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What is Signalling

In technical systems, it refers to the control of different procedures.

With reference to telephony, signalling means the transfer of information relevant to control and monitor telephone connections.

Being operating all the time and being vital for proper operation makes it nervous system in the human body.

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The main purpose of using signalling in modern telecom networks- where different n/w nodes must cooperate and communicate with each other- is to enable transfer of control information between nodes in connection with:

Traffic control procedures as set-up, supervision and release of telecom. Connections and services.

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Database communication, for ex: database queries concerning specific services, roamimg in cellular networks, etc.

Network management procedures, for Ex: blocking, de-blocking trunks.

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Signalling can be divided into two types:

1. Access signalling ( Sub. Signalling )

2. Trunk signalling ( Inter-Exge signalling )

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Signalling types

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Types of access signaling

1. PSTN analogue subscriber line signaling,

2. ISDN Digital Subscriber Signaling System (DSS1),

3. Signaling between the MS and the network in the GSM system

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Access signalling

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Inter-exchange Signaling

The Inter-exchange Signaling information is usually transported on one ofthe time slots in a PCM link, either in association with the speech channelor independently.

There are two commonly used methods for Inter Exchange Signaling:

1. Channel Associated Signaling (CAS)

2. Common Channel Signaling (CCS)

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Channel Associated Signalling (CAS)

The signalling is always sent on the same PCMlink as the traffic.The signalling is associated with the traffic channel.

In an E1 PCM link, 30 time slots are used for speech where TS0 is used for synchronization and TS 16 is used for the line signalling.

Two traffic channels share TS 16 in one frame. However,Register Signalling is carried over the traffic channel

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Channel Associated Signaling (CAS)

Means that the signaling is always sent on the same PCM link as the traffic.

The signaling is associated with the traffic channel.

In an E1 PCM link, 30 time slots are used for speech where TS0 is used for synchronization and TS 16 is used for the line signaling

All 30 traffic connections share TS 16 in a multi-frame consisting of 16 consecutive frames.

On TS 16, each traffic channel has a permanently allocated recurring location for LineSignaling, where two traffic channels share TS 16 in one frame.

However, Register Signaling is carried over the traffic channel.

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LINE SIGNALSIDLE

SEIZURE

SEIZURE ACKNOWLEDGE

B-ANSWER

CLEAR BACK

CLEAR FORWARD

RELEASE GUARD

BLOCKING

UNBLOCKING

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REGISTER SIGNALS

B-NUMBER

A-CATEGORY

A-NUMBER

END-OF-PULSING

CONTROL SIGNALS

B-LINE STATUS

Etc.

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Common Channel Signaling

In Common Channel Signaling, signaling messages are transmitted in the form of data packets over time slots in a PCM link reserved for the purpose of signaling, instead of Line Signals and Register Signals used for Channel Associated Signaling. The system is designed to use a common data channel named signaling link as the carrier of all signals, required by up to 4096 traffic channels.

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CCS

In 1968, CCITT specified a Common Channel Signaling system called CCS System No. 6, which was designed especially for international analogue telephony networks. However, very few installations of this system remain today. It has, as already mentioned,been replaced by Signaling System No. 7. The first version of Signaling System No. 7 (1980) was designed for telephony and data

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In the 80’s the demand for new services dramatically increased and the SS7 was therefore developed to meet the signaling requirements, specified for all these new services.Today SS7 is used by many different networks and related services, typically within PSTN, ISDN, PLMN, and IN all over the world.

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Application Layer

This layer provides services for support of the user’s application process and for control ofall communication between applications. Examples of layer 7 functions are file transfer, message handling, directory services, and operation and maintenance.

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Presentation Layer

This layer defines how data is to be represented, that is, the syntax. The presentation layertransforms the syntax used in the application into the common syntax needed for thecommunication between applications. Layer 6 contains data compression

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Session Layer

This layer establishes connections between presentation layers in different systems. It alsocontrols the connection, the synchronization and the disconnection of the dialogue. It allowsthe presentation layer to determine checkpoints, from which the retransmission will startwhen the data transmission has been interrupted

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Transport Layer

This layer guarantees that the bearer service has the quality required by the application inquestion. Examples of functions are error detection and correction (end-to-end), and flowcontrol. The transport layer optimizes the data communication, for example by multiplexingor splitting data streams before they reach the network

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Network Layer

The Network Layer establishes, maintains, and releases connections between the nodes inthe network and handles addressing and routing of circuits..Open System Interconnection ModelPDF created with FinePrint pdfFactory Pro trial versionhttp://www.fineprint.com

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Data Link Layer

This layer provides error free point to point circuit between network layers. The layercontains resources for error detection, error correction, flow control, and retransmission.

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Physical Layer

This layer provides mechanical, electrical, functional, and procedural resources for activating, maintaining, and blocking physical circuits for the transmission of bits between data link layers. The physical layer contains functions for converting data into signalscompatible with the transmission medium.

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CCITT SS No. 7 is intended primarily for digital networks, both national and international, where the high transmission rates (64 kbps) can be exploited. It may also be used on analogue lines especially on international trunks (CCITT SS No 6).

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CCS was initially meant for telephony only, but has now evolved into non-telephony and non-connection related applications (Location updating of a mobile subscriber). A dialogue with a database or between two databases is a typical application for CCS in GSM. Thus, there is a need for a generic system that is able to support a wide variety of applicationsin telecommunication. 

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The variety of applications is increasing as new types of telephonysystems and a wider use of databases in the network become necessary (mobile telephonynetworks, ISDN, IN, etc.)

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Layered Model in GSM Application

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Signaling Point (SP)

It is the switching or processing node in a signaling network where the functionsof the SS 7 are implemented. The node may have the applicationfunctionality, for example, the MSC in GSM or transit exchange in the PSTN. Every SP is identified by a number determined by the Network Indicator (NI) and Signaling Point Code (SPC).

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The network indicator provides discrimination between international and nationalmessages or discrimination between two national signaling networks.The Signaling Point Code (SPC) uniquely identifies an SP within the signaling network

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Signaling Link (SL)

The packet data link that connects two SPs is a Signaling Link (SL).It is not necessary to have a Signaling Link (SL) in each PCM line.According to the maximum load, there may be more than one SL between two SPs depending on the network structure. One SL can serve up to 4096 connections.Link Set (LS)A number of parallel Signaling Links (SLs) connecting the same Signaling Points(SPs) is referred to as a signaling Link Set (LS)

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Originating Point (OP)

The Signaling Point (SP) at which the signaling message is generated. It is identified by an Originating Point Code (OPC).Destination Point (DP)The Signaling Point (SP) to which the signaling message is destined. It is identified by a Destination Point Code (DPC)Note that OP/OPC and DP/DPC relate to MTPmessages. 

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Signaling Transfer Point (STP)

This is a Signaling Point (SP) that is able to route the signaling messages.In GSM, every SP is an STP as soon as it routes the signaling message that must be delivered to a different destination point. In this case only the MTP is used. The upper levels are not involved

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Signaling Route (SR)The predetermined path a message takes through the signaling network between the Origination Point (OP) and the Destination Point (DP) is called a Signaling Route(SR). A Signaling Route is defined as a link set which is assigned to carry traffic to a particular destination. A link set may carry several signaling routes and hence convey traffic to several destinations

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Signaling Route Set (SRS)

The signaling network groups all Signaling Routes (SRs) that maybe used for message traversing between an Origination Point (OP) and a Destination Point (DP), and refers to them as a Signaling Route Set (SRS) for that signaling relation

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Signalling Modes

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MTP Functions

Ensures that incoming messages are either distributed to a User Part in its own node or routed and forwarded to the next SP/STPEnsures that outgoing messages are directed to the appropriate signaling linkControls and manages all functions that are needed on each signaling link to providereliable transfer

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Level 3 Signalling N/W Functions

1. Signalling message handling:

a. Message discrimination

b. Message Routing

c. Message distribution

3. Signalling N/W management:

a. signalling link management

b. Network Control

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Level 2 Signalling Link Functions

Signal unit delimitation Initial alignment

Signal unit alignment LSSU messages

Error detection Congestion ind. To

Acknowledgements Level 3

Sequence Numbers Link monitoring

Error correction

Buffer functions

Signalling terminal

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Level 1 Signalling Data Link

Interface to and from swg. & Transn. Equip.

Typically 64 Kbps digital signalling chl.

Other types of links, for Ex. Analogue bit rate 4.8 kbps or higher.

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signaling data link

A signaling data link is a bi-directional transmission path for signaling, that is, two datachannels working together in opposite directions at the same bitrate.Usually, one channel of a PCM system is used as a signaling datalink.CCITT recommends time slot 16, but states that if channel 16 is unavailable, any of the channels1-31 may be used.The bit rate of the transmission is thus 64 kb/s

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Signalling UnitEach frame must contain all information to reach the destination point and to invoke the desired reaction, which is the actual reason for sending the signaling message. A signal unit consists of several fields. Each field contains a certain number of bits representing specific information.The user signaling message is carried in a data field called theSignaling Information Field (SIF)within the signal unit frame.

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Types of SU’s

1. MSU

2. LSSU

3. FiSU

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MSU

To carry the signal Information (or data units) between UP’sThe MSU is retransmitted when an error is detected. In addition, there are MSUs that are used for signaling network management and signaling network testing and maintenance. 

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MSU

However, these are not sent to a user part, but stay in the MTP level 3.Minimum Length of this Signal Unit is 9 Octets Maximum Length of this Signal Unit is 279 Octets

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LSSU

A Status Field (SF) contains one or two octets and is generated by the signaling terminal.Minimum Length of this Signal Unit is 7 OctetsMaximum Length of this Signal Unit is 8 Octets

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FISU

Fill-In Signal Unit (FISU) is used for error supervision of the link and to keep the link runningwhen there are no MSUs to be sent.Length of this Signal Unit is 6 Octets

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Flag

Flag “01111110”

To ensure that the bit pattern cannot be imitated elsewhere in the signal unit, bit stuffing isused. Bit stuffing means adding an extra zero after five consecutive ones in the message.These extra zeroes are deleted in the signaling terminal at the receiving end

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EC

Backward Sequence Number (BSN)7 bits reserved for sequence numbers from 0 to 127. used to acknowledge the correct transmission of a signal unit.Backward Indicator Bit (BIB) The BIB (1 bit) marks the signal unit as:²Positive acknowledged if the logical value of the BIB bit is the same as that received in the latest signal unit. Negative acknowledged if the value of BIB is not equal to the value in thelatest received signal uni

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ECFSN is the 7-bit field for sequence numbers from 0 to 127.The FSN is used to recognize the signal units, which have been received “out ofsequence”.Forward Indicator Bit (FIB) If the logical value of the FIB is equal to the one in the previous signal unit, the receiver is informed that the signal unit is sent for the first time.If the logical value of FIB is not equal to the one in the previous signal unit, the receiver is informed that it is a repetition of a previously sent signal unit

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LI

Thje LI octate consists of 6 bits used to indicate the No. of Octates in the fields between the LI & CK. 2 bits kept as spare.

If LI = 0 FISU

LI = 1 or 2 LSSU

LI >2 MSU

For MSU, SIF has 64 or more octates, LI is set to 63.

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CK

The error detection function is performed by means of 16 check bits (CK), provided at the end of each signal unit. The checksum (check bits) is generated by the transmitting signaling terminal by means of aspecified algorithm. On the receiving signaling terminal the same algorithm is used to calculate the checksum. Thischecksum is then compared with the received checksum

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SIO

Network Indicator:

00 International N/w

01 spare ( reserved for international use)

10 National N/w

11 Reserved for national use

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SIO

Service Indicator:

0000 – Sig. N/w mgmt Message

0001 – Sig. N/w Testing & Mtce Mrssage

0010 – spare 0011 – SCCP message.

0100 – TUP message 0101 – ISUP message

0110 – DUP Call related message

0111 – DUP Facility Regn. & Cancellation

1000 – Reserved for MTP testing UP

1001 - Broadband ISDN UP 1010 – Satelite ISDN UP

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SIO

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SIF

The Signal Information Field (SIF) only exists in the MSU and contains the actual user informationand the routing label for MTP level 3. The format and codes of the signaling information field are defined for each user part

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SIF

User information The user information is generally the telephony messages used for connections and other tasks in the SS7 network.Routing label The structure of the label is separately defined in each User Part (UP) and it maylook somehow different depending on the user of the MTP (TUP,ISUP or ManagementMessages).

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LSSU

Status Indication:

000 – Out of Alignment SIO

001 – Normal alignment SIN

010 – Emergency alignment SIE

011 – out of service. SIOS

100 – Processor outage SIPO

101 – BUSY SIB

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Signalling message Handling

MTP Management messages Label type A

TUP

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ISUP TYPE-C

SCCP TYPE D

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Load SharingIf two or more link sets have the same priority, a load sharing is performed between the linksets. After link set selection, a signaling link belonging to this link set is chosen. If there are two or more active signaling links within the link set, a load sharing between the signaling links is performed. Load sharing is applied to distribute the signaling traffic over several signaling links and linksets. The information in the SLS field in the routing label is used to control load sharing.

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SCCP

The Signaling Connection Control Part (SCCP) is added as a functional layer above the MessageTransfer Part in order to meet a number of new requirements, especially concerning the transfer functions, that have emerged after the introduction of the first set of SS7specifications. The SCCP provides transfer capability for non-circuit-related signaling, but can also handle circuit-related signaling

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SCCP

Another purpose of the SCCP is to cater for both connectionless (CL) and connection-oriented(CO) network services

The SCCP also extends addressing capabilities of the SS7 network. Global, flexible addressing makes it possible to send a message between two nodes, and no layer above the SCCP is needed to handle the message.The full OSI Layer 3 functions, offered by the SCCP, make it possible to send messages across many network nodes and countries

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ISUP Basic services

SpeechFax3.1 kHz audio64 kb/s preferred64 kb/s unrestrictedAccess delivery informationSuspend and resumeFallback capabilityetc. .com

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ISUP Supplementary services

Call forwarding on busyCall forwarding on no replyCall waitingCalling line identity presentationCalling line identity restriction Closed user groupConference callingUser-to-user signaling,etc

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Subsystem Number

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SSN’s

SSN0  unknown, not used SSN1  SCCP management messagesSSN6  MAP in the HLRSSN7  MAP in the VLRSSN8  MAP in the MSCSSN9  MAP in the EIRSSN10  MAP in the AUCSSN11  Supplementary services in ISUPSSN12  SMSCSSN146  CAP (CAMEL Application Part)

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SSN149  MAP in the SGSN (GRPS)SSN150  MAP in the GGSN (GPRS)SSN224  Redundant HLRSSN 251 SSF in the IN networkSSN252  SCF in the IN networkSSN253  FNRSSN254  BSSAP

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SCCP in Switching system

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SCCP in the Base Station system

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TCAP

The Transaction Capabilities Application Part (TCAP) is the part of the TC concept which provides the application layer (according to the OSI model) in a certain node with services and protocols for the dialogue with another corresponding node.Functionally, TC resides above the Network Service Part (NSP), that is, the SCCP and MTP.InGSM TC, functions are utilized by MAP and IN  protocols

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Common protocol fns.replaced by TCAP

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TCAP

The overall objective of Transaction Capabilities is to provide the means of transferring information between nodes, and to provide generic services to applications. The main purpose of Transaction Capabilities is to provide support for interactive applicationsin a distributed environment. One example is the communication within the Switching System part of the GSM network, forexample, between the HLR and the MSC/VLR. TC serves as an interface between the applications (TC-Users) and the SCCP. It collects messages from different TC-Users and transfers them using the transportfunctions of the Network Service Part (NSP)

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TC provides an association between the local andthe remote TC.This association is called a transaction.The TC-user data related to its operations andresults are called components. TC receives data from the TC-User, packs it andhands it over to the SCCP. The SCCP adds somemore information that is necessary for routing the message to the destination through the signaling network, and gives it to the MTP.The MTP puts the data into the Signaling Information Field (SIF). The MTP prepares the Message Signal Unit (MSU)and transmits on a Signaling Link (SL)

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TCAP messagesThere are five TCAP messages: BeginContinueEndAbortUnidirectionalThe first four messages are used in structured dialogues, the last messages are used to transfer TC-user data of unstructured dialogues

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MAPThe Mobile Application Part (MAP) protocol is specially designed to support GSM signaling requirements. In the OSI model, the MAP resides above the TCAP.

The MAP protocol is installed in GSM Switching System nodes which can communicate incase of:Location registration and cancellationHandling, management, retrieval, and deregistration of subscriber services and their dataHandoversTransfer of security and authentication data

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Mobility services

location management services (location update, etc.)  handoverand access management services (handover preparation and execution, mobile signaling forwarding, etc.), authentication and security management services (triplets retrieval, IMEIcheck, etc.), subscriber data and information management, and fault recovery services. Call handling services HLR interrogation and roaming number provision belong to this group

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Supplementary servicesServices for registration, erasure, activation, deactivation, and interrogation of supplementary services and also Password management, unstructured supplementary services, etc

SMS management HLR interrogation, SMS transfer, MS status reporting, and Service Center alerting are included.

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Signalling in ISUP