Signaling System 7 (SS7) By : Ali Mustafa

Signaling System 7 (SS7) By : Ali Mustafa

Contents Types of Signaling SS7 Signaling SS7 Protocol Architecture SS7 Network Architecture Basic Call Setup SS7 Applications SS7/IP Inter-working VoIP Network Configuration SIGTRAN Protocol Stack Model Network Evolution to an All-IP Network

Types of Signaling Signaling in Telecommunications Network Channel Associated Signaling (CAS) Common Channel Signaling (CCS) Signaling System Number (SS7) is a form of Common Channel Signaling.

Channel Associated Signaling (CAS) Used for In-Band Signaling Signaling is transmitted in the same frequency band as used by voice. Voice path is established when the call setup is complete, using the same path that the call setup signals used.

Common Channel Signaling Out of Band signaling Employs separate, dedicated path for signaling. Voice trunks are used only when a connection is established, not before. Faster Call Setup. Voice Trunks Switch A Switch B Signaling Link

Advantage of CCS over CAS Faster call setup No interference between signaling tones by network and frequency of human speech pattern. Greater Trunking Efficiency:- CCS has shorter call set up and tear down times that result in less call holding time, thereby reducing the traffic on the network. Information Transfer:- CCS allows the transfer of additional information along with the signaling traffic providing facilities such as caller identification and voice or data identification

SS7 History CCITT developed a digital signaling standard called Signaling System 6 SS6 was based on Packet-Switched, proprietary data network. Uses 2.4 Kbps data links to send packets of data to distant switches to request service. SS7 began deployment in 1983, was initially used for inter office network, but now it is deployed in local central offices. Provide a global standard for call setup, routing, control and database access.

SS7 Principle Out of band Signaling Higher Signaling data rates (56Kbps & 64 Kbps) Signaling traffic is bursty and of short duration, hence operates in connectionless mode using packet switching Variable length signal units with maximum size limitation Optimum use of bandwidth Reliability and flexibility

SS7 Protocol Stack OSI SS7

Protocols Message Transfer Part (MTP Level 1) Physical Provides an interface to the actual physical channel over which communication takes place CCITT recommends 64Kbps transmission whereas ANSI recommends 56 Kbps

MTP Level 2 (Data Link) Protocols Ensures accurate end-to-end transmission of a message across a signaling link Variable Length Packet Messages are defined here Implements flow control, message sequence validation, error checking and message retransmission Monitor links and reports their status Test links before allowing their use Provides sequence numbers for outgoing messages

Protocols MTP Level 3 (Network) Message routing between signaling points in the SS7 network Signaling network management that provides traffic, links and routing management, as well as congestion (flow) control Re-routes traffic away from failed links and signaling points, controls traffic when congestion occurs

Protocols Signaling Connection Control Part (SCCP) Provides connectionless and connection-oriented network services Provides global title translation (GTT) capabilities above MTP level 3; translates numbers to DPCs and subsystem numbers Provides more detailed addressing information than MTPs Used as transport layer for TCAP (Transaction capabilities applications part) based services

Protocols Transaction Capabilities Applications Part (TCAP) Exchange of non-circuit related data Between applications across the SS#7 network Using the SCCP service Queries and responses sent between Signaling Switching Point (SSPs) and Signaling Control Point (SCPs) Sends and receives database information Credit card validation Routing information

Protocols Telephone User Part (TUP) Basic call setup and tear down In many countries, ISUP has replaced TUP for call management ISDN User Part (ISUP) Necessary messaging for setup and tear down of all circuits (voice and digital) Messages are sent from a switch, to the switch where the next circuit connection is required Call circuits are identified using circuit identification code (CIC)

SS7 Networks STP STP STP STP STP

SS7 Components

Service switching point (SSP) SSPs are switches that have SS7 software and terminating signaling links SSPs create packets (signal units) and send those messages to other SSPs, as well as queries to remote shared databases to find out how to route calls SSPs communicate with the voice switch via the use of primitives and have the ability to send messages using ISUP (call setup and teardown) and TCAP (database lookup) protocols. The switch can originate, terminate, or switch calls

Signaling transfer point (STP) STPs are packet switches, and act like routers in the SS7 network. Routes each incoming message to an outgoing signaling link, based on routing information contained in the SS#7 message and a pre-defined route table Does not offer termination services STPs are paired to ensure redundancy There are three levels of STPs. National Signal Transfer Point International Signal Transfer Point Gateway Signal Transfer Point

STP Levels SSP ITU-TS SSP INTERNATIONAL STP GATEWAY STP NATIONAL SSP STP ANSI SSP

National STP exists within the national network Protocol converters often interconnect a National and an International STP by converting from ANSI to ITU-TS. International STP functions within an international network. All nodes connecting to an International STP must use the ITU- TS protocol standard. Gateway STP converts signaling data from one protocol to another. Gateway STPs are often used as an access point to the international network. Depending on its location, the Gateway STP must be able to use both the International and National protocol standards.

Service control point (SCP) An SCP is usually a computer used as a front end to a database system. It is an interface to application-specific databases. The address of an SCP is a point code, and the address of the database it interfaces with is a subsystem number. The database is an application entity which is accessed via the TCAP protocol. Databases that provides information necessary for advanced call processing capabilities Accepts a query for information from a subsystem at another node Used by STP to perform a function called global title translation

Databases Accessible via SCP HLR LNP Home Location Register Local Number Portability Used in cellular networks to store subscriber information. Allows people to change service providers but keep their same telephone number OSS Operation Support System Associated with remote maintenance center for monitoring and managing SS7 and voice networks. VLR Visitor Location Register Used when a cell phone is not recognized by the mobile switching center (MSC).

SS7 Link Types STP STP STP STP STP STP STP STP

SS7 Link Types A link (access) B link (bridge) C link (cross) D link (diagonal) E link (extended) F link (fully associated) Connects signaling end point (SCP or SSP) to STP Connects an STP to another STP; typically, a quad of B links interconnect peer (or primary) STPs (STPs from a network connect to STPs of another network) Connects STPs performing identical functions, forming a mated pair (for greater reliability) Connects a secondary (local or regional) STP pair to a primary (inter-network gateway) STP pair in a quad-link configuration; the distinction between B and D links is arbitrary Connects an SSP to an alternate STP Connects two signaling end points (SSPs and SCPs) in the same local network

Applications Prepaid Calling Local Number Portability (LNP) Global Roaming International Callback Virtual Office Internet Call Waiting/ Caller ID Least Cost Routing Toll Bypass Unified Messaging 800 / Free Phone Services Short Message Service (SMS) Tele-voting Location-based Services Caller Ring Back Tone (CRBT)

SS7 Implementation of SS7 in GSM radio subsystem network and switching subsystem Fixed partner networks MS MS U m MSC ISDN PSTN BTS BTS A bis BSC EIR HLR BTS BTS BSS BSC A MSC IWF VLR ISDN PSTN PDN

SS7/IP Inter-working IP offers an economical solution to challenges created by high volume traffic at network edge. Operators off load voice calls from PSTN to VoIP networks because it is less costly to carry voice traffic over IP network than over Switched circuit network. In VoIP network, digitized voice data is highly compressed and carried in packets over IP network, thereby efficiently utilizing the bandwidth and increasing the number of voice calls carried. Saving realized in using VoIP network are passed on to users in the form of lower cost.

Flow Control Features of SS7 over IP In-Sequence Delivery of Signaling Messages within a single control stream Identification of the originating and terminating signaling points Identification of voice circuits Error detection, re-transmission and other error correcting procedures. Controls to avoid congestion on the internet. Detection of status of peer entities. Support for security mechanism.

Signaling in VoIP networks VoIP Network carry SS7 over IP using protocols defined by Signaling Transport (SIGTRAN) working group of the Internet Engineering Task Force (IETF). In IP telephony networks, signaling information is exchanged between the following functional elements. Media Gateway Media Gateway Controller Signaling Gateway.

VoIP Network Configuration

Media Gateway- Terminates voice calls on Inter-switch Trunks from the PSTN, compresses and packetizes the voice data and delivers compress voice packet to the IP network. For voice calls originating in an IP network, it performs these functions in reverse order. Media Gateway Controller- It handles the registration and management of resources at Media Gateway(s), also knows as Soft Switch. Signaling Gateway- It provides Transparent interworking of signaling between switched ckt. and IP network. It may terminate SS7 signaling or translate and relay messages over an IP network to a media gateway controller or another signaling gateway.

SIGTRAN Protocol Stack Model The SIGTRAN s protocol specify the means by which SS7 messages can be reliably transported over IP network. The architecture identifies three component A standard IP A common signaling transport protocol for SS7 protocol layer being carried. An adaptation module to emulate lower layers of the protocol.

Stream Control Transmission Protocol(SCTP) Allows the reliable transfer of signaling messages between signaling end points in an IP network Allows signaling messages to be independently ordered with in multiple streams to ensure in sequence delivery between associated end points. SIGTRAN recommends SCTP rather than TCP/IP for transmission of signaling messages over IP networks as TCP/IP does enforce Head-ofline Blocking.

Network Evolution to an All-IP Network Both traditional Circuit switch and IP based services need to be supported by single network infrastructure simultaneously. Hybrid architecture may be the best solution. Transition to All-IP network will not happen overnight. IP Network

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