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 SignalingSignaling in Telecommunications NetworkChannel Associated Signaling (CAS)Common Channel Signaling (CCS)Signaling System Number (SS7) is a formof 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 Switch A 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
ProtocolsMessage 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
ProtocolsMTP Level 2 (Data Link) 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
ProtocolsMTP 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
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 numbers Provides more detailed addressing information than MTPs Used as transport layer for TCAP (Transaction capabilities applications part) based services
ProtocolsTransaction 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
ProtocolsTelephone User Part (TUP) Basic call setup and tear down In many countries, ISUP has replaced TUP for call managementISDN 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)
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 theSS7 network.–Routes each incoming message to an outgoingsignaling link, based on routing information containedin the SS#7 message and a pre-defined route table–Does not offer termination services–STPs are paired to ensure redundancyThere are three levels of STPs.•National Signal Transfer Point•International Signal Transfer Point•Gateway Signal Transfer Point
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 Home Location Used in cellular networks to storeHLR Register subscriber information. Local Number Allows people to change serviceLNP Portability providers but keep their same telephone number Operation Support Associated with remote maintenanceOSS System center for monitoring and managing SS7 and voice networks. Visitor Location Used when a cell phone is notVLR Register recognized by the mobile switching center (MSC).
SS7 Link TypesA 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 Connects a secondary (local or regional) STP pair(diagonal) to a primary (inter-network gateway) STP pair in a quad-link configuration; the distinction between B and D links is arbitraryE link Connects an SSP to an alternate STP(extended)F link Connects two signaling end points (SSPs and SCPs)(fully in the same local networkassociated)
Addressing in SS7 NetworkPoint Codes are carried in signaling messages andexchanged between signaling points to identify thesource and destination of each message (24 bitaddress). 1 1 1 NETWORK CLUSTER MEMBER IDENTITY IDENTITY IDENTITY
What goes over Signaling LinkSignaling information is passed over thesignaling link in form of messages, which arecalled signaling units (SUs)3 Types of SUs are:Message signal units(MSUs)Link status signal units(LSSUs)Fill-in signal units(FISUs)
Signaling Units Message signal unit (MSU)Carries signaling associated with call setup & teardown,database query and response and SS7 network input 1 1 1 1 1 8-272 1 FLAG BSN/ FSN/ Length Service Signaling Check BIB FIB Indicator Info. octet Info. field sum
Service Information Octet1 2 3 4 5 6 7 8• Bit 1 - 4 Type of Information in Signaling Information Field• Bit 5 - 6 Whether Message is intended for National or International Network• Bit 7 - 8 To identify Message Priority
Signaling Information Field 8 – 272 Octets The first portion of this field is routing label Routing Label Identifies Message Originator, Intended destination & Signaling Link Selected. Routing Label is of 7 Octets – Destination Point Code – 3 Octets – Originating Point Code – 3 Octets – Signaling Link Selection – 1 Octet
Signaling Units Link status signaling units (LSSU) – Inform the far end about the changes in status of link – Message length can be 1 or 2 bytes 1 1 1 1 1 or 2 1FLAG BSN/ FSN/ Length Status Check BIB FIB Indicator Field Sum
Signaling Units Fill-In Signal Units (FISU)– Fill the gaps between MSU and LSSU messages– Sent only when the buffer is empty, to keep the signaling link active– Facilitate in constant monitoring of link quality. 1 1 1 1 1 FLAG BSN/ FSN/ Length Check BIB FIB Indicator Sum
ISUP Messages Initial address message (IAM): contains all necessary information for a switch to establish a connection Address 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 phone Release (REL): sent by the switch sensing that the phone hung up Release complete (RLC): each exchange that receives REL, sends an RLC message back (this acknowledges receipt of REL)
Applications Prepaid Calling Unified Messaging Local Number 800 / Free Phone Portability (LNP) Services Global Roaming Short Message International Callback Service (SMS) Virtual Office Tele-voting Internet Call Waiting/ Location-based Caller ID Services Least Cost Routing Caller Ring Back Toll Bypass Tone (CRBT)
Implementation of SS7 in GSMradio network and switching Fixed partner networkssubsystem subsystemMS MS ISDN PSTN Um MSC BTS Abis BSC EIR BTS SS7 HLR BTS VLR BSC ISDN BTS MSC A PSTN BSS IWF 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.
Features of SS7 over IP Flow Control 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 networksVoIP Network carry SS7 over IP using protocols defined bySignaling Transport (SIGTRAN) working group of theInternet Engineering Task Force (IETF).In IP telephony networks, signaling information is exchangedbetween the following functional elements.Media GatewayMedia Gateway ControllerSignaling Gateway.
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-of- line Blocking.
Transporting MTP over IP ITU specified following requirements – MTP 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.
M2PA M2PA:- MTP2 User Peer-to- Peer Adaptation Layer –Support the transport of SS7 MTP3 signaling messages over IP using the services of SCTP –Allows full MTP Level3 message handling and network management capabilities between any 2 SS7 nodes communicating over IP –Used b/w signaling gateway & media gateway controller, signaling gateway & IP signaling point, 2 IP signaling points.
M2UA M2UA:- MTP2 User Adaptation Layer – Transports SS7 MTP Level2 user messages over IP using SCTP – Provides services as MTP Level2 provides to MTP Level3 – Used between signaling gateway and media gateway controller.
M3UA M3UA:- MTP 3 User Adaptation Layer – Transports SS7 MTP Level3 user signaling messages over IP using SCTP – Provides services as MTP Level3 provides to ISUP,TUP,SCCP – Used between signaling gateway and media gateway controller or IP telephony database
SUA SUA: SCCP User Adaptation Layer – Transports SS7 SCCP User Part Signaling Message over IP using SCTP – Used between signaling gateway and IP signaling end point and between IP signaling end points.
Performance consideration & Security Requirement for SS7/IP SS7 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 services – Authentication – Integrity – Confidentiality – Availability
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