ss7 module 2 v1-0

7/28/2019 SS7 Module 2 v1-0

1/21

Version 1.0 T.O.P BusinessInteractive GmbH Page 1 of 21

2 SS7 Protocol Stacks

7/28/2019 SS7 Module 2 v1-0

2/21


2.1 OSI Reference Model (1/5).................................................................................32.1 OSI Reference Model (2/5).................................................................................42.1 OSI Reference Model (3/5).................................................................................52.1 OSI Reference Model (4/5).................................................................................62.1 OSI Reference Model (5/5).................................................................................72.2 Basic SS7 Protocol Stack ...................................................................................82.3 SS7 Protocol Stack (1/4) ....................................................................................92.3 SS7 Protocol Stack (2/4) ..................................................................................102.3 SS7 Protocol Stack (3/4) ..................................................................................112.3 SS7 Protocol Stack (4/4) ..................................................................................122.4 SS7 Protocol Stacks in PSTN (1/2) ..................................................................132.4 SS7 Protocol Stacks in PSTN (2/2) ..................................................................142.5 GSM Network Review ......................................................................................152.6 SS7 Protocol Stacks in GSM (1/6)....................................................................162.6 SS7 Protocol Stacks in GSM (2/6)....................................................................172.6 SS7 Protocol Stacks in GSM (3/6)....................................................................182.6 SS7 Protocol Stacks in GSM (4/6)....................................................................192.6 SS7 Protocol Stacks in GSM (5/6)....................................................................202.6 SS7 Protocol Stacks in GSM (6/6)....................................................................21

7/28/2019 SS7 Module 2 v1-0

3/21


2.1 OSI Reference Model (1/5)

To make sure that the signaling points through which the information travels canunderstand each other, they must, as it were, agree on a common official language.This language, in our case SS7 is specified by protocols.

SS7 information is arranged according to the Open System Interconnection model,also called the OSI reference model. This has been used since the early 70ies of thelast century for the functional description and classification of computer andtelecomms network elements.

7/28/2019 SS7 Module 2 v1-0

4/21



We'll now illustrate in general terms the OSI reference model with an example frombusiness life.A car manufacturer B orders 1000 tyres from supplier A. This deal is concluded and

signed by two managers at the highest level. For the two managers, only theoutcome of this business deal is important. The process that takes place in the lowerhierarchy to get the tyres from the supplier to the car manufacturer does not interestthem. The managers rely on their purchasing- and sales departments, which willdeal with practical details. The car manufacturer's purchasing department, however,only communicates with the supplier's sales department. As soon as the financialtransactions are concluded, the goods can be delivered from A to B.

7/28/2019 SS7 Module 2 v1-0

5/21



The purchasing and sales departments are not interested in the practical details ofdelivery. At the supplier, the warehouse workers must pack the tyres and load themon trucks, to get them ready for transport. As soon as the tyres arrive at the

manufacturer, the warehouse workers will unpack the tyres and store them.In summary, we can say: It's always several levels of a company that collaborate ina business transaction. The higher levels give the lower levels instructions, withoutpaying attention to the details of the processes. Communication between the twocompanies takes place only between peer levels. With the OSI model, it's similar.

7/28/2019 SS7 Module 2 v1-0

6/21



OSI is a reference model consisting of 7 layers that are based on each other. Eachlayer has its own tasks. The lower layer always provides support functions for thelayer above. For a layer, the data transported in the layers underneath is irrelevant.

Communication only takes place between the elements of the same layer. This typeof communication between elements belonging to the same layer in differentsystems is known as peer-to-peer communication.

7/28/2019 SS7 Module 2 v1-0

7/21



The layers take on the following tasks:

The lowest layer, layer No 1, is the Physical layer. It's responsible for

transmission, encoding, and modulation. Layer 2 is the Data Link layer. It's responsible for the signalling link

management and data security. Layer 3 is the Network layer. It contains the information needed for switching

and routing and handles call set-up, -supervision, and -clear down. Layer 4 is the Transport layer. Here, the peer-to-peer connections' dataflow

is controlled. Layer 5 is the Session layer. It handles the connections for application

processes as well as charging. Layer 6 is the Presentation layer. It takes over the transfer of application-

oriented formats, as well as encryption and translation. At the top resides layer No 7, the Application layer. It is responsible for the

application protocols and the user interfaces.

7/28/2019 SS7 Module 2 v1-0

8/21


2.2 Basic SS7 Protocol Stack

The Basic OSI refrence model structure was created more than 30 years ago.Modern telecommunication systems can no longer be described properly using thismodel as given functionalities overlap within the defined layer structure.

The SS7 protocol can be split into two basic areas of functionality: the lower protocolparts that represent OSI layer 1 to 3 functionalities and some higher protocol partsthat contain information that can't easily be assigend to the higher OSI layers.Thus, SS7 uses four different levels to describe message functionalities with levels1-3 for the lower protocol parts and level 4 for all parts residing on top of the basicinformation.

7/28/2019 SS7 Module 2 v1-0

9/21


2.3 SS7 Protocol Stack (1/4)

The basic SS7 version consists of two parts:

The Message Transfer Part (MTP), just responsible for message transfer

and The Telephone User Part (TUP) on the user's side, which receives, sends,

and acts on these messages.

Let's turn our attention to MTP first.

7/28/2019 SS7 Module 2 v1-0

10/21



The Message Transfer Part (MTP), represents the basis for the entire SS7 system.It transmits messages between network elements. MTP is composed of three levels.

MTP level 1 defines the physical and electrical characteristics of theconnection.

MTP level 2 supports the error free transmission of signaling messagesbetween neighbouring network elements.

MTP level 3 is responsible for taking the message from any element in asignaling network to any other element within the same network.

7/28/2019 SS7 Module 2 v1-0

11/21



While MTP is responsible for message transfer, the Telephone User Part (TUP)represents the protocol used for sending, receiving, and acting on these messagesfrom the user's point of view. TUP handles call set-up, call supervision and clear

down, and exists for normal public fixed networks, which are also known as PublicSwitched Telephone Networks, or PSTN. With the introduction of the more capableISDN network, some extra sets of messages became necessary. These features arecontained in the ISUP which replaces the TUP.

7/28/2019 SS7 Module 2 v1-0

12/21



To guarantee virtual connections and connectionless signalling, that is signallingwhich is not bound to a call, another protocol layer on top of MTP is required,parallel to TUP. This is the Signalling Connection and Control Part, SCCP. TUP and

SCCP take over different tasks, but both make use of the services provided by MTP.In contrast to MTP, SCCP uses sequence numbers to make sure that messagesarrive at the receiver in a determined order, so a virtual connection can beguaranteed. SCCP also enables the routing of signalling messages across multiplenetworks and in the absence of a call.In order to support network outbound calls, e.g. into foreign PSTNs or PLMNs ISUPnormally resides on top of the SCCP instead of its place on top of the MTP.

7/28/2019 SS7 Module 2 v1-0

13/21


2.4 SS7 Protocol Stacks in PSTN (1/2)

Keeping this in mind, we now have a simplified impression of the SS7 protocol stackwhich signaling messages use to support calls in a PSTN.The MTP represents physical layer information, data link control messages and

basic routing information. It supports either TUP messages if the network is a legacyPSTN or SSCP messages that in turn supports ISDN if we consider an ISDNnetwork.In a more abstract view, we can identify logical interworkings between the differentprocessor units in each signaling point involved: MTP messages are processedbetween MTP processors, SCCP messages between SCCP processors etc.

7/28/2019 SS7 Module 2 v1-0

14/21


2.4 SS7 Protocol Stacks in PSTN (2/2)

So far, SS7 signaling message construction seems to be pretty simple as we do nothave any subscriber mobility that must be supported.This changes dramatically as soon as we consider mobile networks such as a GSM

PLMN.

7/28/2019 SS7 Module 2 v1-0

15/21


2.5 GSM Network Review

For the subscriber, a mobile telephone call is a simple process. In reality, though,this call is only possible thanks to a complex network architecture consisting ofvarious different network elements. Let's have a quick review of the individual

elements of the GSM network and their basic functions before focussing on thesignaling aspects.The Base Station Subsystem BSS provides the connection between the mobilestations and the Network Subsystem NSS. The NSS forwards user signals to othermobiles via the BSS or subscribers in the Public Switched Telephone Network(PSTN) and provides the necessary customer data. The Operation & MaintenanceSubsystem (OMS) monitors BSS and NSS performance, and remotely debugs faultsthat occur in the network elements.Additional components such as interface elements to data networks, the ShortMessage Service Center or the Voice Mail System complete the GSM systemarchitecture.

7/28/2019 SS7 Module 2 v1-0

16/21


2.6 SS7 Protocol Stacks in GSM (1/6)

In GSM networks, signalling is not as easy as in a fixed network. This is because,due to the network architecture, a digital mobile radio network makes much higherdemands on signalling. GSM requires a considerably higher amount of non-call-

related signalling information.

After all, it must be considered that the GSM customer is mobile, in contrast to theuser of a fixed network, who telephones from a fixed device. Therefore, the mobilestation must be continuously provided with localization signals, to enable theLocation Update. The Location Update is an example of a non-call-relatedcommunication between the phone and the network. To guarantee that thesignalling demands in GSM networks are met, additional standard sets of messagesare required.The following protocol layers are necessary:

The Base Station Subsystem Application Part (BSSAP) The Transaction Capabilities Application Part (TCAP) and The Mobile Application Part (MAP)

7/28/2019 SS7 Module 2 v1-0

17/21



The Base Station Subsystem Application Part (BSSAP) is a protocol layerresponsible for communication between the MSC and the BSC in GSM. BSSAP isresponsible for the entire management and control of the radio resources in the

BSS. It resides on top of the Signalling Connection and Control Part, SCCP.

7/28/2019 SS7 Module 2 v1-0

18/21



The Transaction Capabilities Application Part (TCAP) is a protocol layer whichresides directly on top of SCCP. TCAP is able, for example, to organize a complexdialogue between an MSC and an HLR, including a sequence of successiverequests and replies.TCAP functions like a secretary's office, where many different requests are broughtinto the correct sequence and distributed. TCAP handles the access to data baseslike the HLR or the VLR. It must exist so that a higher protocol - the MobileApplication Part (MAP) - can be used.

7/28/2019 SS7 Module 2 v1-0

19/21



The Mobile Application Part (MAP) is a GSM specific protocol for non-call-relatedapplications between elements in the NSS. MAP resides directly on top of TCAP,which can be used as a "secretary's office" by the MAP, and which coordinates and

guarantees smooth MAP communication.A MAP-based communication takes place when data is exchanged between NSSelements in the absence of a call. This is the case for example with normal call set-up. To put a call through to the subscriber, the Gateway MSC must requestnecessary routing data from the HLR. Thus, there is no data exchange between theGMSC and the HLR, without the actual call being routed to the HLR.In such cases, the network relies on MAP, which is used for signallingcommunication between NSS elements. Please note: in MSC-MSC communication,MAP is only used for non-call-related signalling. To forward a call from an MSC toanother MSC, TUP or ISUP is used.

7/28/2019 SS7 Module 2 v1-0

20/21



Today's GSM networks offer a variety of sophisticated supplementary and valueadded services that require additional network elements with enhanced servicelogic. Call forwarding can be designed individually using a web interface to define

during which time of the day the call is delivered or forwarded to a secretary or intothe voice mailbox.These databases and front end computer systems that by the way are also availablein PSTNs are part of the so-called intelligent network. This requires a specialprotocol stack to provide this specific signaling information. It's called IntelligentNetwork Application Part, INAP and also resides on top of the TCAP as the MAPdoes.

7/28/2019 SS7 Module 2 v1-0

21/21

Version 1 0 T O P BusinessInteractive GmbH Page 21 of 21


Not every GSM element must be able to understand every language. Consequently,only those protocol layers need to be implemented which the network elementactually requires for carrying out its task.

A BSC, for example, will never need the TUP protocol, because call supervision -which this layer supports - is not its task. In the following lessons, the SS7requirements of the individual GSM elements will be introduced.

ss7 module 2 v1-0

Documents