SS7
- 1. Signaling System 7 (SS7) BY M Furqan [email protected] QUICK LINX WIRELESS
- 2. What is SS7? • SS7, signaling system number 7, was originally designed to support the PSTN (public switched telephone network) in: – Call setup, – Call management, and – Call release between telephone offices and CPE (customer premises equipment) to simply transport voice traffic.
- 3. What is SS7? • But now it has been further developed to transport not only voice but also data/video traffic. • In the original SS7, the “in-band” signaling mechanism was used. • It means that the signaling information and the user data (e.g. voice) are carried on the same channel.
- 4. What is SS7? • Nowadays, many new applications of SS7 have emerged, such as SS7 intelligent network. • Because these applications require a high quality transfer of the signaling information, the signaling in SS7 has changed to the “out-of-band” signaling. • In the “out-of-band” signaling, the signaling information is carried in a separate and dedicated physical channel, and the user data is carried in other channels, as shown in Figures 1 & 2.
- 5. What is SS7? Figure 1: Out-of-band Signaling
- 6. What is SS7? Figure 1.2: In-band Signaling
- 7. What is SS7? • As shown in Figure 3, the SS7 protocol stack consists of many different layers. • In the following, each of the layers that are involved are discussed in this lecture.
- 8. What is SS7? Figure 3: SS7 Protocol Layers
- 9. MTP1 • The MTP1 (message transfer part level 1) is the physical layer. • It provides a transport for an individual signaling data link. • It defines the physical, electrical, and functional characteristics of a signaling data link and the means to access it. • In a digital environment, 56 or 64 kbps digital paths are used for the signaling data link.
- 10. MTP2 • The MTP2 (message transfer part level 2) is the data link layer. • It provides the functions and procedures to transfer signaling messages over an individual signaling data link. • It is primarily responsible for error free transmission of data for some basic communication functions.
- 11. MTP2 • Once information enters the network, it must be transferred in the proper sequence and without error between network nodes over each segment of the transmission path.
- 12. MTP3 • The MTP3 (message transfer part level 3) is the network layer. • It provides functions and procedures that are common to (but independent from) the operation of individual signaling links. • It provides services that transmit the data through the network from originator to its final destination.
- 13. MTP3 • The message routing is implemented at this layer through the routing labels maintained in MTP3 signal units and at a specific signaling point. • The OPC (origination point code) or DPC (destination point code) in a routing label consists of three bytes which are used to identify the node on network. • The SLS (signaling link selector) in a routing label is used to balance the traffic load among signaling links.
- 14. MTP3 • MTP3 also provides the means to establish, maintain, and terminate connections between systems. • For example, in case of signaling link failure detection and link recovery.
- 15. SCCP • On the top of MTP3 is the SCCP (signaling connection control part), which is a combination of parts of the network layer and parts of the transport layer. • It relies on the MTP level 1, 2, 3 to provide additional functions for both connectionless and connection- oriented network services to transfer circuit-related and non-circuit-related signaling information between switches .
- 16. SCCP • When coupled with the MTP layers, SCCP provides specialized routing and management and control functions for the transfer of higher level messages between the MTP layers and the SCCP users. • For example, TCAP (Transaction Capabilities Application Part).
- 17. SCCP • There are four classes of service in SCCP, two for connectionless services and the other two for connection-oriented services: – basic connectionless class (class 0) – sequenced connectionless class (class 1) – basic connection-oriented class (class 2) – flow control connection-oriented class (class 3)
- 18. SS7 Network • Figure 4 shows a typical topology of an SS7 network, where the major physical components include: – SSP (service switching point), – STP (signaling transfer point), – SCP (service control point) and – SL (signaling link).
- 19. SS7 Network Figure 4: SS7 Network Topology
- 20. SSP • SSP is the local exchange to the subscriber and the interface to the telephone network. • The SS7 signaling information is produced at the sending SSP and is handled at the receiving SSP. • It converts voice signaling into the SS7 signal units, and vice versa. • It also supports database access queries, such as for 800 service.
- 21. STP • STP is the network node. • It works like a router to perform the routing of messages to other signaling points. • It is usually paired to provide redundancy for a reliable message transfer. • It is adjunct to a voice switch, and might stand alone as a separate machine.
- 22. SCP • SCP works as the interface with the databases, which contain the information of the subscribers, such as 800 service, calling cards, fraud data, etc. • When receiving a request, it is triggered to handle the database queries, and returns a response with the results of the queries to the originating SSP.
- 23. Signaling Links(SL) • SL is the link interconnecting the signaling points of SS7 network. • A SL is made up of digital transmission channels. • In each channel, the standard bit rate on a bearer service is 56 or 64 kbps. • But these channels may be multiplexed into a transmission rate of 1.544, 2.048, or 8.448 Mbps.
- 24. Signaling Links(SL) • All the links between two signaling points (e.g. SSP, STP, SCP) compose a “link set”. • The network traffic between two signaling points are evenly distributed to reach a load sharing within a link set. • These different link sets can provide the redundancy to ensure the high reliability of message transfer.
- 25. Signaling Links(SL) • There are six types of SL: • A, B, C,D,E, F links which are shown in Figure 4. • D link which is not shown in Figure 4. • What are D links…..Check it at home?
- 26. Signaling Links(SL) Figure 4: SS7 Network Topology
- 27. What is TCAP? • The TCAP is transaction capabilities application part. • It provides application level functions for special SS7 services. • Service information exchanged between the SSP and the network database would typically be defined within the TCAP.
- 28. What is TCAP? • The TCAP protocol layer resides between the application layer (e.g. a 800 service), and the SCCP layer. • An ASE (application service element) at a SSP represents an application uses the TCAP to initiate a transaction by sending a query message to a remote SCP. • While an ASE at the SCP uses the TCAP to end the transaction by returning a TCAP response message to the originating SSP.
- 29. What is TCAP? • The TCAP are supported by SCCP. • TCAP messages are encapsulated into the SCCP header part. • As shown in Figure 5 , the TCAP layer is composed of two sublayers: – Transaction sublayer, and – Component sublayer.
- 30. What is TCAP? Figure 1.5: TCAP Layer Structure
- 31. What is TCAP? • A TCAP message is composed of a: – “transaction portion” and a – “component portion” • The transaction portion contains the Package Type Identifier. • There are seven package types.
- 32. What is TCAP? • Unidirectional: transfers components in one direction only (no reply expected). • Query with Permission: initiates a TCAP transaction (e.g. a 800 call service). The destination node may end the transaction. • Query without Permission: initiates a TCAP transaction. The destination node may not end the transaction.
- 33. What is TCAP? • Response: ends the TCAP transaction. A response to query with permission may contain the information requested. • Conversation with Permission: continues a TCAP transaction. The destination node may end the transaction. • Conversation without Permission: continues a TCAP transaction. The destination node may nor end the transaction.
- 34. What is TCAP? • Abort: terminates a transaction due to an abnormal situation. • The transaction portion also contains the Originating Transaction ID and Responding Transaction ID. • These fields which can associate the TCAP transaction with a specific application at the origination and destination signaling points respectively.
- 35. What is TCAP? • The component portion contains six kinds of components: • Invoke (Last): invokes an operation. The component is the “last” component in the query. • Invoke (Not Last): similar to the Invoke (Last) component except that the component is followed by one or more components. • Return Result (Last): returns the result of an invoked operation. The component is the “last” component in the response.
- 36. What is TCAP? • Return Result (Not Last): similar to the Return Result (Last) component except that the component is followed by one or more components. • Return Error: reports the unsuccessful completion of an invoked operation. • Reject: indicates that an incorrect package type or component was received.
- 37. Assignment Q # 01: Give a comparison of SS7 & TCP/IP/ATM? (25 marks) Q # 02: How interworking is achieved between SS7 & TCP/IP/ATM networks? (50 marks) Note: • Last submission date: 16-04-2007(Next Monday) • Late submissions shall be marked ZERO… • A VIVA,QUIZ or a PRESENTATION may be expected after the submission…so prepare your on assignment…!!!
- 38. References & Recommended Reading • ANSI T1.1 11-1992, Signaling System No. 7 (SS7) Message Transfer Pan (MTP), 1992. • ANSI T1.1 12-1992, Signaling System No. 7 (SS7) Signaling Connection Control Part (SCCP), 1992. • ANSIT1.1 14-1992, Signaling System No. 7 (SS7) Transaction Capabilities Application Part (TCAP), 1992. • Black, U. ISDN & SS7. New Jersey: Prentice Hall, 1997. • Tanenbaum, Andrew S. Computer Networks, 3rd Ed. New Jersey: Prentice Hall, 1996. Must read the read ones at home for an assignment or quiz…….