaneous transmission in both directions.
Links always should be Terrestrial, although Satellite Links are also supported.
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41. Continue…
Signaling Links (SL) are labeled according to their relationship in the network.
There is no technical difference between the various Links.
They are only different in the way the links are used during message transfer and how network management interacts with the links.
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42. SS7 Link Types
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43. Continue… Links are placed into groups called Linksets.
All the links in a linkset must have the same adjacent node.
The Switching Equipment alternates transmission across all the links in a linkset to ensure equal use of all facilities.
A Signaling Point (SP) must define Routes.
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44. Continue…
A Route: is a collection of Linksets used to reach a particular destination.
A Linkset can belong to more than one route.
Routeset: is a collection of routes.
A Routeset provides alternate routes to the same destination in the event that any one route becomes unavailable.
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45. Continue… Access Links (A-Links):
Six different types of links are used in SS7:
Access Links (A-Links):
Are used between the SSP and the STP or the SCP and the STP.
These links provide access into the network and to Databases through the STP.
The maximum number of A-links to any one STP from SSP is 16, usually, one A-link for 9600 subscribers served by SSP.
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46. A-links connect end-signaling points to the SS7 network
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47. Continue… Bridge Links (B-Links):
Are deployed in a quad fashion and are used to connect mated STPs to other mated STPs at the same hierarchical level.
A maximum of eight B-links can be deployed between mated STPs.
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48. B-links connect a mated pair of STPs to another mated pair of STPs
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49. A typical SS7 network with multiple B-links and A-links
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50. Continue… Cross Links (C-Links): Connect an STP to its mate STP.
They are always deployed in pairs to maintain redundancy on the network.
Normal SS7 traffic is not routed over these links, except in congestion conditions.
The only messages to travel between mated STPs during normal conditions are Network Management Messages.
A maximum of eight C-links can be deployed between STP pairs.
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51. C-links connect an STP to its mate STP, creating a mated pair
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52. Continue… Diagonal Links (D-Links):
Are used to connect mated STP pairs at a Primary Hierarchical Level to another STP mated pair at a Secondary Hierarchical Level.
D-links are deployed in a quad arrangement like B-links.
A maximum of eight D-links can be used between two mated STP pairs.
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53. D-links connect a mated pair of STPs to another mated pair of STPs that are deployed on a higher level in the network hierarchy
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54. Continue… Extended Links (E-Links):
Are used to connect to remote STP pairs from an SSP.
The SSP connects to its home STP pair but, for diversity, also may be connected to a remote STP pair using E-links.
E-links then become the alternate route for SS7 messages in the event that congestion occurs within the home STP pairs.
A maximum of 16 E-links can be used between any remote STP pairs.
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55. E-links connect an SSP with an STP that is not considered its home STP
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56. Continue… Fully Associated Links (F-Links):
Are used when a large amount of traffic exists between two SSPs or when an SSP cannot be connected directly to an STP.
F-links enable SSPs to use the SS7 protocol and access SS7 Databases even when it is not economical to provide a direct connection to an STP pair.
Only call setup and tear-down procedures would be sent over this Linkset.
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57. F-links connect two STPs directly
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58. 5 The SS7 Protocol
The Hardware and Software functions of the SS7 protocol are divided into functional abstractions called “Levels.“
These Levels map loosely to the Open Systems Interconnect (OSI) 7-Layer Model defined by the International Standards Organization (ISO).
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59. The OSI Reference Model and the SS7 Protocol Stack
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60. 5.1 Message Transfer Part
Its purpose is to Reliably Transfer Messages on behalf of the User Parts across the SS7 network.
The MTP is divided into three levels:
MTP Level 1:
Is equivalent to the OSI Physical Layer.
It defines the Physical, Electrical and Functional characteristics of the Digital Signaling Link.
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61. Continue…
It defines a bidirectional transmission path for signaling, comprising two data channels operating together in opposite directions at the same data rate.
Physical interfaces defined include E1 (2.048 Mbps), DS1 (1.544 Mbps), DS0 (64 kbps) and DS-0A (56 kbps).
MTP Level 2:
Is equivalent to the OSI Data Link Layer.
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62. Continue…
It ensures accurate End-to-End transmission of a message across a Signaling Link (SL) between SPs.
It implements Flow Control, Message Sequence Validation and Error Checking.
When an error occurs on a Signaling Link (SL), the Message (or set of messages) is Retransmitted.
This Signaling Message is called FISU.
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63. Continue… MTP Level 3: Is equivalent to the OSI Network Layer.
It provides Message Routing between Signaling Points (SPs) in the SS7 Network.
It Reroutes Traffic away from failed links and SP and Controls Traffic when Congestion occurs.
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64. 5.2 ISDN User Part (ISUP)
Defines the Protocol (Signaling) used to set-up, manage and release Trunk Circuits that carry voice and data between terminating line exchanges (e.g., between a calling party and a called party).
ISUP is used for both ISDN and non-ISDN calls.
However, calls that Originate and Terminate at the same Switch do not use ISUP Signaling.
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65. 5.3 Telephone User Part (TUP)
Is used to support basic call setup and tear-down.
TUP handles Analog Circuits only.
In many countries, ISUP has replaced TUP for Call Management.
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66. 5.4 Signaling Connection Control Part (SCCP)
It provides connectionless and connection-oriented network services and Global Title Translation (GTT) capabilities above MTP Level 3.
A Global Title is an Address (e.g. a dialed 800 number, calling card number, or mobile subscriber identification number) that is translated by SCCP into a Destination Point Code and Subsystem Number.
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67. Continue…
A Subsystem Number uniquely identifies an application (Database) at the Destination Signaling Point (SP).
SCCP is used as the Transport Layer for TCAP-based services (TCAP is a protocol used by SCP to access the a Database Entity).
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68. 5.5 Transaction Capabilities Applications Part (TCAP)
TCAP supports the exchange of Non-Circuit Related Data (Queries from the Database) between applications across the SS7 Network using the SCCP connectionless service.
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69. Continue…
Queries and responses sent between SSPs and SCPs are carried in TCAP Messages.
For example, an SSP sends a TCAP Query to determine the Routing Number associated with a dialed 800/888 number and to check the Personal Identification Number (PIN) of a Calling Card User.
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70. 6 The SS7 Messages
SS7 uses three different structures of Packets, which are called Signal Units (SU) or SS7 Messages.
There are three kinds of Signal Units:
Fill-In Signal Units (FISUs).
Link Status Signal Units (LSSUs).
Message Signal Units (MSUs).
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71. SS7 Signal Units
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72. 6.1.1 FISU
FISUs are transmitted continuously on a SL in both directions unless other SUs (MSUs or LSSUs) are present.
FISUs carry basic Level 2 information only (e.g. Acknowledgment of SU receipt by a Remote SP for accurate End-to-End transmission).
Because a CRC checksum is calculated for each FISU, SL quality is checked continuously by both SPs at either end of the Link.
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73. 6.1.2 LSSU
LSSUs carry one or two octets (8-bit bytes) of Link Status information between SPs at either end of a Link.
The Link Status is used to Control Link Alignment and to Indicate the status of a SP (e.g. Local Processor Outage) to the Remote SP.
LSSUs originate and terminate at Level 3.
It defines the Routing Protocol.
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74. 6.1.3 MSU
MSUs carry all Call Control, Database Query and Response, Network Management, and Network Maintenance Data in the Signaling Information Field (SIF).
MSUs have a Routing Label, which allows an Originating SP to send information to a Destination SP across the Network.
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75. 6.2 The SS7 Packet Overview Flag:
The Flag is an 8-bit (1-byte) pattern beginning with 0, followed by six consecutive 1s, and ending in 0 ( ).
The Flag is used to signify the beginning of a Signal Unit and the end of the preceding Signal Unit.
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76. Continue… Indicators:
The Indicator are used to request a Retransmission.
They are also used for Positive and Negative Acknowledgments.
There are three types of Indicators:
Indicator Bit.
Sequence Number.
Length Indicator.
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77. Continue… Indicator Bit: There are two types of Indicator Bit:
Backward Indicator Bit (BIB).
Forward Indicator Bit (FIB).
During normal conditions, both Indicator Bit (BIB and FIB) should be of the same value (0 or 1).
When a Retransmission is being requested, the SU being sent by the SP requesting the Retransmission will have an inverted BIB.
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78. Continue…
The FIB retains its original value, this indicates to the distant SP that an error occurred and Retransmission must take place.
The Sequence Numbers:
There are two Sequence Numbers:
The Backward Sequence Number (BSN).
The Forward Sequence Number (FSN).
BSN and FSN are used for Sequencing Packets and are used by Level 2 to ensure that all transmitted packets are received.
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79. Continue…
The FSN and BSN are numbers in binary code from a cyclic sequence ranging from 0 to 127.
The FSN Indicates the Number of the SU now being sent.
This Sequence Number is incremented by 1 after every SU transmission, except in the case of the SUs FISU or LSSU.
The FISU and LSSU assume the FSN of the last MSU or LSSU is sent and is never incremented.
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80. Continue… The BSN Indicates the last SU received without error.
It is used to Acknowledge the receipt of SUs by the Remote SP.
The BSN contains the Sequence Number of the SU being Acknowledged.
The transmitting SP maintains all transmitted SUs in its transmit buffer until an Acknowledgment is received.
When a SU is received, the BSN is examined to determine which SUs are being Acknowledged.
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81. Continue…
All Acknowledged SUs are then dropped from the transmit buffer.
For example, if sequence numbers 1 through 7 have been sent and received by the distant SP, the next SU sent by the receiving SP could have a BSN of 7, which Acknowledges that all SUs, 1 through 7, were received without error.
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82. Continue…
In case of Retransmission Request, the receiver of the Retransmission Request then retransmits everything in its transmit buffer with a sequence number higher than the BSN of the Retransmission Request.
The Indicator Bits in the retransmitted message are independent of the originating exchange’s Indicator Bits and do not provide any indication.
Therefore, they will both be of the same value during the retransmission.
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83. Continue… The Length Indicator (LI):
LI is used by Level 2 to determine which type of Signal Unit (SU) is being sent.
The values of the LI can be:
0 for (FISU).
1 or 2 for (LSSU).
3 or more for (MSU).
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84. Continue… Cyclic Redundancy Check (CRC):
The CRC field is the last field in the SU.
This field is calculated using the fields immediately following the Flag up to the check-bit field itself.
The transmitting SP performs the check before bit stuffing and transmission.
The receiving SP then performs a similar calculation and compares the remainder to the CRC field of the received SU.
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85. Continue… Service Information Octet (SIO):
The SIO is used by Level 3 to identify the type of protocol used at Level 4 such as the ISUP or the TCAP and the type of subservice standard (ITU-T or ANSI) at Level 4.
The SIO field in an MSU contains the 4-bit subservice field followed by the 4-bit service indicator.
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86. Continue…
This information is used by Level 3 and the message discrimination function to determine the type of SU, the protocol, and how it should be decoded.
FISUs and LSSUs do not contain an SIO.
Signaling Information Field (SIF):
It is the Payload of SS7.
This field can contain up to 272 octets and is used by all Upper-Level Protocols such as ISUP and TCAP.
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87. Continue…
The SIF in an MSU contains the routing label and signaling information (e.g., SCCP, TCAP and ISUP message data).
The SIF is used to transfer control information and the routing label used by Level 3.
LSSUs and FISUs contain neither a routing label nor an SIO as they are sent between two directly connected SPs.
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88. 6.3 MTP Level 3 Messaging
MTP Level 3 provides message routing between SPs in the SS7 Network.
MTP Level 3 routes messages based on the Routing Label in the Signaling Information Field (SIF) of message SUs.
The Routing Label is comprised of the Destination Point Code (DPC), Originating Point Code (OPC) and Signaling Link Selection (SLS) field.
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89. Continue…
Point Codes are numeric addresses that uniquely identify each SP in the SS7 Network.
When the DPC in a message indicates the receiving SP, the message is distributed to the appropriate user part (e.g. ISUP or SCCP) indicated by the service indicator in the SIO.
Messages destined for other SPs are transferred provided that the receiving SP has message transfer capabilities (i.e. STP).
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90. ANSI vs. ITU-T SIO and SIF
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91. Continue…
An ANSI routing label uses Seven Octets; an ITU-T routing label uses Four Octets.
ANSI point codes use 24 bits (three octets); ITU-T point codes typically use 14 bits.
ITU-T point codes are pure binary numbers, which may be stated in terms of zone, area/network and signaling point identification numbers.
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92. 7 Basic ISUP Call Control
The ISUP defines the protocol and procedures used to Set-Up, Manage and Release Trunk Circuits that carry voice and data calls over the PSTN.
ISUP is used for both ISDN and Non-ISDN Calls.
Intra-exchange calls do not use ISUP signaling.
Figure on the next slide depicts the ISUP signaling associated with a basic call.
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93. Basic ISUP Signaling 1) Initial Address Message (IAM)
2) Address Complete Message (ACM)
3) Answer Message (ANM)
4) Release Message (REL)
5) Release Complete Message (RLC)
Basic ISUP Signaling
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Bahman R. Alyaei

94. Continue… When an Inter-exchange call is placed:
The Originating SSP transmits an ISUP Initial Address Message (IAM) to reserve an idle Trunk Circuit from the Originating Switch to the Destination Switch (1a).
The IAM includes the OPC, DPC, Circuit Identification Code (Circuit "5" in the Figure), Dialed Digits and optionally, the Calling Party Number and Name.
19/04/2013
Bahman R. Alyaei

95. Continue…
The IAM is routed via the home STP of the Originating Switch to the Destination Switch (1b).
The Destination Switch examines the dialed number, determines that it serves the called party and if a line is available for Ringing.
The Destination Switch Rings the called party line and transmits an ISUP Address Complete Message (ACM) to the Originating Switch (2a) via its home STP to indicate that the remote end of the Trunk Circuit has been reserved.
19/04/2013
Bahman R. Alyaei

96. Continue…
The STP routes the ACM to the Originating Switch (2b), which Rings the calling party's line and connects it to the Trunk to complete the Voice Circuit from the calling party to the called party.
If the Originating and Destination Switches are not directly connected with trunks, the Originating Switch transmits an IAM to reserve a Trunk Circuit to an Intermediate Switch.
19/04/2013
Bahman R. Alyaei

97. Continue…
The Intermediate Switch sends an ACM to Acknowledge the Circuit reservation request and then transmits an IAM to reserve a Trunk Circuit to another Switch.
When the called party picks up the phone, the Destination Switch terminates the Ringing Tone and transmits an ISUP Answer Message (ANM) to the Originating Switch via its home STP (3a).
The STP routes the ANM to the Originating Switch (3b), which verifies that the calling party's line is connected to the reserved Trunk and, if so, initiates billing.
19/04/2013
Bahman R. Alyaei

98. Continue…
If the calling party hangs-up first, the Originating Switch sends an ISUP Release Message (REL) to release the Trunk Circuit between the Switches (4a).
The STP routes the REL to the Destination Switch (4b).
If the called party hangs-up first, or if the line is busy, the Destination Switch sends an REL to the Originating Switch indicating the release cause (e.g. normal release or busy).
19/04/2013
Bahman R. Alyaei

99. Continue…
Upon receiving the REL, the Destination Switch disconnects the Trunk from the called party's line, sets the Trunk state to idle and transmits an ISUP Release Complete Message (RLC) to the Originating Switch (5a) to Acknowledge the release of the remote end of the Trunk Circuit.
When the Originating Switch receives (or generates) the RLC (5b), it terminates the billing cycle and sets the Trunk state to idle in preparation for the next call.
19/04/2013
Bahman R. Alyaei

100. 8 ISUP Message Format
ISUP information is carried in the Signaling Information Field (SIF) of an MSU.
The SIF contains the routing label followed by a 14-bit (ANSI) or 12-bit (ITU) Circuit Identification Code (CIC).
The CIC indicates the Trunk Circuit reserved by the Originating Switch to carry the call.
The CIC is followed by the message type field (e.g., IAM, ACM, ANM, REL, RLC), which defines the contents of the remainder of the message (see Figure on the next slide).
19/04/2013
Bahman R. Alyaei

101. SIO and SIF, and ISUP Message
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Bahman R. Alyaei

102. ISUP Message Format
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Bahman R. Alyaei

103. 9 Remarks
An Initial Address Message (IAM) is sent in the “Forward" direction by each switch needed to complete the circuit between the calling party and called party until the circuit connects to the destination switch.
An IAM contains the called party number in the mandatory variable part and may contain the calling party name and number in the optional part.
19/04/2013
Bahman R. Alyaei

104. Continue…
An Address Complete Message (ACM) is sent in the " Backward" direction to indicate that the remote end of a Trunk Circuit has been reserved.
The Originating Switch responds to an ACM message by connecting the calling party's line to the Trunk to complete the Voice Circuit from the Calling Party to the Called Party.
The Originating Switch also sends a ringing tone to the calling party's line.
19/04/2013
Bahman R. Alyaei

105. Continue…
When the called party answers, the Destination Switch terminates the ringing tone and sends an Answer Message (ANM) to the Originating Switch.
The Originating Switch initiates billing after verifying that the calling party's line is connected to the Reserved Trunk.
A Release Message (REL) is sent in either direction indicating that the Circuit is being released.
19/04/2013
Bahman R. Alyaei

106. Continue…
An REL is sent when either the calling or called party "hangs up" the call.
An REL is also sent in the backward direction if the called party line is busy.
A Release Complete Message (RLC) is sent in the opposite direction of the REL to Acknowledge the release of the remote end of a Trunk Circuit and end the billing cycle as appropriate.
19/04/2013
Bahman R. Alyaei