Ss7 Introduction Li In


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This is an introductury SS7 presentation. More of a guide than a full presentation from a data(TCP/IP) person perspective

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  • Each SP has one or more addresses associated with it. Those addresses are called Signaling Point Codes, PC . It is also possible to use an alias which is not PC . That is called Global Title ( GT ). Global title is not an SS7 address and needs to be translated to one to be useful. Nodes in the SS7 network are addressed by their Signaling Point Code ( SPC ). When locations like an SCP (we will explain later what this is) are addressed, Point Code is not enough. Another value must be used to identify the service application that is sought. For this purpose, the SS7 simply uses a value (represented in the message packet by a byte, and therefore, in the range of 0 to 255) which is called a Subsystem Number ( SSN ). Subsystem numbers typically identify databases. However, they may also identify other services. For example, a switch may offer several features. Because several features are offered, simply sending the request to the SPC of the switch is not enough. A subsystem number will be used as well in order to specifically address the exact service that is required.
  • We will show all of the above when we go deeper into the protocol.
  • In the third case where the dialled number is, say an 800 number, the SSP reaches to SCP with the help of an STP using TCAP protocol. In turn SCP returns a new Global Title (destination address) that can be routed in the SS7 network. A global title is an address (e.g., a dialled 800 number, calling card number or mobile subscriber identification number) that is translated by SCCP into a destination point code and subsystem number
  • In SS7, Global Title is an alias for PC or (PC + SSN). If the SSP does not know the destination of a global title, it sends a query in the form of TCAP messages to its local STP. When the query gets to the STP, the global title digits are given to the STP's SCCP. The STP will then translate the SCCP address fields and determine, through its own translation tables the address of the application (SCP). The Global Title Translation provides the SSN of the database and point code of the SCP that interfaces that database application.
  • One, two or all three may be present in an SS7 message as Calling or Called party address. If the associated message requires to be routed over the SS7 network, translation is required. Translation of the GT will result in a DPC being produced and possibly also a new SSN and GT. A field is also included in the address indicator to identify the format of the global title. The MTP part of SS7 stack only needs PCs but at higher levels other addressing types in various combinations are used. When a DPC (Destination Point Code) has more than one application running on it we need something additional to distinguish between them. So comes the usage of SSN. The different applications in a node (Usually an SCP) can be Call Center, Credit card billing application, 1-800 data base etc. So SSN can be an application identifier and/or a data base identifier.
  • ISUP functionality can be further broken down into three procedural categories. The first of these is Signalling Procedure Control (SPRC) which directly interfaces with the services of the MTP. The SPRC, in turn, provides support for Circuit Supervision Control (CSC) and for Call Processing Control (CPC). The application which deals with the circuit connection requirements of the switch, and simultaneously with SS7 signalling, is usually referred to as a Call Control application.
  • IAM- Initial Address Message. The ISUP message with all the info required to establish a connection and also to reserve a circuit for that purpose. In addition to Called address, BW requirement, type of call etc. is also relayed. SAM- For cases where all the digits were not included in IAM an additional message ( SAM ) is sent with the additional digits. ACM- Acknowledgment to IAM. It means that the receiving switch has reserved circuit asked in IAM. When ACM is received a “Ringing”, ring back tone is sent to the connection initiator. CPG- An optional, Call progress advancing message. ANM- When the destination user picks up the phone the switch send a DC current to the phone and also send back to the neighbouring switch the ANM message which is carried back from switch to switch all the way back the source switch. REL- If the destination is Busy the destination exchange sends back that message. As a result the source exchange sends back to the originating client a Busy tone. To make sure that Busy signal does not arrive before a ringing tone a delay is introduced before sending a ringing tone.
  • Taken from
  • ST is last digit state.
  • tacm - This is the Receipt of A ddress C omplete M essage at outgoing international exchange. T9 in ISUP . Time out value is 2-4 minutes.
  • When the called party answers, the destination switch terminates power ringing of the called line, removes audible ringing tone from the calling line and sends an Answer Message ( ANM ) to the originating switch. The originating switch initiates billing after verifying that the calling party's line is connected to the reserved trunk .
  • PT (Payload Type) 0 above in “m=audio 49172 RTP/AVP 0” is PCMU according to RFC3551. Above “a=rtpmap:0 PCMU/8000 “ the rtpmap can be found in RFC4566 . The format for attribute (a) is : a=rtpmap:<payload type> <encoding name>/<clock rate> [/<encoding parameters>] If the Alice INVITE request did not include a max-request header filed than the proxy must add one with the value of 70! When a UAC sends a request to a proxy server, the proxy server may decide to authenticate the originator before the request is processed. The proxy can challenge the originator to by returning a 407 response (Proxy Authentication Required) with a Proxy-Authenticate header containing the challenge. The client can re-send the request with a Proxy-Authorization header which provides the credentials that match the challenge. A client may provide the credentials also before being challenged in order to avoid the delay and extra processing of the 407 response (the credentials may be built according to cached challenges). Both challenge and credentials are built using a cryptographic hash so that certain values, such as password, are not sent in the clear
  • AOR- Address of Record in SIP protocol- RFC3261. It is a SIP or SIPS URI that points to a domain with a location service that can map the URI to another URI where the user might be available. Typically, the location service is populated through registrations. An AOR is frequently thought of as the "public address" of the user. If a proxy wishes to remain on the path of future requests in a dialog created by this request (assuming the request creates a dialog), it MUST insert a Record-Route header field value into the copy before any existing Record-Route header field values, even if a Route header field is already present.
  • Satellite Call Indicator is used to indicate if one or more satellite links are used. Excessive use of satellite might reduce the call quality. Forward Call Indicators include National/International Call End to End method (Pass along or SCCP-Connectionless) SUP indicator (Whether ISUP is used for every leg of the connection) Calling Party Category can be “Regular Subscriber”, “payphone”, “operator” or “test call”
  • An MTP-L2 message is called Signalling Unit.
  • LSSU- Link Status Signalling Unit. This unit is transmitted in both direction and carries Link Status information between Signalling Points. MSU- Message Signalling Unit. This unit carries all call control, data base query answer/info FIU- Fill In Signalling unit. This unit is transmitted in the absence of MSU/LSSU frames.
  • Ss7 Introduction Li In

    1. 1. Basic Introduction to SS7 Moshe Haviv September 2009 [email_address]
    2. 2. Contents <ul><li>Signalling and methodology </li></ul><ul><li>Introduction to SS7 </li></ul><ul><li>SS7 protocol stack </li></ul>
    3. 3. Signalling and methodology <ul><li>Two basic approaches: </li></ul><ul><li>Channel Associated Signalling- CAS </li></ul><ul><li>Signalling and voice/media take the same path/Time slot </li></ul><ul><ul><li>Robbed bit/Wink </li></ul></ul><ul><ul><li>MFCR2/TS16 Multiframe </li></ul></ul><ul><li>Common Channel Signalling- CCS </li></ul><ul><ul><li>Q.921/Q.931 </li></ul></ul>
    4. 4. Introduction to SS7 <ul><li>Network wide CCS for PSTN </li></ul><ul><li>SS7 Defines the elements and procedures for </li></ul><ul><ul><li>User Identification </li></ul></ul><ul><ul><li>Routing </li></ul></ul><ul><ul><li>Billing </li></ul></ul><ul><ul><li>Setting up/disconnect calls </li></ul></ul><ul><ul><li>Manage Calls </li></ul></ul>
    5. 5. Basic SS7 properties <ul><li>A private network consisting well defined Network Elements and links </li></ul><ul><li>Network Elements are called Signalling Point (SP) </li></ul><ul><li>Each SP is identified with a unique address-Point Code (PC) </li></ul><ul><li>Various types of SPs and Links are defined </li></ul><ul><li>Basis for Intelligent Networking (IN) </li></ul>
    6. 6. <ul><li>Out of Band Signalling (CCS) </li></ul><ul><li>High Signalling data rates- 56KBPS/64KBPS and higher </li></ul><ul><li>Packet Switching </li></ul><ul><li>Connectionless, bursty and short duration messages </li></ul><ul><li>Reliable and Flexible </li></ul>Basic SS7 properties (Contd.)
    7. 7. SS7 Features <ul><li>Flow Control </li></ul><ul><li>Error detection and retransmission capability </li></ul><ul><li>Controls for congestion control </li></ul><ul><li>Peer entity status detection </li></ul><ul><li>Security mechanisms </li></ul>
    8. 8. A General SS7-PSTN-IP network
    9. 9. And now an SS7 network- Standalone
    10. 10. SSP – Service Switching Point <ul><li>It is basically a switch with SS7 interfaces </li></ul><ul><ul><li>Class 5 - Local Exchange </li></ul></ul><ul><ul><li>Class 4 - Tandem switch </li></ul></ul><ul><li>Determines how to setup, manage and terminate a call </li></ul><ul><li>Based on dialled numbers, sets up SS7 messages </li></ul><ul><li>Sends messages using </li></ul><ul><ul><li>ISUP - ISDN User Part </li></ul></ul><ul><ul><li>TCAP - Transaction Capabilities Application Program </li></ul></ul>
    11. 11. SSP Call setup scenarios <ul><li>Call originating and terminating on the same switch (SSP) </li></ul><ul><li>No SS7 signalling required </li></ul><ul><li>Call targeting a known SSP </li></ul><ul><li>The SSP routes the call with the help of its routing tables </li></ul><ul><li>using ISUP messages </li></ul><ul><li>Call destined to an unknown SSP </li></ul><ul><ul><li>The Global Title (dialled number) is either an 800 number or ported one </li></ul></ul><ul><ul><li>The SSP gives the address of the next hop SSP </li></ul></ul><ul><ul><li>The IAM message is sent to that SSP with the help of STP(s) </li></ul></ul>
    12. 12. STP – Signalling Transfer Point <ul><li>Router/switch/Gateway of the SS7 Network </li></ul><ul><li>They are normally paired for reliability </li></ul><ul><li>Routes messages based on destination Point Codes on the message header </li></ul><ul><li>For cases where the local SSP does not know the destination GT it must provide GT translation </li></ul><ul><li>Can provide multiple Signalling routes to destination </li></ul>
    13. 13. SCP – Service Control Point <ul><li>Provides Application access such as to a data base </li></ul><ul><li>An SCP communicates with applications using primitives </li></ul><ul><li>The protocol to use and access a data base is called TCAP </li></ul><ul><li>SCP may consist of various Sub Systems each with its own SSN (Sub system Number ) </li></ul>
    14. 14. SS7 Protocol Stack
    15. 15. SS7 transported over other networks
    16. 16. Addressing in SS7 <ul><li>Point Code (PC) is the Basic Addressing units for each SP. </li></ul><ul><li>It should be unique throughout the network. </li></ul><ul><li>Two major versions: </li></ul><ul><li>ITU </li></ul><ul><ul><li>Basic - A number in the range of 0- 16383 </li></ul></ul><ul><ul><li>International- 14 bits [3-8-3] </li></ul></ul><ul><ul><li>[Zone: 0-7]- [Area: 0-255]-[Point: 0-7] </li></ul></ul><ul><li>ANSI </li></ul><ul><ul><li>24 bits [8-8-8 bits] </li></ul></ul><ul><ul><li>[Network ID]-[Cluster ID]-[Network Cluster Member] </li></ul></ul>
    17. 17. Addressing in SS7 (Contd.) <ul><li>Addressing in SS7 makes use of three distinct elements </li></ul><ul><ul><li>Point Code – PC </li></ul></ul><ul><ul><li>Sub-System Number - SSN </li></ul></ul><ul><ul><li>Global Title - GT </li></ul></ul><ul><ul><li>The Global Title (GT) may comprise of dialed digits or another form of address that is not recognized in the SS7 network. It is basically an alias for (PC + SSN) </li></ul></ul>
    18. 18. GT Usage example
    19. 19. SS7 Protocol Stack elements <ul><li>ISUP </li></ul><ul><li>TCAP </li></ul><ul><li>MTP-L1 </li></ul><ul><li>MTP-L2 </li></ul><ul><li>MTP-L3 </li></ul>
    20. 20. ISUP <ul><li>Most widely used part of SS7- ITU Q.764 </li></ul><ul><li>Protocol to establish/tear down connections </li></ul><ul><li>It defines two types of services </li></ul><ul><ul><li>Basic Services- To establish tear down connections </li></ul></ul><ul><ul><li>Supplementary Services- To maintain connections </li></ul></ul>
    21. 21. ISUP Elements in detail
    22. 22. Basic ISUP messages <ul><li>IAM – Initial Address Message </li></ul><ul><li>SAM- Subsequent Address Message </li></ul><ul><li>ACM – Address Complete Message </li></ul><ul><li>CPG – Call Progress Message </li></ul><ul><li>ANM – Answer Message </li></ul><ul><li>REL - Release Message </li></ul><ul><li>RLC - Release Complete Message </li></ul>
    23. 23. A basic ISUP call scenario
    24. 24. ISUP-ISUP Successful call trace
    25. 25. ISUP-ISUP Successful Call trace
    26. 29. ISUP messages list 05 00000101 Continuity 07 00000111 Connect 2F 00101111 Confusion 31 00110001 Charge information 1B 00011011 Circuit group unblocking acknowledgement 19 00011001 Circuit group unblocking 29 00101001 Circuit group reset acknowledgement 17 00010111 Circuit group reset 2B 00101011 Circuit group query response 2A 00101010 Circuit group query 1A 00011010 Circuit group blocking acknowledgement 18 00011000 Circuit group blocking 2C 00101100 Call progress 15 00010101 Blocking acknowledgement 13 00010011 Blocking 09 00001001 Answer 06 00000110 Address complete HEX Binary ISUP Messages
    27. 30. ISUP messages list (Contd.) 0C 00001100 Release 28 00101000 Pass-along 30 00110000 Overload 32 00110010 Network resource management 24 00100100 Loop back acknowledgement 01 00000001 Initial address 03 00000011 Information request 04 00000100 Information 37 00110111 Identification response 36 00110110 Identification request 08 00001000 Forward transfer 1F 00011111 Facility request 21 00100001 Facility reject 20 00100000 Facility accepted 33 00110011 Facility 11 00010001 Continuity check request 05 00000101 Continuity HEX Binary ISUP Messages
    28. 31. ISUP messages list (Contd.) 2D 00101101 User-to-User information 34 00110100 User Part test 35 00110101 User Part available 2E 00101110 Unequipped CIC 16 00010110 Unblocking acknowledgement 14 00010100 Unblocking 0D 00001101 Suspend 02 00000010 Subsequent address 38 00111000 Segmentation 0E 00001110 Resume 12 00010010 Reset circuit 10 00010000 Release complete HEX Binary ISUP Messages
    29. 32. SIP to PSTN Call trace- brief
    30. 37. SIP to PSTN Call trace- detailed
    31. 46. General ISUP Message format
    32. 47. IAM message contains <ul><li>Nature of Connection Indicator (NOC) </li></ul><ul><ul><li>Satellite Indicator </li></ul></ul><ul><ul><li>Continuity Indicator </li></ul></ul><ul><ul><li>Echo Indicator </li></ul></ul><ul><li>Forward Call Indicators </li></ul><ul><li>Calling Party’s category </li></ul><ul><li>Transmission Media Requirement (ITU) </li></ul><ul><li>User Service Info (ANSI) </li></ul><ul><li>Called Party Number </li></ul><ul><li>Optional Parameters </li></ul>
    33. 49. MTP L1
    34. 50. MTP L1 <ul><li>The SS7 Physical layer that defines the electrical/optical signal cable definitions </li></ul><ul><li>The physical interfaces include </li></ul><ul><ul><li>E1 - 2048 KBPS 32 *64KBPS channels </li></ul></ul><ul><ul><li>DS1 - 1544 KBPS 24 *64KBPS </li></ul></ul><ul><ul><li>V35 - 64 KBPS </li></ul></ul><ul><ul><li>DS-0 - 64 KBPS </li></ul></ul><ul><ul><li>DS-0A – 56KBPS </li></ul></ul>
    35. 51. MTP L2
    36. 52. MTP-L2 <ul><li>This layer provides a reliable means of data (signalling info) transfer between adjacent SPs </li></ul><ul><li>Equivalent to Layer 2 in OSI model </li></ul><ul><li>ITU-T Q.703 standard </li></ul><ul><li>Signal Unit Delimitation and Alignment </li></ul><ul><li>Make sure blocks of data are delivered in correct order: Adding the correct sequence number to each frame sent </li></ul><ul><li>Check the integrity of link. </li></ul><ul><li>Signal Unit Error detection and correction </li></ul><ul><li>Flow control: Send back busy signal when the buffer exceeds a threshold </li></ul>
    37. 53. MTP-L2(Contd.) Three kinds of Signalling Units