2b. circuit switching - ss7

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CIRCUIT SWITCHING SIGNALING

SIGNALING SYSTEM 7

(SS7)

Overview

Signaling between exchanges:

Channel Associated Signaling (CAS)

Common Channel Signaling (CCS)

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OverviewChannel Associated Signaling

The key feature that distinguishes Channel Associated Signaling (CAS) from Common Channel Signaling (CCS) that a dedicated fixed signaling capacity is set aside for each and every trunk in a fixed, pre-determined way.

CAS can be implemented using the following related systems: Bell Systems MF, R2, R1, and C5. Single-frequency (SF) in-band and out-of-band

signaling Robbed bit signaling

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OverviewChannel Associated Signaling

Limitations of CAS: Susceptibility to Fraud: CAS employing in-band

supervisory signaling is extremely susceptible to fraud because the subscriber can generate these signals by simply using a tone generator down a handset mouthpiece.

Limited Signaling Information: CAS is limited by the amount of information that can be signaled using the voice channel. Because only a small portion of the voice band is used for signaling.

Inefficient Use of Resources: CAS systems are inefficient because they require either continuous signaling or, in the case of digital CAS, at regular intervals even without new signals.

Signaling is limited: to call set-up and release phases only. This means that signaling cannot take place during the call connection phase.

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OverviewCommon Channel Signaling

CCS refers to the situation in which the signaling capacity is provided in a common pool, with the capacity being used as and when necessary.

The signaling channel can usually carry signaling information for thousands of traffic circuits.

CCS systems are packet-based, transferring over 200 bytes in a single SS7 packet, as opposed to a few bits allocated to act as indicators in digital CAS. The signaling information is transferred by means of messages, which is a block of information that is divided into fields that define a certain parameter or further sub-field.

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Circuit-related and non-circuit-related Circuit-Related Signaling: refers to the original

functionality of signaling, which is to establish, supervise, and release trunks. In other words, it is used to set up, manage, and clear down basic telephone service calls.

Non-Circuit-Related Signaling: refers to signaling that is not related to the establishment, supervision, and release of trunks. Due to the advent of supplementary services and the need for database communication in cellular networks and Intelligent Networks. Non-circuit-related signaling allows the transfer of information

that is not related to a particular circuit, typically for the purpose of transmitting both the query and response to and from telecommunication databases 6

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There are three types of CCS signaling modes: Associated Quasi-associated Non-associated

SS7 runs in associated or quasi-associated mode, but not in non-associated mode. Associated and quasi-associated signaling modes ensure sequential delivery, while non-associated does not. SS7 does not run in non-associated mode because it does not have procedures for reordering out-of-sequence messages.

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Associated signaling both the signaling and the corresponding user traffic

take the same route through the network. Associated mode requires every network switch to

have signaling links to every other interconnected switch (this is known as a fully meshed network design).

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Quasi-associated signaling

In quasi-associated mode, signaling follows a different route than the switched traffic to which it refers, requiring the signaling to traverse at least one intermediate node. Quasi-associated networks tend to make better use of the signaling links.

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Non-associated signaling Because the path is not fixed at a given point in time

in non-associated mode, the signaling has many possible routes through the network for a given call or transaction. Therefore, the packets might arrive out of sequence because different routes might have been traversed.

SS7 does not run in non-associated mode because no procedures exist for reordering out-of-sequence messages. Associated and quasi-associated signaling modes assure sequential delivery, while non-associated signaling does not.

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Limitations of CCS: CSS has the following disadvantages in comparison to CAS:

CCS links can be a single point of failure—a single link can control thousands of voice circuits, so if a link fails and no alternative routes are found, thousands of calls could be lost.

There is no inherent testing of speech path by call set-up signaling, so elaborate Continuity Test procedures are required.

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Pre-SS7 system CCITT R1 (regional 1) C6 (CCITT Signaling System No. 6), also called SS6,

was the first system to employ Common Channel Signaling (CCS).

AT&T developed SS7/C7 in 1975, and the International Telegraph and Telephone Consultative Committee (CCITT) adopted it in 1980 as a worldwide standard.

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Signaling System 7 (SS7)

SS7/C7 is the protocol suite that is employed globally, across telecommunications networks, to provide signaling.

It is a packet-switched network, as well as a service platform. Being a signaling protocol, it provides the mechanisms to allow the telecommunication network elements to exchange control information.

SS7/C7 is the key enabler of the public switched telephone network (PSTN), the integrated services digital network (ISDN), intelligent networks (INs), and public land mobile networks (PLMNs).

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Each time a cellular phone is powered up, SS7/C7-based transactions identify, authenticate, and register the subscriber.

SS7/C7 network tracks the cellular subscriber to allow call delivery, as well as to allow a call that is already in progress to remain connected, even when the subscriber is mobile.

SS7/C7 is possibly the most important element from a quality of service (QoS) perspective, as perceived by the subscriber.

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SS7 based services: Telephone-marketing numbers such as toll-free and

free-phone Tele-voting (mass calling) Single Directory Number Supplementary services Calling name (CNAM) Local number portability (LNP) Cellular network mobility management and roaming

Short Message Service (SMS) Enhanced Messaging Service (EMS)— Ringtone, logo, and

cellular game delivery

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SS7/C7 is invested with Internet and other data-centric technologies to: Internet Call Waiting Internet Calling Name Services Click-to-Dial Applications Web-Browser-Based Telecommunication Services WLAN "Hotspot" Billing Location-Based Games

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SS7 Network Architecture

The worldwide signaling network has two functionally independent levels: International National

SS7 network nodes are called signaling points (SPs).

Each SP is addressed by an integer called a point code (PC). The international network uses a 14-bit PC. The national networks also use a 14-bit PC except

North America and China, which use an incompatible 24-bit PC.

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Node type: There are three different types of SP

Signal Transfer Point

Service Switching Point

Service Control Point

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Signaling transfer point (STP) A Signal Transfer Point (STP) is responsible for the

transfer of SS7 messages between other SS7 nodes, acting somewhat like a router in an IP network.

An STP is neither the ultimate source nor the destination for most signaling messages.

An STP can exist in one of two forms: Standalone STP: deployed in "mated" pairs for the purposes

of redundancy. Under normal operation, the mated pair shares the load. If one of the STPs fails or isolation occurs because of signaling link failure, the other STP takes the full load until the problem with its mate has been rectified.

Integrated STP (SP with STP): combine the functionality of an SSP and an STP. They are both the source and destination for MTP user traffic. They also can transfer incoming messages to other nodes. 20

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Service Switching Point A Service Switching Point (SSP) is a voice switch that

incorporates SS7 functionality. An SSP can originate and terminate messages, but it

cannot transfer them. If a message is received with a point code that does not match the point code of the receiving SSP, the message is discarded.

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Service Control Point A Service Control Point (SCP) acts as an interface

between telecommunications databases and the SS7 network.

Telephone companies and other telecommunication service providers employ a number of databases that can be queried for service data for the provision of services.

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Signaling Links and Linksets

Signaling links and linksets SPs are connected to each other by signaling links

over which signaling takes place. The bandwidth of a signaling link is normally 64

kilobits per second (kbps). To provide more bandwidth and/or for redundancy, up

to 16 links between two SPs can be used. A group of links between two SP is called a linkset.

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Route and Routeset

Route and Routeset

SS7 routes are statically provisioned at each SP. There are no mechanisms for route discovery.

A route is defined as a pre-provisioned path between source and destination for a particular relation.

All the pre-provisioned routes to a particular SP destination are called the routeset.

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ExampleSignaling a POTS Call

1. caller goes offhook, dials callee. SSP A decides to route call via SSP B. Assigns idle trunk A-B

A B

W

X

Y

2. SSP A formulates Initial Address Message (IAM), forwards to STP W

3. STP W forwards IAM to STP X

4. STP X forwards IAM SSP B

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5. B determines it serves callee, creates address completion message (ACM[A,B,trunk]), rings callee phone, sends ringing sound on trunk to A

A B

W

XY

Z7. SSP A receives ACM,

connects subscriber line to allocated A-B trunk (caller hears ringing)

6. ACM routed to Z to Y to A

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8. Callee goes off hook, B creates, sends answer message to A (ANM[A,B,trunk])

A B

W

XY

Z

10. SSP A receives ANM, checks caller is connected in both directions to trunk. Call is connected!

9. ANM routed to A

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Link Types

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A Link

An "A" (access) link connects a signaling end point (e.g., an SCP or SSP) to an STP. Only messages originating from or destined to the signaling end point are transmitted on an "A" link.

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C Link

A "C" (cross) link connects STPs performing identical functions into a mated pair. A "C" link is used only when an STP has no other route available to a destination signaling point due to link failure(s). Note: SCPs may also be deployed in pairs to improve reliability; unlike STPs however, mated SCPs are not interconnected by signaling links.

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B Link

A "B" (bridge) link connects one STP to another. Typically, a quad of "B" links interconnect peer (or primary) STPs (e.g., the STPs from one network to the STPs of another network).

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D Link

A "D" (diagonal) link connects a secondary (e.g., local or regional) STP pair to a primary (e.g., inter-network gateway) STP pair in a quad-link configuration. Secondary STPs within the same network are connected via a quad of "D" links.

The distinction between a "B" link and a "D" link is rather arbitrary. For this reason, such links may be referred to as "B/D" links.

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E Link

An "E" (extended) link connects an SSP to an alternate STP. "E" links provide an alternate signaling path if an SSP’s "home" STP cannot be reached via an "A" link. "E" links are not usually provisioned unless the benefit of a marginally higher degree of reliability justifies the added expense.

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F Link An "F" (fully associated) link connects two

signaling end points (i.e., SSPs and SCPs). "F“ links are not usually used in networks with STPs. In networks without STPs, "F" links directly connect signaling points.

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SS7 Protocol Overview

The number of possible protocol stack combinations is growing. The main protocols are: Message Transfer Parts

(MTP 1, 2, and 3) Signaling Connection

Control Part (SCCP) Transaction Capabilities

Application Part (TCAP) Telephony User Part (TUP) ISDN User Part (ISUP)

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The SS7 physical layer is called MTP level 1 (MTP1) The data link layer is called MTP level 2 (MTP2), The network layer is called MTP level 3 (MTP3).

Collectively they are called the Message Transfer Part (MTP).

The MTP transfers the signaling message, in the correct sequence, without loss or duplication.

The MTP provides reliable transfer and delivery of signaling messages.

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MTP2

MTP2 ensures reliable transfer of signaling messages. It encapsulates signaling messages into variable-length

SS7 packets. SS7 packets are called signal units (SUs). MTP2 provides signaling link error monitoring, error

correction by retransmission, and flow control. The MTP2 protocol is specific to narrowband links Physical interfaces defined include E-1 (2048 kb/s; 32 64

kb/s channels), DS-1 (1544 kb/s; 24 64 kp/s channels), V.35 (64 kb/s), DS- 0 (64 kb/s) and DS-0A (56 kb/s).

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MTP3 MTP3 performs two functions:

Signaling Message Handling (SMH) Delivers incoming messages to their intended User Part and routes outgoing messages toward their destination. MTP3 uses the PC to identify the correct node for message delivery. Each message has both an Origination Point Code (OPC) and a DPC. The OPC is inserted into messages at the MTP3 level to identify the SP that originated the message. The DPC is inserted to identify the address of the destination SP. Routing tables within an SS7 node are used to route messages.

Signaling Network Management (SNM): Monitors linksets and routesets, providing status to network nodes so that traffic can be rerouted when necessary. SNM also provides procedures to take corrective action when failures occur, providing a self-healing mechanism for the SS7 network.

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TUP and ISUP

TUP and ISUP sit on top of MTP to provide circuit-related signaling to set up, maintain, and tear down calls.

Both TUP and ISUP are used to perform interswitch call signaling.

ISUP also has inherent support for supplementary services, such as automatic callback.

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SCCP

SCCP provides a more flexible means of routing and provides mechanisms to transfer data over the SS7 network.

Such additional features are used to support noncircuit-related signaling, which is mostly used to interact with databases (SCPs). It is also used to connect the radiorelated components in cellular networks and for inter-SSP communication supporting CLASS services.

For example, in cellular networks, SCCP transfers queries and responses between the Visitor Location Register (VLR) and Home Location Register (HLR) databases.

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TCAP

TCAP allows applications (called subsystems) to communicate with each other (over the SS7 network) using agreed-upon data elements.

These data elements are called components. Components can be viewed as instructions sent

between applications. TCAP also provides transaction management,

allowing multiple messages to be associated with a particular communications exchange, known as a transaction.

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Protocol Standards

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MTP Level 2 (Data Link Layer)

The DATA-link level provides a reliable transfer of signaling messages between two directly connected signaling points over one individual signaling data link.

The link-level functions include: Delimiting of frames. Alignment of frames. Error detection.(Basic & PCR) Error correction by retransmission. Initial alignment of data link. Error monitoring and reporting. Link-flow control.

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MTP Level 2

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MTP Level 2 Frame Format

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F CK SIF SIO LI Control F

F CK SF LI Control F

F CK LI Control F

MSU (Message Signal Unit)

LSSU (Link Status Signal Unit)

FISU (Fill-In Signal Unit)

Level 3 user information

Network:

• National• International

User part:

• ISUP• SCCP

• Signalling network managementMSBLSB

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F

SPARE

SIFBIB

LICK BSN FFSNFIB

SIO

Direction of transmission

F

SPARE

SFBIB

LICK BSN FFSNFIB

F

SPARE

BIB

LICK BSN FFSNFIB

Format of Message Signal Unit (MSU)

Format of Link Status Signal Unit (LSSU)

Format of Fill-in Signal Unit (FISU)

F – Flag (8) SIF – Signaling Information Field (8n, n>2)CK – Checksum (16) SIO – Service Information Octet (8)LI – Length Indicator (6) FIB – Forward Indicator Bit (1)FSN – Forward Sequence Number (7) BIB – Backward Indicator Bit (1)BSN – Backward Sequence Number (7) SF – Status Field (8 or 16)

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MTP Level 2 Frames

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MSU (Message Signal Unit):Contains actual SS7 signalling messagesThe received frame is MSU if LI > 2(LI = number of octets)

LSSU (Link Status Signal Unit):Contains signalling messages for MTP level 2 (signalling link) supervisionThe received frame is LSSU if LI = 1 or 2

FISU (Fill-In Signal Unit):Can be used to monitor quality of signalling linkat receiving endThe received frame is FISU if LI = 0

••

•

•

•

•

MTP Level 2

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SP

Normal, No-error

SP

Error Control

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MTP Level 2

Length indicator

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LI = 0 indicates a FISU.

LI = 1 or 2 indicates an LSSU.

LI > 2 indicates a MSU

MTP Level 3 (Network Layer)

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MessageDistribution

MessageDiscrimination

MessageRouting

SignallingTraffic

Management

SignallingRoute

Management

SignallingLink

Management

Signalling Network Management

Signalling Message Handling

Signalling Network Functions

Testing & MaintenanceSignalling Message FlowIndications and Control

Level 2 MTP

Level 4User Parts

MTP Level 3LSSU

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MTP Level 3Aligning and Proving

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8.2 Sec-Normal Alignment0.5 Sec-Emergency Alignment

MTP Level 3SIO

In message signal units (MSUs), the service information octet (SIO) is used to perform message distribution. This octet is divided into a four-bit service indicator (SI) and a four-bit subservice field. This subservice field is further divided into a two-bit network-indicator code and two bits that are spare if the indicator code is 00 or 01, or are available for national use if the indicator code is 10 or 11.

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MTP Level 3Signaling Network Functions

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D C B A D C B A

Sub-service field (4 bits) Service indicator (4 bits)

Direction of transmission

Bit D Bit C Network Indicator

0

0

1

1

0

1

0

1

International Network

Not used

National network

Reserved for national use

Sub-service field = Network Indicator(Bits A & B are spare)

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D C B A Network Indicator (National /International Signaling Network)

0000000011111111

0000111100001111

0011001100110011

0101010101010101

Signaling network management messagesSignaling network testing and maintenance messagesSpareSCCPTelephone user partISDN User PartData User Part (call and circuit-related messages)Data User Part (facility registration and cancellation messages)Reserved for MTP Testing User PartBroadband ISDN User PartSatellite ISDN User Part

Spare

MTP Level 3Signaling Network Functions

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MTP Level 3Routing Labels

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SLS (4 bits) OPC (14 bits) DPC (14 bits)

SLS – Signaling Link Selection Direction of transmissionOPC – Originating Point CodeDPC – Destination Point CodeCIC - Circuit Identification code

CIC (12 bits)SIF (2-272 bytes)

MTP Level 3Routing Labels

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SS7 signalling messages contain MTP level 3 routing information in the form of a routing label:

SIO octet

DPC

DPC

LSBMSB

OPC

OPC

OPC SLS

Signalling message payload

International (and most national) signalling networks (ITU-T):

14-bit Destination Point Code (DPC)14-bit Originating Point Code (OPC)4-bit Signalling Link Selection (SLS)

North American national signalling network (ANSI):

24-bit DPC and OPC, 5-bit SLS code

Format for international SPC:

Zone Area/Network SP

3 bits 3 bits8 bits

For examples, see:www.numberingplans.comFor examples, see:www.numberingplans.com

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MTP Level 3Heading Codes-Network Management

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MTP Level 3Example of Changeover of an MSU Containing Changeover Message

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SS7 Application Layer

ISUP (ISDN User Part) The ISDN User Part is the Signaling System No. 7 protocol which

provides the signaling functions required to support basic bearer services and supplementary services for voice and non-voice applications in an integrated services digital network. The ISDN User Part is also suited for application in dedicated telephone and circuit switched data networks and in analogue and mixed analogue/digital networks.

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ISUP

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Message Hex Code Binary Code

Address Complete 06 00000110

Answer 09 00001001

Blocking 13 00010011

Blocking acknowledgement 15 00010101

Call progress 2C 00101100

Circuit group blocking 18 00011000

Circuit group blocking acknowledgement 1A 00011010

Circuit group query 2A 00101010

Circuit group query response 2B 00101011

Circuit group reset 17 00010111

Circuit group reset acknowledgement 29 00101001

Circuit group unblocking 19 00011001

Circuit group unblocking acknowledgement 1B 00011011

Charge information (National specific) 31 00110001

Confusion 2F 00101111

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ISUP

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Connect 07 00000111

Continuity 05 00000101

Continuity check request 11 00010001

Facility 33 00110011

Facility accepted 20 00100000

Facility reject 21 00100001

Facility request 1F 00011111

Forward transfer 08 00001000

Identification request 36 00110110

Identification response 37 00110111

Information 04 00000100

Information request 03 00000011

Initial address 01 00000001

Loop back acknowledgement 24 00100100

Network resource management 32 00110010

ISUP

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Overload 30 00110000

Pass-along 28 00101000

Release 0C 00001100

Release complete 10 00010000

Reset Circuit 12 00010010

Resume 0E 00001110

Segmentation 38 00111000

Subsequent address 2 00000010

Suspend 0D 00001101

Unblocking 14 00010100

Unblocking acknowledgement 16 00010110

Unequipped CIC 2E 00101110

User part available 35 00110101

User part test 34 00110100

User to User information 2D 00101101

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ISUP

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ISUP is a signalling application protocol that is used for establishing and releasing circuit-switched connections (calls).

Only for signalling between exchanges (ISUP can never be used between an exchange and a stand-alone database)

Not only for ISDN (=> ISUP is generally used in the PSTN)

•

•

Structure of ISUP message:

SIO (one octet)

Routing label (four octets)

CIC (two octets)

Message type (one octet) Mandatory fixed part

Mandatory variable part

Optional part

Must always be included in ISUP message

E.g., IAM message

E.g., contains called (user B) number in IAM message

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MTP Routing Label and Circuit Identification Code

Message TypeMandatory Parameter A

Mandatory Parameter FPointer to Parameter M

Pointer to Parameter P

Length Indicator of PParameter P

Pointer to Optional PartLength Indicator of M

Parameter M

Parameter name XLength Indicator of XOptional Parameter X

Parameter name ZLength Indicator Z

Optional Parameter ZEnd of Optional Part

Optional Part

MandatoryVariable Part

MandatoryFixed Part

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Format of IAM Message

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ISUP

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Address Complete Message (ACM). Sent in the

backward direction to indicate that all the required

address signals have been received.

Answer Message (ANM). Sent in the backward

direction to indicate that the call has been answered

and that metering or measurement of call duration can

start.

Call Progress Message (CPG). Sent in either direction

during the setup or active phase of the call, indicating

that an event has occurred which is of significance and

which should be relayed to the originating or

terminating access.

Initial Address Message (IAM). Sent in the forward

direction to initiate seizure of an outgoing circuit and

to transmit the number and other information related

to the routing and handling of the call.

Subsequent Address Message (SAM). Sent in the

forward direction to convey additional called-party

number information.

Release Message (REL). Sent to indicate that the

circuit is being released.

Release Complete Message (RLC). Sent in response

to a release message to indicate that the circuit has

been released and brought into the idle condition.

Charge Information Message (CIM). Sent for

accounting and/or charging purposes.

Confusion Message (CFN). Sent in response to any

message the exchange does not recognize.

Circuit Group Blocking Message (CGB). Sent to

cause an engaged condition for a group of circuits for

subsequent outgoing calls.

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ISUP

Basic ISUP signalling messages:

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Call setup:

IAM (Initial address message)

ACM (Address complete message)

ANM (Answer message)

From LE A to LE B

From LE B to LE A

Call release:

REL (Release message)

RLC (Release complete message)

Direction depends on releasing party (user A or user B)

ISUP

Difference between SLS and CIC The four-bit signalling link selection (SLS) field in the routing

label defines the signalling link which is used for transfer of the signalling information.

The 16-bit circuit identification code (CIC) contained in the ISUP message defines the TDM time slot or circuit with which the ISUP message is associated.

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Exchange

STP

Exchange

Circuit

Signalling link

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ISUP

Signaling using IAM message

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Exchange ExchangeExchange

SPC = 82

Circuit 14

SPC = 22 SPC = 60Circuit 20

STP

SL 4

SL 7

STP

Outgoing message:OPC = 82 CIC = 14DPC = 22 SLS = 4

Processing in (transit) exchange(s):Received IAM message contains B-number. Exchange performs number analysis (not part of ISUP) and selects new DPC (60) and CIC (20).

ISUP

Setup a call using ISUP

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LE A LE BTransit exchange User A User B

Setup IAMIAM

Setup

Alert

Connect

ACM

ANM

ACM

ANM

Alert

Connect

Charging of call starts now

Number analysisDSS1

signalling assumed

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Example: Call Setup

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User A User B

Off hook

Dial tone

B number

Local exchange detects setup request and returns dial tone

Local exchange:

analyzes B number

determines that call should be routed via transit exchange (TE)

LE A LE BTE

•

•

Example: Call Setup

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User A User BLE A LE BTE

Initial address message (IAM)

ISUP message IAM is sent to transit exchange (TE).

TE analyzes B number and determines that call should be routed to local exchange of user B (LE B).

IAM message is sent to LE B.

There now exists a circuit-switched path (the path is “cut through”) between user A and LE B.

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Example: Call Setup

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Address complete message (ACM)

Ringing signalRingback tone

Ringing signal is sent to user B (=> user B is alerted).

Ringback tone (or busy tone) is sent to user A.

(Ringback/busy tone is generated locally at LE A or is sent from LE B through circuit switched path.)

or

Example: Call Setup

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Answer message (ANM)User B answers

User B answers, connection is cut through at LE B.

Charging of the call starts when ISUP message ANM is received at LE A (the normal case).

The 64 kbit/s bi-directional circuit switched connection is now established.

Charging starts now

Conversation over this “pipe”

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Example: Call Setup

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00

0

358 9

9

1234567

1234567

1234567

International number

National number

User number

Prefix

Country code

Area code

358

9

In each exchange, the B number is analyzed at call setup (after the IAM message containing the number has been received) and a routing program (not part of ISUP) selects the next exchange to which the call is routed.

or mobile network code, e.g. 40

Example: Call Setup

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00 358 9 1234567

Prefix

For examples, see:www.numberingplans.comFor examples, see:www.numberingplans.com

Country code (1-3 digits)

National destination code (1-3 digits)

Max. 15 digits

Subscriber number

Area code, e.g. 9

Mobile network code, e.g. 40

MSISDN number

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Example: Call Release

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On hookRelease message (REL)

Release complete message (RLC)

The circuits between exchanges are released one by one.

(The generation of “hanging circuits” should be avoided, since these are blocked from further use.)

Charging stops

Conversation over this “pipe”

SS7 Signaling ISUP Message Flow between Exchanges

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User partISUP

Usermessage

MessageTransfer part

User partISUP

Usermessage

MessageTransfer partSignalling data link

MTPcomponent

Userinformation Address

MTPcomponent

User message

Speech circuits

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Example

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tel LS TS TS LS tel

Off hook

DT

Dialling1st digit Seizure

A5, A1Sequence

IAMIAM

ACMACM

ANM ANM

Ringing

Off hook

Answer

Metering pulse

A3

B6

Diallinglast digit

RBT

Conversation

Signaling Connection Control Part (SCCP)

SCCP is required when signalling information is carried between exchanges and databases in the network.

An important task of SCCP is global title translation (GTT):

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STP DatabaseExchange

STP with GTT capability

Exchange knows the global title (e.g. 0800 number or IMSI number in a mobile network) but does not know the DPC of the database related to this global title.

1.

SCCP performs global title translation in the STP (0800 or IMSI number => DPC) and the SCCP message can now be routed to the database.

2.

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Signaling Connection Control Part (SCCP)

Global title translation (GTT) is usually done in an STP.

Advantage: Advanced routing functionality (= GTT) needed only in a few STPs with large packet handling capacity, instead of many exchanges.

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Exchange

STP

Database

Exchange

Exchange

ExchangeExchange

Database

Exchange

SCCPExample: SCCP Usage in Mobile Call

Mobile switching center (MSC) needs to contact the home location register (HLR) of a mobile user identified by his/her International Mobile Subscriber Identity (IMSI) number.

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SCCPSCCP

MSC located in Espoo HLR located in Oslo

STP

SPC = 82 SPC = 99

SPC = 32

SCCP/GTT functionality

Outgoing message:OPC = 82 DPC = 32SCCP: IMSI global title

Processing in STP:Received message is given to SCCP for GTT. SCCP finds the DPC of the HLR: DPC = 99

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SCCP

SCCP services are divided into 2 groups:

Connection-oriented services

Connectionless services

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SCCP

SCCP connectionless services: Class 0: Basic Connectionless Class.

Data are transported independently of each other and may therefore be delivered out of sequence. This corresponds to a pure connectionless network service.

Class 1:Class 1: Sequenced Connectionless Class.

In protocol class 1 the features of class 0 are complemented by a sequence control. By use of the signaling link selection field, the same link is selected for all messages in one call. This secures sequence control and is identical to the standard service provided by the MTP to the user parts.

The connectionless protocol classes 0 and 1 provide functions necessary to transfer one network service data unit (NSDU). The maximum length of an NSDU is restricted to 32 octets in the international network and 256 octets in the national network.

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SCCP

Class 2: Basic Connection-oriented Class. In protocol class 2, bi-directional transfer of NSDUs is done by

setting up a temporary or permanent signaling connection. This corresponds to a simple connection-oriented network service.

Class 3: Flow Control Connection-oriented Class.

In protocol class 3, the features of protocol class 2 are complemented by the inclusion of flow control, with its associated capability of expedited data transfer. Moreover, an additional capability of detecting message loss and mis-sequencing is included. In such circumstances, the signaling connection is reset and a corresponding notification is given by the SCCP to the higher layers.

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SCCPConnection-oriented Data Transfer

Data is being transferred only after a Virtual signaling connection is made Between the Source Node to the destination Node

Example: BSC to MSC connectivity for MAP messages

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SCCPConnectionless Data Transfer

No Virtual Signaling connection is made the UDT messages contains the actual data.

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SCCPFormat of SCCP Message

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Format of a CgPA and CdPA in the SCCP.

Transaction Capabilities Allocation Part (TCAP)

The overall objective of the ITU-T specified transaction capabilities application part (TCAP) is to provide means for the transfer of information between nodes (exchanges and/or service centers), and to provide generic services to applications (distributed over the exchanges and service centers)

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Transaction Capabilities Allocation Part (TCAP)

98Relation between INAP, OMAP, GSM, MAP, TCAP and the ISO OSI model

TCAP Sub-layers

Component sub-layer deals with components that are the application protocol data units (APDU) which convey remote operations and their responses.

Transaction sub-layer deals with the exchange of messages containing components and, optionally, a dialogue portion between two TC users.

TCAP is normally used to make a session with the DATA BSE entities Like HLR, VLR, SCP, etc. thru which the MAP/INAP can speak to the DB

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Intelligent Network (INAP)

Architecture

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Mobile Application (MAP)

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2b. circuit switching - ss7

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