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ISDN Integrated Services Digital Network

ISDN Integrated Services Digital Network. What is ISDN ?. 1. End-to-end digital connectivity 2. Enhanced subscriber signaling 3. A wide variety of new services (due to 1 and 2) 4. Standardized access interfaces and terminals. Original idea in the 1980’s.

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ISDN Integrated Services Digital Network

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  1. ISDN Integrated Services Digital Network

  2. What is ISDN ? 1. End-to-end digital connectivity 2. Enhanced subscriber signaling 3. A wide variety of new services (due to 1 and 2) 4. Standardized access interfaces and terminals Original idea in the 1980’s ISDN is not a “new” network separated from the PSTN. Interworking with “normal” PSTN equipment is very important. interaction is possible ISDN terminal PSTN terminal

  3. Evolution of the PSTN / ISDN Step 1: all-analogue network (before 1960)

  4. Evolution of the PSTN / ISDN Step 2: digital transmission in the core network (1960 - 1980) PDH transmission systems (2 - 140 Mbit/s)

  5. Evolution of the PSTN / ISDN Step 3: digital switching of 64 kbit/s channels (1970 - 1990) Time switching technology

  6. Evolution of the PSTN / ISDN Step 4: common channel signalling in the core network (1980 - 2000) SS7

  7. Evolution of the PSTN / ISDN Step 5: PDH systems are being replaced by SDH (1990 ...) SDH transmission systems (155, 620 Mb/s)

  8. Evolution of the PSTN / ISDN Step 6: digital access lines (ISDN, ADSL) installed (1990 ...) End-to-end digital user data End-to-end digital signalling

  9. Success of a new concept depends on: Public network => standardization important (there may be different equipment vendors, operators …) => open interfaces Critical mass of services, subscribers, and terminal equipment is needed before concept can be made comercially attractive (chicken and egg problem) Problem-free evolution & concept integration Does the user need this new concept?

  10. Digital access: several alternatives ISDN modem ADSL Bit rate (kb/s) 2 x 64 max. 50 much larger Connection fast slow fast setup time Popularity little great increasing However, large impact on signalling protocols

  11. PSTN vs. ISDN user access 300 … 3400 Hz analogue transmission band “poor-performance” subscriber signaling PSTN Basic Rate Access ISDN 2 x 64 kbit/s digital channels (B channels) 16 kbit/s channel for signaling (D channel) 30 x 64 kbit/s digital channels (B channels) 64 kbit/s channel for signaling (D channel) concatenation of B channels possible Primary Rate Access ISDN

  12. Telecommunication services Basic telecommunication services Bearer services provide the capability of transmitting signals between network access points. Higher-level functionality of user terminals is not specified. Teleservices provide the full communication capability by means of network functions, terminals, dedicated network elements, etc. Supplementary services A supplementary service modifies or supplements a basic telecommunication service. It cannot be offered to a customer as a stand-alone service.

  13. Services examples • Some typical teleservices • Telephony (normal, high quality) • Telefax (Group 3, Group 4) • Video-telephony • Some typical bearer services • Speech (transparency not guaranteed) • 64 kbit/s unrestricted • 3.1 kHz audio (non-ISDN interworking) • Some typical supplementary services • CLIP / CLIR • Call forwarding / waiting / hold • Charging supplementary services

  14. Basic rate access – user interface Exchange S/T Network Termination U Terminal Adaptor Line Interface Circuit Bi-directional 192 kbit/s 160 kbit/s echo canceling or time compression R Non-ISDN terminal ISDN terminal Exchange Subscriber (premises) network

  15. Primary rate access – user interface PBX U Line Termination PBX equipment manufacturer specific solutions Standard 2 Mb/s TDM connection (PDH or SDH) Exchange 64 kb/s D channel in one PCM time slot

  16. ISDN Signalling Protocols

  17. OSI reference model 7. Application A (protocol residing in a) lower layer provides certain services to a (protocol in a) higher layer 6. Presentation 5. Session Protocol Data Units (PDU) 4. Transport 3. Network Headers Rx end Tx end 2. Data Link 1. Physical

  18. Relation between network connection elements and OSI model 7. Application Some examples: 6. Presentation Gateway (GW), Interworking function (IWF) 5. Session 4. Transport 3. Network Router, Switching function 2. Data Link Bridge, Relaying function 1. Physical SDH cross-connect

  19. Tasks of OSI layers: 7. Application User application 6. Presentation Compression & coding 5. Session Dialogue control 4. Transport End-to-end flow & error control 3. Network Switching & routing 2. Data Link Link-layer flow & error control 1. Physical Multiplexing & transport of bits

  20. Typical protocol interaction App App : : Switch, router Relay, bridge Tran Tran Net Net Net Link Link Link Link Link Phy Phy Phy Phy Phy Phy End node End node Intermediate nodes

  21. Signalling protocols for end-to-end connection User interface PSTN Network User interface SS7 Q.931 Q.931 ISUP ISUP Q.931 Q.931 DSS1 DSS1 MTP 3 MTP 3 Q.921 Q.921 MTP 2 MTP 2 Q.921 Q.921 I.430 I.430 MTP 1 MTP 1 I.430 I.430 contains the signalling messages for call control

  22. Layered DSS1 signaling structure DSS1 = Digital Subscriber Signalling system no.1 Layer 1: Bit sequence structure, framing & multiplexing Layer 2: Link control (HDLC-type protocol called LAPD) Layer 3: Signaling messages (application layer) I.430 Q.921 Q.931

  23. LAPD (Q.921) is used for Establishing data link connections identified by the Data Link Connection Identifier (DLCI = SAPI + TEI) Frame delimiting, alignment and transparency, allowing recognition of frames transmitted over the D-channel Flow control: (a) to maintain the sequential order of frames across a data link connection, (b) temporarily stopping transmission Error Control: detection of errors on a data link connection, recovery from errors, and notification to the management entity of unrecoverable errors

  24. Q.931 Call-related messages Call establishment messages: ALERTING CALL PROCEEDING CONNECT CONNECT ACKNOWLEDGE PROGRESS SETUP SETUP ACKNOWLEDGE Call clearing messages: DISCONNECT RELEASE RELEASE COMPLETE Similar functions as ISUP in SS7

  25. Typical content of ISDN Set-up message Called party (user B) number & numbering plan Calling party (user A) number (+ CLIP/CLIR) Bearer capability (64 kbit/s unrestricted, speech, 3.1 kHz audio, packet mode B-channel, packet mode D-channel) Channel identification (B1, B2, or D channel request) Low-layer compatibility (type of bit rate adaptation, type of modem …) High-layer compatibility (teleservice-related issues) Keypad facility Show to B?

  26. Structure of Q.931 message (Release) Message type: RELEASESignificance: LocalDirection: Both Info ElementDirection Type Length Protocol Both M 1 discriminator Call referenceBoth M 2- Message typeBoth M 1 CauseBoth O 2-32 Display n  u O Signal n  u O 2-3 Cause description may require many bytes

  27. Setup of a PSTN call User A Exchange A Exchange B User B off-hook SS7 ISUP dial tone B number ringing tone ringing tone user B answers connection ok

  28. Setup of an ISDN call using Q.931 User A Exchange A Exchange B User B Setup off-hook Setup Call proceed SS7 ISUP Alert “ring” Alert Connect user B answers Connect connection ok

  29. SS7 Common Channel Signalling System Nr. 7 IN Intelligent Network concept

  30. History of inter-exchange signalling Before 1970, only channel-associated signalling (CAS) was used. In CAS systems, signalling always occurs in-band (i.e. over voice channels). CAS SS6 = CCIS (common channel interoffice signaling) was widely deployed in North America, but not in Europe (=> concentrating on SS7 instead). CCIS Starting from 1980 (mainly in Europe), CAS was being replaced by SS7. The use of stored program control (SPC) exchanges made this possible. Like CCIS, signalling messages are transmitted over separate signalling channels. Unlike CCIS, SS7 technology is based on protocol stacks. SS7

  31. Channel-associated signalling (CAS) CAS means in-band signalling over voice channels. signalling possible signalling not possible (yet) Exchange Exchange Exchange circuit switched connection CAS has two serious draw-backs: 1) Setting up a circuit switched connection is very slow. 2) Signalling to/from databases is not possible (setting up a circuit switched connection to the database would be extremely inconvenient).

  32. Common channel signalling (CCS) In practice, CCS = SS7 (except maybe North America). In Finnish: CCS = yhteiskanavamerkinanto (YKM) signalling possible anywhere anytime Exchange Exchange Database The packet-switched signalling network is separated from circuit switched connections. Consequently: 1) Signalling to/from databases is possible anytime. 2) End-to-end signallingis possible before call setup and also during the conversation phase of a call.

  33. CAS vs. CCS Tokyo Oulu Exch User A (calling user) Exch Exch User B (called user) Exch Database 1) Accessing database Espoo 2) Signalling before call setup 3) Signalling during conversation phase (user-to-user => digital access technology required)

  34. Signalling points (SP) in SS7 Every SP is identified by a unique signalling point code Signalling Transfer Point (only related to SS7 network) STP STP Signalling Point (in a database, such as HLR in GSM) SP MAP INAP CAP Application protocols used in SS7 STP Exchange SP Signalling Point (signalling termination in an exchange) ISUP

  35. Intelligent Network (IN) Concept Intelligence => access to various databases Operator implements service logic (IN Service) STP SCP Service Control Point (a network element containing the service logic, is often also called database or register) MAP INAP CAP Exchange SSP Service Switching Point (enables service triggering in an exchange) ISUP

  36. Typical call-related IN procedure (1) 3. SCP 2. 4. SSP 5. 1. Exchange Exchange 1. Call routing proceeds up to Exchange 2. Trigger activated in Basic Call State Model at SSP 3. SSP requests information from SCP (database) 4. SCP provides information 5. Call routing continues (routing to next exchange)

  37. Typical call-related IN procedure (2) 3. SCP 2. 4. SSP 5. 1. Exchange Exchange 2. Trigger activated in Basic Call State Model at SSP Typical triggers: Called number (or part of number) Access code or ID information Time (hour, day) or location (mobile system) Calling number (or part of number)

  38. Typical call-related IN procedure (3) 3. SCP 2. 4. SSP 5. 1. Exchange Exchange 4. SCP provides information Example: Number translation in SCP SSP sends 800 number (0800 1234) SCP translates into ”real” number which can be used for routing the call (+358 9 4512343) translation may be based on several variables

  39. IN service examples “Traditional” IN services: - Freephone / customised charging schemes - Virtual Privat Network (VPN) - Number portability - Televoting “IN” in mobile networks: - Mobility management (HLR, VLR = databases) - Security management (Authentication ...) - CAMEL  IN in mobile networks (Customised Applications for Mobile networks Enhanced Logic)

  40. Protocol layers (”levels”) of SS7 Application protocols TUP ISUP MAP INAP CAP TCAP SCCP routing MTP level 3 MTP level 2 (link-layer protocol) MTP level 1 (64 kbit/s PCM time slot) • MTP - Message Transfer Part • SCCP - Signalling Connection Control Part • UP - User Part AP - Application Part

  41. Application protocols in SS7 • TUP (Telephone User Part) – is being replaced by ISUP • ISUP (ISDN User Part) – for all signalling related to setting up, maintaining, and releasing circuit switched connections • MAP (Mobile User Part) – for transactions between exchanges (MSC, GMSC) and databases (HLR, EIR, AuC) in mobile networks • INAP (Intelligent Network Application Part) for IN applications in fixed networks • CAP (CAMEL Application Part) for extended IN functionality in mobile networks (where MAP is not sufficient ...)

  42. MTP functions • MTP level 1 (signalling data link level): • MTP level 2 (signalling link level): • MTP level 3 (signalling network level): Physical transmission (e.g. 64 kbit/s PCM time slot) HDLC-type frame-based protocol for flow control, error control (using ARQ), and signalling network supervision and maintenance functions. Routing in the signalling network (using OPC,DPC) between SPs with level 4 users (see SIO at level 2).

  43. MTP level 2 frame formats Level 3 signalling message MSU (Message Signal Unit) F CK SIF SIO LI Control F Network: National International User part: TUP ISUP SCCP Network management LSSU (Link Status Signal Unit) F CK SF LI Control F FISU (Fill-In Signal Unit) F CK LI Control F

  44. MTP level 2 frames • MSU (Message Signal Unit): • Contains signalling messages (User Part SIO) • The received frame is MSU if LI > 2 (number of octets) • LSSU (Link Status Signal Unit): • Contains signalling messages for link supervision • The received frame is LSSU if LI = 1 or 2 • FISU (Fill-In Signal Unit): • Can be used to monitor quality of signalling link • The received frame is FISU if LI = 0

  45. Routing information in SS7 message Level 3 signalling message in SIF (Signalling Information Field) Routing label MTP management message: SLC – 4 bit signalling link code SLC OPC DPC MTP SCCP message: SLS – 4 bit signalling link selection SLS OPC DPC MTP TUP message: CIC – 12 bit circuit ID code CIC OPC DPC

  46. Structure of SS7 ISUP message Level 3 signalling message in SIF (Signalling Information Field) Routing label MTP ISUP message: SLS – 4 bit CIC – 12 bit Max 256 + 1 octets CIC SLS OPC DPC ITU-T structure ANSI => different OpP MaVP MaFP MTC MTC: Message Type Code (name of ISUP message) MaFP: Mandatory Fixed Part (no LI, no parameter names required) MaVP: Mandatory Variable Part (LI, no parameter names required) OpP: Optional Part (LI and parameter names required)

  47. Difference between SLS and CIC SLS defines the signalling link which is used for transfer of signalling information. CIC defines the circuit (used for a certain circuit switched connection) with which the ISUP message is associated. signalling link STP Exchange SSP Exchange SSP circuit

  48. F CK SIF SIO LI Control F Role of DPC and OPC in SS7 • DPC – Destination Point Code (14 bit  16384 SPs) • Termination point of application transaction • Key information for routing within SS7 network • DPC is inserted by the originating MTP ”user”. • OPC – Originating Point Code (14 bit) • Originating point of application transaction • The ”network indicator” in the SIO octet determines whether the DPC or OPC is an international, national, or network dependent SP identifier.

  49. Same signalling point codes can be reused at different network levels International SPC = 277 SPC = 277 National SPC = 277 Network specific SPC = 277 means different SPs at different network levels

  50. Functions at signalling network level MTP user ISUP SCCP Signalling link MTP level 2 Message distribution Message discrimination Message routing Signalling message handling Signalling network management

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