02 Tw2101eu01_eg0001 Structure of CCS7

8/3/2019 02 Tw2101eu01_eg0001 Structure of CCS7

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Structure of SS7 Siemens

Objectives

The participant is able to

describe the architecture of SS7 describe the 4-level model describe the 7-layer model explain how the 7-layer model is applied to SS7

Contents

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Structure of SS7

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Siemens

Structure of SS7

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1 Architecture

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Structure of SS7

Despite their complexity, modern signaling systems have to be flexible enough to allow the easymanagement of new and enhanced services. For this reason a structured architecture was chosen todescribe these systems. Not only does structuring facilitate the description and implementation ofthe system, it also increases their flexibility for future enhancements.

The requirements from such a structure are diverse. The following list covers no more than a fewaspects.

Inexpensive integration of new applications must be possible.

Interworking with the preceding version must be assured.

It must be possible to improve individual components without having to modify all the othercomponents at the same time.

The signaling system is expected to support a range of applications, e.g. telephony, ISDN,mobile network as well as network management, in both the national and internationalnetworks.

The specifications for the individual components must be so exact that different manufacturerscan develop and market such signaling systems, whereby communication capability betweensignaling points made by different manufacturers must be guaranteed.

These requirements made it expedient to describe modern common channel signaling systems onthe basis of entities.

Let us now consider two exchanges that want to exchange information with each other. Thesignaling function should be implemented as entities A, B and C. Two entities within one nodecan communicate with each other (vertical communication) by means of Service Primitives(information units that were specified for this purpose). A single entity can be optimized andmodified without influencing the others. In combination with the immediately inferior entity in the

vertical hierarchy, each entity offers to the immediately superior entity a service which the higher-ranking entity can use.

In our example the entity C can use the functions of entities A and B.

Apart from this inner-exchange communication, inter-node communication is possible too.However, inter-node communication is possible only between entities of the same layer(horizontal communication).

Entities conducting inter-node communication are termed "peer entities". The appearance of themessages used for communication between peer entities is known as the "format". The logicalsequence of the messages exchanged by two peer entities is described as the "procedure" and isstandardized likewise.

The description of the

service primitives

formats and

procedures

is termed the protocol.

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Entity

C

Entity

B

EntityA

"Primitives"

Exchange A

Entity

C

Entity

B

EntityA

"Primitives"

Exchange B

Logical

connectionrotocol

Physical

transmission path

Fig. 1 Communication between entities

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Structure of SS7

Let us assume that Exchange A needs to set up a connection to ExchangeB.

Entity A is responsible for detecting the call request, and therefore

compiles the relevant information (message) and forwards it to Entity Bwith a service primitive. Entity B is responsible for routing, and thereforeadds the necessary routing information to the message and forwards theresultant message to Entity C. Entity C is responsible for path selection,and therefore performs its task and causes the message to be transferredto Exchange B. When the message is received by Exchange B its entitiesrepeat the functions performed in Exchange A.

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Entity

A

Entity

B

Entity

C

Exchange A

Entity

A

Entity

B

Entity

C

Exchange B

C B A Inform.

A Inform. A Inform.

B A Inform. B A Inform.

Fig. 2 Example

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Structure of SS7

An architecture model of the type shown on the previous page can beused to describe any mode of common channel signaling.

Only circuit-related applications existed when the architecture model for

SS7 was specified. The resultant model comprised 4 levels (ITU-TRecommendation Q.701). Some time after this structure was specified,the International Standards Organization (ISO) specified the 7-layer OpenSystems Interconnection Model (OSI model).

The OSI model comprises 7 layers, whereas the SS7 model has merely 4levels. The correlation and mapping between the two models will beexamined more closely later in this description.

The SS7 signaling system has meanwhile been extended to include noncircuit-related applications. The original 4-level model is not suitable forsuch applications, however, meaning both models are now used for SS7.

In other words: some applications use the 4-level model, while others usethe 7-layer model.

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.1.1 4-Level Model

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Structure of SS7

The 4-level model can be used in national and international networks. It isvery flexible, supports a large number of applications and performancefeatures, and has been optimized for usage in a digital environment.

However, it must be emphasized that the 4-level model is only suitable fordescribing circuit-related applications.

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Level 1

Level 1 (signaling data link) defines the physical, electrical and functional attributes of asignaling data link as well as the access equipment. The incorporation of these attributes in

Level 1 has the major advantage of making the Levels 2-4 independent of the transmissionmedium. Level 1 represents the carrier for a signaling link. Normally, 64-kbit/s channels areused as signaling data links in a digital network. Alternatively, analog channels (preferablywith bit rate 4.8 kbit/s) can be used via modem as signaling data link.

Level 2

Level 2 (signaling link) defines the function and procedure for error-free signaling messageexchange between two adjacent signaling points via a signaling link. This definition includesthe control of signaling links in the event of a failure, and the monitoring of signaling links.

The Level-2 functions can be optimized separately without an automatic adaptationrequirement for the adjacent Levels 1 and 3. However, this rule applies only if nomodification of the interfaces takes place.

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Structure of SS7

Level 3

Level 3 (signaling network) defines functions that are valid for several signaling links. Adistinction is made between the following two functional groups:

message handling, i.e. directing messages to the required signaling link or, as applicable,to the correct user part.

signaling network management, i.e. controlling message traffic by measures such aschangeover of signaling links in the case of a failure and restoring normal operationfollowing repair.

The various Level-3 functions can interoperate with each other, with functions belonging toother levels, and with the equivalent functions in other signaling points.

Level 4

Level 4 contains the User Part (UP). The UP defines the message format as well as thefunctional procedures for specific applications (e.g. call setup for an ISDN user part call).Level 4 therefore includes call control functions. All Level-4 user parts can use Levels 1-3 asa standardized option for error-free message transfer. ITU-T have so far defined the followinguser parts:

Data User Part (DUP)

Telephone User Part (TUP) and

ISDN User Part (ISUP).

Should these specified user parts prove inadequate for future requirements, or if new user

parts become necessary, it will easily be possible to adapt the existing user parts or specifynew ones as long as the lower levels are not influenced.

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ISDN-UP,

Level 4

DATA-UP,

Level 4

TUP,

Level 4

Si nalin network functions, level 3

Si nalin link functions, level 2

Ph sical and electrical attributes, level 1

Examples

ofuser parts

Message

transfer art(MTP)

Fig. 3 SS7 levels

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Structure of SS7

Message transfer part (MTP)

The message transport part includes the levels 1-3 and in SS7 is used by all user parts as thetransport system for message exchange. A user part hands over to the message transfer part

the messages requiring respectively to be transmitted to another user part. The messagetransfer part guarantees that the messages reach the addressed user part with no informationloss, no duplication, no changes in the message sequence and no bit errors. The MTP isspecified in ITU-T.

Fig. 4 shows the exchange of a user message between signaling points A and B.

The call control function in Exchange A wants to set up a call, for example, to Exchange B,and therefore prompts Level 4 to generate the appropriate message. Level 4 then uses theMTP to execute the error-free transfer of the message to Exchange B. On receiving themessage, Exchange B checks whether the message is error-free and if so forwards it to its

own Level 4. Level 4 passes the message as an event to the call control function, whichperforms the requested action and generates an appropriate response.

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User part

(e.g. ISDN-UP)

Signalingpoint A

User

message

Message

transfer part

Call control

CircuitsUser part

(e.g. ISDN-UP)

Signalingpoint B

User

message

Message

transfer part

Call control

User message

Signalingdatalink

Level 4Level 4

Level 3

2

1

Level 3

2

1

Fig. 4 Transmission of user message

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Structure of SS7

In this example one circuit connects Exchanges A and B directly, but thesignaling message must follow the route via signaling transfer point C.The signaling transfer function includes the task of determining whether

or not the message is addressed to the own exchange. After determiningthat the message is not addressed to its own exchange, Level 3 selects asignaling link that will transport the message to the required signalingpoint (destination), which is B in our example. Thus, Level 4 in thesignaling transfer point does not become involved.

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U s e r p a r t

( e . g . I S D N - U P )

S i g n a l i n g p o i n t A

U s e r m e s s a g e

M T P

C a l l c o n t r o l

C i r c u i t sU s e r p a r t

( e . g . I S D N - U P )

S i g n a l i n g p o i n t B

U s e r m e s s a g e

M T P

C a l l c o n t r o l

S i g n a l i n g

d a t a l i n k

L e v e l 4

L e v e l 3

2

1

L e v e l 4

L e v e l 3

2

1

S i g n a l in g

t r a n s f e r p o i n t C

M T PL e v e l 32

1

S i g n a l i n g

d a t a l in k

Fig. 5 Message exchange via a signaling transfer point

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Structure of SS7

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2 7-Layer Model

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Structure of SS7

The structural concept of the 7-layer model resembles that of the 4-levelmodel. However, the functions are distributed over 7 layers thatcomplement each other from 1 to 7 onward. Each layer provides (if

necessary supported by lower layers) specific services for theimmediately superior layer. This rule applies from Layer 1 to Layer 7without exception. The functions of the lower layers are preconditions forthe functions of the higher layers. Like communication between levels,inter-layer communication takes place with primitives. The primitivesRequest, Indication, Response and Confirmation were used for the OSImodel. CCITT / ITU Recommendation X200 describes the functions ofLayers 1 to 7. A brief description of the main functions of the layers isgiven below.

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Communication

functions

7

6

5

4

3

2

1

ApplicationLayer

PresentationLayer

SessionLayer

Transport Layer

NetworkLayer

DataLinkLayer

Physical Layer

OSI referencemodel

Fig. 6 7-layer model

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Structure of SS7

Layer 1 (physical layer)

The physical layer provides the unprotected transfer of a stream of information units (bits) viathe physical transmission medium. This layer is concerned with issues such as the electrical

matching of terminal equipment to transmission devices and medium. A transmissionmedium - a cable, for instance - must exist so that the functions of this layer can be executed.However, the cable is not part of the 7-layer model.

Layer 2 (data link layer)

The data link layer converts unprotected transmission links into protected transmission links.This layer defines how the protected transmission of a message can be effected on a part-route.

Layer 3 (network layer)

The network layer connects (protected) part-routes

from the first end system to the transit system

between transit systems and

from the transit system to the second end system.

The manner in which a network connection is established and structured along the entiretransmission path is defined in this layer, i.e. the way in which the part-routes are joined up.Layer 3 makes a distinction between two modes of communication:

connection-oriented and

connectionless.

In the case of connection-oriented communication we need a relation between the two usersthat assures the transfer will be coordinated. In the case of the connectionless mode thisservice cannot be guaranteed by the network layer. If required in connectionless mode,therefore, the service must be handled by a higher layer.

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Layer 4 (transport layer)

The transport layer creates, controls and terminates the transport connections leading fromone partner to the other. The manner in which the transport connection, which is known also

as the logical connection, is established is defined in this layer, and also the manner in whichthe transport connection is monitored and cleared down. An imaginary end-to-end connectionfrom message source to message sink is described here as transport connection.

Layer 5 (session layer)

The session layer provides functions required to open a communication relation (termed"session"), and to conduct and terminate this session in an orderly manner. The manner inwhich communication must be handled in a session is defined in this layer. Such a definitionspecifies whether both parties can exchange information simultaneously or only alternately,for example, or even if only unidirectional communication is permitted. The definition also

specifies how a session will be opened, controlled and monitored, as well as the transportconnections to be allocated to the session.

Layer 6 (presentation layer)

The presentation layer defines the rules relating to the manner of exchanging and presentinginformation in a common language. A neutral form of data is generated in this layer, i.e.

parties dispatch information they wish to exchange in their respective language and theinformation is converted into the neutral language of the communications system. If thereceiving party understands the neutral language, no translation is required on its side.

Layer 7 (application layer)

The manner in which two communication partners interoperate to implement an application isdefined in this layer. Many different application layer protocols exist for the variousapplications that can be implemented per telecommunication.

Two entities belonging to the same layer communicate according to a common protocol.

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Structure of SS7

The Fig. 7 shows a connection between a person at a teletypewriter and a process in a dataprocessing system. The users are the person and the process. In the diagram they are

connected by a line. The terminal units are the teletypewriter and the data processing system.Entities belonging to the transmitting terminal unit add protocol elements "P" to theinformation requiring to be transmitted. The entities belonging to the receiving terminal unitevaluate every protocol element before removing them.

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Structure of SS7

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3 Application to SS7 of the 7-

Layer Model

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Structure of SS7

The original OSI model is specified in such a way that the users arecomputers which use the telecommunications network to transmit data. Inthis case Layers 1-3 are serviced by the communications network, while

Layers 4-7 are serviced by the user. From the viewpoint of SS7, however,the insufficient exactness of this organization makes it unsuitable for thenon-circuit-related SS7 applications. For this reason, all 7 layers in themodel are now applied inside the exchange. In consequence, thecustomer regards the network as offering Layers 1-3 only, but in fact all 7layers are applied between the actual exchanges. If one now applies the7-layer model to SS7 employing non-circuit-related applications, it isevident that the message transfer part (Levels 1-3) alone cannotimplement all functions of OSI Layer 3. This deficiency made it necessaryto introduce a Signaling Connection Control Part (SCCP) which issupplementary to the MTP. The SCCP allows SS7 to offer the full

application layer functions in conformity with the 7-layer model.

For the circuit-related user parts the respective user part includes those parts of Layer 3which are missing still as well as the missing Layers 4-7.

Fornon-circuit-related applications the Layers 1-3 are defined with MTP and SCCP. Thelayers 4-7 are defined as Transaction Capabilities (TC). These transaction capabilities aresubdivided into two sub-units: the Transaction Capacity Application Part (TCAP) and theIntermediate Service Part (ISP).

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U s e r s o f C C I T T N o 7

T C U s e r s

T C A p p l ic a t io n

P a r t

T C

I n te r m e d i a t e

S e r v i c e

P a r t

S C C P

I S D N

U s e r

P a r t

T e l e p h o n e

U s e r

P a r t

D a t a

U s e r

P a r t

M e s s a g e T r a n s f e r P a r t ( M T P )

O S I

l a y e r

7

4 - 6

1 - 3

L e v e l

4

1 - 3

Fig. 8 Level and layer model for SS7

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Structure of SS7

The ISP implements the functions of Layers 4-6. In conjunction with itsusers, the TCAP implements the functions of Layer 7.

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T r a n s a c t i o n

C a p a b i l i t i e s

U s e r

T r a n s a c t i o n

C a p a b i l i t i e s

A p p l i c a t i o n

P a r t

I n t e r m e d i a t e

S e r v i c e

P a r t

S i g n a l i n g

C o n n e c t i o n

C o n t r o lP a r t

M e s s a g e

T r a n s f e r

P a r t

L a y e r

7

4 - 6

1 - 3

T r a n s a c t i oC a p a b i l i t i e

Fig. 9 7-layers

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Structure of SS7

The Fig. 10 again shows the architecture employed in the SS7 network, this time fordifferent applications. Normally, an exchange supports one circuit-related user part (e.g.ISUP) and one non-circuit-related application. Such an exchange would support the call setupand clear down of circuits, but would also offer the option of requesting centrally stored data(e.g. Service 800).

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TCTC

SCCPS C C P

ISUP ISUPTU P D U P D U P TU P

Exchange A Ex change B

M TPMT P

Fig. 10 Architecture in the SS7 network

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Structure of SS7

The question we must now ask is: what specifications have to be made ifthese models or architecture need to be used for one type of commonchannel signaling?

It is necessary to specify the procedures which take place between anentity in one terminal and the corresponding entity in a second terminal(message flow supporting a specific service). Furthermore, the format forthe messages required for this purpose must be defined and include themessage structure and coding of individual message fields. The primitivesexchanged by two entities within one terminal must be defined likewise.

The sum of all these definitions is termed a protocol.

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E n t i t y

P r i m i t i v e

T e r m i n a l

M e s s a g e n M e s s a g e 1

T e r m i n a l

P r i m i t i v e

F i e l d n F i e l d 1

E n t i t y

E n t i t y

E n t i t y

Fig. 11 Specification of a model

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Structure of SS7

Documents

02 Tw2101eu01_eg0001 Structure of CCS7