RK-4 Signaling System

Signaling System #7

ContentsTypes of SignalingSS7 SignalingSS7 Protocol ArchitectureSS7 Network ArchitectureBasic Call SetupSS7 ApplicationsSS7/IP Inter-workingVoIP Network ConfigurationSIGTRAN Protocol Stack Model Network Evolution to an All-IP Network

Types of Signaling Signaling in Telecommunications NetworkChannel 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 SignalingOut of Band signalingEmploys separate, dedicated path for signaling.Voice trunks are used only when a connection is established, not before.Faster Call Setup.

Advantage of CCS over CASFaster call setupNo 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 HistoryCCITT developed a digital signaling standard called Signaling System 6SS6 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 PrincipleOut of band SignalingHigher Signaling data rates (56Kbps & 64 Kbps)Signaling traffic is bursty and of short duration, hence operates in connectionless mode using packet switchingVariable length signal units with maximum size limitationOptimum use of bandwidthReliability and flexibility

SS7 Protocol Stack

ProtocolsMessage Transfer Part (MTP Level 1) Physical

Provides an interface to the actual physical channel over which communication takes placeCCITT recommends 64Kbps transmission whereas ANSI recommends 56 Kbps

ProtocolsMTP Level 2 (Data Link)

Ensures accurate end-to-end transmission of a message across a signaling linkVariable Length Packet Messages are defined hereImplements flow control, message sequence validation, error checking and message retransmissionMonitor links and reports their statusTest links before allowing their useProvides sequence numbers for outgoing messages

ProtocolsMTP Level 3 (Network)

Message routing between signaling points in the SS7 networkSignaling network management that provides traffic, links and routing management, as well as congestion (flow) controlRe-routes traffic away from failed links and signaling points, controls traffic when congestion occurs

ProtocolsSignaling 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 numbersProvides more detailed addressing information than MTPsUsed as transport layer for TCAP (Transaction capabilities applications part) based services

ProtocolsTransaction Capabilities Applications Part (TCAP)

Exchange of non-circuit related dataBetween applications across the SS#7 network Using the SCCP serviceQueries and responses sent between Signaling Switching Point (SSPs) and Signaling Control Point (SCPs)Sends and receives database informationCredit card validation Routing information

ProtocolsTelephone User Part (TUP)Basic call setup and tear downIn 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 requiredCall circuits are identified using circuit identification code (CIC)

SS7 Networks

SS7 Components

Service switching point (SSP)SSPs are switches that have SS7 software and terminating signaling linksSSPs 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 callsSSPs 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 tableDoes not offer termination servicesSTPs are paired to ensure redundancy There are three levels of STPs.National Signal Transfer PointInternational Signal Transfer PointGateway Signal Transfer Point

STP LevelsSSPSSPSSPSSPSTPSTPSTPNATIONALINTERNATIONALGATEWAYITU-TSANSI

National STP exists within the national networkProtocol 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 capabilitiesAccepts a query for information from a subsystem at another nodeUsed by STP to perform a function called global title translation

Databases Accessible via SCP

HLRHome Location RegisterUsed in cellular networks to store subscriber information.LNPLocal Number PortabilityAllows people to change service providers but keep their same telephone numberOSSOperation Support SystemAssociated with remote maintenance center for monitoring and managing SS7 and voice networks.VLRVisitor Location RegisterUsed when a cell phone is not recognized by the mobile switching center (MSC).

SS7 Link TypesSTPSTPSTPSTPSTPSTPSTPSTP

SS7 Link Types

A link (access)Connects signaling end point (SCP or SSP) to STPB link (bridge)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)C link (cross)Connects STPs performing identical functions, forming a mated pair (for greater reliability)D link (diagonal)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 E link (extended) Connects an SSP to an alternate STP F link (fully associated) Connects two signaling end points (SSPs and SCPs) in the same local network

Addressing in SS7 NetworkPoint Codes are carried in signaling messages and exchanged between signaling points to identify the source and destination of each message (24 bit address). NETWORK IDENTITYMEMBER IDENTITY111CLUSTERIDENTITY

What goes over Signaling LinkSignaling information is passed over the signaling link in form of messages, which are called signaling units (SUs)

3 Types of SUs are:Message signal units(MSUs)Link status signal units(LSSUs)Fill-in signal units(FISUs)

Signaling UnitsMessage signal unit (MSU)

Carries signaling associated with call setup & teardown, database query and response and SS7 network inputBSN/BIBFLAGLengthIndicatorFSN/FIBChecksumSignalingInfo. fieldServiceInfo. octet111118-2721

Service Information OctetBit 1 - 4 Type of Information in Signaling Information FieldBit 5 - 6 Whether Message is intended for National or International NetworkBit 7 - 8 To identify Message Priority

Signaling Information Field8 272 OctetsThe first portion of this field is routing label Routing Label Identifies Message Originator, Intended destination & Signaling Link Selected.Routing Label is of 7 OctetsDestination Point Code 3 OctetsOriginating Point Code 3 OctetsSignaling Link Selection 1 Octet

Signaling UnitsLink status signaling units (LSSU)

Inform the far end about the changes in status of linkMessage length can be 1 or 2 bytes

Signaling UnitsFill-In Signal Units (FISU)Fill the gaps between MSU and LSSU messagesSent only when the buffer is empty, to keep the signaling link activeFacilitate in constant monitoring of link quality.

Basic Call Setup Example

ISUP Messages Initial address message (IAM): contains all necessary information for a switch to establish a connectionAddress complete message (ACM): acknowledge to IAM; the required circuit is reserved and the phone is ringing (ring back tone) Answer message (ANM): occurs when the called party picks up the phoneRelease (REL): sent by the switch sensing that the phone hung upRelease complete (RLC): each exchange that receives REL, sends an RLC message back (this acknowledges receipt of REL)

ApplicationsPrepaid CallingLocal Number Portability (LNP)Global RoamingInternational CallbackVirtual OfficeInternet Call Waiting/ Caller IDLeast Cost RoutingToll BypassUnified Messaging800 / Free Phone Services Short Message Service (SMS)Tele-votingLocation-based Services Caller Ring Back Tone (CRBT)

Implementation of SS7 in GSM

SS7/IP Inter-workingIP 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.

Features of SS7 over IPFlow ControlIn-Sequence Delivery of Signaling Messages within a single control streamIdentification of the originating and terminating signaling pointsIdentification of voice circuitsError 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 networksVoIP 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 GatewayMedia Gateway ControllerSignaling 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 ModelThe SIGTRANs protocol specify the means by which SS7 messages can be reliably transported over IP network.

The architecture identifies three componentA standard IPA 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 networkAllows 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-of-line Blocking.

Transporting MTP over IPITU specified following requirementsMTP Level 3 peer to peer procedure require response time with in 0.5 1.2 sec.No more than 1 in 10 million messages will be lost due to transport failure.No more than 1 in 10 billion messages will be delivered out of sequence.No more than 1 in 10 billion messages will contain an error.Availability of any signaling route set is 99.9998%.Message length is 272 Bytes for SS7 (MAXIMUM)

IETF SIGTRAN working group recommends 3 new protocols: M2UA, M2PA and M3UA.

M2PAM2PA:- MTP2 User Peer-to-Peer Adaptation LayerSupport the transport of SS7 MTP3 signaling messages over IP using the services of SCTPAllows full MTP Level3 message handling and network management capabilities between any 2 SS7 nodes communicating over IPUsed b/w signaling gateway & media gateway controller, signaling gateway & IP signaling point, 2 IP signaling points.

M2UAM2UA:- MTP2 User Adaptation Layer Transports SS7 MTP Level2 user messages over IP using SCTPProvides services as MTP Level2 provides to MTP Level3Used between signaling gateway and media gateway controller.

M3UAM3UA:- MTP 3 User Adaptation Layer Transports SS7 MTP Level3 user signaling messages over IP using SCTPProvides services as MTP Level3 provides to ISUP,TUP,SCCPUsed between signaling gateway and media gateway controller or IP telephony database

SUASUA: SCCP User Adaptation LayerTransports SS7 SCCP User Part Signaling Message over IP using SCTPUsed between signaling gateway and IP signaling end point and between IP signaling end points.

Performance consideration & Security Requirement for SS7/IPSS7 over IP network must meet ITU standard and user expectations, eg. ITU specifies end to end call setup delay not more than 20-30 sec after IAM is transmitted.For transmission of signaling information over internet, SIGTRAN recommends use of IPSEC, which provides following security servicesAuthenticationIntegrityConfidentialityAvailability

Network Evolution to an All-IP NetworkBoth 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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