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CCS7 Overview

In the beginning, there was

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CCS7 Overview

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    1. CCS7 Overview Stephanie Sanderson January 18, 2001

    2. In the beginning, there was… PTS (Per-trunk signaling) – known as in-band signaling because the signaling and voice/data components are transmitted on the same trunk Requires the voice path to be established completely even if the call does not complete successfully Also known as Channel Associated Signaling (CAS)

    3. Now, there is… CCS7 (Common Channel Signaling Number 7) – a message-based signaling protocol that controls the information traveling between signaling points in a network. Two separate paths are used for transmitting information : one path for voice/data, one path for all signaling The maximum bandwidth per voice/data channel in PTS is 56 kbps. The maximum bandwidth per voice/data channel in CCS7 is 64 kbpsThe maximum bandwidth per voice/data channel in PTS is 56 kbps. The maximum bandwidth per voice/data channel in CCS7 is 64 kbps

    4. Advantages of CCS7 Voice/data traffic and signaling traffic travel on separate paths thus increasing bandwidth. Signaling traffic travels faster and can “look ahead” to make sure long distance and local channels are available. The improved call setup time results in increased network capacity. One signaling link can support several voice/data calls. The voice path is not set up until the signaling indicates a successful call set-up. There is room for more voice/data traffic because the signaling is on a separate trunk More efficient set-up and takedown timesThere is room for more voice/data traffic because the signaling is on a separate trunk More efficient set-up and takedown times

    5. Specifications for CCS CCS7 (Common Channel Signaling #7) North American standard for telecommunications applications Defined by American National Standards Institute (ANSI) SS7 (Signaling System #7) Defined by International Telecommunications Union – Telecommunication Standardization Sector (ITU-T) ITU-T was formerly the International Consultative Committee for Telephone and Telegraph (CCITT) CCS7 and SS7 are nearly identical and the terms are used interchangeably. In SS6 (1972) the voice and signaling followed the exact same path, but on different trunks. In SS7 (1970s), the signaling can take a completely different path from the voice by utilizing the STPs.In SS6 (1972) the voice and signaling followed the exact same path, but on different trunks. In SS7 (1970s), the signaling can take a completely different path from the voice by utilizing the STPs.

    6. Entities of CCS7 Network Service Switching Point A switching office with CCS7 voice trunk capability which can interact with service databases. Service Control Point A network database which provides a centralized place for access to network information. Signal Transfer Point A tandem point for messages destined to other nodes in the network All MSCs are SSPs. They can be integrated SSP/STPs, which means they have routing capabilities as well as database query capabilities. SCPs are databases such as an 800 or calling card database STPs are always in pairs (Mated Pairs) and are for signaling ONLY, voice can not pass through an STP.All MSCs are SSPs. They can be integrated SSP/STPs, which means they have routing capabilities as well as database query capabilities. SCPs are databases such as an 800 or calling card database STPs are always in pairs (Mated Pairs) and are for signaling ONLY, voice can not pass through an STP.

    7. More CCS7 Entities Signaling Link – Signaling connection between two nodes Link Set – Set of links connection two nodes Route – Signaling path from one node to another Linksets are made of 1 to 16 signaling links. The STP routes messages from an incoming linkset to an outgoing linkset on the message’s way to its final destination.Linksets are made of 1 to 16 signaling links. The STP routes messages from an incoming linkset to an outgoing linkset on the message’s way to its final destination.

    8. And More CCS7 Entities Route Set – Set of all routes from one node to another which is made up of linksets Associated Signaling – Direct Access Quasi-Associated Signaling – Requires at least one STP tandem of the signaling messages Routesets are the first level of redundancy, providing up to 6 routes from A to B Associated signaling is similar to SS6 in that the voice and the signaling follow the same path Quasi-Associated signaling allows for multiple paths from A to B; Paths are ranked according to cost (most efficient first)Routesets are the first level of redundancy, providing up to 6 routes from A to B Associated signaling is similar to SS6 in that the voice and the signaling follow the same path Quasi-Associated signaling allows for multiple paths from A to B; Paths are ranked according to cost (most efficient first)

    9. CCS7 Network Configuration * The mesh has 100% redundancy so if there is a point of failure anywhere in the network, the message can still reach its destination.* The mesh has 100% redundancy so if there is a point of failure anywhere in the network, the message can still reach its destination.

    10. Example of GSM CCS7 Network

    11. CCS7 Link Notation Terms A link – Access link; SSP or SCP or STP B link – Bridge link; interconnects STPs in different regions C link – Cross link; connects mated STPs D link – Diagonal link; connects primary and secondary STPs in different regions E link – Extension link; connects an SSP to an STP in a different region F link – Forward link; used for a fully associated link; directly connects an SSP to another SSP, or connects an SSP to an SCP without going through an STP

    12. SS7 Software Architecture Transaction Capabilities Application Part – Layer used to provide services to support database transaction type applications Signaling Connection Control Part – Provides additional functions to the MTP to enhance routing capabilities Message Transfer Part – Serves as a transport system providing reliable transfer of signaling messages between the network nodes * Can be mapped onto the OSI layered protocol architecture. Developed by ISO (International Standards Organization) to standardize communications protocols.* Can be mapped onto the OSI layered protocol architecture. Developed by ISO (International Standards Organization) to standardize communications protocols.

    13. The Message Transfer Part Serves as a transport system providing reliable transfer of signaling messages, in the correct sequence without loss or duplication between two signaling nodes of the network. Enables the significant information of the User Part to be transferred across the SS7 network to the required destination (Q.701) Encompasses the 3 lowest protocol layers: Level 1: Signaling Data Link Level (Physical Link) Level 2: Signaling Link Level Level 3: Signaling Network Level

    14. MTP Level 1 Concerned with the raw transmission of bits over a physical medium which can be satellite, microwave, fiber optics, etc. Transmission of bits occurs over a bi-directional path, comprised of two data channels operating together in opposite directions at the same data rate. The recommended bit-rate for ANSI is 56 kbps and for CCITT is 64 kbps, although this is dependant on the physical medium used.

    15. MTP Level 2 Together with Level 1, this level provides a signaling link between two directly connected signaling points in the SS7 network. Functions are as follows: Transfer signaling information in variable length messages (20 – 279 bytes) called Signaling Units (SU) which come as MSUs, LSSUs, or FISUs Signaling Unit synchronization and alignment Error detection and correction Error monitoring by the FSN, BSN, FIB, BIB Flow Control MTP Level 2 is point-to-point. In the following diagram: A – B – C – D ? A knows about B, B knows about A and C, C knows about B and D, and D knows about C Each node is identified by a Point Code Level 2 uses positive and negative acknowledgements to indicate when a signal unit has been correctly received or to request retransmission of corrupted signal units. For example: The FIB is a “1” for new MSUs, and a “0” for retransmitted MSUs. The FSN, BSN and BIBs are also used for error detection and correction. Error correction is done by retransmission of MSUs and LSSUs (FISUs are detected but not corrected.)MTP Level 2 is point-to-point. In the following diagram: A – B – C – D ? A knows about B, B knows about A and C, C knows about B and D, and D knows about C Each node is identified by a Point Code Level 2 uses positive and negative acknowledgements to indicate when a signal unit has been correctly received or to request retransmission of corrupted signal units. For example: The FIB is a “1” for new MSUs, and a “0” for retransmitted MSUs. The FSN, BSN and BIBs are also used for error detection and correction. Error correction is done by retransmission of MSUs and LSSUs (FISUs are detected but not corrected.)

    16. MTP Level 2 Message Types Message Signal Unit (MSU) – Used to transfer “user” signaling information from one SS7 node to another. This is the only signaling unit that is of interest in the upper layers (users) of MTP. Link Status Signal Unit (LSSU) – Used to report the status of a signaling unit. It is always placed at the beginning of the transmit queue. Fill In Signal Unit (FISU) – Used to “fill-in” gaps between “useful” messages. These are sent only when the transmit buffer is empty. FISU is mainly used for monitoring the status of a signaling link in the absence of any signaling traffic.

    17. MTP Level 2 Message Structure

    18. Signaling Unit Fields Flag – This is a specific predetermined byte which delimits two signaling units. Check – 16-bit Cyclic Redundancy Check (CRC) checksum is used for error detection Signaling Information Field (SIF) “User” specific signaling message where the user has a specific format for SCCP, TCAP, or ISUP Consists of an integral number of octets (2 – 272) First seven octects are used for Routing Level (address of the message) The standard CCITT Flag is 01111110 and is used to open and close signaling units. The standard CCITT Flag is 01111110 and is used to open and close signaling units.

    19. Signaling Unit Fields (cont) Signaling Information Octet (SIO) – Present only in the MSU and indicates: Different Message Types (Natl vs Intl) Priority of the MSU Additional routing info to the SCCP for routing to different users of TCAP Length Indicator (LI) – Indicates the number of octets between the end of the two unused octects to the start of the check field; Differentiates between the types of the SU: FISU – LI = 0 LSSU – LI = 1 or 2 MSU – LI = 3 to 63

    20. Signaling Unit Fields (cont) Status – LSSU specific field; Indicates the status of the signaling link to the far end. Sequence Numbers (FSN, BSN) – From 0 to 127 and work as follows: Forward Sequence Number is the sequence number of a message which is sent. Backward Sequence Number is the sequence number of the message which is being acknowledged Indicator Bits (FIB, BIB) – Used in conjunction with FSN and BSN for sequence and error control Sequence numbers ex: If we have already setn 3 FISUs, the next message has FSN = 4. Sequence numbers ex: Prior to our message, we have received 2 FISUs, so our BSN = 2. FIB and BIB are 0 as long as everything runs smoothly. Turns to 1 when two nodes do not agree about the number of messages sent and received (If BIB = 1, then we will resend the last message with FIB = 1 and FSN same as the first time we sent the message)Sequence numbers ex: If we have already setn 3 FISUs, the next message has FSN = 4. Sequence numbers ex: Prior to our message, we have received 2 FISUs, so our BSN = 2. FIB and BIB are 0 as long as everything runs smoothly. Turns to 1 when two nodes do not agree about the number of messages sent and received (If BIB = 1, then we will resend the last message with FIB = 1 and FSN same as the first time we sent the message)

    21. MTP Level 3 Ensures reliable transfer of messages, even in the event of signaling link and STP failure. For elaboration, the two parts are: Signaling Message Handling Signaling Network Management

    22. Signaling Message Handling Consists of three parts: Discrimination – Determines if the incoming message is destined for the point itself; If not, it will activate the routing function Routing – Determines the outgoing signaling link on which an outgoing message is to be sent towards its final destination Distribution – Delivers an incoming message to the appropriate MTP “user” once DISCRIMINATION has determined the message belongs on the node These functions are performed at each signaling point in the SS7 network based on the first 7 bytes of the SIF known as the routing level.These functions are performed at each signaling point in the SS7 network based on the first 7 bytes of the SIF known as the routing level.

    23. Routing Level (SMH, cont) All functions of Signaling Message Handling are based on the routing address encoded in the first 7 bytes of the SIF of the MSU. Routing consists of 3 fields: Destination Point Code (DPC) – Indicates final destination of the message. Originating Point Code (OPC) – Indicates the originating node in the network Signaling Link Selector (SLS) – When more than one link is available for transmission of a message between two nodes, the SLS id used to select a particular link; Messages which need to be sequenced have the same SLS

    24. Routing Level (cont) Network Identifier : Identify a network to which a node belongs (ex: city, state) Network Cluster : A group of signaling nodes that are connected to the same “Home” STP pair form a cluster (ex: zip code) Network Cluster Member : To identify a single member of the cluster (ex: street address)Network Identifier : Identify a network to which a node belongs (ex: city, state) Network Cluster : A group of signaling nodes that are connected to the same “Home” STP pair form a cluster (ex: zip code) Network Cluster Member : To identify a single member of the cluster (ex: street address)

    25. Signaling Network Management When a failure in a signaling link or STP occurs, reconfigurations are carried out so that the messages are not lost, duplicated or put out of sequence Signaling Network Management does this in three functions: Signaling Traffic Management – Diverts signaling traffic from the unavailable link or route to one or more alternative links or routes Signaling Route Management – Distributes information about the signaling network status in order to block or unblock signaling traffic Signaling Link Management – Controls the locally connected signaling links by link Activation, Deactivation and Restoration Link Activation : Used to activate a signaling link Link Deactivation : Takes the signaling link out of service provided there is no active signaling occurring on that link Link Restore (RTS) : Used to recover a failed signaling linkLink Activation : Used to activate a signaling link Link Deactivation : Takes the signaling link out of service provided there is no active signaling occurring on that link Link Restore (RTS) : Used to recover a failed signaling link

    26. Signaling Connection Control Part Enhances services of the MTP by providing addressing capability to User services through routing by DPC and SSN or GT Subsystem Number (SSN) – Local addressing information used by SCCP to identify each of the SCCP users at a node Global Title (GT) – Addresses, such as dialed digits, that do not contain information that would allow routing by MTP; Requires translation capability to translate the GT into a DPC and SSN If the DPC is like the street address, the SSN is like the name associated with that address For a full GTT (Global Title Translation), the SSP knows exactly where to route this call For a partial GTT, the SSP needs information from the SCP to route the call.If the DPC is like the street address, the SSN is like the name associated with that address For a full GTT (Global Title Translation), the SSP knows exactly where to route this call For a partial GTT, the SSP needs information from the SCP to route the call.

    27. Services of SCCP Provides four classes of service beyond addressing enhancements: Class 0 – Basic Connectionless Class: User-to-User information blocks (called Network Services Data Units, NSDUs) are delivered independently without sequencing Class 1 – Sequenced Connectionless Class: NSDUs are delivered in sequence Class 2 – Basic Connection Oriented Class: Transfer of NSDUs is performed by setting up a temporary or permanent signaling connection. Class 3 – Flow Control Connection Oriented Class: Capabilities of Class 2 with flow control, the detection of message loss and misqueuing Connectionless protocol : query without a response Connection oriented protocol : I will send a message, but you must respond (ex: IAM -> ACM)Connectionless protocol : query without a response Connection oriented protocol : I will send a message, but you must respond (ex: IAM -> ACM)

    28. Transaction Capabilities Application Part Provides a connectionless environment which can be used by an application at one SS7 node to invoke execution of a procedure in another node Consists of two sublayers: Transaction portion Component Portion * The primary use of TCAP in an SS7 network is for invoking remote procedures in support of IN services.* The primary use of TCAP in an SS7 network is for invoking remote procedures in support of IN services.

    29. Transaction Portion (TCAP) Provides means of associating messages with a specific Application Process transaction Consists of the package type and the transaction identifier which uniquely defines the transaction on both sides. A transaction consists of one or more messages exchanged between application process on different SS7 nodes. A transaction is a collection of operations, sent in components, used to perform a specific task. The package type identifies the nature of the TCAP message and the transaction termination permission of the receiving end.A transaction consists of one or more messages exchanged between application process on different SS7 nodes. A transaction is a collection of operations, sent in components, used to perform a specific task. The package type identifies the nature of the TCAP message and the transaction termination permission of the receiving end.

    30. Transaction Portion (TCAP,cont) TCAP defines six package types: Unidirectional Query with permission Query without permission Conversation with permission Conversation without permission Response Query with permission : here’s the info, you can hang-up at any time Conversation with permission : here’s the info you wanted and you can hang up at any time Conversation without permission : here’s the info you wanted, but I need info from you so you can not hang up; on-going conversation Response – closing connection, last message Query with permission ; Conversation without permission ; conversation with permission ; responseQuery with permission : here’s the info, you can hang-up at any time Conversation with permission : here’s the info you wanted and you can hang up at any time Conversation without permission : here’s the info you wanted, but I need info from you so you can not hang up; on-going conversation Response – closing connection, last message Query with permission ; Conversation without permission ; conversation with permission ; response

    31. Component Portion (TCAP) Ensures components are formatted and exchanged properly Contain either requests for action or a response to a requested action Identifies an operation with instructions for the receiving end Four component types: Invoke, Return Result, Error, Reject

    32. Integrated Services Digital Network User Part (ISDN-UP or ISUP) Provides the signaling functions that are needed to support the basic bearer service and supplementary bearer service for switched voice and non-voice applications in an ISDN environment MTP layer transfers the signaling information between ISUP parts located at different nodes

    33. ISUP (cont) Allows voice and data to be integrated into a single signal All ISUP messages include: Message Type Mandatory Fixed Part Mandatory Variable Part Optional Part

    34. ISUP Message Format

    35. Telephone User Part (TUP) Pre-cursor to ISUP (not as “smart”); Main differences in messaging: Either IAM or IAI can be used for call setup ISUP REL message contains a certain release cause whereas any number of Unsuccessful Backward Messages are used in TUP The SIF of the MSU contains TUP specific information and a TUP label

    36. TUP Message Format

    37. SS7 Layers in DMS Software MTP Level 1: LIU7 MTP Level 2: LIU7 MTP Level 3: LIU7/LIM SCCP: LIU7/CM TCAP: CM ISUP/TUP: CM and DTC7

    38. Tables related to SS7 Datafill CLLI : Common Line Language Identifier TRKGRP : Trunk Group TRKSGRP : Trunk Subgroup TRKMEM : The members of the trunk group and the physical resource that will be used (ie: which PDTC, which PCM30 of the PDTC, which timeslot of the PCM30) C7NETWRK : The point code of the node and the network to which it belongs C7RTESET : The route sets, the routes within each routeset and the destination point code (DPC). C7LKSET : Defines each link set and associates them with the Far End Point Code (FEPC). C7LINK : Each link in the linkset and it associated HW (LIU7)

    39. Tables related to SS7 Datafill (cont) C7TIMER : Various Timing Values for SS7 for Error Monitoring (MTP Level 2) ISUPDEST : Associates the CCS7 trunk with the routeset C7TRKMEM : Defines the CIC of each ISUP trunk member ADJNODE : Allows the operating company to identify restrictions present at connected exchanges. The index specified is used in table TRKSGRP. C7UPTMR : Defines the specific user part protocol timer values to be used by the trunkgroup as defined within table TRKSGRP. C7CNGSTN : Congestion Levels data for Signaling Network Management (MTP Level 3) FAILMSG : Associates the cause value received on one protocol type to map to a DMS treatment or release to be applied to another protocol type during an interworking

    40. Tables related to SS7 Datafill (cont) TMTMAP : Associates what action is to be performed when a DMS treatment is to be applied to a particular protocol type PETATTR : Allows the customer to selectively remove optional ISUP parms from outgoing signals PETSIG : Specifies ISU-T ISUP signaling options PETSERVS : Contains the services for that trunk group LTCINV : PDTCs are defined here as well as the C-side links, the physical location of the PM and hardware LIUINV : Defines the location and properties of the LIU7s LTCPSINV : Defines the P-side of the PM (ie, carriers and spans) CARRMTC : Defines the properties of the carrier C7NETSSN : Defines the far end subsystems or the Remote SS C7LOCSSN : Defines the local subsystem for a particular node

    41. ISUP Message Type Codes

    42. ISUP Message Type Codes (cont)

    43. ISUP Message Type Codes (cont)

    44. ISUP Message Type Codes (cont)

    45. ISUP Message Type Codes (cont)

    46. TUP Message Groups

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