Signaling System #7

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. Switch A B Voice Trunks 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

Protocols MTP 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

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

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 SSP ITU-TS INTERNATIONAL STP GATEWAY STP NATIONAL STP ANSI SSP 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 Home Location Register Used in cellular networks to store subscriber information. LNP Local Number Portability 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) Connects signaling end point (SCP or SSP) to STP B 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 Network Point Codes are carried in signaling messages and exchanged between signaling points to identify the source and destination of each message (24 bit address). 1 1 1 NETWORK IDENTITY CLUSTER IDENTITY MEMBER IDENTITY

What goes over Signaling Link Signaling 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)

Message signal unit (MSU) 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/ BIB FSN/ FIB Length Indicator Service Info. octet Signaling Info. field Check sum

Service Information Octet 1 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

Link status signaling units (LSSU) Inform the far end about the changes in status of link Message length can be 1 or 2 bytes FLAG BSN/ BIB FSN/ FIB Length Indicator Check Sum 1 Status Field 1 or 2

Fill-In Signal Units (FISU) 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. FLAG BSN/ BIB FSN/ FIB Length Indicator Check Sum 1

Basic Call Setup Example 1 6,10 9 2 5 13 15

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 Tele-voting 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)

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

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 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-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

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