SS7 Module 2 v1-0

2 SS7 Protocol Stacks

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 T.O.P BusinessInteractive GmbH

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2.1 OSI Reference Model (1/5)................................................................................. 3 2.1 OSI Reference Model (2/5)................................................................................. 4 2.1 OSI Reference Model (3/5)................................................................................. 5 2.1 OSI Reference Model (4/5)................................................................................. 6 2.1 OSI Reference Model (5/5)................................................................................. 7 2.2 Basic SS7 Protocol Stack ................................................................................... 8 2.3 SS7 Protocol Stack (1/4) .................................................................................... 9 2.3 SS7 Protocol Stack (2/4) .................................................................................. 10 2.3 SS7 Protocol Stack (3/4) .................................................................................. 11 2.3 SS7 Protocol Stack (4/4) .................................................................................. 12 2.4 SS7 Protocol Stacks in PSTN (1/2) .................................................................. 13 2.4 SS7 Protocol Stacks in PSTN (2/2) .................................................................. 14 2.5 GSM Network Review ...................................................................................... 15 2.6 SS7 Protocol Stacks in GSM (1/6).................................................................... 16 2.6 SS7 Protocol Stacks in GSM (2/6).................................................................... 17 2.6 SS7 Protocol Stacks in GSM (3/6).................................................................... 18 2.6 SS7 Protocol Stacks in GSM (4/6).................................................................... 19 2.6 SS7 Protocol Stacks in GSM (5/6).................................................................... 20 2.6 SS7 Protocol Stacks in GSM (6/6).................................................................... 21

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2.1 OSI Reference Model (1/5)

To make sure that the signaling points through which the information travels can understand 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 the last century for the functional description and classification of computer and telecomms network elements.

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We'll now illustrate in general terms the OSI reference model with an example from business 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 the outcome of this business deal is important. The process that takes place in the lower hierarchy to get the tyres from the supplier to the car manufacturer does not interest them. The managers rely on their purchasing- and sales departments, which will deal with practical details. The car manufacturer's purchasing department, however, only communicates with the supplier's sales department. As soon as the financial transactions are concluded, the goods can be delivered from A to B.

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The purchasing and sales departments are not interested in the practical details of delivery. At the supplier, the warehouse workers must pack the tyres and load them on 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 in a business transaction. The higher levels give the lower levels instructions, without paying attention to the details of the processes. Communication between the two companies takes place only between peer levels. With the OSI model, it's similar.

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OSI is a reference model consisting of 7 layers that are based on each other. Each layer has its own tasks. The lower layer always provides support functions for the layer 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 type of communication between elements belonging to the same layer in different systems is known as peer-to-peer communication.

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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 applicationoriented 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.

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

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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.

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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 the connection. MTP level 2 supports the error free transmission of signaling messages between neighbouring network elements. MTP level 3 is responsible for taking the message from any element in a signaling network to any other element within the same network.

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While MTP is responsible for message transfer, the Telephone User Part (TUP) represents the protocol used for sending, receiving, and acting on these messages from 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 Public Switched Telephone Networks, or PSTN. With the introduction of the more capable ISDN network, some extra sets of messages became necessary. These features are contained in the ISUP which replaces the TUP.

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To guarantee virtual connections and connectionless signalling, that is signalling which 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 messages arrive at the receiver in a determined order, so a virtual connection can be guaranteed. SCCP also enables the routing of signalling messages across multiple networks and in the absence of a call. In order to support network outbound calls, e.g. into foreign PSTNs or PLMNs ISUP normally resides on top of the SCCP instead of its place on top of the MTP.

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2.4 SS7 Protocol Stacks in PSTN (1/2)

Keeping this in mind, we now have a simplified impression of the SS7 protocol stack which 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 legacy PSTN or SSCP messages that in turn supports ISDN if we consider an ISDN network. In a more abstract view, we can identify logical interworkings between the different processor units in each signaling point involved: MTP messages are processed between MTP processors, SCCP messages between SCCP processors etc.

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2.4 SS7 Protocol Stacks in PSTN (2/2)

So far, SS7 signaling message construction seems to be pretty simple as we do not have any subscriber mobility that must be supported. This changes dramatically as soon as we consider mobile networks such as a GSM PLMN.

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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 of various different network elements. Let's have a quick review of the individual elements of the GSM network and their basic functions before focussing on the signaling aspects. The Base Station Subsystem BSS provides the connection between the mobile stations and the Network Subsystem NSS. The NSS forwards user signals to other mobiles via the BSS or subscribers in the Public Switched Telephone Network (PSTN) and provides the necessary customer data. The Operation & Maintenance Subsystem (OMS) monitors BSS and NSS performance, and remotely debugs faults that occur in the network elements. Additional components such as interface elements to data networks, the Short Message Service Center or the Voice Mail System complete the GSM system architecture.

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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 higher demands on signalling. GSM requires a considerably higher amount of non-callrelated signalling information. After all, it must be considered that the GSM customer is mobile, in contrast to the user of a fixed network, who telephones from a fixed device. Therefore, the mobile station must be continuously provided with localization signals, to enable the Location Update. The Location Update is an example of a non-call-related communication between the phone and the network. To guarantee that the signalling demands in GSM networks are met, additional standard sets of messages are 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)

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The Base Station Subsystem Application Part (BSSAP) is a protocol layer responsible for communication between the MSC and the BSC in GSM. BSSAP is responsible 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.

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

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The Mobile Application Part (MAP) is a GSM specific protocol for non-call-related applications 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 NSS elements in the absence of a call. This is the case for example with normal call setup. To put a call through to the subscriber, the Gateway MSC must request necessary routing data from the HLR. Thus, there is no data exchange between the GMSC and the HLR, without the actual call being routed to the HLR. In such cases, the network relies on MAP, which is used for signalling communication 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 to another MSC, TUP or ISUP is used.

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Today's GSM networks offer a variety of sophisticated supplementary and value added services that require additional network elements with enhanced service logic. 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 into the voice mailbox. These databases and front end computer systems that by the way are also available in PSTNs are part of the so-called intelligent network. This requires a special protocol stack to provide this specific signaling information. It's called Intelligent Network Application Part, INAP and also resides on top of the TCAP as the MAP does.

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Not every GSM element must be able to understand every language. Consequently, only those protocol layers need to be implemented which the network element actually 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 SS7 requirements of the individual GSM elements will be introduced.

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SS7 Module 2 v1-0

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