TELECOMMUNICATION SYSTEMS By Syed Bakhtawar Shah Abid Lecturer in Computer Science 1
Signaling System 7 Architecture Signaling System 7 Protocol Stacks Overview Level 1: Physical Connection SS7 Level 2: Data Link SS7 Level 3: Network SS7 Level 4: Protocols, User & Application 2
SS7 Protocol Stacks Traditional SS7 refers to a stack consisting of the protocols widely deployed from the 1980s to the present Message Transfer Parts (MTP 1, 2, and 3) Signaling Connection Control Part (SCCP) Transaction Capabilities Application Part (TCAP) Telephony User Part (TUP) ISDN User Part (ISUP) The SS7 physical layer is called MTP level 1 (MTP1), the data link layer is called MTP level 2 (MTP2), and the network layer is called MTP level 3 (MTP3). Collectively they are called the Message Transfer Part (MTP). 3
SS7 Protocol Stacks With reference to the Open System Interconnection (OSI) seven-layer reference model, SS7 uses a four-level protocol stack. OSI Layer 1 to 3 services are provided by the MTP together with the SCCP. The SS7 architecture currently has no protocols that map into OSI Layers 4 through 6. TUP, ISUP, and TCAP are considered as corresponding to OSI Layer 7. 4
SS7 Protocol Stacks 5 SS7 uses the term levels when referring to its architecture. The term levels should not be confused with OSI layers, because they do not directly correspond to each other. Levels was a term introduced to help in the discussion and presentation of the SS7 protocol stack. Levels 1, 2, and 3 correspond to MTP 1, 2, and 3, respectively. Level 4 refers to an MTP user. The term user refers to any protocol that directly uses the transport capability provided by the MTP namely, TUP, ISUP, and SCCP in traditional SS7.
SS7 Protocol Stacks call setup SCP db query 6
SS7 Level 1: Physical Connection This is the physical level of connectivity. Physical connection for signaling. Virtually the same as Layer 1 of the OSI model. SS7 specifies what interfaces will be used, i.e. the DS0 or single channel in E1 line Interfaces used to connect signaling links to network equipment Other interfaces are used to connect equipment such as terminals, modems, etc. 7
SS7 Level 2: Data Link 8 Data link level provides the network with sequenced delivery of all SS7 message packets. (to make the frame with the sequence number). Like the OSI data link layer, it is only concerned with the transmission of data from one node to the next, not to its final destination in the network Sequential numbering is used to determine if any messages have been lost during transmission. Each link uses its own message numbering series independent of other links. SS7 uses CRC-16 error checking
SS7 Level 3: Network Level 9 Network level depends on the services of Level 2 to provide routing, message discrimination and message distribution functions. This level provides end-to-end communication. Message Discrimination Message discrimination is the task of determining whether an incoming message is destined for the node that is currently processing the message. This function determines whether a message is local or remote using the point code and data contained in a lookup table. Messages to remote destinations are passed using message routing function for additional processing. In SS7 the information is a Point code, having source & destination information which is stored in the STP and STP acts as like a router. On the basis of point code, local networks are differentiated from remote networks. e.g. National 051, 091, 021., Local 585-170, 581-, 882- International 001, 0044, 0097
SS7 Level 3: Network Level Message Distribution Message distribution provides link, route and traffic management functions. If remote distribution then provides a path. Link Management This function uses the Link Status Signal Unit (LSSU) to notify adjacent nodes of link problems. Level 3 will send LSSUs via Level 2 to the adjacent node, notifying it of the problems with the link and its status. Diagnostics consists of realigning and resynchronizing the link. Traffic is removed from the link, counters are reset to zero, timers are reset and Fill-In Signal Units (FISUs) are sent in the meantime (called the proving period). Proving Period is the amount of time FISUs are sent during link realignment. The duration of the proving period depends on the type of link used. Bellcore specifies the proving period for a 64 Kbps DS0 link is 2.3 seconds for normal proving and 0.6 seconds for emergency proving. 10
Message Transfer Part The MTP include the functions to transport information from one SP to another. Level 1, 2, and 3 are combined into one part, the Message Transfer Part. The MTP transfers the signaling message, in the correct sequence, without loss or duplication, between the SPs that make up the SS7 network. SS7 Level Network Data link Physical MTP Level 3 Level 2 Level 1 11
Message Transfer Part MTP provides the rest of the levels with node-to-node transmission Including basic error detection and correction schemes and message sequencing. It provides routing, message discrimination and distribution functions within a node. The recommendations refer to MTP1, MTP2, and MTP3 as the physical layer, data link layer, and network layer 12
SS7 Level 4: Protocols, User and Application Parts Transactional Capabilities Application Part (TCAP) Facilitates connection to an external database Queries and responses sent between SSP and SCP (e.g. 800, calling card) TCAP allows applications (called subsystems) to communicate with each other (over the SS7 network) using agreed-upon data elements. These data elements are called components. Components can be viewed as instructions sent between applications. For example, when a subscriber changes Visitor Location Register (VLR) location in a GSM cellular network, his or her Home Location Register (HLR) is updated with the new VLR location by means of a Update Location component. 13
SS7 Level 4: Protocols, User and Application Parts There are a number of subsystems; the most common are Toll-free (E800) Advanced Intelligent Network (AIN) Intelligent Network Application Protocol (INAP) Customizable Applications for Mobile Enhanced Logic (CAMEL) Mobile Application Part (MAP) 14
SS7 Level 4: Protocols, User and Application Parts OMAP (Operations, Maintenance and Administrative Part) An applications entity that uses TCAP services for communications and control functions through the network via a remote terminal MAP (Mobile Application Part) MAP is used to share cellular subscriber information among different networks 15
SS7 Level 4: Protocols, User and Application Parts Application Service Part (ASP) Provides the functions of Layers 4 through 6 of the OSI model These functions are not presently required in the SS7 network, and are under further study Signaling Connection Control Part (SCCP) A higher level protocol than MTP that provides end-to-end routing. SCCP is required for routing TCAP messages to their proper database SCCP supports both connection-oriented (seldom used) and connectionless (mostly) 16
SS7 Level 4: Protocols, User and Application Parts Telephone User Part (TUP) An analog protocol that performs basic telephone call connect and disconnect. It has been replaced by ISUP, but is still used in some parts of the world (China). ISDN User Part (ISUP) It supports basic telephone call connect/disconnect between end offices. Used primarily in North America, ISUP was derived from TUP, but supports ISDN and intelligent networking functions. ISUP also links the cellular and PCS network to the PSTN. BISUP (Broadband ISUP) will gradually replace ISUP. Basic services call set up, tear down, management Supplementary services caller ID, call forwarding, call waiting, call blocking Broadband ISDN User Part (BISUP) is an ATM protocol intended to support services such as, voice and image storage and retrieval, video conferencing, high-speed LANs and multimedia 17
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