SS7 Signaling System 7 PDF

SS7 SIGNALING SYSTEM 7  INTRODUCTION Signalling is the process of generating and exchanging information among components of a telecom system to establish, monitor or release connections and to control related network and system operations.  SIGNALLING TYPES...

SS7 SIGNALING SYSTEM 7  INTRODUCTION Signalling is the process of generating and exchanging information among components of a telecom system to establish, monitor or release connections and to control related network and system operations.  SIGNALLING TYPES Signalling Common Inchannel channel Low Voice freq PCM DC Associated Nonassociated freq (for FDM) (Dig sig scheme) Inband Outband  ARCHITECTURE:Message Transfer Part (MTP Level 1) SIGNALLING DATA LINK OR Physical Layer  Defines phy & electrical characteristics of sig links.  Signalling data links are full duplex links. Provides an interface to the actual physical channel over which communication takes place CCITT recommends 64Kbps transmission whereas ANSI recommends 56 Kbps ARCHITECTURE:MTP Level 2 (SIGNALLING LINK)  This layer converts a potentially unreliable physical link to reliable data link. Functions are : – Data to be delivered with no loss or duplication. – Blocks of data to be delivered in same order in which they originate. – There is a match between rec capacity & txn rates-flow control.  Blocks of data called Signaling Units are used. Variable Length Packet Messages are defined here Implements flow control, message sequence validation, error checking and message retransmission Monitors links and reports their status Tests links before allowing their use Provides sequence numbers for outgoing messages 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 8 7 1 7 1 6 2 8 8n, n>4 16 8 B F FLAG BSN I FSN I LI SIO SIF CRC FLAG B B BSN = Backward Sequence number BIB = Backward indicator bit FSN = Forward Sequence number FIB = Forward indicator bit LI = Length indicator SIO = Signaling information octet SIF = Signaling info field CRC = Cyclic redundancy code MESSAGE SIGNAL UNIT : MSU 8 7 1 7 1 6 2 8 8n, n>4 16 8 B F FLAG BSN I FSN I LI SIO SIF CRC FLAG B B 01111110 Bit stuffing in the data part to avoid appearance of flag in frame. Transmitter inserts extra ‘0’ whenever it comes across five consecutive ones in the data part of the SU.  the rec on detecting five ones deletes the ‘0’ following it. MESSAGE SIGNAL UNIT : MSU 8 7 1 7 1 6 2 8 8n, n>4 16 8 B F FLAG BSN I FSN I LI SIO SIF CRC FLAG B B Backward Sequence Number (BSN). Number of last MSU successfully recd. Piggybacked ack. - ve ack by inverting Backward Indication Bit (BIB). Forward Sequence Number (FSN). Number of MSU For retrans, Forward Indication Bit (FIB) is inverted. FIB and BIB remains unchanged until another retrans or –ve ack. MESSAGE SIGNAL UNIT : MSU 8 7 1 7 1 6 2 8 8n, n>4 16 8 B F FLAG BSN I FSN I LI SIO SIF CRC FLAG B B Length Indicator (LI). Length in octets of SIF. Max value 63. SU type indicator 0=FISU 1-2=LSSU 3-63=MSU  MESSAGE SIGNAL UNIT : MSU 8 7 1 7 1 6 2 8 8n, n>4 16 8 B F FLAG BSN I FSN I LI SIO SIF CK FLAG B B 4 4 Service Subservice Indicator Fd Service Information octet specifies type of msg and whether msg relates to national/inter-national n/w.  MESSAGE SIGNAL UNIT : MSU Service Info Octet 8 7 1 7 1 6 2 8 8n, n>4 16 8 B F FLAG BSN I FSN I LI SIO SIF CK FLAG B B Service Indicator – Specifies user of MTP 4 4 0000 Sig NW mgmt msg $ - (Level – 3 info) Service Subservice 0001 Sig NW testing & maint msg Indicator Fd 0011 SCCP @ - (Level – 4 info) Subservice Fd 0100 TUP 00XX International NW 0101 ISUP 01XX Spare 0110 Call & cct related msg 10XX National 0111 Facility registration and cancellation 11XX Reserved for 1000 MTP testing user part national use 1001 Broadband ISDN user part XX : may be used for msg 1010 Satellite ISDN user part priority, used in case of 1011- Spare congestion only 1111 MESSAGE SIGNAL UNIT : MSU Signalling Info field 8 7 1 7 1 6 2 8 16 8 8n, n>4 B F FLAG BSN I FSN I LI SIO SIF CK FLAG B B 14 14 4 8n,n>4 Destination Origination User part SLS Pt Code Pt Code Or NW mgmt Routing Lable Signalling link selection 14 bit source and destination node address. SLS (Signaling Link Selection) used to select path among alt routes. Second part is user data from some SS7 application or NW mgmt data i.e. ISDN User Part or Maint Data. MESSAGE SIGNAL UNIT : MSU 8 7 1 7 1 6 2 8 8n, n>4 16 8 B F FLAG BSN I FSN I LI SIO SIF CK FLAG B B Error detecting code. Uses CRC 16 Bit. CRC is sent by Tx. At Rx, it is calculated and compared. LINK STATUS SIGNAL UNIT : LSSU 8 7 1 7 1 6 2 8 or 16 16 8 B F FLAG BSN I FSN I LI SF CK FLAG B B Status Fd (SF) carried as user data instead of SIO & SIF in MSU. Indicates senders view of the actual status of link. Info used for NW mgmt. Status Fd 000 Out of alignment 001 Normal alignment 010 Emergency alignment 011 Out of service 100 Processor outage 101 Busy  FILL-IN SIGNAL UNIT : FISU 8 7 1 7 1 6 2 16 8 B F FLAG BSN I FSN I LI CK FLAG B B Same structure as MSU and LSSU with no user fd. Transmitted when no other signal units are available. Allows consistent error monitoring to detect and remove faulty links. SIGNALING NW LEVEL-3 SIGNALING SYSTEM 7  SIGNALING N/W FUNCTIONS Level - 3 Signaling n/w fns are Msg handling and N/w mgmt. Msg handling Discrimination Routing Distribution N/W Management Tfc management Link management Route management SIGNALING MSG HANDLING FUNCTIONS MSG HANDLING Discrimination Analyses the destn code in the add fd, decide if a msg is to be routed to another node or distributed to one of the user parts in the loc node. If destn, give it to distribution function else to routing function. This fn needed only in STPs Routing Based upon info in sig info fd (SIF-SLS,4 bit fd=16 routes). In gen all the control sigs associated with a single call follow the same route ensuring sequential arrival of the msgs. Distribution Analyses Service indicator in service info octet (SIO). Determines the user part to which it is to be given. SIGNALING NW MGMT FNS Main objective of level 3 sig n/w fns is the n/w mgmt fn. To monitor sig links and overcome link failures or degradation. Communicate alt routes to the affected nodes. Aim is to achieve non-availability fig of not more than 10 min in a year. Signaling NW mgmt functions. Signaling tfc mgmt. Signaling link mgmt. Signaling route mgmt. SIGNALING NW MGMT FNS SIGNALING TFC MANAGEMENT Changeover – Divert tfc to one or more alt links in the event of link unavailability(failure). Changeback – Reestablish tfc on a signaling link that becomes avbl. Forced rerouting – Divert tfc to an alt route whn a route becomes unavbl(congesion). Controlled rerouting – Divert tfc to a route that has been made avbl. Signaling point restart Management inhibiting Signaling tfc flow control SIGNALING NW MGMT FNS SIGNALING LINK MANAGEMENT Signalink link activation, restoration & deactivation – Restore failed links, activate new links, & deactivate links. Link set activation – Activate a link set not having any links in service. Automatic allocation of signaling terminals & signaling data links – Allocate terminals to links. SIGNALING NW MGMT FNS SIGNALING ROUTE MANAGEMENT Transfer controlled procedure – Performed at STP in case of link congestion. Msg sources told to stop sending msgs having a congestion priority less than congestion level of the link. Transfer prohibited procedure - Performed at STP to inform adjacent signaling points that they must no longer route to a particular destination via this STP. Transfer allowed procedure – Informs adjacent signaling points that routing to a given destination is now normal. Transfer restricted procedure – If possible adjacent signaling points should no longer route to a particular destination via this STP. Signaling route set test procedure Signaling route set congestion test procedure 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 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 SS7 NETWORK STP STP STP STP STP SS7 NETWORK SS7 Signaling Links  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 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 arbitrary E link Connects an SSP to an alternate STP (extended) F link Connects two signaling end points (SSPs and SCPs) (fully in the same local network associated) ARCHITECTURE: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  LEVEL 4 SIGNALING SYSTEM 7  PROTOCOL ARCHITECTURE Sig connection control part (SCCP) : two main fns 1. Capabilities to address applications within a node MTP n/w mgmt msgs and basic call set up msgs are addressed to node as a whole, other msgs are used by separate applications (sub-sys) within a node. SCCP allows these sub-sys to be addsd explicitly. Eg : 1600 call processing, sig services (callback), billing etc  PROTOCOL ARCHITECTURE ISDN User Part (ISUP)/ Tele User Part (TUP) It defines the msgs and protocol used in estab and tear down of voice and data calls, and to manage tk n/ws on which they rely. TUP invoked in response to actions by subs at tele. Control sig associated with TUP deals with estab, maint and termination of calls. Similarly ISUP deals with ISDN subs and related fns. These msg relies exclusively on MTP to transport msg b/n concerned node.  PROTOCOL ARCHITECTURE Op, Maint and Adm Part (OMAP) It defines the msgs and protocol designed to assist, administrators of SS7 n/w. Msg relating to n/w mgmt, ops and mgmt (validating n/w routing tables, diagnosing link troubles etc)  ARCHITECTURE 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  ARCHITECTURE 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)  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) Signaling System 7 Basic call setup 1 IAM initial address message STP STP SS7 LINKS 2 ACM address complete message 2a. ACM 1a. IAM 3 ANM answer message 3a. ANM (initiate billing) 4a. REL 2b. ACM 5a. RLC * CONVERSATION TAKES PLACE 3b. ANM 1b. IAM 4 REL release 5b. RLC 4b. REL message, cause SSP SSP 5 RLC release complete Voice Circuit message (terminate billing) Presenter: Bob Wright  Basic call setup (using ISUP) SSP DTMF SSP SSP Signaling AN M M RI AC N I AM AM NG RE M AC SP ON M SE IA STP IAM Initial Address Message MTP L3 I AM SSP ACM Address Complete Message packet forwarding ANM Answer Message 42  Call setup with advanced services (using TCAP) 516-555-1278 SCP STP STP Local Carrier ISUP Local Carrier IAM Long distance carrier 5 1 6- Switch Switch 55 5- Long Distance 400 SCP 5-4 STP STP SCP 1278 -55 800 00 44 5- Switch 0- 55 Switch ring 8 0 Signaling links Voice trunks 800-555-4400 STP: Signal Transfer Point 516-555-1278 SCP: Service Control Point 43  SS7 Architecture 3 important devices: 1. Signal Switching point: SS7 capable telephone switches. They originate, terminate, or switch calls. SSP 2. Signal Transfer point: SS7 data packet switches. They receive and route incoming signaling messages to the proper location. STP 3. Signal Control point: Databases distributed along the network. They allow advanced call processing capabilities. SCP The intelligence in the network is now distributed among 3 types of equipment.  SS7 Architecture The signal transfer (packet switch) and control points (databases) always come in pairs to provide redundancy. They are exact copies of each other. This redundancy was put in place so that the telephone network could have a high level quality of service (QOS). Since the devices come in pairs, they actually have 4 lines among them. There aren’t necessarily connections to the SCP (database) at every switch. They aren’t needed at every switch.  SS7 Signaling Links We are talking about the packet switched network, NOT the voice network. The voice network is still the digital T1 type interface. The links among the packet switches (STP) are actually fairly slow by our standards: 56 Kbps, full duplex. However, this system has been in place since the mid 1990’s, and it suits current needs.  SS7 Signaling Links 3 types: Associated Nonassociated Quasi-associated All have to deal with how many packet switches and voice switches that are used.  SS7 Signaling Links Associated: There is a static 1 to 1 correspondence to the voice lines and the signaling packet line. This occurs when 23 lines of a T1 are used to carry voice and the 24th is used to carry the SS7 signaling. The most wasteful method by far as the packet channel can often sit empty.  SS7 Signaling Links Nonassociated: The voice path is completely separate from the signaling path. Many nodes (STPs) must be passed through before the call routing signal is passed to the correct place. Very often used.  SS7 Signaling Links Quasi-associated: Reduces the number of STPs that the signaling information must be transported through. Requires much more cabling, but it can be much faster. The signal is only allowed to pass through 1 STP between the voice switches.  SS7 Signal Links Each type of link is given a different name in the architecture depending on what it does. The names are labeled A through F. For example, a C link is the link that occurs between a mated pair of STPs. These designations just help us to manage what type of links we are looking at on the network.  SS7 Link Types “A” Link: An access link connects a SSP or SCP to an STP. “B” Link: A bridge link connects an STP to another STP. “C” Link: A cross link connects two “mated pair” STP. This is done to improve reliability “D” Link: A diagonal links similarly to a B link “E” Link: An extended link connects an SSP to an extra STP in the event that the A link cannot reach one. “F” Link: A fully associated link connects SSP’s  Different message flows ITU ISUP message exchange during a call Local Local Exchange Exchange IAM ACM CPG ANM Voice Transfer REL RLC ANSI ISUP: CPG before ACM Gonzalo Camarillo Fall ‘99 VON Atlanta September 28th, 1999 VON protocols - SIP Advanced Signalling Research Lab.  Signaling System 7 Basic call setup 1 IAM initial address message STP STP SS7 LINKS 2 ACM address complete message 2a. ACM 1a. IAM 3 ANM answer message 3a. ANM (initiate billing) 4a. REL 2b. ACM 5a. RLC * CONVERSATION TAKES PLACE 3b. ANM 1b. IAM 4 REL release 5b. RLC 4b. REL message, cause SSP SSP 5 RLC release complete Voice Circuit message (terminate billing) Presenter: Bob Wright Signaling System 7 Slide 54